J. Eng. Mater. Technol. 1988;110(1):1-8. doi:10.1115/1.3226003.

Modeling of hardening and thermal recovery in metals is considered within the context of unified elastic-viscoplastic theories. Specifically, the choices of internal variables and hardening measures, and the resulting hardening response obtained by incorporating saturation-type evolution equations into two general forms of the flow law are examined. Based on the analytical considerations, a procedure for delineating directional and isotropic hardening from uniaxial hardening data has been developed for the Bodner-Partom model and applied to a nickel-base superalloy, B1900 + Hf. Predictions based on the directional hardening properties deduced from the monotonic loading data are shown to be a good agreement with results of cyclic tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):9-16. doi:10.1115/1.3226013.

Surface small crack growth behavior of Type 304 stainless steel during low cycle fatigue under fast-fast and slow-fast cyclings was investigated at a temperature of 873 K by using the smooth specimens which had the different grain sizes. It was shown that the crack, which had already grown up to a few grain size, predominantly propagated with strain cycling, and that it was very important for the safety assessment of the components in service to detect the crack of a few grain sizes. It was also shown that small crack growth rate showed the minimum when they arrived at the grain boundaries. Above behavior resulted from that the grain boundaries temporarily impeded the small crack growth. The crack length below which the grain boundaries notably affected the small crack growth rate was also given as the function of relative length to the average grain size. Furthermore, the small crack growth rate was compared with the macroscopic crack growth one. In fast-fast cycling, the small crack growth rate was about ten times as large as the macroscopic crack growth one, where its length was comparable to the grain size. Based on the results thus obtained, the application limit of macroscopic crack growth law to the surface small crack growth was discussed. The application limit proportionally increased with the grain size, and it was about ten times average grain size in both fast-fast and slow-fast cyclings.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):17-21. doi:10.1115/1.3226002.

The dependence of the temperature distribution during hot rolling of a steel slab on the speed of rolling, reduction and initial temperature is investigated. It is observed that while the center of the slab cools, the surface loses heat at a much higher rate following which significant reheating occurs. Because of that different parts of the slab receive significantly different thermal-mechanical treatments, possibly resulting in a nonhomogeneous product.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):22-27. doi:10.1115/1.3226004.

Using an Eulerian formulation, a finite element solution for the flat rolling problem is presented. Calculations are performed to establish the effects of roll deformation and of the variation of the coefficient of friction in the roll gap on the predictive capabilities of the model. Comparison to the data of Al-Salehi et al. (1973) and Shida and Awazuhara (1973) indicates that the differences between measurements and calculations decrease when the above-mentioned effects are accounted for.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):28-34. doi:10.1115/1.3226006.

A method of analysis for the multishock compaction process of die-contained powder media with a plug at one end and an impacting punch at the other end is presented. In the method assumptions are made that the media are of a simple rigid-plastic type, and compressed only at the fronts of the shock waves passing through, and furthermore the punch and plug are rigid bodies. Based on the assumptions, particle velocities of elements between the punch surface and a shock wave front are the same and equal to the punch velocity, while velocities of elements between the front and the plug surface are equal to a velocity of the plug surface, i.e., zero. Therefore, it is possible to use jump conditions at the front and equations of motion for the punch and medium moving with the same velocity as it, instead of partial differential equations, i.e., conservation equations which were used in other methods. The equations of motion, together with the jump conditions and rigid-plastic constitutive relation equations provide two sets of equations governing the process. It is shown that there exist unique solutions of the equations of motion, and the equations are analyzed for a copper powder medium. Exact solutions obtained are compared with approximate solutions analyzed previously by the von Neumann and Richtmyer method. A fairly good agreement of the solutions by both the methods indicates that the approximate solutions are effective.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):35-40. doi:10.1115/1.3226007.

The effect of a transient thermal load on a coating which is bonded to a cylindrical substrate is analyzed using fracture mechanics by considering the presence of a circumferential edge crack normal to the inner boundary of the coating. The solution is obtained using the finite element method and is compared to the exact solution of the problem. The analysis is then used to show that smaller heat transfer rates at the boundary result in smaller stress intensity factors. For three different materials combinations, including two ceramic coatings on metal, the nondimensional stress intensity factor has a similar magnitude for short crack lengths, but varies appreciably as the crack length becomes longer. It is also determined that inner coatings result in smaller or comparable stress intensity factors than thicker ones.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):41-47. doi:10.1115/1.3226008.

Novel hybrid-matrix composites with alternating metallic matrices of different plastic flow resistance offer excellent potential for a superior strength and toughness combination than traditional monomatrix composites. The local stress concentrations in this class of composites can be controlled by proper tailoring of the metal matrices. The free edge accentuated stress state which govern inter-matrix interfacial cracking in such hybrid metal matrix composites has been solved. Determined through asymptotic expansion and numerical methods, the local decohesion stress, σθθ , is found to be always positive for far field tensile loading. The power of the stress singularity is found to depend on the ratio of the plastic resistances of the two matrix metals. A larger difference in resistance to onset of plastic flow between the two matrix metals leads to stronger stress singularity. The work hardening behavior of the matrices also affects the power of the stress singularity. At the limit, the interfacial stress becomes nonsingular for non-workhardening matrices. Detailed results of both the power of the stress singularity, and its angular variation have been determined for a range of matrix combinations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):48-54. doi:10.1115/1.3226009.

Push-pull and reversed torsion tests were carried out for notched, precracked, and smooth hollow cylindrical specimens of type 304 stainless steel at 923K in air. This paper describes the crack direction in the three types of specimens and the parameter that correlates the biaxial low cycle fatigue failure data. All types of specimens, except the smooth specimen in the reversed torsion test, failed by mode I cracking. Failure life of the specimens was discussed in connection with the crack mode. The equivalent stress based on COD could correlate the biaxial fatigue data whereas the Mises’ equivalent stress and the maximum principal stress could not.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):55-58. doi:10.1115/1.3226010.

Using extensive data on forty-seven materials including steels, aluminum, and titanium alloys, a new equation has been derived for estimating fatigue properties from simple tensile data at the temperature of interest in the sub-creep range. Compared to the Universal Slopes Equation previously proposed by Manson and Hirschberg, the new equation provides predictions that agree more closely with the measured fatigue properties over the entire life range. The earlier Universal Slopes Equation now in common use involves only ductility in the estimation of the plastic line whereas the improved equation involves both tensile strength and ductility in the estimation of the plastic line. A new equation was also derived for a large class of materials suitable for use in the cryogenic temperature range (ten different structural alloys and metals tested at liquid helium and liquid nitrogen temperatures). Here, as in the original Universal Slopes Equation, the plastic line did not require a tensile strength term. Although new constants were derived for the cryogenic range, the final predictions provided about the same degree of accuracy as the original Universal Slopes Equation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):59-62. doi:10.1115/1.3226011.

In automatic metalworking systems, in-process tool-life monitoring and quality control of the parts produced play a crucial role. This paper is on the architecture and performance of an opto-electronic sensor designed for automatic tool-wear monitoring in Computer Numerical Controlled (CNC) lathe applications. Tool wear is sensed by detecting the wear land image, which is captured by an analogic camera, digitized and processed using a computer system. The computer system, linked to the lathe control module, implements a real-time procedure supporting an optimal tool replacement strategy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1988;110(1):63-68. doi:10.1115/1.3226012.

A multiaxial fatigue failure criterion is proposed based on the strain energy density damage law. The proposed criterion is hydrostatic pressure sensitive; includes the effect of the mean stress, and applies to materials which do not obey the idealized Masing type description. The material constants can be evaluated from two simple test results, e.g., uniaxial tension, and torsion fatigue tests. The predicted results are compared with biaxial tests and the agreement is found to be fairly good. A desirable feature of this criterion is its unifying nature for both short and long cyclic lives. It is also consistent with the crack initiation and propagation phases of the fatigue life, in the sense that both of these phases can be related to the strain energy density either locally or globally.

Commentary by Dr. Valentin Fuster


Commentary by Dr. Valentin Fuster

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