Most resins have properties in the elastic and plastic regions, as shown in the diagram below, expressed as an S-S curve.

In the elastic region, the strain returns to zero after dividing the stress, so the behaviour is linear.

In the plastic region, the strain does not return to zero after the stress is removed, resulting in non-linear behaviour.

In order to perform calculations in the non-linear region in structural analysis, material properties must be entered.

To input the material properties of the non-linear region, it is necessary to input the amount of strain at each stress point, which is very difficult. This also increases calculation time.

To simplify the input of material properties, a method called the two-line approximation is used to reduce calculation time.

This method replaces the physical properties of the resin from the yield point to the rupture point with a straight line (tangent coefficient). The disadvantage of this method is that the actual test results may differ slightly from the actual S-S curve because the values entered are different from the actual S-S curve, but it is a good way to check the design values for a model with a large number of elements and a large calculation scale.

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