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A specimen loaded in bending when rigidly clamped on the ends by specimen grips will undergo an axial force due to the conflict between the grips' resistance to move and the specimen ends' intention to move inwards. By mounting the grips on flexures and allowing them to move with the specimen ends while still applying a bending moment, pure bending on the specimen will result. A constant moment along the length of the specimen can be obtained by applying equal and opposite force couples to the ends of the specimen via the grips.
This paper examines the effects of specimen end conditions and alignment on test results obtained from three- and four-point reversing bend tests. Various end conditions can exist, such as fixed-fixed, fixed-free, free-free, and a special case in which the end condition is a combination of fixed and free. Each of these end conditions imposed on a specimen in bending is evaluated as to its contribution to axial forces and its ability to provide a true bending moment on the specimen. The effects of bending out of plane with respect to the specimen axis are also discussed.
This paper also describes an apparatus for testing specimens in three- and four-point reverse bending in which the axial force in the specimen is reduced.
three- and four-point bend test, reverse bend test, specimen alignment, flexures, boundary conditions, end constraints
Product engineer, MTS Systems Corp., Minneapolis, MN