# Structural robustness

The strength is a mechanical constant that the resistance behavior of a material to - characterizes deformation - elastic or plastic.

The strength is the tension that is calculated from the maximum force reached at a defined strain or at a defined flow, each relative to the original (that is unstressed and undeformed ) cross-sectional area of the material.

A distinction strengths

- According to the type of mechanical stress, among others: tensile strength
- Compressive strength
- Compressive strength
- Flexural strength
- Torsional
- Shear strength.

- Static strength
- Increasing strength
- Term strength
- Fatigue strength
- Fatigue strength

If one measures, one obtains measurement curves from which determines the technically relevant strength parameters and stress-strain diagrams can be created the strains on a component as a function of different applied forces. Particularly important is the Zugverfestigungskurven be viewed from the uniaxial tensile test in this context. Depending on the type of material, material condition, temperature, strain and strain rate different strengths can be achieved.

When a force stresses, a distinction between the terms

- Strength Symbols ( for metals ):
- Unit symbol: MPa (= N / mm ²)

- Symbols ( for metals ): or
- Unit characters: N / mm ² ( = MPa)

The voltage value in relation to the strain is assigned to a specific plastic deformation, for example 0.2% of permanent elongation. Is then noted this so-called " yield point " in relation to the technical 0.2% proof stress, that is, in relation to the technical elongation value of 0.2%. The ( strong ) Yield strength is only relevant for un - and low alloyed steels in certain heat treatment conditions play a role, especially for mild steel.

In the mechanical design of components, the minimum value or guaranteed value of the strengths flows.

The minimum tensile strength is for example a steel (S235JR - formerly St37 -2), which is used in steel building construction, depending on the quality at 370 N / mm ². Its minimum yield strength, however, at 235 N / mm ². If one were now in a tensile test, a sample of this steel, which has a cross section of 1 mm ², charge with a force, it would have to at least 370 N are tearing around the sample. 370 N correspond to the earth the weight of a mass of 37.7 kg. It can be concluded, that can not be excluded in an attempt to raise greater with this steel wire has a mass of 37.7 kg and a failure of the material. At this load, the wire is already stable ( plastic) is deformed. Since this usually will not be allowed to be commonly used in the mechanical layout of components, the minimum yield stress ( ). This value indicates the stress in the material of up to only occurs a resilient deformation. This means when a tensile force of 235 N on a specimen with a cross- section of 1 mm ², this sample expands, but she returns without binding ( plastic) to deform, in their original state. Here you can have a mass of 23.9 kg determine with their weight, this material can be loaded in the tensile test, but behaves elastically.

For security reasons, the above-mentioned characteristic values in the technical applications are generally still divided by a safety factor, which takes into account the uncertainties in the assessment of stress and the dispersion of the resistance values , but also depends on the potential damage in case of failure of the component. In steel construction, the safety factor for the material is usually 1.1. It should be noted that the loads are each protected by its own factors ( partial safety factors ).

Since the parameters are always determined only in the uniaxial tensile test, components but often claimed multiaxial (eg waves on bending and torsion, the bending means already a multi-axial stress on strictly speaking ) it is, with the aid of a strength hypothesis uniaxial equivalent stress to determine, which can then be compared with the known resistance.

Swinging and many generally moving parts are loaded periodically. These loads can not be adequately described by means of the above-mentioned characteristics, as it is already there at significantly lower loads to failure of the material. Such stresses are recorded using the fatigue strength.