Adherend thickness, effect

S Adherend thickness effect on bonded joint strength for different joint geometries (adapted from Hart-Smith, 1974a). 

In a similar manner, differences in adhesive thickness also complicate the comparison between adhesive joints. Say, for example, a person was comparing the lap joint strength of two adhesives in which the amount of overlap and the adherends were identical, but the thickness of the adhesives differed. If the first adhesive thickness was 1.3 mm and the second was 6.4 mm. the adhesive thickness effect would likely swamp any differences due to the adhesive type. Similar effects of thickness are noted for tensile tests. Not only does the relative thickness of the adhesive affect the load at failure, but it may also influence the point from which cracks are likely to grow. 

The effective adhesive transverse modulus Ea takes into account the fact that adhesive properties are different in bulk and film forms. In a bonded joint the adhesive layer is a film. When loaded using through thickness tensile or compressive loads the adherends restrict the deformations of the adhesive, unlike when the bulk form of the adhesive is being similarly loaded. These restrictions in deformations tend to increase the adhesive modulus measured between the relatively stiff adherends. This effective modulus is difficult to establish theoretically. 

The most common adhesive bond is the overlap or lap type. There are many variations of the overlap joint but the simplest type is the single-lap joint. This configuration has been the subject of much work (see Section III) so the geometric factors which effect joint design, such as bond area or overlap length and adherend thickness are well known. 

To eonfirm the theoretical predictions that the 7-stress affects the directional stability of cracks, DCB specimens with adhesive C and adherend thicknesses of 4.8 mm were prepared. The surface preparation for the adherends was simply an acetone wipe, and various levels of residual stress were achieved among the specimens using the stretching method. Specimens were then tested quasi-statically according to the procedure discussed earlier. The resulting fracture surfaces of the specimens were carefully examined and three representative specimens were selected as shown in Fig. 9, from which, the effect of the 7-stress on the directional stability of cracks can be inferred. The initial residual stress... 

Fig. 12. The effect of adherend thickness on the 7-stress level and the directional stability of cracks. The crack tends to be more directionally unstable when the thickness of adherend decreases.

As outlined in Section 1.2, mode mixity can significantly alter the locus of failure, even driving debonds away from weaker regions of the joint. To investigate the effect of fracture mode mixity on the locus of failure in adhesive bonds, quasistatic DCB and ENF tests were conducted. Specimens were made of adhesive C with acetone wipe surface preparation and they were all as-produced, therefore the T-stresses were all negative. For the ENF tests, specimens were symmetric and for the DCB tests, both symmetric and asymmetric specimens were used with three different adherend thickness ratios, i.e. h/H = 0.5, 0.75, and 1. Finite element analysis was used to quantify the mode mixity and the results are shown in Fig. 14 adherend thickness ratios of h/H = 0.5, 0.75, and 1 correspond to fracture mode mixity of = 22°, 10°, and 0° or jj = 14%, 3%, and 0%, respectively. 

Fig. 20 shows the failure surfaces of three typical specimens selected from each specimen group of different adherend thickness ratio. The plastic deformation in the adherends introduced before the tests in order to alter the 7-stress was 1.3% for all the three specimens. As a result, the 7-stresses for all the three specimens are positive according the FEA results in Section 2.3.1 and Eq. 7, and their values are shown in Fig. 20. Due to the positive 7-stress level (35 MPa), the crack trajectory in specimen a, in which the fracture is mode I since the specimen is symmetric h/H = 1), is alternating, highly directionally unstable. In specimen b, the 7-stress has increased slightly to 38 MPa due to the low-level fracture mode mixity with G /G = 3% introduced by the asymmetric adherends h/H = 0.75). However, the crack trajectory is predominantly directionally stable except in limited locations where alternating cracks were observed. This effect...

All curves in Fig. 28 are monotonically decreasing with increasing adherend thickness, indicating an effect of adherend bending on the directional stability of cracks. For instance, the curve with gp = 1.3% and dashed line when H is less than 4 mm and is lower than the dashed line when H is greater than 4 mm. Since the dashed line represents the threshold of the directional stability of crack propagation, this curve indicates that for specimens with gp = 1.3% and oq = 13 MPa, the directional stability of cracks varies with the thickness of the adherends. This prediction is also consistent with the experimental results discussed in Section 2.3.1 where DCB specimens with adherend thicknesses H of mm and 3.2 mm, respectively, were tested to... 

Fig. 20. Effect of adherend thickness on (apparent) lap-shear strength of adhesive bonds.

The reason why properly designed bonded joints do not suffer from mechanical fatigue failures is that the most critical conditions are not developed where the adhesive is protected by the adherends. This can be understood by characterizing the minimum and maximum adhesive shear strains as a function first of bonded overlap and secondly as a function of adherend thickness, accounting for the effect of the environment in each case. This is illustrated in Tig. 44.17, for room temperature. 

Fig. 1.6. (a) Effect of adherend thickness on the mean failure shear stress for single lap joints abraded surface / = 15 mm, b = 25 mm. (b) Effect of adhesive thickness on the mean failure shear stress for single lap joints abraded surface / = 15 mm, = 1 55 mm. 

Fig. 1.6 — contd. (c) Effect of lap length on the mean failure shear stress for a single lap joint for a set of adhesive thicknesses, (d) Effects of surface roughness on the strength ofsingle and double lapjoints.adherend thickness = 1 925 mm adhesive thickness =150 mm. 

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

Since the effectiveness of the water adsorbed on the adherend surfaces only suffices for the polymerization of adhesive layers with limited thickness, such layers should not exceed 0.2 mm. Furthermore, the relative humidity of the processing rooms should range between approximately 40 and 70%. 

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