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The resistance of a material to fatigue crack growth is an important property that can define the usable life of a component. The crack growth rate under given fatigue loading conditions is termed da/dN, where da represents the change in crack length in a small period of time and dN represents the number of fatigue cycles applied in that period. To measure this characteristic, specimens are prepared with a sharp-tipped notch, providing a stress concentration to stimulate crack growth when fatigue loading is applied. The specimen is then cycled under given loading conditions while crack length and the number of applied cycles are logged.
This article provides details about testing for fatigue crack growth, including test parameters, test objectives, and step-by-step procedures.
Test parameters
* Specimen type: The most common specimen type is CT (Compact Tension). Other types include three-point or four-point bend, Center Cracked (or Middle Tension), and Corner Cracked specimens.
* Applicable standards: The most common test standard is ASTM E647 -Standard Test Method for Measurement of Fatigue Crack Growth Rates. This standard provides guidelines covering specimen geometry, loading control modes, crack measurement methods, calculations, and results.
Test objectives
Two objectives are typically sought from the test. The first is to measure how many cycles (under specified loading conditions) the specimen can withstand. This determines the fatigue life of the specimen. The second is to find the critical loading condition, defined as the load below which a crack will not grow.
* Fatigue life is easily determined by cycling the specimen under specified loading conditions until the crack has grown to a certain length. This crack length is the maximum acceptable crack length (considering the material's service conditions), and is not necessarily the ultimate fracture of the specimen. Results are typically shown in a graph in which crack length is plotted against the number of cycles.
* Calculation of da/dN: Fitting a straight line to these data enables us to calculate da/dN, the gradient or slope of the trendline. The da/dN shown in Fig. 2 is equal to 0.0003 mm per cycle. This allows the estimation of the service life of a component for which loading conditions and material imperfections can be specified.
* Critical loading conditions may be determined by a...