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J. Eng. Mater. Technol. 1976;98(1):1. doi:10.1115/1.3443328.
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Abstract
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):1. doi:10.1115/1.3443329.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. Eng. Mater. Technol. 1976;98(1):2-8. doi:10.1115/1.3443331.

Creep tests on two heats of a CrMoV steel at 811K (1000°F) are analyzed using a new graphical procedure to solve the equation σ = C (t ) H (ε). This enables prediction of both constant load and constant stress creep behavior at different stresses within the range examined which included tests lasting in excess of 10,000 hr. An analytical solution of the equation is also described and used to generate isochronous stress-strain curves. Attempts to predict the response in relaxation and constant strain rate tests where the load is continuously changing are described. Reasonable agreement was obtained in the former type of test using a total strain rule, which assumes a unique relationship between stress, strain and time, when the stress range covered was within the range used in the creep analysis. For constant strain rate testing, where a much broader stress range was involved, it was necessary to use incremental analysis to achieve good agreement between predicted and experimental curves up to about 2 percent strain.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):9-16. doi:10.1115/1.3443345.

Conventional mechanical properties tests (tensile, stress/rupture, and impact toughness) have been performed on annealed specimens of austenitic stainless steels (0.152 cm thick, 0.635 cm wide) which have been carburized in high temperature (538–593°C) sodium for times up to 2000 hr. Included in the work were alloys which might have been more resistant to carburization than conventional alloys, Type 310SS, 18-8-2, Uranus-S and Inconel-601. Results indicate that the tensile and stress rupture ductility of all the alloys decreased to less than 5 percent in tests at 538–593°C when the bulk carbon content of the specimens was 0.4 percent or greater. The decrease in ductility of surface-carburized specimens is much greater than that of alloys of the same nominal composition and same bulk carbon content but with the carbides homogeneously distributed throughout the specimen. It is shown that the brittle surface layer fractures in a transgranular, cleavage mode while the center of the specimens fail in the normal shear mode.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):17-23. doi:10.1115/1.3443327.

The plastic zones associated with single overloads of cyclically loaded specimens have been mapped using electron channeling patterns. The zones are asymmetric with respect to the crack tip, and are complex in shape. Crack retardation subsequent to an overload is closely related to the size and shape of the overload zone, but has no apparent relationship to the maximum zone dimension. Following an overload, cracks try to exit from the monotonic zone by moving toward the nearest elastic-plastic boundary. The size of the overload zone is predicted by a plane strain rather than plane stress relationship. The minimum retarded growth rate corresponds to an effective stress intensity factor no greater than the threshold value for Stage II growth. This is caused by crack closure, with minimal crack tip shear strains and an absence of crack tip opening and blunting. Since the crack growth rate quickly approaches the preoverload rate once the crack crosses the overload boundary, it appears that residual stress within the overload plastic zone is the key factor in governing crack retardation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):24-29. doi:10.1115/1.3443330.

The techniques of selected area electron channeling and positive replica examination have been used to study the plastic zones attending fatigue crack propagation in 304 SS, 6061-T6 aluminum alloy, and Fe-3Si steel. These observations allowed the strain distribution at the crack tip to be determined. The results indicate that the concepts of a monotonic and a cyclic plastic zone are essentially correct, with the strains at demarcation between these two zones being 3 to 6 percent. Strain distribution varies as r −1/2 in the cyclic zone and as ln r in the monotonic plastic zone. The strain distributions for all materials studied may be made approximately coincident by using a dimensionless parameter related to distance from the crack tip.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):30-35. doi:10.1115/1.3443332.

The model of Ritchie, Knott, and Rice [10], relating the critical tensile stress for initiation of unstable cleavage fracture of mild steel to the cryogenic temperature-dependence of plane strain fracture toughness (KIc ), is applied to a pressure vessel steel, ASTM A533B. It is shown that the fracture criterion of achieving the critical tensile stress over a characteristic microstructural distance is essentially unaffected by neutron irradiation. Thus, it appears that the effects of irradiation on the temperature-dependence of KIc in the cleavage range can be quantitatively assessed solely in terms of the effects on the yield and flow properties of the material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):37-46. doi:10.1115/1.3443335.

The effects of work hardening rate and inclusion content on the initiation and propagation of ductile fracture in low strength steels have been investigated. A model has been proposed based on these results, which describes the difference between the energy required for initiation and propagation of ductile fractures in such steels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):47-51. doi:10.1115/1.3443336.

The paper discusses the constitutive relations for the inelastic deformation of a polycrystalline metal at high temperature. Commencing from a description of a dislocation structure in terms of strain hardening and thermal softening, the general form of the constitutive relation is developed in terms of a potential function. The existence of a stationary state potential is established and generalizations of isotropic and kinematic hardening are described.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):52-59. doi:10.1115/1.3443337.

Further analysis of a model in which creep is attributed to the movement generation, immobilization, and recovery of dislocations has permitted the anelastic (recoverable) component of strain to be calculated. The earlier conclusions are unaffected by this refinement and so the model equations continue correctly to predict the effects of raising, reducing, removing and reversing the stress during the creep of a range of materials including stainless steel, zirconium alloys, copper, tungsten, nickel, and aluminum. In addition, however, it is now (correctly) predicted that removal of a tensile stress will permit compressive creep to occur, producing an anelastic strain whose magnitude increases towards an upper asymptote as the magnitude of the prior tensile stress increases. Anelasticity is found to become an increasingly large proportion of the total strain at low stresses: the model predicts this. It also predicts the pattern of behavior observed in “stress-dip” tests: in particular that there is a critical magnitude of stress reduction which causes momentary cessation of creep.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):60-68. doi:10.1115/1.3443338.

Limiting solutions are discussed for elastic-plastic deformation around rigid particles of both equiaxed and greatly elongated shapes. It is shown that if the matrix can be characterized as a rigid nonhardening continuum the stress concentration at the particle interface and interior is less than two for either equiaxed or elongated particles. In a rapidly strain hardening matrix, however, while the interfacial stress concentration relative to the distant boundary traction remains at a factor of two for the equiaxed particles, it rises nearly linearly with aspect ratio for slender platelets and rods. Interaction between particles can occur when the local volume fraction of particles is high. Such interactions raise the interface tractions for a given state of shear of the matrix and hasten void formation, and are often discerned as a particle size effect. Another particle size effect based on flawed particles is also discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):69-75. doi:10.1115/1.3443339.

Observations of microscale behavior of cast irons indicate that considerable plastic flow occurs between microflaws prior to rupture. Therefore, the plastic limit load for small representative volume elements of material treated as structures should be a close approximation to the load at rupture. A representative volume element of cast iron is analyzed using limit analysis techniques to obtain bounds on the limit, or failure, surface under three dimensional average stress. Comparisons with other cast iron failure criteria and experimental data are made.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):76-85. doi:10.1115/1.3443340.

The true stress-strain curves of polycrystalline aluminum, copper, and stainless steel are shown to be adequately represented by an exponential approach to a saturation stress over a significant range. This empirical law, which was first proposed by Voce, is expanded to describe the temperature and strain-rate dependence, and is put on a physical foundation in the framework of dislocation storage and dynamic recovery rates. The formalism can be applied to the steady-state limit of creep in the same range of temperatures and strain rates; the stress exponent of the creep rate must, as a consequence, be strongly temperature dependent, the activation energy weakly stress dependent. Near half the melting temperature, where available work-hardening data and available creep data overlap, they match. Extrapolation of the proposed law to higher temperatures suggests that no new mechanisms may be necessary to describe high-temperature creep. A new differential equation for transient creep also follows from the empirical work-hardening law.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):86-91. doi:10.1115/1.3443341.

In this paper we report the behavior of the plastic deformation of an idealized crystal made by stacking parallel slip planes. Each slip plane is assumed to contain active sources of dislocations leading to a constant density of non-interacting dislocations in the plane which glide through randomly distributed localized point obstacles, representing small precipitates. The dislocation is assumed to have a constant line tension and the dislocation-obstacle interaction is taken to have a simple step form. Using results of computer simulation of thermally activated glide through random arrays of point obstacles we modeled deformation as a function of temperature and applied stress, determining the strain rate and the morphological characteristics of slip.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):92-95. doi:10.1115/1.3443342.

Fiber-reinforced epoxy laminates were exposed to several combinations of temperature and relative humidity. The purpose was to determine rates and extent of moisture absorption. Diffusion rates varied with temperature, and equilibrium moisture contents varied with relative humidity. Data analysis provided values for the material properties that determined diffusion behavior. A nonlinear diffusion model containing these values was shown to describe and predict absorption behavior for any combination of temperature and humidity. The model also describes desorption and correlates well with experimental data from changing humidity conditions.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

BOOK REVIEWS

J. Eng. Mater. Technol. 1976;98(1):96. doi:10.1115/1.3443343.
FREE TO VIEW
Abstract
Topics: Metal fatigue
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1976;98(1):96. doi:10.1115/1.3443344.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

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