Modeling, Testing, and Validation of the 2007 Chevy Silverado Finite Element Model

The National Crash Analysis Center (NCAC) at the George Washington University (GWU) has been developing and maintaining a public domain library of finite element (FE) vehicle models for use in transportation safety research. Using the NCAC’s unique and innovative reverse-engineering process, an FE model of the 2007 Chevrolet Silverado pick-up truck was developed. This pick-up truck satisfies the requirements for a 2270P test vehicle under the soon to be adopted crashworthiness evaluation criteria specified in the Manual for Assessing Safety Hardware (MASH). Since this FE model will be extensively used to design safer roadside barriers, the representation of the suspension system and its dynamic response becomes a critical factor influencing the performance of the roadside barrier. To improve the FE model fidelity and applicability to the roadside hardware impact scenarios it is important to validate and verify the model to a multitude of component and full-scale tests. A series of highly instrumented, non-destructive, full-scale tests and destructive, component-level suspension tests were conducted to gather data for validating the suspension system of the FE model. This paper provides a description of the vehicle FE model, the various component and full-scale tests that were performed, and the current status of the model validation to these physical tests. Working Paper NCAC 2009-W-005 Oct 2009 Modeling, Testing, and Validation of the 2007 Chevy Silverado Finite Element Model INTRODUCTION Full-scale physical crash tests are conducted by several agencies worldwide to evaluate vehicle and roadside hardware crashworthiness. It is not economically feasible to perform full-scale crash tests on the wide range of parameters that influences safety performance. With the advent of high-speed, high-memory capacity computers in the early 1990s, computer technology has reached the point where vehicle crash outcomes can be accurately predicted using the computer. Specialized FE software – appropriate for depicting the physics of motor vehicle collisions – is now used extensively to predict collision outcomes. Because of this new technology, impact simulations utilizing nonlinear FE analysis have become effective tools in designing and evaluating crashworthy vehicles and roadside safety hardware among other things. Once successfully validated, the FE models can be linked to analyze new impact scenarios (e.g., incremental changes in design parameters, a range of impact conditions). As an update to the National Cooperative Highway Research Program’s (NCHRP) Report 350 [1], new test vehicles have been designated to be representative of the current fleet in the Manual for Assessing Safety Hardware (MASH). One of the test vehicles has been upgraded from a pick-up truck weighing 2000 kg to a pickup truck weighing 2270 kg. Since the 2007 Chevy Silverado meets the requirements of the new pick-up truck test vehicle, an FE model representing this vehicle has been developed using the NCAC’s reverse-engineering process. The three main requirements of a highly viable FE model are accuracy, robustness, and processing speed. The FE model should be accurate enough to yield reasonable predictions of the essential features being sought, robust enough to successfully simulate many impact scenarios, and fast enough to allow for many iterations and parameter studies. Verifying and validating the FE model to a range of component and full-scale impact tests is necessary to ensure adequate accuracy of the model. FE MODEL DESCRIPTION The NCAC has been developing vehicle and roadside hardware FE models for over 15 years. Over the years, the reverse-engineering methodology has evolved and a unique process for vehicle model development has been developed. The reverse-engineering process involves methodical disassembly of the physical vehicle to create a computer model of each and every part in the vehicle. The geometrical data for each part is converted to an FE mesh and carefully re-assembled with the appropriate consideration of connections and constraints between elements to create a full vehicle FE model. A series of material level characterization tests, on coupons extracted from various locations of the vehicle, are performed to gather the required input for the material models in the FE program. The fully assembled model of the 2007 Chevy Silverado pick-up truck consisting of 930,000 elements is shown in Figure 1. Since this vehicle FE model will be extensively used in the analysis and improvement of roadside safety features, special emphasis was given to accurate representation of the suspension components and its connections. For the front suspension system (Figure 2a), the upper and lower control arms, the coil spring and damper are explicitly modeled. Their connections to the wheel spindle are modeled with appropriate joint degrees of freedom so that it would function as in the physical system. For the rear suspension system (Figure 2b), the individual leaf springs with varying thickness are explicitly modeled. The leaf springs are connected to the rear axle using the U-bolts modeled as beam elements. The ends of the leaf springs are connected to the truck frame with the appropriate joint degrees of freedom. Figure 1: 2007 Chevy Silverado FE Model aFront Suspension bRear Suspension Figure 2: Suspension Details MODEL VALIDATION The Silverado model was subjected to a more extensive validation than had previously been applied to NCAC vehicle models. This multi-stage validation effort included: 1. Detailed measurement of inertial properties 2. Suspension system component tests 3. Non-destructive bump and terrain tests 4. Comparisons to New Car Assessment Program (NCAP) full frontal rigid wall impact tests 5. Comparisons to recent crash tests for six common roadside barriers. This paper describes the current validation results associated with items 1, 2, and 3 above. All of these tests [2] [3] were designed and developed to gather the required data for model validation. The majority of these tests focused on the suspension system characterization since it plays a critical role in predicting the overall response of the vehicle and performance of the roadside safety barrier [4] [5]. In addition to the impact tests, the physical vehicle’s center of gravity location and the roll, pitch, and yaw moments of inertia were measured for use in model validation. 1. Mass, Inertia, and CG Comparisons The FE model was validated to a series of component and full-scale impact tests to ensure accurate, predictable response. However, the first step in the validation process was to ensure that the FE model has the correct mass distribution and inertia measurements compared to the physical vehicle. The Chevy Silverado’s center of gravity height and roll, pitch, and yaw moments of inertia were not available in the open literature. These measurements play a critical role in predicting the overall response of the vehicle when impacting roadside safety hardware at oblique impact angles. Before the reverse-engineering process, the Chevy Silverado pick-up truck was sent to a specialized laboratory (SEA Limited) that could measure the center of gravity height and roll, pitch, and yaw moments of inertia. The center of gravity height and the three moments of inertia of the completed FE model was compared to the laboratory test results (Table 1). The inertia test of the physical vehicle was conducted with a full tank of gas. In order to meet the weight requirements of the MASH test vehicle, the fuel mass in the FE model was removed. The difference in measurements between the physical vehicle and the FE model was within 3%, ensuring that the total mass and mass distribution closely matched that of the physical vehicle. Table 1: Mass, Inertia, and CG Comparison Physical Vehicle FE Model % difference Weight, kg 2337 2270 2.86 Pitch inertia, kg-m^2 6155 6028 2.06 Yaw inertia, kg-m^2 6453 6490 0.57 Roll inertia, kg-m^2 1051 1050 0.09 Vehicle CG height, in 27.96 28.64 2.32 Based upon the small differences between the actual vehicle and FE model for the mass, inertial properties, and center of gravity measures, it was concluded that the model is a valid representation of the actual vehicle. 2. Pendulum Impact Tests A series of pendulum impact tests were conducted on the front and rear suspension assemblies of the 2007 Chevy Silverado after the reverse-engineering was complete. The objective of these tests was to gather dynamic suspension deflection data (bottoming out) for validating the 2007 Chevy Silverado FE model to improve its range of applicability in evaluating roadside safety features. The suspension and its connections to the frame rail were preserved during the vehicle reverse-engineering process. The frame rails were then cut in the center to separate the front and rear suspensions, so that the tests could be done independently. The 2007 Silverado suspension assembly was tested in the following three different impact configurations a. Front suspension vertical loading b. Rear suspension vertical loading c. Rear suspension lateral loading The impactor used in these tests was a 2000 kg pendulum mass suspended by four cables to a steel framework. The pendulum is released to be free falling from a predetermined height based to achieve a desired impact velocity. The tests were conducted with the pendulum located at the FHWA Federal Outdoor Impact Lab (FOIL) in McLean, Virginia. a. Front Suspension Vertical Loading The front suspension assembly along with the frame rails was firmly attached to a specially fabricated support structure to prevent any movement during the pendulum impact (Figure 3a). The structure was positioned such that the pendulum impacted the bottom of the front tire to induce vertical loads in the suspension assembly. The tire pressure was set to 35 psi for all tests. Three different impact speeds (3, 6 and 9 km/h) were chosen for this test series. The impact velocity was gradually increased to get full compression of the suspension assembly. (a) Front Suspension Vertical Loading (b) Schematic of Front Suspension Support Structure Instrumentation Locations Figure 3: Front Suspension Pendulum Test Setup The pendulum and the front suspension assembly were instrumented with accelerometers and string pot potentiometers at several locations. Specific locations of the accelerometers and string pot potentiometers at the front suspension are shown in Figure 3b. A series of high speed digital cameras were used to capture the test event. Each of these cameras captured the test event at 500 frames per second. Time zero for data acquisition and high speed imaging was initiated when the pendulum impacted the contact switch on the tire surface. The front suspension sub-system along with the frame rails was extracted from the full vehicle FE model of the Chevy Silverado. The frame rails were constrained as in the test set-up. The front tire was impacted with a rigid pendulum at representative impact velocities. The initial model response did not compare well with the physical test. This was expected since the model was assembled based on past experience. Component level test data were not available at that time for model verification and validation. (a) Simulation Setup (b) Displacement Time-History Comparison Figure 4: Front Suspension Setup The suspension system model was updated as part of the validation process. Each individual part of the suspension assembly and its connections were rechecked for accuracy. The lower A-arm was originally modeled using shell elements. In order to represent the correct mass, stiffness, and inertia the lower A-arm was re-meshed using solid elements. Quasi-static uniaxial compression tests were conducted on the coil spring to investigate its stiffness. Subsequently, the coil spring had to be re-meshed from shell to solid elements to match the stiffness measured in the compression test. A revolute joint was added to represent the rotational degree of freedom provided by the pin that connects the shock damper to the lower A-arm. A nonlinear damping curve was used to represent the characteristics of the shock damper. This damping curve was optimized iteratively to match the suspension system response observed in the physical tests. The simulation set-up and the comparison of the timehistory response of the front suspension dynamic deflection are shown in Figures 4a & 4b. All of the above updates to the FE model improved the suspension system response and a good correlation was observed with respect to the physical test. Peak dynamic deflection of 80 mm was measured at a pendulum impact velocity of 9 km/h. b. Rear Suspension Vertical Loading Similar to the front suspension vertical tests, the rear suspension assembly along with the frame rails was firmly attached to a specially fabricated support structure to prevent any movement during the pendulum impact (Figure 5a). The structure was positioned such that the pendulum impacted the bottom of the tire to induce vertical loads in the suspension assembly. The tire pressure was set to 35 psi for all tests. A range of impact speeds from 2 to 6 km/h was chosen for this test series. The impact velocity was gradually increased to get full compression of the suspension assembly. The tests were conducted with and without the shock dampers to isolate its effect on the overall suspension response. Specific locations of the accelerometers and string pot potentiometers on the rear suspension are shown in Figure 5b. shock damper Ladder frame Rear axle High-speed camera Rear end of ladder frame