Papers by Keyword: Impact Behaviour

Abstract: Composite materials are increasingly used in those fields where it is necessary to achieve the requirements of lightweight and high mechanical properties. Even though their high specific strength which get these materials very attractive, especially for the transport field, there are several critical issues that still limit their application in primary structures. Among these, dynamic loading conditions play a critical role because they can significantly lower their residual strength. This paper aims to investigate experimentally the structural response of a 25 mm thick Omega composite structure under different impact loading conditions. The investigated test article consists of E-glass fibres (40% volume fraction) reinforced polyester matrix. The structure is covered by a HELIOPOL 1401 M AGC W 11 gelcoat layer and it has been impacted through a drop mass of 3.94 kg, dropped from heights of 50 mm, 75 mm, 100 mm, 150 mm, 200 mm, 250 mm, 350 mm and 500 mm.

Abstract: Low velocity impacts induce concurring failure phenomena in unidirectional fiber reinforced composites. Hence a refined methodology able to predict the different failure modes and their interaction is mandatory to correctly predict the damage onset and evolution. Indeed, intra-laminar damage and inter-laminar damage often take place concurrently, causing a significant strength reduction up to composite structure collapse. In this paper, a numerical study is proposed which, by means of non-linear explicit FEM analysis, aims to completely characterize the composite reinforced laminates damage under low velocity impacts by introducing a user defined material model in the FEM code ABAQUS. The proposed 3-D numerical investigation allowed to obtain an exhaustive insight on the different phases of the impact event considering the damage formation and evolution.

Abstract: The investigation of fiber-reinforced composite laminates mechanical response under impact loads can be very difficult due to simultaneous failure phenomena. Indeed, as a consequence of low velocity impacts, intra-laminar damage as fiber and matrix cracking and inter-laminar damage, such as delamination, often take place concurrently, leading to significant reductions in terms of strength and stability for composite structure. In this paper a numerical study is proposed which, by means of non-linear explicit FEM analysis, aims to completely characterize the composite reinforced laminates damage under low velocity impacts. The numerical investigation allowed to obtain an exhaustive insight on the different phases of the impact event considering the damage formation and evolution. Five different impact locations with the same impact energy are taken into account to investigate the influence on the onset and growth of damage.

Abstract: The aim of this study was to investigate the valuable impact damage parameters from quasi-static indentation testing to access the low-velocity impact behaviour of ex-situ toughened composites by comparing low-velocity impact and quasi-static test results (the same boundary conditions). In terms of the delamination damage threshold load and indentation depth, quasi-static tests predicted the impact damage resistance well. However, only very conservative estimates of maximum load due to the final fibre failure under higher energy level were achieved. This phenomenon is attributed to two factors. First, energy during quasi-static indentation event is completely transformed or absorbed by the laminate, where it is stored elastically in panel bending or absorbed by the creation of damage, without the energy in the form of vibration, heat, inelastic behaviour of the impactor or the supports. Second, strain rate effect may have a remarkable influence on the fibre failure but on undamaged and delaminated damage.

Abstract: The demand for energy-absorbing lightweight structures for impact applications in automotive, aerospace and defence industry is rapidly growing, posing a challenge for innovative engineering design to maintain lightweight without reducing damage tolerance and impact and shock absorption. In this context, biological materials offer a source of inspiration for the design of new materials. Nacre, commonly known as the mother-of-pearl, is a biological material that exhibits outstanding mechanical properties due to its hierarchical structure, which includes a brick-like pattern, layer waviness and interface. Although nacre is made of 95% of aragonite, a brittle material, its toughness is about 3000 larger than that of aragonite. Research addressing the behaviour of nacre-like engineering composites is limited and this work intends to contribute to the understanding of such materials under impact loading. In this paper, the study of the impact behaviour of layered nacre-like plates made of 1-mm thick tablets of aluminium alloy 7075 glued with toughened epoxy resin is performed using Abaqus/Explicit. A 9-mm steel spherical projectile with initial impact velocities in the range of 400-900 m/s is used. The epoxy material is modelled using a user-defined cohesive element that accounts for the experimentally observable increase in both strength and toughness in compression. Target thicknesses of 5 and 7 mm are modelled. The ballistic performance of bulk plates made of bulk Al-7075 is compared with that of nacre-like composite plates of the same thickness. It is found that the nacre-like structures performed slightly better than the bulk plate for high impact velocities with a reduction of about 9% in the residual velocity; however, for lower impact velocities close to the ballistic limit, nacre-like plates performed worse than the bulk plate. The higher performance at higher impact velocities of the nacre-like composites is attributed to the hierarchical structure that enables both localized energy absorption by deformation of the metallic tablet and tablet interlocking due to the waviness and inter-layered delamination, which allows plastic deformation further away from the impact zone. It is concluded that nacre-like aluminium composite plates should be further investigated for their potential in designing protective structures because they could enable substantial improvements in weight-savings and in the ballistic performance of the structure. However, a quantitative assessment of their benefit warrants further numerical and experimental research.

Abstract: There are two main factors that need to be considered as important parameters that affect the response of a structure: kinetic energy (E=1/2mv^2 ) and potential energy (E=mgh). For instance, if one has a large mass but with lower height, the amount of damage produced on the structure may not be the same as if one has a smaller mass with a higher dropping height although the potential energies will be the same. Therefore, before performing tests on the structures, the selection for the appropriate test apparatus and test procedures must be made carefully to ensure that the test conditions are similar to the actual impact conditions. In this present work, a study was conducted to fully understand the damage progression and growth, not only should the impacted surface be evaluated, but also the cross sectional defects on the impacted area must be accurately identified and examined. In this current work, the impacted test specimens will be observed at different magnifications to distinguish the types of failure mechanisms using Scanning Electron Microscopy (SEM). To perform this, the impacted specimens will be examined by two different approaches: surface defects and cross-sectional defects. This allows the failure mechanism to be observed more precisely.

Abstract: This paper presents impact behaviour and energy absorption response of car door safety beams. Low carbon steel of thickness 2.25 mm, designed into four different shapes of, tube-beam, I-beam and II-beam were used in this experiment to study the effect of impact load on the crash characteristic of the door beams in terms of load bearing and attenuation of energy. The tube-beam is the conventional beam commonly used in cars today. The reason propelling the investigation of other beams is to draw a parallel comparison with the conventional tube beam and possibly obtain an optimised design in terms of impact absorption capability. Masses of impactors used in the impact load simulations were 10 kg, 20 kg, 30 kg, 40 kg and 50 kg at an impact speed of 30 km/h. Analysis were carried out on all samples focusing on energy absorption and deformation characteristics of the beam structures using Pam Crash^TM finite element analysis software. Results from this studies indicated that the II-beam design is better than the other beams in terms of the energy absorption and deformation. The proposed II-beam design may be able to prolong the useful life of passenger car door.

Abstract: In this paper, the impact deformation behavior of the as-rolled and annealed Mg-3%Li-1%Nd alloy was investigated by using the Hopkinson compressive bar. The effects of rolling and annealing on the high speed impact deformation behavior were analyzed. The relationship between microstructure evolution and impact fracture mechanism was discussed.

Abstract: Projectile impact test is carried out to investigate damage and failure behaviour under different impact velocity from 90m/s to 160m/s. Strain-time history curve on the control points are analysed in this paper. Sandwich beam dynamic response and the degree of structural degeneration under impact loading both depends on the thickness of metallic skins. The projectile impact test demonstrate difference damage characteristics between the sandwich beams with different thickness skins. The peak stress value are estimated approximately to determine the skin deformation and sandwich beam global damage degree.

Abstract: To explore the dynamic impact fracture behavior of nanoparticle-reinforced composites, a bottom-up numerical method was proposed and verified through the fracture process simulation of nano-SiO2/epoxy sample in Charpy impact test. At the nano-scale, a parametric micromechanics model having interphase was built. And the effective material properties of the nanocomposites with variant volume fractions were obtained. Based on the homogenization theory, the macro-scale model of impact sample was established. It is demonstrated that this proposed bottom-up method can predict the locations and directions of cracks at macro-scale, and the growth process of rupture can also be visualized dynamically. The impact strength obtained from this method has a good agreement with the measuring results in literature. And this simulation method can also be used as an assistant tool for comparing the crack propagation rate of nanocomposites with variant particle contents.