Tensile Test Sample Preparation Techniques for Hot Dip Galvanized, Light Gauge Steel

Abstract

Tensile testing specifications (ISO, ASTM, DIN, JIS) are well established for defining the geometry of specimens and the conditions for conducting the tensile test; however, sample preparation is a topic that is not well defined in published standards. There are numerous techniques for creating tensile bars, including die cutting, machining, electric discharge machining (EDM), grinding with abrasive paper, and using a specially designed instrument such as a Tensilkut. In all cases, the goal is to create a sample that represents the true properties of the material. An issue that has continued to be the subject of debate based on varying sample preparation techniques, which have led to inconsistent results, is the preparation of thin (\0.25 mm, or 0.01 in.), coated steel tensile test specimens. The coating most often encountered on this type of steel is hot-dipped galvanized (HDG) zinc. While other coated steel, including tin-plate and aluminized, also create sample preparation difficulties, the focus here will be on galvanized steel. Sample preparation of light gauge (thin) steel requires substantial effort in order to avoid deforming the specimen while simultaneously creating an edge that is free of cracks, tool marks, heat distortion, and edge burrs. Typically, die cutting test specimens creates an unacceptable edge on light gauge steel, as substantial deformation is created. Many machining operations also result in specimens with tool marks on the edge and large burrs present. The edges of thin steel specimens are often finished by hand (a process referred to as dressing the edge) using a fine file, abrasive stone, or abrasive grit paper. While this will remove tool marks and burrs, dressing the edges is not without problems. The technique used to finish the edge may vary from person to person and in the case of light gauge steel, rounding of the edges may cause a change in the geometry of the specimen that is not insignificant. A sample preparation technique that results in defect-free edges with no post preparation dressing would be ideal. To the best of the author’s knowledge, no technique has been proven effective for this to date. The other issue with HDG light gauge steel is whether or not to remove (strip) the Zn from the specimen. The author’s laboratory often sends samples to external labs as a check to ensure that the results we generate in-house are consistent with others. There is broad disagreement on whether or not to remove the Zn, which is typically done by immersing the entire sample in a hydrochloric acid solution. Several labs always remove the Zn, while others & Elvin Beach Elvin.Beach@worthingtonindustries.com