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Bend Test

Bend tests deform the test material at the midpoint causing a concave surface or a bend to form without the occurrence of fracture and are typically performed to determine the ductility or resistance to fracture of that material. Unlike in a flexure test the goal is not to load the material until failure but rather to deform the sample into a specific shape. The test sample is loaded in a way that creates a concave surface at the midpoint with a specified radius of curvature according to the standard in relation to which the test is performed. Bending tests are as popular as tensile test, compression test, and fatigue tests.

Purpose of Bend Testing:

Bend testing a material allows for the determination of that materials ductility, bend strength, fracture strength and resistance to fracture. These characteristics can be used to determine whether a material will fail under pressure and are especially important in any construction process involving ductile materials loaded with bending forces. If a material begins to fracture or completely fractures during a three or four point bend test it is valid to assume that the material will fail under a similar in any application, which may lead to catastrophic failure.

Four Common Bend Tests:

There are four common types of bend tests. In a guided bend test, the sample is placed horizontally across two supports and then a force applied to the top of the midpoint deforming the sample into a “U” shape. In a semi-guided bend test, the specimen’s midpoint is bent to a specific angle or inside radius. In a free bend test, the ends of the sample are pushed together, but no force is applied to the bend itself. Finally, a common fracture toughness test (ASTM E399) consists of a sample with a precracked starter crack on the bottom side of the midpoint which is loaded into a 3-point bend fixture so that the midpoint force is applied on the opposite face from the fracture.

Types of Materials Used in Bending Applications:

Generally a bending test is performed on metals or metallic materials but can also be applied to any substance that can experience plastic deformation, such as polymers and plastics. These materials can take any feasible shape but when used in a bend test most commonly appear in sheets, strips, bars, shells, and pipes. Bend test machines are normally used on materials that have an acceptably high ductility.

Common Use of this test:

One of the more popular uses of bend testing is in the area of welds. The purpose of bend testing welds is to make sure that the weld has properly fused to the parent metal and that the weld itself does not contain any defects that may cause it to fail when it experiences bending stresses. The sample weld is deformed using a guided bend test so that it forms a “U” subjecting the material on the outer surface to a tensile force and the material on the inside to a compressive force. If the weld holds and shows no sign of fracture it has passed the test and is deemed an acceptable weld.

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