Civil and Environmental Engineering


Date of this Version

Spring 4-21-2016


Humphrey, B.M., "Improved Methodologies in Modeling and Predicting Failure in AASHTO M-180 Guardrail Steel Using Finite Element Analysis - Phase I," A Thesis Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, April 2016.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Civil Engineering, Under the Supervision of Professor Ronald K. Faller, Lincoln, Nebraska, April, 2016

Copyright © 2016 Brandt M. Humphrey


Steel guardrail systems have historic and widespread applications throughout the nation’s highways and roadways. However, catastrophic system failure can occur if the guardrail element ruptures, thus allowing an errant vehicle to pass uncontrolled through the system and potentially allow fractured ends to pierce the occupant compartment. To aid in the analysis and design of guardrail systems, further efforts are needed to develop and implement more reliable material failure criteria to predict and model guardrail steel rupture under all vehicle impact loading scenarios within impact simulation finite element method (FEM) software, such as LS-DYNA.

This Phase I study accomplished a number of tasks to aid in this objective. First, historical and state-of-the-art failure criteria with emphasis on stress state dependent failure criteria were reviewed. Next, various failure surface methods that provide estimations on the triaxiality and Lode parameter vs. effective plastic strain at failure were review and analyzed. It was determined that more flexible failure surface fitting methods may provide better estimations, and larger more diverse testing programs are required to estimate the failure surface through all stress states. A failure surface method using a Smoothed, Thin-Plate Spline was also proposed to overcome short comings in existing failure surface estimation methods. Based on the review of the existing failure surfaces’ performance, a steel material testing program was developed, and testing was performed on 21 different specimen configurations that represent a range of stress states. The specimens were prepared using ASTM A572 Grade 50 steel with similar material properties as AASHTO M-180 guardrail steel. Test results and calculated material properties were presented herein. Lastly, a preliminary FEM modeling effort was conducted. Various modeling parameters were examined, including the effects from hourglass controls, mesh-size effects, inertial effects from load rate, and solid vs. shell behavior. Based on this analysis, preliminary models of the testing specimen were developed. Also, a preliminary material model was calibrated and presented herein. Conclusions were made, and recommendations were provided for continuing a Phase II effort.

Advisor: Ronald K. Faller