Technical info

Ultimate tensile & yield strengths

Ultimate tensile strength measures the maximum stress a metal can withstand while being stretched before breaking; in other words, it is measured as the tensile load required to fracture something. Some metals can break sharply, while some others will deform or elongate before breaking.

The yield strength is defined as the level of stress that produces a specific amount of permanent deformation, before breaking. The yield strength for a material is lower that its ultimate tensile strength. A material that is loaded to a stress level below its elastic limit will completely return to its original size and shape, if the load is released immediately. On the other hand, when material is loaded to a stress level greater than the elastic limit, it will experience some degree of permanent deformation.

The 0.2% offset yield strength is defined as the amount of stress that will result in a plastic extension of 0.2%. This is the yield strength that is most often used by metal suppliers and used by engineers to design engine components, car parts, fasteners, etc...

A maximum stress level of 75% of the yield strength has empirically been used when designing an object, as a safety measure in order to ensure that it does not yield in service. Recently, the use of finite element analysis has allowed engineers to proceed with safety factors approaching 1, on certain situations.

The most frequently used units of stress for metals are the kilopound-force per square inch (ski) - a force of one thousand pounds-force applied to an area of one square inch - and the megapascal (MPa) -  a force of one million Newtons  applied to an area of one square meter.


Temperature dependence

Temperature has an effect on the mechanical properties of metals, including their yield and ultimate strengths. In general, both tend to decrease as the temperature increases.

Some materials can retain their strength over a wider temperature range, which attractive for high temperature applications where others would succumb to creep as a result of thermally induced crystal vacancies, such as in exhaust applications.

Figure 1 shows the effects of temperature on ultimate tensile strength for five different metals and alloys.

Figure 2 shows the effects of temperature on 0.2% offset yield strength for the same materials.

As observed, Ti6Al4V and Inconel 718 have a far superior 0.2% offset yield strength than the steel employed for stock exhaust studs, and much of that strength is retained at higher temperatures; which especially true for Inconel 718.

Furthermore, both Ti6Al4V and Inconel 718 have excellent oxidation resistance at low and high temperatures, and in most corrosive environments.