Finite element model and validation of a surrogate crash test vehicle for impacts with roadside objects

Highway and roadside safety features are crash tested for compliance with certain safety criteria. US Federal Highway Administration (FHWA) uses a surrogate reusable test vehicle, Bogie, to conduct crash tests of roadside safety features. Bogie is intended to replace an impacting vehicle in order to reduce the cost of the test. Bogie can be configured with different crushable impact noses representing various vehicle fronts. Typically, the crushable nose employed by Bogie is a multi-compartment honeycomb material. Different configurations of honeycomb arrangement are considered for different impact velocities. Recently, the advances in crashworthiness and dynamic finite element analysis have allowed considerable modelling and simulation of vehicle impacts with roadside hardware. This paper describes, in detail, the finite element models and the validation of Bogie and its honeycomb material in impacts with an instrumented rigid pole. This model can be exercised in various simulations of crash scenarios for design optimization of roadside hardware. This validation also allows the use of the model for impacts with narrow objects, which is a critical aspect of crashes with roadside safety devices. NOTATION ρ Density ν Possion’s ratio E Elastic or Young’s modulus G Elastic shear modulus μ Material viscosity coefficient σy Yield stress for fully compacted honeycomb material τ Shear stress V Relative volume, the ratio of the current volume over the initial volume of honeycomb Vf Relative volume at which the honeycomb is fully compacted a,b,c Subscripts defining the honeycomb cell depth, ribbon, and transverse direction, respectively i,j Subscript index for a,b, and c directions u Subscript designating uncompacted honeycomb β Coefficient for calculation of Eii and Gij Eaau, Ebbu, Eccu Three elastic moduli of honeycomb in uncompressed configuration Gabu, Gbcu, Gcau Three elastic shear moduli of honeycomb in uncompressed configuration max (value1, value2) Function returns the maximum of two values min (value1, value2) Function returns the minimum of two values σaa, σbb, σcc, σab, σbc, and σca Stress components INTRODUCTION Roadside safety appurtenances, e.g., guard rails, bridge rails, median barriers, and roadside hardware (sign supports, luminare poles, etc.) are designed to redirect the vehicles, reduce the vehicle speed, or break upon impact. These safety features are intended to reduce the occupant injuries in vehicle crashes with roadside devices. The performance of the roadside safety appurtenances traditionally is evaluated by full-scale crash tests. These devices are continuously undergoing design changes and being improved upon for safety compliance. The changes are made based on the evaluation of the field performance and the resulting accident/injury statistics. US Federal Highway Administration (FHWA) and the State Departments of Transportation play a major role in the development of safety criteria and the testing of the roadside appurtenances. The crash testing of roadside devices using vehicles is a very costly undertaking. The variety of devices to be tested, the different classes of vehicles for which safety compliance must be checked, and the number of various impact conditions that need to be evaluated require an extensive matrix of crash tests to be conducted. This is definitely a prohibitive cost requirement. Therefore, improvements in the design of many roadside safety features are only incremental due to the lack of comprehensive test data and performance measures. The vehicle is a major cost element in the crash testing of roadside appurtenances. In many cases, the reusable surrogate test vehicle, Bogie, is used in lieu of the impacting vehicles to reduce the test cost. The use of Bogie also expedites the test setup and preparation because the acquisition, instrumentation, and preparation of new vehicles usually requires a longer lead time and often delays the test schedule. Bogie, on the other hand, is equipped with all the necessary instrumentation and can be easily reconfigured with various impact noses to represent different vehicle frontal crush characteristics. Many tests for the performance evaluation of roadside safety features are therefore conducted with Bogie. Advances in crashworthiness research [1-4] and particularly in dynamic finite element analysis of impacts [5-10] has opened up new possibilities for the evaluation of the crash performance of roadside appurtenances. In recent years, non-linear explicit finite element codes have significantly advanced the computer modelling and simulation of automobile crashes [11-20]. This capability additionally allows the application of the software to model and analyse the performance of roadside objects in crashes [21-24]. The simulations can assist the redesign and optimization of these devices for the purpose of reducing injuries in highway accidents. The codes used are based on various public domain and commercial versions of the DYNA3D program[25-27]. The authors have been applying these codes in various highway safety analysis projects [10-13, 2123], in particular, in addressing collisions with highway narrow objects like sign support systems. At present, many models of roadside safety features are being developed by various researchers using DYNA3D software in a coordinated FHWA program. The unavailability of functional, computationally efficient and validated, impacting vehicle models is a major impediment to these roadside hardware crash simulation efforts. Since most of the tests are conducted using Bogie, the need for a validated model for the bogie is evident. To this end, a DYNA3D model of the FHWA’s Bogie has been developed. The model incorporates a multi-compartment crushable honeycomb nose. The experimental testing and the process for validation of the individual honeycomb segments as well as for the validation of the entire vehicle model in impacts with an instrumented rigid pole is described. The validation with the rigid pole, as opposed to the rigid wall, is of special interest because it is a better measure of performance of the honeycomb in impacts with highway narrow objects, because roadside narrow objects (poles, signs, etc.) are a major cause of severe injury in highway crashes [28]. Much testing is conducted by FHWA to evaluate the performance of these objects in crashes. FHWA crash tests the roadside appurtenances to discover shortcomings in the existing or proposed designs, and to assure compliance with the requirements of the NCHRP (National Highway Cooperative Research Program) Report 350 [29]. In the NCHRP 350 test criteria, various roadside hardware must be evaluated in crashes with various classes of vehicles including the Small Size Vehicle denoted by 700C and 820C representing 700 and 820 kg vehicle weights, respectively. Certain devices are allowed to be tested using surrogate vehicles like Bogie. For this application, Bogie simulates the small or light weight vehicle class. The goal of this research is to arrive at a validated DYNA3D finite element model of Bogie that can be used (1) initially as a predictor for the crash tests and (2) eventually as a tool for design parametric studies to optimize the performance of the roadside safety appurtenances in reducing the crash pulse intensity in highway collisions. FHWA SURROGATE TEST VEHICLE, BOGIE The Federal Highway Administration’s (FHWA) Bogie is a surrogate vehicle used for the full scale crash test of highway appurtenances [30-31]. Bogie is used in crash tests by FHWA to simulate the impact dynamics of a small size vehicle. It can be configured with different noses to represent different crush characteristics. Each frontal nose represents a different class of vehicle or impactor [32]. Its weight can also be slightly adjusted with ballast to represent weight variations within a vehicle class category. Bogie used here, shown in Figure 1, simulates NCHRP’s 820C vehicle. Figure 1. Bogiesurrogate crash test vehicle Bogie is basically an un-powered four-wheeled rigid structure which is guided and accelerated with an external drive mechanism, e.g., a weight pulley system or a towing winch system, to arrive at a desired impact angle and velocity. Bogie is equipped with an accelerometer at the approximate location of the C.G.. The accelerometer is contained in an instrument box which is connected to the off-board data acquisition system via an umbilical cord. The rigidity of Bogie’s structure behind the nose assuages concerns about the accuracy of acceleration measurement and simplifies some of the complexities of the FE modeling. In impact tests of roadside hardware, two noses are widely used: 1) a rigid nose, and 2) a flexible honeycomb nose. The rigid nose model does not truly require a validation since both the structure and the impactor are rigid and practically act as a rigid body with the specified weight and inertia distribution. Figure 2 is a schematic drawing showing the dimensions of Bogie. The flexible honeycomb nose consists of multiple compartments of honeycomb material, possibly of different consistencies, i.e., different crushability. The compartments are separated by 12.7 mm (0.5 in) fiberglass plates. The nose assembly is attached to two shafts which are guided to slide through Bogie’s front structure (two structural tubes). This arrangement allows the plates and honeycomb segments to be compacted consecutively upon impact as the whole assembly slides through Bogie’s rigid structure. A fore bumper (frontal block) honeycomb segment is also attached to reduce the initial impact spike. Figure 3 shows the sketch of the honeycomb nose. Figure 2. Dimensions of Bogie Cartridge # Size in (mm) Type Static Crush Strength (KPa) 1 70x406x76 HC130 896 2 102x127x51 HC025 172 3 203x203x76 HC23