Testing of Bars and Rods

Bars and rods are long round or square products with cross-sections up to 240 x 320 mm² and are used widely in rolling and forging: for use as base product for wire rod and sections or for transport industry forgings such as connecting rods, crankshafts and steel rails; generator and turbine shafts in energy technology; as end products in bridge building and shipbuilding or in equipment manufacturing and storage tank construction. The demands on their mechanical characteristics are as varied as their applications: from high tensile for structural materials to ductile for subsequent forming processes.

Tensile Tests

Tensile tests on bars and rods are usually performed according to ISO 6892-1 and ASTM E8, which are internationally recognized and widespread standards. ISO 6892-1 is also a European standard (EN ISO 6892-1) that is identical in wording and thus applicable in the European Union (for example, as DIN EN ISO 6892-1 in Germany). Specimens are taken from the product and prepared for the tensile test in accordance with the product shape or as specified in the product standards. Sections of products can be used as specimens for testing if the cross-section of the product permits. The required loads can easily exceed 2500 kN and customized Zwick systems up to 5000 kN are operating successfully. This places particular demands on specimen grips and clamping, and Zwick develops specimen grips backed by new technologies to ensure that specimens do not fail early due to the effects of clamping.

Standard-compliant strain measurement

Zwick's parallel closing, hydraulic specimen grips ensure that perfect clamping and positioning of specimens are maintained throughout the entire test, preventing the specimen from slipping or sliding. In most cases, standard-compliant strain measurement is performed by automatic contact or optical (non-contact) extensometers. Zwick's makroXtens is the classic and tested solution for the testing of bars and rods. Thanks to its mechanical construction featuring high resolution, and a very high level of accuracy and robustness, makroXtens withstands harsh environments. It's robust mechanical construction enables continuous strain measurement up to specimen break. Automated determination of strain at break is possible without onerous markings and manual measuring after specimen remains are sorted.

Strain measurement up to specimen break

laserXtens is an innovative solution for strain measurement up to specimen break, fulfilling standard requirements (ISO 6892-1, ASTM E8, ISO 9513, and ASTM E83) for bars and rods with flying colors. laserXtens does not require specimen markings; using a laser light, it uses the pattern it creates itself as surface markings. The optical evaluation of this "self-marking" is done so that even cinders and the occasional spalling of cinder does not disturb the markings. Since 2009, ISO 6892-1 and ASTM E8 allow the test speed to be automatically controlled and regulated by the strain rate. The tolerances called for in the standards for strain rate control (in particular those relevant to closed-loop strain rate control) can be easily met by both the makroXtens and laserXtens extensometers.

Hardness Testing

Hardness tests are performed on bars and rods for characterization of micro-structures (micro hardness testing), to determine the surface hardness, and for rough estimation of strength. A wide variety of methods with very small and large test forces are used. All of the classic methods to Brinell (DIN EN ISO 6506-1), Rockwell, Vickers (DIN EN ISO 6507-1) and Knoop (DIN EN ISO 4545-1) are relevant hardness testing methods for these pre-products. The relevant ASTM standards are ASTM E10 (Brinell), ASTM E 384 (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). Because the hardness test is simple, fast, and reliable, it is frequently performed and its results are also compared with other characteristics. With long products the hardenability of the material is often determined by establishing the hardness distribution along a rod following a tempering and quenching test (Jominy test, DIN EN ISO 642). Zwick’s product portfolio contains testing machines for all required hardness testing methods, including those for automated Jominy testing in particular.

Testing and determining the average global hardness value

An additional aspect of hardness testing is the testing and determination of the average global hardness value after rolling. Rolling is a thermo-mechanical process used to determine width and thickness, as well as mechanical properties. Hardness methods that employ higher forces are used to determine the average values of these sometimes coarse structures. Preferable methods are Brinell or Rockwell. In the testing of bars and rods, portable hardness testers are also often used. They can be used on site on the original part.

Determining the grain structure of metallographic constituents with hardness tests

Another aspect of hardness testing is the determination of the grain structure by performing hardness tests on metallographic constituents. Due to the small size of metallographic constituents, hardness testers with small to very small forces are used—generally speaking, stationary microhardness testers with indention sizes and depths that can be adjusted via the indention force to the dimensions of the metallographic constituents. 
The Zwick product portfolio offers hardness testers and devices for all applications and test methods. Zwick hardness testers and devices meet the requirements of all common international standards and can also be calibrated to international standards. As a calibration lab, Zwick is accredited for the calibration of hardness testers by the German national accreditation body, DAkkS. 

Fatigue Testing Under Torsion Loading

The behavior of round products under torsion is important in materials testing. For static testing, Zwick supplies drives, which when used in combination with materials testing machines, can apply torque to the specimen and determine the corresponding material properties. It is also possible to superimpose multiple load axes and test the material according to its use. For fatigue testing under high torque alternating load, frequencies up to over 200 Hz can be achieved in a high-frequency pulsator (Vibrophore) equipped with special test fixtures. Using resonance conditions means that the test is both quick and, due to extremely low power consumption, cost-effective. The advantages when testing with high-frequency pulsators (Vibrophore) are

  • Customized fixtures and devices
  • Faster testing
  • Higher energy efficiency
  • Very low maintenance requirements