To describe the dynamic cracking process of the CLT vertical layer, the correlation between a load-displacement curve, specimen cracking, and planar shear failure mechanism of the CLT were explored. A three-point bending test and an improved planar shear test are used to evaluate the shear performance of the CLT. In this study, the load-displacement curve is recorded, the experimental part is synchronized with the video, the dynamic process of cracking of the vertical layer is observed and analyzed throughout the test. From the load-displacement curve, the image characteristics of the initial cracking and the sudden increase of the cracking of the specimen are summarized. The description results of the whole dynamic process of the CLT vertical layer cracking are analyzed by planar shear strength value, cracking phenomenon, and azimuth angle of cracking surface. The main conclusions show that the three-point bending test and the improved plain shear test can be used to test the plain shear strength of the CLT, with a difference of only 5.7%. The original crack and the new crack expansion account for 18.9% and 81.1% of the main cracking surface, respectively. And the vertical layer of the CLT specimen under three-point bending has three cracking morphologies, such as radial shake, ring shake, neither along with the radial shake nor along with the ring shake. The azimuth angle of the cracking surface of the CLT vertical layer under planar shear is quite consistent with the first main plane azimuth of the vertical layer of the CLT specimens under the three-point bending test and the shearing test. The shape in the cracking direction of the left half-span or the right half-span of the vertical layer of the specimen is similar to the Chinese character eight.
Timber structure buildings have a long history in construction and occupy a very important position in the history of world architecture. In recent years, a new generation of heavy timber structures, represented by Cross Laminated Timber (CLT), is partially replacing reinforced concrete and brick-mixed structures and being widely used in the construction of low, middle, and even high-rise civil housing and public buildings and other non-civilian buildings. Therefore, the CLT planar shear (rolling shear) strength is an indicator used to evaluate the mechanical properties of the CLT layers. The CLT is subjected to vertical loads when used as a floor and wind load when used as a wall panel [
Based on the definition of planar shear in the standard ASTM D2718 [
This study considers not only obtaining accurate strength values but also, exploring the correlation between the load-displacement curve and specimen cracking, and the new planar shear failure phenomenon of the CLT, through the description of the dynamic process of the CLT vertical layer cracking and analyzing and summaring the failure mode of the specimen during the whole experiment. Since the sawn timber used in this experiment is processed and manufactured in a conventional manner and has the same specifications, the influence of the position of the pith, the size of the annual ring, and the width of the annual ring will not be discussed further [
In this study, a Canadian hemlock (
In this study, according to ANSI APA PRG320-2012 [
The test specimennumber | Quantity(block) | Specifications | Span-depthratio | Method |
---|---|---|---|---|
A4-A18 | 15 | 735 mm × 305 mm × 105 mm | 7 | Three-point bending test |
B1-B6 | 6 | 735 mm × 210 mm × 105 mm | 7 | Three-point bending test |
C1–C4, C6–C10 | 9 | 270 mm × 135 mm × 105 mm | 2.67 | Improved planar shear test |
According to ANSI APA PRG320-2012 [
The advantage of the CLT’s improved planar shear test compared to the conventional planar shear tests specified in the ASTM D2718 [
The formula for determining the tilt angle
In
It is supposed that the inclination angle of the lead line at the center of the CLT specimen for the improved planar shearing and its centerline is
That is, the following triangular equations are satisfied:
When calculating the angle
In this article, the size of C specimen is 270 mm × 135 mm × 105 mm, the calculated inclination angle
JAW-2000 multi-channel structure test loading system 1 set, including load-displacement analysis software, the maximum test force of 300 kN, manufactured by Hangzhou Bonway Electrical and Mechanical Control Engineering Co., Ltd., China. This device is used for the three-point bending test. AG-IC-type electronic universal test machine 1 set, the maximum test force of 100 KN, manufactured by Shimadzu International Trade (Shanghai) Co., Ltd., China; This device is used for improved planar shear tests.
A formula for calculating the shear strength of the CLT plane from the maximum load value obtained from the load-displacement curve.
A planar shear strength formula of the CLT three-point bending test [
In formula
A planar shear strength formula of CLT improved planar shear testis as follows:
In formula
To explore the cracking behavior before the failure of the CLT under the three-point bending test, the load-displacement curve of specimens A and B, the test part, as well as the whole process of vertical cracking, crack expansion, and failure of the specimens are recorded in real time. The video and loading process of the specimen are synchronized until the end of the experiment.
In the Excel file of the load-displacement curve formed in this experiment, the load and load point displacement applied to the specimen corresponding to the loading moment are provided; in this study, the description of the whole dynamic process of the test will be carried out with the A9 specimen with a load speed of 0.5 mm/min as an example.
In this paper, the relationship between the load-displacement curve of the A9 specimen and the vertical layer cracking, crack expansion, and destruction is described by the time node as the contact point.
The load-displacement diagram of the A9 specimen and its key node results are shown in
Key nodes | The name of the project | ||
---|---|---|---|
The time node | Load value (kN) | Load point displacement (mm) | |
Point A | 12’01’’ | 40.58 | 6.08 |
Point B | 14’40’’ | 45.03 | 7.41 |
Initiation point | 14’49’’ | 45.55 | 7.98 |
Point C | 15’40’’ | 46.63 | 7.90 |
The crack suddenly increases | 15’57’’ | 45.11 | 8.07 |
Point D | 16’13’’ | 41.61 | 8.17 |
Point E | 34’52’’ | 58.79 | 17.32 |
Point F | 36’37’’ | 49.64 | 18.22 |
In
In
Curve stage | Appearance description of specimens | Cracking time | Video screenshot |
---|---|---|---|
O–A | No change. | ||
A–B | No change. | ||
B–C | The crack appeared on the vertical layer in the direction of the annual ring. | 14’49’’ | |
C–D | The crack suddenly increased, and ran through the width of the specimen. changed from the initial direction of the ring to 53°, and the mesh cracks appear between the parallel layer and the vertical layer. | 15’57’’ | |
D–E | A new small crack appeared in the vertical layer near the center of the specimen length. | 27’22’’ | |
E–F | A large displacement occurred between the layers, but no pull damage was observed in the parallel layer. |
To explore the difference between the results of test methods, and to obtain a more suitable test method combined with theoretical analysis, the planar shear strength of the CLT specimen is tested by the improved planar shearing. A comprehensive analysis is conducted by test values, the crack expansion in the dynamic process of shear failure in the load-displacement curve, the plane stress calculation, as well as the crack and its morphology, and its shear failure mechanism.
The shear specimen is placed up and down on the positioning aluminum block, the position is adjusted to ensure that the loading direction passes through the center of the specimen, the loading speed of the plane cut is 0.5 mm/min, using displacement control loading and recording its load-displacement curve. The specimen loading and load-displacement curve will be synchronized by video recording.
In the Excel file of the load-displacement curve formed in this experiment, the load and load point displacement applied to the specimen corresponding to the loading moment are provided. Using the specimen video synchronized with the loading of the testing machine, and taking each time node on the load-displacement curve as the contact point, it is possible to describe the cracking, crack expansion, and the final failure form of the vertical layer at each stage of the load-displacement curve. Taking the C3 specimen as an example, the results and analysis of its shear performance are carried out using an improved planar shear test method.
In this paper, with its video observation, the relationship between the load-displacement curve of the C3 specimen and the vertical layer cracking, crack expansion, and failure is described by the time node as the contact point. The load-displacement diagram of the C3 specimen and its key node results are shown in
Key nodes | The name of the project | ||
---|---|---|---|
The time node | Load value (kN) | Load point displacement (mm) | |
Point A | 4’46’’ | 43.65 | 2.39 |
Point B | 6’09’’ | 51.92 | 3.07 |
The starting point of the new crack | 6’10’’ | 51.4 | 3.08 |
Point C | 6’14’’ | 51.02 | 3.12 |
Point D | 6’37’’ | 52.13 | 3.31 |
Complete destruction point | 6’51’’ | 49.55 | 3.43 |
By the maximum peak load of 52.13 kN substituted into formula
In
Curve stage | Appearance description of specimens | Cracking time | Video screenshot |
---|---|---|---|
O–A | No change. | ||
A–B | No change. | ||
B–C | The new crack suddenly increased along the direction of the annual ring, from which the angle of the crack changes by 45° and continues again along the annual ring and all the way to the layer. | 6’10’’ | |
C–D | The cracks in the specimen kept increasing. | 15’57’’ | |
D–unload | Both the original crack and the new crack at this stage increased until the specimen was completely destroyed. |
Hemlock CLT specimens A and B were subjected to the three-point bending test, the maximum load value of the load-displacement curve can be obtained by the destruction test and then based on the formula
Hemlock CLT specimen C was subjected to the shear test, the maximum load value of the load-displacement curve can be obtained by the destruction test, and then according to the formula
Thus, the planar shear strength of hemlock CLT under the three-point bending test is quite consistent with the planar shear strength of hemlock CLT under the improved planar shear test, with a relative error of only 5.7%.
The crack morphology and the azimuth angle of the cracking surface of the CLT three-point bending specimens and the plane shear specimens are shown in
Number | Initial state | Final state | Dynamic analysis of overall crack morphology | Whether the crack penetrates the thickness of the vertical layer | Wood chips deviation after interlayer cracking |
---|---|---|---|---|---|
C1 | (1) The original crack 1 has slightly expanded; |
Yes | The upper half of the specimen cracked between layers, and the wood chips were inclined to the vertical layer. | ||
C2 | (1) The original crack 1 has slightly expanded; |
No | - | ||
C3 | (1) No change in original crack 1; |
Yes | The whole layer of the specimen was cracked, and the wood chips were inclined to the parallel layer. | ||
C4 | (1) The original crack 1 expands; |
Yes | - | ||
C6 | (1) The original cracks on the vertical layer expand along the 50° direction of wood ray, and finally stop at the interlayer cracking; |
Yes | The whole layer of the specimen was cracked, and the wood chips were inclined to the parallel layer. | ||
C7 | (1) No change in original crack 1; |
No | The whole layer of the specimen was cracked, and the wood chips were inclined to the parallel layer. | ||
C8 | (1) The new crack (top) extends from the interlayer crack to the middle of the vertical layer along the 30° direction of the annual ring; |
Yes | - | ||
C9 | (1) New cracks appearing first (top): Extend along the 60°directon of the wood ray on the vertical layer to coincide with the cracks between the layers; |
Yes | The whole layer of the specimen was cracked, and the wood chips were inclined to the parallel layer. | ||
C10 | The new cracks start at 45° direction along the wood ray on the vertical layer; they expand and penetrate the vertical layer. | Yes | The whole layer of the specimen was cracked, and the wood chips were inclined to the vertical layer. |
Among the CLT three-point bending specimens (A and B), the specimens where the crack did not penetrate the vertical layer are: A7, A8, A10, A13, B4. The vertical layers of other specimens were all penetrated by cracks. The crack angle and shape are shown in
The crack morphology and crack orientation of CLT planar shear specimen (C) are shown in
Before the test, the cracks that already existed on the vertical layer of the specimen were called original cracks. During the test, the newly generated cracks on the vertical layer of the specimen are called as new cracks (the cracks formed by the expansion of the original cracks are not included).
The total amount of CLT A and B specimens under the three-point bending and CLT C specimens under shearing constituted 30 pieces, of which, 20 original cracks were observed on 11 samples. During the test, the cracks did not expand in 5 places, while in 8 places there was a slight expansion, and in 7 places the expansion of the main cracks caused damage to the surface of the specimen. On the CLT A and B samples under the three-point bending, 26 cracks of the main surface were observed, that is, the expansion of the original cracks to the main surface was in 4 places, while the expansion of the new cracks to the main surface occurred in 22 places.
The CLT A and B specimens under the three-point bending and CLT C specimens under the shearing showed 37 main cracks on the surface. Among them, the expansion of the original crack to the main surface crack was observed in 7 places, and the expansion of new cracks to the main surface crack was observed in 30 places. The expansion of the original cracks and new cracks accounted for 18.9% and 81.1% of the main surface cracks, respectively.
Obviously, the data listed above show that the new crack causes the dominant damage of the specimens, since the original crack load is not as sensitive as the new ones. Wood is different from metal, cracks in the wood cause material damage characteristics.
The statistics and analysis of the cracking morphology and azimuth angle of the CLT vertical layer in this study are only carried out for the main cracking surface that is finally generated in the experiment. Each specimen has one main cracking surface, two and three main cracking surfaces are rare.
The cracking morphology of the wood grain on the cross-section of wood is usually divided into two types: ring shake and heart shake [
The vertical layer of the CLT specimens under three-point bending has the shape of the Chinese character eight in the cracking direction of the left half-span or the right half-span.
The cracking shape of the vertical layer of the wood and azimuth angle of CLT specimens A and B at three-point bending test and CLT specimen C at the shear test are shown in
As indicated in
Cracking morphology | Cracking direction | |
---|---|---|
(40–50o) | (0–40 or 50–90o) | |
Ring shake | A4 (50°), A5 (40°), A6 (48°), A9 (50°), A11 (40°), A12 (45°), A14 (45°), A17 (40°), B1 (50°), B5 (45°, 50°) | B2 (60°), C2 (60°), C8 (30°), C9 (55°) |
Heart shake | A10 (50°), A16 (40°), B3 (50°), C1 (40°), C4 (50°), C6 (50°), C10 (45°) | A7 (25°), A8 (65°), A13 (78°), B1 (35°), B4 (85°), C3 (30°), C7 (60°–30°), C8 (30°), C9 (60°) |
Neither ring nor heart shake | A15 (45°), A16 (50°), B6 (40°) | A10 (25°), A18 (30°) |
Through the analysis of the cracking shape of the vertical layer and azimuth angle of the CLT specimen at the three-point bending test, it is found that: ① the vertical layer cracking of the CLT specimen at the three-point bending test has three morphologies: along the wood ray (diameter crack), along the annual ring or the tangent line of the annual ring (ring shake), neither along the ring shake nor along the heart shake. ② The azimuth angle of the CLT specimen’s vertical layer cracking at the three-point bending test is between 40°–50°, a total number is 15 specimens (18 places). There are three types of cracking: ring shake (10 blocks, 12 places), heart shake (2 blocks, 3 places), and neither ring shake nor heart shake (3 blocks, 3 places). ③ The angle of the vertical layer cracking in 8 specimens (8 places) is outside 40°–50°. There are three types of cracking: ring shake (1 block, 1 place), heart shake (5 blocks, 5 places), and neither ring shake nor heart shake (2 blocks, 2 places). ④ The specimens with two cracks are: A10 (50°, 25°), B1 (45°, 35°), B5 (45°, 50°), A16 (40°, 50°); The specimens with three cracks are: A15(The ring shake is 45°; neither the ring shake nor the heart shake is 45°; there is also a crack along the wood ray-annual ring-wood ray.). ⑤ The basic cracking form on the wood cross-section is ring shake and heart shake, and this study found that in addition to these, there are cracking forms that are neither ring shake nor heart shake, such as A15 (45°), B6 (40°), A16 (50°), A10 (25°), A18 (30°). There are 5 pieces in total, accounting for 23.8% of the total number of three-point bending specimens. ⑥ Among 15 CLT specimens A at three-point bending test, the angle of the vertical layer cracking of 12 specimens is between 40°−50°, and 11 blocks crack in the thickness of the vertical layer; The angle of the vertical layer cracking of the CLT specimen A and B at the three-point bending test is between 40°–50°, the number of specimens is 16 pieces, accounting for 76.2% of the total number of three-point bending test specimens. (The cracking angle between 40° and 50° is calculated by block. It means that if there are two cracks with a cracking angle between 40°–50° on a piece of the specimen, it is calculated as one block) ⑦ Different cracking features, such as A6, are also observed on both ends of the vertical layer of the same specimen if the cracking surface runs through the width of the test specimen. The cracking surface of A6 is located on the left half of the three-point bending specimen, and the direction of the cracking surface on the front and back end faces of the vertical layer of the same specimen is in a straight shape of the Chinese character eight.
Through the analysis of the vertical layer cracking shape and azimuth angle of the CLT specimen at the shear test, it is learned that ① the cracking position of the CLT specimen in the vertical layer is not along the shear stress surface, but presents a certain angle between the parallel layer and the vertical layer interface. The cracking shape is ring shake and heart shake, and no cracking shape is found that is neither ring shake nor heart shake. ② Among the 9 pieces of CLT shear specimens, the morphology and azimuth angle of 11 main cracking surfaces are summarized as 4 blocks (4 places) with the azimuth angle of the cracking surface between 40°–50°, accounting for 44.4% of the total number of specimens. There are cracking forms along the wood ray (heart shake) such as C1 (40°), C4 (50°), C6 (50°), C10 (45°). There are 5 blocks (7 places) of the azimuth angle of the cracking surface which is outside 40°–50°, accounting for 55.6% of the total number of specimens. There are cracking forms along the wood ray (heart shake) such as C3 (30°), C7 (60°), C8 (30°), C9 (60°). As well as along the annual ring or the tangent of the annual ring (ring shake) such as C2 (60°), C8 (30°), C9 (55°). Among them, there are 3 specimens that exhibited the expansion from the original crack to the main crack surface such as C3, C6, C7. ③ 9 blocks of CLT specimens at shear test exhibited 12 original cracks, of which 5 places did not expand, 4 places expanded slightly, and 3 places expanded to the main cracking surface. ④ The azimuth angle of the original crack in the C7 specimen is 60°, and it expands along the wood ray (heart shake).
In the three-point bending test, at the point B on the load-displacement curve (
The load-displacement curve of the CLT specimen in the improved planar shear test is different from the load-displacement curve of the three-point bending test, there is a slight “shake” of the falling stage (CD stage of C3 in
The load-displacement curve of the specimen C1 also shows a “shake” phenomenon several times ( The load-displacement curves of the three-point bending test and the improved planar shear test have different degrees of falling segments, which are in line with the characteristics of fracture mechanics theory and wood orthotropy.
Key nodes | The name of the project | ||
---|---|---|---|
The time node | Load value (kN) | Load point displacement (mm) | |
Point A (sound) | 4’58’’ | 37.22 | 2.49 |
Point B (new crack 1) | 5’54’’ | 44.35 | 2.95 |
Point C | 6’53’’ | 49.61 | 3.45 |
Initiation point of new crack 2 | 6’55’’ | 49.2 | 3.46 |
Point D (new crack 3) | 7’29’’ | 51.44 | 3.74 |
Point E | 7’47’’ | 51.59 | 3.89 |
Point F | 7’52’’ | 49.91 | 3.94 |
Obviously, the test breakpoint of the three-point bending test and the improved planar shear test is before or at the initial drop point. A “shake” appears on the load-displacement curve of the three-point bending test and the improved planar shear test, indicating that the test specimen is cracking. On the load-displacement curve, there is a sudden increase in cracks in the three-point bending test, and the sudden increase in cracks in the improved planar shear test specimens is not clear or unclear, reflecting the cracking characteristics in the process of realizing planar shear deformation under two different loading methods. The crack angles on the front and back sides of the test specimen are consistent, such as A17 and B3. Some are not exactly the same, such as A9, which also reflects the orthotropic characteristics of wood.
Both the three-point bending test and the improved planar shear test can realize the in-planar shear of the CLT vertical laminate cross-section, which can be used to test the planar shear strength of CLT. The planar shear strength of the hemlock CLT tested by two methods is quite consistent, with a difference of only 5.7% in value.
The original cracks and new cracks expanded to the main crack surface account for 18.9% and 81.1% of the total cracking surface, respectively.
The vertical layer cracks of CLT specimens under three-point bending have three morphologies: heart shakes, ring shakes, and neither ring shakes nor heart shakes. Among them, CLT specimens A and B have heart shakes and ring shakes on the vertical layers accounting for 76.2% of the total number of specimens, and the cracking morphology with neither ring shakes nor heart shakes accounts for 23.8% of the total number of specimens.
The azimuth of the cracking surface of the CLT vertical layer in planar shear is quite consistent with the first principal plane azimuth of the vertical layer of the CLT three-point bending specimen and the shear specimen. The vertical layer of the specimen is shaped like Chinese character eight, in the cracking direction of the left half-span or the right half-span. In this study, 76.2% of the CLT specimens A and B under three-point bending and 44.4% of the CLT specimens C under shearing have the cracking surface of the vertical layer with the azimuth between 40°–50°.
Compared with the new cracks, the original cracks are more stable during the loading process, and most of the damage of the specimens is caused by the new cracks.
Density
Tilt angle of the specimen
Planar shear strength
Maximum peak load
Width of the specimen
Thickness of the specimen
Length of the sample