Joints tensile tests

In a modification of the napkin ring shear test, the adherends are solid bars and the adhesive forms a penny-shaped slab similar to the butt joint tensile test. Such a test will give the relationship between torque and twist, but whereas in the napkin ring test it may be assumed that all the adhesive is at the same stress and strain, with a solid butt joint there is a radial variation of strain, but a non-linear variation of stress after yield. It is then necessary to use the Nadai correction (see Adams and Wake") to determine the true stress-strain curve of the adhesive. 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Tensile Tests. The tensile strength of an adhesive joint is seldom reported in the adhesive supplier s literature because pure tensile stress is not often encountered in actual production. An exception to this is the tensile test of the bonds between the skins and core of a... 

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. 

In the case of the dynamic tensile test, co-cured single lap joint specimens selected in this paper... 

The bond length of the specimens under cyclic tensile test was 20mm. Cyclic tensile tests were performed under the condition of stress ratio 7 = 0.1 and a loading frequency / = 5 Hz. Cyclic tensile loads applied to the co-cured single lap joint specimens were 30%, 40%, 50%, 60%, and 70% of the tensile load bearing capacity obtained from the static tensile load test. 

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

This test is carried out according to the standard DIN EN 1465 Adhesives - Determination of the tensile shear strength of high strength lap joints. The test piece has the dimensions according to Figure 10.2. 

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 work of Bussu and Irving on 6.35 mm (0.25 in.) thick 2024-T351 Al sheet is illustrative of hardness variations following FSW in 2024-T351 Al (Ref 12). In their work (Fig. 5.3), hardness is illustrated both as a function of distance from the joint interface and depth from the top surface. As shown, a typical W -shaped hardness curve is created. Due to the close relationship between hardness profiles and tensile test results, this composite of hardness results has implications for resultant mechanical properties. The studies in this work show four distinct hardness zones ... 

Cyclic tests provide the best representation of the conditions to which sealants are subjected in practice. They are very complex tests, however, and can be designed satisfactorily only if the material properties are well known from the results of tests using simpler loading patterns and if the rates are related to those of actual joints. Tensile extension at constant rate, stress relaxation under constant strain, and creep under constant stress are three of the simpler tests used to obtain the material properties of polymers. Tensile extension is not the simplest of the three tests (of the four basic variables only temperature can be kept constant), but it has been chosen because it is this type of loading that occurs in the sealant in a joint when the chance of failure is most probable. There is less likelihood of failure when the sealant is compressed in summer than when it is extended in winter. In addition, the tensile test is the least time-consuming and most laboratories are equipped for it. 

Transverse weld joint strength in all weldments exceeded 665 MPa at room temperature. Tensile tests of the two types of steel showed different behaviors. For the nickel steels, 25°C specimens fractured in the base metal, and all cryogenic specimens fractured in the weld metal. This performance is typical for nickel steel weldments prepared with undermatching low-temperature strength Inconel filler metals. In the stainless weldment, the test performance was reversed, with weld metal fractures at 25°C and base metal fractures at -160 and -196°C. 

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