A B S T R A C TThe endurance fatigue strength of structural steel S355 was investigated in fatigue tests according to the method of increasing stress amplitude. The so-called 'load increasing test' is based on the direct correlation between the fatigue limit and the temperature changes caused by local plastic deformation ahead of the tip of a micro crack, which was initiated as a result of cyclic loading. In the present work the fatigue limit for testing temperatures 40 • C and −20 • C was estimated not only from the temperature measurements but also from the electrical potential measurements. Further, the obtained results were validated in standard fatigue tests with constant stress amplitude and a very good agreement was found.Keywords endurance fatigue strength; fatigue test methods; fatigue limit; load increasing test
N O M E N C L A T U R ER = stress ratio T = temperature change T 1 , T 2 , T 3 = temperatures measured by thermocouples T 1 , T 2 , T 3 U = measured potential U 0 = initial potential U/U 0 = potential ratio
I N T R O D U C T I O NThe fatigue behaviour at low temperatures plays a very important role for many industrial applications. For example, in the aircraft industry the knowledge of fatigue properties at low and high temperatures allows for precise fatigue design of structural elements without unwanted oversizing.For determination of endurance fatigue limit the standard constant amplitude test is widely used. The results obtained using this approach show sometimes a broad scattering, so that for the statistical validation a large number of samples should be investigated. Moreover, the formerly used fatigue limit for endurance strength (minimal number of cycles) of 2 million cycles has been extended recently to 10 million cycles. These make the standard method time consuming and expensive. Especially the investigations at low temperatures involve high costs as they require prolonged and strict temperature control. These drawbacks were an indication for many researchers to * Correspondence: P. Kucharczyk. develop alternative methods allowing for the estimation of endurance fatigue limit. Some attempts were made in order to find the relationships between the fatigue behaviour and the high loading frequencies up to 20 kHz. 1-3 The goal of the accelerated test procedure was determination of the fatigue resistance at normal loading frequencies on the basis of high frequencies tests.In Ref. 4 Williams and Reifsnider measured the acoustic emission during strain-and stress-controlled fatigue tests. They found a good correlation between the extent of specimen damage and totalized acoustic emission. The electrical potential method has been used for a long time for the study of crack propagation in CT specimens. The relationship between electrical resistance change and damage in metals was analysed under different testing conditions for diverse materials. 5-9 In Ref. 10 the structural health monitoring of non-insulating composites and joints by electrical resistance measurement was reviewed.In recent ye...
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