Bend Test

The bend test is a popular test method that is found in many welding standards and specifications throughout the world due the simplicity of the test method and equipment required. The history of the bend test dates back to the early years of wrought iron and steel testing before the advent of modern testing equipment. Bend specimens are prepared typically from a test plate rather than from an expensive finished product and are used to evaluate the ductility and soundness of welded joints.

There are two different bend testing methods:

Guided bend test

Free bend test

Guided Bend Test

The guided bend test is commonly used in welder and procedure qualification tests to determine the ability of the welder to make sound welds. The test is performed by bending prepared specimens of a specific dimension (usually specified in the relevant code) in a special jig. The dimensions of the jig will vary with specimen thickness and material

It is important to note that the strain applied to the test specimen depends on the spacing of the rollers and the radius of the member. The strain on the outside fiber of the bend specimen can be approximated from the following formula:

e = 100 t / (2R + t)

Where

e = strain, %

t = bend test specimen thickness, mm. (in)

R = inside bend radius, mm. (in)

When performing qualification tests the specimen thickness and bend radius are chosen according to the ductility of the metal being tested. An elongation in the outside fiber of 20 percent can be easily achieved on sound mild steel welds. Bend tests will consistently fail if the specimens contain weld discontinuities that are on are near the surface of the material. After bending, the welds are examined for the presence of discontinuities. Many welding standards and specifications consider that a bend specimen has failed if on examination of the convex surface after bending there is a crack or open defect exceeding 3mm (1/8 in.).

There are three types of guided bend tests:

Root bends tests

Face bend tests

Side bend tests

A root bend test puts the weld root in tension while a face bend test does the same for the weld face. Both types are generally used on material thickness of 10 mm (3/8 in.) or less. When the material thickness is greater than 10 mm (3/8 in.) side bend test specimens are usually chosen due to the difficulty in bending the thicker material.

Side bend test specimens are typically 10mm (3/8 in.) thick. This test strains the entire weld cross section, and thus is especially useful for exposing defects near the mid-thickness that might not contribute to failure in a face or root bend test.

Bend Test Limitations

The same weakness that tensile tests suffer from also affects bend tests. Non uniform properties along the length of the specimen can cause non uniform bending. Bend testing is sensitive to the relative strengths of the weld metal, the heat-affected zone, and the base metal.

Many problems can develop in transverse bend tests such as over matching weld strength may prevent the weld zone from conforming exactly to the bending die radius, and thus may force the base metal to deform to a smaller radius. This will not produce the desired elongation in the weld. Alternatively, with an under matching weld strength, the specimen may bend in the weld to a radius smaller than the bending die. In this case failure may result when the weld metal ductility is exceeded, and not because the weld metal contained a defect.

These problems with weld strength mismatch can be avoided by using longitudinal bend specimens which have the bend axis perpendicular to the weld axis. In this case all zones of the welded joint (weld, heat affected zone, and base metal) are strained equally and simultaneously. This test is usually used for the evaluation of joints in dissimilar metals.

Weld discontinuities in longitudinal bend tests that are oriented parallel to the weld axis such as incomplete fusion, inadequate joint penetration, or undercut are only moderately strained and may not cause failure.