Bondline stresses

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

Adhesion is often discussed in relation to the strength of joints, but the force required to fracture a joint is resisted by a complex interaction of internally generated bondline stresses. Attempts to use joint strengths as a measure of adhesion can therefore be extremely misleading. In the ensuing discussion, the term adhesion is reserved for bonding across interfaces, and there are many useful recent publications on the science of adhesion(6-15). 

In summary, the stress (typically at the edge of the die) increases with die size, modulus of the adhesive, temperature, difference in expansion coefficients, and increasing bondline thickness." ... 

Figure 4 (a) Fracture process zone (area of stress concentration) surrounding the area or volume of the bondline immediately ahead of the crack tip when the joint is subjected to cleavage, shear, or shrinkage forces (b) small process zone and high stress concentration with rigid adherend and adhesive (c) large process zone and low stress concentration with flexible adhesive and adherend. 

Adhesives as a class of materials are designed to hold substrates together by surface attachment. The products distribute a load over an entire bondline rather than concentrating stress at specific points like mechanical fasteners. Adhesive bonding also can significantly reduce part weight and assembly time compared to mechanical fasteners. 

The test is based on ASTM D 3535-90. Multiple bondline specimens are subjected to a constant load equal to 3870 N (inducing shear stresses on the bondline of 3 N/mm ) under a climatic cycle composed of 3 parts described in Table 10. 

Yet, athird equation defines maximum stress for rectangular devices taking into account the size of the device, bondline thickness, and cure and exposure temperatures in addition to modulus and CTEs. ... 

Based on these equations, stresses that can result in failures are a function of many parameters including modulus of elasticity, expansion coefficients of the adhesive and adherends, glass-transition temperature, cure temperature, operating or exposure temperatures, and bondline thickness. 

Moisture is absorbed by adhesives and other plastic materials to various degrees and can accumulate in voids within the bondline during assembly, testing, and operation. The absorption of moisture in adhesives as well as in other polymeric materials, such as molding compounds, has been proven to cause failures when parts are subsequently solder reflowed and exposed to the high solder melt temperatures of 200°C and above. The rapid evaporation and expulsion of the moisture results in stresses that cause cracking and delamination, a phenomenon referred to as popcoming. 

The methods developed over the last decade or so to predict failure employ in-bondline non-linear adhesive characterisation. This in turn requires some fairly sophisticated experimental techniques, carefully conducted for a range of test conditions and environments. Hart-Smith concluded that a precise representation of the adhesive stress-strain characteristic is not important. He maintains that the... 

A close look at the stress state indueed within this joint indicates clearly a complex interaction of strain, and there are many useful commentaries on the limitations of this test(4, 5. 25, 31). If the adherend and adhesive moduli are very different (e.g. the ratio of epoxy to steel moduli, E E = 40), the axial strains in the adhesive will be about 40 times greater than those in the adherend with a similar ratio for the lateral (Poisson s) strains. Where the two materials join, this conflict is resolved by generating large interfacial radial shear stresses (Fig. 4.11(c)). Joint strength inereases with a decrease in adhesive thickness, and in a thin bondline affected completely by adherend restraint a eomplex stress arises. The ratio of the applied stress to the strain across the adhesive is then defined as the apparent or constrained Young s modulus, a(5)-... 

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