William J. Mills

Review and synthesis of stress intensity factor solutions applicable to cracks in bolts

Abstract The available literature for stress intensity factor solutions for cracks in round bars, both threaded and unthreaded, subjected to either tension or bending, is reviewed. The results are synthesized into a form that is appropriate for the analysis of bolts and studs.

Effect of heat treatment and heat-to-heat variations in the fatigue-crack growth response of Alloy 718

The fatigue-crack growth behavior of seven heats of Alloy 718 was studied at five different test temperatures. These seven heats represented at least four different producers, four different product forms, two melt practices, and most of the heat were tested in two different heat-treated conditions. Heat-to-heat variations were noted; these were most obvious in material given the conventional heat-treatment. 8 figs., 5 tabs.

Heat-to-heat variations in the fracture toughness of austenitic stainless steels

Abstract The effects of heat-to-heat variations on the fracture toughness behavior for Types 304 and 316 stainless steel (SS) were examined at 24, 427 and 538°C using the multiplespecimen J R - curve procedure. These alloys exhibited considerable variability in fracture toughness, with J c initiation toughness values ranging from 178 to 781 kJ/m2 and tearing moduli from 272 to 676 at elevated temperatures. The high initiation toughness coupled with the very steep J R curves demonstrate that base metals are exceptionally resistant to unstable fracture. Minimum-expected fracture properties were defined from lower-bound J c and tearing modulus values generated here and in previous studies. Fractographic examination revealed that microvoid coalescence was the operative fracture mechanism. Variations in fracture toughness behavior were correlated with the depth of the microvoids.

Character of fatigue fracture surface micromorphology in the ultra-low growth rate regime

The fatigue fracture surface micromorphology in the ultra-low growth rate regime has been examined in aluminum, copper, nickel, titanium, and iron based alloys. A transition from striated to faceted fracture surface appearance is found for all materials examined. For the aluminum alloys a faceted appearance was observed over a three decade growth rate regime down to 5 x 10 - 9 mm/cycle for which macroscopic test data are available. At intermediate crack growth rates (2 x 10 - 5 to 1 x 10 - 4 mm/cycle) where the striation mode usually dominates, evidence of the faceted morphology is also seen and persists to higher growth rates within this growth rate range the lower the stacking fault energy of the material. This is believed related to a preference for faceted growth in planar slip materials. Faceted growth is produced at low ΔK a p p levels under uniform loading conditions and at low ΔK e f f levels associated with complex load interactions. The fracture facets generated by these two conditions are not readily distinguishable. Finally, it is not yet possible to identify the increment of discontinuous crack extension on the fracture facets in the crack growth regime where discontinuous crack extension must occur.

Effect of Temperature on the Fracture Toughness Behavior of Inconel X-750

The elastic-plastic J 1 c fracture toughness response of precipitation heat-treated Inconel X-750 was evaluated by the multispecimen resistance-curve (R-curve) technique at room temperature and elevated temperatures. The fracture toughness of this nickel-base superalloy was found to increase slightly under intermediate temperature conditions (427°C), but it decreased dramatically at the highest test temperature (649°C). Metallographic and electron fractographic examination of fracture surfaces revealed that the slight increase in J 1 c at intermediate temperatures was associated with an intergranular-to-transgranular fracture mechanism transition. At room temperature, crack extension occurred primarily by an intergranular dimple rupture mechanism attributed to microvoid coalescence along a grain-boundary denuded region. In the 427 to 538°C regime, the fracture surface was dominated by a transgranular morphology consisting of poorly defined dimples coupled with a rather flat. faceted appearance. At 649°C, intense heterogeneous slip initiated localized separation, which resulted in a very faceted fracture surface morphology and a severe degradation in fracture toughness.

Fatigue-crack propagation and fracture toughness behavior of cast stainless steels

Abstract The fatigue-crack growth and fracture toughness behavior of two cast stainless steels, ASME SA 351 Grades CF8 and CF8M, was investigated over a wide range of temperatures. The effect of a number of material-related parameters such as ferrite level, crack orientation, and heat-to-heat and alloy-to-alloy variations was studied. In addition, the crack growth and J- integral toughness of cast CF8 subjected to neutron irradiation was characterized.

Postirradiation fracture toughness of inconel X-750

Abstract The effect of fast-neutron irradiation on the fracture toughness response of Inconel X-750 was characterized at 427°C using the J-R curve technique. Irradiation exposures ranging from 3 to 16 displacements per atom resulted in a reduction in JIc from 130 to 76 kJ/m2 and a reduction in tearing modulus from 32 to 2.6. Postirradiation fractographic examination revealed that an intergranular fracture mechanism was dominant, in contrast to the extensive transgranular cracking mode found on unirradiated fracture surfaces. The enhanced intergranular failure observed after irradiation was caused by extensive heterogeneous slip in a matrix that was greatly strengthened by an irradiation-induced dislocation substructure. Specifically, intense planar slip bands impinged on the grain boundaries and generated large stress concentrations. Since the stress concentrations could not be relaxed by the hardened matrix, the grain boundaries “unzipped” readily, resulting in the low toughness and tearing resistance.

Fracture and tensile properties of irradiated Zircaloy-2 pressure tubes

The mechanical properties of nuclear reactor components degrade as a result of long service exposure in high-temperature, irradiation, and corrosive environments. Fracture toughness and tensile testing are conducted on the pressure tubes of Zircaloy-2 to evaluate the effects of neutron fluence, hydrogen content, and temperature on the mechanical properties. Tensile tests are performed on the base metal, and fracture toughness tests are performed on both the base and weld metals. Neutron irradiation increases the strength, reduces ductility, and significantly degrades fracture toughness. The postirradiation fracture toughness increases substantially as the test temperature is increased from room temperature to 250 C. Hydrogen levels up to 250 ppm are found to have little or no effect on the postirradiation fracture toughness. Because of its anisotropic nature, Zircaloy-2 displays different fracture resistances, depending on crack orientation. In the base metal, the fracture toughness in the longitudinal orientation is higher than that for the circumferential orientation, and this difference is enhanced at higher temperatures. The weld exhibits lower toughness in the longitudinal direction.

An Evaluation of the Round Compact Specimen for Fatigue Crack Growth Rate Testing

A proposed round compact specimen was evaluated for its suitability for fatigue-crack growth rate testing. The results were compared to results from standard compact specimens (per ASTM E647-78T), and the two specimens were found to yield equivalent results. A number of K-solution have also been proposed for the round compact specimens, and these are reviewed. The agreement between the various solutions was quite good, and the Newman Equation was used to evaluate the present results.

Fracture Toughness of Stainless Steel Welds

The effects of temperature, composition, and weld-process variations on the fracture toughness behavior for Types 308 and 16-8-2 stainless steel (SS) welds were examined using the multiple-specimen J-resistance-curve procedure. Fracture characteristics were found to be dependent on temperature and weld process, but not on filler material. Gas-tungsten-arc (GTA) welds exhibited the highest fracture toughness, a shielded-metal-arc (SMA) weld exhibited an intermediate toughness, and submerged-arc (SA) welds yielded the lowest toughness. Minimum expected fracture properties were defined from lower bound fracture toughness and tearing modulus values generated here and in previous studies. Fractographic examination revealed that microvoid coalescence was the operative fracture mechanism for all welds. Second-phase particles of manganese silicide were found to be detrimental to ductile fracture behavior because they separated from the matrix during the initial stages of plastic straining. In SA welds, the high density of inclusions resulting from silicon pickup from the flux promoted premature dimple rupture. The weld produced by the SMA process contained substantially less manganese silicide, while GTA welds contained no silicide inclusions. Delta ferrite particles, present in all welds, were substantially more resistant to local failure than the silicide phase. In welds containing little or no manganese silicide, delta ferrite particles initiated microvoid coalescence, but only after extensive plastic straining.