Fatigue Testing

Fatigue Testing

History Regarding Fatigue • Fatigue as a specific failure mechanism recognized since the early part of the nineteenth century. • The development of rail travel that resulted in a major increase of interest in this type of fracture. • The premature failure of wagon axles led to investigating fatigue failure under rotating loading. • This led to the design of the first standardized rotating fatigue test.

Wohler rotating fatigue test

Fatigue Testing • Failure under cyclic or repeated stress. • The value of stress is lower than the static stress required to cause fracture. • It occurs in metals and non metals a like. • Generally characterized by local crack propagation. • The component often showing no sign of failure before the final fracture.

• Most fatigue failure occurs in relatively common components such as gear teeth, crankshaft, axel and so on. • Catastrophic failures causing extensive lives. • In practice ninety percent of all services failures are due to fatigue. • Most of failures are due to poor design.

• Data obtained under constant load are normally plotted on S-N curve. • S is the amplitude of alternating stress. • N is the No. of cycles to failure. • Two types of S-N curve • Fatigue limit and endurance limit. • Better to draw S-N-P graph.

Variables affecting fatigue life • • • •

Amplitude of the stress cycle: Surface condition: Effect of T: High fatigue strength at low T Frequency of stress cycle: little effect on fatigue life. lowering frequency reduced fatigue life. • Environment:

• Failure can occur at a fluctuating load well below the yield point of the metal and below the allowable static design stress. The number of cycles at which failure occurs may vary from a couple of hundreds to millions. There will be little or no deformation at failure and the fracture has a characteristic surface, as shown in Fig.2.

• Why Do a Fatigue Test? • In many applications, materials are subjected to vibrating or oscillating forces. • The behavior of materials under such load conditions differs from the behavior under a static load. • Because the material is subjected to repeated load cycles (fatigue) • in actual use, designers are faced with predicting fatigue life, which is defined as the total number of cycles to failure under specified loading conditions. • Fatigue testing gives much better data to predict the inservice life of materials.

• It should be mentioned that, in service, few structures experience purely static loads and that most will be subjected to some fluctuations in applied stresses • Therefore be regarded as being fatigue loaded. Motorway gantries, for example, are buffeted by the slipstream from large lorries and offshore oilrigs by wave action. • Process pressure vessels will experience pressure fluctuations and may also be thermally cycled. • If these loads are not accounted for in the design, fatigue failure may occur in as few as a couple of tens of cycles or several million and the result may be catastrophic when it does.

• It may be thought that the use of a higher strength material will be of benefit in increasing fatigue life. • The rate of crack propagation, however, is determined by Young's Modulus - a measure of the elastic behaviour of the metal - and not simply by tensile strength. • Alloying or heat treatment to increase the strength of a metal has very little effect on Young's Modulus and therefore very little effect on crack propagation rates. • Since the bulk of a welded component's life is spent in propagating a crack, strength has little or no influence on the fatigue life of a welded item. • There is thus no benefit to be gained by using high strength alloys if the design is fatigue limited.

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The figure shows several types of loading that could initiate a fatigue crack. The upper left figure shows sinusoidal loading going from a tensile stress to a compressive stress. For this type of stress cycle the maximum and minimum stresses are equal. Tensile stress is considered positive, and compressive stress is negative. The figure in the upper right shows sinusoidal loading with the minimum and maximum stresses both in the tensile realm. Cyclic compression loading can also cause fatigue. The lower figure shows variable-amplitude loading, which might be experienced by a bridge or airplane wing or any other component that experiences changing loading patterns. In variable-amplitude loading, only those cycles exceeding some peak threshold will contribute to fatigue cracking.

• There are two general types of fatigue tests conducted. One test focuses on the nominal stress required to cause a fatigue failure in some number of cycles. This test results in data presented as a plot of stress (S) against the number of cycles to failure (N), which is known as an S-N curve. A log scale is almost always used for N.

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The data is obtained by cycling smooth or notched specimens until failure. The usual procedure is to test the first specimen at a high peak stress where failure is expected in a fairly short number of cycles. The test stress is decreased for each succeeding specimen until one or two specimens do not fail in the specified numbers of cycles, which is usually at least 107 cycles. The highest stress at which a runout (non-failure) occurs is taken as the fatigue threshold. Not all materials have a fatigue threshold (most nonferrous metallic alloys do not) and for these materials the test is usually terminated after about 108 or 5x108 cycles.

• Since the amplitude of the cyclic loading has a major effect on the fatigue performance, the S-N relationship is determined for one specific loading amplitude. The amplitude is express as the R ratio value, which is the minimum peak stress divided by the maximum peak stress. (R=σmin/σmax). It is most common to test at an R ratio of 0.1 but families of curves, with each curve at a different R ratio, are often developed.