The document discusses factors of safety in engineering design. It defines factors such as ultimate stress, yield stress, and working stress. Factor of safety is the ratio of the maximum stress to the working stress. For ductile materials like mild steel, factor of safety is based on yield point stress. For brittle materials like cast iron, it is based on ultimate stress. The appropriate factor of safety depends on material properties, test results, load reliability, failure mode certainty, assumptions made, stress concentrations, manufacturing stresses, consequences of failure, and loss of property if failure occurs. Common factor of safety values vary according to material and load conditions.