Peel test adherend plasticity

Fig. 2. Schematic of energy dissipation in a commonly used peel test. The energy dissipation can occur in the adhesive and/or the adherends. The extent of energy dissipation depends on the elasto-plastic properties of the adhesive and the adherends under the test conditions as well as the local stresses and strains near the crack tip.

A T-peel test is shown in Fig. 27.2. The specimen is usually 1 in. (2.54 cm) wide and is described in Standard Test Method ASTM D1876 [6]. This specimen is symmetrical (both adherends are the same thickness). Other peel test specimens are not symmetrical, such as the floating roller peel test [7] or the climbing drum peel test [8]. The test measures the fracture resistance of an adhesive under conditions in which the adherends may plastically deform. In the tables presented later, the peel strength is given in Newtons per centimeter of width (N/cm) and in tmits of pounds per inch width (piw). The latter is shown in parenthesis. In some cases, the peel strength is derived from climbing drum peel measurements in which the results are presented in torque, in. Ib/in. For pressure sensitive adhe-... 

The study of fundamental adhesion has been hampered because standard Tests of adhesion provide a result that is a complicated combination of fundamental adhesion, the physical properties of the adherend and the viscoelastic/plastic character of the adhesive (see Adhesion - fundamental and practical, Peel tests). Our understanding of adhesion has been significantly improved with the advent of mechanical devices that are able to probe the forces of adhesion under conditions that minimize all of the confounding effects of adherend, viscoelasticity, and so on. The Surface Forces Apparatus (SFA) as developed by Israelachvili and Tabor is a mechanical device that has allowed adhesion scientists to directly measure the forces of adhesion under very low rate, light loading, almost equilibrium conditions. Attention is also drawn to Atomic force microscopy. 

Clearly, the peel strength is not a fundamental property for an adhesive. The value of force per unit width required to initiate or sustain peel is not only a function of the adhesive type, but also depends on the particular test method, rate of loading, thickness and stiffness of the adherend(s) and adhesive as well as other factors. Thus, peel tests generally do not yield results that may be used in quantitative design. This does not imply, however, that the peel test is not a useful test. Peel tests provide quantitative comparisons between different adhesive systems, insight into rate and temperature effects, etc. Additionally, peel tests can be used to provide fracture mechanics information as will be discussed in the next section. In the author s opinion, the latter aspect of peel tests has been perhaps most adroitly exploited by Gent and Hamed [18-20] who used peel tests in conjunction with fracture mechanics to obtain insights into time-temperature effects, the role of plasticity, and many other aspects of adhesive fracture. 

D-0903. Test Method for Peel or Stripping Strength of Adhesive Bonds. This is the standard 180 peel test (Fig. 12) applicable where one of the adherends is sufficiently flexible to endure the 180° fold back. Tests the resistance to peel separation of flexible foils, films, or tapes from relatively rigid base adherends (metal, wood, plastic, glass, etc. of adequate thickness). 

As expected, the energy release rate J and lump-sum cohesive law can be experimentally determined if the crack tip separation 8, the loadhne rotation Op of the adherends, and the global peel load P are simultaneously recorded during the fracture test. It is noted that this interface constitutive relationship is the equivalent interface cohesive law, not necessarily the intrinsic cohesive law. This is because, in addition to the intrinsic cohesive separation, possible plastic deformation in the adhesive layer contributes to the entire normal separation between the two adherends during the fracture test. Of course, with the decrease of the adhesive thickness, it is expected that this equivalent interface cohesive law will finally approach the intrinsic cohesive law [66]. 

The focus in testing was to describe briefly the vast number of tests available to determine tensile, shear, peel, fatigue, creep, and impact properties. Wood, metal, plastic, and fiber-reinforced composite adherends were considered. Attention was also given to precautions which must be taken to obtain valid results. 

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus... 

Dom and Liu (1993) investigated the influence of the spew fillet in plastic/metal joints. The study includes an FE analysis and experimental tests and they conclude that the spew fillet reduces the peak shear and peel adhesive stresses and decreases stress and strain concentrations in the adherends in the most critical regions. They also studied the influence of different adhesive and different metal adherends. A ductile adhesive and a more balanced joint (aluminum/plastic instead of steel/plastic) give a better stress distribution. 

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