Mechanical Tests

The Charpy impact test is a common, cost-effective and semi-quantitative test method used to determine a material’s relative impact toughness at a specific temperature. Specimens are first machined to a specific dimension and surface finish before being notched and placed in a liquid bath till the specimens have reached the test temperature. The specimen is then loaded into the test machine and struck with a weighted pendulum released from a set height within 5 seconds of leaving the liquid bath. The higher the absorbed energy of the specimen, the greater the relative impact toughness of the material. The test is usually requested when qualifying welding procedures, evaluating heat treatment processes or verifying material type and grade, typically on welds or materials intended for low temperature (0 degrees Celsius or lower) applications such as ship hulls or marine structures.

The bend test is to verify the ductility of the material. If the material does not fracture after bending, the piece is visually checked for cracking or other surface irregularities. The specimen shall be placed on the die member of the jig with the weld at midspan. Face bend specimens shall be placed with the face of the weld directed toward the gap. Root bend and fillet weld soundness specimens shall be placed with the root of the weld directed toward the gap. Side bend specimens shall be placed with that side showing the greater discontinuity, if any, directed toward the gap. The plunger shall force the specimen into the die until the specimen becomes U-shaped. The weld and HAZs shall be centered and completely within the bent portion of the specimen after testing. When using the wraparound jig, the specimen shall be firmly clamped on one end so that there is no sliding of the specimen during the bending operation. The weld and HAZs shall be completely in the bent portion of the specimen after testing. Test specimens shall be removed from the jig when the outer roll has been moved 180° from the starting point.

The Fracture or Nick Break Test is used to determine the quality of a weld, and involves breaking open the weld length-wise, and then visually inspecting the exposed weld interior for any internal imperfections such as lack of sidewall fusion, lack of inter-run fusion and porosity. The test can be carried out for both Butt Welds and Fillet Welds Test coupons. For Butt Welds, a groove is machined along the weld centreline to provide a convenient fracture path before the specimen split open via a hydraulic press or a conventional hammer. For Fillet Welds, use of a hydraulic press to apply sufficient force to the opposite side of the weld fillet is typically sufficient to fracture the weld for inspection. The test is often requested when inspection of the entire weld length is required, as opposed to inspecting only a cross sectional area of the welded specimen. The test typically forms part of a suite of tests used to evaluate a welder’s skills for Welder Qualification.

Flange the end of the tube symmetrically by means of a hardened conical steel mandrel having an included angle of approximately 90o. The mandrel is to be forced into the test specimen until the drifted portion has formed a flange perpendicular to the axis of the test specimen.
The percentage increase in the outside diameter of the end of the test specimen is not less than the value given in the specification requirements.

Expansion the end of the tube symmetrically by means of a hardened conical steel mandrel having a total included angle of 60o. The mandrel is to be forced into the test specimen until the percentage increase in the outside diameter of the end of the test specimen is not less than the value given in the specification requirements.

The tube shall be flattened between two plain parallel and rigid platens which extend over both the full length and the width after flattening of the test specimen. Flattening is to be continued until the distance between the platens, measured under load, is not greater than the requirements. The flattening shall be continued until the specimen breaks or the opposite walls of the pipe meet.

Shear Test is used to determine the shear strength of materials. It is generally performed on specimens such as bolts, screws and rivets. An appropriate shear set up is used depending on the specimen and is then placed in the tensile machine. Lateral shear force is applied to the specimen until failure occurs.

Proof Load Tests are typically performed on finished components such as bolts and nuts as part of a quality control process to verify if the batch of components are able to hold their rated design load without failure. To perform this test, the specimens are loaded individually in the tensile machine and, depending on the relevant product specifications, a specified load is applied to it for a set time. If the product breaks, yields or show signs of cracking, the product is considered to have failed the test. NDT tests are often used in conjunction with the Proof Load Test to ensure any fine cracks are detected before and after testing.

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A hardness test is a means of determining resistance to penetration and is occasionally employed to obtain a quick approximation of tensile strength. There are 3 types of hardness tests:

  • Vickers
  • Rockwell
  • Brinell

The tension test related to the mechanical testing of steel products subjects a machined or full-section specimen of the material under examination to a measured load sufficient to cause rupture.

  • Transverse Weld Tensile
  • All-Weld Tensile
  • Through Thickness Tensile
  • Tensile Test at Elevated Temperature (up to 1200ºC)
  • Reinforcement Bar / Wire Mesh

A Fracture Mechanics Toughness Test is a test used to evaluate a material’s true fracture toughness property, which is the material’s ability to resist stable crack extension under load. It is commonly known as a CTOD Test, due to the Crack Tip Opening Displacement (CTOD) value that derived from the Test. CTOD tests are commonly carried out using 2 specimen forms, the single edge notch bend (SENB), which is typically tested in a chilled liquid bath using a 3-point bend fixture, and the single edge notch tension specimen (SENT), which is tested in an environment chamber fitted around a tensile machine. In both cases, the specimens are of the same thickness as the final product and have a sharp, artificially induced crack in the middle. Specimens are then tested using a slowly increasing load, in order to maintain the stress concentration at the tip of the crack. This is unlike the Charpy Impact Test, which utilized a standardized specimen dimension with a “V” shaped notch and applies a suddenly heavy force via a swinging pendulum. CTOD Tests, while more expensive, can give a more accurate assessment of the material’s fracture toughness properties, allowing the data to then be used in an Engineering Critical Assessment to determine the critical defect size in a component, estimate the remaining lifespan of a component or design more accurate safety factors. CTOD Tests are therefore commonly specified for steels used in subsea or cross-country pipelines, or for heavy steel plate used in large boilers, pressure vessels or ship hulls and decking.

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