Substrate failure bonded joint

Figure 4 shows failure types of the adhesive bonded joint under a pull strength test. The cohesive failures in Fig. 4(a) and (b) occur when fracture is developed either within the adhesive and substrate, while the adhesive failure in Fig. 4(c) separates the substrate and adhesive at the interface. 

Thus, the adhesive contacts the substrate via a layer of substances that frequently differ from the adhesive in composition. If the cohesion strength of these substances is less than that of the adhesive, this will determine the failure stress of the adhesive-bonded joint. The adhesive, which has the same composition as that of the adhesive in bulk, can form a weak zone in the substrate surface. Adhesives are polymers and the particular nature of a polymer must have effects at all stages of formation and operation of an adhesive-bonded joint. 

Thus, despite unsoundness of the structure of the polymer boundary layers, their mechanical properties can be high. The zone of failure of the adhesive-bonded joint in this case will depend on the correlation of the weakening and strengthening effects of the substrate on the polymer layer in contact with it. 

One of the principal reasons for failure of the adhesion bonds is a specific adsorption reaction of the medium with the material to be cemented at the boundary with the adhesive. There is an adsorption substitution of adhesive-substrate bonds by medium-substrate bonds. Surface structural defects that are present in each solid are the first to be subjected to adsorption. It is to be expected that the probability of appearance of such defects is higher at an interface of two materials with different properties. The rate of penetration of the medium along the polymer-substrate interface frequently substantially exceeds the rate of diffusion of the medium in pure polymer [212]. Adsorption substitution of the polymer macromolecules by water molecules on the metal surface explains the low water resistance of such adhesive-bonded joints as fluoroplastic-steel or polyethylene-steel [34]. The adhesion strength, which decreases during hold-up of adhesive-bonded joints in water, is frequently reestablished after the joints are dried [213]. 

Adhesive materials are applied as thin layers of polymeric materials capable of transmitting stresses between two substrates. They can be classified according to their functions into physical or chemical adhesive forms. Adhesives must behave as fluids before they set and become solid. Thus, the solid adhesive is formed from (a) its solution by solvent evaporation, (b) hot-melting by coohng and (c) reactive liquid precursor by in-situ thermosetting reactions. The purpose of adhesives is the transmission of forces from one adherent to the other. Thus, adhesive performance is always described in terms of mechanical adhesion in which the strength of the polymer interface with the adherents is evaluated. The distribution of stresses in bonded joints depends on the overall bond geometry and on the loads applied to the bonded structnre. The initiation and development of failure is most certainly associated with stress distribntion. 

Surface preparation is also very important for long term durability of bonded joints. Laek of proper surface preparation of substrates prior to adhesive bonding ean attribute to surface contamination which leads to adhesive failure. 

Another basic major advantage is that the cyclic-fatigue fracture-mechanics data may be gathered in a relatively short time-period, but may be applied to other designs of bonded joints and components, whose lifetime may then be predicted over a far longer time-span. Obviously, the fracture-mechanics tests need to be conducted under similar test conditions and environments as the joints, or components, whose service-life is to be predicted. This is important since the fracture-mechanics test specimens do need to exhibit a similar mechanism and locus of failure (e.g. cohesively through the adhesive layer, or interfacially between the adhesive and substrate, or through the oxide layer on the metallic substrate, etc.) as observed in the joints, or components, whose lifetime is to be ranked and predicted. 

In order to analyze the effects of rate and temperature on the interfacial strength of bonded joints, one can initially use a simple energy approach in the following maimer A critical energy level, Wc, is used to represent interfacial failure. In the presence of adhesive, substrate, and interphase (a distinct material layer between the substrate and the adhesive layer which transmits the rigid substrate s energy to the adhesive layer), the combined elastic energy can be written as ... 

When designing a bonded joint, the goal is to achieve at least a break within the adhesive. This guarantees that the adhesion is higher than the cohesion and the adhesive is fully utilized. If failure in one of the substrate occurs, the bonding fiilfils the requirements. Such failures can be much better predicted due to the inherent material properties of the substrates or the adhesive. 

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