Surface defects and scratches lead to fatigue failure of structures. Fatigue failure starts on the surface of a component and grows as a crack under cyclic loading until it finally fails. In order to extend the number of load cycles a component is able to withstand in service, it is therefore necessary to reduce surface defects or to counteract them.
Where Cracks Begin
Surface defects or scratches on a component’s surface act as stress raisers on the surface of the part. As a component goes through cyclic loading, these scratches or pits can concentrate stress at the tip of the scratch or pit, far in excess of the bulk material’s stress. This is where the fatigue crack will start to open up. Thus, reducing or neutralizing surface damage can extend a part’s fatigue life.
Compressive Stress Keeps Cracks Closed
The process of shot peening creates a shallow layer of plastic deformation on the surface of a part. When this layer is ‘set’ into a state of residual compressive stress it has the effect of fatigue cracks only being able to propagate when in a state of tension. In order to continue to propagate the crack must ‘overcome’ the compressive stress in the surface layer. The depth and magnitude of the residual compressive zone can be controlled by the parameters of shot size, shot velocity and coverage. This allows the designer to ‘tailor’ the surface treatment to the specific component stress and design requirements.
Hard Coatings and Surface Quality
HVOF is a surface treatment that is used to create a hard surface coating, usually made of tungsten carbide or a similar hard wearing material. This material is sprayed at high velocity creating a dense, low porosity coating. The surface finish of the coating is also very smooth which is ideal for many applications. Another surface treatment that is often used to create a hard surface on a component is hard chrome electroplating. Although hard chrome creates a very smooth surface, it also creates a large amount of tensile stress in the coating. This can have a large effect on the fatigue life of the component, particularly if the majority of the component is not coated. In these cases a shot peen treatment is required to try to recover the stress. If you are looking for Aerospace Surface Treatments, see //www.poeton.co.uk/about/industries/aerospace-defence/.
Anodising Aluminium: a Trade-off
Aluminium parts that are subjected to fatigue are often anodized to attempt to and improve their fatigue life. The anodising process increases the surface hardness of the aluminium however the formed oxide layer is very brittle and can act as a crack initiation site. Sealed hard anodising does not provide as good a fatigue life as unsealed hard anodising as the sealing process softens the oxide however does not reduce its brittleness. For fatigue critical parts therefore anodising to a controlled depth can be acceptable.
Matching Treatment to the Part
In situations where parts are to be used in high-cycle fatigue applications (e.g. aircraft landing gear or engine attachments) it is generally recommended that a specialist in aerospace surface treatment is contacted to outline an appropriate treatment sequence. For example shot peening could be used after plating processes to produce the highest value compressive residual stress in critical areas of the part. As with all other aspects of cost effective design it is best to get the surface treatment right at the design stage – this is likely to cost only a fraction of the cost of a failure in service.
Design and treatment of surfaces at the design stage is cost effective compared to investigating a fatigue failure at a later date.