Influence of Key Parameters on the Crack Growth Resistance Curve (R-Curve) Behavior of End-Notched Flexure (ENF) Specimens of Laminated Composites: A Review

1  Introduction

Composite materials are a specialized category of advanced materials formed by combining two or more components, each exhibiting distinct properties. The primary objective behind the development of composite materials is to achieve a specific set of properties that cannot be realized by any individual component alone [1]. It is worth noting that over the past few decades, this approach has positioned composite materials as vital materials across various industries [2,3]. Typically, the components of a composite material can be categorized into two phases: the matrix phase and the reinforcing phase [1], which, based on these phases, composites are classified into several major groups. For instance, based on the type of matrix material used, composite materials are generally categorized into three main types: polymer matrix composites (PMCs, which are valued for their ease of processing and high strength-to-weight ratio) [4,5], metal matrix composites (MMCs, known for their applications in high-temperature environments and their excellent wear resistance) [6,7], and ceramic matrix composites (CMCs, which are distinguished by their ability to withstand extremely harsh thermal and corrosive conditions) [8,9]. It should be noted that, among these, PMCs are the most common type of composite materials, primarily due to their unique advantages and cost-effectiveness [5,10]. Additionally, within the broader category of PMC materials, there are further classifications based on the size of the reinforcement (micro or nano) and the type used, resulting in materials such as nanocomposites and laminated composites. In this field, although nanocomposite materials possess attractive mechanical [11–13], thermal [14,15], and chemical properties [16,17], their widespread use is often limited by structural challenges during manufacturing [18–20], as well as concerns related to cost and biocompatibility [21–23]. In contrast, laminated composite materials have gained popularity and widespread acceptance in various industries due to their distinct advantages and broad range of applications across different fields [24–26].

In polymeric laminated composite materials, fibers act as the primary load-bearing elements, while the resin serves as the binding matrix that holds them together. In fact, this combination creates a structure that is lightweight, strong, and flexible [4,5,10]. One of the key advantages of these materials is their design flexibility and the simplicity of fabrication, further enhancing their appeal [27]. In addition to these benefits, laminated composite materials offer a high strength-to-weight ratio, excellent corrosion resistance, and the ability to be molded into complex shapes, all of which significantly contribute to their popularity [27,28]. These qualities have made laminated composites effective alternatives in various industries, including aerospace, marine engineering, automotive manufacturing, and even sporting equipment. Nevertheless, despite these advantages, laminated composite materials are not without limitations [28]. For instance, a significant challenge is their susceptibility to out-of-plane loading due to inherent heterogeneity and discrepancies in mechanical, thermal, and chemical properties between the matrix and the reinforcement in these materials [29]. These circumstances can lead to stress concentrations and uneven stress distributions at the interlaminar interfaces, resulting in delamination, the gradual or sudden separation of adjacent layers in laminated composite materials [29].

Delamination is recognized as one of the most critical types of damage in laminated composite materials. It poses a significant failure by drastically reducing load-bearing capacity and stiffness, and it often develops without any visible warning signs before a catastrophic failure occurs [30–32]. Delamination damage typically begins when mechanical or thermal loading weakens the bond at the matrix-fiber interfaces in laminated composites, leading to discontinuities and microcracks at the interlaminar regions [33,34]. This fact means that in safety-critical structures, such as aircraft or ships, even a small, imperceptible interlaminar crack can escalate into irreparable damage. It should be noted that factors such as insufficient matrix-fiber contact, the presence of voids or impurities, and mismatches in the coefficients of thermal expansion among the laminated composite materials can accelerate this process [35,36]. As a result, it has become one of the key research challenges in this field [28]. Therefore, since the late twentieth century, numerous strategies have been proposed and explored to enhance delamination resistance [37–40]. These strategies can be broadly categorized into two main approaches, i.e., chemical modifications of the matrix or matrix-fiber interfaces [41–43], and physical changes in the laminated composite architectures [44–47]. It is worth noting that the details of these approaches will be discussed in subsequent sections. Additionally, in this field, it is essential to highlight the role of the crack growth resistance curve (R-Curve) in studying the evolution of delamination damage [48,49]. The R-Curve illustrates the variation in strain energy release rate (SERR), also known as interlaminar fracture toughness (ILFT), during the different stages of crack growth, providing valuable insights into interlaminar properties and resistance to delamination. Therefore, based on this note, the primary objective of many researchers has been to shift the behavior of the R-Curve from a linear trend to a rising, non-linear trend, thereby improving the overall delamination resistance of laminated composite materials.

In conclusion, generally, it can be succinctly stated that delamination in laminated composites is one of the most critical and performance-limiting damage mechanisms. Therefore, a clear understanding of this phenomenon, particularly through the use of the R-Curve, is crucial for developing effective strategies to mitigate it. Also, this understanding is essential for ensuring the reliability of laminated composite materials in sensitive and safety-critical applications. Therefore, this study takes a comprehensive and review-oriented approach to examine both the crack growth resistance in composite laminates under the shearing mode of loading (mode II, Fig. 1b) and the various strategies proposed to enhance delamination resistance. It should be noted that while extensive investigations have been conducted on delamination under opening-mode loading (mode I, Fig. 1a), a focused new review of mode II crack-growth resistance, which is considered one of the most relevant and practical loading modes in engineering applications [49], was found to be essential. Also, this need prompted the selection of the present research topic, particularly in light of recent advancements aimed at improving this property. To this end, the present study begins with a comprehensive overview of the R-Curve in polymeric laminated composite materials, including its significance and the experimental methods and data analysis techniques used for evaluation. It then provides an in-depth discussion of the mechanisms, conventional approaches, and emerging strategies designed to improve the R-Curve and enhance mode II crack growth resistance in these materials. Importantly, this review aims to provide both researchers and engineers with a broad and practical perspective by clarifying effective mechanisms and applicable solutions. This will help identify and adopt innovative and optimized approaches for achieving superior performance, durability, and reliability in polymeric laminated composite materials.

Figure 1: Schematic of interlaminar damage and crack growth under: (a) Mode I loading; (b) Mode II loading.

Therefore, it can be said that this review aims to answer three concrete questions:

•   How do different interface designs (e.g., interlayers, yarn architectures) influence the shape and magnitude of the mode II crack growth resistance curve?

•   Are current standard tests and data reduction methods, i.e., compliance calibration method (CCM) and the compliance-based beam method (CBBM), still reliable when toughening mechanisms create extensive bridging and large fracture process zones (FPZ)?

•   Which toughening strategies are mature enough for structural application, and which remain limited to laboratory-scale coupons?

2  R-Curve and Its Importance

From fracture mechanics, when a crack exists in a material, the strain energy stored around the crack tip is released to create new crack surfaces and facilitate crack growth. To quantify this process, fracture mechanics introduces a parameter known as the strain energy release rate (SERR, G), which measures the energy released per unit increment of crack extension. In simple terms, if the energy provided by external loading exceeds the intrinsic resistance of the material at the crack interface, crack propagation will inevitably occur [50]. It is important to note that this parameter, for pure mode II loading, it is denoted as GII. It should be noted that for certain types of materials, such as monolithic metals and some nanocomposites, this value tends to be constant. However, in many advanced materials, especially laminated composites, the resistance to crack propagation is not a constant value. In fact, as the crack propagates in these materials, the interactions at the crack tip and along the delamination interfaces may change, altering the crack growth resistance and SERR. In this field, to capture this behavior more accurately, the concept of the crack growth resistance curve is used. In other words, the R-Curve illustrates the relationship between crack extension (Δa) and the corresponding SERR required for further crack growth (See Fig. 2).

Figure 2: Schematic of common different types of R-curve: (a) rising-steady-state R-curve; (b) steadily rising R-curve; (c) flat R-curve; (d) declining R-curve.

It should be noted that, as illustrated in Fig. 2, the crack-growth resistance of laminated composites can vary significantly after the initial crack initiation and during subsequent crack extension. This variation depends on the material characteristics and the active micromechanical or nanomechanical toughening mechanisms at the crack front and delamination surfaces in laminated composite materials [50–52]. For a more detailed explanation, in a rising R-Curve, the crack growth resistance increases with crack growth due to the activation of additional toughening mechanisms along the interfaces, ultimately reaching a saturation or steady-state level (see Fig. 2a,b). In addition, another type of R-Curve, i.e., a flat R-Curve, indicates that the resistance remains nearly constant after crack initiation, suggesting a lack of significant toughening mechanisms (refer to Fig. 2c). Conversely, a declining R-Curve, as demonstrated in Fig. 2d, shows that crack growth resistance decreases as the crack grows, which can result in sudden delamination, unstable crack propagation, and abrupt load drops in the affected structural component. It’s also important to note that in some composite systems with complex architectures, a mixed response may occur.

Additionally, for a comprehensive understanding and practical application of the R-Curve in laminated composite materials, several key parameters are crucial (Fig. 2). For further explanation, one of the fundamental parameters is the ILFT value at crack initiation, denoted as GIICini.. This represents the energy required to initiate crack growth from the initial crack tip. It should be noted that in laminated composites, this value displays significant sensitivity to interfacial adhesion quality, fiber-matrix bonding, resin-rich regions, and microscopic imperfections [53,54]. Another important parameter is the propagation value of ILFT, represented as GIICprop.. This parameter defines the critical energy level at which stable, steady-state crack growth occurs throughout the composite laminates, serving as an indicator of the structural safety margin and illustrating the extent to which the material can withstand a load before catastrophic delamination occurs. It is noteworthy that in systems exhibiting a rising R-Curve, GIICprop. consistently exceeds GIICini. (as illustrated in Fig. 2a,b). In this field, it should be noted that a sustained rising R-Curve is usually linked to mechanisms that keep carrying load across the delamination surface behind the crack tip. Typical examples include fibre/yarn bridging (often stronger in woven composites due to transverse yarns), plastic deformation of thermoplastic or porous interlayers, and crack deflection or pinning caused by interleaves. These effects expand the FPZ and increase the energy dissipated as the crack grows, which leads to a gradual rise in GII.

Conversely, in instances where no toughening mechanisms are active, GIICprop. may equal (refer to Fig. 2c) or even fall below (refer to Fig. 2d) the initiation value. In fact, it can be said that if delamination growth is mainly controlled by matrix shear cracking with little bridging or limited load transfer at the interface, the FPZ stays relatively small, and GII usually reaches a steady plateau quickly.

Additionally, in this field, the fracture process zone length, lFPZ, is another key parameter associated with the R-Curve. This length reflects the extent of the region ahead of the crack tip where micro-damage and toughening mechanisms operate. It is worth noting that lFPZ is specifically defined as the difference in crack length between the GIICini. and GIICprop. [55,56].

3  Achieving a Laminated Composite R-Curve under Mode II Loading

To obtain the R-Curve and evaluate the SERR in laminated composite specimens under mode II loading, where crack surfaces experience in-plane shear sliding, standardized experimental procedures, along with conventional data reduction methods, are commonly employed. Each of these approaches will be discussed in detail in the following sections.

3.1 Common Experimental Tests

One of the earliest experimental methodologies employed to investigate mode II ILFT in laminated composites was the short beam shear (SBS) test [57–59]. However, this method presented several inherent limitations, such as a relatively small span-to-thickness ratio and non-uniform stress distributions resulting from the applied load. These factors contradicted the assumptions of beam theory, which served as the foundation for most data reduction schemes [57,59]. In light of these challenges, subsequent refinements were introduced, and the classical framework of linear elastic fracture mechanics (LEFM) was utilized to ascertain the mode II ILFT/SERR in polymeric composite laminates. In this context, Barrett and Foschi [38], in 1977, employed the end-notched flexure (ENF) specimen to examine mode II ILFT in cracked wood beams. Subsequently, in 1982, Russel and Street [60] adopted the same ENF configuration to assess the critical ILFT/SERR of laminated composites.

It should be noted that, currently, the ENF specimen, in conjunction with the ASTM D7905 standard [61], is considered one of the most widely recognized experimental methods for determining mode II SERR of polymeric laminated composite materials. However, in addition to the primary methods, several other test configurations have been introduced over the years, although they have not gained widespread application like the ENF test specimens and ASTM D7905 standards. For example, in 1989 and 1990, Maikuma et al. [62,63] developed the center notch flexural (CNF) specimen to investigate mode II ILFT under both static and impact loading conditions.

3.2 Common Data Reduction Methods

As previously discussed, this review primarily focuses on the widely used ENF configuration for deriving R-Curves in composite laminates. Therefore, it particularly addresses the common data reduction methods associated with this specimen. In this context, it is essential to emphasize that mode II SERR/ILFT is typically determined in accordance with ASTM D7905 [61], utilizing the compliance calibration method (CCM). It should be noted that this method involves measuring the specimen’s compliance at five different pre-crack lengths (i.e., 20, 25, 30, 35, and 40 mm). To achieve this, five-point compliance curves are created by conducting three-point bending tests at four non-critical crack lengths (20, 25, 35, and 40 mm). It is worth noting that, during these tests, the applied load is half of the fracture load. Subsequently, a mode II fracture test is performed at a crack length of 30 mm (see Fig. 3).

Figure 3: Calculation method of the m parameter.

The CCM data reduction method is based on the study results of Irwin and Kies [64], as shown in Eq. (1). It should be noted that in this equation, the parameters P, a, and B are defined as critical load at a specified crack length, interlaminar crack length, and ENF specimen width, respectively.

G=P22BdCda(1)

However, the compliance parameter (C), based on Eq. (2), is experimentally calibrated using the initial compliance (C0), the cube of the crack length (a3), and the slope of the fitted line derived from the relationship between C and a3 (m, as shown in Fig. 3).

C=C0+ma3(2)

Finally, the mode II ILFT, as determined using the CCM (GIIcCCM), is calculated by combining Eqs. (1) and (2), as summarized in Eq. (3) [61].

GIIcCCM=3mP2a22B(3)

It is important to note that accurately determining the mode II SERR/ILFT, based on CCM, requires simultaneous monitoring of load, load-point displacement, and visual crack length. This process can be experimentally challenging for specific materials and often results in significant errors. To overcome these limitations, de Moura et al. [65,66] proposed an alternative method based on beam theory, known as the compliance-based beam method (CBBM). This approach eliminates the need for direct crack length measurements by introducing the concept of an equivalent crack length (aeq). Within this framework, the mode II ILFT, based on CBBM (GIIcCBBM), is directly derived from load-displacement data using Eq. (4), where h represents half the ENF specimen thickness [65,66].

GIIcCBBM=9P2aeq216B2h3Ef(4)

Also, the aeq and equivalent flexural modulus (Ef) value in Eq. (4) are obtained from Eqs. (5) and (6), respectively, where L is the span length of the mode II ILFT test [65,66].

aeq=[CcC0ca03+23(CcC0c−1)(L32)]13(5)

Ef=3a03+2(L32)8Bh3×(C0c)−1(6)

Additionally, the equivalent compliances associated with the initial compliance of the ENF specimen (C0C) and the load-displacement curve (Cc) are calculated using Eqs. (7) and (8), respectively [65,66]. In these equations, the parameters C0, C, and G13 represent the initial compliance, the equivalent compliance derived from the load-displacement data, and the out-of-plane shear modulus, respectively.

Notably, previous studies [65–67] suggest that the influence of G13 on the mode II SERR of laminated composites is minimal. Therefore, these parameters can be reasonably approximated using values reported in existing literature for similar materials, which negates the need for additional experimental measurements [65–67].

C0c=C0−3(L2)10BhG13(7)

CC=C−3(L2)10BhG13(8)

3.3 Limitation of Data Reduction Methods

The mode II ILFT test, based on ENF specimens, is commonly used for characterizing mode II fracture behavior. However, its reliability depends heavily on the accurate observation and measurement of delamination growth. In mode II loading, the delamination front is often hidden within the laminate and primarily propagates through interfacial sliding. This makes direct measurement of the crack length particularly challenging. Additionally, factors such as frictional sliding and local stick-slip behavior along the interface can complicate this process. Furthermore, the crack front may lose its uniformity or even branch, especially in laminates with toughened interfaces, such as those that include interleaves, through-thickness reinforcements (like Z-pins), or highly textured interfacial structures. When considered together, these phenomena create uncertainty in accurately determining the effective crack length and, more importantly, in identifying the true onset of crack propagation. Since the evaluation of SERR directly relies on these measurements, any ambiguity in tracking the crack front or detecting the initiation of cracks can significantly impact the reported values. Also, it should also be noted that extensive bridging and long FPZs may challenge classical LEFM assumptions, particularly the small-scale process zone requirement and the notion of self-similar crack advance. As a result, reported SERR values can represent an apparent toughness that includes bridging contributions and may exhibit sensitivity to specimen geometry and the adopted data-reduction procedure. This observation motivates the careful reporting of both initiation and propagation values.

In this context, selecting the appropriate data-reduction methodology is crucial. The CCM, as outlined in ASTM D7905, relies on direct measurements of crack length. Therefore, CCM is generally effective for determining the SERR when the crack front is easily identifiable, the FPZ is relatively small, and bridging effects are minimal. Under these conditions, the interpretation aligns well with classical LEFM assumptions. However, when the crack front is obscured, uneven, or accompanied by branching or sub-cracking, conditions often observed in highly toughened interfaces, the reliability of crack-length tracking diminishes. In these scenarios, CCM becomes more dependent on the operator, and its accuracy in determining initiation values may be compromised, which can affect the comparability of results across different studies.

In contrast, the CBBM addresses a significant experimental challenge by eliminating the need for direct measurements of crack length. Instead, it utilizes an equivalent crack length that is inferred from the measured compliance. This feature makes CBBM particularly beneficial for systems where the physical crack tip is hard to observe or where crack-front irregularities are anticipated. Additionally, because it does not depend on visual tracking, CBBM reduces operator variability and is often regarded as a convenient method for constructing R-curves and assessing crack propagation.

3.4 Comparing the CCM and CBBM Data Reduction Methods

In summary, both the CCM and the CBBM have unique advantages and limitations. CCM is aligned with ASTM practices and is particularly effective for determining the SERR when the crack front is visible and bridging effects are minimal. However, its reliability decreases in scenarios involving hidden or branching crack fronts, and the results can be more dependent on the operator’s skill.

On the other hand, CBBM removes the need for direct measurement of crack length, making it advantageous for analyzing propagation and evaluating R-curves. Nevertheless, in cases with significant bridging or long process zones, CBBM may produce higher apparent values of SERR because the compliance loss accounts for both crack advancement and the effects of bridging and the process zone.

Therefore, the choice between these methods should be based on the specific fracture mechanisms and interfacial architecture of the laminate being studied, as well as the intended focus on initiation vs. propagation behavior.

4  Mechanisms and Enhancement Factors

Up to this point, an overview was provided of laminated composites, delamination damage, the concept and importance of the R-Curve in delamination damage, and common standard experimental and data reduction methods used to characterize the R-Curve. Now, in this section, the main focus shifts to the parameters that govern the R-Curve in laminated composite materials subjected to mode II loading. In fact, this section explores explicitly the mechanisms that modify this curve, as well as proposed strategies to enhance it. To this end, the key factors, mechanisms, and approaches that contribute to the formation of the R-Curve and improve this behaviour will be discussed in detail. It is essential to emphasize that, given the critical need to enhance resistance to delamination in laminated composites, this review will focus on methods that result in either a consistently rising R-Curve or a two-stage response, i.e., an initial rising trend followed by a steady-state growth trend.

In this context, numerous studies have proposed various strategies to achieve an upward trend in the R-Curve and improve its performance under mode II loading. The primary goal of these approaches is to increase interlaminar friction, which enhances interlaminar energy absorption and slows the sliding of delamination interfaces. Additionally, these strategies aim to improve interlaminar interaction, which strengthens bonding, facilitates more efficient stress transfer across interfaces, and reduces stress concentration at the crack tip. Also, they seek to activate toughening mechanisms that resist crack propagation, such as fiber breakage, fiber bridging, crack pinning, and crack arresting. Therefore, in the following sections, several practical and widely adopted strategies in this field will be discussed in detail.

4.1 Stitching and Z-Pinning

In other words, a major weakness of laminated composites is the lack of reinforcing elements in the thickness direction, which results in poor performance in out-of-plane applications. Therefore, through-thickness reinforcement of laminated composites is an effective strategy for reducing delamination damage, commonly known as Z-pinning or stitching [46,68]. This approach aims to enhance the interaction between layers and improve the interfacial bonding in delamination areas of laminated composites. As a result, it boosts both the out-of-plane mechanical properties and the resistance to interlaminar crack propagation in laminated composites [69]. It is worth noting that the diameter of Z-pins is crucial in determining the effectiveness of this technique. For instance, Che et al. [70] studied how different Z-pin diameters impacted performance by using two sizes of carbon pins. Their results showed that reducing the pin diameter to 0.1 mm led to a 176% increase in mode II ILFT, while also reducing stress concentrations at the delamination interfaces. Also, this change shifted the primary fracture mechanism from Z-pin pullout to shear fracture.

Finally, it can be said that the through-thickness reinforcements, such as stitching and Z-pinning, effectively enhance delamination resistance by creating mechanical bridges and facilitating load transfer across interfaces. However, these advantages come with certain drawbacks that limit their universal application. For instance, disturbances to in-plane fibers, such as fiber waviness and localized damage, can impair in-plane properties, particularly compressive performance. Additionally, the introduction of extra materials and resin-rich zones may lead to increased mass, while the processes of insertion and manufacturing raise complexity and costs [70,71].

4.2 Changing in Fabrication Parameters

Processing and fabrication parameters are other critical factors that influence delamination resistance under mode II loading, as well as the overall trend of the R-Curve in laminated composites. In these fields, key variables such as curing temperature, heating rate of the curing process, post-curing conditions, and curing pressure [72–75] play a crucial role in determining the ILFT and delamination behavior of the composite laminate. In fact, it can be stated that by optimizing the curing temperature or pressure, or by applying an appropriate post-curing cycle, it is possible to enhance resin flow, promote interlaminar interaction and entanglement, and ultimately improve ILFT and the behavior of the R-Curve. It should be noted that when the specified parameters exceed their optimal limits, it can lead to several challenges, such as fiber degradation, microcracking within the matrix, or thermal damage. These challenges can negatively impact both ILFT and in-plane mechanical performance. For example, Liu et al. [76] found that increasing the curing temperature of carbon fiber-reinforced polymer laminates up to an optimal limit improves mode II ILFT of these composite laminates. This enhancement occurs due to improved resin flowability and better interfacial bonding. However, if the curing temperature is raised beyond this optimal point, it can cause excessive fluidity of the resin. This increased fluidity can expedite the lay-up process and boost the chances of bubble entrapment and porosity formation, ultimately leading to early defects that jeopardize the mechanical properties of the polymeric laminated composites. Also, Li et al. [77] examined how different curing pressures affect interlaminar properties in laminated composites. They found that applying a moderate pressure significantly enhances interlaminar entanglement, which in turn improves mode II ILFT. However, using excessive curing pressure can lead to fiber distortion and interfacial damage, ultimately decreasing delamination resistance in these composite laminates.

In other words, when optimizing the cure for thermosets, several key parameters, including cure temperature, pressure, heating rate, and post-cure conditions, play a crucial role. These factors influence resin flow, void content, and the quality of interlaminar bonds, all of which directly affect mode II delamination behavior. Also, in the case of thermoplastic laminates, in-situ consolidation parameters significantly impact the level of intimate contact, healing between plies, and void formation. In fact, these aspects are critical for determining mode II resistance.

In conclusion, it should be noted that because of these sensitivities, it is crucial to control processing parameters precisely. In fact, this necessitates carefully designed manufacturing schedules and extensive optimization through repeated testing, which can pose practical challenges for large-scale industrial implementation.

4.3 Specific Stacking Sequence and Wave Pattern

The stacking sequence and ply orientation are crucial factors that determine the delamination resistance and ILFT of laminated composites [44,45,78–82]. Gong et al. [83] investigated the effects of various stacking sequences and demonstrated that the fiber orientation at the delamination interface of laminated composites plays a significant role in influencing mode II ILFT and R-Curve behavior. In this context, it can be stated that when the fiber lay-up is arranged so that transverse yarn bundles are aligned against the crack front, mechanisms such as fiber bridging and fiber breakage become more effectively activated, thus enhancing resistance to delamination and ILFT [84–86]. For instance, Ozdil et al. [87] found that polymeric composite laminates with a [±45]5 lay-up demonstrated higher ILFT values compared to those with [±30]5 and unidirectional configurations. This enhancement stems from the transverse alignment of fiber bundles at the delamination interface, which fosters a more uniform stress distribution and enhances resistance to both crack initiation and propagation. Also, Salamat-Talab et al. [88] discovered that incorporating glass fibers with an 8-harness (8H) satin weave in a transverse orientation at the delamination interfaces led to a fully rising R-Curve and enhanced mode II ILFT by about 27% compared to woven glass fabrics positioned at the same interface. However, when the same type of fiber (8H) was used with a longitudinal fiber orientation, the limited number of transverse yarn bundles resulted in moderate toughness improvement and failed to maintain a fully rising R-Curve due to the lack of continuous toughening mechanisms along the entire crack propagation path. Additionally, in another study, Ogasawara et al. [85] examined three different stacking sequences based on the orientation of yarn bundles in a 5H satin weave fiber at the delamination interface. Their findings indicated that when the majority of yarn bundles in the 5H satin weave fiber were oriented transversely, the ILFT was significantly enhanced, due to increased interaction between the fibers and matrix, as well as the activation of fiber bridging and fiber fracture mechanisms during the delamination process. It should be noted that in addition to the stacking sequence and yarn bundle orientation, factors such as the type of warp-weft pattern also affect delamination resistance [84,89–93].

4.4 Functionalization or Fiber Treatment

One of the key factors that enhances mode II delamination resistance in laminated composites is the surface functionalization or treatment of reinforcing fibers. These methods aim to improve the interlocking and interaction between fibers and the matrix, leading to stronger adhesion between the fibers and resin during crack propagation. As a result, more interlaminar energy can be absorbed through mechanisms like fiber bridging, fiber pull-out, and fiber breakage. This not only delays crack growth but also promotes a rising R-Curve and a more progressive fracture response [94–96]. For example, Qian et al. [97] demonstrated that applying oxygen plasma treatment for secondary layer functionalization significantly enhanced fiber-matrix adhesion, resulting in a 34% increase in mode II ILFT for glass/epoxy laminates. In fact, this improvement was primarily due to better interfacial bonding and the activation of energy-dissipating mechanisms, such as fiber bridging. Also, Zhao et al. [98] found that surface treatment of reinforcing fibers resulted in increases in mode II ILFT of laminated composites. These enhancements were linked to the formation of new functional groups on the fiber surfaces, which improved fiber-matrix interfacial adhesion and encouraged mechanisms that resist cracks, thereby enhancing delamination resistance in the treated composites.

It is important to note that the physical parameters used during these treatments can significantly affect various mechanical properties of the polymeric composite laminates. In fact, if the processing parameters are not set correctly, they may damage the fiber surfaces, leading to a reduction in both interlaminar and in-plane properties, as well as an increase in fabrication costs [99–101].

4.5 Interleaving

In recent years, using interlayers in polymeric laminated composites has garnered significant attention as an effective method to improve ILFT or R-Curve behavior and reduce the risk of delamination in these materials. In fact, placing an interlayer at the fiber-matrix interfaces in laminated composites helps alleviate stress concentrations and interfacial discontinuities, ultimately improving the out-of-plane performance of these materials. Additionally, a key benefit of this approach is that while the out-of-plane behavior is greatly enhanced, the in-plane mechanical properties of the laminates are either maintained or may even be slightly improved [102–105].

When utilizing an interlayer as a secondary component between adjacent layers in laminated composites, it is crucial to have a thorough understanding of the primary damage modes and loading conditions. This knowledge allows for the design of an optimized interlayer that is specifically tailored to address the typical failure mechanisms in a given application, thereby ensuring enhanced delamination resistance and improved mechanical performance. For instance, for a mode II loading condition, an ideal interlayer should not only create a secondary bonding mechanism between the fibers and the resin but also improve interfacial friction. Additionally, it should activate interlaminar energy-absorbing mechanisms, such as fiber bridging and other processes that help arrest cracks. Accordingly, the choice of interlayer material system, along with its fabrication method and geometric configuration, is critically important.

Interlayers can be broadly classified based on their composition and structural characteristics into three categories: simple (single-component), hybrid (multi-component), and advanced or specific interlayers. Therefore, the following sections will discuss some of the most effective and widely studied interlayer configurations reported in recent literature.

4.5.1 Simple Interlayer

One category of interlayers consists of single-component materials, which typically include nanofibers or nano- and micro-scale particles. The main purpose of using these interlayers is to improve the interaction between adjacent layers in laminated composites and activate interlaminar mechanisms that resist cracking. For instance, nanofibrous interlayers can promote nanofiber bridging, while incorporating nano and micro particles into the structure can facilitate crack pinning [106].

In this field, it is noteworthy that a combination of structural and processing factors influences the efficiency of these interlayers in improving mode II ILFT. These include thickness [107–109], surface area density [110], the number of interlayer sheets [111], and the inherent properties of the materials used [106]. By optimizing these factors, it is possible to achieve a balance between interfacial adhesion, resin infiltration, and structural integrity. This optimization ultimately leads to a significant enhancement in the Mode II ILFT of laminated composites. For instance, Kheirkhah Barzoki et al. [107] demonstrated that using an electrospun polyvinyl Butyral (PVB) interlayer with an optimal thickness of 0.025 mm significantly enhanced mode II ILFT and resulted in an increasing R-Curve behavior in glass fiber reinforced composites. This improvement was attributed to adequate interfacial bonding and the activation of energy-absorbing mechanisms. In contrast, increasing the interlayer thickness had a negative impact due to disrupted resin flow throughout the laminate. Similarly, Salimi-Mofrad et al. [108] reported that electrospun polyamide (PA) interlayers with a thickness of 0.11 mm resulted in an 84% increase in mode II ILFT. In another study, Bahmani et al. [110] explored the use of nonwoven polypropylene (PP) interlayers within glass/epoxy laminated composites. They discovered that incorporating an interlayer with a surface weight of 60% increased mode II ILFT by 56%. However, further increasing the areal density resulted in reduced toughness. This decline was attributed to hindered resin flow and inadequate interfacial contact.

Also, in the context of nano/micro fillers used as interlayers, Li and Li [111] reported that the incorporation of carbon nanotubes (CNTs) interlayers progressively improved mode II ILFT, achieving enhancements of up to 100% under optimal configurations. Shan et al. [112] demonstrated that applying CNTs interlayers onto carbon/epoxy composite laminates could increase ILFT by approximately 11% under optimized conditions. This enhancement was mainly attributed to the activation of mechanisms such as crack pinning and crack deflection. However, an excessive amount of CNTs disrupted interfacial bonding and adversely affected the interlaminar properties.

Finally, based on the aforementioned studies, it can be concluded that nanofibers primarily enhance mode II ILFT by introducing physical barriers that promote crack pinning and nanofiber bridging. However, the efficacy of these interlayers is fundamentally governed by a strict geometric trade-off regarding resin infiltration (which dictates interlaminar adhesion) and the mechanisms resisting crack propagation. For instance, as evidenced by the previously discussed studies, while optimizing the thickness and areal density of the interlayers yields significant improvements in crack growth resistance, exceeding the critical thresholds of these parameters inevitably disrupts resin flowability between the layers. This disruption weakens interlaminar contact and can even lead to a reduction in mode II ILFT. Similarly, while the incorporation of nanoparticles and the use of nanoparticle-based interlayers can enhance mode II ILFT under conditions of ideal dispersion, an excessive concentration of these materials rapidly makes the interface brittle, diminishes resin flowability, disrupts bonding, and intensifies stress concentration at the delamination interface, ultimately decreasing crack growth resistance

4.5.2 Hybrid Interlayer

As previously mentioned, single-component interlayers typically cannot activate all possible toughening mechanisms at the same time, which limits their effectiveness in preventing crack propagation. As a result, hybrid interlayers have gained significant attention in recent years. In fact, these interlayers offer combined material properties and can create multiscale interactions within the laminated composite structure. In this regard, Liu et al. [113] investigated a porous poly(ether sulfone) (PES)/CNTs hybrid interlayer and found that incorporating this interlayer significantly improved the mode II ILFT of carbon/epoxy laminated composites. Also, Zhang et al. [114] used an electrospun hybrid interlayer made from PES, polyphthalamide ether nylon ketone (PPENK), and polyphthalamide ether ketone (PBPESK) hybridized with zinc oxide nanowires. They reported that optimizing the nanowire deposition enhanced the ILFT. Also, in a related study, Kazemi et al. [106] developed an innovative green composite nanoparticle system that features a silica core and a magnesium hydroxide shell. They incorporated 0.75 wt% of these nanoparticles into electrospun poly(vinyl alcohol) (PVA) nanofibers [106]. Their findings demonstrated a 122% increase in mode II ILFT, along with a fully ascending R-Curve in glass/epoxy composite laminates. In this regard, it should be noted that this significant resistance to crack propagation was attributed to enhanced interfacial adhesion and secondary bonding between the PVA nanofibers and the epoxy matrix. Additionally, factors such as the breakage of the nanofibers, nanofiber bridging, and crack pinning (facilitated by the green composite nanoparticles) contributed to this improved performance.

4.5.3 Advanced and Specific Interlayer

In addition to all the aforementioned interlayers, recently, new interlayer architectures and innovative fabrication methods have gained attention, particularly three-dimensional (3D)-printed and mesh-structured interlayers. It should be noted that these approaches offer significant environmental and economic benefits [115,116]. In this context, Beylergil and Duman [115] used a 3D-printed PA interlayer designed with a mesh-like structure, resulting in an impressive 81% improvement in mode II ILFT for carbon/epoxy laminated composites. Similarly, Salamat-Talab and Kazemi [116] investigated how the type of filament material (wood/Poly(lactic Acid) (PLA) and PVA) and the thickness of 3D-printed interlayers, with cellular structure, affect the R-Curve behavior and mode II ILFT of polymeric laminated composites. They found that increasing the 3D-printed interlayer thickness up to 0.4 mm resulted in an improvement in mode II ILFT by as much as 70%, leading to a fully rising R-Curve (particularly with PVA interlayer). Generally, it can be stated that the resin-rich areas within these 3D-printed interlayers developed stronger adhesion, while the reinforcing filaments served as microstructural barriers, activating mechanisms for crack pinning and arresting during the propagation of mode II delamination. Furthermore, their results showed that the 3D-printed wood/PLA interlayers were effective in resisting crack growth, even at higher loading rates of up to 50 mm/min [53]. Also, Zhang et al. [117] reported a 69% increase in mode II ILFT for glass fiber reinforced polymer composites that included a mesh-like PA interlayer. They attributed this improvement to several energy-absorbing interlaminar mechanisms, such as PA fiber bridging, plastic deformation, fiber fracture, and the interlayer’s capability to induce controlled crack deflection and guidance.

In this section, we can conclude that interlayers, specifically simple, hybrid, and architectured types, are among the most promising methods for enhancing mode II delamination resistance. This is due to their ability to be specifically tailored to selected interfaces without the need to modify the overall resin system. However, several important questions remain regarding their structural adoption, particularly concerning durability, manufacturability, and scalability.

4.6 Using Particle-Reinforced in Matrix

The incorporation of reinforcements, particularly micro- and nano-fillers, into the matrix of laminated composites is a widely adopted method for enhancing their thermal, electrical, chemical, and mechanical properties, including resistance to interlaminar delamination [17,118]. In this section, it is important to highlight the distinction between matrix modification and local interface-engineering approaches. Matrix modification involves altering the resin throughout the entire laminate, such as by adding nanoparticles or fillers. This can impact viscosity, manufacturing conditions, cost, qualification requirements, and potentially the in-plane properties of the laminate. On the other hand, local interface design focuses on enhancing toughness at specific interfaces that are prone to delamination, without altering the overall resin system of the laminate.

In using particle-reinforced in matrix field, since the early 1990s, nanomaterials such as CNTs and graphene have emerged as innovative additives [119,120]. These materials have demonstrated their effectiveness in improving delamination resistance of polymeric composite laminates, under various loading conditions, and in facilitating the formation of R-Curves [121–123]. In this context, Akhavan-Safar et al. [124] demonstrated that incorporating micro-cork particles, as fillers, within the matrix phase of glass/epoxy laminates can significantly enhance the mode II ILFT. Liu et al. [125] explored the synergistic effects of hybrid multi-walled CNTs (MWCNTs) and graphene oxide, as nanofillers, on mode II ILFT and reported remarkable improvements of up to 215% compared to reference specimens. It should be mentioned that these enhancements were attributed to the activation of reinforcing mechanisms such as improved fiber-matrix interfacial interactions and crack arresting. Similarly, Soyugüzel et al. [126] found that adding nitrogen-doped reduced graphene oxide particles to carbon/epoxy laminated composites activated crack-pinning mechanisms at the delamination interface, resulting in a 45% increase in mode II ILFT.

Generally, the studies demonstrate that the improvements in delamination resistance and crack growth behavior due to matrix reinforcements can be primarily attributed to several factors:

•   Stronger interlaminar bonding, resulting from the infiltration of nanoparticles into both the matrix and the reinforcing fibers.

•   Increased interfacial friction at the interfaces of the laminated composite layers, which is enhanced by the presence of nanoparticles.

•   Activation of interlaminar energy-dissipating mechanisms, including crack bridging and crack arresting.

•   Redistribution of stress, which helps alleviate stress concentration at the crack tip.

Nevertheless, this approach has its limitations and potential drawbacks. In other words, some nanofillers or excessive filler loading can increase the hydrophilicity of the matrix, reduce the operational temperature range, elevate moisture uptake, and raise resin viscosity, potentially hindering the uniform dispersion of particles. As a result, particle accumulation within the resin can create zones of stress concentration, negatively impacting crack growth resistance and decreasing both interlaminar and in-plane mechanical properties of laminated composites [127,128]. For example, Srivastava et al. [129] demonstrated that adding 3 wt.% of multi-walled CNTs or graphene enhanced mode II ILFT in carbon/epoxy composites by 29% and 53%, respectively. In contrast, the addition of 3 wt.% carbon black, due to its high aggregation and poor dispersion, disrupted interlaminar bonding and resin flow, leading to a 43% decrease in mode II ILFT.

In conclusion, while the addition of micro- and nanofillers typically increases ILFT and encourages the formation of favorable R-Curves, achieving uniform dispersion of particles within the matrix is essential, as this uniformity allows for the full realization of benefits from mechanisms such as crack arresting, fiber bridging, and stress redistribution. In fact, these factors collectively contribute to reliable improvements in interlaminar delamination resistance and the development of R-Curves in polymeric laminated composite materials.

Furthermore, it is worth noting that the incorporation of high-surface-area fillers fundamentally alters the rheology of the matrix. This creates significant challenges in ensuring uniform particle dispersion, and often severely limits the use of infusion-based manufacturing processes due to premature resin gelation and the creation of localized stress concentrations in areas where particles agglomerate.

4.7 A Comparison of the Most Common Methods

While the strategies discussed earlier improve mode II ILFT and R Curve behavior, choosing a specific method for practical use requires carefully considering the trade-offs between mechanical performance, technological readiness, and industrial feasibility. For instance, macroscale techniques like stitching and Z pinning, though proven to enhance ILFT, often lead to a noticeable reduction in in-plane properties and increased manufacturing complexity, both of which create significant operational challenges.

Also, matrix modification through the incorporation of micro and nanoscale fillers offers a pathway toward achieving multifunctional properties throughout the composite laminates. Nevertheless, this approach substantially alters the rheological characteristics of the resin bulk, frequently leading to increased viscosity, a higher likelihood of particle agglomeration, the formation of stress concentration, and ultimately the initiation of microcracks in these laminated composite structures. Collectively, these factors may deteriorate the mechanical performance of the final structure and thus constitute important barriers to the broader im-plementation of this strategy. Similarly, approaches based on interleaving directly target the delamina-tion prone interfaces while preserving in plane performance. Consequently, these approaches can substantially improve resistance to crack propagation and mode II ILFT. However, a notable difference persists in how these methods are technologically advanced, especially when considering more complex versions. Indeed, despite the impressive performance of these technologies in laboratory scale, their implementation in industrial production is still impeded by challenges associated with scalability and long-term reliability

Finally, it should be noted that, in addition to the aspects outlined above, a more comprehensive understanding and systematic comparison of these strategies is facilitated in the following section. Table 1 provides a concise overview of the mechanisms, advantages, limitations and trade-offs, industrial maturity, and the most suitable application domains for the crack-propagation resistance enhancement approaches discussed.

Also, based on the previous mentions and Table 1, it can be stated that among the numerous strategies available, one of the most commonly used methods to enhance out-of-plane properties and increase delamination resistance in polymeric laminated composite materials, without significantly affecting in-plane performance, is the incorporation of secondary layers (i.e., interlayers) or local modifications at the delamination interface [102–104]. Consequently, to provide a comprehensive overview, Table 2 summarizes various interleaving and interfacial modification techniques documented in the literature, along with their respective effects on mode II ILFT of polymeric laminated composites. It is important to note that all reported improvements are expressed as percentage changes relative to the reference laminates that did not undergo interlayer and interfacial modifications.

5  Conclusions, Limitations, and Outlook

5.1 Key Conclusions from the Current Literature

In recent years, polymeric laminated composites have been essential in high-performance structures. However, their reliability is significantly limited by weak interlaminar bonding. In fact, the lack of through-thickness reinforcement in laminated composite materials, along with the variability at the fiber-matrix interface, creates stress discontinuities that severely compromise performance in out-of-plane directions. This makes delamination a major and often undetected failure mode. Therefore, it is crucial to understand the mechanisms that govern interlaminar crack growth and interlaminar delamination failure. It is important to note that delamination behavior is highly dependent on the loading mode, and among these modes, mode II loading conditions hold particular significance, due to significant shear stresses encountered in practical applications. In this field, the review of related studies indicates that the primary goal of various strategies is to enhance mode II ILFT and delamination resistance in laminated composites by increasing the interaction and friction between layers in laminated composites. For instance, some traditional methods, such as stitching, Z-pinning, matrix toughening, and fiber surface functionalization or treatment, aim to improve the mode II ILFT of laminated composites. However, these approaches often come with trade-offs, including issues like stress concentrations, agglomeration effects, increased costs, environmental concerns, and potential degradation of in-plane properties. Therefore, a notable shift in perspective has emerged: rather than using some traditional methods, recent research is increasingly focusing on interface engineering. For example, more efficient alternatives have emerged in the field of composite materials, such as the modification of ply stacking sequences. This approach aims to introduce additional transverse yarns, which activate more bridging and crack-arresting mechanisms. Another effective strategy is the incorporation of interlayers, which improve interlaminar interaction, reduce stress discontinuities, and activate multiple toughening mechanisms. In fact, these methods can enhance out-of-plane performance without compromising, and in some cases even improving, the in-plane mechanical properties. It is worth mentioning that in the field of interleaving, recent research indicates that early single-component interlayers, although beneficial, are progressively being replaced by more complex hybrid or architectured interlayers. These advanced configurations can promote multiple toughening mechanisms simultaneously, such as increased interfacial friction, enhanced chemical bonding, and the formation of physical barriers to crack propagation. In conclusion, this approach represents a crucial pathway for developing high-performance composite materials that can resist hidden damages, such as delamination, while also extending their long-term structural durability in challenging service environments.

5.2 Limitations and Outlook

5.2.1 Limitations of Data Reduction

The mechanisms that enhance the apparent mode II ILFT, such as engineered interlayers, tailored stacking sequences, and interface-driven toughening, can also lead to non-classical fracture behavior. This means that in highly toughened systems, we may increasingly observe features like extended FPZs, strong fiber bridging, friction-dominated sliding, crack-front non-uniformity, and non-self-similar crack propagation. These characteristics fundamentally challenge the core assumptions of linear elastic fracture mechanics (LEFM), which include small-scale yielding and self-similar crack growth. As a result, standard characterization procedures that rely on the ENF method and beam-theory-based data reduction methods, such as ASTM D7905 using the CCM or CBBM, may not accurately represent the true fracture mechanics when large FPZs and bridging-dominated mechanisms are at play. In these situations, parameters like initiation and propagation value of ILFT can become sensitive to interpretation, dependent on geometry, and potentially non-transferable to structural design allowables.

5.2.2 Durability and Service Realism

Beyond the limitations of formulation, these interface-engineering strategies present several practical challenges. While results from laboratory-scale experiments show significant increases in delamination resistance and prolonged rising R-Curves, questions remain about their durability in service environments and long-term stability in aerospace or automotive structures. Therefore, future research should focus not only on enhancing toughness incrementally but also on establishing methodological rigor and scalability.

Also, most of the reported improvements have been demonstrated through quasi-static ENF testing. However, comprehensive validation under conditions such as shear fatigue, cyclic degradation of bridging mechanisms, environmental aging (particularly in hot and wet conditions), impact loading, and compression after impact is still limited. Since the contributions from friction and bridging may change with repeated loading and environmental exposure, their long-term stability has not yet been fully established.

5.2.3 Manufacturability and Large-Scale Implementation

While architected and hybrid interlayers enhance design possibilities, challenges remain in achieving consistent industrial-scale production. Key concerns include their integration with automated lay-up or out-of-autoclave processes, sensitivity to porosity and areas with excess resin, and ensuring quality assurance in thick or complex components. Conducting demonstrations beyond the laboratory scale is crucial to validate their robustness, reproducibility, and readiness for certification.

Finally, it should be noted that addressing these interconnected challenges will shape the next generation of laminated composites that can resist hidden delamination damage while maintaining reliable performance at real structural scales.

Acknowledgement: Not applicable.

Funding Statement: The authors received no specific funding for this study.

Author Contributions: The authors confirm contribution to the paper as follows: Conceptualization, Mazaher Salamat-Talab, Hossein Kazemi and Mehdi Safari; Data collection, Hossein Kazemi; Interpretation of results, Hossein Kazemi; Reviewing it critically for important intellectual content, Hossein Kazemi, Mazaher Salamat-Talab and Mehdi Safari; Writing—original draft preparation, Hossein Kazemi; Writing—review and editing, Mazaher Salamat-Talab, Hossein Kazemi and Mehdi Safari; Supervision, Mazaher Salamat-Talab and Mehdi Safari; Project administration, Mehdi Safari. All authors reviewed and approved the final version of the manuscript.

Availability of Data and Materials: The data that support the findings of this study are available from the Corresponding Author, Mazaher Salamat-Talab, upon reasonable request.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest.

Nomenclature and Abbreviations

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