Experimental study on the deformation and failure mechanism of parallel bamboo Strand Lumber under drop-weight penetration impact

Construction and Building Materials 242 (2020) 118135

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Experimental study on the deformation and failure mechanism of parallel bamboo Strand Lumber under drop-weight penetration impact Xin Li a,b, Mahmud Ashraf a,b,⇑, Haitao Li a,⇑, Xiaoyan Zheng a, Safat Al-Deen c, Hongxu Wang c, Paul J. Hazell c a

College of Civil Engineering, Nanjing Forestry University, Nanjing 210037, China Deakin University Geelong, School of Engineering, Waurn Ponds, VIC 3216, Australia c School of Engineering and Information Technology, The University of New South Wales Canberra, ACT 2612, Australia b

h i g h l i g h t s
Specimen height under penetration impact showed almost negligible effect on material responses.
Failure mechanism of PBSL relies strongly on the fibre orientation.
Specimens impacted parallel to the fibre exhibited higher critical failure energy.

a r t i c l e

i n f o

Article history: Received 8 October 2019 Received in revised form 6 January 2020 Accepted 7 January 2020

Keywords: Parallel Bamboo Strand Lumber (PBSL) Penetration impact Damage evaluation Failure mechanism

a b s t r a c t This paper reports an experimental investigation on impact response of Parallel Bamboo Strand Lumber (PBSL) samples when subjected to drop-weight penetration on partial cross-section. A total of 42 samples with 40 mm square cross-sections were tested with 20 mm hemispherical impactor to simulate partial impact resistance with special emphasis on height of specimens and the corresponding fibre orientations. Two typical material responses were observed i.e. penetration-rebound under low impact level (25.4 J) and penetration-failure under high impact level (76.3 J). Since there is currently no standard available for testing of PBSL under impact loading, effect of specimen height was investigated in regards to the ultimate load carrying capacity, deformation capacity and energy absorption ability in both response modes. In addition, penetration-critical failure modes were investigated using electron microscopes. Obtained results indicated that the ultimate load carrying capacity of PBSL was limited by their critical failure energy; however, their deformability and energy absorption capacity increased with increasing impact levels. Failure mechanism of PBSL relies strongly on the fibre orientation i.e. bulking failure with column-like separations was observed when impact load was applied parallel to the fibres, whilst tearing failure with inverted triangular separations was observed in specimens that were tested perpendicular to the fibres. Microscopic images were taken to conduct an in-depth analysis of the observed differences between failure patterns. Specimens with grains perpendicular to the impact load were observed to be more brittle and sensitive to impact load. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Bamboo has been widely accepted as an environmental-friendly construction material. Although it has been used for various types of structural applications [1–5], its natural tubular shape, limited size and low rigidity restrict its widespread use in general purpose construction [6]. A new type of engineered bamboo composite

⇑ Corresponding authors at: College of Civil Engineering, Nanjing Forestry University, Nanjing 210037, China (M. Ashraf). E-mail addresses: [email protected] (M. Ashraf), lhaitao1982@126. com (H. Li). https://doi.org/10.1016/j.conbuildmat.2020.118135 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

manufactured by compressing adhesive and bamboo culm under hot-pressing technique, known as parallel bamboo strand lumber (PBSL), has been drawing attention from researchers [7,8]. As a result of this special engineering treatment [9,10], bamboo composite is reported to have enhanced strength as well as better uniformity and process-ability [11]. PBSL can complement as well as can compete with other commonly used building materials, and may be used as a timber substitute based on local availability, whilst offering renewable characteristics [12–16]. Majority of existing research were reported on engineered bamboo with primary focus on evaluating its flexural and compressive performance through quasi-static tests [17–27]. In a

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recent research programme, axial and eccentric compression behaviour of PBSL columns were recently investigated [8,28,29]. Although impact loading, during service life, is quite common in all types of structures [30,31], there has been no comprehensive study reported to investigate the impact resistance of PBSL, especially under low-velocity impact. PBSL is harder than timber but could be more vulnerable to indentation or penetration due to impact loads when compared to its traditional counterparts such as metals and concrete. Appropriate understanding of impact responses, including knowledge of relevant deformation and failure mechanisms, are crucial for design applications [32]. To our knowledge, the dynamic mechanical properties, and particularly the resistance to drop-weight penetration of PBSL, have not been previously studied with sufficient details, but require significant research to ensure efficient use of these innovative and renewable composites in construction. In a recent publication [33], the authors reported useful yet basic insight into PBSL’s response under quasi-static and impact loading for different fibre orientations. Obtained material response data from quasi-static and simple drop-weight rebound impact tests (using a flat surfaced impactor) were used to identify failure mechanisms under static and dynamic conditions. The current manuscript is an advanced study on the impact resistance of PBSL, which is built upon the basic understanding developed through the preliminary investigation reported earlier [33]. As part of the current study, comprehensive investigation was conducted on the impact resistance PBSL using drop-weight penetration tests (using a hemispherical shaped impactor) to reveal in-depth understanding of failure mechanisms and relevant failure patterns; observed results were critically evaluated both at macro and micro (fibre) scale. 40 mm square specimens representing four different heights – 25 mm, 50 mm, 75 mm and 100 mm, were used to investigate impact response of PBSL due to variations in fibre length (height of specimen) and orientations through penetration-rebound tests and penetration-failure tests. Obtained results were carefully analysed to develop useful understanding of this unexplored loading regime for PBSL.

Fig. 1. Typical PBSL speciemns used for tests.

2. Experimental investigations 2.1. Material PBSL used in the current study, same as our previous research [33], were manufactured from commercial columns with a mean characteristic density of 1260 kg/m3. 40 mm square crosssectional samples with varying height between 25 mm and 100 mm were carefully cut with a manufacturing tolerance less than ± 1 mm. As an anisotropic material [34], fibre orientation of PBSL is one of the major concerns in determining mechanical properties. Two subgroups i.e. perpendicular to the fibre (PER specimens) and parallel to the fibre (PAL specimens) were classified for further investigations, as illustrated in Fig. 1. 2.2. Drop-weight impact testing Impact testing were conducted using an INSTRON drop tower (CEAST 9350). Impactor equipped with a hemispherical striker (20 mm in diameter, 5.482 kg of total weight) were used herein, as illustrated in Fig. 2. The initial energy for impact was determined as 25.38 J corresponding to an impact initial velocity of 3 m/s based on our previous research [33] - this has been defined as the basic energy level (B) in the current study. Two test series were considered to investigate effects of specimen size and fibre orientation on failure modes, and to reveal the influences of impact on failure mechanism. Tables 1 and 2 summarize specimen dimen-

Fig. 2. Drop-weight compression impact setup.

sions with designations and important test parameters for each considered test series; designation used for specimens include material (PBSL) - height of the sample - fibre orientation (PAL/ PER) and impact energy level.

3. Effects of size and fibre orientation on failure modes Following sections present details of the experimental investigation conducted as part of the current study. A total number of 30 specimens were tested as shown in Table. 1. Key results recorded for all tested specimens are summarized in Table 3.

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X. Li et al. / Construction and Building Materials 242 (2020) 118135 Table 1 Details of test specimens used for investigating failure modes. Specimen Designation

Cross-section Area (mm)

Height (mm)

Number of Specimen

Fibre Orientation

Impact Energy (J)

Equivalent Velocity (m/s)

Characteristic Density (kg/m3)

PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL

40  40

25

3 3 3 3 3 3 3 3 3 3

Parallel Perpendicular Parallel Perpendicular Parallel Perpendicular Parallel Perpendicular Parallel Perpendicular

25.38 25.38 25.38 25.38 76.26 76.26 76.26 76.26 76.26 76.26

3.00 3.00 3.00 3.00 5.20 5.20 5.20 5.20 5.20 5.20

1272 1270 1250 1263 1250 1263 1258 1252 1234 1222

Characteristic Density (kg/m3)

25 PAL B 25 PER B 50 PAL B 50 PER B 50 PAL 3B 50 PER 3B 75 PAL 3B 75 PER 3B 100 PAL 3B 100 PER 3B

50

75 100

Table 2 Details of test specimens used for in-depth failure mechanism under varying impact levels. Specimen Designation

Cross-section Area (mm)

Height (mm)

Number of Specimen

Fibre Orientation

Impact Energy (J)

Equivalent Velocity (m/s)

PBSL PBSL PBSL PBSL PBSL PBSL

40  40

25

3 3 3 3 3 3

Parallel Parallel Parallel Perpendicular Perpendicular Perpendicular

25.38 50.70 63.37 25.38 37.99 50.70

3.00 4.24 4.74 3.00 3.67 4.24

25 25 25 25 25 25

PAL PAL PAL PER PER PER

B 2B 2.5B B 1.5B 2B

1272

1270

Table 3 Details of test results for investigating failure modes. Specimen Designation

Maximum Force, Pu (kN)

Time at ultimate force, tPu (ms)

Maximum Displacement, dmax (mm)

Time at ultimate displacement, tdmax (ms)

Absorbed Energy (J)

PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL

22.77 22.65 22.71 22.19 19.76 21.10 21.54 21.64 23.24 27.89 29.94 32.08 19.58 20.43 21.71 20.07 30.62 31.17 30.69 26.63 29.90 29.01 29.23 35.10 30.64 29.91 29.17 31.27 25.93 29.42

0.92 0.93 0.92 0.95 1.01 1.08 0.99 1.00 0.95 0.42 0.49 0.57 1.07 1.09 1.09 0.43 0.63 0.67 0.57 0.60 0.70 0.62 0.59 0.85 0.70 0.75 0.77 0.58 0.75 0.59

2.10 2.15 2.10 2.21 2.50 2.42 2.09 2.11 2.05 4.28 4.12 3.88 2.49 2.45 2.36 3.92 4.10 3.93 4.08 4.46 4.23 4.10 4.19 4.03 4.27 4.34 4.73 4.01 4.70 3.76

1.12 1.14 1.13 1.17 1.31 1.26 1.13 1.13 1.10 1.23 1.19 1.38 1.31 1.30 1.24 0.88 1.04 1.07 1.44 1.30 1.43 1.08 1.08 1.22 1.41 1.40 1.61 1.06 1.23 0.95

20.17 20.63 20.88 19.01 20.04 19.48 20.70 20.29 19.92 61.79 61.45 72.05 19.86 20.17 19.35 37.50 53.27 60.73 70.97 63.75 69.98 56.98 54.35 69.42 70.71 70.07 70.01 56.39 56.30 57.94

25 PAL-1 B 25 PAL-2 B 25 PAL-3 B 25 PER-1 B 25 PER-2 B 25 PER-3 B 50 PAL-1 B 50 PAL-2 B 50 PAL-3 B 50 PAL-1 3B 50 PAL-2 3B 50 PAL-3 3B 50 PER-1 B 50 PER-2 B 50 PER-3 B 50 PER-1 3B 50 PER-2 3B 50 PER-3 3B 75 PAL-1 3B 75 PAL-2 3B 75 PAL-3 3B 75 PER-1 3B 75 PER-2 3B 75 PER-3 3B 100 PAL-1 3B 100 PAL-2 3B 100 PAL-3 3B 100 PER-1 3B 100 PER-2 3B 100 PER-3 3B

3.1. Material responses Two typical response modes i.e. penetration-rebound and penetration-failure were observed for all aforementioned tested specimens. Penetration-rebound response was characterised by the specimen maintaining its integrity with a distinct local indentation due to impact; this response was observed under lower impact energy levels. Penetration-failure response, on the other hand, referred to cases when specimens lost its shape and were

separated into pieces due to change in fibre orientations and/or higher impact energy levels. Typical examples of observed failure patterns are shown in Fig. 3. 3.2. Size effects and fibre orientations Sample size and fibre orientation are two basic parameters that influenced the observed responses of PBSL. Penetration-rebound response was observed for all samples conducted under basic

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(a) Typical penetration-rebound specimen

(b) Typical penetration-failure specimen

Fig. 3. Typical response modes of specimens.

impact level of B (representing 25.38 J of energy). When the energy level was increased to 3 times the basic energy level i.e. 3B, penetration failure mode was obvious for most samples. These two typical modes are thoroughly discussed in following subsections.

3.2.1. Penetration-rebound response 3.2.1.1. Load versus time response. Typical penetration-rebound force versus time curves recorded for 25 mm and 50 mm samples are shown in Fig. 4(a). Fig. 4(b) presents all data for 25 mm and 50 mm samples as shown in Table 2. Size of specimens seemed to have almost negligible influence on the load-carrying capacity for all samples with same fibre orientation. Load response curves showed very good consistency during the whole loading period with only 3% coefficient of variations (COV) for maximum force of PAL specimens (load applied parallel to the grain) and that for PER specimens (load applied perpendicular to the grain) was only 5%.

In terms of fibre orientation, PAL samples showed somewhat higher (approximately 7%) load carrying capacity than the corresponding PER samples but with shorter response time. Height of the samples did not show any noticeable effect. 3.2.1.2. Deformation versus time response. Typical penetrationrebound deformation-time curves of 25 mm and 50 mm samples are shown in Fig. 5(a), which can be generally obtained by Eq. (1) using a double integration method. Relevant maximum deformations are graphically presented in Fig. 5(b).

ZZ D¼

F ðtÞ  Mg 2 d t M

ð1Þ

where D represents the central deformation; M and g are the total weight of the impactor and the acceleration of gravity (9.8 m/s2), respectively, F(t) is the recorded force; Obtained curves were almost overlapping with each other, as shown in Fig. 5(a). The COVs of maximum deformation for PAL and PER specimens were 0.015 and 0.044, respectively; maximum deformations from PER specimens were somewhat higher than

(a) Typical penetration-rebound force versus time curves (25 mm and 50 mm samples) (a) Typical penetration-rebound deformation versus time curves (25 mm and 50 mm samples)

(b) Maximum penetration-rebound force versus time (25 mm and 50 mm samples)

(b) Maximum penetration-rebound deformation versus time (25 mm and 50 mm samples)

Fig. 4. Typical penetration-rebound load versus time responses, subjected to 25.38 J of energy (B).

Fig. 5. Typical penetration-rebound deformation versus time responses, subjected to 25.38 J of energy (B).

X. Li et al. / Construction and Building Materials 242 (2020) 118135

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those obtained from PAL specimens. However, all tested samples under identical impact level exhibited almost perfect linear fit regardless of fibre orientation and height of specimen. 3.2.1.3. Load-deformation response and energy absorption. Typical penetration-rebound deformation versus force curves are shown in Fig. 6(a). Fig. 6(b) summarizes the amount of energy absorbed by each specimen. Numerical integration using the area enclosed by the force-deformation curve are applied for calculation following Eq. (2):

Z Eabsorbed ¼

F ðDÞdD

ð2Þ (a) Typical penetration-failure force-time curves (50 mm, 75 mm and 100 mm samples)

where Eabsorbed is the amount of energy absorption; F(D) represents the relevant deformation versus force curve. Fig. 6 shows that specimen height did not have any considerable effect on the deformation-force curves for the considered cases. Although loading procedures of specimen were varied between fibre orientations, the amount of total energy absorbed were nearly the same with an average value of 20.04 J and a mean COV of 0.02. This implies that the amplitude of absorbed energy from penetration-rebound specimen was primarily controlled by the initial impact level rather than height or fibre orientation of specimens. 3.2.2. Penetration-failure response 3.2.2.1. Load-time response. Typical penetration-failure force versus time responses obtained for 50 mm, 75 mm and 100 mm specimens under impact level of 3B (representing 76.14 J of energy) are shown in Fig. 7. It is obvious from Fig. 7 that fibre orientation played a major role in load-deformation characteristics for penetration-failure responses. PER specimens (with grains perpendicular to the

(a) Typical penetration-rebound deformation-force curves (25 mm and 50 mm samples)

(b) Energy absorption of each specimen under penetration-rebound test Fig. 6. Typical penetration-rebound deformation-force responses and relevant energy absorption, subjected to 25.38 J of energy (B).

(b) Maximum penetration force versus time (50 mm, 75 mm and 100 mm samples) Fig. 7. Typical penetration-failure load versus time responses, subjected to 76.14 J of energy (3B).

impact) demostrated shorter plateau stage and failed abruptly once the load exceeded its carrying limit showing brittleness to dynamic loading when compared against those observed for PAL specimens (with grains parallel to the impact). However, maximum forces recorded from each sample, as shown in Fig. 7(b), are very consistent with less that 5% COV, and the maximum load for both PER and PAL specimens were almost identical.

3.2.2.2. Deformation-time response. Fig. 8 shows typical penetration-failure deformation versus time responses. Fibre orientation showed some influence on the deformation capacity of PBSL with PER samples experiencing relatively rapid deformation when compared with those observed for PAL specimens; this observation supports abrupt unloading behaviour shown in Fig. 7(a). Overall, specimen size did not show any significant effect as can be observed from Fig. 8(b).

3.2.2.3. Load-deformation response and energy absorption. Typical penetration-failure force versus deformation curves and the amount of energy absorbed by each tested specimen are summarized in Fig. 9. Although failure modes of penetration-failure specimens were different between fibre orientations (PAL vs PER), specimen height did not significantly affect the deformation-force curves. Similar observations were also observed for penetration-rebound specimens. The amount of total energy absorbed of PER specimens were smaller due to its shorter plateau and larger brittleness when compared with PAL counterparts, as illustrated in Fig. 9(b); however, they were stablized at certain values in terms of initial impact level, with a mean of 56.52 J and 67.86 J, respectively.

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3.3. Summary of load-deformation response Following two observations may be summarised based on the aforementioned results:

(a) Typical penetration-failure displacement-time curves (50 mm, 75 mm and 100 mm samples)

1) The height of specimen did not seem to have any significant influence on penetration impact tests conducted on PBSL. Specimen height showed negligible effects on maximum load, deformation capacity and energy absorption ability; 2) Fibre orientation showed insignificant effect on energy absorption in penetration-rebound responses, but had clear influence on penetration-failure response; this is due to the difference in failure mechanisms but this observed difference would require further investigation for complete understanding. Hence, analysis of impact effects and revelvant failure mechanism of PBSL, presented in the following sections, were conducted based on 25 mm specimens with varing impact levels.

4. Effect of impact on failure mechanism 4.1. Effects of impact level on material response (b) Maximum penetration-failure displacement versus time (50 mm, 75 mm and 100 mm samples)

Fig. 8. Typical penetration-failure deformation-time responses, subjected to 76.14 J of energy (3B).

(a) Typical penetration-failure deformation versus force curves (50 mm, 75 mm and 100 mm samples)

A total number of 18 specimens were tested as summerized in Table 2. Initial impact energies were chosen as 25.38 J (B) and 2.5B (63.37 J) for specimens with grains paralle to loading (PAL), and B (25.38 J) and 2B (63.37 J) for specimens with grains perpendicular to loading (PER). Since penetration-failure were observed for PAL 2.5B and PE 2B specimens, intermediate impact levels i.e. PAL 2B and PER 1.5B were further conducted to reveal the critical failure between penetration-rebound and penetration-failure. Key data for each test configuration are presented in Table 4. Fig. 10 compares some force, deformation and energy characteristics obtained from conducted tests. Following are some general conclusions that may be drawn from test observations: 1) Initial impact levels showed some considerable influence on the load carrying capacity of PBSL specimens considered in the current study. The maximum force for 25 mm PER specimens increased with increasing initial impact levels, whilst those for PAL specimens showed intial increase but then dropped as impact level was increased; 2) The overall deformation capacity of PBSL showed significant increase as the impact level was increased; 3) Energy absorption of PBSL increased with higher impact levels for both PAL and PER specimens. Although similar 2.4 times increments in absorbed energy, smaller amplification of impact energy (B to 2B for PER and B to 2.5B for PAL) indicated higher sensitivity of PER specimens when subjected to dynamic impacts. 4.2. Damage evaluation

(b) Energy absorption of each specimen under penetration-failure test Fig. 9. Typical penetration-failure deformation-force responses and relevant energy absorption, subjected to 76.14 J of energy (3B).

4.2.1. Response mode Three types of response modes were observed from current tests. They were low impact (penetration-rebound), medium impact (penetration-critical failure) and high impact (penetration-failure). Unlike penetration-rebound and penetration-failure, which have already been elaborated in previous sections, penetration-critical failure is characterized by the specimen with well-developed cracks penetrated through the entire cross-section whilst main-

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X. Li et al. / Construction and Building Materials 242 (2020) 118135 Table 4 Details of test results for investigating failure mechanism under varying impact levels. Specimen Designation

Maximum Force, Pu (kN)

Time at ultimate force, tPu (ms)

Maximum Displacement, dmax (mm)

Time at ultimate displacement, tdmax (ms)

Absorbed Energy (J)

PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL PBSL

22.77 22.65 22.71 30.49 29.05 30.90 27.95 28.36 26.87 22.19 19.76 21.10 25.49 28.73 27.97 30.38 31.34 28.83

0.92 0.93 0.92 0.85 0.86 0.85 0.50 0.59 0.50 0.95 1.01 1.08 0.92 0.91 0.90 0.88 0.88 0.74

2.10 2.15 2.10 2.95 3.06 2.93 3.86 3.63 3.52 2.21 2.50 2.42 2.89 2.57 2.60 3.11 3.07 3.32

1.12 1.14 1.13 1.13 1.18 1.13 1.20 1.19 1.03 1.17 1.31 1.26 1.35 1.11 1.13 1.17 1.25 1.25

20.17 20.63 20.88 43.11 44.07 43.12 49.44 55.10 47.16 19.01 20.04 19.48 36.17 28.84 28.96 45.36 49.27 46.55

25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25

PAL-1 PAL-2 PAL-3 PAL-1 PAL-2 PAL-3 PAL-1 PAL-2 PAL-3 PER-1 PER-2 PER-3 PER-1 PER-2 PER-3 PER-1 PER-2 PER-3

B B B 2B 2B 2B 2.5B 2.5B 2.5B B B B 1.5B 1.5B 1.5B 2B 2B 2B

(a) PER specimens

(b) PAL specimens Fig. 11. Typical responses modes of 25 mm samples subjected to various impact levels.

taining its integrity at the end of impact. Typical curves for each response mode are shown in Fig. 11. In this study, critical failure energies were observed at around 2B (50.76 J) for PER specimens and 2B-2.5B (50.76 J  63.45 J) for PAL specimens.

Fig. 10. Typical responses of 25 mm specimens subjected to various impact levels.

4.2.2. Failure mechanism The failure mechanism of PBSL was fairly affected by the impactor. Hemispherical striker used in this study concentrated the load into densification area resulting in local damages through the

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impact period. Microscopic technique was then used to investigate the general process of failure propagations. Considered response modes were low impact (penetration-rebound), medium impact (penetration-critical failure) and high impact (penetration-failure) as described in Section 4.2.1. 4.2.2.1. Failure characteristics observed in PAL specimens. Typical microstructures of PAL specimens i.e. specimens subjected to different response modes parallel to the fibres, are shown in Fig. 12. At low impact, the striker caused an obvious deformation (indentation) at the upper surface of the specimen but fully rebounded as shown in Fig. 12(a). The indentation was confined within the impact area and there was no distinct separation at the fibre level. At medium impact, indentations were more pronounced. Fibre bundles within this area were stripped due to the uneven centripetal tractions; clear separations were observed at the fibre level, which resulted in a ‘Bulking Inner Ring’, as shown in Fig. 12(b). At high impact, the resulting separations were continuously extending, and the ‘Bulking Inner Ring’ was unable to support additional load. Self-bulking of column-like separations dominated the failure once cracks penetrated through the entire cross-section. Surrounding components suffered from extrusion, and the specimen lost its integrity and separated into pieces, as illustrated in Fig. 12(c). Fig. 13 shows the schematic diagram of PAL failure sections. The bamboo fibres were mostly affected and dislocated within the impacted zone (Fig. 13(a)), and pure delamination between bamboo bundles and adhesive were observed (Fig. 13(b)). However, other parts of the specimen were not affected by the impact. Scan results show that the carrying capacity of PBSL specimens impacted parallel to fibres relied strongly on the resistance of core indentation. Specimens failed abruptly as soon as separation was initiated; this phenomenon somewhat explains the underlying reason of why the height of specimen did not have any significant influence on penetration impact tests conducted for PBSL.

4.2.2.2. Failure procedure of PER specimens. Typical microstructures of PER specimens i.e. specimens subjected to different response modes perpendicular to the fibres, are shown in Fig. 14. At low impact, similar to PAL response, obvious indentation at the upper surface was caused by the striker. However, no distinct separation at the fibre level was observed as shown in Fig. 14(a). At medium impact, broken edge occurred with the further densification of fibre bundles resulting a clear ‘Crashed Ring’ (Fig. 14 (b)). Main cracks developed along the fibre direction towards each end, but the specimen was still able to maintain its shape after the rebound of the striker. At high impact, main cracks caused tearing of the upper surface, which subsequently extended to adjacent fibres. Finally vertical separation occurred once the applied loading exceeded the specimen’s carrying capacity. Fig. 15 shows typical failure sections of PER samples. Impact affected zone was limited within a certain distance from the upper surface with approximately inverted triangular separations as shown in Fig. 15(a). Stress redistribution may have contributed to the shape of crack propagations, and may have accelerated the abrupt failure of PRE specimen. However, beyond this region, similar pure delamination between fibre bundles and adhesive were observed (Fig. 15(b)). This observation also supports our previous analysis that the height of PER specimen does not have any significant influence on penetration impact tests. 4.2.2.3. Comparison. The relationships between the maximum load (Fmax), maximum deformation (Dmax) and impact levels as obtained from the conducted tests on PBSL specimens are shown in Fig. 16. Based on the analysis in pervious sections, it might be reasonable to conclude that height of specimen does not have any significant influence on penetration impact tests; therefore, mean values for 50 mm, 75 mm and 100 mm specimens subjected to 3B impacts were also considered for comparison. Mean maximum force increased almost linearly as the impact level was increased but reached a steady peak at around 2B impact level. Once the critical failure energy was reached, there was no

Fig. 12. Microstructure of 25 mm PAL Specimens subjected to different response mode.

X. Li et al. / Construction and Building Materials 242 (2020) 118135

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(a) Failure section of PAL specimen

(b) Scanned section outside affected area Fig. 13. Schematic diagram of PAL failure sections.

Fig. 14. Microstructure of 25 mm PER Specimens subjected to different response mode.

increase in maximum force. Mean maximum deformation also increased with increasing impact for all considered cases. This implies that the overall deformation capacity of PBSL specimens were unaffected by their failure modes; influenced fibres were fully deformed within the indentation area during the loading period.

As mentioned in Section 3.2, energy absorption capacity of PBSL was strongly dependent on its force–deformation curve. Fig. 16 illustrates that before reaching the critical failure energy, both maximum force and maximum deformation increased as the impact energy was increased. When the applied load of PBSL reached a steady state, the deformability still kept increasing and

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(a) Failure section of PER specimen

(b) Scanned section outside affected area Fig. 15. Schematic diagram of PER failure sections.

uncertainties. Specimens with 25 mm height and 40 mm square cross-sections produced consistent results in the current study. The impact-affected areas were most limited within the area under direct impact i.e. column-like area for PAL specimens and inverted triangular area for PER specimens. Obtained results indicated an increase of 50.76 J (2B) in impact energy resulted in a significant increase of ultimate load by 37% and maximum displacement by 86%. Peak load was observed around the critical failure energy at around 50.76 J–63.45 J, and dynamic sensitivity was more obvious for PER specimens. The strength of adhesive may influence the material properties of PBSL under dynamic loading, but would require further investigations. CRediT authorship contribution statement Fig. 16. Relationships of maximum load/impact energy and maximum deformation/impact energy.

hence the amount of absorbed energy of PBSL still kept increasing albeit at a slower rate. PER specimens were somewhat more brittle than PAL counterparts as the amplitude of critical failure energy was smaller (2B for PER specimen and 2B-2.5B for PAL specimen), but showed a higher sensitivity to dynamic loading. 5. Conclusion The current paper presented detailed experimental investigations on the deformation and failure mechanism of PBSL under penetration impacts, which has been an unexplored topic in this field of research. Penetration-rebound response, penetrationfailure response as well as penetration-critical failure responses were revealed by using INSTRON drop tower and microscopic technique. All tested specimens conducted under same conditions showed great repeatability indicating to the dynamic stability of PBSL. The size effect was of no importance on penetration behaviour of PBSL, but use of samples with the same height is suggested to reduce

Xin Li: Methodology, Investigation, Validation, Writing - original draft. Mahmud Ashraf: Conceptualization, Supervision, Writing - review & editing. Haitao Li: Conceptualization, Supervision. Xiaoyan Zheng: Resources. Safat Al-Deen: Resources. Hongxu Wang: Investigation. Paul J. Hazell: Resources. Acknowledgements The study in this paper is based upon work supported by a Project Funded by the Natural Science Foundation of Jiangsu Province (No. BK20181402), the National Natural Science Foundation of China (51878354) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors gratefully acknowledge Ali Ameri, Zongjun Li, M.A.Islam, Bohan JIN, Xiaocan SUO and others from the UNSW at ADFA, the Nanjing Forestry University and Deakin University for helping with this work. References [1] E.A. Nurdiah, The potential of bamboo as building material in organic shaped buildings, Procedia-Soc. Behav. Sci. 216 (2016) 30–38. [2] D. Jayanetti, P. Follett, Bamboo in construction, in: Modern Bamboo Structures, CRC Press, 2008, pp. 35–44.

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