Maximum tensile joint loadings

Table 13 Calculated and measured maximum tensile joint loadings.

The ASTM D 897 tensile button test is widely used to measure the tensile strength of a butt joint made with cylindrical specimens (Fig. 20.3). The tensile strength of this bond is defined as the maximum tensile load per unit area required to break the bond (measured in pounds per squre inch). The cross-sectional bond area is usually specified to be equal to 1 in.2. The specimen is loaded by means of two grips that are designed to keep the loads axially in line. The tensile test specimen requires considerable machining to ensure parallel surfaces. 

Hart-Smith (reference 5.26) has presented the situation where the joint is loaded by tensile or compressive and in-plane shear loadings simultaneously. Joint failure will occur when the bondline displacement resultant caused by these actions exceeds the capacity of the adhesive. If the tensile (compressive) loading develops a maximum bondline displacement of ta(yt,c)max and the shear loading induces an orthogonal displacement of ta(ys)max at the same end of the joint, the displacement at failure is ... 

An adhesive bond, joint, or design is considered in tension when the loads or forces are applied perpendicular to the adhesive layer, as illustrated in Figure 1. The value of the tensile strength, a-c is determined by the maximum tensile load, at fracture and by the cross-sectional area. A, of the bond (Eq. 1). 

Let us also consider the titanium adherend joint with FM350NA adhesive, because it showed the widest variation of applied load. Fig. 12 shows the maximum principal stresses, averaged across the adhesive thickness, for joints loaded to the failure load normalised by dividing by the applied tensile stress. 

Modem adhesives usually exhibit a degree of plasticity before failure. Typically, we might find 20% shear strain at a shear stress, t, of 40 MPa. One simple predictive tool is to say that the absolute maximum strength for a lap joint is when the whole of the adhesive layer is at the shear yield strength. Thus, the maximum tensile load P which could be carried by a lap Joint of width b and length / is given by... 

The influence of the applied load on a 25 4 mm (1 in) wide lap joint is shown in Fig. 9. The maximum tensile stresses in the adhesive per unit load of both the original and corrected solutions, and the maximum adhesive shear stress per unit load are shown. The maximum stresses occur at the ends of the overlap. It can be seen that the maximum stress per unit load in the joint decreases as the load is increased. The greatest decrease occurs in the value of the corrected... 

A significant aspect of hip joint biomechanics is that the stmctural components are not normally subjected to constant loads. Rather, this joint is subject to unique compressive, torsion, tensile, and shear stress, sometimes simultaneously. Maximum loading occurs when the heel strikes down and the toe pushes off in walking. When an implant is in place its abiUty to withstand this repetitive loading is called its fatigue strength. If an implant is placed properly, its load is shared in an anatomically correct fashion with the bone. 

Shear tests are very common because samples are simple to construct and closely dupUcate the geometry and service conditions for many structural adhesives. As with tensile tests, the stress distribution is not uniform and, while it is often conventional to give the failure shear stress as the load divided by the bonding area (Table 11.1), the maximum stress at the bond line may be considerably higher than the average stress. The stress in the adhesive may also differ from pure shear. Depending on such factors as adhesive thickness and adherend stiffness, the failure of the adhesive shear joint can be dominated by either shear or tensioa ... 

Finite-element methods have also been used to evaluate new test equipment to measure shear strength under impact loads. In the equipment, two rectangular plates, bonded opposite faces of a vertical hexagonal prismatic rod, bear on a firm surface. The top of the central rod is subjected to an impact load. To prove the validity of the method, the maximum shear stress was compared with the impact shear strength, which was measured using a cylindrical butt joint subjected to impact torsional loads (see Tensile tests). 

Two cracking loads have been identified. One is associated with flexural cracking of the beam and the other with a diagonal crack in the joint which occurs when the maximum principal stress exceeds the tensile strength of concrete in the joint itself. 

Another tensile test, ASTM D2095-72, involves the testing of bar and rod butt joint specimens. ASTM 2094-69 describes the preparation of these specimens (see Figure 3). This test and the samples can be used with substrates comprised of metals, plastics, or reinforced plastics. Loads are applied through fixtures connected to the samples by dowel pins. The standard test rate is 2400 to 2800 lb per square inch of bond area. The maximum load at failure is used to calculate the tensile strength, the same as for the pi-tensile specimens. 

The lap joint test is the most commonly used adhesive test, likely because test specimens are simple to construct and resemble the geometry of many practical joints. While this test is commonly referred to as the lap shear test, this is generally a misnomer, since failure is often more closely related to the induced tensile stresses than to the shear stresses. Further, it is conventional to report the apparent adhesive strength as the load at failure divided by the area of overlap, even though the maximum stress will almost always differ markedly from this average value. Thus, while the lap joint test is commonly performed, the results from this test must be interpreted with caution. Issues associated with the single-lap joint test are discussed in ASTM D4896. 

Adams and Peppiatt [60] have considered the problem of the in-plane transverse stresses and to ascertain the magnitude of such stresses they have used experimental models and analytical and finite-element analyses solutions of the Volkersen theory, but in three dimensions. They demonstrated that Poisson s ratio strains generated in the substrates cause shear stresses, T13, in the adhesive layer and tensile stresses, 0-33, in the substrate acting transverse to the direction of the applied load, but in the plane of the joint. For metal-to-metal joints the transverse shear stress, has a maximum value of about one-third of the maximum longitudinal shear stress, ri2(max), and this occurs at the corners of the overlap. This, therefore, enhances the shear stress concentration which exists at this point due to the effects described above. Bonding substrates of dissimilar stiffness produces greater stress concentration in the adhesive than when similar substrates are employed. 

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