Fastener Testing

By Alan Thomas, marketing manager, ZwickRoell

Mechanical testing plays a major role in research, product design and quality control. Tests can be conducted using a wide range of static and dynamic materials testing machines, which engineers frequently use during product development. Mechanical testing incorporates a wide range of techniques, from common tensile and compression tests, to flexural, or torsional, characterisation. Testing may also occur under ambient, or non-ambient conditions, with widely varied temperatures and environmental conditions.

Tensile testing is a destructive test process that provides information about the tensile strength, yield strength and ductility of a material. It measures the force required to break a test specimen and the extent to which the specimen stretches, or elongates, to that breaking point. Universal materials testing machines, capable of conducting static, or monotonic, tests in tension, or compression, play a vital role in product development and quality control of materials and components. The ability of such equipment to provide ‘raw data’, which can be used by design engineers, is essential in the development of new products. 

The manufacturing sector faces tough global competition and, for this reason, the use of computer-based development methods continues to grow. However, these simulation methods must be accompanied by real mechanical tests in the various development phases, to determine the important material data.

Many materials and components are required to have a defined fatigue strength, which must be verified to ensure fitness for purpose. These fatigue tests can be accommodated in a servo-hydraulic test machine, but are more quickly and economically performed using high-frequency, resonance pulsators. Whichever test method is used, consideration must be given to special gripping arrangements, in order to satisfactorily complete fatigue tests on a wide range of sample types and sizes.

The operating principle of the high-frequency resonance pulsator is based on the concept of a mechanical resonator with electro-magnetic drive. The mean force is applied by moving the upper crosshead with a lead-screw drive. Due to their design, these machines have been used in the past, solely as dynamic materials testing machines, to determine durability with regard to fatigue life and fatigue limit, for example for fatigue testing to DIN 50100 (S-N curve), under tensile, compression, pulsating and alternating loads. 

The new generation of machines can be used as both dynamic and fully fledged static materials testing machines, which can accommodate test loads of up to 1,000kN. Typical applications are material fatigue tests and durability tests on standardised specimens and components, e.g piston rods, crankshafts, and fasteners, as well as production and quality control of components exposed to dynamic loads during their anticipated service lives.

Servo-hydraulic testing machines have universal application for materials and component testing under pulsating, or alternating, loads –
 with constant amplitude, or pseudorandom, signals. Servo-hydraulic testing machines may be used in static, dynamic or fatigue testing applications. They operate through a closed loop system consisting of a bi-directional hydraulic actuator linked to the test specimen; a servo valve and controller for adjusting actuator position, velocity and force; a load frame with an hydraulic power source; and an electrical feedback loop enabling the test variables to be controlled using position and load transducers. Although they require infrastructure for both electrical and hydraulic connections, servo-hydraulic testing machines can be a cost-effective, static testing choice, at very high forces, or if a high testing speed is required.

Elevated temperature tensile testing is a reliable process used to evaluate the behaviour of metallic materials when subjected to a combination of high temperature and tensile forces. High temperature, tensile testing procedures are performed routinely in many industries for assessing high performance steel, and other metals, that will be exposed to high temperatures while in service. In applications such as engine design, the material behaviour under increased temperatures, up to approximately 1,600°C and higher, is of vital importance.

The Charpy impact test, establishes the relationship of ductile-to-brittle transition in absorbed energy at a series of test temperatures. Since a variety of metals undergo a transition from ductile behaviour at higher temperatures, to brittle behaviour at lower temperatures, the Charpy test is now specified for a range of steel products, including steel hull plates used in shipbuilding, pressure vessels in nuclear plants, and forgings for electric power plant components.

The Charpy test is usually performed using precisely machined specimens, typically measuring 10mm x 10mm x 55mm, with a 2mm deep V-notch in the middle of one of the specimen faces. Specimens are usually tested over a range of sub-ambient and elevated temperatures. Once a specimen reaches the required test temperature, it is quickly located onto a special fixture in the pendulum impact tester, with the notch oriented vertically. Following the release of the test machine pendulum, the specimen is struck by the tup attached to the swinging pendulum of appropriate design and mass. On impact, the specimen breaks at its notched cross-section and the upward, return swing of the pendulum is then used to determine the amount of energy absorbed in the process.

Indentation hardness tests are used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic, or microscopic, scale. There are three commonly used hardness testing methods: Rockwell, Vickers and Brinell.

The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load, compared with the penetration made by a preload. There are different scales, denoted by a single letter, that use different loads, or indenters.

The Vickers hardness test was developed as an alternative to the Brinell method, to measure the hardness of materials. The Vickers test is often easier to use than other hardness testing methods, as the required calculations are independent of the size of the indenter and the indenter can be used for all materials, irrespective of hardness. The Vickers test has one of the widest scales among hardness tests and the unit of hardness given by the test is known as the Vickers Pyramid Number. 

The Brinell hardness test method is used to determine Brinell hardness. Most commonly, it is used to test materials that have a structure that is too coarse, or that have a surface that is too rough, to be tested using another test method, e.g. castings and forgings. Brinell testing often uses a very high-test load, (3,000kgf), and a 10mm diameter indenter, so that the resulting indentation averages out most surface and sub-surface inconsistencies. 

Today’s state of the art hardness testing machines can be used in the widest range of applications. They use innovative mechatronic technology for high-precision testing, particularly for quality assurance, production line testing and in the laboratory.

Mechanical testing helps verify that the product meets the stipulated industry standards and regulations, ensuring its quality and reliability. It helps optimise the material selection process by identifying the most suitable material for a particular application, based on its mechanical properties.