J. Eng. Mater. Technol. 2006;129(4):497-504. doi:10.1115/1.2772323.

This paper is on the characterization of the mechanical properties of Newtonian-type shock absorbing elastomeric composites. This composite material is a blend of elastomeric capsules or beads in a matrix of a Newtonian liquid. The material can be considered as a liquid analogy to elastomeric foams. It exhibits bulk compression characteristics and acts like an elastic liquid during an impact, unlike elastic foams, which exhibit uniaxial compression characteristics. A test cell consisting of an instrumented metal cylinder and a piston was designed. A sample of the material was placed in the instrumented cylinder, which was located at the base of a drop test rig. A drop mass of 17.3kg was subsequently released from a desired height to impact the piston. From measurements of the acceleration histories of the drop mass and the piston, and from the displacement history of the piston, the force-displacement curves and the associated impact energies absorbed were derived. These are compared to the corresponding characteristics derived from measurements of pressure of the fluid medium inside the cylinder. The results are compared for blends of different bead types, and the different aspects contributing to their performance are discussed. It is shown that the performance curves derived from the accelerometer measurements matched those derived from the pressure measurements. Blends of this composite material of different types of beads showed distinctively different characteristics.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):505-512. doi:10.1115/1.2744430.

Traditionally, the hardness of materials is determined from indentation tests at low loading rates (static). However, considerably less work has been conducted in studying the dynamic hardness of materials using relatively high loading rates. In the present work, two models are used to predict strain rate dependency in hardness. The first model is a power law expression that is based on the dependence of the yield stress on the strain rate. This model is relatively simple in implementation, and it is quite easy to determine its parameters from simple uniaxial experiments. The second model is a micromechanical based model using Taylor’s hardening law. It utilizes the behavior of dislocation densities at high strain rates in metals in order to relate dynamic hardness to strain rates. The latter model also accounts for any changes in temperature that could exist. A finite element is also run and compared with the two models proposed in this work. Results from both models are compared with available experimental results for oxygen-free high-conductivity copper and 1018 cold rolled steel, and both models show reasonably good agreement with the experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):513-522. doi:10.1115/1.2772328.

Two methods have been used to simulate 2D elastic wave scattering in nickel aluminide (NiAl) bicrystals to study effects of grain boundaries and material anisotropy on elastic wave propagation. Scattering angles and amplitude ratios of the reflected and refracted waves produced at the grain boundary were calculated via slowness curves for both grains, which were plotted in the plane of incidence containing the grain boundary normal. From these curves, scattering angles were measured graphically and amplitude ratios were calculated based on the continuity of tractions and displacements at the boundary. To support these calculations, finite element simulations were performed with ABAQUS /EXPLICIT to obtain time- and space-dependent stresses. The results of each method correlated well with each other for four bicrystals. It was found that for bicrystals where the transmitted quasi-longitudinal (TQL) wave amplitude decreased across the boundary, diminished stresses were found in the finite element models for the same bicrystal. Conversely, where an increase in amplitude of the TQL wave was found, the finite element simulations showed that stress under the boundary increased. In general, the amplitude of the TQL wave was found to have a strong connection to the ratio of incident and TQL sound speeds. However, other directions in each grain are believed to contribute strongly to the overall scattering process since the pairs of bicrystals in this investigation had somewhat similar sound speeds. These findings correlated well with free surface cracking observed in a previous paper (Loomis, E., Peralta, P., Swift, D., and McClellan, K., 2005, Mater. Sci. Eng., Ser. A., 404(1-2), pp. 291–300), where cracks nucleated and propagated due to the focusing of scattered waves at the boundary. Specifically, in bicrystals oriented for shielding, the grain boundary was protected forcing cracks to grow outside of the shielded region.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(4):523-529. doi:10.1115/1.2744429.

The chemical composition of Stellite® 21 alloy was modified by doubling the molybdenum (Mo) content for enhanced corrosion and wear resistance. The specimens were fabricated using a casting technique. Half of the specimens experienced a heat treatment at 1050°C for an hour. The microstructure and phase analyses of the specimens were conducted using electron scanning microscopy and X-ray diffraction. The mechanical properties of the specimens were determined in terms of the ASTM Standard Test Method for Tension Testing of Metallic Materials (E8-96). The mechanical behaviors of individual phases in the specimen materials were investigated using a nano-indentation technique. The wear resistance of the specimens was evaluated on a ball-on-disk tribometer. The experimental results revealed that the increased Mo content had significant effects on the mechanical and tribological properties of the low-carbon Stellite® alloy and the heat treatment also influenced these properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):530-537. doi:10.1115/1.2772329.

The influence of temperature on the forming behavior of an aluminum/polypropylene/aluminum (APA) sandwich sheet was studied. Shear and tensile tests were performed to determine the mechanical properties of the laminate and the component materials as a function of process temperature. The forming limit diagram (FLD) of the laminate was established for two different temperatures, and its springback behavior was examined in four-point bend and channel bend tests. Cup forming tests were performed at various test temperatures to determine the limiting drawing ratio (LDR) and the tendency for wrinkling at these temperatures. Although there was only a minor influence of temperature on the mechanical properties and the FLD values of the laminate, the bend test results reveal that springback can be reduced by forming at higher temperature. The decreasing strength of the core material with rising process temperature led to an increased tendency of the laminate to wrinkle in the heated cup drawing tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):538-549. doi:10.1115/1.2744431.

Ultrasonic consolidation, an emerging additive manufacturing technology, is one of the most recent technologies considered for fabrication of metal matrix composites (MMCs). This study was performed to identify the optimum combination of processing parameters, including oscillation amplitude, welding speed, normal force, operating temperature, and fiber orientation, for manufacture of long-fiber-reinforced MMCs. A design of experiments approach (Taguchi L25 orthogonal array) was adopted to statistically determine the influences of individual process parameters. SiC fibers of 0.1mm diameter were successfully embedded into an Al 3003 metal matrix. Push-out testing was employed to evaluate the bond strength between the fiber and the matrix. Data from push-out tests and microstructural studies were analyzed and an optimum combination of parameters was achieved. The effects of process parameters on bond formation and fiber/matrix bond strength are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):550-558. doi:10.1115/1.2772327.

The mechanical threshold stress (MTS) model is not commonly used in industrial applications due to its complexity. The Zener–Hollomon parameter Z was utilized to develop a simplified and compact formulation similar to the MTS model. The predictions of the proposed formulation are compared to the results obtained by the original MTS model and experimental data. The flow stresses of three cold-rolled steels frequently used in automotive industries were analyzed for both formulations over a wide range of strain rates (8.103s1ε¯̇p103s1).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):559-566. doi:10.1115/1.2772330.

Industrial boilers experience bulbous cracks in carbon steel water-wall tubes and other water-touched surfaces. Because these cracks are blunt and different from sharp fatigue cracks, they are generally referred to as stress-assisted corrosion (SAC) cracks. The performance of carbon steels in industrial boilers strongly depends on the formation and stability of the magnetite film on the waterside surface. To understand the mechanism for SAC crack initiation and propagation, slow strain rate tests were conducted in a recirculation autoclave under industrial boiler water conditions. The dissolved oxygen in the water was maintained from a negligible amount (5ppb) to 3ppm. The SAC crack initiation and propagation mechanism involves magnetite film damage and requires the presence of dissolved oxygen in the water. Increasing the test temperature accelerates the process. A mechanism for SAC cracking is proposed, and interrupted slow strain rate tests were carried out to validate this mechanism. Temperature and dissolved oxygen in boiler water are important factors in initiation and propagation of stress assisted corrosion cracks. SAC in boilers can be controlled by controlling the dissolved oxygen levels around 5ppb.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):567-579. doi:10.1115/1.2772338.

Finite element analysis was used in the current study to examine the effects of strain hardening and initial yield strength of workpiece material on machining-induced residual stresses (RS). An arbitrary–Lagrangian–Eulerian finite element model was built to simulate orthogonal dry cutting with continuous chip formation, then a pure Lagrangian analysis was used to predict the induced RS. The current work was validated by comparing the predicted RS profiles in four workpiece materials to their corresponding experimental profiles obtained under similar cutting conditions. These materials were AISI H13 tool steel, AISI 316L stainless steel, AISI 52100 hardened steel, and AISI 4340 steel. The Johnson–Cook (J–C) constitutive equation was used to model the plastic behavior of the workpiece material. Different values were assigned to the J-C parameters representing the studied properties. Three values were assigned to each of the initial yield strength (A) and strain hardening coefficient (B), and two values were assigned to the strain hardening exponent (n). Therefore, the full test matrix had 18 different materials, covering a wide range of commercial steels. The yield strength and strain hardening properties had opposite effects on RS, where higher A and lower B or n decreased the tendency for surface tensile RS. Because of the opposite effects of A and (B and n), maximum surface tensile RS was induced in the material with minimum A and maximum B and n values. A physical explanation was provided for the effects of A, B, and n on cutting temperatures, strains, and stresses, which was subsequently used to explain their effects on RS. Finally, the current results were used to predict the type of surface RS in different workpiece materials based on their A, B, and n values.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):580-587. doi:10.1115/1.2772340.

We investigate properties that govern interfacial fracture within the framework of linear elastic fracture mechanics, including interfacial fracture toughness, mode mixity, and the associated reference length. The reference length describes the arbitrary location where the mode mixity is evaluated, ahead of the crack tip, in a bimaterial system. A method for establishing a reference length that is fixed for a given bimaterial system is proposed. This is referred to as the “characteristic reference length,” with the associated “characteristic mode mixity.” The proposed method is illustrated with an experimental investigation, utilizing a four-point bend test of a bimaterial system.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):588-593. doi:10.1115/1.2772325.

The corrosion fatigue performance of a superduplex stainless steel in two different chloride-bearing solutions (30gl and 60glNaCl) is explored in the present paper. Fatigue life results and surface damage analysis show a strong influence of the applied strain amplitude, as well as of the chloride content, on the cyclic response. Moreover, there is an interaction between both factors because corrosion fatigue mechanisms at low and high strain amplitudes are not comparable, since ferrite is only fully plastically active at high strains. Whereas a detrimental effect of the 30glNaCl solution on the fatigue strength is observed, longer fatigue lives were attained in 60glNaCl solution, even longer than in air. Surface damage inspection and residual solutions analysis support a critical anodic dissolution rate, which would act as a polishing effect, as the main reason for this unexpected result.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(4):594-602. doi:10.1115/1.2772324.

Details are provided for an experimental approach to study the tensile fatigue crack growth behavior of very thin metallic foils. The technique utilizes a center-notched specimen and a hemispherical bearing alignment system to minimize bending strains. To illustrate the technique, the constant amplitude fatigue crack growth behavior of a Ni-base superalloy foil was studied at temperatures from 20°C to 760°C. The constant amplitude fatigue tests were performed at a frequency of 2Hz and stress ratio of 0.2. The crack growth rate versus stress intensity range data followed a Paris relation with a stress intensity range exponent m between 5 and 6; this exponent is significantly higher than what is commonly observed for thicker materials and indicates very rapid fatigue crack propagation rates can occur in thin metallic foils.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2006;129(4):603-608. doi:10.1115/1.2744428.

A method that can determine uniquely the elastoplastic properties from indentation loading and unloading curves has been developed. It is based on finite element modeling and inverse analysis of two separate indenters. The approach was validated by numerical experiments using a fictitious material. It was demonstrated that the proposed method can uniquely recover the elastoplastic properties using only indentation load-displacement curves of two indenters. Although the proposed procedure has been used to predict elastoplastic strain hardening behavior, it is also applicable to estimate other mechanical properties where there are more than two unknown parameters, such as rate-dependent behavior.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2007;129(4):609-613. doi:10.1115/1.2772339.

A new surface modification process was developed to introduce compressive residual stresses at the surface of components. In this process, instead of oil droplets a high-velocity cavitation jet (cloud of oil bubbles) impinges on the surface of the component to be peened. The impact pressure generated during implosion of cavitation bubbles causes severe plastic deformation at the surface. Consequently, beneficial compressive stresses are developed at the surface. In order to find the potential of this process, aluminum alloy AA6063-T6 specimens were peened at a constant cavitation number with various nozzle-traveling velocities. Residual stress induced by oil jet cavitation peening was measured using X-ray diffraction. Oil cavitation jet peening results in a smooth and hard surface. The developed compressive residual stresses at the peened surface are about 52%, 42%, and 35% of yield strength in samples for peened at nozzle traveling velocities of 0.05mms, 0.10mms, and 0.15mms, respectively.

Commentary by Dr. Valentin Fuster

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