What term is used for the stress at which fracture of the material occurs?

The term used for the stress at which fracture of the material occurs is known as ultimate stress. Ultimate stress, often referred to as tensile strength, is the maximum stress that a material can withstand while being stretched or pulled before breaking. This property is crucial for engineers as it indicates the capacity of a material to endure loads without failing, which ensures safety and reliability in applications.

Ultimate stress is determined during tensile testing, where a sample is subjected to increasing tensile forces until it ultimately fails. This peak value is vital for material selection in engineering design, as it helps to define the limits within which materials can function effectively.

Other options, such as elastic limit, relate to the point where a material will return to its original shape upon unloading, and allowable stress is a calculated value used in design to ensure safety by accounting for factors like service conditions and material variability. The factor of safety is a design principle that provides a safety margin between actual material performance and the loads that will be applied, but it does not describe a specific point of stress where fracture occurs. Thus, the term ultimate stress directly pertains to the fracture point of a material, making it the correct choice.