Testing in Bending

As mentioned earlier, a bend test is more commonly used for ceramics than a tensile test. The advantages of conducting bend tests are  

is the modulus of rupture, M, is the moment in the beam at rupture, h is the height of the beam, and W is its width. These dimensions are 

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As part of an investigation, in 1952, of the failure of some electrical condensers, Treuting found whiskers of tin (about 2 x 10 6m diameter) growing from the condenser walls, as shown in Figure 5.11. When these whiskers were tested in bending, (a/2)[ 10] -> (a/6)[2ll] + (a/6)[121], it was found that these whiskers could be bent to a strain of 2 to 3% without plastic deformation. This meant that the yield strength was more than 2.5% of... 

The onset and extent of plastic deformation are often measured when the 0-8 behavior of a material is being determined. We showed some general o-e curves in Chapter 16. In Figure 17.1 0-8 curves obtained for crystals of KBr and MgO tested in bending are shown. From these curves we can identify several parameters that may already be familiar to you from the discussion of the mechanical properties of metals. 

Fracture strength, Op, is the stress at a fracture. Because ceramics are often tested in bending we do not see any reduction in cross-sectional area during the test as we often see in a tensile test with a metal. As a result we would not expect to see a maximum in the a-8 curve corresponding to the tensile strength or ultimate tensile strength. 

FIGURE 17.1 Stress-strain curves for KBr and MgO crystals tested in bending. 

The bipartite beams manufactured with the adhesive 027 could be tested in bending as planned. The adhesive layer was never the cause of failure. The beams all failed on the tensile face, usually by wood fracture at knots (Fig. 34). 

This test method can be used to measure the relative susceptibility of materials by "Determining the Threshold Stress Intensity Factor for Environment-Assisted Cracking EAC of Metallic Materials. Constant load specimens, both the fatigue precracked single edge beam SE(B) and precracked compact specimens are tested in bending. The SE(B) was originally referred to as the Cantilever Beam Test. 

Table 25 Results of stress corrosion cracking tests in bending samples after 12 months exposure time in the immersion zone (number of cracked samples in 3 parallel samples) [66]...

Rarely does composite testing include actual mechanical verification of the final product. The exceptions are filament-wound pressure vessels and some small composite structures, such as golf shafts, that are tested in bending. Many pressure vessels experience some deterioration due to the proof pressure test, which causes transverse stress in the hoop plies that ends up fracturing the matrix in the outer plies and results in a first-ply failure. This is one reason why another failure criterion is invoked for pressure vessels, namely the last-ply failure criterion. However, the failure in the outermost plies reduces the composite s resistance to water intrusion, and consequently the product may have to be recoated with a moisture-resistant paint to satisfy some environmental fears. 

Machining needed to make the sample may introduce surface flaws. Fracture of the samples normally takes place by the cracks nucleated at the surface flaws. Perfecf alignment is necessary because ceramics fail after about 0.1% strain. Because of fhese difficulties, ceramics are more commonly tested in bending rather than in tension. 

Each specimen was tested in bending 3 times. For this purpose special extenders were mounted on 2 halves of the specimen. The extenders had counterweights for a more exact load requirement by taking sample dead weights into account. This bend test method is reported in detail in Ref. (2). 

Since FRC composites, are, in practice, frequently subjected to flexural loading and are usually tested in bending, it is important to understand the processes... 

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