The four main loading modes of bonded joints are (see Figure 1):
In practice, a bonded joint will simultaneously experiences several of these loading components.
Figure 1: Basic loading modes experienced by adhesive joints
The aim of designing adhesive joints is to maintain the adhesive in a state of shear or compression. Bonded joints are strongest under these loading conditions. Tension, cleavage or peel forces should be avoided, or their effect minimised. The presence of the stresses will compromise joint strength and fatigue performance. Structural adhesives have relatively poor resistance to through-thickness (peel) stresses, and therefore to obtain maximum efficiency, joints need to be designed to minimise tensile stresses. For composite laminates, resistance to peel stresses may be considerably lower, so even greater care must be taken with these materials to minimise these stresses.
The aim is to design a joint to fail by bulk failure of the adherends. A margin of safety is generally incorporated in the design to account for factors, such as service environment, type of loading, degree of control in adhesive application, etc. It is important to ensure that the adhesive is not the weakest link. This is because of the high variability in adhesive strength and concern as to the speed of damage growth that can occur under cyclic loading. For composite adherends, failure is often observed to occur in the near surface plies of laminate materials. This is due to the low toughness associated with the thin resin layer present at the surface of these materials. Considerable care needs to be taken to ensure that the thin surface resin layer does not become the weakest link.
There are a number of potential failure modes for adhesively bonded metallic or composite joints, including (see Figure 2):
Adhesive failure is the rupture of the adhesive bond, such that separation occurs at the adherend/adhesive interface. This form of failure, which can result from either inadequate surface treatment or material mismatch, should be avoided. Information on interfacial strength, although qualitative, is normally obtained from adhesive joint tests (i.e. lap shear). The term “interface” is used for the layer of material bordering the adherend and adhesive, which encompasses the true interface, the interphase and the near surface area. The material properties in this region tend to differ significantly from the bulk adhesive.
Cohesive failure of the adhesive occurs when the load exceeds the adhesive strength. This tends to be a localised effect, occurring near stress concentrations (ends of joints).
Cohesive failure of the adherend occurs when the load exceeds the adherend strength. For metals, adherend strength usually corresponds to the yield strength. In laminate materials, this form of failure generally initiates from the matrix between layers as a result of out-of-plane tensile or interlaminar shear stresses. Other forms of failure can occur if the composite adherend is not a layered structure (e.g. through-thickness tensile cracking).
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Interfacial failure |
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Cohesive failure of adherend |
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Cohesive failure of adhesive |
Figure 2: Locations of failure initiation in a laminated bonded joint
Note: Generally, failure tends to be mixed mode - a combination of interfacial and cohesive - location of initial failure can also be difficult to detect.
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