Bend-beam specimens four-point loaded

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. 

For linear elastic materials, deflection at the load-point of a four-point bending beam is given by two different contributions, accounting for both bending and shear deformation. In the case of notched specimens, a third term accounting for crack length arises. The resulting analytical expression for the specimen compliance C is as follows ... 

The bending is achieved by the movement of the centre load point(s) in vertical direction perpendicular to the longitudinal axis of the specimen. The two end points of the beam remain fixed (clamped). The applied periodical loading (force) is sinusoidal to obtain the required strain, e, amplitude of 50 3 microstrain. A schematic representation of the four-point bending test is given in Figure 7.6. 

This test method determines the asphalt s behaviour under repetitive fatigue loading in a four-point bending test equipment in which the inner and outer clamps are symmetrically placed and a slender rectangular-shaped specimen (prismatic beam) is used. The prismatic beam is subjected to four-point periodic sinusoidal bending with free rotation and translation at all load and reaction points. 

Bending is achieved by applying load to two points of the prismatic beam at L/3 and L/3 distance from the clamped ends of the specimen. The layout of the test device is similar to the one shown in Section 7.4.4 (Figure 7.6) for four-point bending. The periodic loading is sinusoidal and it can cause a constant moment and hence a constant strain between the inner clamps (in the middle of the specimen). 

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