Adhesive joints tensile tests

In technical data sheets, the strength of an adhesive is generally stated in terms of its tensile lap-shear strength which is determined by performing tests on a singlelap adhesive joint. The test piece is subjected to a shearing stress by applying a tensile load axially to the two lapped substrates (Fig. 28). 

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 ... 

A sealant s adhesion is commonly studied by 180 degree peel tests such as ASTM C794 or by tensile/adhesion joints tests such as ASTM C719. The adhesion test protocol should simulate actual field conditions as closely as possible. Sealants often have good adhesion to dry substrates, but this adhesion may be quickly destroyed by water. Because most sealants are exposed to water over their lifetime, adhesion testing should include exposure to water for some length of time. ASTM C719 is one of the better tests to determine a sealant s adhesion durability as it exposes sealants to seven days of water immersion. 

Structural adhesives usually require curing by the application of heat, the addition of a catalyst, the addition of pressure, or a combination of the three. The strength developed in the adhesive joint at various times during the curing process may be determined by lap shear tensile specimens. This test is commonly used to determine when an adhesive or sealant is fully cured or when the system reaches a handling strength so that the assembled product can be moved with moderate care. 

ASTM D 1144 provides a recommended practice for determining the rate of bond strength development for either tensile or lap shear specimens. However, peel and can-teliever tests can also be used effectively. Measured bond strength values of partially cured test specimens are compared with those of a reference (i.e., fully cured adhesive joint) to assess the extent of cure. This method may suit some applications, but it is limited in accuracy because it does not directly measure the degree of cure in the adhesive, and the effect on the joint design and substrates may override the effect of cure development. 

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... 

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 tensile strength and the elongation-at-break of the polymer films were measured according to EN ISO 527 [22], Mechanical tests were performed on the adhesive joints as described in the standard [8] by adhesively bonding two beech wood specimens using a pneumatic press. In particular, density, moisture content, dimensions, thickness and fabrication conditions are specified in the specific standards. 

Despite the fact that adhesive joints are rarely designed to be loaded directly in tensile mode, tensile tests are eommon for evaluating adhesives. The axially-loaded butt (or poker chip ) joint geometry, as recommended by ASTM D897(52), is depicted in Fig. 4.11. 

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 ... 

Tensile tests and so on. It can be a peel strength or a Fracture mechanics parameter. These measures are sometimes referred to as the practical adhesion of a particular joint they more or less satisfactorily answer the question, How strong is the joint ... 

Structural adhesives such as epoxy resins can be treated as any rigid polymer and samples can be machined from cast sheets to produce test-pieces. These can then be used to measure typical tensile properties such as failure stress and strain. Using accurate exten-sometry, it is possible to characterize completely the uniaxial properties of an adhesive. The Creep of adhesive joints is especially important for structural adhesives maintained at high temperature. It is possible to determine the creep resistance of such materials by applying suitable loads at an appropriate temperature to samples of the adhesive, and to record the deformation with time. From such data, it will soon be evident if the adhesive is suitable for use or if it will cause a joint to deform with time. It is important to remember that humidity is likely to affect the properties of the adhesive, and in a long-term creep experiment, the humidity could cause premature failure. 

The tensile test piece can take several forms one simple configuration consists of two right-circular cylinders whose ends are then bonded together. Such a joint is loaded to failure at right angles to the plane of the adhesive and the failure stress is determined from the loaded area and failure load. 

This chapter will discuss the testing, analysis, and design of structural adhesive joints. Adhesive bond test techniques to be considered include tensile, shear, peel, impact, creep, and fatigue. Some considerations will also be given to the effect of environment and test rate. A continuum approach to the analysis of adhesive joints will discuss tensile, shear, and peel stresses which arise in various joint geometries. Classical theories by Volkersen, Goland and Reissner, and others will be included. References to finite element analysis will be made where appropriate throughout the chapter. 

As with tensile tests, it is sometimes necessary to test dissimilar materials in shear or materials which cannot be easily fabricated into adherends. For these cases, ASTM D3164-73 describes the testing of a lap shear sandwich joint. The test specimens consist of two metal adherends, typically 0.064 inch thick aluminum, the structural adhesive, and a 0.010-inch thin film of plastic. The sandwich joint is shown in Figure 8. 

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