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IN THIS ISSUE

### Research Papers

J. Eng. Mater. Technol. 2010;132(3):031001-031001-9. doi:10.1115/1.4001261.

A variant of the equal channel angular pressing (ECAP) process is examined in this paper where the channels are of rectangular shape with different thicknesses while the widths of the channels are the same. The process is named nonequal channel angular pressing and it is similar to the earlier introduced dissimilar channel angular pressing (DCAP) process. In DCAP, however, the diameters are near values, with the exit channel being slightly larger, while in NECAP, the exit channel is much smaller attributing several advantages to nonequal channel angular pressing (NECAP) with respect to ECAP. In this work an analysis is performed to determine the strain mode in a 90 deg NECAP die. A new flow line function is also presented to better describe the deformation field. The proposed flow line function is validated using finite element simulations. A comparison is made between ECAP and NECAP. Finally, texture predictions are presented for NECAP of fcc polycrystals. The advantages of this severe plastic deformation process are the following: (i) significantly larger strains can be obtained in one pass with respect to the classical ECAP process, (ii) grains become more elongated that enhances their fragmentation, and (iii) large hydrostatic stresses develop that improve the stability of the deformation process for difficult-to-work materials. The results obtained concerning the deformation field are also applicable in the machining process for the plastic strains that imparted into the chips.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031002-031002-11. doi:10.1115/1.4001262.

Stress whitening problem in thermoformed alumina trihydrate (ATH) reinforced poly(methyl methacrylate) (PMMA) was studied. In situ heavy-gage thermoforming of acrylics was entirely replicated under laboratory controlled conditions at different operation parameters. Samples were monitored with optical microscope after the completion of the thermoforming operation. For stress whitening quantification, a new index was proposed from image histograms of processed optical micrographs. Results indicated that stress whitening in PMMA/ATH samples increases with level of plastic deformation at all thermoforming conditions. The influence of the forming rate and forming temperature on the degree of stress whitening was explained in terms of change in material behavior and microdeformation mechanisms around two characteristic temperatures of PMMA/ATH. Developed method for stress whitening quantification characterizes different levels of stress whitening with single numeric values. It is shown that stress whitening index and density of microdeformation features display a strong correlation. Higher density of particle cracks at low forming temperatures results in higher stress whitening levels. Increased surface irregularity and large size voids at high forming temperatures produce lower stress whitening.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031003-031003-16. doi:10.1115/1.4001263.

$Al2O3$ coatings have been deposited onto an Al alloy by the fluidized bed (FB) technique using alumina powder. Film formation through a cold deposition process and its growth kinetics have been investigated by varying the deposition time. This allowed us to establish how the morphology, microstructure, hardness, scratch resistance, and adhesion strength of the $Al2O3$ films were progressively imprinted. The FB process led to the deposition of a good-looking and well-adhered graded $Al2O3$ coating, which was found to be progressively richer in $Al2O3$ on moving from the interface with the Al alloy toward the outermost layers. The resulting $Al2O3$ coatings have been shown to produce a consistent improvement of the overall mechanical and tribological performance of the Al alloy, thus leading to the build-up of an overlying hard and tough protective layer.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031004-031004-11. doi:10.1115/1.4001264.

Sandwich structures are gaining wide applications in aeronautical, marine, automotive, and civil engineering. Since such sheets can be subjected to stamping processes, it is crucial to characterize their forming behavior before trying out any conventional forming process. To achieve this goal, sandwich sheets of Al 3105/polymer/Al 3105 were prepared using thin film hot melt adheres. Different sandwich specimens with different thickness ratios (of polymer core to aluminum face sheet) were prepared. Throughout an experimental effort, the limiting drawing ratios (LDRs) of the sandwich sheets were determined. Besides, the LDR of the sandwich sheets were predicted using finite element analysis simulations by considering Gurson–Tvergaard–Needleman damage model. The results show the capability of the damage model to predict the LDR and the location of damaged zone in a workpiece during a forming operation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031005-031005-7. doi:10.1115/1.4001301.

This work concerns single crystal Ni-based superalloy welded through the capacitor discharge welding (CDW) process. The static and fatigue behavior of CMSX-4 type single crystal alloy has been analyzed, performing mechanical tests on square specimens in order to establish in detail the superalloy behavior at the expected service stresses. Tests have been conducted both at room and working temperatures $(800°C)$, comparing base material and capacitor discharge welded joints. The material microstructure has been observed with optical and scanning electron microscopy in order to characterize welding parameters effects on the weld characteristics, analyzing base material and as-welded specimens; this way, the CDW process has been studied to perform butt joint with the better characteristics ad achieving sound welds. Microstructural modifications and the heat affected zones have been observed in the welded section as well as the interaction between weld material and parent metal. The research activities have been conducted in the frame of AWFORS project, financed by the European community.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031006-031006-10. doi:10.1115/1.4001302.

An experimental study was conducted to identify acceptable welding parameters for friction stir welding Ti-6Al-4V butt joints, ranging from 3 mm to 12 mm in thickness. The primary parameters of interest were the spindle speed and feed rate. Welds were produced using spindle speeds of 140–320 rpm and feed rates between 40 mm/min and 125 mm/min. Joints were evaluated by macro- and micrometallurgical examination along with limited fatigue and tensile testing. The weld parameters were found to influence the microstructure, penetration, void formation, and tool wear among other things. A process window was identified for combinations of the feed rate and spindle speed capable of achieving defect free joints for a given tooling configuration and thickness. It was found that the tensile and fatigue properties of the welds produced in this study were comparable to Ti–6Al–4V base material properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031007-031007-9. doi:10.1115/1.4001444.

Asymmetric three-point bending of a layered beam containing an interior interface crack is analyzed on the basis of the classical beam theory. Axial compressive and tensile forces are induced by bending in the parts of the beam above and below the delamination, and they are determined by modeling the cracked part as two lapped beams jointed together at the corners of both beams. When the magnitude of the applied load is small, the beam deflects, retaining the mutual contact of whole crack faces, but as the applied load reaches a critical value, local delamination buckling of the compressed part occurs. The relation between the magnitude of the applied load and the deflection at the point of load application is found to be nearly bilinear. The validity of this prediction is confirmed by experiments. It is also shown that once the delamination buckling occurs, the energy release rate generally becomes larger as compared with the case of a perfect contact of delaminated surfaces.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031008-031008-9. doi:10.1115/1.4001445.

Much of the damage mechanics literature has focused on void growth due to tensile hydrostatic stress. To clarify the effect of combined shear stress and hydrostatic stress on the development of damage, specimens of various geometries were employed in an experimental program to cover a wide range of triaxiality and shear stress. Digital image correlation (DIC) is utilized to measure the fracture strain of the 2D specimens. Experiments are paired with simulations utilizing $J2$ plasticity theory to complement the experiments and relate the fracture strain with combined hydrostatic and shear stresses. The results display accelerated damage for cases dominated by shear at low triaxiality. Crystal plasticity simulations were carried out using boundary conditions based on the DIC displacement field. These simulations indicate that tensile hydrostatic stress develops due to grain-to-grain interaction.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031009-031009-6. doi:10.1115/1.4001591.

The cold-expansion process is used routinely for improving the fatigue life of holes in a variety of components. The expansion process involves drawing a slightly oversized tapered mandrel through the hole. Upon expansion, the material near the hole deforms plastically while material away from the hole undergoes elastic deformation. Upon removal of the mandrel, the ensuing elastic recovery of the surrounding material results in the development of a compressive residual stress field around the hole. Since the magnitude of plastic deformation sustained by the material near the hole depends on the severity of the expansion, the elastic-plastic boundary radius (EPBR) during the expansion process can be used to characterize the extent of cold expansion. The elastic-plastic boundary is an important parameter in characterizing the residual stress distribution around cold-expanded holes, as well as in determining required spacing between successively expanded holes. This paper presents a new method for determining the EPBR using strain measurements within the elastic region. Analytical equations are developed relating elastic strains measured away from the hole to EPBR. A methodology is presented for employing strain data (obtained via miniature resistance strain gauges located away from the hole and within the elastic region) to be used as an input variable into the developed equations for determining EPBR. Using the method described in this paper, an average normalized EPBR of 2.38 (normalized with respect to the initial hole radius) was calculated utilizing elastic strain measurements during 4.0% cold expansion of a set of 4.826 mm thick 7075-T6 aluminum specimens containing a 6.0 mm diameter hole. The results showed excellent agreement with numerical simulations using a nonlinear elastic-plastic finite element code (ABAQUS ). The deviation between the average EPBR determined by the analytical-experimental method and the finite element analysis was about 4.0%. The proposed method for using elastic strain measurements away from the hole provides improvement over earlier methods that rely on fringe observations or strain measurements within the relatively narrow plastic zone that has an uneven surface near the hole.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031010-031010-5. doi:10.1115/1.4001593.

In this paper, mechanical and tribological properties of a Ti-50.3 at % Ni alloy were investigated. The transformation temperatures of the alloy were determined using differential scanning calorimetry. Three-point bending tests were performed to characterize the pseudoelasticity and shape memory effects. Uni-axial compression tests were also performed at different testing temperatures. The wear tests were conducted using a pin-on-disk tribometer at testing temperatures ranging from $0°C$ to $80°C$. The wear results showed that with increasing the testing temperature from $0°C$ to $50°C$, the wear of the alloy was decreased, which could be attributed to the higher pseudoelasticity of the alloy at a testing temperature of $50°C$. The pseudoelasticity of the alloy decreased at a higher testing temperature of $80°C$; however, its wear resistance increased considerably due to higher ultimate strength and work hardening.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031011-031011-8. doi:10.1115/1.4001594.

Using a three-dimensional crystal plasticity model for cyclic deformation of lath martensitic steel, a simplified scheme is adopted to simulate the effects of shot peening on inducing initial compressive residual stresses. The model is utilized to investigate the subsequent cyclic relaxation of compressive residual stresses in shot peened lath martensitic gear steel in the high cycle fatigue (HCF) regime. A strategy is identified to model both shot peening and cyclic loading processes for polycrystalline ensembles. The relaxation of residual stress field during cyclic bending is analyzed for strain ratios $Rε=0$ and $−1$ for multiple realizations of polycrystalline microstructure. Cyclic microplasticity in favorably oriented martensite grains is the primary driver for the relaxation of residual stresses in HCF. For the case of $Rε=−1$, the cyclic plasticity occurs throughout the microstructure (macroplasticity) during the first loading cycle, resulting in substantial relaxation of compressive residual stresses at the surface and certain subsurface depths. The initial magnitude of residual stress is observed to influence the degree (percentage) of relaxation. Describing the differential intergranular yielding is necessary to capture the experimentally observed residual stress relaxation trends.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031012-031012-11. doi:10.1115/1.4001833.

In this paper, a hydraulic forming machine with the functions of axial feeding, counter punch, and internal pressurization is designed and developed. This self-designed forming machine has a capacity of 50 tons for axial feeding and counter punch, 70 MPa for internal pressurization, and $300°C$ for forming temperature. Using this testing machine, experiments of T-shape protrusion of magnesium alloy AZ61 tubes at elevated temperatures are carried out. A commercial finite element code DEFORM 3D is used to simulate the plastic deformation of the tube within the die during the T-shape protrusion process. Different kinds of loading paths for the pressurization profile and the strokes of the axial feeding and the counterpunch are scheduled for analyses and experiments of protrusion processes at $150°C$ and $250°C$. The numerical thickness distributions and the flow line configurations of the formed product are compared with the experimental results to validate this finite element modeling. The thickness distribution of the formed product or the flowability of AZ61 tubes at $150°C$ and $250°C$ is discussed. The effects of the forming rate on tube flowability at $250°C$ are also investigated. Through the observation of the flow line configurations of the tube material, adequate backward speeds of the counter punch relative to the axial feeding for preventing the material from accumulating at the die entrance region are verified. Finally, a sound product with a protrusion height of 49 mm is obtained.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031013-031013-8. doi:10.1115/1.3078392.

The use of composite materials with continuous fibers in the aeronautic and aerospace industries requires reliable and precise methods for the prediction of failure. Predicting failure stresses and failure modes in composite laminates is very difficult. The choice between failure criteria is complex, and there is a lack of experimental study to validate the result obtained partly because the biaxial tests are still difficult to perform. This work employs a mixed methodology based on a theoretical and an experimental approach to develop a procedure for the choice and the validation of the failure criterion. The comparison is concerned not only with the macroscopic failure but also with the succession of the failure, the failure mode, and the effect of the geometrical parameters of the test specimen. The most general failure criteria are tested by using two approaches of the stiffness reduction. A finite element code has been elaborated within our laboratory for postfailure treatment. The numerical simulation results are compared with the experimental ones and permit us to make a conclusion on the validity of the failure criteria used.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031014-031014-11. doi:10.1115/1.4001834.

Nitriding is usually applied to increase the surface properties of mechanical components and can also enhance the fatigue resistance. The aim of this paper is to investigate, by means of numerical models and experimental tests, the effects of residual stresses induced by nitriding on the fatigue behavior of a marine diesel engine crankshaft. The residual stress gradient induced by the thermochemical treatment was taken into account by means of finite element models. Experimental tests were carried out with an axial testing machine in order to validate the numerical models and assess the crankshaft mechanical parameters such as the yield strength and the fatigue limit. An experimental innovative method applied to evaluate the crankshaft residual stresses by means of strain measurements under bending was also developed. This methodology proved to be useful to determine the magnitude of the residual stresses induced by the thermal treatment into the crankshaft, and it could be applied for the evaluation of the residual stress field in several cases.

Topics: Stress , Fatigue
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031015-031015-6. doi:10.1115/1.4001835.

We have studied the fabrication of hierarchical periodic microstructures on metals by means of direct laser interference patterning. A nanosecond pulsed Nd:YAG laser at 355 nm wavelength was used to produce the microstructures with grating periods ranging from $1 μm$ to $10 μm$ on stainless steel, titanium, and aluminum. The results indicate that the geometrical characteristics of the interference patterns as well as the thermal properties of the substrates determine the quality of the fabricated structures. In particular, the best structures were obtained when the material at the interference minima position remained in the solid state and the temperature at the interference maxima is below the vaporization temperature. Thermal simulations by finite element method were carried out modeling photothermal interactions of the interference pattern with the metallic substrates to evaluate laser induced thermal effects, such as temperature distribution and temperature gradients and, thus, enabling us to explain the obtained results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2010;132(3):031016-031016-8. doi:10.1115/1.4001836.

The environmental stability of three room temperature cure epoxy adhesives was evaluated following exposure to temperatures of $20°C$, $30°C$, and $50°C$ at 95%RH, to $50°C$ in air and soaked in water for up to 90 days. The adhesives contained nano- and microparticles and were especially formulated for bonded-in timber connections, and the properties of bulk adhesives and adhesively bonded block shear specimens were evaluated. After 90 days of aging the results demonstrate critical temperature effects controlled by the glass transition temperature. The apparent free volume for all the adhesives remains constant as moisture is absorbed but plasticization takes place at high temperature and relative humidity, evidenced by the increased elongation and yield observed by strain values and scanning electron microscope. Exposure at $50°C$ in air causes the adhesives to postcure enhancing strength but high humidity causes degradation. Nanofiller additions enhance environmental stability but the addition of microparticles provides better moisture resistance.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Mater. Technol. 2010;132(3):034501-034501-5. doi:10.1115/1.4001300.

The present paper presents a relation between the vickers hardness HV and the loading stiffness C of instrumented vickers indentation of metal substrates. The relation is based on the fact that the nonaxisymmetry of the plastic deformation of the vickers indenter leaves the corners of the indentation imprint at the surface of metal substrates after complete unloading. This relation can transform available HV data for metals to C data. It is also shown that the strain hardening details are important in the estimation of material properties and investigators should be cautions when using power-law strain hardening in all cases.

Commentary by Dr. Valentin Fuster