J. Eng. Mater. Technol. 2004;126(4):329-338. doi:10.1115/1.1789967.

Motivated by the strong dependence of strain-hardening processes on slip system activity, a slip system hardening formulation that explicitly employs accumulated slip system shear strains and net crystal shearing rates is introduced within a polycrystal plasticity modeling formulation for predicting material response during cyclic deformation. The model, which is a slight modification of the Voce hardening model commonly employed for large strain forming simulations, was employed to model the behavior of 304L stainless steel subjected to uniaxial and multiaxial nonproportional multiple-step experiments and multiaxial multiple-phase angle experiments. The model successfully captured the pseudosaturation response that is common during the multiple-step tests and captured many of the loop-shape and stress-level features of the multiple-phase angle experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):339-352. doi:10.1115/1.1789966.

High strength low alloy (HSLA) steels, used in a wide variety of applications as structural components are subjected to cyclic loading during their service lives. Understanding the cyclic deformation behavior of HSLA steels is of importance, since it affects the fatigue life of components. This paper combines experiments with finite element based simulations to develop a crystal plasticity model for prediction of the cyclic deformation behavior of HSLA-50 steels. The experiments involve orientation imaging microscopy (OIM) for microstructural characterization and mechanical testing under uniaxial and stress–strain controlled cyclic loading. The computational models incorporate crystallographic orientation distributions from the OIM data. The crystal plasticity model for bcc materials uses a thermally activated energy theory for plastic flow, self and latent hardening, kinematic hardening, as well as yield point phenomena. Material parameters are calibrated from experiments using a genetic algorithm based minimization process. The computational model is validated with experiments on stress and strain controlled cyclic loading. The effect of grain orientation distributions and overall loading conditions on the evolution of microstructural stresses and strains are investigated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):353-359. doi:10.1115/1.1789962.

α-brass and aluminum microhardness and deep nanoindentation data when fitted to the Taylor dislocation-hardening (TDH) model produced a straight-line behavior consistent with the model. Literature data, including copper, silver, and tungsten when also fitted to TDH model, exhibited results similar to the ones produced by the α-brass and aluminum data. The nanohardness data obtained at shallower depths also exhibited straight-line behavior but with a shallower slope. Taken together, the nano-microindentation data constituted what we term a “bilinear behavior,” and we shall discuss possible mechanisms for this behavior.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):360-367. doi:10.1115/1.1789955.

Commercial methods for the mechanical design of part and die shapes often rely on trial-and-error methods applied to either experiments or simulation, as guided by intuition. Academic alternatives based on optimization techniques have had slow acceptance because they require programming access to finite element analysis programs. Two practical design methods were developed for use in conjunction with standard finite element software. The first of these produces a part shape that will exhibit a specified contact area and pressure when in contact with a deformable body. The second procedure produces die shape compensated for springback, to form a specified target part shape. The simplicity and effectiveness of these techniques were illustrated by a case study for the design of a contact heating device. The methods were shown to be robust and efficient, and an automated procedure was implemented to illustrate their practicality for a production environment. Use of these techniques can substantially reduce the cost and lead time required to produce optimal sheet-formed parts while improving performance. Extensions to other design criteria and situations can be envisioned.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):368-377. doi:10.1115/1.1789954.

Experimental evidence indicates that nickel-base alloys fail in the presence of hydrogen by ductile intergranular fracture. The degradation mechanism involves void nucleation at grain boundary carbides and grain boundary decohesion. In this study, a micromechanical model is suggested to understand the interaction of void nucleation and growth with the failure of the grain boundaries. The analysis is carried out at a unit cell comprising an elastic particle imbedded in a ductile matrix, a grain boundary along a plane of symmetry of the cell, and loaded in plane strain perpendicularly to the grain boundary. A phenomenological model for hydrogen-induced decohesion calibrated at the fast-separation limit of the decohesion theory of Rice [1], Hirth and Rice [2], and Rice and Wang [3] was used to describe the hydrogen effect on the cohesive properties of the particle/matrix interface and grain boundary. The finite element results indicate that hydrogen embrittlement of the alloy 690 is controlled by hydrogen assisted void nucleation at the carbides. The effect of hydrogen on grain boundary cohesion is almost negligible. The grain boundary decohesion, which proceeds almost instantaneously upon initiation, is caused by normal stress elevation due to the interaction of the void with the applied load. Lastly evaluative statements are made on the quantitative effect of hydrogen on the fracture toughness of the alloy 690.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):378-383. doi:10.1115/1.1789953.

Commercial, aluminum die-cast alloys are subject to long-term stresses leading to viscoelastic material responses resulting in inefficient engine operation and failure. Constant load creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al-Si and a 17% Si-Al alloys. Rupture occurred in the primary creep regime, with the eutectic alloy having the longest times to failure. Primary creep was modeled by J(t)=A+Btn with A, B, and n dependent on stress. Poor creep performance is linked to the brittle fracture of the primary silicon phase as well as other casting defects.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):384-391. doi:10.1115/1.1789952.

The tensile deformation and rotating-bending fatigue properties of a highelastic thin wire, a superelastic thin wire and a superelastic thin tube, all made of NiTi alloys, were investigated experimentally. The results obtained are summarized as follows: (1) The stress-strain curve of the highelastic thin wire is approximately linear up to a strain of 4 percent with a stress of 1400 MPa and shows little dependency on temperature and strain rate; (2) The modulus of elasticity for the initial loading stage of both the highelastic wire and the superelastic tube is low, showing superior bending flexibility as is necessary for medical applications; (3) The slopes of the strain-life curves of the alloys are steep in the low-cycle fatigue region (the strain amplitude of the fatigue limit is in the region of 0.6–0.8 percent); and (4) In the tube, fatigue cracking initiates on the rougher inner surface, resulting in a shorter fatigue life than in the case of the wire.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):392-397. doi:10.1115/1.1790542.

There is a compelling desire by power generating plants to continue running existing stations and components for several more years, despite many of them have surpassed their design service life. The idea is to avoid premature retirement, on the basis of the so-called design life, because actual useful life could often be well in excess of the design life. This can most readily be achieved by utilizing nondestructive monitoring methods to monitor the degradation of the microstructure, either when a station is down for maintenance or preferably when it is under operation. This study evaluates the use of quasi static hysteresis measurements as a possible procedure to evaluate creep in a 410 martensitic stainless steel, a material utilized in power plant components. The creep rupture tests were conducted at stresses of 100 and 200 MPa, temperatures of 500°C and 620°C, and the times varied between 48 and 120 hours. Following the creep tests all specimens were evaluated magnetically and then metallurgically by optical and scanning electron microscopy, x-ray diffraction (XRD) and by energy dispersive spectroscopy (EDS). The microstructural changes were compared with the magnetization changes. It was determined that the changes in the hysteresis curves were clearly detectable and correlated with the creep-induced damage.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):398-405. doi:10.1115/1.1789960.

The objective of this study is to predict the Brinell hardness distribution in cold formed parts by relating the plastic strains found by finite element (FE) analysis to hardness. Based on the material’s plastic flow curve, an analytical relation was established between the plastic strain induced in the metal during cold working and its Brinell hardness so that its hardness can be determined from numerically obtained plastic strains without producing the part and taking measurements. In order to verify the model developed in this study, cold extrusion experiments were performed on samples made of two different metals at five different extrusion ratios. These samples were cut, and at their centers Brinell hardness measurements were taken, which were then compared with the analytical predictions. The results of the analytical model compared quite well with the data obtained from experiments. The model was also verified by comparing its predictions with the experimentally determined values of hardness reported by previous researchers. The results showed that within the applicable range of Brinell hardness test, which covers a great percentage of hardness levels resulting from cold forming operations, the analytical model can reliably be used.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):406-412. doi:10.1115/1.1631029.

In the present study, a mathematical model has been developed to evaluate temperature and strain fields as well as dynamic and static microstructural changes during the nonisothermal forging process. To do so, a finite element analysis and a microstructural model based on Bergstrom’s model have been coupled for predicting temperature history, velocity and strain fields as well as phase transformations within the metal during and after hot forging. To verify the results of the model, theoretical predictions for loadstroke behavior and austenite grain size have been compared with experimental results for two grades of steel.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):413-419. doi:10.1115/1.1789956.

The equivalent time temperature method (ETT) is a novel extension of the equivalent time method. ETT is developed in this work to deal with time-temperature shifting of long-term polymer and polymer composite creep data, including the effects of physical aging at nonuniform temperature. Modifications to classical testing methods and protocols are presented to obtain accurate and repeatable data that can support long-term predictions with nonuniform temperature conditions through time. These techniques are used to generate momentary Time temperature superposition (TTSP) master curves, temperature shift factor rates, and aging shift factor rates. Novel interpretation and techniques are presented to deal with the coupled age-temperature behavior over long times. Validation of predictions against over 20,000 Hr of long-term data in field conditions is presented.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):420-426. doi:10.1115/1.1789958.

Performance of composite materials usually suffers from process-induced defects such as dry spots and microscopic voids. While effects of void content in molded composites have been studied extensively, knowledge of void morphology and spatial distribution of voids in composites manufactured by resin transfer molding (RTM) remains limited. In this study, through-the-thickness void distribution for a disk-shaped, E-glass/epoxy composite part manufactured by resin transfer molding is investigated. Microscopic image analysis is conducted through-the-thickness of a radial sample obtained from the molded composite disk. Voids are found to concentrate primarily within or adjacent to the fiber preforms. More than 93% of the voids are observed within the preform or in a so-called transition zone, next to a fibrous region. In addition, void content was found to fluctuate through-the-thickness of the composite. Variation up to 17% of the average void content of 2.15% is observed through-the-thicknesses of the eight layers studied. Microscopic analysis revealed that average size of voids near the mold surfaces is slightly larger than those located at the interior of the composite. In addition, average size of voids that are located within the fiber preform is observed to be smaller than those located in other regions of the composite. Finally, proximity to the surface is found to have no apparent effect on shape of voids within the composite.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):427-435. doi:10.1115/1.1789959.

The common methods used to determine the diffusion coefficients of polymer composites are based on the solution of Fickian diffusion equation in one-dimensional (1D) rectangular domain. However, these diffusivities usually involve errors primarily due to finite sample dimensions and anisotropy introduced by fiber reinforcements. In this study, the solution of transient, three-dimensional (3D) anisotropic Fickian diffusion equation is nondimensionalized using six parameters. The solution is then used to analyze the combined contribution of finite sample dimensions and anisotropy to the errors involved in diffusion constants calculated by 1D methods. The small time solution of the Fickian diffusion equation in 3D domain is used to analyze the slope used in diffusivity calculations. It is shown that the diffusion coefficient calculated by the 1D approach is exact only if the correct slope of percent mass gain versus root square time curve at t=0 is used. However, it has also been shown that depending on the part geometry and degree of anisotropy, there might be considerable differences between the measured slope from the experimental data and the actual slope at t=0. The mismatch between the slopes results in as much as 50% errors in estimates of diffusion coefficients. Using the 3D solution in nondimensional form, the magnitudes of these errors are studied. A least-square curve-fit method, which yields accurate anisotropic diffusion coefficients, is proposed. The method is demonstrated on artificially generated experimental data for a polymer composite containing 50% unidirectional reinforcement. The anisotropic diffusion coefficients used to generate the data are recovered with less than 1% error.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):436-442. doi:10.1115/1.1789961.

Mechanical properties of silicon carbide particle reinforced aluminum alloy 6061 composites with 5% and 10% (SiCp/Al) as well as aluminum alloy 6061 (AA6061) degraded by neutral 3.5% NaCl solution were examined by tensile tests and micro Vickers hardness measurement. The samples were degraded in the neutral 3.5% NaCl solution for a month (≑720 h) at 23°C (room temperature corrosion, RC) and 100°C (high temperature corrosion, HC). It is noted from surface observation of the corrosively degraded samples that the RC-samples (SiCp/Al and AA6061) were degraded by pitting around intermetallic compounds and SiC particles while corrosive degradation of the HC-samples was caused by synergy effect of pitting and intergranular corrosion. Corrosion reaction of the RC-samples was limited to their surface but the HC-samples were received severe corrosion damage until inside part. Thereby, mechanical strength of the latter (a maximum of 220 MPa) was lower than that of the former (a maximum of 330 MPa). Reduction of proof stress, σ0.2%, and ultimate tensile strength, σUTS, was greater in the SiCp/Al than in the AA6061 in the case of the same condition. The result to analyze the experimental data regressively showed that reduction of tensile strength for the RC-samples was proportional to the size of pit while tensile strength of the HC-samples was proportional to the ratio of corroded area to cross-section area. The empirical equations to evaluate the mechanical strength of both cases of the corrosively degraded samples (RC- and HC-samples) were proposed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):443-449. doi:10.1115/1.1789963.

Engineering test data occasionally violate assumptions underlying standard material model identification. Consequently, one has to apply appropriate remedies with respect to each violation to enhance the reliability of identified material parameters. This paper generalizes the use of the signal-to-noise weighting scheme when heteroscedasticity of test data are suspected. Different mathematical and practical aspects of the approach are discussed. Additionally, the ensuing weighted identification process is simplified to an equivalent standard form by means of a space transformation. Finally, the approach is applied to the identification of a nonlinear material model for textile composites, on both qualitative and quantitative levels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):450-456. doi:10.1115/1.1789964.

This paper considers the mechanical problem of a semi-infinite piezoelectric medium under sudden thermal load. The medium contains an electrically conducting crack perpendicular to its surface. The transient stresses and electric fields in an uncracked medium are calculated first. Then, these stresses and electric fields are used as the crack surface traction and electric field loads with opposite signs to formulate the mixed boundary value problem. Numerical results for the stress and electric field intensity factors are calculated as a function of normalized time and crack size. Crack propagation behavior is discussed. The parameters that control the transient thermal stress and electric fields are also identified. The maximum thermal shock strength that the material can sustain without catastrophic failure is established according to two distinct criteria: (i) maximum local tensile stress equals the tensile strength of the medium, and (ii) maximum stress intensity factor for the preexisting representative crack equals the fracture toughness of the medium. The results show that the influence of the piezoelectric effects on the thermal stress intensity factor is insignificant.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):457-464. doi:10.1115/1.1789965.

Recently, a new method for residual stress measurement in thin films by using the focused ion beam (FIB) has been proposed by the authors. It is based on the combined capability of the FIB imaging system and of high-resolution strain mapping software (VIC-2D). A simple equation based on two-dimensional elasticity is used to evaluate the residual stress from the displacements due to introducing a slot. The slot length is assumed to be much larger than the slot width or depth. And the effect of the slot width was neglected. However, it is often hard, depending on film materials, to introduce a narrow and deep slot by FIB. In this work some practical issues regarding the slot geometry are addressed. Through two- and three-dimensional finite element analyses, it is explored how the slot length, width and measurement location affect the displacements which are the basic data for residual stress evaluation. As a result, the validity and limit of the equations based on two-dimensional elasticity are evaluated. Also, the effect of material dissimilarity between film and substrate is explored. Finally, examples for a diamond-like carbon film on glass substrate and an aluminum oxide film thermally grown upon an alloy are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2004;126(4):465-473. doi:10.1115/1.1789957.

This paper examines the effects of laser peening on Alloy 22 (UNS N06022), which is the proposed material for use as the outer layer on the spent-fuel nuclear waste canisters to be stored at Yucca Mountain. Stress corrosion cracking (SCC) is a primary concern in the design of these canisters because tensile residual stresses will be left behind by the closure weld. Alloy 22 is a nickel-based material that is particularly resistant to corrosion; however, there is a chance that stress corrosion cracking could develop given the right environmental conditions. Laser peening is an emerging surface treatment technology that has been identified as an effective tool for mitigating tensile redisual stresses in the storage canisters. The results of laser-peening experiments on Alloy 22 base material and a sample 33 mm thick double-V groove butt-weld made with gas tungsten arc welding (GTAW) are presented. Residual stress profiles were measured in Alloy 22 base material using the slitting method (also known as the crack-compliance method), and a full 2D map of longitudinal residual stress was measured in the sample welds using the contour method. Laser peening was found to produce compressive residual stress to a depth of 3.8 mm in 20 mm thick base material coupons. The depth of compressive residual stress was found to have a significant dependence on the number of peening layers and a slight dependence on the level of irradiance. Additionally, laser peening produced compressive residual stresses to a depth of 4.3 mm in the 33 mm thick weld at the center of the weld bead where high levels of tensile stress were initially present.

Commentary by Dr. Valentin Fuster

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