Residual crashworthiness of CFRP structures with pre-impact damage – An experimental and numerical study

Highlights

• Effects of transverse impact on axial compress behavior of square CFRP tubes are investigated.

• The crashworthiness decreases gradually with the increase of impact energy.

• Increase of impact energy (after 20 J) mainly affects energy absorption of axial compression.

• The residual crushing capacity are dependent on the impact energy, regardless of impact position.

Abstract

This study investigated the effect of transverse pre-impact damage on the load bearing capacity and failure behavior of square carbon fiber reinforced plastic (CFRP) tubes for axial crushing. The CFRP tubes were impacted transversely in different levels of impact energies to generate initial damage, and then the specimens were further crushed axially to evaluate the relation between transverse impact energies/positions and the residual axial crashworthiness. A finite element (FE) model was also developed to simulate the complex damage behavior of the CFRP tubes under these two different loading processes, based upon the continuum damage model (CDM) with user-defined material subroutine in Abaqus. A combined failure mode was observed in the transverse pre-impact tests, in which delamination was combined with partial or complete fiber breakage when increasing the impact energy from 10 J to 30 J. In the axial compression tests, two typical failure modes with circumferential fracture near the pre-impact position were identified for the damaged tubes, exhibiting significant difference from the progressive folding failure seen in undamaged tubes. Further, the damaged tubes yielded up to 38%, 58.5% and 58.3% reduction in terms of the peak load, mean load and energy absorption respectively in comparison with the specimens without pre-impact damages. It is also found that the residual crushing capacity decreased with increase in the transverse pre-impact energy; nevertheless the residual axial crushing properties were insensitive to single or double impacts on different circumferential positions. The failure modes of fiber breakage, delamination and matrix crack were investigated in detail by using the FE analysis.

Introduction

Higher requirements of fuel efficiency and structural safety are in strong demand with intensifying socioeconomic legislation and industrial standard [1], [2], [3]. In general, these two aspects often conflict with each other [4]. One way to tackle this challenge is to replace traditional heavy metals with light-weight materials while remaining or even enhancing the structural performances. In this regard, carbon fiber reinforced plastic (CFRP) composites have been exhibiting significant potential thanks to their extraordinary capacity of weight to specific stiffness, strength and energy absorption [5].

To understand crash behavior of CFRP structures, substantial experimental studies have been conducted on crashworthiness of various tubal components. For example, Mamalis et al. [6], [7] investigated the different collapse modes of square CFRP composite tubes subjected to static and dynamic axial crashes, and they found that only the progressive crushing mode absorbed the highest energy. They also identified that the peak crushing load increased with increase of wall thickness and fiber volume content. Jia et al. [8] investigated the quasi-static crushing behavior of filament wound CFRP cylinder with different geometric parameters, winding angles and pre-crack angles. They concluded that the evolution process of failure mostly depended on the pre-crack angle which would cause the crack initiation. Siromani et al. [9] studied three typical failure trigger modes to identify their effects on initial peak load and specific energy absorption (SEA); and they showed that combing a chamfered tube with an inward-folding crush-cap yielded the lowest initial peak load and the highest SEA. Liu et al. [5] analyzed the effects of wall thickness and lengths of the double hat shaped CFRP tubes; they identified two distinctive failure modes through the dynamic tests, which differed with a typical mode of continuous splaying fronds in quasi-static tests. They also reported that increasing impact velocity would increase the peak load but decrease the energy absorption (EA) and specific energy absorption (SEA). Meredith et al. [10] explored the effects of manufacture processes (e.g. vacuum assisted oven cure and autoclave cure) on crash performance of CFRP cones through dynamic impact tests, in which performance versus cost analysis was conducted, revealing enormous potential for cost reduction of prepreg carbon fiber epoxy cones through use of heavier areal weight fabrics with fewer plies as well as through use of oven cured prepreg.

Numerical modeling represented by finite element method has gained growing popularity for its advantages in simulating mechanical behaviors of composite tubes in detail. For example, McGregor et al. [11] adopted the continuum damage mechanics model (CDM) to predict axial impact of two-ply and four-ply square tubes with and without an external plug initiator using LS-DYNA; and good agreement was obtained in terms of the failure characteristics and energy absorption. Zhu et al. [12] proposed two different numerical models, namely multi-layer stacked model and single-layer shell model, to simulate the crushing process of CFRP structures; and they found that the multi-layer stacking model exhibited a better capability of predicting the main failure modes and crashworthiness of the CFRP structures. Obradovic et al. [3] carried out the experimental, analytical and numerical studies on the crash analysis of composite structures under frontal impact, demonstrating the critical importance of selecting failure criteria for predicting brittle collapse. In literature, some typical composite damage models, e.g. MAT58 and MAT54 in LS-DYNA, were validated and have proven to be effective for simulating the inter-ply delamination under axial crushing [13], [14]. Two different finite element (FE) models, namely stacked shell model and crushing zone model, were developed for predicting the energy absorption in the crushing process [15]. However, these abovementioned FE models have not model complex failure mechanism of CFRP tubes for predicting crushing process and energy absorption.

It is well known that CFRP composite structures are very sensitive to dynamic loading; and even a minor, invisible damage could cause noticeable reduction in the strength and stiffness [16]. Therefore, it is crucial to evaluate the load-bearing capacity of composite structures with any pre-existed damage at different levels. In literature, there have been some studies available on evaluating the residual performance of CFRP structures with some pre-generated holes, defects and/or damages. In this regard, Liu et al. [17] undertook an experimental and numerical study on the load bearing behavior of square CFRP tubes with open holes subjected to axial compression; the effects of hole sizes, shape and distribution on the first peak force, failure modes and SEA were explored. It exhibited that the hole size had the stronger effect on peak load and SEA than hole shape and distribution. Guades and Aravinthan [18] conducted an experimental study on the residual properties of square pultruded tubes made of E-glass fiber composites subjected to axial impact, in which the coupons taken around the impacted area were tested with compressive, tensile and flexural loadings. Their study revealed that the residual strength of the pre-impacted tubes degraded with the pre-impact energy, number of impacts and mass of the impactor, whilst little effect appeared on the residual modulus. Sebaey and Mahdi [19] studied the quasi-static transverse crushing characteristics of glass/epoxy pipes by introducing impact damage. They found that the peak load was reduced by 23% for top/bottom pre-impacted tube and 15% for the side pre-impacted tube in comparison with those of the non-impacted ones, meaning that the capacity of resisting crash was reduced due to the pre-impacted damage. With the increase in the impact energy and impact numbers, such a reduction trend could be also seen in the peak force and average crushing load.

In literature, most of the existing numerical studies on residual mechanical response of composites have been focused on laminates [20], [21], [22], [23], [24], [25]. For example, Wang et al. [20] developed a FE model to simulate the low-velocity impact characteristics and predicted the residual tensile strength, in which a progressive damage model with stress-based Hashin criteria was used to model the fiber and matrix failures of the CFRP laminates under impact load. Abir et al. [21] investigated the effects of impact damage on crushing performance of CFRP laminates numerically, in which the continuum damage mechanics (CDM) model and cohesive interface elements were adopted to characterize the fiber failure and inter-laminar fracture behavior. Tan et al. [22] adopted a three-dimensional composite damage model to simulate the fiber failure and delamination behavior under the so-called compression-after-impact (CAI) test; and good agreement was obtained between the numerical and experimental results in terms of force–displacement curves, damage contours and permanent indentation. As for the composite laminates reinforced by unidirectional fibers, the cohesive connections were introduced only in the areas between the plies with different fiber orientations for reducing the computational cost [23]; and the simulated results showed fairly good accuracy on modeling the complex failure phenomena during crushing after the initial impact tests.

The previous study indicated that the CFRP tubes are of sizeable advantages on crashworthiness than aluminum counterparts under quasi-static axial loading [26]. There is a great potential of replacing traditional metallic energy absorbers with CFRP structures attributable to its high energy absorption and lightweight performance. Crash box, as one important application for energy absorption, could be subjected to various impact loading during its life cycle, such as tools dropping (assembling process) or collision from ground pebbles (travel process), representing accumulative damage from different pre-impacts. Unfortunately, only few studies have been available in literature for evaluating the residual crushing behavior of composite tubes with pre-introduced damage. In this regard, Liu et al. [17] investigated the damage mechanisms of perforated CFRP tubes under quasi-static crushing. Their FE model was able to model the crack initiation, propagation and strain distribution around the pre-perforated hole edges accurately. Deniz et al. [27] explored the low-velocity crushing with pre-impacted damage in the [±55]₃ filament-wound glass/epoxy composite circular tubes. To the author's best knowledge, nevertheless, there has been no study available to explore the complex damage mechanism for the axial crash with pre-impacted damage laterally. It remains to identify the residual axial crashworthiness for the CFRP tubes with pre-impacted damage in the transverse direction.

This study aimed to provide an experimental and numerical investigation into the residual load bearing capacity of the CFRP tubes with lateral pre-impacted damage. The failure modes and force–displacement curves characterized by different impact energies were analyzed, and then the damage mechanisms induced by different loading levels were discussed based upon the optical observations on the fractured area. The effects of impact parameters such as impact energy and impact position on the residual axial crushing properties were investigated in detail. The study is expected to provide a guideline for quantifying residual crashworthiness of CFRP tubes with pre-damage in a different direction.