This article on tensile strength was written by Dr.Charlie Geddes for Hardie Polymers.
When scanning a thermoplastics data sheet for a new project or finding a replacement for an existing grade, the eye tends to get drawn to the values quoted for tensile strength (or more correctly ‘tensile stress at break’) as an indication of the material’s mechanical properties. Although tensile stress is a useful property for metals, it is of less value when dealing with thermoplastics. Applied loads during product service life are more likely to be in compression or in bend than in pure tension… with a few notable exceptions. Secondly, rupture under an applied tensile load is very seldom the cause of product failure in a plastics component. A more likely cause is unacceptable deformation or distortion. That is an entirely different property, related to the tensile or flexural modulus (stiffness) and component geometry.
Another reason for not reading too much into tensile strength data? The value listed in data sheets is a snapshot. It is derived from a standard test. This is carried out under a specific combination of temperature and strain rate. (This is the rate at which the test piece is stretched). Changing temperature and strain rate can significantly alter the resulting value for tensile strength (and also the elongation at break). The ultimate elongation is also influenced by the presence of flaws and imperfections in the material.
Is the product is going to be subjected to mechanical stresses during its service life? It would be prudent to pay more attention to the values of modulus (stiffness) and impact data rather than tensile strength. However, data for impact can give a false sense of security and needs careful interpretation.
Written by Dr.Charlie Geddes for Hardie Polymers
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