Лако-красочные материалы — производство

The true shear strength of an adhesive can be determined only if normal stresses are entirely absent. These conditions can be approached under special conditions, but not in single-lap specimens made with thin adherents, which are normally used in manufacturing and in most standard test specimens (Figure 7.3).

Due to the specimen geometry, the application of force is eccentric and the adherents rotate as a result of bending moment, as shown in Figure 7.4.

This rotation introduces adhesive peel stress in the vicinity of the joint edges. Due to the eccentric sample geometry, the stress distribution within a lap joint is not uniform in the load direction. Furthermore, the shear stress nonuniformity in the through-thickness direction becomes appreciable if the bond-line thickness exceeds certain values. In most cases the tensile stress in the adhesive layer controls joint failure and, as a consequence, the strength value obtained by single lap shear specimens is unrelated to — and an unreliable measure of — the true shear strength

of an adhesive. Other commonly used ASTM standard shear tests on adhesives include:

• ASTM E 229: Standard Method of Test for Shear Strength and Shear Modulus of Structural Adhesive.

• ASTM D1002: Standard Method of Test for Strength Properties of Adhesives in Shear by Tension Loadings (Metal-to-Metal).

• ASTM D 3165: Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies.

• ASTM D 5656: Thick-Adherend Metal Lap-Shear Joints for Determination of Stress-Strain Behaviour of Adhesives in Shear by Tension Loading.

The so-called ‘thick adherent specimen’ (TAS) as described in ISO 11003-2 or ASTM D3983 specifies a test method for determining the shear behavior of an adhesive in a single lap joint-bonded assembly when subjected to a tensile force.

The test is performed on specimens consisting of thick, rigid adherents with a short length of overlap, in order to obtain the most uniform distribution of shear stresses possible and to minimize other stress states which may initiate failure (Figure 7.5).

The design of the test specimen is based upon the theoretical analysis by Goland and Reissner relating stress concentrations (i. e. nonuniformity) in single-lap joints to the geometry of the joint and the mechanical properties of the materials involved. The controlling factor in the Goland and Reissner equations is a composite of essentially two ratios which can be manipulated to improve the stress uniformity in the joint, and thereby control the accuracy of measurement. Stress uniformity is improved by:

(i) increasing the adherent tensile modulus in relation to the shear modulus of the adhesive; and (ii) increasing the adherent and adhesive thickness while minimizing the overlap length. The test method is capable of providing shear modulus and shear strength values for adhesives with accuracy suitable for the use by design engineers in predicting the characteristics of building assemblies bonded with nonrigid adhesives. In general, the thick adherent lap-shear test has become a useful tool in research during studies of both short — and long-term load-deformation

properties of adhesives. However, it should be noted that pure shear strength cannot be monitored by this test method, because even in this joint geometry some tensile and compression stresses and stress concentrations are present at the edges. Hence, the estimation of shear strength using this test method will be conservative. If the measurement of pure shear strength is demanded, then Test Method E 229 should be applied [2].

Both, ISO 110003-1 andASTM E 229 relate to theso-called‘napkin ring test’, where specimens are loaded in torsion (T). An additional tensile stress (F) can be super­imposed to assess the mechanical adhesive properties under multiaxial load condi­tions (Figure 7.6).

Torsional shear forces are applied to the adhesive through a circular specimen which produces a peripherally uniform stress distribution. The maximum stress in the adhesive at failure represents the shear strength of the adhesive. By measuring the adhesive strain as a function of load, a stress-strain curve can be established. Production cleaning and bonding processes should be used when applicable. The basic material properties obtained from this test method can be used in the control of the quality of adhesives, in the theoretical equations for designing bonded joints, and in the evaluation of new adhesives.

7.2.3