Legform Impactor Research Articles

On the design of vehicle front-ends for pedestrian safety with the advanced Pedestrian Legform Impactor (aPLI)

The latest Euro NCAP Vulnerable Road Users Assessment Protocol (v11.3) introduces the advanced Pedestrian Legform Impactor (aPLI) to reduce the risk of lower extremity injury to pedestrians in a vehicle collision. The addition of femur injury criteria and the added simplified upper body mass, compared to its predecessor –the FlexPLI–, allows a more comprehensive assessment of passive pedestrian safety. However, this presents new challenges for vehicle manufacturers as these additions must be accounted for. We perform four sensitivity analyses at two test positions (centre and at the headlight) on two vehicles (compact car and SUV). Thereby, we investigate five design parameters that represent the main front-end components and identify that the vehicle type determines the sensitive design parameters significantly. Furthermore, we show that the influence of the lower stiffener and energy-absorbing foam diminishes compared to older legform impactors, while that of the bonnet increases.

CORA+: an objective rating method for the pedestrian protection leg impact

Numerical simulation is an important tool in the development process of vehicles in order to predict the passive vehicle safety extensively. Based on simulation results, physical tests are carried out to evaluate critical points. The validity of numerical simulations is analysed by objective rating methods. Predominantly CORA (Correlation and Analysis) is used to compare the similarity of response time histories between simulation and physical test. In pedestrian protection, the peaks of the response time histories measured during the test are the injury criteria values. Additionally, the risk of injury is determined by these peak values. However, response time histories are found to be rated inadequately by CORA because this method is not emphasising on peak values. We investigate Flexible Pedestrian Legform Impactor (FlexPLI) and advanced Pedestrian Legform Impactor (aPLI) data with different CORA parameter settings in order to achieve adequate ratings. Thereby, we try to focus on the peak values of the curves while keeping the parameter variation as objective as possible. Further, we show a heuristic approach by adding peak value deviation as an additional element to the CORA metric – so called CORA+.

Injury biomechanics-based nondeterministic optimization of front-end structures for safety in pedestrian–vehicle impact

Lower extremity is the most frequently injured body region in a pedestrian–vehicle impact. To evaluate lower extremity injuries, both the Flexible Pedestrian Legform Impactor (FlexPLI) and the Flexible Pedestrian Legform Impactor with Upper Body Mass (FlexPLI-UBM) have been used in practice. In general, UBM would have considerable influence on the design of front-end structures. In this study, a sedan was used to perform the experimental tests first for evaluating the effects of different lower extremities. The experimental results indicated that placement of UBM can lead to a higher risk evaluation of knee ligament damage and a more significant increase in femur bending moment than that in tibia bending moment. Second, a new multiobjective discrete robust optimization (MODRO) algorithm was developed to optimize front-end structures subject to FlexPLI-UBM impact involving uncertainties. In the proposed MODRO algorithm, the order preference by similarity to ideal solution (TOPSIS) was coupled with the fuzzy approach for developing a fuzzy multiple attribute decision making (MADM) model for converting multiple conflicting objectives into a single unified cost function. The presented optimization procedure is iterated using the successive orthogonal experiment to deal with a large number of design variables and design levels. The optimal results showed that in contrast to the structures subject to the FlexPLI impact, the front-end structures under FlexPLI-UBM impact require a higher stiffness of tibia contact area but a lower stiffness of knee and femur contact area. This study provides automotive engineers with new insights into the injury biomechanics-based design of frontal structure from a road safety perspective.

Nondeterministic multi-objective and multi-case discrete optimization of functionally-graded front-bumper structures for pedestrian protection

Pedestrian lower-leg protection and lower-speed crashworthiness often present two important yet competing criteria on the design of front-bumper structures. Conventional design optimization is largely focused on a single loading condition without considering multiple impact cases. Furthermore, design of front-bumper structures is usually discrete in engineering practice and impacting conditions are commonly random. To cope with such a sophisticated nondeterministic design problem, this study aimed to develop a successive multiple attribute decision making (MADM) algorithm for optimizing a functionally graded thickness (FGT) front-bumper structure subject to multiple impact loading cases. The finite element (FE) model of front-end vehicle was constructed and validated with the in-house experimental tests under the loads of both Flexible Pedestrian Legform Impactor (Flex-PLI) impact and lower-speed impact. In the proposed successive MADM algorithm, the order preference by similarity to ideal solution (TOPSIS) based upon relative entropy was coupled with the analytic hierarchy process (AHP) to develop a MADM model for converting multiple conflicting objectives into a unified single cost function. The presented optimization procedure is algorithmically iterated using the successive Taguchi method to deal with a large number of design variables and design levels. The results showed that not only the algorithm enabled to generate an optimal design efficiently, but also the robustness of Flex-PLI impact is significantly enhanced. The proposed algorithm can be potentially used for other engineering design problems with similar complexity.

Multi-criteria decision making for lower bumper stiffener material change in a sports utility vehicle with CAE simulation

This article describes the steps to replace the lower bumper stiffener material of a sport utility vehicle. It was taken as the main characteristics to obtain the behaviour principles as the lower bumper stiffener. A static structural simulation was developed according to the regulation of physical trials of the EuroNCAP. Three multi-criteria selection methods called TOPSIS, VIKOR and ENTROPY were applied. Polylactic acid was determined as the optimal material. A dynamic simulation between of the contact of the leg form impactor and pedestrian protection system, at a speed of 40 km/h was performed. The simulation evaluates the acceleration of the tibia, equivalent strain of Von Misses and deformation in the three nodes of the lower bumper stiffener. It was determined that the optimal material is the PLA, where it is appreciated that the main factors to consider for the decision making is the acceleration and the nodal deformation of the dynamic simulation.

Automotive frontal body structure optimisation in the Upper Legform to Bonnet Leading Edge test with the aim of reducing the pedestrian injuries

Automotive frontal body structure design and optimisation are absolutely essential towards reducing the risk of pedestrian injuries in road accidents. In this paper, a two-step optimisation process in combination with the finite element method (FEM) is utilized to enhance pedestrian protection for a class-B passenger car. In this way, the finite element (FE) model of the Upper Legform Impactor has been developed and verified according to the European Commission (EC) Regulation No 631/2009. The Upper Legform to Bonnet Leading Edge test has been numerically simulated using LS-DYNA software. Material and geometry of the frontal body structure components are considered as the design variables and optimum design points are identified using the Taguchi method through a two-step framework. The Analysis of variance (ANOVA) is also used to determine the percentage effect of each controllable factors on the desired output. The obtained results revealed that in the optimum design point in comparison with the base vehicle, more than 45% improvement in the pedestrian protection criterion is observed, while selected components experienced mass reduction about 9%.

Could an isolated human body lower limb model predict leg biomechanical response of Chinese pedestrians in vehicle collisions?

The purpose of the current study was to investigate whether an isolated human body lower limb FE model could predict leg kinematics and biomechanical response of a full body Chinese pedestrian model in vehicle collisions. A human body lower limb FE model representing midsize Chinese adult male anthropometry was employed with different upper body weight attachments being evaluated by comparing the predictions to those of a full body pedestrian model in vehicle-to-pedestrian collisions considering different front-end shapes. The results indicate that upper body mass has a significant influence on pedestrian lower limb injury risk, the effect varies from vehicle front-end shape and is more remarkable to the femur and knee ligaments than to the tibia. In particular, the upper body mass can generally increase femur and knee ligaments injury risk, but has no obvious effect on the injury risk of tibia. The results also show that a higher attached buttock mass is needed for isolated pedestrian lower limb model for impacts with vehicles of higher bonnet leading edge. The findings of this study may suggest that it is necessary to consider vehicle shape variation in assessment of vehicle pedestrian protection performance and leg-form impactors with adaptive upper body mass should be used for vehicles with different front-end shapes, and the use of regional leg-form impactor modeling the local anthropometry to evaluate the actual lower limb injury of pedestrians in different countries and regions.

Multi-criteria selection methods (MCDM) for lower bumper stiffener material change in a Sports Utility Vehicle with CAE simulation

This article describes the steps to replace the lower bumper stiffener material of a sport utility vehicle. It was taken as the main characteristics to obtain the behaviour principles as the lower bumper stiffener. A static structural simulation was developed according to the regulation of physical trials of the EuroNCAP. Three multi-criteria selection methods called TOPSIS, VIKOR and ENTROPY were applied. Polylactic acid was determined as the optimal material. A dynamic simulation between of the contact of the leg form impactor and pedestrian protection system, at a speed of 40 km/h was performed. The simulation evaluates the acceleration of the tibia, equivalent strain of Von Misses and deformation in the three nodes of the lower bumper stiffener. It was determined that the optimal material is the PLA, where it is appreciated that the main factors to consider for the decision making is the acceleration and the nodal deformation of the dynamic simulation.

Optimal Specifications for the Advanced Pedestrian Legform Impactor.

This study addresses the virtual optimization of the technical specifications for a recently developed Advanced Pedestrian Legform Impactor (aPLI). The aPLI incorporates a number of enhancements for improved lower limb injury predictability with respect to its predecessor, the FlexPLI. It also incorporates an attached Simplified Upper Body Part (SUBP) that enables the impactor's applicability to evaluate pedestrian's lower limb injury risk also with high-bumper cars. The response surface methodology was applied to optimize both the aPLI's lower limb and SUBP specifications, while imposing a total mass upper limit of 25 kg that complies with international standards for maximum weight lifting allowed for a single operator in the laboratory setting. All parameters were virtually optimized considering variable interaction, which proved critical to avoid misleading specifications. The results from this study can be used to construct physical aPLIs that are expected to be used in future car-to-pedestrian crash safety testing programs worldwide.

Reliability optimization of a novel negative Poisson’s ratio forepart for pedestrian protection

The bumper system always directly causes the injury of the pedestrian leg in the car–pedestrian accidents. Therefore, it is of practical significance to optimize the bumper structure to enhance the protective effect of the pedestrian leg. Based on the original bumper system, this work proposes a novel negative Poisson’s ratio energy-absorbing forepart with inner hexagonal cellular structure, which is designed at the lower leg impact zone between the bumper beam and the cover. The parametric model of the negative Poisson’s ratio forepart, the lower legform impactor model, and the car–pedestrian collision model are firstly established. Then, through integrating the response surface method, second-order reliability method, and archive-based micro genetic algorithm, a reliability optimization design is further conducted for the negative Poisson’s ratio forepart based on the deterministic optimization results. Simulation results show that the optimized negative Poisson’s ratio forepart not only can significantly enhance the reliability and robustness, but also improve the protective effect of the pedestrian leg.

Numerical investigation on automotive bumper structure improvements for pedestrian protection

ABSTRACTVehicle bumper design remains a key automotive engineering design challenge for ensuring pedestrian safety in car accidents. In this study, a three-dimensional finite-element lower legform is simulated and validated using static and dynamic tests according to three main criteria, namely, the upper tibia acceleration, knee shearing displacement and bending angle. Important design parameters such as material, thickness and location of different parts of the front vehicle structure were studied. The lower legform and dynamic impactor were simulated in LS-DYNA according to European commission (EC) regulation No 631/2009. Then, the legform and a Class-B car front bumper were used in collision tests at three different distances from the bumper centre point. Finally, the design was optimised in order to meet all requirements of the EC regulation related to vehicle pedestrian collision tests, and an improved bumper model which guarantees complete pedestrian protection is presented and discussed along with guidelines which can have significant applications in the design of pedestrian-friendly bumper structure components.

Dynamic Modeling of Pedestrian Legform and Prediction of Pedestrian Injuries

Decreasing the death toll of pedestrians in traffic accidents is one of the most urgent tasks to be solved all over the world. This paper describes the prediction of pedestrian injuries for the TRL legform impactor using MATLAB. The TRL legform impactor consists of three parts: a femur, a tibia, and a ligament connecting them. The impactor was physically modelled with springs, dampers and two masses as a dynamic model. The impactor behaves in a translational and rotational motion during the collision with a vehicle. The behavior of the impactor during the crash event was captured by a high speed camera and is regarded as the four-degree-of-freedom system in terms of translational and rotational motions. Pedestrian injuries are evaluated by three physical quantities indexes: the acceleration of the tibia, both the displacement and the bending angle between the femur and the tibia. The physical model for the impactor was expressed mathematically by differential equations. In the case of modelling, the ligament connecting both the femur and the tibia in particular plays an important role. Shear forces were applied to the ligament in translational motions and the bending moments in rotational motion. Differential equations were expressed in the form of a state equation and an output equation by MATLAB. Numerical solutions were obtained by a block diagram with Simulink. As a result, it was found that the predicted injuries agree quite well with their experimented data in terms of acceleration, displacement, and the bending angle mentioned above.

INVESTIGATION ON THE KNEE STIFFNESS UNDER THE LATERAL IMPACT FOR THE DESIGN OF PEDESTRIAN LEGFORM IMPACTOR

The objective of the present study is to investigate the stiffness of the knee joint, in order to provide a basis for developing a biofidelic pedestrian legform impactor. A biofidelic lower limb model was employed to replicate structural responses of the human knee joint by finite element simulations. In the simulation, a single displacement was imposed on the thigh or leg, and constrained six freedoms of the other part. As a result, nonlinear load-displacement data were approximated by a linear regression to determine the stiffness. Considering knee joint kinematics under lateral car-pedestrian impacts, a stiffness matrix was established including lateral bending, lateral shearing and torsion effects that significantly influence pedestrian lower limb injuries. Then, this stiffness matrix was applied in a developed legform model. The structural responses of the legform were obtained by the impact with a family car model. Finally, the legform biofidelity was evaluated by comparing the global kinematics of the pedestrian lower limb.

New vehicle bumper design for pedestrian protection during Impact

This study discusses the influence of the active bumper design on the performance of the selected current bumper in terms of pedestrian protection by using finite element method. The legform impactor is used to test the bumper models created according to EEVC/WG regulations (European Enhanced Vehicle-Safety Committee). The simulation was performed using LS-DYNA. The lower leg injury risk was discussed based on the performance of the bumper. Results of the study show a significant improvement in the bumper performance to mitigate the impact injury of the pedestrian's lower leg.

Reliability design optimization of vehicle front-end structure for pedestrian lower extremity protection under multiple impact cases

Injuries to the lower extremities are one of the major issues in vehicle to pedestrian collisions. To evaluate pedestrian lower extremity protection, the Transport Research Laboratory Pedestrian Legform Impactor (TRL-PLI) test has been conducted according to the specifications in European Union (EU) regulation. At the same time, a Flexible Pedestrian Legform Impactor (Flex-PLI), which has flexible femur and tibia, is examined in European New Car Assessment Program (Euro-NCAP) rating system. To minimize injury risks of pedestrian lower extremity, this paper presents the design optimization of a typical vehicle front-end structure subjected to two different impact cases of TRL-PLI and Flex-PLI. Several approaches involving sampling techniques, surrogate model, multiobjective optimization algorithm and reliability analysis are introduced and applied. Four different basis functions of radial basis function (RBF) surrogate model are adopted for achieving more accurate solutions for structural optimization and reliability analysis. It shows that the accuracy of the basis function is different and the best one is selected for approximate the objective and constraint functions. In order to take into account the effect of design variables uncertainty, the reliability-based design optimization (RBDO) is conducted, and a Monte Carlo Simulation (MCS) is adopted to generate random distributions of the constraint functions for each design. The differences of the different Pareto fronts of the deterministic optimization and RBDO are compared and analyzed in this study. Finally, the reliability-based optimum design result is verified by using test validation. It is shown that the pedestrian lower extremity injury can be substantially improved for meeting product development requirements through the proposed approach.

The Effect of Upper Body Mass and Initial Knee Flexion on the Injury Outcome of Post Mortem Human Subject Pedestrian Isolated Legs.

In the ECE 127 Regulation on pedestrian leg protection, as well as in the Euro NCAP test protocol, a legform impactor hits the vehicle at the speed of 40 kph. In these tests, the knee is fully extended and the leg is not coupled to the upper body. However, the typical configuration of a pedestrian impact differs since the knee is flexed during most of the gait cycle and the hip joint applies an unknown force to the femur. This study aimed at investigating the influence of the inertia of the upper body (modelled using an upper body mass fixed at the proximal end of the femur) and the initial knee flexion angle on the lower limb injury outcome. In total, 18 tests were conducted on 18 legs from 9 Post Mortem Human Subjects (PMHS). The principle of these tests was to impact the leg at 40 kph using a sled equipped with 3 crushing steel tubes, the stiffness of which were representative of the front face of a European sedan (bonnet leading edge, bumper and spoiler). The mass of the equipped sled was 74.5 kg. The test matrix was designed to perform 4 tests in 4 configurations combining two upper body masses (either 0 or 3 kg) and two knee angles (0 or 20 degrees) at 40 kph (11 m/s) plus 2 tests at 9 m/s. Autopsies were performed on the lower limbs and an injury assessment was established. The findings of this study were first that the increase of the upper body mass resulted in more severe injuries, second that an initial flexion of the knee, corresponding to its natural position during the gait cycle, decreased the severity of the injuries, and third that based on the injury outcome, a test conducted with no upper body mass and the knee fully extended was as severe as a test conducted with a 3 kg upper body mass and an initial knee flexion of 20°.

Safety assessment characteristics of pedestrian legform impactors in vehicle-front impact tests

This study investigated the characteristics of safety assessment results of front-area vehicle impact tests carried out using the Transport Research Laboratory (TRL) legform impactor and a flexible legform impactor (FLEX legform impactor). Different types of vehicles (sedan, sport utility vehicle, high-roof K-car, and light cargo van) were examined. The impact locations in the study were the center of the bumper and an extremely stiff structure of the bumper (i.e., in front of the side member) of each tested vehicle. The measured injury criteria were normalized by injury assessment reference values of each legform impactor. The test results for center and side-member impacts indicated that there were no significant differences in ligament injury assessments derived from the normalized knee ligament injury measures between the TRL legform impactor and the FLEX legform impactor. Evaluations made using the TRL legform impactor and the FLEX legform impactor are thus similar in the vehicle safety investigation for knee ligament injury. Vehicle-center impact test results revealed that the tibia fracture assessments derived from the normalized tibia fracture measures did not significantly differ between the TRL legform impactor and the FLEX legform impactor. However, for an impact against an extremely stiff structure, there was a difference in the tibia fracture assessment between the FLEX legform impactor and the TRL legform impactor owing to their different sensor types.

Robust design optimization of frontal structures for minimizing injury risks of Flex Pedestrian Legform Impactor

The Flexible Pedestrian Legform Impactor (Flex-PLI) consisting of a flexible femur and tibia will be tested for pedestrian protection by Euro NCAP within the next couple of years as a potential replacement for the Transport Research Laboratory (TRL) legform impactor. The injury risks that are measured when using Flex-PLI are the elongation of the anterior/posterior cruciate ligament (A/PCL), elongation of the medial collateral ligament (MCL), and tibia bending moment (TBM). In this study, we used a correlated computer-aided engineering (CAE) model to conduct a contribution analysis of each injury with regard to the changes in the location of the frontal structures based on the results of a design of experiments (DOE) and analysis of variance (ANOVA). The frontal structures that were selected as control factors were the energy absorber (EA), lower bumper stiffener (LBS), and hood angle. A kriging interpolation model was developed using the DOE results, and its results were compared with those of the CAE model. Furthermore, for robust design optimization, the speed and height of Flex-PLI were used as the noise factors. Finally, a robust design optimization was carried out using the optimal combination of the discrete control factors for minimizing MCL elongation.

Effect of humidity on dynamic characteristics of foam CF45 for the TRL pedestrian legform impactor

The improvement of computer simulation technology allows car designers to evaluate injury risks of newly designed bumpers using models of a car and the Transport Research Laboratory (TRL) legform impactor. Since the Confor™ foam type CF45 of the TRL legform impactor is not a non-ferrous material, the material's dynamic compression characteristics should be clarified for the development of a highly reliable TRL legform impactor model. The stress–strain characteristics of CF45 of varying thickness were investigated under dynamic compression conditions. The humidity conditions were set at RH30% and RH60%. The effect of humidity on the stress–strain characteristics of CF45 was clarified. We proposed a stress–strain curve for the TRL legform impactor model.

Effect of humidity on quasi-static characteristics of foam CF-45 for TRL pedestrian legform impactor

The improvement of computer simulation technology allows car designers to evaluate injury risks of newly designed bumpers by using models of a car and TRL legform impactor. Since a conforTM foam type CF45 of the TRL legform impactor is not a non-ferrous material, the material property of compression characteristics should be clarified for the development of highly reliable TRL legform impactor model. The stress-strain characteristics of CF45 having 25 mm in thickness were investigated under quasi-static compression conditions. The humidity conditions were set at RH20%, RH35%, RH50%, RH65% and RH80%. The effect of humidity on the stress-strain characteristics of CF45 was discussed considering the stiffness of stress-strain.