Comparative study on metal/CFRP hybrid structures under static and dynamic loading
Introduction
Over the past few decades, rising requirements in fuel consumption, environmental regulations and road safety have presented great challenge for automotive industry. Accordingly, more advanced energy-absorbing components are being developed by applying high performance and lightweight materials and structures [1]. Tailor rolled blank [2,3], auxetic structures [4], [5], [6], porous materials [7,8] and composite materials [9], [10], [11] are some typical examples in balancing lightweight and performance. Of the abovementioned lightweight materials and structures, carbon fiber reinforced plastics (CFRP) composites have attracted significant attention attributable to their unique mechanical properties and excellent design flexibility, in which substantial research efforts have been increasingly devoted on how to use CFRP for structural design of automotive industry. For example, Zhu et al. [12] numerically investigated crashworthiness characteristics of CFRP bumper beam with variable cross-sections, and they found that the developed novel bumper beam was of superior mechanical performance. Liu et al. [13] experimentally studied crushing behaviors of double hat shaped CFRP tubes, and they found that tubal wall thickness is a critical parameter determining the failure mode and energy-absorbing capability. Boria et al. [14] experimentally explored crushing behaviors and energy-absorbing capability of CFRP truncated cones, in which two typical crushing failure modes were identified. Waimer et al. [15] developed the numerical approach to modeling failure mechanisms of CFRP components under dynamic crash loads. Zhu et al. explored energy-absorbing mechanisms of CFRP multi-cell structures by using experimental and numerical approaches [16]. Mamalis et al. [17] carried out a series of axial compressive tests to explore the crushing behaviors, collapse modes and crashworthiness characteristics of CFRP square tubes. Zhao et al. [18] numerically investigated crushing behavior of several CFRP tapered tubes with various cross-sectional profiles and tapered angles under multiple load cases, and they found that there was a positive correlation between total energy-absorbing capacity and number of edges in the cross section. Ren et al. [19] and Jiang et al. [20] developed an accurate progressive failure model based upon continuum damage mechanics to investigate crushing behaviors of CFRP corrugated beam under quasi-static axial crushing load. Ataabadi et al. [21] experimentally explored the influences of lay-up configuration on energy-absorbing capability of CFRP tubes under axial loading conditions. In light of these abovementioned studies, it is known that CFRP materials demonstrate considerable potential in lightweight design for crashworthiness design of vehicle structures.
Nevertheless, recent research has indicated that the high material cost of CFRP is not suitable for mass production of vehicle components [22]. For this reason, an appropriate trade-off between material cost and weight reduction needs to be made. In addition, it is also found that CFRP structures are susceptible to damage from impact loading, leading to significant reduction in energy absorption [23]. Fortunately, these shortcomings could be overcome by a rationally designed hybrid structure, in which each constituent phase could present its own characteristics contributing to the overall performance synergistically. For example, the metal/CFRP hybrid structures, which combine the low density and high strength CFRP materials with low cost and high toughness metallic materials, exhibit great potential in achieving mingled features of lightweight and competitive material cost.
Over the past few decades, metal/CFRP hybrid structures have been developed for crashworthiness purposes. In this regard, for example, Song et al. [24] studied dynamic crushing behaviors of the hybrid tubes, which were fabricated with the metallic tubes externally wrapped with composite layers. Bambach et al. [25], [26], [27] experimentally studied the crashworthiness of hybrid square tubes, and then they numerically investigated these hybrid structures for vehicular applications. Their study exhibited that the hybrid components were able to improve crashworthiness and reduce weight. Zhu et al. [28,29] and Wang et al. [30] studied crashing characteristics of different hybrid columns under axial and oblique quasi-static loading conditions; and they demonstrated the superior crashworthiness characteristics and competitive material costs of such hybrid structures. Imran et al. [31] explored axial compression capacity of steel columns externally reinforced by CFRP. They found that axial crushing capacity of the hybrid column increased up to 2.6 times of that of the steel columns, indicating the superior performance of the hybrid columns. Dlugosch et al. [32] studied the axial crashing characteristics and crashworthiness of several metal/CFRP hybrid square tubes; and they further developed a simplified modeling approach to effectively evaluating structural behavior in an early stage of crashworthiness design. Lee et al. [33] applied steel/CFRP hybrid composites in center-pillar reinforcements through experimental and numerical approaches, in which considerable advantages in lightweight and crashworthiness were demonstrated by comparing with the tailor welded blank (TWB) counterparts. From these aforementioned studies, it has demonstrated that the metal/CFRP hybrid structures possess superior mechanical performance and competitive material cost; and consequently, they are more suitable to be used for vehicular components.
It is noted that most of the metal/CFRP hybrid structures considered in the previous studies consisted of an inner metallic column and externally bonded CFRP layers. Nevertheless, it was shown that the outer CFRP layers of these hybrid columns are bent externally with formation of considerably large fragments during crushing process, which might result in a relatively lower damage level of CFRP [29]. Thus, the configuration of such a hybrid column may not take a full advantage of CFRP. In other words, it is worth exploring other hybrid configurations for achieving better crashworthiness behavior.
This work aims to study axial crashing characteristics of aluminum/CFRP columns with different hybrid configurations under different loading conditions by comparing with net aluminum and net CFRP counterparts (i.e. individual tubes made of single material). Specifically, two groups of aluminum/CFRP columns with different hybrid schemes are fabricated. The first group comprises an inner aluminum tube and externally wrapped with the CFRP layers, named as H-I hybrid tubes; and the second group comprises an outer aluminum tube and internally adhered CFRP layers, named as H-II hybrid tubes. First, a series of axial crushing tests with quasi-static and dynamic loading conditions are respectively carried out to explore their crushing behavior; and their energy-absorbing characteristics and crashworthiness are compared with those of net aluminum and net CFRP tubes. Second, the numerical simulations are conducted to provide further insights into the underlying energy-absorbing mechanisms which are unobservable with the experimental means available. Finally, a systematic parametric study is conducted to quantitatively explore the influences of tubal wall thickness, fiber orientation and interfacial strength on crushing behavior of the hybrid tubes.
Section snippets
Specimens fabrication
Two groups of aluminum/CFRP hybrid tubes are fabricated to perform the experimental study here. To enable the comparison, the corresponding net aluminum circular tube and CFRP tube are prepared as well, and all the different specimens are summarized in Fig. 1. According to Fig. 1, the CFRP tube with a larger diameter, named as "CFRP-L", has the same geometric dimensions as the outer CFRP layers of the type H-I hybrid tubes; and the CFRP tube with a smaller diameter, named as "CFRP-S", have the
Crushing process and force-displacement curves
The crushing processes and force-displacement curves are compared for the tests under quasi-static and dynamic loading, in which the deformation behavior, crashworthiness and the effect of loading rate are investigated in detail.
Numerical modeling
Fig. 24(a) presents a representative finite element (FE) model with specific boundary and loading conditions in commercial FE code ABAQUS/Explicit. The tube is compressed by the simulated moving upper rigid platen, and the lower rigid platen is fixed. To be consistency with the experimental tests, a lumped mass of 320 kg is established in the center of the upper platen, which has the initial impact velocity of 5 m/s; and the bottom end of the column is fixed to the lower platen. The CFRP-S tube
Parametric studies
Based upon the validated FE models, this sub-section further explores the influences of design parameters such as aluminum wall thickness, fiber orientation and interfacial strength on crashing characteristics of the H-II hybrid tubes by comparing their crashworthiness indicators, crushing process and energy-absorbing mechanisms.
Conclusion
This study explores the crushing behavior of two groups of aluminum/CFRP hybrid tubes subjected to quasi-static and dynamic loading conditions by using experimental and numerical techniques, respectively. In the experimental studies, several key crashworthiness indicators and the crushing process of the hybrid tubes are investigated by comparing with corresponding individual tubes. The underlying energy-absorbing mechanisms and the influence factors on crashworthiness of the H-II hybrid tubes
Author contribution statement
Guangyong Sun and Guohua Zhu designed the research scheme and wrote the manuscript, Jiapeng Liao and Guohua Zhu conducted the experimental tests and numerical simulations, Qing Li analyzed the numerical results and revised the manuscript.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgement
This work is supported by National Natural Science Foundation of China (51575172, 51905042). Dr Guangyong Sun is a recipient of the Australian Research Council (ARC) Discovery Early Career Researcher Award (DECRA).
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