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RESEARCH PAPERS

J. Eng. Mater. Technol. 1989;111(4):331-337. doi:10.1115/1.3226475.

The longitudinal coefficient of thermal expansion (α1 ) of unidirectional T300/934 and AS/3501-5A graphite/epoxy (Gr/Ep) composite changes within a thousand thermal cycles at ranges greater than 210°C. A phenomenological model is developed to extrapolate α1 to a smaller thermal range (< 100°C) and longer duration (2×105 cycles) typical of service in low earth orbit. Factors affecting fiber/matrix interface cracking are considered, and the utility of calculating α1 from an “effective” fiber volume fraction, which excludes from the volume fiber segments disbonded from the matrix, is demonstrated. This analysis is used to estimate the effect of fiber volume on α1 of T300/934 Gr/Ep during thermal fatigue.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):338-344. doi:10.1115/1.3226477.

Fatigue crack growth under random amplitude and sequence loading with peaks following the Rayleigh probability density function is simulated using the probabilistic model. Another attempt at fatigue life prediction under the above loads is made by converting random loads in to equivalent constant amplitude. Prediction results are compared with experimental findings. Empirical data for fatigue crack growth under random loads at different frequencies are compared with the results of prediction using the above techniques. Experimental results of three steels are used in this study to compare with the findings of the above prediction techniques. These steels are AISI 1018, AISI 4340 and stainless pH 17-7. It is seen that the probabilistic model produces reliable results. It conservatively predicts fatigue crack growth when no delay mechanism to retard crack growth is introduced.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):345-353. doi:10.1115/1.3226478.

The elevated temperature, uniaxial inelastic deformation behavior of a Ni-base alloy, B1900 + Hf, has been investigated by performing isothermal tensile, creep, cyclic, stress relaxation, and thermomechanical fatigue tests. The range of strain rates examined are 10−7 to 10−2 s−1 , while the test temperatures range from 25°C to 1093°C. This extensive constitutive data base has been used for evaluating the unified constitutive models of Bodner-Partom and of Walker which apply for the small strain regime. Comparison of test results with independent model predictions indicates good agreement over a broad range of loading conditions demonstrating the applicability of the unified constitutive equation approach for describing the strongly nonlinear, time, and temperature-dependent response of metals under a wide range of deformation and thermal histories. Thus, the results give confidence that the unified approach is an effective and efficient approach in which complex, history-dependent, thermo-viscoplastic flow can be represented within a single inelastic strain-rate term.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):354-362. doi:10.1115/1.3226479.

This paper reviews the experimental activities that were performed in recent years at the Department of Mechanical and Nuclear Engineering (DCMN) of the University of Pisa, mainly during research programs sponsored by the Italian Committee for Nuclear Energy (ENEA), in the field of life assessment under high temperature low-cycle fatigue and creep-fatigue conditions in AISI 316 weldments. These activities comprised tests on tubular, welded model structures of AISI 316, carried out in air, under either uniaxial or biaxial loading conditions, at 550°C and 650°C. Small smooth specimen testing was also performed mainly for characterizing the stress-strain behavior of the weldments. Extensive post-test analysis was focused on correlating the experimental results with life predictions based on commonly used or recently proposed design rules. Some of the main results have already been published but a complete report has not yet been delivered to the international literature. This paper is therefore aimed at giving a fairly complete picture of both the experimental results and the life prediction work.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):363-367. doi:10.1115/1.3226480.

The fatigue crack initiation and growth characteristics of short cracks emanating from blunt notches with root radius varying from 1.6 to 6.35 mm at various depths in woven fiber-glass reinforced polymeric composites have been investigated. It is demonstrated that the initiation and growth rate of short cracks emanating from blunt notches can be accurately described by an effective stress intensity factor range ΔKeff . The ply orientations studied, did not have any effect on the analysis. The results provide an adequate engineering approach for designing against failure from range of stress concentrations, at least for this composite system.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):368-371. doi:10.1115/1.3226481.

An approximate constitutive relation is derived for plane strain deformation in a power-law viscous matrix reinforced with long rigid inclusions with elliptical cross sections. The analysis is based on the differential self-consistent scheme and numerical calculations estimating the influence of a single inclusion on the material surrounding it. In particular, the effects of particle shape and material nonlinearity are reported.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):372-377. doi:10.1115/1.3226482.

A Nb-V steel’s response to constant true strain rate compression was compared to its behavior when the strain rate varied as during a rolling pass. The constitutive behavior during the two types of tests was found not to differ in a significant manner.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):378-383. doi:10.1115/1.3226483.

The zero-to-tension ratchetting behavior was investigated under uniaxial loading at room temperature and at 550, 600, and 650°C. In History I the maximum stress level of ratchetting was equal to the stress reached in a tensile test at one percent strain. For History II the maximum stress level was established as the stress reached after a 2100 s relaxation at one percent strain. Significant ratchetting was observed for History I at room temperature but not at the elevated temperatures. The accumulated ratchet strain increases with decreasing stress rate. Independent of the stress rates used insignificant ratchet strain was observed at room temperature for History II. This observation is explained in the context of the viscoplasticity theory based on overstress by the exhaustion of the viscous contribution to the stress during relaxation. The viscous part of the stress is the driving force for the ratchetting in History I. Strain aging is presumably responsible for the lack of short-time inelastic deformation resulting in a nearly rate-independent behavior at the elevated temperatures.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):384-392. doi:10.1115/1.3226484.

The constitutive modeling of cyclic plasticity has made great progress during the past twenty years. One of the incompletely solved problems concerns ratchetting, that is the progressive strain accumulation, cycle-by-cycle, induced by the superposition of a cyclic secondary load to a constant primary load. The paper discusses the main experimental facts and identifies the main inadequacies of the classical cyclic constitutive equations when they are used to predict ratchetting effects.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):393-398. doi:10.1115/1.3226485.

Detailed studies of the filling process of the die with liquid metal and the solidification are necessary to put this technology on a firm scientific basis. An experimental study of the fluid flow, heat transfer, and solidification encounters, however, enormous difficulties. It is extremely fast [in order of milliseconds], the small scale of the die makes local measurements difficult, and the temperature range and the nature of the liquid metal does not lend itself readily to experimentation. This paper explores whether similarity analysis is useful for the design of model experiments which reduce these difficulties and which reproduce the actual occurrence faithfully. The study is carried out in two steps. During the initial period, the whole cavity of the die is available for the fluid. Reynolds and Weber numbers which have to have the same value for the model experiment and for the die casting process permit the use of any fluid and of a large scale model which decreases the injection velocity and increases the filling time. During the later period of the filling process the cavity available for the liquid is reduced by the solidified metal. The energy conservation equation results in two more dimensionless numbers, the Prandtl and Jakob numbers which prescribe that model experiments have now to use a liquid metal but use of a metal with a low melting point and of a large scale decrease again the required injection velocity and increase the filling time by orders of magnitude, conditions beneficial for detailed and accurate experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):399-403. doi:10.1115/1.3226486.

For the purpose of comparing crack growth characteristics on small indentation cracks with those for long cracks, subcritical crack growth data on soda-lime glass were obtained using the compact type (CT) specimens with long cracks and the indentation cracks. It was found that there is apparently a small crack effect in the as-indented cracks which increases crack growth. However, the annealed indentation crack shows the same trend of crack growth as that for the CT specimens. A residual stress effect can be used to explain this anomalous growth behavior of the as-indented cracks.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):404-408. doi:10.1115/1.3226487.

The influence of pore shape on the porosity-pressure behavior of powders undergoing compaction is studied theoretically and numerically. Idealized two-dimensional models are used to compare the compression of circular pores, representing elevated temperature compaction, with cusp-shaped pores typically associated with cold compaction. An attempt is made to correlate porosity-pressure response with a logarithmic relation proven useful in the characterization of experimental compactions. A preliminary investigation on the effects of introducing a macroscopic shear into the compaction process is conducted.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):409-416. doi:10.1115/1.3226488.

The modeling possibilities of several linear and nonlinear-kinematic hardening rules are studied in detail in the particular case of tension-compression and in the framework of time-independent plasticity. The main difficulty is to reduce the predicted ratchetting without disturbing the tensile plastic flow. Among several modifications, the model proposed by Rousselier offers the best compromise. Incorporating the strain memory effect allows the correct description of many experimental facts observed on type 316 stainless steels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):417-423. doi:10.1115/1.3226489.

To formulate the constitutive equation for nonproportional cyclic loading, the influence of preloading on subsequent loading should be investigated experimentally. In this paper, both the stress-strain relation and the equi-plastic strain surface of subsequent loading after cyclic preloading have been examined carefully from combined tension-compression and torsion test with SUS304 stainless steel. The equiplastic strain surface could be represented by the quadratic form of stress when it was probed from the current center of the yield surface after cyclic preloading. Then, all the stress-strain curves of subsequent radial loading from the center were confirmed to be represented by the modified Ramberg-Osgood law. As a result, the modified Ramberg-Osgood law obtained shows availability to derive the constitutive equations of cyclic plasticity.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):424-430. doi:10.1115/1.3226490.

This paper deals with the monotonic and cyclic behavior of austenitic stainless steels and the way to model it. Developments made on constitutive equations led to elaborate sophisticated models justified by the complexity of the real behavior of these materials, especially at high temperature. In the first part, a survey of the various components introduced in the ONERA model is made in terms of a hierarchical presentation. The second part is devoted to several applications on different types of stainless steels in order to illustrate the capabilities of the model for various loading situations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):431-437. doi:10.1115/1.3226491.

Completely reversed, strain-controlled, low-cycle fatigue behavior at room temperature is investigated for steels and their weld metals. Weld metal specimens were taken from multi-pass weld metal deposited by shield metal arc welding (SMAW) and gas metal arc welding (GMAW), such that their gage length consisted entirely of the weld metal. Results indicate that there is a trend toward reduction in the low-cycle fatigue life of weld metals as compared with the base metals. In low carbon steel weld metals, the tendency described above is explained in terms of local plastic strain concentration by lack of uniformity of the multi-pass weld metals. The weld metals do not have the same mechanical properties anywhere as confirmed by hardness distribution, and the fatigue crack grows preferentially through the temper softened region in the multi-pass welds. In Type 308 stainless steel weld metals, the ductility reduction causes reductions in low-cycle fatigue life. This study leads to the conclusion that fairly accurate estimates of the low-cycle fatigue life of weld metals can be obtained using Manson’s universal slope method. However, life estimates of the Type 304 stainless steel is difficult due to a lack of ductility caused by a deformation-induced martensitic transformation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):438-439. doi:10.1115/1.3226492.

Moisture in composite materials has been the subject of considerable interest in the past decade. Many studies have shown that moisture directly causes damage in graphite/epoxy composite materials. With the increasing use of such composites in various environments, an assessment of the effects of sub-freezing and/or cryogenic temperatures on composites containing moisture is needed. This article presents a summary of these effects and demonstrates that reductions in strength at low temperatures are not due to the formation of ice crystals, but instead are due to changes in the residual stresses caused by absorbed moisture.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):440-442. doi:10.1115/1.3226493.
Abstract
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):443-449. doi:10.1115/1.3226494.

An incremental three-dimensional stress-strain relationship for concrete with induced anisotropy has been developed. The nonlinearity and path-dependency are modeled by expressing the elastic moduli at each increment as function of the octahedral and deviatoric strains, based on a uniaxial stochastic model developed earlier. Predictions of multiaxial response under proportional and nonproportional loading are in good agreement with experimental results.

Commentary by Dr. Valentin Fuster

ERRATA

Commentary by Dr. Valentin Fuster

BOOK REVIEWS

J. Eng. Mater. Technol. 1989;111(4):450. doi:10.1115/1.3226495.
FREE TO VIEW
Abstract
Topics: Ceramics
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1989;111(4):450. doi:10.1115/1.3226496.
FREE TO VIEW
Abstract
Topics: Ceramics
Commentary by Dr. Valentin Fuster

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