4-point bend test
All bending tests are based on beam theory, which relates the stress, strain and deformation of a sample to its dimensions and applied load. A simple form of bending is called a pure bend, and it's what we aim for in a four-point bend test. Often this form of bend testing is used to determine the failure strain of interconnects on circuit boards. Four-point bending can also be used to determine material parameters, such as flexural modulus, and composite materials for circuit boards.
Three-point bending test
Three-point bending is recommended to be understood by looking at the strain in a simple cantilever where one end is fixed and the other deflects with an applied load. In three-point bending, the maximum stress occurs at the center of the beam, under the loading anvil. The stress at this time is the same as that at the fixed end of the cantilever, and the length is equal to the distance between the support and the central anvil. Increasing the span of the anvil increases the maximum stress for a given applied load, and in this way, larger stresses can be produced for moderate applied loads.
Under load, the yellow square deforms so that the bottom stretches (stretch) and the top shrinks (compression). The centerline of the square remains the same; this is the neutral plane, which is at the middle thickness of a simple rectangular beam.
