Strip refers to steel and non-ferrous products that are rolled into coils, for example, hot-rolled strip, which is used as primary material for cold-rolled sheets.
Hot-rolled strip is produced in thicknesses up to approximately 15 mm and in widths up to approximately 2,200 mm. Hot-rolled plate is sheet cut out of hot-rolled strip and is up to approximately 15 mm thick and up to approximately 2,000 mm wide.
Tensile tests on sheets and strips are predominantly performed to the international standard ISO 6892-1 and the American standard ASTM E8. In addition, there are many national standards that have aligned more and more with the international standard ISO 6892-1 over the last few years. EN ISO 6892-1 applies to Europe and is identical in wording. All EU member states must adopt it as their national standard (for example, as DIN EN ISO 6892-1). The characteristic values defined in these standards and described in their determination methods are comparable in their end results. Important values for the characterization of materials are the yield point, yield strength, maximum strength, strain at maximum force and/or strain at maximum stress, as well as strain at break. These characteristic values are used by engineers as the basis of their work and are important criteria for product acceptance in the customer-supplier relationship.
A wide range of testing solutions
ZwickRoell supplies a wide range of static materials testing machines for determining material properties from tensile tests; these systems provide high-precision testing under high loads. Our comprehensive range of axial and transverse strain extensometers allow the optimum combination to be selected in line with your requirements and testing conditions, for example, a makroXtens digital extensometer combined with a videoXtens transverse strain extensometer. This combination offers robustness and a high level of automation plus easy specimen handling. makroXtens measures the extension to break. The mechanical version of the knife edge holders prevents the measuring system from being negatively influenced by a sudden tension release of the specimen at break. With the optional break location identification feature, the optical transverse strain extensometer measures the transverse strain up to break along the specimen, thus making classification of the break location as accurate as possible.
Determining the r- and n-value
R- and n-values are often also determined in tensile tests in order to characterize forming properties; the n-value describes the work hardening—increase in stress—during plastic deformation up to uniform elongation, while the r-value describes the vertical anisotropy. The n-value is determined from the tensile stress data and strain values; for the r-value, the transverse strain on the tensile specimen is additionally measured. Tensile specimens are taken from the strip or sheet at set angles to the rolling direction. One factor affecting the r-value is the rolling direction. This can be particularly important when using the sheet to manufacture components through bulk metal forming. The sheet thickness is retained as specimen thickness in a tensile test so that the material does not undergo any changes during surface processing. The parallel length is produced by milling or punching plus finishing so that the material does not undergo any changes during processing.
Biaxial tensile tests
A distinctive feature is the ability to perform a two-axis tensile test, which is used to determine additional deformation properties of the material. It is primarily employed in research and development, as it allows defined stress values to be set and investigated at the intersection point of the specimen. ZwickRoell offers three different models of biaxial testing machines. In most cases, strain is measured optically and without contact. There are two different standardized solutions available from ZwickRoell.
Hardness testing on sheet and strips are mainly performed to characterize the overall surface hardness of a specimen and for metallographical testing. Depending on the application, hardness tests are performed to ISO 6506-1 (Brinell), ISO 6507-1 (Vickers), ISO 6508-1 (Rockwell), as well as ASTM E10 (Brinell), ASTM E384 (Vickers and Knoop), and ASTM E18 (Rockwell). In addition, other methods or specifications are used for certain application areas (for example, the European standard EN 2002-7 is used in aerospace).
Hardness testing is also used for weld seam tests on sheet and strips if large constructions are welded together from these pieces. Striking examples of this is ship construction, offshore facilities, and pipeline construction for oil and gas transport.
The DIN EN ISO 9015 series of standards defines the test types. Typically, hardness tests to Vickers HV5 or HV10 (with test loads of 49 N or 98 N) are performed. Hardness must be tested and monitored in the parent metal, the heat-affected zones and in the weld metal itself. This is why numerous indentions must be made in the cross-sections and traverse cross-sections of a welded seam. For this type of test, automatic hardness testers are often used. They make indentions automatically using programmable sequences at preset positions and then measure them automatically.
The ZwickRoell product portfolio offers hardness testers and devices for all test methods. Fully automated systems are specifically used for weld seam testing. These can perform hundreds of hardness tests without manual intervention by the operator, autonomously and efficiently. ZwickRoell hardness testers and devices meet the requirements of all common international standards and can also be calibrated to international standards. As a calibration lab, ZwickRoell is accredited for the calibration of hardness testers by the German national accreditation body DAkkS.
The 3-point flexure test serves to not only determine the characteristics of the flexural properties but also visually evaluates the flexural edge. In particular, the behavior of the weld seams are visually examined during the flexure test. ZwickRoell’s range of flexure test kit options combined with adaptations to existing specimen holders provide an ideal solution.
In applications such as engine design, power plant design, powertrains, and chemical plants, the material behavior under increased temperatures up to approx. 1,200 °C are of vital importance. Tensile tests are primarily performed for these applications under increased temperature, although flexure tests are as well. ZwickRoell’s complete solutions for these tests comprise temperature-controlled furnaces, specimen-grip pull-rods, high-temperature strain measurement and other essential accessories for integration into ZwickRoell testing machines.