13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Vienna [19] is used. The system is stimulating the specimen in its resonance frequency and is with about 21 kHz many times higher than loading frequencies of conventional systems. Tests have been performed in pulse-pause-sequences to avoid excessive heating of the specimen. Thus, experiments with a fatigue life up to 109 cycles take between 0.6 and 6 days depending on the pulse- and pause-sequence. Using the ultrasonic fatigue testing technique no load time history could be subjected to the specimen. Because of increasing vibration amplitudes at the beginning and decreasing amplitudes at the end of a pulse, only block loading tests can be realized. Thus, for systematic studies firstly two block loading tests are performed with different block loading length. Furthermore, a reconstruction algorithm, generating adequate cumulative frequency distributions of load cycles, is used to perform fatigue tests on the basis of a load time history. Therefore, the software Ultrasonic Fatigue Testing Software for Variable Amplitude Loading (UFaTeSVAL), which has been developed at the Institute of Structural Mechanics at the University of Rostock, is used for data acquisition and controlling of the VAL experiments. 2.3. Variable amplitude loading tests A high demand is requested for investigating the influence of variable amplitude loadings, and load interaction effects in particular, on short crack growth and crack initiation in the VHCF. 2.3.1. Two-step loading tests For systematic studies of the influence of load interaction effects on the fatigue life, particularly with regard to short crack growth, two-step loading tests have been performed (Fig. 2). The amount of numbers n1 of the maximum load amplitude of the cumulative distribution is 10.000 cycles and constant for all experiments. Tests have been carried out with different maximum load amplitudes with 110% and 120% of the fatigue strength σD for each experiment. The low block load is 90% of the fatigue strength and the number of cycles n2 was varied, characterized by the ratio Rbl = n2/n1. Three different ratios of 1, 10 and 100 have been investigated. However, the ratio was kept constant within one test. The two blocks are repeated until the specimen fails or the limit of 109 cycles is reached. Figure 2. Schematic procedure of two-step loading tests 2.3.1. Tests with load-time history FELIX ⎯Sa1 Sa ⎯Sa2 n1 = 10.000 n2 H 0.9·σD 1.1·σD 1.2·σD Sa0 = const.
RkJQdWJsaXNoZXIy MjM0NDE=