Simonetta Boria

Numerical investigation of energy absorbers in composite materials for automotive applications

Nowadays, there is increasing interest in lightweight automotive structures capable of absorbing large quantities of energy in case of a crash phenomenon. These requirements are satisfied by composite devices, provided they are properly designed. The aim of the present paper is the investigation of the crashworthy behaviour of composite material tubes with woven laminae subjected to dynamic axial compression. The research was done by combining experimental and numerical analysis; without any experimental feedback, in fact, engineers might not accurately design an innovative structure. After the numerical characterisation of the used CFRP (carbon fibre-reinforced polymer) material, different simulations with the non-linear explicit dynamic code LS-DYNA have been done in order to understand how the structure absorbs energy by varying its geometrical and material parameters. In particular, circular and square tubes have been investigated with different resistant section, wall thickness, fibres orientation and staking sequence. The numerical analysis has been carried out taking into account different composite material models present in the LS-DYNA library, where each of them implements a different damage criterion. The choice of model to be used was made only after performing crash tests on the same tubes using a drop tower, appropriately instrumented in order to measure the main impact characteristics. The comparison between numerical and experimental results gave satisfactory outcomes, providing the basis for the design methodology of impact attenuators that are geometrically more complex.

Energy Absorbing Sacrificial Structures Made of Composite Materials for Vehicle Crash Design

Nowadays thin-walled components in CFRP composite materials are considered in order to progressively replace metals for crashworthy applications in the automotive industry, thanks to their undoubted advantages such as high strength to weight ratio.

Energy absorption capability of laminated plates made of fully thermoplastic composite

The behaviour of composites materials, made of synthetic fibres embedded in a thermoplastic resin, subjected to low velocity impacts, was largely studied in the past. However, in the last years, the use of thermoplastic composites has been increased due to the considerable advantages in terms of recyclability of this family of materials. Thermoplastic composites are composed of polymers with different material’s structure if compared to the more traditional thermoset composite. Consequently, the behaviour of these materials can be different in some loading conditions. Moreover, considering the wide range of thermoplastic composites that have been developed in the last years, the study of the behaviour of these materials, in case of impact, has not been yet widely analysed, in particular considering materials where both the matrix and the reinforcement are made of thermoplastic. In this perspective, the goal of this work is to study the behaviour of a new thermoplastic composite (PURE thermoplastic) in conditions of low velocity impact. In this material, the matrix and the fibre reinforcement are made of polypropylene both. The paper presents the results of an experimental investigation. In particular, a series of impact tests with a drop dart equipment have been carried out on laminates made of PURE thermoplastic. Laminates with different thicknesses have been taken into consideration. The influence of the impact conditions on the material’s behaviour has been investigated and the capability of energy absorption has been studied. The PURE thermoplastic showed a different behaviour in terms of energy absorption and damage mechanisms if compared to the composites presented in the literature. The thickness of the laminate has had influence on the deformation and the damage mechanism of the specimens: with low thickness, the perforation of the specimen has been obtained, whereas, with the higher thickness, the specimens have shown a ductile behaviour and extended plasticity without crack tip. The contact force between the dart and the specimen has been influenced by the energy level of the impact, but with an opposite trend if compared to that of the composites studied in the literature.

Repeated impact behaviour of fully thermoplastic laminates

The use of laminated composites are spreading in engineering applications, respect to heavier metallic materials, thanks to their excellent advantages of weight/strength and weight/stiffness ratio. Even if up to now the attention was focused on fibers reinforced with thermosetting matrix, in the last years composites made with thermoplastic matrix have gained consideration. This is mainly due to their advantages in terms of low density and, more important, recyclability. The use of thermoplastics composites are of interest not only for the replace of the non structural parts, but also for the structural components located in areas potentially subjected to impacts. Depending on the design geometry and the field of application, some composite components may be subjected to repeated impacts at localised sites either during fabrication, routine maintenance activities or during service conditions. Even though the impact damage associated to the single impact event maybe slight, the accumulation of the damage over time may seriously impair the mechanical performance of the structure. In this paper, experimental data of repeated impact tests performed on fully thermoplastic thick laminates are presented. Repeated impacts at different energy levels are considered. The experimental data are analyzed evaluting the damage index (DI). This parameter has been previously defined and used for thermosetting composites in order to overcome shortcomings of the damage degree (DD) in case of thick composite laminates. The rate of initial steady damage accumulation as well as the onset of severe damage modes are analyzed and discussed in the paper. When applied to repeated impact tests, the DI allows to distinguish between an initial steady phase of damage progression and the onset of severe damage mechanisms. For the initial damage, the damage index increases linearly with the impact number whereas the severe damage mechanisms lead to laminate performation in few impacts. In this work, the values of the DI at different stages are discussed: at first impact, in the steady phase and at the onset of the unsteady phase. Moreover, the total energy absorbed by the laminate in the steady phase is also computed for all the performed tests and compared with the results of other thermosetting composites previously tested by other researchers.

Thin-walled truncated conical structures under axial collapse: Analysis of crushing parameters

Abstract Axially crushed thin-walled tubular structures are extensively used as energy absorbers in various automotive and aerospace applications because of their high energy absorption efficiency and long strokes. Moreover the necessity to reduce weight of structural components to contain gas emissions and reduce pollution impels to use lightweight engineering materials, such as composites. Although significant numerical and experimental works on the collapse of fiber-reinforced composite shells have been carried out, studies on the theoretical modeling of the crushing process are quite limited given the complex and brittle fracture mechanisms of composite materials. A mathematical and finite element approach on the failure mechanisms, pertaining to the stable mode of collapse of thin-walled composite frusta subjected to axial loading, are investigated. The theoretical analysis is conducted from an energetic point of view. The main energy contributions to the absorption (bending, petal formation, circumferential delamination, friction) are identified and then, by the conservation law, the total internal energy is equated to the work done by the external load. The total crushing process can be seen as a succession of deformation states, each responsible for a partial absorption. The minimum configuration for each impact force in the specific state of deformation, function of several variables and dependent on geometric and material parameters, is obtained. Moreover the numerical modeling through a finite element analysis is conducted; all parametric values and cards set in the explicit dynamic code LS-DYNA is described in detail. Finally a comparison between theory, finite element approach, and experiments concerning crushing loads and total displacements is presented, showing how the proposed strategy is able to capture the crushing mechanisms of such composite structures despite the simplification adopted.

Lightweight design and crash analysis of composites

The design of composite materials seems more challenging compared with that of their metallic counterparts because of their microstructural heterogeneity and sophisticated behavior. The ever-wider use of composite materials for structural applications in the automotive industry makes it necessary to predict their behavior under varying load conditions. This chapter is focused on the experimental and numerical crashworthiness design of carbon fiber-reinforced plastic structures under quasi-static and dynamic axial loading. The material used for this research is a prepreg obtained from high-strength carbon fibers embedded in an epoxy resin. Initially, simple geometries were taken into account to evaluate the influence of several geometric parameters and loading conditions on the final deformation. Afterward, a more complex geometry, equal to that adopted for the frontal impact attenuator of a Formula SAE car, was investigated. For each analysis, the numerical results were compared with those obtained from real axial crushing tests. The study indicates that the deformation behavior of the laminate calculated by using numerical models was in good agreement with experimental results; despite the simplification adopted, the finite element models were able to capture the main crushing parameters with maximum relative errors of 10%.

Crashworthiness and Lightweight Design of an Innovative Microcar

The increase in road accidents involving microcar is raising public awareness, pushing the manufacturers to increase their safety performance even without current regulations. Nevertheless, lightweight and small dimensions seem to hinder the pursuit of crashworthiness. Passenger decelerations can be reduced through two design solutions: thin-walled internal devices able to absorb kinetic energy and a frame that fulfils the role of survival cell. The present research work concerns the choice of design solutions for improving road safety of a microcar, through numerical and experimental analysis. In the first step, the original frame was optimised in terms of stiffness. Subsequently, thin-walled cylindrical tubes, with different thickness/diameter ratios and materials (aluminium and carbon fibre reinforced plastic composite) were modelled and tested in order to capture the real crushing. Finally, an optimised frontal impact attenuator was modelled with both materials, in order to choose the best solution in terms of specific energy absorption capacity.

Honeycomb Sandwich Material Modelling for Dynamic Simulations of a Crash-box for a Racing Car

The aim of this study is to investigate, through experiment as well as numerical approaches, the energy absorbing capabilities of a thin-walled crash-box, made of sandwich material, for a racing car. The basic considered structures are panels composed of 2 aluminium alloy sheets and an aluminium hexagonal cells honeycomb core. Several crash tests were performed, in the conditions related to a frontal impact at the velocity of 12 m/s, in order to acquire information on the dynamic behavior of the mentioned structure; during these tests the load-deformation diagram, the deceleration, and the energy absorbed by the structure were measured. A finite element model is then developed using the nonlinear, explicit dynamic code LS-DYNA. In order to characterize the material and set up the numerical model, strength tests were conducted on aluminium honeycomb-cored sandwich panel specimens. By means of these preliminary tests some necessary material parameters were determined. Simulation results accurately predicted the average deceleration, the specific absorbed energy, and the total deformation of the specimen, but appeared to slightly overestimate the initial peak load obtained in the crash tests. Therefore, the performed investigation can help to build confidence in the future possibility of using the nonlinear dynamic FE code LS-DYNA for design of sandwich primary structures subjected to crash loads, especially after a further tuning up of the models and material characteristics.