How does the residual stress increase the strength

The tensile strenght is the stress that is calculated in the tensile test from the maximum tensile force achieved in relation to the original cross-section of the specimen. The symbol of the tensile strength is R.m. The dimension of tensile strength is force per area. Frequently used units of measurement are N / mm² or MPa (MegaPascal). In the stress-strain diagram, the tensile strength can be calculated directly from the maximum force achieved.

The stress values ​​read from the stress-strain diagram (tensile strength, yield point) do not correspond to the true stress in the material. This is because when calculating the tension, the tensile force is related to the initial cross-section. The real cross-section in the tensile test is smaller than the initial cross-section (transverse contraction, constriction). In the case of elastic-plastic deformation (for samples made of ductile materials), this deformation (elongation and constriction) is visible and measurable after the test. A distinction is often made between the "true" stress σwaHr and the "nominal" stress σnOminell ("Engineer Tension").

The tensile strength is therefore not the true stress in the specimen at the moment of breakage, but is lower.

The true maximum stress arises in the constriction area of ​​the sample. In this area, the deformation and, if necessary, the solidification increases until it breaks. In the so-called instrumented tensile test, the specimen cross-section is measured continuously and the force is related to the real cross-section. Samples examined in this way show a continuous increase in the true stress up to breakage. However, the value determined in this way is only of theoretical importance.

In the past, tensile strength was often used to characterize materials. An example of this is the designation of structural steels. Steel 52 (St52, today S355) was named after its tensile strength of 52 kp / mm² (510 N / mm²). Due to the harmonization of European and international standards, many steels are now called according to the yield point, which from a constructional point of view is a better characteristic value for the load-bearing capacity of a material.

See also:Strength, tear length, stress-strain diagram

Categories: Material Properties | Strength theory