Peel joints

The static stress distribution that results when a flexible member is peeled away from an adhesive layer supported on a rigid substrate has been considered by a number of authors [91-102]. These analyses have considered the adhesive and 

Equation 6.24 assumes that the value of cm in the adhesive layer is constant through the adhesive layer thickness and across the width of the joint. The 

Kaelble has also shown that peel force, P, per unit width (which is equivalent to an energy per unit area) is given by  

However, Gent and Hamed [104] have shown that the theory of small bending deformations used to derive Equation 6.24 is only valid to peel mechanics when )8pmp sin a, i.e. when Kp = 1. The results from these analyses have not been widely used in the interpretation of peel test data and the fracture mechanics approach, based upon an energy balance argument, which avoids the necessity for developing a detailed stress analysis has been far more widely applied. This approach is considered in Section 7.3.2. 

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It is evident from this model that the lap joint peels in essentially the same way as the T joint, except for the extra elastic stretching of the material caused by the higher force required. If we assume that the force F depends on the work of adhesion W in the same manner as the peeling joint, but also on the elastic modulus E of the strips and on the size L of the strips, then it is obvious from a simple dimensional analysis that the force must be given by an equation of the kind... 

Subjected to stress, moisture and temperature uses peel joint As above, but uses single-lap shear joint loaded in tension As above but uses a wedge test Exposure to moisture and temperature... 

FIGURE 11. High-resolution SEM analysis of fracture surface of anodized 2024-T3 A1 clad peel joints showing poor wetting by the adhesive to the nonpeeling face. Reprinted with permission from Reference 17. Copyright 1984 Butterworth Scientific Ltd. 

G. F. Carter, Durability of adhesive peel joints while stressed in water, ASTM STP 401, p. 28 (August 1966). 

Fig. 2.14. Some joints used in laboratory testing of adhesives, (a) Single lap, (b) double lap, (c) butt, and (d) 90° peel joints.

If plasticity is included in the adhesive characteristics, then an alternative failure criterion was proposed and implemented by Crocombe and Adams [9] using critical values of effective uniaxial plastic strain. The triaxial strain was expressed as an effective uniaxial strain, and then compared with the strain to failure of the bulk adhesive. Unfortunately, this too was seen to be dependent on the density of the finite element mesh, and in reality was a critical strain at a distance criterion. The choice of critical strain was also dependent on the adherends present as different plastic zone sizes will be given by different adherend combinations. The geometry considered in this paper was the peel joint. 

The development of SPM and particularly AFM has not been ignored by the adhesion world. The analysis of failure surfaces by AFM is still in its infancy but it is already clear that AFM may sometimes be more useful than SEM. Thus AFM has been used successfully in the analysis of the failure surfaces of adhesively bonded galvanized steel [149] and in the study of the adsorption of polymers on conducting polymer substrates [150,151]. Some of the author s unpublished studies have shown that the corona treatment of polyolefines gives rise to a characteristic morphology and that failure interfaces from peel joints of polyolefine substrates can yield more useful information from AFM than from SEM investigations [152]. 

Method for assessing the effect of stress and moisture and temperature uses a peel joint test... 

Fig. 70. Dealing with unavoidable load in T-peel (a) brittle adhesive (b) ductile (tough) adhesive (c) strapped peel joint.

For the complete T-peel joint combining the individual contributions above leads to ... 

Single- and double-lap Joints, single- and double-strap Joints, peel Joints and cylindrical Joints... 

Bijlmmer PA (1978) In Allen KW (ed) Adhesion, 2nd edn. Applied Science Publishers, London, 45 p BorkU, Challis R (1995) NDE of the adhesive fillet size in a T-peel joint using ultrasonic Lamb waves and a linear network for data discrmination. Meas Sci Technol 6 72-84... 

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