Modeling the Thermo-Structural Response of Railcar Floor Assemblies During Standard Fire Resistance Tests

Abstract

Performing fire endurance tests of railcar floor assemblies in accordance with NFPA 130 is expensive given the minimum size requirements of 3.7 m (12 ft) in length and the full vehicle width. Often it is not financially viable to conduct such tests on several iterations of designs for the purpose of design optimization. Simulations of the fire endurance tests can be performed in place of experiments to provide predictions of floor assembly response of multiple designs at much lower cost. However, capturing the thermo-structural response of the floor assembly requires the ability to model the relevant physical phenomena including softening and weakening of the steel frame, degradation of the fire insulation, and failure of the composite floor. A methodology for performing such simulations was developed under this research addressing each of these phenomena. Temperature dependent thermal and mechanical properties of all modeled materials captures material softening and weakening. Degradation of the insulation was handled through a novel temperature dependent shrinkage approach. Failure models for the sandwich composite floor panels were obtained from literature to predict shear fracture of the core based on a maximum principal shear stress approach and delamination of the core/facesheet based on a maximum strain energy approach. The developed methodology was applied to the simulation of a fire endurance test of an exemplar railcar floor assembly using the commercial finite element solver Abaqus. The assembly was known to hold a passing rating for a 30-minute fire endurance test according to NFPA 130. The floor assembly consisted of a stainless-steel frame, fiberglass insulation, and a ply-metal composite floor. Sequentially coupled thermal and structural models were developed to predict the thermostructural response of the floor assembly for a 30-minute exposure to the ASTM E119 prescriptive fire curve. User-subroutines were utilized to implement the sandwich composite failure models developed for predicted core shear fracture and core/facesheet delamination. The predicted temperature rise on the unexposed surface of the floor assembly after a 30-minute exposure ranged from 50°C to 90°C. The floor assembly was also predicted to maintain structural integrity with the applied crush load, having a center-point vertical deflection of 161 mm after the 30-minute exposure. This resulted in a predicted pass rating for a 30-minute exposure which agrees with the floor assembly’s actual fire rating.