Fatigue testing of threaded fasteners

Threaded fasteners are used in the construction of complex machinery and equipment, and widely employed in the aerospace and automotive industries. Since mechanical stress is concentrated on these types of connection points, the integrity of fasteners is extremely important. A wide variety of mechanical tests apply to evaluating the performance of screws, bolts and nuts and includes tension, torsion, impact, hardness and importantly, fatigue.

Threaded fasteners are selected very carefully with regard to their intended industrial application and integrated precisely into the design. Accurate determination, via tensile tests, of the elastic tensile modulus and the limit of elastic loading is essential, as it is from these values that the limit forces for a secure, reliable threaded joint are established.

For fatigue tests on threaded fasteners a distinction is made between routine tests and testing for research and development purposes. Fasteners are normally tested in accordance with internationally recognised standards; ISO 3800, DIN 969 and NASM 1312.??

With routine tests the focus is on quality control and threaded fasteners are tested with regard to fatigue life. Depending on the design or material of the fastener, specimens can be subjected to half a million load cycles during the tests. The general requirements for the test are specified with reference to the mechanical properties of the material. The mean load used to test the fasteners is defined by DIN 969 as 70% of the yield point (Rp0.2) with reference to stress area As. Routine fatigue tests are used for pre-delivery product quality assurance.?

Fatigue limit tests, from approximately 5 million load cycles, are also performed for research and development relating to new threaded fasteners, with the aim of producing Haigh or Smith diagrams. Mechanical properties are initially determined via static or monotonic tests. The dynamic properties of the fasteners are specified by the designers and verified using appropriate fatigue tests. For example, fatigue limit is determined using a staircase, arcsin or threshold value method.?

In the staircase method, the specimen is subjected to dynamic loading at a constant stress amplitude. If the specimen fails before the pre-determined number of cycles is reached, a reduced stress amplitude is used for the next test. If the specimen does not fail, these specimens are referred to as ‘run outs’, the stress amplitude is increased for the next test. The results for each individual specimen, stress amplitudes and number of load cycles completed, are entered into the relevant diagram. They are distributed around the?mean value of the fatigue limit for the target number of cycles.

Specimen data distributed around the mean value, represents a fracture probability of 50%. Between 15 and 20 specimens must be tested in these experiments in order to determine the fatigue limit with a tolerance of 5%.?

The definition of the dynamic loading of a threaded fastener can be illustrated in what is known as a joint diagram, which shows extension (X-axis) against force (Y-axis). The curve in a joint diagram is basically a load deflection?curve.?

fSM – elongation of the bolt due to the clamping force FM?

fPM – compression of the clamped elements due to the clamping force FM

FM – original clamping force in the connection

FA – external axial force

FPA – reduction in clamping force due to FA

FSA – increase in bolt load due to FA

FKR – residual clamping force in the connection

FS – total load on the bolt

Tightening a bolt causes it to be elongated, while the bolted material is compressed. In this context the curve for the bolt is indicated with a positive gradient and that for the bolted material with a negative gradient. In this way the positive and negative changes in length, i.e., elongation and compression, are illustrated.?

Dynamically loaded threaded fasteners can be found in a variety of components, for example in internal combustion engines and aircraft engines. The aviation and automotive industries, motor racing in particular, are constantly striving to achieve an optimum combination of low weight and high long-term load capacity. Dynamic testing is therefore essential.?

The test conditions and test sequence for fatigue tests on threaded fasteners are generally specified by the customer (e.g., on the basis of in-house standards). Where no test sequence is defined by the customer the following standards are generally applicable:?

• Threaded fasteners – axial load fatigue testing – test methods and evaluation of results - test methods DIN 969 or ISO 3800?
• Mechanical properties of fasteners made of carbon steel and alloy steel – Part1: bolts, screws, and studs with specified property classes – coarse thread and fine-pitch thread -?test method EN ISO 898-1?
• Mechanical properties of fasteners; Part 2: nuts with specified proof load values - test method EN 20898-2?
• Fastener Test Methods - test method NASM 1312 (MIL-STD-1312 B)?
• Aerospace fasteners made of steel with nominal tensile strength of 1100 N/mm and 1250 N/mm, for temperatures up to 235°C - technical specifications LN 65009?

Aircraft and automotive design continue to evolve, with ever-increasing demands on fastener performance, such as reducing weight while maintaining load-bearing capability. Development data obtained from fatigue testing allow improvements in the design and durability of fastener products to meet these tougher requirements. For fastener manufacturers and for end users working in production environments, fatigue testing is essential for maintaining quality assurance and product integrity.