Pure bend testing

Rotating Beam Fatigue Test for Steel Cords. The purpose of this test method is to evaluate steel cord for pure bending fatigue (121). The test sample consists of a 3-mm diameter mbber embedded with steel cord. Different bending stress levels are appHed and the time to failure is recorded. The test stops at 1.44 million cycles. The fatigue limit is calculated from S—N (stress—number of cycles) curve. 

Shear strength is measured via a simple single overlap shear specimen of standard dimensions (Fig. 9). In contrast to its simple appearance, the forces in a thin-adherend shear specimen can be quite complex due to the inherent offset loading of the specimen and subsequent bending in the substrates. The single overlap shear test is anything but a pure shear test, but the configuration is easy to manufacture, simple to test and is firmly entrenched in the industry as a primary examination technique for materials qualifications, inspection and process control. 

Flexural strength is determined using beam-shaped specimens that are supported longways between two rollers. The load is then applied by either one or two rollers. These variants are called the three-point bend test and the four-point bend test, respectively. The stresses set up in the beam are complex and include compressive, shear and tensile forces. However, at the convex surface of the beam, where maximum tension exists, the material is in a state of pure tension (Berenbaum Brodie, 1959). The disadvantage of the method appears to be one of sensitivity to the condition of the surface, which is not surprising since the maximum tensile forces occur in the convex surface layer. 

Figure 12.2 Schematic representations of (a) pure shear test (b) plane strain compression test and (c) three-point bending test.

Prismatic beam samples (30 x 60 x 700 mm) of RubCon were tested for a cross, so-called pure bend, by two equal concentrated forces symmetrically located at an average third of the samples span and simultaneous temperature influence. Foading was carried out step by step to the attainment of the normal stress in the middle span of 5.7, 6.1, and 6.5 MPa. 

Beams were loaded using two symmetrically located concentrated loads. Such loading draft is appropriate for a so-called pure bend and is optimum for examining the influence of transverse forces in zones between supports and loads. The value of the transverse forces in these zones is constant, and bending moments are increased from zero on supports up to a maximum in cross-sections of a beam under loads. Beam samples were tested on a laboratory press. Loading was done with speeds of500 kg/min up to... 

Images of the sample pieces were taken before and after the test. Figure 7 displays failure plane location in each of the samples. In batch A, aU the samples collapsed between the upper two loading points. In batch B one specimen and in batch C almost three specimen failed out of the pure bending area. This can be dne to the reason that for weaker samples the uniformity of the load becomes more and more important in flexural tests. On the other side, for materials with low Weibull modulus it is normal to have a number of samples(around 10% of them) fracturing outside of the inner gage area. 

A cadaveric study comparing the range of motion in the L5-S1 motion segment in five human spines, before and after implantation with the PRODISC, has indicated that implantation of fhis device did not significantly affect the ROM (Lipman et al. 2003). Specimens were tested in an apparatus that applied pure bending moments. The specimens were cycled in torque to a maximum of 10 Nm in flexion-extension, lateral bending, and torsion with 600 N and 1200 N compressive loads. The ROM at 8 Nm after the fifth cycle of loading was documented and used for comparison between intact and implanted specimens. Summary data is shown in Table 10.3. 

The loading arrangement for structural tests should, where possible, be Kept as simple as possible to aid interpretation of the test results. For example components should be loaded in pure bending or in axial compression. 

To obtain as much information as possible from the test, component parts should be tested whenever possible. By applying simple loading, for example pure bending, the complex loading patterns that occur in the real structure can be avoided. In addition, the uncertainties induced by connection behaviour are eliminated. 

A four point bend test will reduce the likelihood of a compression face failure and will provide pure bending between the center loading points. 

The diagrams, as in Figure 10.18, are characteristic for a test when specimens are loaded by one or two concentrated and symmetrical forces. These two cases are four-point or third-point with two symmetrical loads and three-point or centre-point with one force in the middle of the span. The first system is considered as better reflecting the bending conditions in the central part of the element, where pure bending without shearing is created. The second system is simpler to execute, but in fact the maximum effort is... 

Brandt, A. M., Stroeven, R, Dalhuisen, D., Donkei L. (1989) Fracture mechanics tests of fibre reinforced concrete beams in pure bending, Report 25.1-89-7/C4, Faculty of Civil Eng., Delft University of Technology. 

In-plane shear properties In simple shear loading, two parallel faces move in opposite parallel directions. In pure shear, the plane is subjected to tensile forces on one axis and compressive forces of equal magnitude on the orthogonal axis. Many different techniques have been proposed for the determination of the in-plane and through-plane shear properties with variations appropriate to composite plates, rods and tubes [1]. Inter-laminar shear strength The inter-laminar shear strength (ILSS) test is a three-point bend test at very... 

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