Failure modes/mechanisms adhesive fractures

Failures modes in adhesives joints, attending the locus of failure and the directional stability of crack propagation, have been considered. Special attention has been paid to the microscopical features left on the fractured surfaces. Influence of the strain rate and the loading mode on the joint has been discussed. Fatigue and creep failure mechanisms have been briefly introduced. 

The failure mechanisms which occur due to thermal cycling differ for underfilled parts compared to those with no underfill. In the absence of an underfill, area array solder joints fail due to solder fracture during thermal cycling. With an underfill, solder fracture is suppressed and other parts of the structure fail. These failure modes include chip fracture and delamination of the underfill from a chip, chip carrier, and solder joints. When the underfill adhesion fails, the solder joints locally are subjected to very high strain levels and quickly fracture. 

Flank wear and catastrophic failure are the main tool failure modes when cutting tempered martensitic stainless steel using coated cermet tools. Abrasive and adhesive mechanisms lead to the flank failure mode while a combination of abrasion, adhesion, diffusion, fracture, and plastic deformation results in catastrophic tool breakage. The application of coated carbide... 

Kinloch(4) observed that the selection of appropriate failure criteria for the prediction of joint strength by conventional analysis is fraught with difficulty. The problem is in understanding the mechanisms of failure of bonded joints, and in assigning the relevant adhesive mechanical properties. Current practice is to use the maximum shear-strain or maximum shear-strain energy as the appropriate failure criterion. However, the failure of practical joints occurs by modes including, or other than, shear failure of the adhesive. This difficulty has led to the application of fracture mechanics to joint failure. 

The propensity for image collapse depends on the width and proximity of the features, its aspect ratio, the substrate surface, and the mechanical properties of the imaged resist. Two characteristic failure modes have been identified deformation, where fracture occurs within the resist structure, and adhesion loss, where the resist feature separates from the substrate at their interface. Low surface-tension rinse liquids [in the extreme, supercritical fluids (101)] and the use of rigid and highly adhesive resists reduce the frequency of image collapse. 

Within this volume, the reader will find several approaches within this general framework. Chapter 2 introduces the concept of fracture mechanics, which is treated in more detail in Chapters 7 and 8. Chapter 3 provides an elegant overview of the energy approach to adhesion. Stresses and driving energies for contact problems relevant to adhesion are given in Chapter II, and Chapter 15 uses fracture mechanics concepts to help interpret the failure modes occurring in bonded joints. 

Clearly, interface fracture mechanics coupled with finite element analysis is a powerful tool for modeling failure in adhesive bonds. Perhaps the biggest limitation is the difficulty in knowing the spatial variation in material properties within the adhesive layer and at the interfaces. Further understanding in this area could lead to an even greater understanding and ability to accurately predict the failure mode and locus of failure in adhesive Joints of various types. 

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

The possible delamination modes are (i) loss of adhesion at paint/phosphate interface, (ii) within phosphate layer due to mechanical fracture (iii) due to dissolution of phosphate (iv) dissolution of coating (v) mechanical failure at coating/steel interface. 

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

Fracture mechanics characterisation tests have been performed to determine the mixed mode fracture envelope of an epoxy bonded glass/epoxy composite. Analysis of lap shear, and L-stiffener geometries has shown that for this relatively brittle adhesive reasonable first estimations of failure loads can be obtained for both cracked and uncracked specimens. An image analysis technique has been developed which enables failure mechanisms to be... 

The failure of adhesive joints depends on the combination of peel and shear loading. In fracture mechanics, three basic modes of loading (Fig. 6.10) are the following ... 

---