Brian P. Somerday

Global constraint-insensitive fracture in SiC particulate-reinforced AA 2009

Experimental results prove this hypothesis:The effective plastic fracture strain (∈f¯ p) for a discontinuously reinforced metal matrix composite (MMC) is insensitive to globally imposed triaxial tensile stress, because the elastic reinforcement independently produces high local matrix constraint during plastic deformation. The value∈f¯ p is measured for SiC particulate-reinforced AA 2009-T6, with cylindrical circumferentially notched (global constraint ratio, (σm/¯σ, of 1.0) and smooth tensile specimens (σm/¯σ of 0.3), as a function of temperature. The MMC fractures by microvoid-based processes associated with the SiC at all temperatures. The ratio (r) of smooth to notched specimen ∈f¯p equals unity from 25 °C to 200 °C and is less than values of 2 or higher typical of monolithic Al alloys at similar global constraint. This result establishes that global constraint does not degrade MMC fracture resistance because of the unique effect of local matrix constraint. The ratior for the MMC increases from near 1 at 200 °C to a maximum of 3.3 at 250 °C, indicating a loss of local matrix constraint, possibly due to reduced matrix-particleload transfer or microvoid nucleation at low strains. Uncertainties hinder precise definition of ∈f¯p vs global σm/σ-σ. Global constraint-insensitive fracture for the MMC suggests that plane strain fracture toughness differences between the composite and unreinforced matrix are not well defined by smooth specimen ductilities. Further, MMC fracture toughness should be insensitive to global constraint and associated cracked specimen dimensions.

Effect of strength on environment-assisted cracking of Ti-8V-6Cr-4Mo-4Zr-3Al in aqueous NaCl. Part 1 : Age hardening vs. work hardening

Abstract The objective of this research is to understand the effect of yield strength on the environment-assisted cracking (EAC) susceptibility of a β Ti alloy, Ti–8V–6Cr–4Mo–4Zr–3Al (Beta-C), precracked and stressed actively in aqueous NaCl. Isothermal aging renders immune, β -solution treated Beta-C susceptible to severe EAC coincident with strengthening by α precipitates. The threshold for EAC decreases to 40% of the air fracture toughness, subcritical crack growth rate approaches 100 μm s −1 , and the crack path becomes fully intergranular for aging times in excess of several hours. In contrast, solution treated and cold-worked single-phase β Beta-C retains immunity to EAC for similar high strengths. For several levels of cold work, Beta-C cracks by transgranular microvoid processes, with equal values of EAC threshold stress intensity and air fracture toughness. Cold work reduces the air fracture toughness of β -phase Beta-C compared to values for β/α microstructures at similar strengths. The strength increase due to α precipitation does not uniquely promote EAC susceptibility.

Micromechanical modeling of temperature-dependent initiation fracture toughness in advanced aluminum alloys

The temperature dependence of the plane-strain initiation fracture toughness (K{sub JICi}) is modeled micromechanically for a variety of advanced aluminum alloys that fail by microvoid processes. Materials include precipitation-hardened ingot metallurgy, spray formed, submicron-grain-size powder metallurgy, and metal-matrix composite alloys. A critical-plastic-strain-controlled model, employing tensile yield strength, elastic modulus, work hardening, and reduction of area measurements, successfully predicts K{sub JICi} versus temperature for eight alloys, providing a strong confirmation of this approach. Modeling shows that K{sub JICi} is controlled by the interplay between the temperature dependencies of the intrinsic failure locus {bar {var_epsilon}}{sub f}{sup p}({sigma}{sub m}/{sigma}{sub fl}) and the crack-tip stress/strain fields governed by alloy flow properties. Uncertainties in {bar {var_epsilon}}{sub f}{sup p}({sigma}{sub m}/{sigma}{sub fl}), as well as the critical distance (volume) for crack-tip damage evolution, hinder absolute predictions of K{sub JICi}. Critical distance (calculated from the model) correlates with the nearest-neighbor spacing of void-nucleating particles and with the extent of primary void growth determined from quantitative fractography. These correlations suggest a means to predict absolute plane-strain fracture toughness.

Effect of strength on environment-assisted cracking of Ti–8V–6Cr–4Mo–4Zr–3Al in aqueous NaCl: Part II: Crack tip strain rate

Abstract The objective of this study is to critically examine the intrinsic environment-assisted cracking (EAC) resistances of three microstructures of Ti–8V–6Cr–4Mo–4Zr–3Al (Beta-C) in aqueous NaCl through variation in crack tip strain rate. The single-phase β microstructure of Beta-C cold worked to high strength, as well as a β/α structure from short-term aging at 500°C, are immune to EAC in aqueous NaCl for a range of crack tip strain rates that should allow varying levels of atomic hydrogen production and accumulation. Intergranular EAC occurs in underaged Beta-C, but only if driven by high crack tip strain rate from rapid loading or stable crack growth. Increased process-zone hydrogen from increased crack tip strain rate promotes this embrittlement. The severity of EAC in ST/A Beta-C increases with aging time for constant crack tip strain rate. These results strengthen the Part I finding that increased strength does not uniquely promote intergranular EAC; rather, crack tip strain rate as well as grain boundary characteristics that change during α-precipitation hardening govern the EAC severity of β-Ti alloys.

Observation and prediction of cavities in a lubricant between non-parallel disks in relative rotation

Observed and predicted cavity shapes in a lubricant between two disks are presented. The geometry is similar to that of a thrust bearing or a face seal having non-parallel rotating surfaces. Photographs of cavitated regions are shown and compared with predictions of cavity shapes obtained by an approximate calculation method analogous to the short bearing analysis used for journal bearings. The calculation and the experiment show steady state cavities that are symmetric about the maximum gap thickness. For small angles of disk inclination a single exterior cavity is seen and for larger values of inclination both an interior and an exterior cavity are present. The predicted cavity shape in a given case is a function of the disk inclination, the lower bound pressure for the oil film and rotor speed. General agreement on the cavity shape is obtained between theory and experiment. Presented at the 46th Annual Meeting in Montreal, Quebec, Canada April 29–May 2, 1991