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Failure Analysis – Basics

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Failure analysis – Basics


Failure analysis, evaluation of damage cases

Damage analyses serve to determine the causes of component failure and to draw conclusions from this in order to initiate targeted measures for damage repair and prevention.

A wide range of analytical methods, polymer testing and polymer diagnostics are available for damage analysis on plastics.

Compared to metallic materials, the properties of plastic components are much more strongly influenced by design, material selection and processing and machining conditions. These influencing factors must therefore be given special consideration when performing damage analyses on plastics.

The wide range of possible polymer modifications and the use of fillers and reinforcing materials (see: fibre-reinforced plastics), additives and stabilisers result in a wide variety of failure and damage mechanisms in practical use, which can significantly affect the functional integrity of the component.

Plastic components are often subjected to very complex mechanical, thermal, media and climatic stresses during use, which, in conjunction with ageing and degradation effects, can lead to component failure due to fracture.

As a result of the inherent creep tendency of plastics and the relaxation conditions that occur, either plastic instability with impermissibly large deformations or unstable crack propagation can occur in conjunction with the residual stress and orientation state as well as the viscoelastic deformation behaviour.

Failure analysis, fracture surface analysis

Fracture surface analysis allows initial conclusions to be drawn about the type of failure and the conditions under which it occurred. Even with the naked eye or with the aid of a microscope, it is possible to distinguish between a brittle fracture and a ductile fracture. It is also possible to determine whether the damaged component was subjected to static, impact or cyclic (e.g. oscillating) stress prior to the fracture (see also: fatigue).

Figure: Fracture surface of a component made of high-density polyethylene (abbreviation: PE-HD) after cyclic stress

Failure analysis, complexity

Damage is defined as changes to a component that significantly impair or prevent its intended function, whereby the cause of damage is the sum of the factors that caused the damage.

In damage analysis, the type and cause of damage are determined from the damage pattern – the apparent damage phenomena.

The following damage phenomena occur most frequently in plastic components:

One of the most common causes of catastrophic failure is the brittle fracture of components or component parts (see: types of fracture). Brittle fracture is promoted by various influencing factors, which can occur either individually or in combination (see: brittle fracture promoting factors).

Figure: Factors promoting brittle fracture in plastic components

Component damage occurs when the material's property profile (physical, mechanical, thermal and chemical properties) does not match the component's requirement profile.

Failure analysis, mechanical stress

If a component fails during use as a result of mechanical stress alone, it can be assumed that this component was subjected to stress exceeding the permissible load limit specified in the component design or that it was not used for its intended purpose, i.e. that it was overloaded (see: plastic component, dimensioning).

Figure: Damage caused by mechanical stress – damage phenomena, causes of damage and evidence of damage

Component failure due to mechanical stress in the subcritical range, on the other hand, is an indication that the actual cause of damage is not to be found in the mechanical stress itself, but rather in the component design, the choice of materials, the quality of manufacturing or the effects of external influences (temperature, media, radiation, etc.). The mechanical stress on already damaged components is then ultimately the load that leads to the final component failure. If, for example, media stress causes embrittlement of the material and associated crack formation (see: crack formation), additional mechanical stress leads to the opening of the crack flanks, associated crack propagation and ultimately to fracture, the final failure of the component, which is associated with material separation leading to a loss of load-bearing capacity.

See also

References

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