Cooling debonding

Another consideration is the difference in thermal expansion between the matrix and the reinforcement. Composites are usually manufactured at high temperatures. On cooling any mismatch in the thermal expansion between the reinforcement and the matrix results in residual mismatch stresses in the composite. These stresses can be either beneficial or detrimental if they are tensile, they can aid debonding of the interface if they are compressive, they can retard debonding, which can then lead to bridge failure (25). 

The radial (compressive) stress, qo, is caused by the matrix shrinkage and differential thermal contraction of the constituents upon cooling from the processing temperature. It should be noted that q a, z) is compressive (i.e. negative) when the fiber has a lower Poisson ratio than the matrix (vf < Vm) as is the normal case for most fiber composites. It follows that q (a,z) acts in synergy with the compressive radial stress, 0, as opposed to the case of the fiber pull-out test where the two radial stresses counterbalance, to be demonstrated in Section 4.3. Combining Eqs. (4.11), (4.12), (4,18) and (4.29), and for the boundary conditions at the debonded region... 

Mechanical Failures Cracks or debonding of the catalyst from the substrate material can occur from thermal stresses as well as dynamic forces on the modules. The catalyst must be carefully handled to prevent premature fracturing. Each requires a warm-up and cool-down rate. 

A threshold level of interfacial adhesion is also necessary to produce a triaxial tensile state around rubber particles as the result of the cure process. When the two-phase material is cooled from the cure temperature to room temperature, internal stresses around particles are generated due to the difference of thermal expansion coefficients of both phases. If particles cannot debond from the matrix, this stress field magnifies the effect produced upon mechanical loading. 

Ochiai, S. (1999) Interfacial debonding in single fibre-composite with a cracked matrix - Part 1 Debonding during cooling. International Journal of Materials and Product Technology, 14, 147-166. 

By heating one surface of a bonded sandwich structure and observing the temperature rise of the opposite face, areas of debond, which resist the transfer of heat, show as cool areas. Alternatively, if the heated face is scanned, debonds will show as hot areas. Temperature sensing is normally done with a scanning infra-red camera (e.g. AGA Thermovision). More recently, heat pulses or moving heat sources have been used (Vavilov and Taylor, 1982). Temperature sensitive paints or liquid crystals, and thermoluminescent coatings are also used. 

By heating one surface of a bonded sandwich structure and observing the temperature rise of the opposite face, areas of debond, which resist the transfer of heat, show as cool areas. Alternatively, if the heated face is scanned, debonds will show as hot areas. 

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