Issue 41

M. Sakane et alii, Frattura ed Integrità Strutturale, 41 (2017) 16-23; DOI: 10.3221/IGF-ESIS41.03 17 The objective of this paper is to overview factors influencing cracking direction under multiaxial LCF. The factors that this paper overviews are stress/strain multiaxiality, notch and precrack and stress/strain range. This paper discusses the strain range dependency of cracking directions in torsion LCF in more detail. In torsion LCF, shear cracking occurs at high strain ranges but principal cracking at low strain ranges. The reason of the transition of cracking direction depending on strain range has not been well understood so that this paper discusses this topic from micro crack observations on specimens fatigued in torsion. ϕ        Scale Main crack 1 mm Sub crack 0.1 < 2a < 1.0 mm 0.1 mm Micro crack 2a < 0.1 mm 0.05 mm Axial direction Figure 1 : Crack observation in tension-torsion LCF of SUS304 stainless steel at 923K. I NFLUENTIAL FACTORS TO CRACK DIRECTION Multiaxial strain/stress states ultiaxial strain/stress state has an evident influence on cracking mode and many papers discuss this factor. A systematic research of the author’s [2] on cracking mode in tension-torsion LCF of a SUS 304 austenitic stainless steel is shown in Fig.1. In the figure, cracking directions are classified according to crack length into three categories. The main crack is a crack that brings specimen to failure, the sub-crack a crack having a length between 0.1 mm and 1 mm, and the micro-crack a crack with a length less than 0.1 mm. Fig.2 depicts the crack angle at failure against the principal strain ratio (  =  3 /  1 ) and the strain ratio (  =  /  ) as a summary of the observations shown in Fig.1. The crack angle is the angle from the longitudinal direction of specimen axis,  the shear strain amplitude and  the axial normal strain amplitude. The  =  0.5 test corresponds with the tension test and  =  1.0 with the torsion test, assuming that the Poisson’s ratio being 0.5 in LCF region. The main cracks at the three strain ranges propagated at an angle of 90 degrees (principal crack) in tension tests (  =  0.5 test) but they at an angle of 45 degrees (shear crack) in torsion tests (  =  1.0 test). The transition of the cracking mode from principal crack to shear crack occurred at the principal strain ratio of about  0.74. The cracking mode of sub-cracks is the same as that of the main crack. However, the cracking mode of the micro-crack is different from that of the main and sub-crack, and both principal and shear cracking modes were found in the principal strain ratios of  0.86 and  1.0. Principal strain ratios at which the tension-torsion crack transits from the principal mode to the shear mode with decreasing principal strain ratio are listed in Tab. 1 for several materials listed in the table together with testing temperatures. The table indicates that the principal strain ratio at transition does not significantly depend on material and takes the value around  0.70 whereas a slightly smaller value is found in SUS 304 stainless steel at 823 K. Materials listed in Tab. 1 range over wide types of materials, austenitic stainless steel, low alloy ductile steels, conventional cast superalloy, and directional solidified super alloys. Considering that the steels and the conventional cast superalloy are an isotropic material but the directionally solidified superalloys have a strong anisotropy due to the crystallographic texture the result in the table means that texture and material anisotropy has a weak effect on the principal strain ratio at the transition. M