A Novel Procedure to Study Crack Initiation and Growth in Thermal Fatigue Testing

Abstract

An induction heating procedure coupled with an advanced alternating-current potential drop (ACPD) system to study thermal fatigue crack initiation and growth is presented. This technique can be adapted for various specimen geometries and is well suited for studying isotropic as well as anisotropic alloys. Details of the experimental apparatus and ACPD system are given. To illustrate the procedure, the effect of three cyclic thermal histories on the number of cycles to crack initiation of double-edge wedge specimens of IN-100 and MA-6000 superalloys was studied in air. The thermoelasto plastic finite-element analyses (FEA) to determine the critical locations and their respective stress-strain histories are presented. For this particular geometry of specimen, the initiation of microcracks about 10 to 20 μm in length could be detected and crack growth rates lower than 1 μm/cycle were measured. The experimental results combined with the FEA showed that the peak compressive strains encountered on specimen heat-up are more critical to crack initiation and surface degradation than total stress or strain ranges.

Bauschinger yield in the range 400–1025°C during cyclic deformation of high temperature alloys

“…Temperature is also indicated. Values of Young's modulus are included for completeness and where these could not be measured directly (for tests involving remote extensometry) values have been taken from external sources in the instances of alloy IN718 [22], IN100 [23], SRR99 [24] and CMSX4 [25]. It may be noted that sometimes runs at different temperatures were performed on single specimens.…”

“…Taking values of the constants derived from all Bauschinger-type tests in Table 1, values of yield strain ranges (and associated stress ranges) given by Eqn (23) are listed in Table 2. It is noted that (i) yield strain ranges in the region 0.02-0.05% are predicted for the alloys tested and (ii) where tests were done at several temperatures for a given alloy, the predicted yield strain decreases with increasing temperature.…”

“…It is noted that (i) yield strain ranges in the region 0.02-0.05% are predicted for the alloys tested and (ii) where tests were done at several temperatures for a given alloy, the predicted yield strain decreases with increasing temperature. The sensitivity of Eqn (23) to strain range may also be tested by means of Eqn (15) or strictly, the more accurate values derived from the steady-state (multiple-step) tests presented in Figure 24. Taking (from separate records) the highest and lowest strains for the alloys tested, these are presented in Table 3 together with corresponding values of A and b and the calculated yield strains and stresses.…”