Mechanical Behavior of Adhesives and Joints

The mechanical behavior of adhesively bonded composite joints depends on the type of adhesive, the properties of the substrates, particularly in the through-thickness direction, and the care taken to guarantee the interface between the two. 

The interfaces in bonded structures influence the mechanical behavior of components significantly. Therefore, an important task in nondestructive testing (NOT) is the investigation of the interaction forces in adhesive joints and the development of techniques to evaluate the bond quality. The load capacity of such joints is often limited by regions of weak bonding. As in all materials, the... 

Sound knowledge of the joint behavior is required for a successful design of bonded joints. To characterize the bonded joint, the loading in the joint and the mechanical properties of the substrates and of the adhesives must be properly defined. The behavior of the bonded joint is investigated by finite element (FE) analysis methods. While for the design of large structures a cost-efficient modeling method is necessary, the nonlinear finite element methods with a hyperelastic material model are required for the detailed joint analysis. Our experience of joint analysis is presented below, and compared with test results for mass transportation applications. 

The thermal and dynamic mechanical behaviors of triblock copolymers with a styrene/isoprene/styrene architecture were investigated in order to understand their adhesive properties. Both copolymer free films and films bonding together two titanium alloy plates were found to have thermal and mechanical response that was strongly dependent on joint preparation. Microphase separation in the melts of these triblock materials was felt to contribute to the observed phenomena namely, the presence of residual stresses in thin films which had been cooled while under high pressure. 

Despite these criticisms, however, we are not saying that diffusion-created interfaces or interphases do not exist or do not influence mechanical behavior of joints. They do. But Voyutskii s diffusion theory simply is not, and cannot be, the basis for a general understanding of the mechanical behavior of, e.g., adhesive joints or other adhering systems. 

Zhang Y, Vassilopoulos A Pand KellerT (2010a), Mode I and mode II fracture behavior of adhesively-bonded pultruded composite joints. Engineering Fracture Mechanics, Vol. 77, No. 1, pp. 128-143. 

The stresses in an adhesive joint depend, once a constitutive model is chosen, on the geometry, boundary conditions, the assumed mechanical properties of the regions involved, and the type and distribution of loads acting on the joint. In practice, most adhesives exhibit, depending on the stress levels, nonlinear-viscoelastic behavior, and the adhetends exhibit elastoplastic behavior. Most theoretical studies conducted to date on the stress analysis of adhesively bonded joints have made simplifying assumptions of linear and elastic and/or viscoelastic behavior in the interest of tracking solutions. 

The analysis of the interface mechanics provides useful insights into crack propagation behavior in adhesively bonded joints. A finite element model for the DCB specimen was constructed using Franc2D/L [64], a convenient code for this task because of its capability for automatic remeshing in the vicinity of a growing crack. An adhesive layer (material 2) with thickness of / = 0.5 mm is sandwiched between two adherends (material 1) with thickness of = 6 mm, and... 

It is often seen that the strength of adhesive joints decreases on aging in natural environments. This is usually because of the effect of environmental moisture, which is absorbed by polymeric adhesives and affects both the mechanical behavior of the adhesive and the interface between the adhesive and adherend. These effects are usually increased by increasing temperature or the application of stress. The effect on the interface is potentially the most damaging however. 

There are a lot of specific techniques that provide valuable information related to polymeric adhesives characterization. The more commonly used microscopy techniques are listed in O Table 43.2, including some figures about size ranges and magnifications. Every microstruc-tural characteristic is related to the overall mechanical behavior of the adhesive, and the use of all or some of these techniques can help in the postfracture analysis of the joints. 

The fracture-based approach derives from continuum fracture mechanics theory, which claims the strength of most real solids is governed by flaws within the material [2]. To help predict this type of behavior, many test methods have been developed to determine fracture properties of adhesives. These tests are used to characterize the mode I, II, and III fracture properties of many types of material systems. In this study, the focus will be on the mode I and II characteristics of bonded joints for automotive applications. 

For that reason, the properties of stainless steel joints bonded with epoxy systems are of special interest. According to the literature, the aging behavior of such joints is critical [1, 2]. Mechanical tests reveal that the combined attack of water and temperature causes a strong deterioration in their performance [3]. It is necessary to understand the processes that are going on during aging in order to improve the reliability of structural adhesive bonding. 

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