Material Test

Different material tests could result in different material mechanic character. The typical material test for metal is tensile test. With this test strain-stress curve, yield point, necking point and failure point of material could be observed. For example, Force (F) and Length (l) are measured, as shown below.

material-test_explicit_structural_fea

Engineer strain-stress curve could be generated by:

Where, S0 is the section area in the initial state and l0 is the initial length.

In this Force-elongation curve or engineer stress-strain curve, three point are important.

1.	Yield point: where material begin to yield. Before yield you can assume material is in elastic state (the Young’s modulus E could be measured) and after yield, material plastic strain which is non-reversible.

•	Some material in this test will first reach the upper yield point (ReH) and then drop to the lower yield point (ReL). In engineer stress-strain curve, lower yield stress (conservative value) could be taken.

•	Some material can not easily find yield point. Take the stress of 0.1 or 0.2% plastic strain as yield stress. Rp0.2 is usually taken for high stress steel and aluminum.

2.	Necking point: where the material reaches the maximal stress in engineer stress-strain curve. After this point, the material begins to soften.

3.	Failure point: where material failed.

material-test_necking_point

Rm - Maximum resistance

Ag - Uniform elongation

Fmax - Maximum force

Agt - Total uniform elongation

ReH - Upper yield level

A - Failure strain

ReL - Lower yield level

At - Total failure strain

True stress-strain curve which is requested in most materials in RADIOSS, except in LAW2, where both engineer stress-strain and true stress-strain are possible to input material data.

In the example below, find engineer stress-strain curve (blue) by using:

The result is true stress-strain curve (red). Plastic true stress-strain curve is shown in green, which plastic strain begin from 0. This green plastic true stress-strain curve is what you need, as in LAW36, LAW60, LAW63, etc.

material-test_true_stress_strain_curve

The true stress-strain curve is valid until the necking point of the material. After the necking point, the material curve has to be defined manually for hardening. Using a different material law, RADIOSS will extrapolation the true stress-strain curve to 100%.

•	Linear extrapolation: If stress-strain curve is as function input (LAW36), then stress-strain curve is linearly extrapolated with a slope defined by the last two points of the curve. It is recommended that the list of abscissa value be increased to a value greater than the previous abscissa value.

•	Swift and Voce: After necking point, use one of the following equations to extrapolate the true stress-strain curve.

Swift model:

Voce model:

Combination of Swift and Voce model (LAW84):

material-test_necking_point_2