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IN THIS ISSUE

### Editorial

J. Eng. Mater. Technol. 2008;130(3):030201-030201-1. doi:10.1115/1.2945279.
FREE TO VIEW

It is a great honor and privilege for me to begin my term as Editor of the JEMT. I have served and contributed to the JEMT for over 20 years as a reviewer, author, guest editor for special issues, and associate editor. During my service as associate editor from 1998 through 2004, I had the opportunity to interact closely with two editors (Dr. David McDowell and Dr. Huseyin Sehitoglu), the editorial board and numerous authors. I also learned about the journal and its purpose and scope, editorial management and requirements, and the review process. The previous Editors have established clear editorial and management policies and a fair review process. Thanks to their effort and that of the editorial board, JEMT has become a premier journal in both the mechanics and materials communities. As Editor, I will build on the success of the previous Editors and will work closely with the editorial board to elevate the journal to new heights.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Eng. Mater. Technol. 2008;130(3):031001-031001-6. doi:10.1115/1.2931142.

In this paper, the metallurgical phenomena occurring in friction stir welding processes of AA6082-T6 and AA7075-T6 aluminum alloys are investigated. In particular, to predict the local values of the average grain size, either a simple analytical expression depending on a few material constants or a properly trained neural network is linked to the finite element model of the process. The utilized tools, which take as inputs the local values of strain, strain rate, and temperature, were developed starting from experimental data and numerical results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031002-031002-7. doi:10.1115/1.2931147.

The failure of a cylindrical brittle material impacted by a supersonic air jet is investigated. Gypsum was cast around steel tubes to simulate the deposit formed on tube surfaces in industrial boilers. The breakup behavior of two deposit sizes, positioned at several distances from the nozzle exit, was visualized and documented using a high-speed video camera. Three deposit failure behaviors were observed: (i) crack formation and propagation along the longitudinal axis of the cylinder, (ii) surface pitting followed by axial crack formation, and (iii) surface pitting followed by spalling. These types of failure depend on the ratio of jet diameter to deposit diameter, which affects the magnitudes of compressive, tensile, and shear forces induced within the material. By analyzing the breakup movies, characteristics of the broken deposits, such as the breakup duration and the amount of deposit removed, were measured. Also, the effects of deposit thickness and distance from the nozzle exit on these characteristics were investigated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031003-031003-8. doi:10.1115/1.2931150.

Aerospace aluminum alloy forgings can have the residual stresses arising from heat treatment reduced by modification to the quench cooling rates and subsequent aging treatments. A series of propeller hubs usually made from the alloy 2014 have been closed die forged from the less quench sensitive alloy 7050. These forgings have been subjected to various quenching and aging treatments in an attempt to improve the balance of mechanical properties with the residual stress magnitudes. These forgings were not amenable to stress relieving by cold compression or stretching. Warm water $(60°C)$ and boiling water quenches are investigated in addition to quenching into molten salt $(200°C)$ and uphill quenching from $−196°C$. Various dual aging treatments including retrogression and reaging have been evaluated in an attempt to optimize low residual stress magnitudes with mechanical properties. Residual stresses determined by the center hole-drilling strain-gauge method are reported in addition to electrical conductivity, stress corrosion cracking, fracture toughness, initiation fatigue, and tensile mechanical property variations. It was found that quenching into boiling water and salt at $200°C$ did substantially reduce the residual stress but had only a small detrimental effect on the majority of the properties measured. However, the influence of quench rate on fracture toughness was much more significant. This is attributed to both coarse grain boundary precipitation and heterogeneous precipitation of $η$ on $Al3Zr$ dispersoids within the grains, which promotes easier crack propagation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031004-031004-5. doi:10.1115/1.2931151.

The constitutive behavior of both direct-chill cast (DC) and continuous cast (CC) AA5754 sheet materials has been investigated up to large strains using a modified shear test. The modified shear sample prevents rotation of the shear zone compared to the ASTM standard B831 test (2005, “Standard Test Method for Shear Testing of Thin Aluminum Alloy Products  ,” 2005 Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 02.02, pp. 601–603). The results show that the effective stress and effective strain curves from shear tests match those obtained from uniaxial tension, but only by incorporating the material anisotropy using the Barlat–Lian yield function. The flow stresses of both materials saturate at large strains; however, the fracture strain of the CC material is significantly lower than that of the DC material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031005-031005-9. doi:10.1115/1.2931140.

The state of nanoclay dispersion in a molded epoxy disk and its effects on the thermomechanical properties of the resulting nanocomposite are analyzed. A commercially available nanoclay, Cloisite® 25A, is mechanically mixed at $2wt%$ with EPON 815C epoxy resin. The epoxy∕clay compound is then mixed with EPI-CURE 3282 curing agent by a custom made molding setup and injected into a disk shaped mold cavity. Upon completion of curing, nanoclay dispersion is quantified on a sample cut along the radius of the composite disk. Dispersion of nanoclay clusters larger than $1.5μm$ are analyzed by digital image processing of scanning electron micrographs taken radially along the sample, whereas dispersion at smaller scales is quantified by compositional analysis of clay via wavelength dispersive spectrometry (WDS). Digital images of the microstructure indicate that amount of nanoclay clusters that are larger than $1.5μm$ remain approximately constant along the radius. However, size analysis of nanoclay clusters revealed that they are broken down into finer clusters along the radius, possibly due to the high shear deformation induced through the thickness during mold filling. Compositional analysis by WDS signified that approximately $0.4wt%$ of the nanoclay is dispersed to particles smaller than $1.5μm$, which are not visible in micrographs. Tensile and three-point bending tests are conducted on additional samples cut from the molded disks. Except for slight reduction in flexural strength, up to 9.5% increase in tensile strength, stiffness, and flexural modulus are observed. Glass transition temperature is determined under oscillatory torsion and observed to increase by 4.5% by the addition of nanoclay.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031006-031006-7. doi:10.1115/1.2931141.

Among the multiple stages of the resin transfer molding (RTM) processes, flow and mold filling of injected resin correspond to the most complex and crucial stage. During the latter, air bubble agglomeration must be avoided and complete wetting of fibers must be achieved in order to ensure the maximum quality of the parts at the lowest possible manufacturing time. Focusing on these manufacturing issues, a mathematical model and a numerical resolution are presented to predict the resin flow throughout the fiber reinforcement inside the mold cavity. The methodology employs conventional finite element techniques for solving the flow problem through a porous medium governed by Darcy’s law and mass conservation. Simultaneously, a state of the art numerical scheme known as the discontinuous Galerkin method is implemented to determine the location and shape of the advancing flow fronts ruled by a hyperbolic transport equation. These two schemes are implemented to work with a two-dimensional domain, handling diverse geometries with multiple injection and ventilation ports. The results for key process parameters, such as filling time and position of the advancing flow fronts, show a good agreement with results from analytical solutions for particular cases and from empirical data. When several simulated results are taken into account in the design process of RTM cavities, the overall process could be enhanced.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031007-031007-15. doi:10.1115/1.2931143.

Friction-stir welding (FSW) promises joints with low porosity, fine microstructures, minimum phase transformation, and low oxidation compared with conventional welding techniques. It is capable of joining combinations of alloys not amenable to conventional welding. Certain combinations of FSW parameters were used to create FSWs of aluminum alloys 5083-H18 and 6111-T4, and the physical weld defects were measured. The mechanical behavior of FSW welds made under the most favorable choice of parameters was determined using tensile tests and hardness measurements and was correlated to the microstructures of the weld and base material. Stir zones (SZs) in the 5083 specimens were much softer than the strain-hardened base materials. SZs in the 6111 material are approximately as hard as the base material. Natural aging of 6111 FSW specimens occurred in some parts of the heat-affected zone and produced hardening for up to $12weeks$ after welding. Annealing of 5083 FSW specimens produced abnormal grain growth (AGG) for welds produced under certain welding conditions and in certain parts of the weld zone. AGG is more severe for low-heat conditions, i.e., higher tool travel speed but lower rotational speed. The conditions for most favorable FSW are presented, as well as the expected microstructures and mechanical properties, along with the weld conditions that promote AGG.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031008-031008-7. doi:10.1115/1.2931145.

An electromagnetic sensor was assessed as a possible instrument for nondestructive detection and monitoring of corrosion in structural carbon steels. In this study, the magnetic response of three structural carbon steel rods (AISI 1018, AISI 1045, and AISI 1045-High Mn), was evaluated in the as-received (uncorroded) and corroded conditions. Initially, the material was systematically machined out from each steel rod, followed by the magnetic evaluation of each specimen. Other set of metal rods were exposed to uniform corrosion and later examined by the electromagnetic sensor. Correlations have been established between the degree of mass loss and magnetic response of the test specimen. Based on the results, it can be said that the electromagnetic sensor has the potential to be used as a reliable nondestructive tool to detect corrosion at early stages based on the variation in magnetic properties. A metallurgical analysis of all test rods was also undertaken, which showed that microstructures have an important effect of the magnetic properties of the steels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031009-031009-13. doi:10.1115/1.2931146.

The semiphysics constitutive model developed by Cherkaoui (2006, “From Micro to Macroscopic Description of Martensitic Transformation in Steels: A Viscoplastic Model  ,” Philos. Mag., in press;Philos. Mag. Lett. in press) has been implemented in the user’s material subroutine of the finite element code ABAQUS∕EXPLICIT (2006, Version 6.6 Manuals, Dassault Systemes) to predict the thermomechanical behavior of unstable transformation induced plasticity (TRIP) steel sheets under conditions of forming, which are essentially composite materials with evolving volume fractions of the individual phases. These steels undergoing $α′$ martensitic phase transformation exhibit an additional inelastic strain resulting from the phase transformation itself and from the plastic accommodation in parent (austenite) and product (martensite) phases due to different sources of internal stresses. This inelastic strain known as the TRIP strain enhances ductility at an appropriate strength level due to the typical properties of the martensite. A numerical analysis of the effects of the martensitic phase transformation on formability is performed. A validation of the stress-strain behavior and the volume fraction of the martensite are carried out in the case of multiaxial paths at various temperatures. The effects of the stress state and of the kinetics of the martensite phase transformation are analyzed in the case of the cup drawing test. Finally, the numerical predictions are compared to experimental tests on type AISI304 austenitic stainless steels and TRIP800 multiphase industrial steels.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031010-031010-8. doi:10.1115/1.2931148.

In this paper, both the dissipation of the plastic-strain energy and the exhaustion of the static toughness during high-temperature low-cycle fatigue of GH4145/SQ superalloy were investigated. Together with the analysis of the microscopic aspects of the material, an energy-based damage mechanics model was developed for the prediction of the residual fatigue life of the high-temperature fastened parts in power plant. Experimental results show that the static toughness is a parameter that is highly sensitive to the fatigue damage process. The deterioration of the static toughness during fatigue process reveals the exhaustion of the materials’s ability to absorb energy, which is essentially associated with the irreversible energy dissipation process of the fatigue failure. Based on the dissipation of the plastic-strain energy and the exhaustion of the static toughness during fatigue, a damage variable is defined that is consistent with the fatigue damage mechanism. The variable is sensitive to the fatigue process and can be measured with a simple experimental procedure. A fatigue damage evolution equation is derived on the basis of Lemaitre’s potential of dissipation in the framework of continuum damage mechanics. Furthermore, an equation for the determination of the residual fatigue life is deduced. The fatigue damage mechanics model is verified by comparing the predicted results with the experimental observations. The fatigue damage mechanics model developed may provide a feasible approach to determining the residual fatigue life of the high-temperature fastened parts in power plant.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031011-031011-7. doi:10.1115/1.2931149.

The design of metal-polymer foam adhesion and load transfer characteristics is carried out in this research work. The metal inserts are used as the load transfer component, while the foam is used as the structural element of the system. The inserts are embedded in the foam during the foaming process. Flexural testing was conducted on different metal foam configurations to establish the typical interaction trends. The load-deflection response and the mode of failure of the structure were documented. Moduli of elasticity of the system for various geometries and embedded lengths were evaluated, and behavior patterns were gleaned. Rectangular, circular, and triangular (taper-/wedgelike) inserts were used. Results show that simple taper inserts embedded in foam slabs perform better than the other shapes. Finite element analyses of the interaction under different loads were carried out. The modeling results coincided with the experimental ones hence validating the model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031012-031012-7. doi:10.1115/1.2931153.

This paper discusses the dependence of the mechanical properties and microstructure of sintered hydroxyapatite (HA) on the sintering temperature and pressure. A set of specimens was prepared from as-received HA powder and sintered by using a spark plasma sintering (SPS) process. The sintering pressures were set at $22.3MPa$, $44.6MPa$, and $66.9MPa$, and sintering was performed in the temperature range from $800°Cto1000°C$ at each pressure. Mechanisms underlying the interrelated temperature-mechanical and pressure-mechanical properties of dense HA were investigated. The effects of temperature and pressure on the flexural strength, Young’s modulus, fracture toughness, relative density, activation energy, phase stability, and microstructure were assessed. The relative density and grain size increased with an increase in the temperature. The flexural strength and Young’s modulus increased with an increase in the temperature, giving maximum values of $131.5MPa$ and $75.6GPa$, respectively, at a critical temperature of $950°C$ and $44.6MPa$, and the fracture toughness was $1.4MPam1∕2$ at $1000°C$ at $44.6MPa$. Increasing the sintering pressure led to acceleration of the densification of HA.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031013-031013-12. doi:10.1115/1.2931154.

Tension-compression, torsion, and axial-torsion fatigue experiments were conducted on the AL6XN alloy to experimentally investigate the cyclic plasticity behavior and the fatigue behavior. The material is found to display significant nonproportional hardening when the equivalent plastic strain amplitude is over $2×10−4$. In addition, the material exhibits overall cyclic softening. Under tension-compression, the cracking plane is perpendicular to the axial loading direction regardless of the loading amplitude. The smooth strain-life curve under fully reversed tension-compression can be described by a three-parameter power equation. However, the shear strain-life curve under pure torsion loading displays a distinct plateau in the fatigue life range approximately from 20,000 to 60,000 loading cycles. The shear strain amplitude corresponding to the plateau is approximately 1.0%. When the shear strain amplitude is above 1.0% under pure shear, the material displays shear cracking. When the shear strain amplitude is below 1.0%, the material displays tensile cracking. A transition from shear cracking to tensile cracking is associated with the plateau in the shear strain-life curve. Three different multiaxial fatigue criteria were evaluated based on the experimental results on the material for the capability of the criteria to predict fatigue life and the cracking direction. Despite the difference in theory, all the three multiaxial criteria can reasonably correlate the experiments in terms of fatigue life. Since the cracking mode of the material subjected to pure torsion is a function of the loading magnitude, the prediction of cracking orientation becomes rather challenging.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031014-031014-7. doi:10.1115/1.2931156.

The cold expansion method is one of the most popular techniques in the fatigue enhancement processes, and it has been widely used as a means of improving the fatigue resistance for aircraft structures with holes. Cold expanded holes have lower compressive residual stresses on the entry surface rather than the middle and exit surfaces. Due to the nonuniform residual stress distribution, fatigue crack initiation often occurs on the entry surface. This study proposes a new approach to increase the compressive residual stress magnitude at the entry of the hole. The new method is to apply chamfers into holes before the cold expansion process. Split mandrel process was used to cold work the hole with and without chamfers. Both numerical and experimental studies were done to verify the effects of hole chamfers on the residual stress distribution of the cold expanded holes. Finite element analysis (FEA) was conducted in order to see the effects of the chamfer geometries on the residual stress distributions. The FEA results showed an improvement of compressive residual stress magnitudes at the entry position of the cold expanded hole. The numerical results were compared with X-ray diffraction measurements. Fatigue tests were done to compare the fatigue life of the holes with various chamfer sizes and angles. The cold expansion chamfered holes showed a clear improvement in fatigue life over cold expanded holes without chamfers.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(3):031015-031015-15. doi:10.1115/1.2931157.

The finite-volume direct averaging micromechanics (FVDAM) theory for periodic heterogeneous materials is extended by incorporating parametric mapping into the theory’s analytical framework. The parametric mapping enables modeling of heterogeneous microstructures using quadrilateral subvolume discretization, in contrast with the standard version based on rectangular subdomains. Thus arbitrarily shaped inclusions or porosities can be efficiently rendered without the artificially induced stress concentrations at fiber/matrix interfaces caused by staircase approximations of curved boundaries. Relatively coarse unit cell discretizations yield effective moduli with comparable accuracy of the finite-element method. The local stress fields require greater, but not exceedingly fine, unit cell refinement to generate results comparable with exact elasticity solutions. The FVDAM theory’s parametric formulation produces a paradigm shift in the continuing evolution of this approach, enabling high-resolution simulation of local fields with much greater efficiency and confidence than the standard theory.

Commentary by Dr. Valentin Fuster