Research Papers

J. Eng. Mater. Technol. 2013;135(3):031001-031001-11. doi:10.1115/1.4023405.

A long-bar apparatus for subjecting relatively small samples to stress-wave loading has been devised for failure characterization. A methodology based on digital image correlation (DIC) used in conjunction with ultra high-speed photography and a long-bar impactor has been developed for determining dynamic crack initiation stress intensity factor (SIF) (KI-inid), as well as SIFs for a rapidly growing crack (KId) during high-strain rate events. By altering the material of the pulse shaper, a range of strain rates has been attained. Commercial grade PMMA was first used to calibrate the device, and then dynamic fracture characterization was performed for the first time on PMMA-based bone cement (BC). Despite several key differences, the two materials performed similarly during quasi-static fracture tests; however, under dynamic loading conditions, bone cement exhibited significantly lower crack initiation SIF (KI-inid), lower dynamic SIFs (KId), and higher crack tip velocities for three different dynamic loading rates (K·=6.5-24×104MPams-1).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031002-031002-11. doi:10.1115/1.4023674.

Determining a material constitutive law that is representative of the extreme conditions found in the cutting zone during machining operations is a very challenging problem. In this study, dynamic shear tests, which reproduce, as faithfully as possible, these conditions in terms of strain, strain rate, and temperature, have been developed using hat-shaped specimens. The objective was to identify the parameters of a Johnson–Cook material behavior model by an inverse method for two titanium alloys: Ti6Al4V and Ti555-3. In order to be as representative as possible of the experimental results, the parameters of the Johnson–Cook model were not considered to be constant over the total range of the strain rate and temperature investigated. This reflects a change in the mechanisms governing the deformation. The shear zones observed in hat-shaped specimens were analyzed and compared to those produced in chips during conventional machining for both materials. It is concluded that the observed shear bands can be classified as white-etching bands only for the Ti555-3 alloy. These white bands are assumed to form more easily in the Ti555-3 alloy due to its predominately β phase microstructure compared to the Ti6Al4V alloy with a α + β microstructure.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031003-031003-12. doi:10.1115/1.4023848.

Sandwich sheet structures are gaining a wide array of applications in the aeronautical, marine, automotive, and civil engineering fields. Since such sheets can be subjected to forming/stamping processes, it is crucial to characterize their limiting amount of deformation before trying out any forming/stamping process. To achieve this goal, sandwich sheets of Al 3105/polymer/Al 3105 were prepared using thin film hot melt adheres. Through an experimental effort, forming limit diagrams (FLDs) of the prepared sandwich sheets were evaluated. In addition, simulation efforts were conducted to predict the FLDs of the sandwich sheets using finite element analysis (FEA) by considering the Gurson–Tvergaard–Needleman (GTN) damage model. The agreement among the experimental results and simulated predictions was promising. The effects of different parameters such as polymer core thickness, aluminum face sheet thickness, and shape constraints were investigated on the FLDs.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031004-031004-7. doi:10.1115/1.4023769.

In this paper, AZ61 magnesium alloy composites containing nanoalumina and micron-sized copper particulates are synthesized using the technique of disintegrated melt deposition followed by hot extrusion. The simultaneous addition of nano-Al2O3 and copper particulates led to an overall improvement in both microstructural characteristics in terms of distribution and morphology of secondary phases and mechanical response of AZ61. The presence of nanoalumina particulates broke down and dispersed the secondary phase Mg17Al12. The 0.2% yield strength increased from 216 MPa to 274 MPa. The ductility increased from 8.4% to 9.3% in the case of the AZ61-1.5Al2O3 sample. The results of aging heat treatment in the case of the AZ61-1.5Al2O3-1Cu sample showed significant improvement in both tensile strength, ductility, and work of fracture (54% increment). An attempt is made to correlate the tensile response of composites with their microstructural characteristics.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031005-031005-8. doi:10.1115/1.4023673.

The low cycle fatigue behavior of Alloy 617 has been evaluated at 850 °C and 950 °C, the temperature range of particular interest for the intermediate heat exchanger on a proposed high-temperature gas-cooled nuclear reactor. Cycles to failure were measured as a function of total strain range and varying strain rate. Results of the current experiments compare well with previous work reported in the literature for a similar range of temperatures and strain rate. The combined data demonstrate a Coffin–Manson relationship, although the slope of the Coffin–Manson fit is close to −1 rather than the typically reported value of −0.5. At 850 °C and a strain rate of 10−3 /s Alloy 617 deforms by a plastic flow mechanism in low cycle fatigue and exhibits some cyclic hardening. At 950 °C for strain rates of 10−3–10−5 /s, Alloy 617 deforms by a solute drag creep mechanism during low cycle fatigue and does not show significant cyclic hardening or softening. At this temperature the strain rate has little influence on the cycles to failure for the strain ranges tested.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031006-031006-10. doi:10.1115/1.4023849.

The incremental slitting or crack compliance method determines a residual stress profile from strain measurements taken as a slit is incrementally extended into the material. To date, the inverse calculation of residual stress from strain data conveniently adopts a two-dimensional, plane strain approximation for the calibration coefficients. This study provides the first characterization of the errors caused by the 2D approximation, which is a concern since inverse analyses tend to magnify such errors. Three-dimensional finite element calculations are used to study the effect of the out-of-plane dimension through a large scale parametric study over the sample width, Poisson's ratio, and strain gauge width. Energy and strain response to point loads at every slit depth is calculated giving pointwise measures of the out-of-plane constraint level (the scale between plane strain and plane stress). It is shown that the pointwise level of constraint varies with slit depth, a factor that makes the effective constraint a function of the residual stress to be measured. Using a series expansion inverse solution, the 3D simulated data of a representative set of residual stress profiles are reduced with 2D calibration coefficients to yield the error in stress. The sample width below which it is better to use plane stress compliances than plane strain is shown to be about 0.7 times the sample thickness; however, even using the better approximation, the rms stress errors sometimes still exceed 3% with peak errors exceeding 6% for Poisson's ratio 0.3, and errors increase sharply for larger Poisson's ratios. The error is significant, yet, error magnification from the inverse analysis in this case is mild compared to, e.g., plasticity based errors. Finally, a scalar correction (effective constraint) over the plane-strain coefficients is derived to minimize the root-mean-square (rms) stress error. Using the posed scalar correction, the error can be further cut in half for all widths and Poisson's ratios.

Topics: Stress , Errors
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031007-031007-6. doi:10.1115/1.4023850.

Hollow particulate composites are lightweight, have high compressive strength, are low moisture absorbent, have high damping materials, and are used extensively in aerospace, marine applications, and in the manufacture of sandwich composites core elements. The high performance of these materials is achieved by adding high strength hollow glass particulates (microballoons) to an epoxy matrix, forming epoxy-syntactic foams. The present study focuses on the effect of volume fraction and microballoon size on the ultrasonic and dynamic properties of Epoxy Syntactic Foams. Ultrasonic attenuation coefficient from an experiment is compared with a previously developed theoretical model for low volume fractions that takes into account attenuation loss due to scattering and absorption. The guidelines of ASTM Standard E 664-93 are used to compute the apparent attenuation. Quasi-static compressive tests were also conducted to fully characterize the material. Both quasi-static and dynamic properties, as well as coefficients of attenuation and ultrasonic velocities are found to be strongly dependent upon the volume fraction and size of the microballoons.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031008-031008-7. doi:10.1115/1.4024116.

Alternatives to quasi-static and dynamic constitutive relationships have been investigated with respect to a previously developed energy-based fatigue lifing method for various load profiles, which states: the total strain energy dissipated during both a quasi-static process and a dynamic process are equivalent and a fundamental material property. Specifically, constitutive relationships developed by Ramberg–Osgood and Halford were modified for application to the existing energy-based framework and were compared to the lifing method originally developed by Stowell. Extensive experimentation performed on Titanium 6Al-4V (Ti-64) combined with experimental data generated for Aluminum (Al) 6061-T6 at various temperatures were utilized in support of this investigation. This effort resulted in considerable improvements to the accuracy of the lifing prediction for materials with an endurance limit through application of a modified-Halford approach. Additionally, the relative equality in predictive accuracy between the modified-Stowell approach the modified-Ramberg–Osgood approach was demonstrated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031009-031009-8. doi:10.1115/1.4023676.

Forming limit diagrams (FLD) have been widely used as a powerful tool for predicting sheet metal forming failure in the industry. The common assumption for forming limits is that the deformation is limited to in-plane loading and through-thickness bending effects are negligible. In practical sheet metal applications, however, a sheet metal blank normally undergoes a combination of stretching, bending, and unbending, so the deformation is invariably three-dimensional. To understand the localized necking phenomenon under this condition, a new extended Marciniak–Kuczynski (M–K) model is proposed in this paper, which combines the FLD theoretical model with finite element analysis to predict the forming limits after a sheet metal undergoes under continuous-bending-under-tension (CBT) loading. In this hybrid approach, a finite element model is constructed to simulate the CBT process. The deformation variables after the sheet metal reaches steady state are then extracted from the simulation. They are carried over as the initial condition of the extended M–K analysis for forming limit predictions. The obtained results from proposed model are compared with experimental data from Yoshida et al. (2005, “Fracture Limits of Sheet Metals Under Stretch Bending,” Int. J. Mech. Sci., 47(12), pp. 1885–1986) under plane strain deformation mode and the Hutchinson and Neale's (1978(a), “Sheet Necking—II: Time-Independent Behavior,” Mech. Sheet Metal Forming, pp. 127–150) M–K model under in-plane deformation assumption. Several cases are studied, and the results under the CBT loading condition show that the forming limits of post-die-entry material largely depends on the strain, stress, and hardening distributions through the thickness direction. Reduced forming limits are observed for small die radius case. Furthermore, the proposed M–K analysis provides a new understanding of the FLD after this complex bending-unbending-stretching loading condition, which also can be used to evaluate the real process design of sheet metal stamping, especially when the ratio of die entry radii to the metal thickness becomes small.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):031010-031010-22. doi:10.1115/1.4024173.

A nonassociative plasticity model of Drucker–Prager yield surface coupled with a generalized nonlinear kinematic hardening is considered. Conforming to the plasticity model, two exponential-based methods, called fully explicit and semi-implicit, are recommended for integrating its constitutive equations. These techniques are proposed for the first time to solve nonlinear hardening materials. The integrations are thoroughly investigated by utilizing stress and strain-updating tests along with a boundary value problem in diverse grounds of accuracy, convergence rate, and efficiency. The results indicate that the fully explicit scheme is more accurate and efficient than the Euler's, but the same convergence rate as the classical integrations is also perceived. Having a quadratic convergence, the semi-implicit is noticeably the most accurate and efficient procedure to use for this plasticity model among the algorithms in question. Since the plasticity model is in a great consistency with discontinuously reinforced aluminum (DRA) composites, the suggested formulations can be utilized pragmatically. The tangent moduli of the proposed and Euler's strategies are derived and examined, as well, due to their vital role in achieving the asymptotic quadratic convergence rate of the Newton–Raphson solution in nonlinear finite-element analyses.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Mater. Technol. 2013;135(3):034501-034501-3. doi:10.1115/1.4023675.

The increase of the luminosity of the Large Hadron Collider (LHC) by 2020 requires the upgrade of the ATLAS inner tracker experiment. Expected to be used as support structures in the design of the inner tracker, the thermal and mechanical properties of POCOFOAM and ALLLCOMP foam needed to be well understood and dimensionally stable in order to allow efficient cooling and accurate track reconstruction. Thermal conductivities of these foams were measured experimentally together with the Young's modulus, yield, and shear stresses of POCOFOAM at low stress. Thermomechanical measurements of POCOFOAM were also achieved. This paper describes briefly the measurement systems used and reports the results obtained.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2013;135(3):034502-034502-6. doi:10.1115/1.4024195.

The deformation of substrate caused by laser scanning pattern during direct laser fabrication (DLF) is ignored in spite of its importance to the final dimension accuracy. In this paper, in order to investigate the effect of deposition pattern on the deformation of substrate during DLF, eight laser scanning patterns are designed to build a cylinder on an asymmetrical IN718 arc substrate, respectively. Deformations of substrates along x, y, z-directions after DLF and heat treatment are compared and discussed. Meanwhile, the maximum displacement in z-direction of each substrate is calculated. Besides, a modified temperature gradient mechanism (TGM) is introduced to understand deformations of substrates under different laser scanning patterns. The results show that the deformation of substrate along z-direction is much larger than other two directions for all scanning patterns. The deformation of substrate strongly depends on x- and y-directional dimension of the substrate when a symmetrical build is fabricated. Compared with contour-offset scanning patterns, raster scanning patterns have distinct directional effect on deformations of substrates. Especially, the deformations of substrates caused by laser fabrication are permanent. In order to improve the fabrication efficiency, to and fro laser scan along the short dimension direction is preferred during DLF for generating the minimum deformation to substrate and reducing unnecessary movements of the working table.

Commentary by Dr. Valentin Fuster

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