Fatigue tests determine the resistance of materials or material capability to resist against the damage, strength loss and failure under the repeated application of load or also known as Cyclic Loading. Design point of view, a material selection is made to meet or exceed service loads which are anticipated in fatigue testing applications. Cyclic fatigue tests generally generate repetitive loading and unloading in tension, compression, bending, torsion, or groupings of these stresses, thus fatigue testing is considered to be one of the critical tests as well as relatively expensive too in contrast to other mechanical tests.
Understanding fatigue properties is one of the most vital aspects of research and development, product safety, and material verification procedures.
Fatigue Test Procedure
A fatigue test calculates how cyclic forces will influence a part or material over time, using fluctuating loads, speeds, and environmental conditions to establish predictive patterns for material behavior. While most fatigue tests are carried out to plot stress (S) against the number of cycles to failure (N) (also known as an S-N curve), they can also be helpful for simulating particular situations and studying actual-world failures.
To carry out a fatigue test a test specimen is loaded into a fatigue tester or fatigue test machine and loaded using the pre-determined test stress, then unloaded to either zero load or an opposite load. This cycle of loading and unloading is then repeated till the end of the test is achieved. The test may be run to a pre-determined number of cycles or until the sample has failed depending on the parameters of the test. The fatigue stress is applied repeatedly to the sample using a range of load waveforms, as exhibited in the below picture. The form of the loading wave is usually sinusoidal, although triangular and block loads are also used. The frequency of the load cycles is usually low, typically 1–20 Hz (that is load cycles per second), to prevent heating of the sample, which can influence the fatigue findings.
Fatigue Testing types
There are several common types of fatigue testing as well as three common test types are:
- High Cycle Fatigue (HCF) or load controlled high cycle and
- Low Cycle Fatigue (LCF) strain controlled low cycle fatigue.
- Thermo-mechanical fatigue (TMF)
A high cycle Fatigue (HCF) test has a tendency to be related to loads in the elastic system and low cycle fatigue (LCF) tests normally include plastic deformations. Thermo-mechanical fatigue (TMF) delivers understandings into material properties under real-time cyclic thermal and mechanical strain.
Types of Loading in Fatigue Test
During fatigue tests, a test specimen or an actual component is tested under cyclic loading. Generally, during Fatigue tests, the test specimens are loaded in:
- Tension – tension,
- Compression – compression and
- Tension into compression and reverse.
What is an S-N Curve in Fatigue Test?
A SN-Curve (sometimes written S-N Curve) is a plot of the magnitude of alternating stress versus the number of cycles to failure for a given material. Typically both the stress and number of cycles are displayed on logarithmic scales.
Given a load time history and an SN-Curve, one can use Miner’s Rule to determine the accumulated damage or fatigue life of a mechanical part.
SN curves are usually developed by using a metal coupon testing machine (shown in the below figure). A small metal coupon is placed into the machine and subjected to a cyclic (or alternating) stress time history until a crack or failure occurs in the metal coupon.
Several coupons must be tested at different stress levels to develop an SN-Curve. The figure below illustrates a typical SN-Curve derived from testing metal coupons.
Fatigue Test Standards/ Specifications
- ASTM C1368 Slow Crack Growth of Ceramics in Flexure
- ASTM C394 Shear Fatigue – Sandwich Core Materials
- ASTM D3479 Tension-Tension Fatigue of Polymer Matrix Composite Materials
- ASTM D7774 Flexural Fatigue Test Equipment for Plastics
- ASTM E2207 Axial-Torsional Fatigue Thin-Walled Tubular Materials Test Equipment
- ASTM E2368 Thermomechanical Fatigue Test Equipment
- ASTM E2714 Creep Fatigue High Temperature Test Equipment
- ASTM E466 Load Controlled Constant Amplitude Fatigue Tests of Metals
- ASTM E467 Constant Force Axial Fatigue Test System Test Machines
- ASTM E606 Strain-Controlled High and Low Cycle Fatigue Testing
- ASTM E647 Fatigue Crack Growth Test Equipment
- ISO 15024 Interlaminar Fracture Toughness of Plastic Composites Test Machines
- NASM 1312-11 Tension Fatigue Test Procedure for Aeronautical Fasteners