Stress

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Stress

General

The stresses to which materials can be exposed temporarily or permanently during use are very diverse. They can occur individually or in combination. In the latter case, one also speaks of complex stresses. Typical basic stresses during use are

mechanical stresses:

→ Effects of forces and moments

chemical stresses:

→ Attacks of a chemical and electrochemical nature on the surface

thermal stresses:

→ Effects of constant and changing temperatures

radiation-physical stresses:

→ Effects of light and ionising radiation

tribological stresses:

→ Surfaces acting on each other in relative motion – contact deformation, friction and wear

biological stresses:

→ Effects on organisms and microorganisms

Mechanical stress

External forces/loads cause localised stresses in components/structures in the form of stresses, moments, strains or displacements. In addition, residual stresses can also become effective, which arise during the production and processing of materials or when joining components, e.g. due to dimensional deviations or during welding.

To ensure safe operation, each component must be designed and tested in accordance with its intended use. The effect of the stress on the material depends on the type and intensity of the stress:

on the type and intensity
on the sequence, duration and temporal progression of the stress
on the interaction with the environment, e.g. temperature, medium

A classification is made between tensile, compressive, bending, torsional, transverse and shear stresses according to the type of force applied. A distinction is made between uniaxial and multiaxial stresses according to the stress state.

According to the time course of the stress, a distinction is made between

quasi-static,
monotonically increasing,
oscillating (cyclic) and
shock/impact

stress (see Fig. 1), whereby in the literature both the oscillating and the shock/impact stresses are referred to as ‘dynamic’ stresses.

Fig. 1:

Typical stress–time functions

Examples of the individual types of stress are

Creep tensile test, static tensile test (static)
Tensile test, compression test, bending test, hardness test (quasi-static)
Fatigue test (vibration test)
Impact and notched Charpy impact bending test, puncture impact test, impact and notched tensile-impact test (impact or shock).

Static stress

'Static' generally refers to a load in which the material is subjected to a static constant load. In the ‘classic’ tensile test, the load is applied continuously and without impact (quasi-static). The upper limit of the loading rate for quasistatic test methods is at strain rates of 1 s^-1.

Cyclic (vibratory) stress

An oscillating load is understood to be a load that changes over time (see fatigue). The material testing method for determining material parameters is known as a fatigue (vibration) test.

Tribological stress

Tribological stress is defined as the stress on the surface of a solid body caused by contact and relative movement of a solid, liquid or gaseous counter-body. Tribological stress is also characterised by the fact that the mechanical stress triggers secondary thermal, tribochemical physical reactions between the elements of the tribological system involved. In addition to the purely mechanical, thermal or medial stress, it is therefore a separate type of loading, concentrated on surface areas of the materials, which decays to-wards the inside of the material.

Tribological stress can cause damage that leads to the failure of structures and components. The damage phenomena are described in detail in the VDI guideline 3822 ‘Failure analysis on thermplastic products’.

General

Mechanical stress

Static stress

Cyclic (vibratory) stress

Tribological stress

See also