Finite element models of the thorax are used to design restraint systems by predicting rib fractures. This study presents a sensitivity analysis of the effect of ultimate strain criteria on the resulting number of rib fractures in a finite element model of the thorax. Plastic failure strain values from 0.6% to 1.8% were used, with impacts from a 23.4 kg blunt impactor using frontal (4.9 m/s) and lateral (5.16 m/s) loading. For both impacts, fractures generally decrease monotonically with increasing failure strain when material properties vary by location, while uniform material properties cause rib fracture numbers to oscillate.