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RESEARCH PAPERS

J. Eng. Mater. Technol. 1974;96(3):145-156. doi:10.1115/1.3443203.

Scanning electron micrographs of different types of commercial abrasive grains used in grinding are discussed. Surface topography is found useful, along with other physical and chemical properties, in characterizing abrasive materials. The results of this study explain some of the differences in performance of several types of abrasive presently used in grinding practice.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):157-162. doi:10.1115/1.3443204.

Dynamic SEM studies of plastic flow in a uni-axial tensile test requires that the loading stage possess more than an ability to apply large loads to the test specimen. In such a test, observations of the details of the fracture processes are feasible only if the transfer of stored energy from the test system to the test specimen is minimal. This requires careful choice of test system compliance. This paper discusses system-specimen interactions and identifies the key criteria to be met for the design of satisfactory SEM tensile stages. The design and use of such a new stage is described. Preliminary results from a 0.08 percent carbon steel sheet tensile specimen are presented to demonstrate the utility of rigid tensile stages. The relevance of such tests, particularly for the characterization of the fracture resistance of test materials, is indicated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):163-167. doi:10.1115/1.3443205.

The effect of cutting speed and wear land length on the phenomenon of microchip formation during machining of quenched and tempered AISI 4340 steel under dry orthogonal conditions is determined. Machined test pieces are examined using scanning electron and optical microscopy. Surface roughness is determined using a profilometer. A possible mechanism of microchip formation based on the interaction between the surfaces of the tool and freshly machined work piece is discussed. It is suggested that the grooves left by the generation of microchips may act as sources for the initiation of failures by creep, fatigue and stress corrosion cracking. It is also suggested that the results obtained using scanning electron microscopy may be more indicative of the true surface condition than surface roughness measurements.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):168-175. doi:10.1115/1.3443206.

The mechanism of formation of top and bottom dross during plasma cutting were studied and a theory was developed for the case of top dross which agrees well with experiment. Techniques were developed, involving secondary gas flows around the plasma jet and a rotating brush following the plasma jet, which very greatly reduced top-dross formation. Similarly the use of jets under the cut was shown to reduce under-dross to negligible proportions. Thus, by using these techniques, the quality of plasma cutting can be greatly improved and the speed greatly increased without the formation of excessive top dross. Tests done with thin gage stainless steel were successful and are described in detail.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):176-181. doi:10.1115/1.3443207.

Laminar composites were prepared of outer layers of 18Ni (200) maraging steel with various inner layer materials. The composites showed reasonable fabricability in both hot and cold working operations. The materials were weldable, and the composite structure could be continued through the weld zone using conventional welding techniques. Yield strengths on the order of 200 ksi were obtained along with excellent crack stopping ability in impact, fracture toughness, stress-corrosion, and fatigue loading. In general, good combinations of fabricability and crack stopping ability can be achieved through the use of soft, relatively thick internal layers that are well bonded to the outer layers.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):182-189. doi:10.1115/1.3443208.

The method of slip-lines has been used historically to solve two-dimensional problems of plane-strain plasticity in homogeneous metals. The application of the theory to obtain solutions for sandwich metals consisting of a hard core and softer clad is described here. The behavior of stress components, strain rate components, and velocity discontinuities at the interface between the two metals is examined. Some slip-line solutions for incipient plastic flow due to indentation of a sandwich strip with flat dies are presented. Three modes of deformation are considered: full penetration of the plastic zone with lateral motion of the rigid overhangs, bulging with clad flow only, and bulging with flow of clad and core. The numerical construction of the slip-line fields and hodagraphs is described and some results given. The differences between these results and the results of using homogeneous theory with an average yield strength are shown to be significant for strips of intermediate clad fraction. In addition, the results show that bulging, rather than full penetration of the plastic zone, is the expected mode of deformtion for a wide range of the strip parameters.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):190-194. doi:10.1115/1.3443209.

Tension tests on solid cylindrical specimens and internal pressure tests on one type of tubular specimen showed that a superimposed hydrostatic pressure increased the ductility. Internal pressure tests on a similar tubular specimen that was supported in a different manner showed that the hydrostatic pressure had almost no effect on the ductility.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):195-200. doi:10.1115/1.3443210.

A diameter gage is described which has been used to obtain true stress-true strain data at elevated temperatures. The device is capable of continuously and automatically measuring diameter of a tensile specimen to a true strain as high as 0.90–0.95. Data are presented which demonstrate the ability of the gage to identify conditions of non-uniform deformation behavior.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):201-206. doi:10.1115/1.3443211.

Uniaxial tensile properties of the niobium-base alloy Cb-752 have been determined as a function of oxygen, nitrogen, and hydrogen content over a temperature range of −196 C to 200 deg C. Each of these impurities increased the temperature at which a ductile-brittle transition occurs. Although ductility was severely reduced, strength parameters were relatively unchanged making detection of embrittlement by hardness testing difficult. Impurity levels for embrittlement were sufficiently low and the affinity of Cb-752 for contamination sufficiently great that processing operations require strict control. The mechanism of this impurity embrittlement is not well understood. However, observations of fracture surfaces of brittle failures reveal mixed intergranular cleavage with a uniform distribution of precipitates throughout grain boundaries. These observations are discussed in the light of current theories.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):207-213. doi:10.1115/1.3443212.

Creep theories with the effect of the third invariant of the deviatoric stress tensor and their accuracy as applied to practical problems are discussed. Constitutive equations for transient creep are first formulated by assuming creep potentials of the Prager-Drucker and the Bailey-Davis type together with the associated equivalent stress functions. Strain-hardening and time-hardening hypotheses are assumed. Experimental results hitherto reported for thin-walled tubes are discussed according to these equations. Then, the creep of a thick-walled tube of mild steel is analyzed and compared with experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):214-221. doi:10.1115/1.3443213.

Analytical comparisons are made between the thermal ratcheting response of a kinematic hardening material, a perfectly plastic, and an isotropic hardening material for a two-element assembly. Significant differences were found in the range of mechanical and thermal loading for which ratcheting occurred and the magnitude of the strain accumulation when ratcheting did occur. The kinematic hardening strain accumulation predicted was always smallest.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):222-226. doi:10.1115/1.3443214.

A new approach to improving the fatigue resistance of loaded holes in sheet material is presented. The material is first dimpled centering about the region where the hole is to be made, thus introducing tensile plastic strain. The dimple is then flattened and the hole drilled so that the region around the hole is under residual compressive stress. Zero to tension fatigue tests indicate considerable improvement in the fatigue strength of loaded holes as a result of dimpling. A fatigue strength improvement factor of nearly three for 106 cycles to failure was obtained based on the median lives of nominally 0.050-in. thick 2024-T3 aluminum alloy specimens.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):227-234. doi:10.1115/1.3443215.

Reference stresses and scaling factors are determined for some common beam structures and the results are presented in practical design charts. One set of charts treats the influence of cross-sectional shape and another set the influence of the structural layout. The approximations of the reference stress technique are discussed and the error in deformation rate is calculated for the structures. The reference stress method and the use of the design charts is demonstrated in the creep design of a simply supported uniformly loaded beam.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Eng. Mater. Technol. 1974;96(3):235-236. doi:10.1115/1.3443216.
Abstract
Topics: Density , Measurement
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):236-238. doi:10.1115/1.3443217.
Abstract
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1974;96(3):238-240. doi:10.1115/1.3443218.

A numerical method based on the modified Hencky and Geiringer equations is described for solving plane-strain metal flow problems of strain-rate sensitive materials. The slip-line field and flow-stress distributions are determined simultaneously using an iterative calculation.

Commentary by Dr. Valentin Fuster

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