When designing with plastics, it is necessary to know the main parameters of the material used.
Strength and Plasticity / brittleness
Mechanical strength is the force that must be applied to exceed a certain critical level of stress leading to fracture or irreversible deformation of the workpiece or component.
Test methods for measuring the strength
Tensile strength in uniaxial pull test
In conventional tensile tests on plastics, stress- and elongation-related parameters are measured in accordance with DIN EN ISO 527-1, here the maximum tensile strength (= maximum stress) and elongation at break (= elongation at break) are the most important.
Tensile tests are also used to test the chemical and thermal resistance of plastics, as even the smallest material changes can be detected in the test results. The strength of polyurethanes at different temperatures is especially important here.
Optionally, the following tests are possible:
In general, the durability of plastics is only estimated at 20 % to 30 % of the tensile strength, while for structural steels this value is 60 % to 80 %.
When determining the fatigue strength, the deformation characteristics of the materials to be tested shall be taken into account. In the case of elastic polymers, the deformation behaviour is following (flexural work), the effective stress does not immediately produce a corresponding deformation. In each load cycle the elastomer absorbs a certain portion of the deformation work and this energy is converted into heat. Plastics are usually poor heat conductors; the energy introduced leads to permanent heating and thus causes a drop in the E-modulus.
The E-module describes the stiffness factor of a plastic as being in the ideal elastic starting range of its stress-strain curve and is measured in N/mm2 or MPa. As the E-modulus value increases, the material becomes stiffer. In simple words, the E-module is a key parameter for the strength with which a material flexes when a force is applied (resistance to deformation). With the same load and geometry, a component made of an elastomer will bend much more than the same component made of steel, for example.
Test methods for measuring the E-module
To test the E-modulus of elasticity, material samples are tested in a tensile test by clamping the sample with a known starting profile in a tensile testing machine and putting a load F on it. This force is then displayed graphically over the length change caused by increasing the force.