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RESEARCH PAPERS

J. Eng. Mater. Technol. 1982;104(3):159-164. doi:10.1115/1.3225059.

Four kinds of creep constitutive models, i.e., strain-hardening, modified strain-hardening, kinematic-hardening, and mixed-hardening theory, are evaluated on the basis of creep-test results on type 304 stainless steel at 650°C under repeated multiaxial loading. The predictions of the four models are compared with the experimental results. It is shown that substantial differences appear among these predictions under large rotations of the principal axes of the deviatoric stress tensor, and that none of them can describe with sufficient accuracy the transient increase of strain-rate and the noncollinearity between the deviatoric stress and creep strain-rate vectors which are observed just after the stress-rotations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):165-173. doi:10.1115/1.3225060.

The aim of this investigation was to develop a method for the prediction of the high-strain multiaxial fatigue life (low-cycle fatigue) of a steel, based on high-strain uniaxial fatigue data. In this study, thinwalled cylindrical specimens were subjected to fully reversed high biaxial strains using differential pressure and axial load. The directions of the principal strains were maintained parallel and perpendicular to the specimen axes. Five different strain ratios were investigated covering both the inphase and fully out-of-phase conditions. The material selected for this investigation was a medium carbon steel; C-1018. It was found that a maximum principal strain theory, based on the plastic components of the applied strains, taking into account the strain in the thickness direction, resulted in a correlation between the biaxial and uniaxial results which was either good or conservative. Under inphase conditions the low-cycle fatigue life could be predicted using Nf = Nf0 a(ε1 +ε2 ) and under fully out-of-phase conditions using Nf = Nf0 aε1 where ε1 and ε2 are the ordered applied plastic strains and the constants “Nf0 ” and “a” are obtained from uniaxial high-strain fatigue tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):174-179. doi:10.1115/1.3225061.

Experimental results are presented for fatigue of closed-ended thick-walled tubes of 2.5 wall ratio and 32.3 mm (1.27 in.) ID for SAE 4333 steel subjected to pulsating internal pressure. Results are given for the following conditions: as-received-polished bore, stress relieved, honed with a 207 MPa (30 ksi) mean pressure, honed with a 276 MPa (40 ksi) mean pressure, nickel plated bore, roller burnished bore, and shot peened bore.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):180-185. doi:10.1115/1.3225062.

Multiaxial fatigue studies of peened helical extension springs made of music wire were performed in a specially designed testing machine. The test regime imposed static torsion and alternating laterial deflection, with the spring ends held parallel. The maximum stresses, which were primarily due to bending, occurred in the first turn out of the specimen grip at an end of the coil axis normal to the direction of lateral motion. The stresses in the middle turn ranged between 88-94 percent of those in the ends turns. The practical fatigue limit was about 100,000-300,000 cycles, with the principal tensile stress averaging 50 percent of initial wire tensile strength and maximum shear stress averaging 28 percent of initial wire tensile strength. For static stresses up to 32 percent of tensile strength, the alternating stress at the fatigue limit was essentially invariant, as predicted by Sines’ criterion for multiaxial fatigue.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):186-191. doi:10.1115/1.3225063.

In the development of better methods of assessing damage accumulation, one of the requirements is an understanding of the cyclic constitutive behavior of the material. It is known that the cyclic stress-strain behavior is affected by temperature and possibly Rε ratio (εmin /εmax ) and that the properties change as cycles are accumulated. This report presents some results, particularly the development of a mean stress in the material, obtained during low cycle fatigue tests of Inconel 718. The tests varied temperature and Rε ratio to determine their effects on the cyclic constitutive relation. Changes in the cyclic stress-strain behavior as a function of cycles were also examined. It was possible to relate the mean stress to either the total or plastic strain ranges for all temperatures. There was sufficient scatter in the data to prevent an unambiguous interpretation of the effect of Rε ratio on the mean stress, however.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):192-199. doi:10.1115/1.3225064.

Tests are carried out on compact tension specimens of AISI 1020 hot-rolled steel at nominal stress levels beyond yield. The results show that crack propagation rates can be correlated successfully using an operational definition of the range ΔJ of the J-integral. The rate of crack growth per cycle da/dN is shown to be related to ΔJ by da/dN=A (ΔJ)α for conditions ranging from linear elastic to limit load. The exponent α agrees well with other published data on low strength steels and the quantity A is found to have a weak dependence on frequency. Examination of the fracture surface reveals that static failure modes are operating at the high crack propagation rates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):200-206. doi:10.1115/1.3225065.

Fracture of beryllium under biaxial states of stress has been studied in the past using thin-walled tubes loaded by combinations of axial load, torsion, and internal pressure. In the present investigation to obtain a triaxial state of stress, notched beams of beryllium with varying dimensions were tested in plane strain four-point bending. The conditions necessary to ensure plane strain are discussed in detail and plane strain finite element analyses are used to determine the stresses and strains at fracture in the notched specimens. Based on the test results a strain dependent, maximum tensile stress fracture criterion is proposed for parts without macroscopic cracks. In addition, the plane strain fracture toughness of beryllium is estimated from the notched bar tests using the RKR model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):207-214. doi:10.1115/1.3225066.

In a previous paper by the authors the minimum elastic strain energy density criterion was studied by considering the general case where both KI and KII -components of the stress intensity factor are operative when the crack is stationary, and static elastic fracture theory is valid. In this paper the dynamic case was considered. The influence of the velocity of a propagating slant crack in a homogeneous and elastic plate was taken into account. The dynamic stress intensity factors KI d and KII d were accurately determined by using the method of reflected caustics and the dynamic initial curve was defined to delineate the boundary of the core region, which is assumed to be the limiting curve, along which the energy density should be evaluated. It was shown that, in particular cases, important corrections to the position of this minimum and its size should be introduced in order to take care of the dynamic influence.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):215-219. doi:10.1115/1.3225067.

Zahoor and Kanninen have recently developed a simple procedure for estimating the magnitude of the J-integral for through-wall cracks in pipes subjected to bending loads. This paper gives consideration to their procedure, but to check its predictions against available numerical results, it is explored in detail for the case of a crack in a solid deforming under plane-strain bending conditions. In this case, an implicit assumption in the procedure is that the plastic rotation depends on the ligament size, and not on any other geometrical dimension. This assumption is strictly valid only for deep cracks, and this paper shows the degree of inaccuracy obtained when it is applied to shallow cracks. The assumption is also shown to correlate with the existence of a unique relation, independent of geometrical parameters, between the ligament net-section stress and the J-integral, and also with the existence of Turner’s plastic η factors.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):220-226. doi:10.1115/1.3225068.

Pressure vessels for coal conversion processes are likely to be made of a 2 1/4 Cr-1Mo steel, covered by ASTM specification A387, class 2, grade 22. Under the projected operating conditions, degradation of the mechanical properties of the steel by temper embrittlement and hydrogen embrittlement is a major concern. The significance of these degradation mechanisms is reviewed here in the light of experimental results recently generated by EPRI projects and other results published in the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):227-233. doi:10.1115/1.3225069.

Oxygen-free copper, 6061 aluminum, and 1100 aluminum bars were drawn from various initial diameters in the annealed state to bars of 11.8 mm (0.464 in.) at maximum reductions in area up to 79.5 percent. It was found that an apparent lack of hardening existed at the higher strains up to the maximum investigated as was determined in tension tests of the drawn bars. The lack of hardening observed was thought to be due, at least in part, to the development of microcracks at the larger strains, a theory proposed by Shaw. It was also found that the oxygen-free copper of the two different heats had appreciable variations in ductility at fracture in tension tests. The reason for this behavior was not determined.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1982;104(3):234-240. doi:10.1115/1.3225070.

A heat treatment has been developed which produces significant improvements in the tensile properties of Cu-15Ni-8Sn spinodal alloy. The treatment involves solution heat treatment in the two-phase region rather than the single-phase region normally used. After quenching and aging, increased strength and ductility of the alloy over single phase solution heat-treated and aged values were found. The mechanical properties obtained were superior to any previously observed for material of the compositions studied in the solution treated, quenched, and aged condition. Also, the alloys’ transformation kinetics were greatly slowed by the two phase heat-treatment. It is suggested that the increase in strength and slow kinetics of transformation observed are caused by grain size effects and by grain boundary modifications. Resistivity data and etching response corroborate these arguments.

Commentary by Dr. Valentin Fuster

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