Crashworthiness Design Research Articles

Body optimization approach of sedan structure for improving small overlap impact rating

Small overlap accidents of sedans often result in severe injuries. In this article, a crashworthiness design method, including engine-room energy management, suspension safety design and passenger compartment enhancement, was evaluated to improve the small overlap rating based on finite element simulation. Longitudinal reinforcement and shotgun redesign techniques were proposed to increase energy-absorption and decrease the impact force that transferred to the passenger compartment. The response surface methodology was utilized to propose the optimal passenger compartment parameters in order to reduce intrusion. The evaluation of the sedan structure performance was conducted by comparing simulated intrusion results with the rating guidelines of the Insurance Institute for Highway Safety (IIHS). The optimal results indicated that the intrusion was decreased by an average of 58.64%, with a minimum percentage of 44.98% around footrest area, and the measurement points for intrusion was upgraded to good. The proposed crashworthiness design approach is effective in vehicle structure optimization for better small overlap impact performance.

Numerical and Experimental Investigation on the Energy Absorption Capability of a Full-Scale Composite Fuselage Section

In the case of catastrophic events, such as an emergency landing, the fuselage structure is demanded to absorb most of the impact energy preserving, at the same time, a survivable space for the passengers. Moreover, the increasing trend of using composites in the aerospace field is pushing the investigation on the passive safety capabilities of such structures in order to get compliance with regulations and crashworthiness requirements. This paper deals with the development of a numerical model, based on the explicit finite element (FE) method, aimed to investigate the energy absorption capability of a full-scale 95% composite made fuselage section of a civil aircraft. A vertical drop test, performed at the Italian Aerospace Research Centre (CIRA), carried out from a height of 14 feet so to achieve a ground contact velocity of 30 feet/s in according to the FAR/CS 25, has been used to assess the prediction capabilities of the developed FE method, allowing verifying the response under dynamic load condition and the energy absorption capabilities of the designed structure. An established finite element model could be used to define the reliable crashworthiness design strategy to improve the survival chance of the passengers in events such as the investigated one.

On the crashworthiness behaviour of a composite fuselage Sub-floor component

In this work, the crashworthiness of the floor subsection of the cargo area of a civil aircraft is investigated. The structural behaviour under dynamic loading conditions of a structural sub-floor component, made of carbon fibre-reinforced composite material, has been studied in detail from a combined experimental/numerical perspective. Indeed, numerical impact tests characterized by different impact energies have been performed, with advanced numerical tools, finalized to the full understanding of the damage onset and propagation during the impact event. Preliminary numerical quasi-static and dynamic analyses have been carried out on the single sub-components. Then, the impact behaviour of the entire subfloor component has been numerically simulated taking into account the inter-laminar and intra-laminar damages onset and propagation. The comparison between the numerical results and the experimental data from the entire subfloor component has allowed to fully characterize the damage evolution in the investigated component and to provide interesting information finalized to a crashworthy design.

Energy-absorbing mechanisms and crashworthiness design of CFRP multi-cell structures

This study aims to investigate crashworthiness and energy-absorbing capacity of CFRP multi-cell structures under the quasi-static axial loading. In the present study, CFRP single-cell and multi-cell tubes are manufactured, and the same overall dimensions and mass for all specimens are guaranteed through allocating different thickness of each side. The crushing process and energy-absorbing capacity of all specimens are experimentally investigated under the quasi-static axial crushing load. According to the experimental results, it is known that the single-cell tube develops unstable local buckling mode, and the multi-cell tubes with two configurations crush progressively. Total energy absorption of the multi-cell tubes are almost 69% higher than that of the single-cell tube. Subsequently, numerical simulations are further conducted to provide additional insights into the underlying energy-absorbing mechanisms of the multi-cell tubes. The numerical results indicate that intra-laminar energy is the primary energy-absorbing mechanism for all configurations, and the energy absorbed by each part in the multi-cell tubes are much higher than the corresponding part in the single-cell tube. Based on the validated numerical models, the influences of wall thickness and cells number (n) on crashworthiness characteristics of multi-cell tubes are further investigated by performing a comparative analysis. It is found that the energy-absorbing capacity is slightly increased with raising cells number, and energy-absorbing capacity gradually increases with increasing layer number of inner cross beam. Finally, the CFRP multi-cell tube with n = 3 is further optimized, and as a result SEA is improved by 4.68% from the initial design. This study is expected to provide guideline for crashworthiness design of CFRP multi-cell structures.

FABRICATION OF FLEXIBLE EP-FG COMPOSITE AUTOMOBILE SHELL STRUCTURE USING COMPRESSION MOULDING PROCESS

Corrugations are believed to enhance the mechanical properties of a shell structure. In this study, the theory of corrugation is applied to enhance the flexibility of epoxy- fibreglass material which is being used widely in the automotive industry. It could fulfil the requirement of using a flexible composite material for automobile shell structure for a more crashworthy design. Two different corrugated profiles were studied in this project which are the damped curve and pyramidal groove profiles. Rubber moulds were fabricated and used for the corrugated epoxy-fibreglass composite preparation. A compression moulding machine was used to form the corrugations on the composite material as it can deliver high pressure and temperature to cure the laminate. In this study, the compression moulding process was carried out under constant parameters: temperature,pressure and holding time. Results obtained from the tensile test of the corrugated epoxy-fibreglass composite sheets were compared with the tensile properties of a flat epoxy-fibreglass composite sheet to determine the effect of corrugations in terms of flexibility. The pyramidal groove corrugation managed to enhance the flexibility of the epoxy-fibre glass composite sheet, however there was a significant decrease in yield load, and no notable changes in tensile strength, yield strength and elongation. On the other hand, the flexibility achieved by the sample with damped curve corrugation is higher than the flat sample but lower than the flexibility possessed by the pyramidal groove sample. Elongation of the damped curve sample prior to fracture is two times higher compared to the elongation achieved by both pyramidal groove and flat samples. In conclusion, the epoxy-fibreglass composite sheet with the damped curve corrugation profile is the best, as it exhibits the desired increase in flexibility with minimal loss in terms of tensile strength, yield load and yield strength.
 Keywords: corrugation, thermoset composite, compression moulding, tensile test, damped curve, the pyramidal groove

Crashworthiness design of self-similar graded honeycomb-filled composite circular structures

Lightweight cellular materials can effectively improve the crashworthiness of thin-walled structure. To further investigate the potential of honeycomb-filled composite structures, an innovative self-similar graded honeycomb-filled thin-walled structure is proposed in this paper. First, the numerical model of the self-similar graded honeycomb-filled circular tube (SGHCT) is established in ABAQUS and then is validated via the existing experiment. To systematically study the bending behavior of this novel structure, eight kinds of SGHCT with different graded patterns are designed and compared with uniform honeycomb-filled circular tube (UHCT). Through parametric study, we find that the novel structure exhibits better energy absorption characteristics than the corresponding UHCT. In addition, various parameters like thickness, mass ratio and yield stress of column wall have a considerable effect on the crashworthiness performance of this innovative structure. Furthermore, the bending behaviors of effective SGHCT are also investigated to increase the weight efficiency. Finally, the SGHT and UHCT are optimized by adopting the non-dominated sorting genetic algorithm II (NSGA-II) and Kriging modeling technique. The optimization results show that the SGHCTs show superior Pareto solutions to the uniform honeycomb-filled tubes, also better crashworthiness than foam-filled tubes under high PCF conditions in the previous researches. Especially, the specific energy absorption of ascending single-direction self-similar tube increases from 1.17 kJ/kg to 1.74 kJ/kg by 48.9%. The findings of this study offer a new route of designing novel crashworthiness structure with high energy absorption capacity.

The mystery of coconut overturns the crashworthiness design of composite materials

The natural structures inspires human beings to find a new composite material design for the crashworthiness. The mystery of outstanding crashworthiness of a coconut is revealed in its multilayered and multiscale structure, including the macroscopically organized pericarp and microscopically disordered mesocarp. The pericarp consists of parallel fibers well-aligned in the tangential direction. The fibers of coconut pericarp is macro ordered. The mesocarp is the thickest layer among three layers of the pericarp. The coconut mesocarp is micro disorderly. In this paper, the coconut free drop experiments and quasi-static experiments were carried out, and the theoretical analysis was carried out to explain the natural rule of the coconut fiber arrangement, and finally the numerical computational model was built to validate the theoretical analysis. This work shows that the bottom part of the coconut is the impact point to the ground during the free drop experiments without initial angel. The fiber orientation in the coconut fruit can increase the energy absorption ability, and such a fiber arrangement can influence the propagation of the stress wave to protect the coconut endocarp. A bio-inspired template has been provided for the functionally gradient composite material design.

Analysis of the crashworthiness design and collision dynamics of a subway train

In this study, the crashworthiness of a subway train was assessed by establishing a finite element model for the first three carriages of the train and the track using the Hypermesh software. By utilising the *MAT_HONEYCOMB material model, a honeycomb in an anti-climbing energy-absorbing device was simulated. Moreover, the process of a subway train – travelling at a speed of 25 km/h – colliding with another identical train in a stationary and non-braking state was simulated by employing the finite element analysis software Hypermesh and LS-DYNA. The process of simulation analysis was divided into two parts: (1) analysis of the anti-climbing energy-absorbing devices under static compression for the investigation of energy absorption and (2) collision analysis of the whole train. The contributions of the proposed energy-absorbing structure – at the end of driver’s cab, the coupler and draft gears on each section – to the overall energy absorption in a train collision were calculated. Furthermore, based on the EN15227 standard, the crashworthiness of the train with respect to the survival space for occupants, train acceleration and uplift of wheels relative to the track was evaluated. The coupler of the first carriage fails in a collision at 25 km/h, and the coupler and draft gear are the main energy-absorbing devices. *MAT_HONEYCOMB was used to define the honeycomb materials in anti-climbing energy-absorbing devices and could simulate the mechanical performance thereof. The crashworthiness of the train meets the relevant standard requirements.

Experimental and numerical study on splitting process of circular steel tube with enhanced crashworthiness performance

Abstract The axial splitting of thin-walled tube is usually considered as an efficient deformation mode to dissipate impact energy thanks to its large stroke ratio. However, the low crushing force and the unstable deformation process, such as crack merging and branching, significantly limit its application in crashworthiness design. In this paper, we propose to enhance the deformation stability through introducing initial kerfs on the inner and outer surfaces of the circular steel tube in its axial direction to guide the propagation of cracks during the splitting process, thus we can improve the crushing force via a significant increase in tube wall thickness. To demonstrate the feasibility of the proposed method, quasi-static compressive experiments on single tube (inner radius r = 55 mm, wall thickness t = 5 mm) and doubled tube (consisted of two tubes with wall thickness t = 5 mm) with kerf depth δ = 0.5 mm split by a radiused die are performed, which exhibit stable deformation processes and high steady-state compression forces (103.32 kN for single tube, and 216.44 kN for doubled tube). Then, finite element simulations are conducted to model the tested samples. It is found that the experimentally observed deformation processes are well captured by simulations, and the relative errors of numerical steady-state compression forces in comparison to experimental results are 0.39% (single tube) and 1.90% (doubled tube), respectively. Finally, based on the validated numerical model, the influence of tube and die dimensions on its crashworthiness performance is discussed. It is observed that the axial load significantly depends on kerf depth, crack number, and tube thickness. The curling radius is nearly not affected by kerf depth, but it almost linearly depends on die radius. Moreover, the tube with larger wall thickness has a higher specific energy absorption.

Design for Crashworthiness of Categorical Multimaterial Structures Using Cluster Analysis and Bayesian Optimization

Abstract This work introduces a cluster-based structural optimization (CBSO) method for the design of categorical multimaterial structures subjected to crushing, dynamic loading. The proposed method consists of three steps: conceptual design generation, design clustering, and Bayesian optimization. In the first step, a conceptual design is generated using the hybrid cellular automaton (HCA) algorithm. In the second step, threshold-based cluster analysis yields a lower-dimensional design. Here, a cluster validity index for structural optimization is introduced in order to qualitatively evaluate the clustered design. In the third step, the optimal design is obtained through Bayesian optimization, minimizing a constrained expected improvement function. This function allows to impose soft constraints by properly redefining the expected improvement based on the maximum constraint violation. The Bayesian optimization algorithm implemented in this work has the ability to search over (i) a real design space for sizing optimization, (ii) a categorical design space for material selection, or (iii) a mixed design space for concurrent sizing optimization and material selection. With the proposed method, materials are optimally selected based on multiple attributes and multiple objectives without the need for material ranking. The effectiveness of this approach is demonstrated with the design for crashworthiness of multimaterial plates and thin-walled structures.

Crashworthiness and failure analysis of steeple-triggered hat-shaped composite structure under the axial and oblique crushing load

A clear understanding of failure mechanisms and coupling interaction of multiple failure modes greatly contributes to crashworthiness design of composite structure. Thus, the crashworthiness and failure of steeple-triggered hat-shaped composite structure (HSCS) is investigated based on finite element method. The stacked-shell model is adopted to capture both intra- and inter-laminar failure modes and validated with available experiments. The effect of oblique crushing on the crashworthiness of HSCS is predicted and analyzed. Further, to improve specific energy absorption (SEA), the HSCS as a sub-structure is used to design cellular structures including unit-cell, double-cell and four-cell structures. From the predicted results, failure process and complex coupling failure mechanisms of HSCS are revealed. It is found that the oblique crushing angle has a significant effect on the crashworthiness and there exists a critical angle. It is also revealed that the SEA increases with the increase of the cell number due to different splaying modes.

Reliability-based design optimization for vehicle body crashworthiness based on copula functions

ABSTRACT Vehicle body crashworthiness is crucial in a traffic accident. It can protect passengers from serious injury or death and reduce vehicle damage. To provide a more effective design framework for ensuring crash safety, a reliability-based design optimization (RBDO) method for vehicle body crashworthiness considering complex parametric correlation is developed by integrating copula functions. An RBDO model is formulated for the vehicle body crashworthiness problem based on a finite element model of a vehicle crash. Taking into account the correlation of random parameters, the optimal copula function is selected by the Bayesian method to model parametric correlation and build the joint probability distribution function. An efficient solution strategy is provided to perform the reliability analysis and optimization design of crashworthiness. The results demonstrate that the proposed method is reliable and has a high accuracy for crashworthiness design.

Theoretical prediction and crashworthiness optimization of top-hat thin-walled structures under transverse loading

In this study, a theoretical model is developed to reveal the bending collapse of top-hat thin-walled structures by dividing a top-hat thin-walled structure into a top-hat element and flat-plate element. A theoretical formula is also developed to describe the bending deformation and energy-absorption of the structures. The theoretical model is capable of predicting the bending collapse and energy-absorption of top-hat thin-walled structures with different thickness and material specification. The accuracy and generality of the theoretical prediction model is validated by performing three-point bending tests and finite element simulations. Then, both theoretical prediction formulas and finite element analysis (FEA) based surrogate models are employed to perform the crashworthiness optimization of top-hat thin-walled structures. The results show that (i) the theoretical prediction model is capable of producing results that can be directly used to optimize the thicknesses, cross-sectional geometry, and material specifications for top-hat thin-walled structures, which will increase the efficiency and shorten the cycle time of crashworthiness design optimization for this type of structure; and (ii) steel-aluminum hybrid top-hat thin-walled structure has a larger energy-absorption capacity than high-strength steel without exceeding the initial weight, whereas a lightweight design is more feasible with an aluminum alloy than with high-strength steel without sacrificing the energy absorption of the baseline design.

Investigation on Composite Energy Absorbers of a Composite Fuselage Section Subjected to Vertical Drop Test

In the aircraft industry, crashworthiness design and certification phases have been and are going to be the most attractive topics for designers, mostly because of the increasing use of composites for primary structural components. It is well known that the cargo subfloor elements of the fuselage structure play a crucial role in absorbing the kinetic energy during a crash. In particular, the stanchions, or struts, are important parts for the structural response; as a matter of fact, they connect the fuselage frames to the cabin’s floor and, ideally, are expected to crush under a compressive force in order to dissipate the impact energy in a controlled way and, consequently, to minimize the energy transferred to the passengers. The aim of this work is to demonstrate, experimentally and numerically, the energy absorption capability of the stanchions, made of both composite material and aluminium alloy, of a full-scale 95% composites made fuselage section under a critical load condition, such as an emergency landing. A Finite Element model allowing estimating the passive safety capabilities of the designed struts has been developed and herein proposed.

Evaluation of various multi-cell design concepts for crashworthiness design of thin-walled aluminum tubes

In this paper, we evaluate various multi-cell design concepts to optimize the crash performances of thin-walled aluminum tubes. The crash performances of the tubes are evaluated by means of two metrics: the crush force efficiency (CFE) and the specific energy absorption (SEA). The CFE and SEA of the tubes are predicted through the use of the finite element analysis software LS-DYNA. Experiments are also conducted to validate the finite element models. Thirty different multi-cell design concepts are evaluated in terms of CFE and SEA, and the best design concept is selected for further evaluation. Next, we perform surrogate-based optimization of the selected design concept, upon which we find that optimum design for maximum CFE which utilizes smaller wall thickness values (except the wall thickness of the inner tube) and larger tube diameters than those of the corresponding ones for the optimum design for maximum SEA. Additionally, the optimized designs exhibit remarkable CFE and SEA performances.

Crashworthiness design and optimisation of windowed tubes under axial impact loading

Thin-walled structures are frequently used as energy absorbers in the automotive, railway and aviation industries. This paper addresses the crashworthiness performance of thin-walled windowed tubes under dynamic impact loading. Different shapes of cut-outs were introduced to thin-walled tubes with different cross-sectional shapes to create windowed tubes. Explicit finite element code, LS-DYNA, was used to simulate the crushing behaviour of the windowed tubes under axial impact loading. The Finite Element (FE) model was validated by conducting experimental tests and showing that the numerical and experimental responses are comparable. The crashworthiness responses of the different windowed tubes were compared and the best performing tube was identified using a multi-criteria decision-making method known as Technique of Order Preference by Similarity to Ideal Solution (TOPSIS). It was found that a circular tube with a square window shape outperforms all other sections and exhibits the best energy absorption characteristics.Subsequently, a multi-objective optimisation analysis was performed to find the optimal configuration of the best tube. Response Surface Methodology (RSM) was used to develop models for the energy absorption responses of the tube. The design variables were selected to describe size, number, and distributions of the windows, while specific energy absorption (SEA) and peak crush force (PCF) were set as design responses. Parametric analysis was conducted to understand the effects of the design variables on the crashworthiness behaviour and the optimal configuration was identified.

Theoretical analysis and crashworthiness optimization of hybrid multi-cell structures

Thin-walled structure is widely used in automobile, aviation and other industrial fields due to its lightweight, high energy absorption efficiency and low cost. Four thin-walled hybrid multi-cell structures with circular and square sections are proposed in this paper. The energy absorption characteristics and crushing deformation of the hybrid structures are investigated by experimental testing and numerical analysis. Meanwhile, the theoretical models of mean crushing force and specific energy absorption of the hybrid structures are developed by simplified super folding element (SSFE) theory. It is found from the sensitivity analysis of parameters that the dimension of the external tube (D) and the wall thickness (T) of the structure have significant effects on energy absorption. Furthermore, the multi-objective optimization including multiple surrogate models is performed to obtain optimal crashworthiness of the hybrid multi-cell structures. The results show that multiple surrogate models are more favorable and accurate for the crashworthiness design, and the hybrid multi-cell structure with the outer circle and inner square section (CS2) has the best crashworthiness performance. Finally, the multi-objective optimization solutions are analyzed and chosen by the normal boundary intersection (NBI) method to carry out the crashworthiness comparison with the typical multi-cell structures of the same mass, and the hybrid structure (CS2) outperforms multi-cell tubes with the single circular or square section.

Crashworthiness analysis and design of composite tapered tubes under multiple load cases

Carbon fibre reinforced plastics (CFRP) are characterized by lightweight, high specific strength and stiffness, superior energy-absorbing capacity, and excellent design flexibility. Thus, use of CFRP thin-walled structure as crashworthy application can improve fuel economics and crashworthiness characteristics compared with their metal counterparts. In this study, we reported crushing behavior of several CFRP tapered tubes with various cross-sectional profiles and tapered angles under multiple load cases through numerical simulations. The influences of cross-sectional profiles, tapered angle and loading angle on crashworthiness of all configurations were explored by comparing their failure modes, load-displacement curves, crashworthiness indicators and energy-absorbing mechanisms. It was found that a positive correlation between total energy-absorbing capacity and the edge number of cross section profile was demonstrated, and the tapered tubes with circular profile showed the best total energy-absorbing capacity under both axial and oblique loadings. Besides, it was indicated that the energy-absorbing capacity of the tubes reduced in different degrees with increasing impact angle, and raising tapered angle of the tube was capable to mitigate this reduction degree. Further, the complex proportional assessment method (COPRAS) was adopted to effectively pick up the optimal configuration from all configurations concerned. It was found that H-6, which is the CFRP tapered tube with hexagonal profiles and tapered angle of 6°, was the best configuration, indicating its superiority in crashworthiness characteristics under multiple load cases. Finally, the CFRP tapered tube H-6 was further optimized by using a discrete optimization method to improve its crashworthiness characteristics, and the results showed that the SEAα was improved by 4.35% from the initial design. This study demonstrated that the proposed two-step process (firstly select the best optimal configuration based on COPRAS; then perform the discrete optimization) was fairly effective to obtain an optimal configuration for crashworthiness design of CFRP tapered tubes under multiple load cases.

The crashworthiness prediction of 2D triaxially braided composite fuselage frame under transverse impact load

The fuselage frame, an apron construction of the typical transport aircraft fuselage, manufactured with carbon fibre/epoxy 2D triaxially braided composites, hinders the crashworthy aircraft analysis and design for the complicated failure behaviour under transverse impact loading. To capture the intricate details, the crashworthiness performance is investigated and compared with different damage models including enhanced composite damage model, composite failure shell model and laminated composite fabric model. Based on the continuum damage mechanics and irreversible thermodynamics, the progressive failure behaviour is implemented to predict the damage behaviour. The damaged constitutive equations with damage state variables and Hashin’s criteria are adopted. Results show that the presence of non-physical parameters in the damage models make it challenging to predict the peak and post-peak responses using enhanced composite damage model and composite shell failure models. The laminated composite fabric model demonstrates good functionality to capture the failure sequences and the load response of the frame under transverse impact loading.

Analysis of Integrated Fuselage Composite Seat for Small Aircraft in Crashworthiness Applications

The research paper is addressed to develop and validate an aircraft seat cushion to be used for the passenger seat directly integrated in the fuselage. For very light aircraft, these systems result to be a good choice in terms of weight reduction and space optimization, but can represent a challenging problem for crashworthiness design and a key energy-absorbing function for human protection from injuries so to improve the level of the safety. The first issue was dedicated to investigate the mechanical properties of four commercial foams, diffused in different industrial applications, which present nonlinear and time-dependent viscoelastic behavior, and finally to determine whether typical seat cushion materials are rate sensitive or not. Then, numerical simulations were validated with measurements to simulate the mechanical properties of foams and to evaluate the contact pressures, stress distribution, and the dependence of stress on the strain rate in order to provide adequate levels of both comfort and energy absorption.