Crashworthy Behavior of Plain Weave Glass/Epoxy Composite Tubes Containing Different Extents
of Overlap of Dropped Plies

1. Kureel , Murari V. ^*

Department of Applied Mechanics, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, 211004, India

1.      Introduction

The utilization of composite materials has expanded a thousand-fold in recent decades. Composites are used in many different fields owing to their advantageous characteristics, their ability to offer a superior strength-to-weight ratio, higher energy absorption capabilities, strong fatigue resistance, corrosion resistance, and lower environmental impact [1-5]. A significant amount of effort has been made to discover energy-absorbing structures that are crashworthy [6]. Crashworthiness is described as the capacity of a vehicle to sustain collisions and its ability to safeguard occupants and passengers from severe or deadly injuries [7]. Particularly in the automotive industry, composite tubes have grown as permanent energy absorbers [8, 9]. Composite tubes have excellent energy absorption capacity due to their complex and multiple failure modes. This makes them ideal for use in automotive crashworthiness applications. The energy-absorption in these structures depends on a number of factors, including reinforcement, matrix material, layer orientation, stacking sequence, cross-sectional shape, and position of trigger [10].

Othman et al. [11] performed crash analysis tests to examine the crushing mechanism and failure pattern in foam-filled tubular structures containing different thicknesses. They estimated the effect of filler material on the crashworthy behavior of pultruded composite tubes and observed that the profiles with and without foam filling have failed through fiber splaying and matrix crushing. Unidirectional composite tubes reinforced with carbon fiber but with varying fiber orientations and wall thicknesses were studied by Wang et al. [12] for their energy absorption rate and crushing mode. They found that the tubes deform via brittle fracture failure mode generate less debris in impact loading than in quasi-static loading. Tube specimens were divided into two groups: one with the same wall thickness (3mm) but containing different fiber orientations (ϕ=15^ο, 60^ο & 75^ο) and the other with the same fiber orientation (45⁰) but with different thicknesses (2mm, 3mm, 4mm, 5mm, and 6mm). The study revealed that certain parameters related to crashworthiness (including mean crushing load, peak load, and energy absorbed) decreased in the first group, while only peak and mean load exhibited a linear increase in the other group, as thickness changed. Zhang et al. [13] examined the effect of thickness on the quasi-static crushing response of square aluminium tubes. The study revealed that in order to achieve an increase of 35% in energy absorption, the thickness of the corner of a square tube needed to be greater than that of its sides, while the maximum load capacity remained unchanged. Thornton et al. [14] inspected the effect of different types of constituent materials on the quasi-static crushing response of composite tubes. It was observed from the study that the specific energy absorption for different thermosetting matrices were found to be decreasing in the following order: epoxy, polyester, and phenolic. The specimens with 0^ο fiber orientation were found to possess a higher SEA in the case of glass fiber reinforced polymer (GFRP) tubes, where one or two outside 90° layers bind the 0° inner layer. In terms of section geometry, the SEA was found to be decreasing in the following order: round, square, and rectangular. Farley et al. [15] tested several composite specimen geometries consisting of carbon and kevlar as reinforcements to determine the energy absorption capability under constant rate crushing. It was concluded that as the ratio of diameter and thickness decreases, the SEA decreases as well. It was found that the specimens were geometrically scalable by varying the diameter and the wall thickness of the specimen at appropriate places in the case of kevlar/epoxy tubes. The same is not true in the case of tubes with carbon reinforcement. Patel et al. [16] examined the impact of several design variables on crush force dissipation parameters in automobile accidents. The peak load and the deformed length were used as crashworthiness parameters when testing a composite tube under axial and oblique compressive loads. Tube arrangement and fiber orientation of ply were influencing the crashworthiness parameter during impact loading. It was found that GFRP material outperformed CFRP (carbon fiber reinforced polymer) material for oblique impact loads. In their research, Song et al. [17] investigated the response of metal-wrapped GFRP tubes under quasi-static compressive and impact loading. The failure mode is found to be influenced by factors such as the loading rate, wall thickness, and fiber orientation, which ultimately determine whether the tube would undergo stable crushing or catastrophic failure. The crushing behavior of laminated shells of different thicknesses in a cone form under axial compression was investigated by Kazemi et al. [18]. The buckling load was found to increase initially but subsequently reduced with an increase in the fiber orientation. Further, it was found that increasing the cone's original thickness has raised the buckling load. Turner et al. [19] examined two different thermoset matrix-based composite materials: a non-crimp fabric (NCF) and a continuous filament mat (CoFRM) with fiber volume fractions of 22% and 38%. The amount of binder has a considerable impact on the fracture toughness of mode I, while the composite tube's SEA seems to be quite forgiving of binder levels. Both materials' SEA levels seem to be tolerant to high void levels, opening the door to significant cost savings through expedited processing. Abosbaia et al. [20] investigated load-carrying capacity and energy absorption for segmented carbon and cotton fibers. Conversely, it was discovered that segmented composite tubes, incorporating tissue mat glass fibers, had a catastrophic failure mechanism and absorbed limited energy. Melo et al. [21] compared the effect of vacuum and non-vacuum production environments of composite tubes on the crushing behavior. The results show that crashworthy properties of polymer-based composite structures were highly influenced by controlled manufacturing processes. The crashworthy parameter was studied by Yan et al. [22] for tubes made of flax/epoxy reinforced composites with three different length-to-diameter (L/D) ratios, three different inner diameters (36, 54, and 82mm), and three different three-ply configurations (single, double, and triple) under quasi-static in-plane compressive loading. Additionally, it was discovered that a 36mm diameter tube with an L/d ratio of 2 and consisting of 3 layers achieved optimum SEA and crush load efficiency (CLE), which is superior to conventional metal tubes and almost as effective as GFRP and CFRP. Under quasi-static compression and drop weight impact, Gupta et al. [23] studied the crushing behavior and energy-absorbing capability of a glass/polyester hemispherical shell of varying thicknesses. As the thickness of the hemispherical shell increases, the average crushing load also increases. Karbhari et al. [24] examined the post-crush response of biaxially and triaxially braided composite tubes made of three different types of fiber (glass, carbon, kevlar). They found that braiding changes the damage mechanism from stable crushing to lateral cracking when specimens were subjected to impact loading prior to quasi-static compression. Another study was reported by Tong et al. [25] on braided carbon/epoxy composite tubes for crash energy absorption. A novel chamfer trigger was developed and investigated under axial compressive loading. Further, it was found that cavities caused by chamfer lead in energy absorption up to 53% when the diameter-to-thickness (D/t) ratio increases. In this context, Farley et al. [26] categorized different crushing behavior of GFRP composite tubes subjected to different crushing speeds. They found that crushing behavior depends on crushing speed, constituent properties, and specimen geometry. The crushing mechanism for both (circular and square) GFRP composites was investigated by Palanivelu et al. [27] under several impact velocities showing a range from 9.3 to 14 meters per second. It is observed that circular tubes with a thickness of around 6% of their diameter exhibit 59% higher energy absorption capacity than the square tubes having a thickness of around 7.5% of their side length. According to Mahdi et al. [28], an optimized fiber orientation was created for GFRP composite tubes under in-plane quasi-static crushing. Tubes having circular cross-sections were prepared using epoxy-impregnated woven glass fiber fabric having different orientations. Further, they suggested that the ideal orientation of layers for stable crushing are 0^ο/90^ο and +45^ο/-45^ο, and the same for higher energy absorption is +15^ο/-75^ο and that for both higher energy absorption and stable crushing is +75^ο/-15^ο. Liu et al. [29] investigated energy absorption in carbon/epoxy composite tubes using quasi-static and dynamic tests. It is found that strain rate significantly influenced crushing behavior, reducing crush load, and resulting in lower SEA values. Thicker tubes with 0°/90° layup and fabric architecture showed superior SEA performance. Diniz et al. [30] examined composite tubes made of different materials with drop-offs. It was found that carbon/aramid fabric had higher longitudinal elasticity modulus than carbon and glass. Carbon fabric tubes had the highest compression load capacity followed by hybrid tubular structures with four plies dropped. Khan & Mahdi [31] examined the effect of trigger mechanisms on glass epoxy/PVC hybrid composite tubes under axial compression. It is found that integrated trigger mechanisms positively affect the crushing behavior and non-trigger specimens showed catastrophic failure modes. The double-step trigger outperformed the chamfer trigger in load-bearing capacity, energy absorption capacity, and crush force efficiency. Chen et al. [32] studied axial crushing response and crashworthiness characteristics of circular carbon/epoxy composite tubes. CFRP tubes achieved the best crashworthiness performance with a Specific Energy Absorption of 78.8 kJ/kg and a Maximum Crushing load of 485.7 kN. Also, it is found that fiber hybridization mainly affects post-crash integrity and crashworthiness characteristics, with negligible influence on crushing failure mode.

Many studies have examined the crashworthiness parameters of composite tubes.  Improving crashworthiness in composite tubes holds the potential for various benefits, including enhanced occupant protection, reduced risk of injuries, and increased structural integrity during collisions. Composite materials provide engineers with the opportunity to tailor their properties to specific crash scenarios, allowing for optimized energy absorption, deformation mitigation, and enhanced crash resilience. By investigating factors such as material composition, fiber orientation, layup configuration, and geometrical parameters, researchers can further advance the understanding and practical application of composite materials in crashworthiness. The goal is to improve safety measures and protect human lives in critical impact situations. To this end, the present work focuses on the effect of geometry parameters on the crashworthiness of composite tubes. Comprehending the compressive behavior of composite tubes with the extent of overlap of dropped-off plies is an area that still needs investigation. To effectively estimate the crashworthiness, understanding the impact of the extent of overlap between dropped-off layers on crashworthiness parameters is crucial. The present research examines the consequences of the extent of overlap and inner diameter on the damage response and the crashworthiness parameters of composite tubular specimens subjected to quasi-static compression and those subjected to impact loading.

2.     Methodology

2.1. Specimen Geometry/Boundary Conditions

To better understand how the extent of overlap affects crushing response, test specimens featuring the basic geometrical characteristics as shown in Figure 1 are analyzed. Throughout the study, the extent of overlapping remained constant along the length of the specimen. In this study, composite tubes with three different inner diameters (20, 25, and 30mm) are considered. The lengths of the composite tubes are fixed so that the L/D ratio turns out to be 2.5.

The different extents of overlaps in each case are defined as the fraction of the circumference of the composite tube. Five different extents of overlap are taken into account: 0, ¼, ½, ¾, and 1 times the circumference of the composite tube. Pictorial representations of different extents of overlap are presented in Figure 2 showing the cross-sections of the specimens.

Three different inner diameters each with five different extents of overlap make fifteen numbers of cases in total. Four test specimens per case are prepared for performing the quasi-static compression test and three test specimens per case are prepared for performing the impact tests. The details of the different cases considered are presented in Table 1. The specimen codes mentioned in the table are to be interpreted as follows. For example, the first three letters ‘WCT’ in code ‘WCT20_NO’ stands for Woven Composite Tube, the next two digits represent the inner diameter of the specimen, the characters/number following the

underscore symbol represents the extent of overlap. Here ‘NO’ indicates no overlap, ‘1.25O’ indicates ¼ overlap, and so on.

During the manufacturing of the specimens, a plain weave woven E-glass 350 GSM fabric is utilized as reinforcement. The matrix is a combination of epoxy (LY556) and amine hardener (HY951) in a 10:1 ratio. All specimens are prepared using the hand lay-up method. Both the ends of the specimen were made flat and parallel to each other, no triggering features for damage initiation such as chamfering, etc. are provided in the specimens. Figure 3 displays photographs of the different specimens constructed for testing.

2.2. Experimental Setup

The specific details regarding the machine used for conducting the tests are as follows:

The in-plane quasi-static crushing is performed on a TINIUS OLSEN (L-series) machine. The machine has a piezo-based load cell of 10 kN and is designed to operate with an external personal computer installed with the software package to obtain the recorded data of the experiment. The videography of the specimen under crushing is done to analyze the failure mechanism. For each case, a group of four samples is subjected to testing. To execute the impact loading, the IMATEK IM10T-20HV drop weight impact test platform [33] is utilized. This equipment runs on the IMATEK IMPACT ANALYSIS software package. A data collection system equipped with a laser is employed to record approximately 4,000 data points within a 20-millisecond timeframe. In the case of quasi-static compression testing the bottom end of the tube is simply supported, while the compressive load is applied to the top end. In the case of drop weight impact testing, the weight is dropped from above to impact the top end of the specimen, with the bottom simply supported. The testing parameters used for impact loading are specified in Table 2. In each case, a total of three samples are tested.

Table 2. Impact test parameters

3.     Results and Discussions

The effect of inner diameter and the extent of overlapping of dropped-off plies on the parameters affecting the crashworthiness of composite tubes are discussed in the following subsections.

3.1.       Effect of Extent of Overlap and Inner Diameter on the Response of Specimen Under Quasi-Static Crushing

In general, the entire crushing process during the quasi-static compression could be split into three different zones: preliminary crushing, crushing, and compaction (refer to Figure 4). During the preliminary crushing stage, the specimens were found to exhibit a linear load-displacement response until peak load (P_peak) was attained. During the crushing stage, a gradual failure process is observed. The damage mostly begins at the top portion of the tubes and the damaged portions start bending inwards and outwards, followed by the damage at the bottom portion of the specimen. This is observed as an initial steep decrease in the load and a subsequent cyclic increase and decrease in the load. These observations correspond to the process of damage formation and the bending of the damaged portion of the crushed specimen at both the loading and the supporting end of the specimen. The crush load has remained almost constant with slight fluctuations around P_mean. Finally, in the compaction zone, debris that got filled inside of the remaining tube densifies, and that results in a sudden rise in load.

The mechanical response of the specimens with different inner diameters containing no overlap is presented in Figure 5 and the same for other cases of overlaps are shown in Figures 6 to 9 respectively. The effect of different extents of overlap of dropped-off plies (such as no overlap, quarter, half, three quarter, and full overlap) on the mechanical response of the test specimen with 20mm inner diameter is presented in Figure 10 and that for the case with other diameters (25 and 30mm) are presented in Figure 11 and 12 respectively. It can be seen from Figure 5 to Figure 9 that the influence of the inner diameter diminishes as the amount of overlap grows. Further, it is observed that the crushing of specimens becomes more uniform with the increasing extent of overlap. From Figure 10 to Figure 12, it is observed, for all the cases of inner diameter considered, that the value of peak load increases around four times with an increase in the extent of overlap from no overlap to full overlap. In addition, it has been shown that an increase in the P_peak and P_mean occurs when the inner diameter of the specimen is increased.

3.1.1.          Energy Absorption (EA)

The amount of energy absorption is determined from the region underneath the load-displacement curve in the crushing zone. Figure 13 shows the amount of energy absorbed by test specimens, with different inner diameters and different extents overlap, under quasi-static compressive loading. After conducting tests on different specimens, it has been found that the total amount of energy absorbed ranges from 20J to 250J, depending on the specific case. An increase in the amount of absorption of energy is noted when there is an increase in both inner diameter and the extent of overlap in the samples. However, for a set of test specimens with a particular diameter, the amount of energy absorption for an increase in the extent of overlap from no overlap to half overlap is smaller than that for an increase in the extent of overlap from half overlap to full overlap.

The study indicates that the geometry of the overlap has a strong influence on the ability of the specimen to absorb energy. Specifically, it has been observed that samples with larger inner diameters, containing larger extents of overlap, have a higher capacity for energy absorption compared to those with smaller inner diameters and smaller extents of overlap.

3.1.2.          Specific Energy Absorption (SEA)

Specific energy absorption (SEA) is calculated by dividing the amount of energy absorption in a specimen by its crushed mass. The SEA is a useful measure for assessing the energy absorption capabilities of specimens that have different inner diameters and different extents of overlap. From Figure 14, it is observed that the SEA for all the cases of inner diameters increases as the extent of overlap increases from no overlap to half overlap. With further increase in the extent of overlap from half overlap to full overlap, a decrease in the SEA is observed for the composite tube with 30mm diameter and only a little change is observed in the case of the composite tube with other smaller diameters. The SEA of tubes with larger diameters are found to be smaller than that of the tubes with smaller diameter. This is observed in all the cases of extents of overlap, except the case with no overlap. These observations suggest that the appropriate choice of extents of overlap and inner diameter will result in a tailored crushing response.

3.1.3.          Mean Crushing Load (P_mean)

The “mean crushing load” (Pmean) is the average load in the crushing zone of the load-displacement curve.

The mean crushing load of test specimens with different inner diameters and different extents of overlap are shown in Figure 15. The results show that for smaller inner diameters (20mm and 25mm), P_mean increases as the extent of overlap increases. This can be attributed to the larger contact area and improved load distribution within the crushing zone as the extent of overlap grows. The increased overlap results in a greater load-bearing area, leading to enhanced load-carrying capacity and higher mean crushing load values. However, for the larger inner diameter (30mm), the P_mean increases when the extent of overlap increases from no overlap to half overlap, and then remains constant for further increases in the extent of overlap. These observations suggest that the relationship between the extent of overlap and the mean crushing load differs based on the inner diameter of the composite tubes. Smaller inner diameters exhibit a continuous increase, while larger inner diameters reach a saturation point where the mean crushing load stabilizes after a certain extent of overlap.

3.1.4.          Crush Load Efficiency (CLE)

Crush load efficiency (CLE) is the amount of crushing load that is effectively prevented from getting transmitted from the loading end to the other end of the energy absorber. It is defined as the ratio of P_mean to P_peak. This measure is used in the crash safety sectors to determine the danger of head and neck injuries during a collision, and also to determine how much load is transferred to the operator or others during a crash. Figure 16 shows the effect of the extent of overlap and inner diameter on the crush load efficiency of test specimens subjected to quasi-static crushing. The crush load efficiency of all the specimens is found to be above 60%, except in the case of specimens with 30mm diameter containing no overlap and the specimen with 25mm diameter containing half overlap. Further, it is found that the crush load efficiency of 30mm diameter composite specimens increases monotonically when the extent of overlap is increased from no overlap to full overlap, whereas the crush load efficiency of the composite tubes with 25mm diameter decreases initially with an increase in the extent of overlap from no overlap to half overlap and then increases with further increase in the extent of overlap to full overlap. In the case of composite tubes with 20mm diameter, an increase in crush load efficiency is noted initially when the extent of overlap is increased from no overlap to half overlap and then has reduced magnitudes with further increase in the extent of overlap to full overlap. In addition to this, crush load efficiency is seen to be at its highest when composite tubes have a 20mm diameter with half overlap. These findings imply that optimizing the inner diameter and the extent of overlap might boost the crush load efficiency of the composite tubes.

3.2.     Effect of Extent of Overlap and Inner Diameter on the Crushing Response of Specimens Under Impact Load

This section discusses the impact loading response of composite tubes with varying inner diameters and extents of overlap. To prevent post-crushing compaction of the test specimens, a specific amount of impact energy (IE) is carefully selected. The results of the impact testing for different test specimen scenarios are compiled in Table 3, which presents various crashworthiness parameters. The effect of inner diameter on the load-displacement responses of the test specimens having different extents of overlap subjected to impact loading are presented in Figure 17 to Figure 21, and the effect of extent of overlap on the load-displacement responses of specimens with different inner diameter subjected to impact loading are presented in Figure 22 to Figure 24. Further, in the case of tubes with 25mm diameter containing quarter, half, and full overlap (refer Figure 23), a gradual increase in the load levels with progressive damage and crushing is observed. This reduced initial peak load and gradually increasing load levels with progressive damage is a desired characteristic for an improved crashworthiness behavior. A similar response is observed in the case of quasi-static compression, for the specimen having a 25 mm diameter containing a quarter overlap (refer to Figure 6).

3.2.1.          Energy Absorption (EA)

Energy absorption in the test specimens with different extents of overlap and different inner diameters subjected to impact loading are presented in Figure 25. An increase in the absorption of energy is noted when the inner diameter and the extent of overlap in the specimen are increased. However, the effect of extent of overlap on energy absorption is more significant than the effect of inner diameter.

3.2.2.          Specific Energy Absorption (SEA)

Figure 26 shows the effect of the different extents of overlap and different inner diameters on the energy absorbed per unit mass of crushed composite tubes under dynamic crushing. The specific energy absorption (SEA) for various test specimens ranges from 7.5 kJ/kg to 32.5 kJ/kg. An increase in the value of SEA is noted when the extent of overlap increases from no overlap to half overlap and thereafter differences in SEA are less. Additionally, composite tubes with larger inner diameters have lower SEA compared to those with smaller inner diameters, which is consistent with the observation from quasi-static loading (see Figure 14). It is worth noting that, except for cases with half overlap, the SEA for composite tubes subjected to impact loading is found to be more than the corresponding quasi-static cases.

3.2.3.          Mean Crushing Load (P_mean)

Figure 27 shows the effect of different extent of overlap and different inner diameters on the mean crushing load (Pmean) of composite tubes subjected to crushing under impact load. The Figure reveals that, except for the case of no overlap, the mean crushing load increases with increasing diameter for all extents of overlap. In terms of loading instance, the mean crushing load for composite tubes subjected to impact loading is smaller than that obtained for quasi-static loading (refer Figure 15), except for the cases of composite tubes with 30 mm diameter containing quarter, three-quarter, and full overlap. The percentage increase in mean crushing load for different cases under impact load is around 2% to 21% as compared to quasi-static crushing. Notably, the mean crushing load is highest for the case of three-quarter overlap, rather than full overlap.

3.2.4.          Crush Load Efficiency (CLE)

The effect of different extents of overlap and inner diameter of the test specimens subjected to impact load on the crush load efficiency (CLE) are shown in Figure 28. The crush load efficiency is found to be above 60% (refer Figure 28) for the specimens with inner diameters of 25 mm and 30 mm that have quarter, half, or full overlap and for the specimens with inner diameter of 20mm containing full overlap. This observation is in contrast to that noted in the case of quasi-static loading (refer to Figure 16), where the crush force efficiency for almost all the cases were above 60%. When compared to the results obtained for the cases under quasi-static load, the crush force efficiency of the specimens with no overlap and those with three-quarter overlap under impact loading was found to be reduced by 60% and 30%, respectively. On the other hand, an increase in the crush force efficiency is noted in the case of specimens with 25 mm inner diameter containing quarter, half, and full overlap; specimens with 20mm diameter containing full overlap and specimens with 30mm diameter containing half overlap.

3.2.5.          Stroke Efficiency (η_stroke)

Stroke efficiency (η_stroke) is defined as the ratio of total crushable length to the total length of the specimens being considered. Figure 29 shows the effect of different extents of overlap and different inner diameters on the stroke efficiency of composite tubes under dynamic crushing. The stroke efficiency (η_stroke) in most of the cases of composite tubes are found to be more than 50%, with a maximum value of 77.4%. in the case of the composite tube with a 30mm diameter containing a quarter overlap. Stroke efficiency of less than 50% is found in the case of composite tubes containing half overlap (with 20mm and 25mm diameter) and those containing three-quarter overlap (with 20mm and 30 mm diameter).

3.3.   Effect of Extent of Overlap and Inner Diameter on Crushing Behavior and Failure Mechanism

The crushing behavior of composite tubes with different inner diameters containing different extents of overlap at different instances during impact loading are presented in Figures 30 to 34. Typically, composite tubes with different inner diameters and different extents of overlap experience gradual crushing, along with fibers on the exterior or interior splaying simultaneously. In the case of the composite tube with a 20 mm inner diameter containing no overlap (refer to Figure 30 (a)), the crushing is seen to be occurring at both ends of the specimen whereas in the case of 25 mm (refer to Figure 30 (b)) crushing is to be occurring only at the top end. In the case of the specimen with a 30 mm diameter containing no overlap, a circumferential crack is seen at the mid-span of the specimen (ref. fig. 30(c)).

The specimens with different inner diameters containing some amount of overlap show a steady progressive crushing at the loading end of the test specimen. Further, no mid-span cracking and no simultaneous crushing at both ends are seen in these cases.

Figures 35 and 36 show the distinct types of failure that are observed in different cases of specimens subjected to quasi-static compression and impact loading respectively. The specimens with a smaller inner diameter and lesser extent of overlap subjected to impact loading fail through combined crushing at the end and delamination/kinking of the overlap portion (refer to Figure 35 (a) & (b)). Splaying fronds are developed up to some length and then they fail through collision with uncrushed portions in composite tubes containing no overlap and quarter overlap. Composite tubes containing larger extents of overlap (half, three-quarters, and full) fail through progressive crushing with petal formation and fiber splaying as shown in Figures 35 (c) & (d). In the case of composite tubes with smaller diameters containing no overlap and quarter overlap, subjected to quasi-static compression, cracks developed at the mid-span of the tube and then propagated around its circumference, as shown in Figures 36 (a) & (b). This resulted in an unstable crushing upon further loading. In the case of composite tubes containing half, three-quarters, and full overlap, progressive crushing that resulted in compaction as shown in Figures 36 (c) and (d) is observed.

4.     Conclusions

The present study aims to systematically understand the influence of inner diameter and the extent of overlap of dropped-off plies on crashworthiness parameters of woven glass/epoxy composite tubes. To the best of the authors’ knowledge, the needed focus has not been given in the literature to the study of the effect of the geometry of the overlaps resulting from ply drops on the crashworthiness of composite tubes. These features are found to influence the crushing behavior and thus can lead to a specific and tailor-made damage behavior, which is a desired factor in utilizing composite specimens for crash applications.

Composite tubes with three different diameters (20mm, 25mm, and 30mm) and various extents of overlap (0, ¼, ½, ¾, 1 overlap) were considered in the study. Quasi-static compression and impact loading tests were conducted on the specimens. The following results indicate that the inner diameter of the composite tubes and the extent of overlapping of dropped-off plies are crucial in achieving tailor-made crashworthiness characteristics.

1. Maximum crush load efficiency was observed in specimens with smaller diameters containing half overlap, while larger diameter specimens with full overlap showed lower efficiency.
2. Impact-loaded specimens exhibited higher crush load efficiency and specific energy absorption compared to specimens subjected to quasi-static crushing.
3. Specimens with different diameters and a three-quarter overlap demonstrated the highest specific energy absorption, while composite tubes with a 30mm diameter and a quarter overlap achieved maximum stroke efficiency.

Availability of Data

The datasets used or analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The author declares that there is no conflict of interest regarding the publication of this article.

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[1] CL= crushed length

[2] IE= impact energy

[3] EA= energy absorb

[4] CFE= crush force efficiency

[5] SEA= specific energy absorption