Research Papers

J. Eng. Mater. Technol. 2009;131(2):021001-021001-12. doi:10.1115/1.3078389.

A combined experimental and analytical approach is used to study damage initiation and evolution in three-dimensional second phase particle fields. A three-dimensional formulation of a damage percolation model is developed to predict damage nucleation and propagation through random-clustered second phase particle fields. The proposed approach is capable of capturing the three-dimensional character of damage phenomena and the three stages of ductile fracture, namely, void nucleation, growth, and coalescence, at the level of discrete particles. An in situ tensile test with X-ray tomography is utilized to quantify material damage during deformation in terms of the number of nucleated voids and porosity. The results of this experiment are used for both the development of a clustering-sensitive nucleation criterion and the validation of the damage percolation predictions. The evolution of damage in aluminum alloy AA5182 has been successfully predicted to match that in the in situ tensile specimen. Two forms of second phase particle field input data were considered: (1) that measured directly with X-ray tomography and (2) fields reconstructed statistically from two-dimensional orthogonal sections. It is demonstrated that the adoption of a cluster-sensitive void nucleation criterion, as opposed to a cluster-insensitive nucleation criterion, has a significant effect in promoting predicted void nucleation to occur within particle clusters. This behavior leads to confinement of void coalescence to within clusters for most of the duration of deformation followed by later development of a macrocrack through intracluster coalescence. The measured and reconstructed second phase particle fields lead to similar rates of predicted damage accumulation and can be used interchangeably in damage percolation simulations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021002-021002-10. doi:10.1115/1.3078390.

Theoretical and experimental studies have shown that stress triaxiality is the key parameter controlling the magnitude of the fracture strain. Smooth and notched round bar specimens are mostly often used to quantify the effect of stress triaxiality on ductile fracture strain. There is a mounting evidence (Bai and Wierzbicki, 2008, “A New Model of Metal Plasticity and Fracture With Pressure and Lode Dependence,” Int. J. Plast., 24(6), pp. 1071–1096) that, in addition to the stress triaxiality, the normalized third deviatoric stress invariant (equivalent to the Lode angle parameter) should also be included in characterization of ductile fracture. The calibration using round notched bars covers only a small range of possible stress states. Plane strain fracture tests provide additional important data. Following Bridgman’s stress analysis inside the necking of a plane strain specimen, a closed-form solution is derived for the stress triaxiality inside the notch of a flat-grooved plane strain specimen. The newly derived formula is verified by finite element simulations. The range of stress triaxiality in round notched bars and flat-grooved specimens is similar, but the values of the Lode angle parameter are different. These two groups of tests are therefore very useful in constructing a general 3D fracture locus. The results of experiments and numerical simulations on 1045 and DH36 steels have proved the applicability of the closed-form solution and have demonstrated the effect of the Lode angle parameter on the fracture locus.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021003-021003-14. doi:10.1115/1.3078309.

A microstructure sensitive criterion for dwell fatigue crack initiation in polycrystalline alloy Ti-6242 is proposed in this paper. Local stress peaks due to load shedding from time dependent plastic deformation fields in neighboring grains are held responsible for crack initiation in dwell fatigue. An accurately calibrated and experimentally validated crystal plasticity finite element (FE) model is employed for predicting slip system level stresses and strains. Vital microstructural features related to the grain morphology and crystallographic orientations are accounted for in the FE model by construction of microstructures that are statistically equivalent to those observed in orientation imaging microscopy scans. The output of the finite element method model is used to evaluate the crack initiation condition in the postprocessing stage. The functional form of the criterion is motivated from the similarities in the stress fields and crack evolution criteria ahead of a crack tip and dislocation pileup. The criterion is calibrated and validated by using experimental data obtained from ultrasonic crack monitoring techniques. It is then used to predict the variation in dwell fatigue lifetime for critical microstructural conditions. The studies are extended to field experiments on β forged Ti-6242. Macroscopic aspects of loading are explored for their effect on dwell fatigue life of Ti-6242.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021004-021004-8. doi:10.1115/1.3078305.

Residual stress relief in ceramic-metal joints produced by active brazing depends primarily on the plastic response of the filler metal. A procedure for the production and mechanical characterization of bulk active filler alloy specimens is developed. In parallel ceramic-metal joints are produced and tested. Residual stresses are measured by X-ray diffraction while the joint strength is assessed by four-point bend tests. The obtained elastoplastic properties of the filler are introduced into finite element models to predict the residual stresses in the joints and their behavior in bending. The results of the simulations show good agreement both with the residual stress measurements and with the results of four-point bend tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021005-021005-10. doi:10.1115/1.3078304.

This research paper describes a specifically constructed Variant A continuous dieless wire-drawing machine to experimentally determine the principal processing parameters for dieless wire drawing using extra low interstitial Ti–6Al–4V wire alloy. It was experimentally determined that the process was limited by the ratio of the ingoing and outgoing axial velocities, also known as the reduction ratio R and influenced by the primary drawing velocity V1. Reductions of up to 36% per pass wire in cross-sectional area (CSA) were achieved. However, a direct relationship between the wire diameter variation and an increase in overall achievable reduction in CSA was observed. The separation distance between the wire heating and cooling devices (S) was identified as one of the principal governing process parameters. It was found that processing in an inert gas environment led to an increased reduction on CSA of approximately 3% per pass when compared with processing in compressed air. This was attributed to a reduction in surface oxidation and stress cracking. The experimentally determined results showed excellent agreement with a proposed mathematical model. It was also determined that the calculated strain rate for the process fell within the boundaries of previously determined strain rates for this particular alloy. The successful operation of this experimental machine effectively illustrates the possible commercial validity of continuous dieless wire drawing.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021006-021006-6. doi:10.1115/1.3078303.

Ultrasonic consolidation of 150μm thick 3003-H18 aluminum foil to a 3003-H18 build plate has been investigated. The effects of the normal load, vibrational amplitude, and sonotrode velocity on consolidation quality as characterized by the total linear weld density (LWD) including both edge and edge defects were examined utilizing two sequential three-level full factorial design of experiments at a constant build plate temperature of 150°C. These showed that the normal load and the vibrational amplitude have a significant influence on LWD, while the sonotrode rotational velocity has only a marginal effect. The formation of edge defects at the foil-build plate interface has been attributed to the nonuniform strain state across the foil width, while the central defects were related to sonotrode-foil contact pressure variations following the sonotrode pattern. In addition, LWD variations in ±10–20% were attributed to a nonuniform sonotrode-foil contact pressure distribution. Finally, representation of the total LWD as a function of the control parameters indicates that high total linear weld densities can be achieved with a control space bounded by high normal loads and intermediate to high vibrational amplitudes. Consideration of only central defects expands this control space allowing utilization of lower normal loads and vibrational amplitudes to achieve similar linear weld densities.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021007-021007-8. doi:10.1115/1.3078302.

The effect of nanoclay on the high strain rate mechanical properties of syntactic foams is studied. Two types of microballoons with different wall thicknesses are used in fabrication of plain and nanoclay syntactic foams. Plain syntactic foams are fabricated with 60% volume fraction of glass microballoons. 1%, 2%, and 5% volume fractions of Nanomer I.30E nanoclay are incorporated to produce nanoclay syntactic foams. High strain rate test using split Hopkinson pressure bar (SHPB) apparatus is performed on all types of plain and nanoclay syntactic foams. Dynamic modulus, strength, and corresponding strain are calculated using the SHPB data. Quasistatic test is also performed and results are compared with the dynamic SHPB results. The results demonstrate the importance of nanoclay and microballoon wall thickness in determination of syntactic foam dynamic properties. It is found that at a high strain rate, the strength and modulus of composite foams having K46 microballoons increase due to addition of 1% volume fraction of nanoclay. However, in composite foams having S22 microballoons, the increase in strength is not significant at a high strain rate. Further increase in nanoclay volume fraction to 2% and 5% reduces the strength and modulus of composite foams having S22 microballoons. Difference in wall thickness of microballoons is found to affect the strength, modulus, strain energy, and deformation of composite foams. Composite foams fabricated with thicker walled microballoons (K46) show comparatively higher values of strength, modulus, and strain energy compared with thin walled (S22) microballoons. Scanning electron microscopy shows that crack propagation behavior is distinct at different strain rates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021008-021008-10. doi:10.1115/1.3030879.

The aim of this paper was to investigate the tensile and flexural properties of hybrid laminates made with titanium sheets and high modulus carbon fiber composites. Grade II titanium was used, which exhibits great high-temperature performance and creep resistance, low weight, and high strength. An inorganic fireproof matrix, known as geopolymer, was used to fabricate the high modulus carbon fiber composites. Previous studies have shown that these composites are strong, durable, lightweight, and can exhibit excellent performance up to 400°C. In the present study, a number of specimens were tested in uniaxial tension and four-point bending after exposure at elevated temperatures. The results indicate that the addition of carbon fibers can reduce the weight and increase the stiffness of the pure titanium. Moreover, the hybrid laminates are stronger and stiffer than the sum of the individual strengths and stiffnesses of the parent materials. An important finding is that the interlaminar bond is strong, and as a result no delamination failures were observed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021009-021009-9. doi:10.1115/1.3078300.

This paper summarizes an attempt to devise an engineering method suitable for predicting fatigue lifetime of metallic materials subjected to both proportional and nonproportional multiaxial cyclic loadings. The proposed approach takes as a starting point the assumption that the plane experiencing the maximum shear strain amplitude (the so-called “critical plane”) is coincident with the micro-/mesocrack initiation plane. In order to correctly account for the presence of both nonzero mean stresses and nonzero out-of-phase angles, the degree of multiaxiality/nonproportionality of the stress state damaging crack initiation sites is suggested here to be evaluated in terms of the ratio between maximum normal stress and shear stress amplitude relative to the critical plane. Such a ratio is used then to define nonconventional Manson–Coffin curves, whose calibration is done through two strain-life curves generated under fully reversed uniaxial and fully reversed torsional fatigue loadings, respectively. The accuracy and reliability of our approach were systematically checked by using approximately 350 experimental data taken from the technical literature and generated by testing 13 different materials under both in-phase and out-of-phase loadings. Moreover, the accuracy of our criterion in estimating lifetime in the presence of nonzero mean stresses was also investigated. Such an extensive validation exercise allowed us to prove that the fatigue life estimation technique formalized in the present paper is a reliable tool capable of correctly evaluating fatigue damage in engineering materials subjected to multiaxial cyclic loading paths.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021010-021010-8. doi:10.1115/1.3078299.

The present study is concerned with the use of the modified Manson–Coffin curve method to estimate the lifetime of notched components subjected to multiaxial cyclic loading. The above criterion postulates that fatigue strength under complex loading paths can efficiently be evaluated in terms of maximum shear strain amplitude, provided that the reference Manson–Coffin curve used to predict the number of cycles to failure is defined by taking into account the actual degree of multiaxiality/nonproportionality of the stress/strain state damaging the assumed crack initiation site. The accuracy and reliability of the above fatigue life estimation technique was checked by considering about 300 experimental results taken from the literature. Such data were generated by testing notched cylindrical samples made of four different metallic materials and subjected to in-phase and out-of-phase biaxial nominal loading. The accuracy of our criterion in taking into account the presence of nonzero mean stresses was also investigated in depth. To calculate the stress/strain quantities needed for the in-field use of the modified Manson–Coffin curve method, notch root stresses and strains were estimated by using not only the well-known analytical tool due to Köttgen (1995, “Pseudo Stress and Pseudo Strain Based Approaches to Multiaxial Notch Analysis,” Fatigue Fract. Eng. Mater. Struct., 18(9), pp. 981–1006) (applied along with the ratchetting plasticity model devised by Jiang and Sehitoglu (1996, “Modelling of Cyclic Ratchetting Plasticity, Part I: Development and Constitutive Relations. Transactions of the ASME,” ASME J. Appl. Mech., 63, pp. 720–725; 1996, “Modelling of Cyclic Ratchetting Plasticity, Part I: Development and Constitutive Relations,” Trans. ASME J. Appl. Mech., 63, pp. 720–725)) but also by taking full advantage of the finite element method to perform some calibration analyses. The systematic use of our approach was seen to result in estimates falling within an error factor of about 3.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021011-021011-14. doi:10.1115/1.3078391.

The approaches traditionally used to quantify creep and creep fracture are critically assessed and reviewed in relation to a new approach proposed by Wilshire and Scharning. The characteristics, limitations, and predictive accuracies of these models are illustrated by reference to information openly available for the bainitic 1Cr–1Mo–0.25V steel. When applied to this comprehensive long-term data set, the estimated 100,000–300,000 h strength obtained from the older so called traditional methods varied considerably. Further, the isothermal predictions from these models became very unstable beyond 100,000 h. In contrast, normalizing the applied stress through an appropriate ultimate tensile strength value not only reduced the melt to melt scatter in rupture life, but also the 100,000 h strengths determined from this model for this large scale test program are predicted very accurately by extrapolation of creep life measurements lasting less than 5000 h. The approach therefore offers the potential for reducing the scale and cost of current procedures for acquisition of long-term engineering design data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021012-021012-11. doi:10.1115/1.3086336.

The present work extends the multicoated micromechanical model of Lipinski (2006, “Micromechanical Modeling of an Arbitrary Ellipsoidal Multi-Coated Inclusion,” Philos. Mag., 86(10), pp. 1305–1326) in the quasistatic domain to compute the effective material moduli of a viscoelastic material containing multicoated spherical inclusions displaying elastic or viscoelastic behavior. Losses are taken into account by introducing the frequency-dependent complex stiffness tensors of the viscoelastic matrix and the multicoated inclusions. The advantage of the micromechanical model is that it is applicable to the case of nonspherical multicoated inclusions embedded in anisotropic materials. The numerical simulations indicate that with proper choice of material properties, it is possible to engineer multiphase polymer system to have a high-loss modulus (good energy dissipation characteristics) for a wide range of frequencies without substantially degrading the stiffness of the composite (storage modulus). The numerical analyses show also that with respect to the relative magnitudes of the loss factors and the storage moduli of the matrix, inclusion and coating, the overall properties of the viscoelastic particulate composite are dominated by the properties of the matrices in some frequency ranges. The model can thus be a suitable tool to explore a wide range of microstructures for the design of materials with high capacity to absorb acoustic and vibrational energies.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021013-021013-8. doi:10.1115/1.3086386.

Unidirectional fiber-reinforced plastic (UD-FRP) laminates have been modeled previously as an equivalent quasihomogeneous monoclinic half-space subjected to an inclined line load on the surface using Lekhnitskii’s formulation simulating the orthogonal edge trimming loads in UD-FRPs. In continuation, failure analysis of the aforementioned composite half-space has been carried out in the present investigation based on Tsai–Wu criterion. In particular, the failure behavior of the half-space laminate with respect to the fiber orientation, load inclination angle, and spatial coordinates has been examined in detail. The motivation behind such a study lies in correlating the failure behavior of the half-space laminate with the machining damage observed during orthogonal edge trimming experiments. The present work strives at identifying this relationship and, in the process, understanding the physics of orthogonal cutting of UD-FRP laminates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021014-021014-11. doi:10.1115/1.3030944.

Effect of fiber volume fraction on occurrence, morphology, and spatial distribution of microvoids in resin transfer molded E-glass/epoxy composites is investigated. Three disk-shaped center-gated composite parts containing 8, 12, and 16 layers of randomly-oriented, E-glass fiber perform are molded, yielding 13.5%, 20.5%, and 27.5% fiber volume fractions. Voids are evaluated by microscopic image analysis of the samples obtained along the radius of these disk-shaped composites. The number of voids is found to decrease moderately with increasing fiber content. Void areal density decreased from 10.5voids/mm2 to 9.5voids/mm2 as fiber content is increased from 13.5% to 27.5%. Similarly, void volume fraction decreased from 3.1% to 2.5%. Increasing fiber volume fraction from 13.5% to 27.5% is found to lower the contribution of irregularly-shaped voids from 40% of total voids down to 22.4%. Along the radial direction, combined effects of void formation by mechanical entrapment and void mobility are shown to yield a spatially complex void distribution. However, increasing fiber content is observed to affect the void formation mechanisms as more voids are able to move toward the exit vents during molding. These findings are believed to be applicable not only to resin transfer molding but generally to liquid composite molding processes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021015-021015-9. doi:10.1115/1.3030945.

In this work, an analytical model based on continuum mixture theories is developed to study the biaxial interfacial shear stresses in adhesive-bonded joints due to thermomechanical loading. The model predicts the effect of adhesive thickness and properties on the interfacial shear stresses. Two sets of governing partial differential equations are solved for the displacement field in each layer of the joint. The interfacial shear stresses between the adhesive and each adherend are determined using the constitutive equations. Numerical results show that both the adhesive thickness and the material properties have a significant effect on the thermomechanically induced interfacial shear stresses between the adherends and the adhesive. The proposed model inherently has the capacity for optimizing the selection of the adhesive thickness and material properties that would yield a more reliable bonded joint.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(2):021016-021016-6. doi:10.1115/1.3030946.

In this paper, continuous SiC fibers were embedded in an Al 6061 O matrix through ultrasonic consolidation at room temperature. The optimum embedding parameters were determined through peel tests and metallographic analysis. The influence of the embedded fiber volume fraction and base metal thickness on the interface bond strength was studied, and the fiber/matrix bond strength was tested through fiber pullout test. The results showed that embedding 0.8% volume fraction of SiC fiber in a 6061 O matrix could significantly increase and even its interfacial strength, but there is a threshold for embedded fiber volume fraction at specific parameters, over which the plastic flow and friction may be insufficient to have a strong bond at foil/foil interfaces between fibers. The study also showed that base metal thickness did not have significant influence on the interfacial strength with an exception of samples with a base metal thickness of 500μm. Based on the results, it was proposed that microfriction at consolidation interfaces plays an important role for joint formation, and localized plastic flow around fibers is important to have fibers fully and safely embedded.

Topics: Fibers , Bond strength
Commentary by Dr. Valentin Fuster


Commentary by Dr. Valentin Fuster

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