Issue34

N.R. Gates et alii, Frattura ed Integrità Strutturale, 34 (2015) 27-41; DOI: 10.3221/IGF-ESIS.34.03 33 Nominal Loading (MPa) On τ max plane (MPa) Crack Orientation vs. Length (mm) Specimen ID † τ amp σ amp σ mean τ amp σ max initiation 0.2 <2c <1 1 <2c <2 2c >2 N f TM56-ST 248.3 0 0 248.3 0 τ max τ max τ max τ max 736 TM63-SC 178.0 300.0 0 232.8 150.0 τ max τ max τ max τ max 2009 TM100-STSA 187.8 0 92.0 187.8 92.0 τ max τ max τ max τ max 6742 TM101-STSA 187.8 0 92.0 187.8 92.0 τ max τ max τ max τ max 15177 TM1-ST* 187.8 0 0 187.8 0 τ max τ max τ max τ max 72397 TM58-ST 187.8 0 0 187.8 0 τ max τ max τ max τ max 68105 TM103-STSA 187.8 0 -92.0 187.8 0 τ max τ max τ max τ max 15283 TM106-STSA 187.8 0 -92.0 187.8 0 τ max τ max τ max τ max 21191 TM53-SC* 130.3 225.0 0 172.1 112.5 τ max σ 1 τ max τ max 25555 TM4-ST 168.2 0 0 168.2 0 τ max σ 1 σ 1 τ max 180288 TM78-ST 150.1 0 0 150.1 0 τ max σ 1 τ max τ max 181299 TM96-STSA 140.3 0 150.0 140.3 150.0 τ max τ max τ max τ max 35095 TM99-STSA 140.3 0 150.0 140.3 150.0 τ max τ max τ max τ max 58046 TM67-ST 140.3 0 0 140.3 0 τ max σ 1 σ 1 τ max 531716 TM105-STSA 140.3 0 -150.0 140.3 0 τ max σ 1 σ 1 τ max 770431 TM37-SC 106.4 175.0 0 137.8 87.5 τ max σ 1 τ max τ max 147725 TM54-SC 90.3 156.0 0 119.3 78.0 σ 1 σ 1 σ 1 σ 1 213138 TM62-SC 90.3 156.0 0 119.3 78.0 τ max σ 1 σ 1 τ max 867295 † Suffix indicates loading condition as follows: ST (pure torsion), STSA (torsion with static axial stress), and SC (in-phase axial-torsion) *crack orientations refer to a secondary crack, not the failure crack Table 1 : Loading conditions, crack orientation vs. crack length, and fatigue life (to crack lengths of approximately 15-20 mm) for smooth specimen fatigue tests. All stresses are given in MPa and all crack lengths are given in mm. These observations support the idea that friction and roughness induced crack closure, caused by crack face interaction, play a key role in determining crack path. At the lower loading levels, the combined effect of friction and roughness seem to reduce the effective mode II driving force at the crack tip until it drops below a critical level. This critical level could either represent a mode II threshold condition, or a value at which the potential for mode I growth exceeds that of the existing mode II crack. At this point, the crack then turns to maximum tensile planes where there is less resistance to mode I crack propagation. This idea echoes the conclusions drawn in several of the studies reviewed in the introduction. At the higher loading levels, however, these factors do not appear to play as large of a role in determining crack path. This is likely due to a combination of factors. For example, at higher stress levels, the effect of friction may not be large enough to drop the effective mode II driving force below its critical value. Additionally, increased plasticity at the crack tip can result in the deformation and destruction of the crack face asperities which would otherwise restrict the effective mode II driving force. Fig. 3 shows that as the shear stress amplitude was increased in the pure torsion tests, a longer mode II crack length was observed before branching occurred. This trend was also reported in Refs. [8, 18] and suggests that the mode II driving force increases at a faster rate than the attenuation effect due to crack face interaction, which eventually leads to a non-branching condition. Such a dependence on load level is consistent with the observed differences between crack paths in smooth and notched specimens. Because of the stress concentration effect in notched specimens, they are often tested at much lower nominal stress amplitudes than smooth specimens. Therefore, once an initial mode II crack propagates out of the notch affected zone and into the lower stressed gage section, frictional effects would significantly restrict its growth and lead to a tendency for the crack to transition into mode I where there is less resistance to crack extension. The same is true for crack growth tests where crack growth is evaluated from a precrack. Because data for a large range of growth rates and stress intensity factors are typically desired in these types of test, they are performed at the lower nominal loadings conducive to mode I growth.