Defects or irregularities in structural materials such as holes, sharp corners, notches and grooves disrupt the flow of stress and cause areas where the stress is considerably higher than the surrounding areas.  These are known as stress concentrations, stress raisers or stress risers. Such irregularities in materials may occur naturally or as a result of manufacturing procedures however, they may also be created intentionally for the purpose of using mechanical fixings such as screws and bolts to join materials together. Fatigue cracks are generally initiated at stress concentrations (such as those from mechanical fixings) when the mechanical stress on the material is not constant but changes in direction and magnitude. After initiation, the fatigue crack will propagate a little with every load cycle. Once the crack has grown to a critical limit the crack will then propagate more quickly and will eventually result in complete failure.
As well as forming at stress concentrations, cracks can also form in structural materials from corrosion, this is known as stress corrosion cracking. Generally mechanical fixings are made of metal or metal alloys and hence can be susceptible to corrosion and stress corrosion cracking. Several mechanisms for initiation of stress corrosion cracking have been suggested. One such mechanism is the rupture of the oxide film which leads to pitting. Pitting initiates crack formation as hydrolysis reactions cause the levels of corrodent in the pits to be significantly higher than the bulk of the material and this creates a climate favourable for crack formation. Furthermore, when metals are joined by metallic mechanical fixings, galvanic corrosion can sometimes occur. Galvanic corrosion occurs when a metal is in contact with a different metal in the presence of an electrolyte. Due to the difference in potential of the two metals current flows between them and consequently corrosion occurs at the metal that is considered the anode.
Joining dissimilar materials improves design flexibility and allows the specific properties of each material to be used in conjunction. However, dissimilar materials expand and contract at different rates with temperature changes. If mechanical fixings are used to join dissimilar materials the clamping force on the materials and the tension in the mechanical fixing will change as the temperature changes. Inappropriate clamping force can lead to breaking of the mechanical fixing or elimination of tension, also known as stress relaxation can occur.
Some manufacturers are taking precautions to limit the risks of failure occurring by the previously discussed methods. One such measure that is becoming popular is combining mechanical fixings with adhesive bonding. Using two or more joining techniques is known as hybrid bonding and there are studies to suggest that this can lead to improved fatigue, strength and stiffness., In hybrid joining, adhesives are particularly used at stress concentrations to enhance joints and reduce fatigue cracking.
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