Off-axis Tensile Test

The [10°] off axis tension specimen shown in Fig 3.23 is another simple specimen similar in geometry to that of the [ 45 ]s tensile test. This test uses a unidirectional laminate with fibers oriented at 10° to the loading direction and the biaxial stress state (i.e. longitudinal, transverse and in-plane shear stresses on the 10° plane) occurs when it is subjected to a uniaxial tension. When this specimen fails under tension, the in-plane shear stress, which is almost uniform through the thickness, is near its critical value and gives the shear strength of the unidirectional fiber composites based on a procedure (Chamis and Sinclair, 1977) similar to the [ 45°]s tensile test. 

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Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

Fig. 3.23. Schematic drawing of specimen for [10°] off-axis tensile test. After Chamis and Sinclair (1976).

It has been mentioned earlier that Soden and McLeish employed the classical tensor calculus to compute the effects of axis rotation and thereby to deduce the shear strength of balsa wood from the results of off-axis tensile tests. 

Using experimental data from three different unidirectional composites, a comparison is made between the theory and measured values from off-axis tensile tests. Table 11.1 presents the in-plane elastic properties for unidirectional composites [14]. 

The theory compares quite well with experimental data, as should be expected. The experimental values for the off-axis shear modulus were computed from the respective G12 measured for each off-axis angle. The deviations found are related to experimental errors, which are much more critical and pronounced in the case of the off-axis tensile tests. However, the measurement of the in-plane shear modulus requires the use of off-axis specimens. Although consistent values of in-plane shear modulus may be obtained, specimen geometry must be optimized to reduce the errors provoked by the end-constraint effect [34]. 

The value of F,2 then depends on the various engineering strengths plus the biaxial tensile failure stress, a. Tsai and Wu also discuss the use of off-axis uniaxial tests to determine the interaction strengths such as F.,2 [2-26]. 

An important implication of the presence of the shear-extension coupling coefficient is that off-axis (non-principal material direction) tensile loadings for composite materials result in shear deformation in addition to the usual axial extension. This subject is investigated further in Section 2.8. At this point, recognize that Equation (2.97) is a quantification of the foregoing implication for tensile tests and of the qualitative observations made in Section 1.2. 

Tensile test specimens have been fabricated based on ASTM D 3039 for off-axis angles. The materials chosen for fiber and matrix were E-Glass UD-weaves and ML-506 Epoxy resin, respectively. UD-weave fiber lay-up and test specimen geometry are shown in Figure 1 and Figure 2 ... 

The determination of the in-plane elastic properties involves four values El, E2, V12 and G12. The measurement of longitudinal and transverse moduli and Poisson s ratio Ey, E2, V12) is made using tensile coupons oriented at 0° and 90°. These tests are quite straightforward to perform and to analyse when measuring the elastic properties. For the in-plane shear modulus (G12) off-axis, 45° and losipescu test coupons are used, but these tests are not so straightforward to analyse since a complex stress/strain state is induced in the coupons further details on this subject are well documented in the literature [33-37]. 

Other less well-known types of nonlinearities include interaction and intermode . In the former, stress-strain response for a fundamental load component (e.g. shear) in a multi-axial stress state is not equivalent to the stress-strain response in simple one component load test (e.g. simple shear). For example. Fig. 10.3 shows that the stress-strain curve under pure shear loading of a composite specimen varies considerably from the shear stress-strain curve obtained from an off-axis specimen. In this type of test, a unidirectional laminate is tested in uniaxial tension where the fiber axis runs 15° to the tensile loading axis. A 90° strain gage rosette is applied to the specimen oriented to the fiber direction and normal to the fiber direction and thus obtain the strain components in the fiber coordinate system. Using simple coordinate transformations, the shear response of the unidirectional composite can be found (Daniel, 1993, Hyer, 1998). At small strains in the linear range, the shear response from the two tests coincide. 

Procedures for this test method are covered in ISO 4624 Paint and Varnish Pull-off Test for adhesion. In preparation for this test a stud, normally made of steel, is glued with the coating and is subjected to axial tension until detachment of the paint film takes place. The result is a maximum tensile stress that is possible at the interface (see Figure 15.6). The maximum shear stress is obtained when a torque is applied about the axis of the stud. 

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