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Research Papers

J. Eng. Mater. Technol. 2012;134(4):041001-041001-10. doi:10.1115/1.4006132.

For a better understanding of the thermomechanical behavior of glasses used for nuclear waste vitrification, the cooling process of a bulk borosilicate glass is modeled using the finite element code Abaqus. During this process, the thermal gradients may have an impact on the solidification process. To evaluate this impact, the simulation was based on thermal experimental data from an inactive nuclear waste package. The thermal calculations were made within a parametric window using different boundary conditions to evaluate the variations of temperature distributions for each case. The temperature differences throughout the thickness of solidified glass were found to be significantly non-uniform throughout the package. The temperature evolution in the bulk glass was highly responsive to the external cooling rates applied; thus emphasizing the role of the thermal inertia for this bulky glass cast.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041002-041002-5. doi:10.1115/1.4004489.

The flow stress of AZ41M (Mg-Al-Zn-Mn-Ca) and ZK 60 (Mg-Zn-Zr) wrought magnesium alloys under the deformation conditions of twin rolling casting and hot compression at different temperature and strain rates was studied. Deformation behavior and failure mechanism of them were discussed. Microstructure evolutions were analyzed by optical microstructure and electron backscatter diffraction technique. The results have indicated that AZ41M and ZK 60 have different strain-stress curve under the same conditions. Working hardening results in occurrence of cracks in or around the shear bands. The recrystallized, equiaxed, and fined grains in shear bands attribute to recovery and recrystallization, grains refinement causes local working hardening as well as decreases of crack tip driving forces. Stress concentrated in shear bands causes crack initiation and propagation. Nucleus of cracks due to casting defects is another failure mode. With the increase of strain, dislocation rearranged in subgrain level while the low angle grain boundaries continuously evolved into high angle grain boundaries.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041003-041003-8. doi:10.1115/1.4006678.

Metal–ceramic composites are an emerging class of materials for use in the next-generation high technology applications due to their ability to sustain plastic deformation and resist failure in extreme mechanical environments. Large scale molecular dynamics simulations are used to investigate the performance of nanocrystalline metal–matrix composites (MMCs) formed by the reinforcement of the nanocrystalline Al matrix with a random distribution of nanoscale ceramic particles. The interatomic interactions are defined by the newly developed angular-dependent embedded atom method (A-EAM) by combining the embedded atom method (EAM) potential for Al with the Stillinger–Weber (SW) potential for Si in one functional form. The molecular dynamics (MD) simulations are aimed to investigate the strengthening behavior and the tension–compression strength asymmetry of these composites as a function of volume fraction of the reinforcing Si phase. MD simulations suggest that the strength of the nanocomposite increases linearly with an increase in the volume fraction of Si in the Al-rich region, whereas the increase is very sharp in the Si-rich region. The higher strength of the nanocomposite is attributed to the reduced sliding/rotation between the Al/Si and the Si/Si grains as compared to the pure nanocrystalline metal.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041004-041004-7. doi:10.1115/1.4006821.

A proposed damage model is used for investigating the deformation and interfacial failure behavior of an adhesively bonded single-lap thick joint made of S2 glass/SC-15 epoxy resin composite material. The bonding material is 3M Scotch-Weld Epoxy Adhesive DP405 Black. Continuum damage mechanics models are used to describe the damage initiation and final failure at or near the interface. The effect of adhesive overlap length, thickness, and plasticity on the interfacial shear and normal stresses is studied. Experimental and analytical data are used to validate the proposed damage models.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041005-041005-8. doi:10.1115/1.4007213.

The stress–strain response of cast iron under tension or compression is nonlinear. This paper examines how the hyperbolic constitutive law can be applied to characterize nonlinear stress–strain behavior of cast iron used in water supply networks. Procedures are described to obtain parameters of the hyperbolic constitutive law from either the response (data) obtained from simple uniaxial tensile and compressive tests or from bending tests. To demonstrate its applicability, this hyperbolic constitutive law is first applied to data obtained from uniaxial tensile and compressive tests conducted by Schlick and Moore (1936, “Strength and Elastic Properties of Cast Iron in Tension, Compression, Flexure, and Combined Tension and Flexure,” Bulletin 127, Iowa Engineering Experiment Station, Ames, IA). In addition, an approach to extract parameters for the hyperbolic constitutive law from bending (beam and pipe rings) tests is proposed and subsequently applied to tests conducted by Talbot (1908, “Tests of Cast-Iron and Reinforced Concrete Culvert Pipe,” Bulletin No. 22, University of Illinois, Urbana, IL). This latter approach is attractive for practical purposes because the test set up is simple and the test coupons are very easy to prepare. The hyperbolic constitutive law in conjunction with maximum normal strain theory as proposed by St. Venant (Collins, J. A., 1993, Failure of Materials in Mechanical Design: Analysis, Prediction, Prevention, John Wiley, New York, NY) was also used to predict failure loads.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041006-041006-7. doi:10.1115/1.4006918.

Friction stir processing (FSP) is a relatively new technology for microstructure refinement of metallic alloys. At high processing speeds, excessive heating due to severe plastic deformation and friction may result in local melting at the interface between the FSP tool and the workpiece. In this work, a computational fluid dynamics (CFD) approach is applied to model material flow and heat evolution during friction stir processing of AZ31B magnesium alloy, taking into consideration the possibility of local melting in the stirring region. This is achieved by introducing the latent heat of fusion into an expression for heat capacity and accounting for possible effects of liquid formation on viscosity and friction. Results show that the temperature in the stirring region increases with the increase in rotational speed and drops slightly with the increase in translational speed. As liquid phase begins to form, the slope of temperature rise with rotational speed decreases and the maximum temperature in the stirring region stabilizes below the liquidus temperature at high rotational speeds. It is also shown that the formation of a semi-molten layer around the tool may result in a reduction in the shearing required for microstructure refinement.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041007-041007-10. doi:10.1115/1.4006822.

Austenitic stainless steels—particularly the 304 and 316 families of alloys—exhibit similar trends in the dependence of yield stress on temperature. Analysis of temperature and strain-rate dependent yield stress literature data in alloys with varying nitrogen content and grain size has enabled the definition of two internal state variables characterizing defect populations. The analysis is based on an internal state variable constitutive law termed the mechanical threshold stress model. One of the state variables varies solely with nitrogen content and is characterized with a larger activation volume. The other state variable is characterized by a much smaller activation volume and may represent interaction of dislocations with solute and interstitial atoms. Analysis of the entire stress–strain curve requires addition of a third internal state variable characterizing the evolving stored dislocation density. Predictions of the model are compared to measurements in 304, 304L, 316, and 316L stainless steels deformed over a wide range of temperatures (up to one-half the melting temperature) and strain rates. Model predictions and experimental measurements deviate at temperatures above ∼600 K where dynamic strain aging has been observed. Application of the model is demonstrated in irradiated 316LN where the defect population induced by irradiation damage is analyzed. This defect population has similarities with the stored dislocation density. The proposed model offers a framework for modeling deformation in stable austenitic stainless steels (i.e., those not prone to a martensitic phase transformation).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041008-041008-9. doi:10.1115/1.4006919.

In finite element analysis of sheet metal forming the use of combined isotropic-kinematic hardening models is advisable to improve stamping simulation and springback prediction. This choice becomes compulsory to model recent materials such as high strength steels. Cyclic tests are strictly required to evaluate the parameters of these constitutive models. However, for sheet metal specimens, in case of simple axial tension-compression tests, buckling occurrence during compression represents a serious drawback. This is the reason why alternative set-ups have been devised. In this paper, two experimental arrangements (a cyclic laterally constrained tension-compression test and a three-point fully reversed bending test) are compared so as to point out the advantages and the disadvantages of their application in tuning the well-known Chaboche’s hardening model. In particular, for tension-compression tests, a new clamping device was specifically designed to inhibit compressive instability. Four high strength steel grades were tested: two dual phases (DP), one transformation induced plasticity (TRIP) and one high strength low alloy material (HSLA). Then, the Chaboche’s model was calibrated through inverse identification methods or by means of analytical expressions when possible. The proposed testing procedure proved to be successful in all investigated materials. The achieved constitutive parameters, obtained independently from the two experimental techniques, were found to be consistent. Their accuracy was also been assessed by applying the parameter set obtained from one test to simulate the other one, and vice versa. Clues on what method provides the better transferability are given.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041009-041009-6. doi:10.1115/1.4006978.

In this study, mixed-mode fatigue tests are conducted using surface-cracked specimens. Slant surface-cracked specimens are prepared with crack angles of 15 deg, 30 deg, 45 deg, and 60 deg. It is shown that a “factory roof” fracture is formed at the deepest point of the surface crack due to ΔKIII and causes the crack growth rate to decrease. Additionally, fatigue crack growth is simulated using the superposition finite element method (FEM) with crack growth criteria. It is shown that conventional crack growth criteria are not applicable to factory roof fractures. Finally, a modified criterion for the prediction of crack growth rate is proposed, fatigue crack growth simulation is conducted using this criterion, and the results are compared with experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041010-041010-9. doi:10.1115/1.4006979.

We review He ion induced radiation damage in several metallic multilayer systems, including Cu/V, Cu/Mo, Fe/W, and Al/Nb up to a peak dose of several displacements per atom (dpa). Size dependent radiation damage is observed in all systems. Nanolayer composites can store a very high concentration of He. Layer interfaces promote the recombination of opposite type of point defects and hence reduce the accumulative defect density, swelling, and lattice distortion. Interfaces also alleviate radiation hardening substantially. The chemical stability of interfaces is an important issue when considering the design of radiation tolerant nanolayer composites. Immiscible and certain miscible systems possess superior stability against He ion irradiation. Challenge and future directions are briefly discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041011-041011-8. doi:10.1115/1.4006177.

The ductile-to-brittle cutting mode transition in single grit diamond scribing of monocrystalline silicon is investigated in this paper. Specifically, the effects of scriber tip geometry, coefficient of friction, and external hydrostatic pressure on the critical depth of cut associated with ductile-to-brittle transition and crack generation are studied via an eXtended Finite Element Method (XFEM) based model, which is experimentally validated. Scribers with a large tip radius are shown to produce lower tensile stresses and a larger critical depth of cut compared with scribers with a sharp tip. Spherical tipped scribers are shown to generate only surface cracks, while sharp tipped scribers (conical, Berkovich and Vickers) are found to create large subsurface tensile stresses, which can lead to nucleation of subsurface median/lateral cracks. Lowering the friction coefficient tends to increase the critical depth of cut and hence the extent of ductile mode cutting. The results also show that larger critical depth of cut can be obtained under external hydrostatic pressure. This knowledge is expected to be useful in optimizing the design and application of the diamond coated wire employed in fixed abrasive diamond wire sawing of photovoltaic silicon wafers.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041012-041012-8. doi:10.1115/1.4007351.

This paper aims to investigate the effect of fabrication processes on fatigue life enhancement of interference-fit pin-loaded glass fiber reinforced plastics (GFRP) composites. In this experimental study, three GFRP composite fabrication processes are used: hand lay-up (HL), vacuum infusion (VI), and hybrid (hand lay-up + vacuum infusion) processes. Stainless steel pins with interference fits ranging from 0% to 1% are inserted into the GFRP samples. The quasi-static and fatigue properties of the pin-loaded composites with interference fit (0.6% and 1%) are then compared to samples with transition-fit (0% of interference fit). Even with possible local damage on the joints, interference fit does not degrade the performance of the composite joints under quasi-static loading, especially when kept under 1% of interference fit. However, fatigue life is highly related to the fabrication processes. Vacuum infusion processed GFRP samples show most visible fatigue life improvement due to interference fit, while hand lay-up or hybrid samples have moderate improvement. Fractography and failure mode of each sample are examined using microscopes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041013-041013-10. doi:10.1115/1.4007260.

This paper is aimed to investigate the size-dependent pull-in behavior of hydrostatically and electrostatically actuated rectangular nanoplates including surface stress effects based on a modified continuum model. To this end, based on the Gurtin–Murdoch theory and Hamilton’s principle, the governing equation and corresponding boundary conditions of an actuated nanoplate are derived; the step-by-step linearization scheme and the differential quadrature (GDQ) method are used to discretize the governing equation and associated boundary conditions. The effects of the thickness of the nanoplate, surface elastic modulus and residual surface stress on the pull-in instability of the nanoplate are investigated. Plates made of two different materials including aluminum (Al) and silicon (Si) are selected to explain the variation of the pull-in voltage and pressure with respect to plate thickness.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041014-041014-9. doi:10.1115/1.4007352.

The Bammann, Chiesa, and Johnson (BCJ) material model predicts unlimited localization of strain and damage, resulting in a zero dissipation energy at failure. This difficulty resolves when the BCJ model is modified to incorporate a nonlocal evolution equation for the damage, as proposed by Pijaudier-Cabot and Bazant (1987, “Nonlocal Damage Theory,” ASCE J. Eng. Mech., 113, pp. 1512–1533.). In this work, we theoretically assess the ability of such a modified BCJ model to prevent unlimited localization of strain and damage. To that end, we investigate two localization problems in nonlocal BCJ metals: appearance of a spatial discontinuity of the velocity gradient in any finite, inhomogeneous body, and localization of the dissipation energy into finite bands. We show that in spite of the softening arising from the damage, no spatial discontinuity occurs in the velocity gradient. Also, we find that the dissipation energy is continuously distributed in nonlocal BCJ metals and therefore cannot localize into zones of vanishing volume. As a result, the appearance of any vanishing width adiabatic shear band is impossible in a nonlocal BCJ metal. Finally, we study the finite element (FE) solution of shear banding in a rectangular plate, deformed in plane strain tension and containing an imperfection, thereby illustrating the effects of imperfections and finite size on the localization of strain and damage.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):041015-041015-15. doi:10.1115/1.4007261.

Unpredicted sheet forming failures of dual-phase (DP) steels can occur in regions of high curvature and with little apparent necking. Such failures are often referred to as “shear fractures”. In order to reproduce such fractures in a laboratory setting, and to understand their origin and the inability to predict them, a novel draw-bend formability (DBF) test was devised using dual displacement rate control. DP steels from several suppliers, with tensile strengths ranging from 590 to 980 MPa, were tested over a range of rates and bend ratios (R/t) along with a TRIP (transformation induced plasticity) steel for comparison. The new test reliably reproduced three kinds of failures identified as types 1, 2, and 3, corresponding to tensile failure, transitional failure, and shear fracture, respectively. The type of failure depends on R/t and strain rate, and presumably on the initial specimen width, which was constant in this study. Two critical factors influencing the lack of accurate failure prediction were identified. The dominant one is deformation-induced heating, which is particularly significant for advanced high strength steels because of their high energy product. Temperature rises of up to 100 deg. C were observed. This factor reduces formability at higher strain rates, and promotes a transition from types 1 to 3. The second factor is related to microstructural features. It was significant in only one material in one test direction (of 11 tested) and only for this case was the local fracture strain different from that in a tensile failure. Alternate measures for assessing draw-bend formability were introduced and compared. They can be used to rank the formability of competing materials and to detect processing problems that lead to unsuitable microstructures.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Mater. Technol. 2012;134(4):044501-044501-7. doi:10.1115/1.4007017.

An innovative manufacturing method, bladder assisted composite manufacturing (BACM), to fabricate geometrically complex, hollow parts made of polymeric composite materials is presented. Unlike the conventional bladder or diaphragm assisted curing processes, BACM uses an internally heated bladder to provide the consolidation pressure at the required cure temperature. The feasibility of this manufacturing method is demonstrated by fabricating laminated composite cylinders using multiple cure pressures and number of plies. The elastic moduli, failure strength, fiber volume fraction, and void contents of the cylinders were found to be comparable to the values obtained from flat laminates produced by hot plate molding of the same material. Compared to conventional bladder manufacturing methods, the BACM process reduced the energy required to cure the cylinders by almost 50% due to internal heating and insulated mold.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2012;134(4):044502-044502-5. doi:10.1115/1.4005404.

In this study, the effectiveness of analytical models which attempt to predict the density of unsintered powder metallurgy (PM) compacts as a function of consolidation pressure is investigated. These models do not incorporate the nonuniform densification of powder compacts and may insufficiently describe the pressure/densification process. Fabrication of uniform and nonuniform Zinc (Zn) tablets is conducted to assess the validity of the pressure/density model developed by Quadrini (Quadrini and Squeo, 2008, “Density Measurement of Powder Metallurgy Compacts by Means of Small Indentation,” J. Manuf. Sci. Eng., 130 (3), pp. 0345031–0345034). Different tablet properties were obtained by varying the compaction pressure and fabrication protocol. Density gradients within Zn tablets result in a spatial dependence of Vickers microhardness (HV) throughout the fabricated specimen. As a result, micro-indentation testing is used extensively in this study as a characterization tool to evaluate the degree of nonuniformity in fabricated Zn tablets. Scanning electron microscopy (SEM) is also employed to verify tablet density by visual examination of surface porosity as compaction pressure is varied and sintering is applied.

Commentary by Dr. Valentin Fuster

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