Bonded joints mechanical properties

Most metals show a low deformability in comparison to nonmetal materials. For bonded joints this property means that adhesive layers, when exposed to mechanical stress (tension, shear, pressure, bending, torsion), are subjected to deformation stress only to the same extent. 

Although numerous studies (1-3) have described work aimed at establishing criteria for the durability of adhesive joints, a thorough understanding of effects of the chemical and mechanical properties, on the durability of adhesive bonds is lacking. More specifically, the effects of surface preparation and dynamic loading, especially under environmental service conditions, has not been explored in detail for automotive structures. In this paper, a description of the effects of environment on the durability of adhesive bonds is presented. Particular attention is given to... 

The DB-procedure was optimised in respect with the kinetic requirements and the high-temperature mechanical properties of the Ni-superalloy. From the kinetic point of view, the bonding temperature should be over 1000°C when alumina and transition metals are directly bonded [6]. The bonding procedure was always carried out in high vacuum, better than 2-10 mbar (0.2 mPa). The typical thermal and axial compression cycles are presented in Fig.la. It was experimentally found that the ambient bonding temperature is 1100"C or less due to the fast creep of the superalloy beyond this. The compression for the tests was selected as 10 MPa in ceramic-metal joints and 20 MPa in ceramic-ceramic joints [6]. 

The crack-free coated alumina specimens were subjected to a diffusion bonding (DB) procedure, described above. The parameters of the process were same as for non-graded specimens. After the bonding to IN-738 superalloy the joints were examined by their appearance and microstructure. For some specimens it was found that good, crack-free microstructure does not guaranteed high mechanical properties, in particular at elevated temperatures under external mechanical load. 

A measure of the damages affecting the bonded joints is the loss of strength of the aged test pieces in contrast to the nonaged test pieces, which may be indicated in the form of reduction factors (Section 10.2.2). Only property testing of bonded joints under these complex stresses composed of mechanical and environmental influences enables an extensive statement on the behavior in practical application. 

Ageing Modification of properties of bonded joints due to mechanical, physical and chemical influences generally resulting in strength reduction. 

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. 

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. 

Figure Q13.12 shows the mechanical properties of two urea-formaldehyde (UF) resins cured with NH4CI (bottom) variation of the shear strength of bonded wood joints with cyclic wet-dry treatments of joints (middle) development of internal stress with duration of resin cure at room temperature (top) dynamic mechanical properties of resins. Discuss the interrelationships between the observed mechanical properties. 

The initial flaw can be a discontinuity, such as a void, or an abrupt change in material properties. By nature, wood contains innumerable discontinuities, such as the cell cavity and transition zones between cell wall layers. An adhesive may contain air bubbles or fillers with properties different from the resin. A rough wood surface may not be completely wetted by the adhesive, leaving voids at the interface. The adhesive and wood also have different mechanical properties. When a joint or bonded material is subjected to some force, the resultant stress is heightened or concentrated around the discontinuities far above the average stress in the joint or material. Fracture results... 

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. 

Let us consider the adhesion properties of ERC. The mechanism of the effect of the low-molecular-weight additives on the properties of the polymers in block has been studied in detail, though their effect on the strength of adhesive-bonded joints has not yet been studied in sufficient detail. 

Well-designed and well-made joints should retain their mechanical properties indefinitely if the wood moisture content stays within reasonable limits (i.e., < 15%) and if the temperature remains within the range of human comfort. However, when bonded joints are exposed either intermittently or continuously to abnormally high or low temperatures for long periods they will eventually deteriorate [23]. 

Table 2.5 summarises typical values of these bulk mechanical properties at around 20 °C for a range of epoxy adhesives. Of particular note is the low strength and stiffness of the epoxy polyamides and polysulphides as compared to those with aliphatic polyamine hardeners. For the design of bonded assemblies, joint tests are often used to determine the relevant mechanical properties. It must be remembered, however, that the results will be highly dependent on the specimen geometry and testing conditions and... 

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