K.S. Chan

Stage I fatigue crack propagation in a titanium alloy

Abstract The effects of crack orientation on Stage I fatigue behavior have been investigated by testing individual Widmanstatten colonies of the α-β Ti alloy Ti-8Al-lMo-lV in laboratory air and dry helium. Crack growth occurs in a crystallographic manner on or near the basal plane of α-phase. The fracture surface is characterized by a cleavage-like appearance and an absence of striations. The crack propagation rates are insensitive to crack orientation in both lab air and dry helium except for those cases in which the crack plane lies nearly perpendicular to the stress axis and a considerable amount of microcracking occurs on or near the α-β interfaces. When compared to the behavior in lab air, crack propagation in a dry helium environment is characterized by Stage I cracks propagating significantly slower and with a fracture surface exhibiting more ductility. The crack growth behavior in the individual colonies is also compared with that of large cracks and with ramifications concerning the small crack behavior.

The role of microstructural dissimilitude in fatigue and fracture of small cracks

Abstract The use of the stress intensity factor, K , as the characteristic driving force parameter for crack extension requires that the conditions of small scale yielding and microstructural similitude be met. The specific effect of microstructural similitude, or its lack, on the crack driving force of both small and short, versus large, cracks is examined in this article. The concept of microstructural dissimilitude is introduced, and its relevance to small crack behavior illustrated by considering the dependence of the number of grains interrogated by the crack front on crack size, and the resulting yield strength variation within the crack tip process zone. It is demonstrated that microstructural dissimilitude can lead to characteristic small crack behavior (anomalous rapid growth) by altering (1) the intrinsic fracture toughness and cyclic crack growth resistance of the process zone, and (2) the local crack driving force. The latter effect is examined in detail by using a modified Barenblatt-Dugdale model to deduce the dissimilitude-induced local crack driving force. The proposed model is then applied for predicting the crack growth and fracture behavior of short and small cracks on the basis of large crack data for a number of alloys. It is shown that the approach may be relevant to subcritical crack growth and fracture in brittle materials like ceramics, as well as to fatigue crack growth in metals.

Viscous cavity growth in ceramics under compressive loads

The cavity growth behavior of liquid phase sintered ceramics containing a continuous, amorphous grain boundary phase and subjected to compressive loads is examined. Based on the assumption that under compressive loads the creep-induced cavities nucleate and grow on grain boundaries which are approximately parallel to the loading axis, it is deduced that the local tensile stress which drives cavity growth in the viscous grain boundary film is likely induced as the result of sliding in the adjacent grain boundaries and, for compatibility reasons, controlled by the constraints and creep behavior of the matrix. On this basis, a viscous cavity growth model is developed by extending the analysis of Raj and Dang to compressive loading conditions through the incorporation of Raj and Ashbys grain boundary sliding analysis and Dysons concept of constrained cavity growth. The present model is used to predict the sizes and shapes of evolving cavities as a function of creep strain. The predicted results are discussed and compared with small-angle neutron scattering measurements of hot-pressed SiC.

Cavity nucleation at grain boundary ledges

Abstract The kinetics of cavity nucleation at grain-boundary ledges is examined for ceramics subjected to compressive loads. By considering grain boundary sliding and diffusion as concurrent processes, the analysis shows that sliding of faceted grain boundaries can induce time-dependent stress concentrations of sufficient magnitude to cause cavity nucleation at the ledges. The transient stress concentration has been found to depend on the ledge height and spacing, and to decrease rapidly with time and with distance from the ledge. Whether or not cavity nucleation occurs at the ledge depends on the local stress, the shape of the critical nucleus, and the characteristic time for cavity formation. The theoretical results are discussed in conjunction with nucleation rate measurements obtained for a sintered alumina using small-angle neutron scattering.

The crystallography of fatigue crack initiation in coarse grained astroloy at 20°C

Abstract The effects of crystallographic orientation on fatigue crack initiation has been examined for coarse-grained Astroloy at 20°C. Specimens were cycled by three-point bending at stress ranges between 5 and 95% of the proportional limit until fatigue cracks were detected. The crystallographic orientation of individual grains within which fatigue cracks initiated was determined by use of selected area electron channeling. Grains forming cracks were found to have surface normals near the 〈100〉, 〈011〉, and 〈113〉 directions. Conversely, grains which did not initiate cracks were not similarly grouped in orientation. Calculations of the Taylor factor using the Bishop-Hill approach revealed that fatigue crack initiation in Astroloy occurred at grains with low values of the Taylor factor.

A new criterion for brittle-to-ductile fracture transition

The phenomenon of brittle-to-ductile fracture in metals has been examined on the basis of the mechanics of stable crack growth. The analysis indicates that the experimentally observed tearing modulus, TR, is the result of the competition between the intrinsic tearing resistance of the material and a tearing term induced by crack extension, both of which can be expressed in terms of the normalized parameter, Ω = Eelf/σ0, where E, elfandσ0 are the elastic modulus, intrinsic fracture strain, and flow stress, respectively. Through the use of the tearing modulus, a new criterion of brittle-to-ductile fracture is proposed, and the boundary for the transition of fracture modes is predicted and compared with the experimental results of Al-Fe-X alloys, steels, conventional Al-, Ti-, and Ni-alloys in the forms of a fracture mode diagram and a tearing resistance, TR-Ω, curve. The proposed methodology is then applied to explain the brittle-ductile transition phenomena resulting from strain rate, temperature, and inclusion effects as well as those associated with grain boundary embrittlement, temper embrittlement, hydrogen embrittlement, and cleavage.

Effect of cross slip on crystallographic cracking in anisotropic single crystals

Abstract The effect of cross slip on cracking along coplanar slip bands in anisotropic single crystals is examined. By considering the activation of slip due to the stress field at a crack tip, it is demonstrated that: 1. (1) cross slip would not be effective in relaxing elastic normal stresses ahead of a crack propagating along coplanar slip bands, and 2. (2) the unrelaxed elastic normal stresses can be plastically relaxed by out-of-plane noncoplanar slip only. This results in a build-up of large normal stresses ahead of the coplanar crack. The magnitude of the maximum normal stress depends on the size of the noncoplanar secondary slip plastic zone and elastic anisotropy. Based on the presence of relatively high normal stresses on both the coplanar and the cross-slip planes, a rationale for explaining the occurrence of simultaneous cracking on two {111} cross-slip planes and the formation of fracture surface ridges in Mar-M200 single crystal is proposed.

Creep Cavitation Behavior in Polycrystalline Ceramics

Small-angle neutron scattering measurements have been performed on compression-crept alumina characterized by a relatively thick, continuous, grain boundary phase, another alumina with essentially no continuous grain boundary phase, and a silicon carbide with a relatively thin, continuous, grain boundary phase. The neutron scattering measurements, which provide a means of characterizing nucleation rates, growth rates, and morphologies of typical grain boundary cavities during the early stages of creep, have shown unequivocally that cavity nucleation does in general occur. They also have identified very significant differences in the relative roles of cavity nucleation and growth in these three ceramic systems. Measured cavitation parameters are outlined and discussed in relation to micro-structural differences and appropriate cavitation models.

Near-tip behavior of deflected creep cracks

Near-tip displacement fields of a creep crack which exhibited moderate deflection from its initially mode I condition have been measured using the stereoimaging technique. From the measured displacement fields, near-tip strains and crack opening displacements (CODs) are obtained and compared with existing asymptotic solutions for stationary, deflected cracks. The comparison reveals that the near-tip strain field and CODs of a stationary deflected creep crack in stainless steel (creep exponent of 8) are of the Riedel-Rice type. The degree of mode mixity is also adequately predicted for the deflected crack. The results for stainless steel are compared with previous results for a glass-ceramic (creep exponent of 1.5), to assess the range of applicability of the RR field. Discrepancies between theory and experiment are discussed in terms of the dominant creep mechanism, which is dislocation creep for the stainless steel and grain boundary sliding for the glass-ceramic.