Fasteners such as screws are exposed to particularly high stresses in the aviation and motorsport industries. The requirements in terms of wear and corrosion resistance, repeated strength and temperature cycles, fluctuations in dynamic loads and high vibrations are correspondingly high. In addition to special manufacturing processes, this also requires extensive testing of the materials in quality assurance. One of the leading German manufacturers of fasteners in the high-end sector is the company Johann Maier in Stuttgart. The applications of the extensive portfolio range from use in the International Space Station and satellites to helicopters, jets, gearboxes and engines.

In order to achieve the required values for the respective applications, sophisticated manufacturing processes such as hot forging, heat treatment or thread rolling are used and the components are checked using destructive and non-destructive testing methods. "We call it proven safety," explains Dr. Oliver Lang, Managing Director of the company. "If we want to guarantee the maximum quality, highest precision and absolute safety of our products, we have to test them accordingly."

Cyclical stresses

In addition to static loads, fasteners are generally also subject to cyclical stresses such as vibrations. To determine the fatigue strength of components using uniaxial fatigue tests, the company uses a servo-hydraulic testing machine from ZwickRoell in Ulm. The strength of this drive principle lies in its wide range of options in terms of force, amplitude and frequency. They can also be used universally; for example, these testing machines can also be used to carry out durability tests or crash tests at speeds of up to 20 m/s.

The servo-hydraulic fatigue testing machine with a dynamic nominal force of 250 kN is part of the HA series, which is available in standard sizes from 50 to 500 kN. It is specially designed for determining material characteristics under oscillating loads and is particularly suitable for test set-ups in which temperature chambers or furnaces are mounted in the test chamber. The arrangement of the cylinder, which is located in the lower crosshead and below the test chamber, prevents heated air from rising upwards and heating up the cylinder. It also prevents oil from dripping into the heated test chamber.

Low-cycle fatigue tests are a typical application for servo-hydraulic testing machines. In these tests, the material is cyclically loaded to a low level of plastic deformation, usually at an elevated temperature. The specimen can only withstand a few thousand load cycles with this type of load. But the machine's controller is also challenged. As the stiffness of the specimen changes rapidly during the transition from elastic to plastic deformation, the controller must react within a very short period of time in order to guarantee a constant increase in the strain rate, for example. The high rigidity of the testing machine also plays a decisive role here.

Tensile strength and elongation at break

One of the central static tests on screws is determining the tensile strength in ^N/mm2. Johann Maier uses an electromechanical testing machine from ZwickRoell's AllroundLine series for these tests. It is specially designed for tensile, compression and flexure tests as well as shear and torsion and is equipped with backlash-free preloaded ball screws and a maintenance-free AC drive with motor feedback system. As a result, it achieves excellent synchronization properties even at low speeds in order to generate precise measured values.

In addition to the parameters of tensile strength, yield strength and elongation, another relevant parameter is elongation at break. This permanent plastic strain is an indicator of the deformability of the material and is measured on twisted bolts. The MakroXtens extensometer developed by ZwickRoell for strain measurement records the elastic and plastic deformations during the entire tensile test via its probes - even with high forces and brittle materials - and can be used up to specimen break. It is calibrated at the factory and fulfills class 0.5 of EN ISO 9513 from 20 μm change in length.

To clamp the specimens in the testing machine, so-called clip-on specimen grips with corresponding head and thread holders are used. This ensures, among other things, that the minimum specimen length specified in the ISO 898 1 standard for carbon steel and alloy steel bolts is adhered to. The specimen grips are available for testing bolts and nuts as well as for testing shoulder and thread head specimens in accordance with the relevant standards. With the aid of special test mandrels and dies, they can also be used for test load tests (tensile and compression tests) on nuts.

Test software and control electronics

The TestXpert III testing software developed by ZwickRoell is already equipped with a corresponding test specification for testing the strength values of bolts and nuts to ISO 898 1 and ISO 898 2. The minimum breaking force required by the standards is already stored and is compared with test results. In addition, the software offers an intuitive operating concept for all testing machines and testing devices and guides the operator from preparing and performing the test through to analyzing the results. The operator is also supported by the TestControl II control electronics, which, in conjunction with the testing software, enables extremely precise control of the strain rate via a closed control loop, even at very low strains.

With its wide range of modular testing machines, ZwickRoell demonstrates that even demanding tests on high-performance bolts can be performed efficiently. Force ranges tailored to the respective testing task and simple adaptation of the fixtures for bolts and nuts to other sizes contribute to this. Another advantage is the quick and easy loading and frontal access to the tools.