J. Eng. Mater. Technol. 2005;127(1):1. doi:10.1115/1.1836794.
Commentary by Dr. Valentin Fuster


J. Eng. Mater. Technol. 2005;127(1):2-7. doi:10.1115/1.1836765.

The fatigue crack growth (FCG) behavior of PWA1484 single crystals was characterized in air under mixed-mode loading at 593°C as a function of crystallographic orientation using an asymmetric four-point bend test technique. Most mixed-mode fatigue cracks deflected from the symmetry plane and propagated as transprecipitate, noncrystallographic cracks, while self-similar fatigue crack growth occurred on the (111) planes in (111)/[011] and (111)/[112̄] oriented crystals. The local stress intensity factors and the crack paths of the deflected mixed-mode cracks were analyzed using the finite-element fracture mechanics code, FRANC2D/L. The results indicated that the deflected crack path was close to being normal to the maximum tensile stress direction where the Mode II component diminishes. Crystallographic analysis of the deflected crack paths revealed that the Mode I and the deflected mixed-mode cracks were usually of different crystallographic orientations and could exhibit different Mode I FCG thresholds when the crystallography of the crack paths differed substantially. These results were used to identify the driving force and conditions for cracking mode transition.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):8-15. doi:10.1115/1.1836766.

High-cycle fatigue behavior of Nicalon™ fiber-reinforced calcium aluminosilicate (CAS) glass–ceramic matrix composites (Nicalon™/CAS) was investigated with the aid of a nondestructive evaluation (NDE) technique. Infrared (IR) thermography was employed to study two different types of Nicalon™/CAS composites: crossply and unidirectional specimens. During fatigue testing, an IR camera was used for in-situ monitoring of temperature evolution of Nicalon™/CAS samples. Stress versus cycles to failure curves were generated for predicting the lifetime of Nicalon™/CAS composites, and the IR camera measured the temperature changes during high-cycle fatigue testing. Microstructural characterizations using scanning electron microscopy (SEM) were performed to investigate fracture modes and failure mechanisms of Nicalon™/CAS samples. In this study, the NDE technique and SEM characterization were used to facilitate a better understanding of damage evolution and progress of Nicalon™/CAS composites during high-cycle fatigue.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):16-22. doi:10.1115/1.1836767.

This paper presents the results of recent studies of cyclic microbend experiments and their consequences for plasticity length-scale phenomena in LIGA Ni microelectromechanical systems (MEMS) thin films. The strain–life fatigue behavior of LIGA Ni thin films is studied by performing fully reversed cyclic microbend experiments that provide insights into cyclic stress/strain evolution and cyclic failure phenomena. The effects of cyclic deformation on the plasticity length-scale parameters are also considered within the context of strain gradient plasticity theories. The implications of the results are then discussed for the analysis of plasticity and cyclic deformation in MEMS structures and other microscale systems.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):23-32. doi:10.1115/1.1836768.

The anomalous behavior of microstructurally “short” cracks that can control fatigue life at very high cycles can be attributed to the local conditions around these cracks, since the length scale involved requires the consideration of anisotropic material behavior and the effect of changes in grain orientation as the crack grows. The effect of local crystallography was studied in multicrystalline Compact-Tension (CT) specimens of pure nickel and a cast Ni-based superalloy. Orientation Imaging Microscopy (OIM) was used to map the crystallography of the grains ahead of the notch. A standard fatigue crack growth test was then carried out to characterize the crack path in relation to the grain orientations. Two extreme cases were identified: at one end cracks grew with small deviations through all the grains ahead of it, whereas at the other end large deflections from a path perpendicular to the applied load were observed. Intergranular cracks were found to prefer high angle boundaries, whereas transgranular cracks had a tendency to nucleate and display stage I growth along slip traces of systems with high Schmid factors, as determined by the uniaxial conditions expected at the notch tips. In addition, crack path tortuosity was more pronounced in grains with loading axes close to 〈111〉. Finally, the influence of changes on slip geometry as cracks moved across grain boundaries is also discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):33-39. doi:10.1115/1.1836769.

This paper presents the results of recent experimental and finite element studies of contact damage in model dental multilayered systems with equivalent elastic properties to those of crown/join/dentin layers that are found in dental restorations. Subsurface radial cracks are observed to form after Hertzian indentation fatigue loading. In order to explain the possible failure mechanisms, the viscous deformation of the foundation (dentinlike ceramic filled polymer) and epoxy join layers are measured. Finite element and analytical models are then developed in an effort to explain the observed contact-induced deformation of the composite multilayered system. Our results suggest that: viscous deformation of the join and foundation layers can give rise to increased tensile stresses in the top elastic layers (glass or zirconia); defects at the bottom of the top layers (induced by grinding steps before crown attachment) are also shown to promote ratcheting phenomena that can lead to stress build-up in the top layers; and viscous flow of the cement can cause the subcritical crack growth in the dental ceramics.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):40-45. doi:10.1115/1.1836770.

This paper presents the results of the combined experimental investigation and digital image correlation (DIC) analysis of the fatigue failure of open cell aluminum foams. Compression–compression cyclic loads were applied to foam specimens under the as-fabricated condition. Following characterization of the S-N curve behavior, the macroscale deformation of the tested foam under fatigue was recorded using an in-situ digital camera. The deformation sequence was then analyzed using DIC technique. It was found that foams failed with an abrupt strain jump when shear bands were formed, and serious deformation up to more than 30% was developed in the center of the shear band. The ex-situ scanning electron microscopy analysis indicated that the abrupt strain jump was due to the microscale damage accumulation in struts where surface cracks were formed and propagated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):46-57. doi:10.1115/1.1836771.

Surface and subsurface crack nucleation and growth mechanisms are elucidated for equiaxed (microstructure 1), elongated (microstructure 2), and colony (microstructure 3) microstructures of Ti6242. Prominent cleavage facets, indicative of a Stroh-type dislocation-pile phenomenon characterize the nucleation sites. Beachmarking and scanning electron microscopy (SEM) techniques are used to study fatigue crack growth rates and crack shape evolution in the short and long crack regimes. The studies reveal that surface crack growth rate data are generally comparable to the through-crack growth rate data in the long crack growth regime. However, the depth crack growth rates are somewhat slower than the through-crack growth rates. Surface crack evolution profiles are shown to exhibit a tendency towards “Preferred Propagation Paths” (PPPs). However, the magnitudes of the aspect ratios along the PPPs are different from those reported for square or rectangular cross sections subjected to cyclic tension or bending loads. Finally, the measured crack lengths and aspect ratios are compared with predictions obtained from a fracture mechanics model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):58-64. doi:10.1115/1.1836791.

A new vibration-based fatigue testing methodology for assessing high-cycle turbine engine material fatigue strength at various stress ratios is presented. The idea is to accumulate fatigue energy on a base-excited plate specimen at high frequency resonant modes and to complete a fatigue test in a much more efficient way at very low cost. The methodology consists of (1) a geometrical design procedure, incorporating a finite-element model to characterize the shape of the specimen for ensuring the required stress state/pattern; (2) a vibration feedback empirical procedure for achieving the high-cycle fatigue experiments with variable-amplitude loading; and finally (3) a pre-strain procedure for achieving various uniaxial stress ratios. The performance of the methodology is demonstrated with experimental results for mild steel, 6061-T6 aluminum, and Ti-6Al-4V plate specimens subjected to a fully reversed bending, uniaxial stress state.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):65-74. doi:10.1115/1.1836792.

The microstructure and mechanical properties of ultrasonically welded AA6111-T4 specimens are discussed. The effects of welding time on the mechanical properties of welded joints are investigated. A longer welding time results in a more continuous welded interface and higher yield and fracture strengths. Accordingly, fatigue properties of the welded specimens with longer welding times are improved. The results of electron microscopy on the cross section of ultrasonically welded joints show three distinct zones: weld zone, weld affected zone and compression zone each with a distinct microstructure. TEM results show nanocrystalline grains along with second phase particles in the range of 15–25 nm in the weld zone. Flow patterns consistent with the geometry of weld–tip were observed at the weld interface.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):75-82. doi:10.1115/1.1836793.

A high-speed and high-sensitivity thermographic-infrared (IR) imaging system has been used to investigate the temperature evolutions of SA533B1 steel specimens during high-cycle fatigue experiments. Both thermodynamics and heat-transfer theories are applied to quantify the relationship between the observed temperature variations and stress–strain states during fatigue. The thermoelastic effect has been utilized to calculate the maximum stress level during fatigue testing. The predicted results matched the experimental data quite well. Different temperature and strain behaviors have been observed between cylindrical and flat specimens during high-cycle fatigue experiments. Explanations have been provided, based on Lüders band evolutions in flat specimens during fatigue, which have been observed in detail by thermography. Numerical methods have been provided to convert the temperature map (thermograph) into heat-dissipation-rate (HDR) map, which illustrates the kinetics of the Lüders-band evolution. Thus, the thermography technology can provide an effective means to “watch” and “quantify” the heat-evolution processes, such as the mechanical-damage behaviors, which can open up new opportunities for in- situ studying mechanical and phase-transformation behaviors in detail.

Commentary by Dr. Valentin Fuster


J. Eng. Mater. Technol. 2005;127(1):83-89. doi:10.1115/1.1839215.

The compressive response of a NiTi shape-memory alloy is investigated at various strain rates using UCSD’s modified 12-in. Hopkinson pressure bar and a conventional Instron machine. To obtain a constant strain rate during the formation of a stress-induced martensite in a Hopkinson test, a copper tube of suitable dimensions is employed as a pulse shaper, since without a pulse shaper the strain rate of the sample varies significantly as its microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. The NiTi shape-memory alloy shows a superelastic response for small strains at all considered strain rates and at room temperature, 296 K. At this temperature and below a certain strain rate, the stress–strain curves of the NiTi shape-memory alloy display two regimes: an elastic austenite regime and a transition (stress-induced martensite) regime. The transition stress of this material and the work-hardening rate in the stress-induced martensite regime increase with increasing strain rate, the latter reaching a steady state level and then rapidly increasing.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):90-96. doi:10.1115/1.1839214.

Plastic deformation of polysilicon at high temperatures under stress due to creep has been demonstrated at the micro scale. This type of material behavior is generally associated with mechanical failure, however it can also be used to permanently deform or position a device. In order for creep in polysilicon to be used for MEMS applications its mechanical properties must be investigated. In this work, an experimental micro test structure is developed and measurements of high temperature plastic deformation within polysilicon are conducted. Both increases in temperature and stress are shown to increase the creep rate within the studied beams in the region of interest of the test device. Immediate plastic deformation of polysilicon has been observed to start at approximately 63% of the absolute melting temperature under moderate stress.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):97-105. doi:10.1115/1.1839187.

The creep behavior of a rotating disc made of isotropic composite containing varying amounts of silicon carbide in the radial direction has been investigated in the presence of a thermal gradient, also in the radial direction. The variation of silicon carbide content has been so tailored as to contain larger amounts of particles in a highly stressed region. This type of inhomogeneous material is known as Functionally Graded Material (FGM). The thermal gradient experienced by the disc is the result of braking action as estimated by FEM analysis. The creep behavior of the disc under stresses developing due to rotation has been determined following Sherby’s law and compared with that of a similar disc following Norton’s law. The difference in the distribution of stresses and strain rates in the discs does not follow any definite trend but the values are somewhat different. The presence of thermal gradient and a linear particle gradient separately or their simultaneous presence result in a significant decrease in steady state creep rates as compared to that in a composite disc with the same average particle content (20 vol %) distributed uniformly and operating under isothermal condition. Further, the study revealed that the creep behavior of a FGM disc could be significantly improved by increasing the gradient of particle distribution while keeping the same average particle content of 20 vol % silicon carbide in the disc.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):106-118. doi:10.1115/1.1839217.

The objective of this work was to use micromechanical finite element models to simulate the static mechanical behavior of a metal matrix composite: a cast Al 359 alloy reinforced with 20% SiC particles, at two different temperatures: room temperature and 150°C. In the simulations, periodic unit cell models incorporating the explicit representation of the matrix, reinforcing particles and precipitated primary silicon crystals in both 2D and 3D were used. Micromechanical models with both idealized and realistic reinforcing particle geometries and distributions were generated. The realistic particle geometries and distributions were inferred from experimental SEM micrographs. The pattern and intensity of the plastic deformation within the matrix was studied and the macroscale behavior of the composite was inferred from average stress and strain values. In order to include the effects of residual stresses due to the processing of the material, a quenching simulation was performed, prior to mechanical loading, and its effects on the macroscopic and microscopic behavior of the MMC was assessed. The effects of introducing the damage mechanisms of ductile void growth and brittle failure of the reinforcing particles was also investigated. The results of the simulations were compared with experimental results for the MMC in terms of macroscopic tensile stress–strain curves and conclusions were drawn.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):119-129. doi:10.1115/1.1839193.

A new approach used for single crystal (SC) materials analysis is described. Its principle is based on the extension of predictive models for isotropic material behavior to anisotropic materials such as SC nickel base superalloys. A viscoplastic model describes the material in the macroscopic level while a factor, based on the crystallographic approach, accounts for the global state of micro slip on the crystal. The combination of both elements defines the so-called “combined approach” (CA). This paper presents the development of the theory and its applications to the determination of initial yielding and tension–compression asymmetry.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):130-135. doi:10.1115/1.1839191.

Thin-walled tubular specimens, made from woven-roving glass fiber-reinforced polyester (GFRP) with two fiber orientations, [±45°]2s and [0,90°]2s, were tested under torsional fatigue tests at negative stress ratios (R),R=−1,−0.75,−0.5,−0.25, 0. The mean-amplitude diagram of the [0,90°]2s specimens was found to be divided into two regions; region (1) in which the mean stress is ineffective and region (2) in which the mean stress has a detrimental effect on the amplitude component. All examined failure criteria were found to be valid for the [0,90°]2s specimens, without any modifications; using the amplitude component and the corresponding fatigue strength in region (1), and the equivalent static stress with the corresponding static strength in region (2). For the [±45]2s specimens, having the mean stress being effective in the whole mean-amplitude diagram, the equivalent static stress was used with the corresponding static strength in different failure criteria. None of the available criteria succeeded in predicting failure for the studied case; consequently, was introduced, which a new modifying term (SWT2/F1sF1f) was introduced, which made Norris-Distortional, Tsai-Hahn, and Tsai-Hill criteria suitable for this case.

Topics: Fibers , Stress , Failure
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):136-144. doi:10.1115/1.1839192.

In machining titanium alloys with cemented carbide cutting tools, crater wear is the predominant wear mechanism influencing tool life and productivity. An analytical wear model that relates crater wear rate to thermally driven cobalt diffusion from cutting tool into the titanium chip is proposed in this paper. This cobalt diffusion is a function of cobalt mole fraction, diffusion coeficient, interface temperature and chip velocity. The wear analysis includes theoretical modeling of the transport-diffusion process, and obtaining tool–chip interface conditions by a nonisothermal visco-plastic finite element method (FEM) model of the cutting process. Comparison of predicted crater wear rate with experimental results from published literature and from high speed turning with WC/Co inserts shows good agreement for different cutting speeds and feed rate. It is seen that wear rates are independent of cutting time.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):145-153. doi:10.1115/1.1839212.

A study of the effect of tool–sheet interaction on damage evolution in electromagnetic forming is presented. Free form and conical die experiments were carried out on 1 mm AA5754 sheet. Safe strains beyond the conventional forming limit diagram (FLD) were observed in a narrow region in the free form experiments, and over a significant region of the part in the conical die experiments. A parametric numerical study was undertaken, that showed that tool–sheet interaction had a significant effect on damage evolution. Metallographic analysis was carried out to quantify damage in the parts and to confirm the numerical results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):154-158. doi:10.1115/1.1839213.

Hardness tests are performed to determine not only hardness but also other properties such as strength, wear resistance, and deformation resistance. They are also performed to predict residual lifetime through analysis of the hardness reduction or hardness ratio. However, hardness tests require observation of the residual indentation, and for that reason are not widely used in industrial fields. This study thus examines obtaining Brinell hardness values without optical observation, using instead quantitative formulas and analyzing the relationship between the indentation depths from the indentation load-depth curve and mechanical properties such as the work-hardening exponent, yield strength, and elastic modulus on the basis of finite-element analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2005;127(1):159-164. doi:10.1115/1.1839216.

Currently available models describing superplastic deformation are mostly based on uniaxial tensile test data and assume isotropic behavior, thus leading to limited predictive capabilities of material deformation and failure. In this work we present a multi-axial microstructure-based constitutive model that describes the anisotropic superplastic deformation within the continuum theory of viscoplasticity with internal variables. The model accounts for microstructural evolution and employs a generalized anisotropic dynamic yield function. The anisotropic yield function can describe the evolution of the initial state of anisotropy through the evolution of unit vectors defining the direction of anisotropy during deformation. The generalized model is then reduced to the plane stress condition to simulate sheet metal stretching in superplastic blow forming using pressurized gas. Different ratios of biaxial stretching were investigated, including the case simulating the uniaxial loading condition, where the model successfully captured the uniaxial experimental data. The model is also used to develop a new forming pressure profile that accounts for anisotropy and microstructural evolution.

Commentary by Dr. Valentin Fuster

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