When evaluating a material for use in a particular application, engineers usually look first at the material properties data sheet or material data sheet for short. These are available on-line for most plastic materials. This sheet will give typical values for a variety of properties. Usually it will include values for physical properties and mechanical properties as well as thermal and electrical properties. Typically, the material properties data sheet will include what ASTM, ISO or UL test was used at to arrive at the value and the units the value is given in as well as the data. An example might be that the density of a particular material was determined by ASTM D792* and the value is given in lb/in2 or g/cm3. Here we will take a closer look at some of the mechanical properties used to evaluate material strength.
- Ultimate Tensile Strength (UTS) or simply “Tensile Strength” is a measurement of how much stress a material can withstand. The term refers to the maximum stress a particular plastic can withstand while being pulled or stretched without breaking. The highest point of the stress strain curve is the UTS. The measure of unit for UTS is MPa or psi. Some plastic materials will break sharply-this is called “brittle failure”. Other materials, such as most metals, will experience some plastic deformation before fracture. The values are most important in brittle materials as there is no yield point.
- Tensile Stress at Break or Ultimate Tensile Stress refers to the elongation of a material that is subject to tension until it breaks. Stress=force/area or F/A. It is commonly expressed in MPa or psi.
- Flexural Modulus, sometimes referred to as the bending modulus, is the tendency of a material to bend. It is the ratio stress to strain in a flexural deformation and is expressed in units of force per area. ASTMD790
- Flexural Strength is the ability of a material to resist deformation under load. ASTMD790
- Young’s Modulus or Tensile Modulus is used to characterize the stiffness of an elastic material. It was named after the 19th century British scientist Thomas Young. It is often referred to as the Modulus of Elasticity. It is a ratio of the stress along an axis to the strain along that axis. A material with a high Young’s modulus is quite rigid.
- Shear Strength is the force per unit area at failure divided by its density. Shear forces are unaligned forces pushing one part of an object in one direction but another part of that body in another direction.
- Rockwell Hardness refers to a method of testing the hardness of materials. In this case hardness is defined as the resistance of a polymer to penetration. The Rockwell scales are the most widely recognized tests for hardness.
- Izod Impact Strength, Notched is an ASTM standard method of determining the impact resistance of materials. The ASTM test determines the energy per unit thickness required to break a test specimen under flexural impact. The test specimen is held as a vertical cantilevered beam and is impacted by a swinging pendulum. The energy lost by the pendulum is equal to the energy absorbed by the test specimen.
- Elongation at Break is the ratio between the initial length and the changed length after breakage of the test specimen. It expresses the capability of a material to resist changes of shape without cracking
*ASTM or the American Society for Testing and Materials is an international standards organization that develops and publishes technical standards for use on a voluntary basis.
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