Jacob Kallivayalil

Compared to cold-formed parts, age-formed parts have lower residual stresses and consequently better stress corrosion resistance. This article addresses the technical issues that arise in the investigations of creep in precipitate-strengthened materials. The issues addressed help in developing alloys and tempers particularly suited for the age-forming process. The different steps involved in the program for predicting the final part shape are discussed. These basic steps involve developing mechanical tests to study creep at low temperatures and low stresses, describing low-temperature creep in terms of a constitutive model, and then using the constitutive model in a process model or finite element analysis to predict the final part shape.

ABSTRACT Cracks propagating at high speeds in metals generate heat at their tips which cannot be conducted away in the very short time span of crack growth. The effect of heating on the crack tip stress fields and on dynamic fracture toughness is analyzed using a method based on the theory of generalized analytic functions. In this method all deviations from a linear elastic, homogeneous strain field are represented as fictitious body forces, allowing one to model inelastic and thermal effects. Numerical computations of the crack tip stress and strain fields are performed assuming steady-state, plane stress, mode-I dynamic crack growth with a fixed crack tip temperature field, but with temperature and strain rate sensitive mechanical properties. The relationship between dynamic stress intensity factor and crack velocity has been determined using competing ductile and brittle fracture criteria. A careful analysis of the trailing wake has been performed to study its effect on the crack tip stress field.

Aluminum foams offer an attractive combination of attributes as engineering materials, such as low density, high rigidity, high energy absorption, and fire resistance. To date, however, metallic foams have achieved only a fraction of the market acceptance enjoyed by polymeric foams, owing largely to size limitations, poor uniformity and, above all, high unit costs. Methods utilizing casting (non-powder) metallurgy, while seemingly offering the potential of economies of scale, often suffer quality issues such as large cell sizes, poor uniformity and insufficient structural integrity. Many of these problems are associated with the rheology of the molten metal itself. While prior efforts to modify melt rheology through extrinsic additions of ceramic particles have been shown to be effective, the costly materials and processing paths used to create such suspensions have limited the economic attractiveness of such products. In this paper, aluminum foams produced through an alternative processing method will be described. The physical and mechanical properties in these fine (< 1 mm) celled aluminum foams will be related to their cellular structure and the properties of the aluminum alloy matrix from which they are produced.