SIMULATION OF SOFTENING AND RUPTURE IN MULTILAYERED FUEL TANK MATERIAL
To be published on November 21, 2019 by SAE International in United States
Event: NuGen Summit
Research and/or Engineering Questions/Objective Plastic automotive fuel tanks made up of blow molded, multi-layered, high-density polyethylene (HDPE) material can take complex shapes with varying thickness. Accidental drop of fuel tank from a height during handling can lead to development of cracks. Damage can also occur due to an impact during a crash. This can be catastrophic due to flammability of the fuel. The objective of this work is to characterize and develop a failure model for the fuel tank material to simulate damage and enhance predictive capability of CAE for chassis and safety load cases. Methodology Different aspects were considered to develop a characterization and modelling strategy for the HDPE fuel tank. Material properties can be influenced by factors such as, service temperature, rate of deformation, state of stress etc. Hence, samples cut-out from different regions of the fuel tank were subjected to a variety of tests such as tensile test at different strain rates viz. 0.01/s, 0.1/s, 1/s, 10/s and 100/s, compression, shear, flexure and instrumented dart impact tests at different temperatures, -40°C, 23°C and 85°C. A novel way of modeling damage was accomplished via progressive damage and failure modeling capability available in ABAQUS. Ductile damage initiation criteria and equivalent plastic displacement for damage evolution were considered. The parameters of the failure model were optimized using Design for Six Sigma (DFSS) principles. The material model was validated by comparing simulation results with test at coupon and component levels. Results Evaluation of test results showed the influence of different factors on the fuel tank material. A marginal increase in ultimate tensile strength could be observed due to increase in strain rate, though, there was a significant drop in displacement to failure. An increase in test temperature reduced the ultimate tensile strength but increased displacement to failure. Optimizing parameters of the failure model using DFSS approach resulted in a good correlation between test and CAE for tensile test simulations at different strain rates and temperatures. A good correlation could also be observed for instrumented dart impact test simulation. The material model was further validated using a pressure-vacuum cycle load case. Limitations of this study Unlike injection molded samples where uniform thickness can be achieved, blow molding, results in varied thickness at different sections of a component. Hence, thickness control was difficult and test coupons were cut from different regions of the fuel tank thus affecting the accuracy of test results to some extent. What does the paper offer that is new in the field including in comparison to other work by the authors? Different factors influencing the material properties such as variation in temperature, strain rates, state of stress etc. were considered while developing a method to characterize and model a blow molded, multilayered, fuel tank material. The effect of each of these factors on the initiation and evolution of damage was studied and a robust failure model was developed to effectively predict failure in a plastic fuel tank. Conclusions Samples cut from different regions was essential to obtain test data for the characterization of a blow molded HDPE fuel tank. While thickness variation did not result in significant change in the stress-strain behavior of samples cut from uniform fuel tank area, it had a significant influence on the properties at the pinch flange area. A good test versus CAE correlation was obtained for coupon level tensile and instrumented drop impact test. Softening and rupture experienced by a fuel tank during pressure-vacuum cycle test was effectively predicted in CAE.