Robert N. Pangborn

Static and High Strain Rate Response of a Glass Fiber Reinforced Thermoplastic

The mechanical response of an impact-modified, discontinuous fiber reinforced styrene-maleic anhydride (S/MA) polymer has been characterized at static and high strain rates and under both dry and wet test conditions. Five different material configurations were tested, including unreinforced S/MA as a reference material and composites incorporating fiber reinforcement with different average diameters, the presence or absence of an interfacial silane coupling agent, and fibers prepared with an acrylonitrile/butadiene latex coating. The ultimate tensile strengths, strains to failure, fracture energies, and effective moduli for each of the materials were evaluated as a function of strain rate, which was varied between 1.67 × 10−3 and 6.0 mm/mm·s. The results of the tests performed on the unreinforced S/MA revealed a 60% increase in the ultimate strength and marked reductions of 80% and 50% in the strain to failure and fracture energy, respectively, with increasing strain rate. While all of the composites exhibited on the order of twice the strength, 2.5 times the stiffness, and less than a tenth of the strain to failure compared to the unreinforced S/MA, the dependence of these properties on the strain rate was much weaker. Significantly, the work of fracture more than doubled with the strain rate for all the composite configurations tested due to the comparatively small reduction in fracture ductility of about 25%. All of the materials showed some degradation in the mechanical properties when tested wet, a result that was particularly evident for the composites having no silane coupling agent which suffered about a 15% loss in strength and stiffness. A simple rule of mixtures calculation revealed that as the rate of testing was increased, more efficient reinforcement by the fibers was realized. Fractographic observations using a scanning electron microscope, viewed in conjunction with the experimental results, indicated that fiber debonding during composite deformation was limited by the inhibition of viscoelastic flow in the matrix material at high strain rates.

Mechanical Behavior of Mullite Fiber Reinforced Aluminum Alloy Composites

Discontinuously reinforced aluminum alloys are viewed as candidate materials for elevated temperature applications because of their attractive high temperature strength properties and wear resistance. The elevated temperature elastic properties and the failure characteristics in relation to the preform flaws, however, have not received much attention in spite of their potential significance. These issues are studied for an aluminum-silicon alloy reinforced with mullite discontinuous fibers, fabricated using the squeeze infiltration technique. The effect of preform flaws (shot) on room temperature strength and ductility is investigated for composites seeded with different amounts of shot. The Youngs modulus of the composite exceeds that of the unreinforced alloy over a wide range of temperatures, and the beneficial influence of the fibers is especially significant at elevated temperatures. The primary contribution to the reduction in the modulus of the composite at higher temperatures is shown to be the degradation in the matrix stiffness. Reinforcing the alloy with mullite fibers results in only a moderate improvement in strength at room temperature but the elongation to failure is reduced considerably. Increasing the amount of shot, although not appreciably degrading strength, further reduces the ductility. Shot is found to play an important role in the damage evolution by fracturing early in the loading process, and thus, the composite integrity when subjected to slow stable crack growth, as in fatigue, for example, could be adversely affected.

The effect of grinding on the flexural strength of a sialon ceramic

The effect of selected grinding parameters on the flexural strength of a sialon ceramic was studied. Support compliance was found to have no significant effect, while depth of incursion and grinding direction did. Weibull statistics and analysis of variance techniques were used to detect these effects which are explained through flaw magnitude and direction.

Dry sliding wear behavior of an Si-Al-O-N ceramic

Abstract Dry sliding wear tests were conducted on (Si-Al-O-N)-cast iron sliding pairs using a wear tester specifically designed to simulate closely the high pressures, velocities and resultant temperatures encountered in heavily loaded machinery and other engineering applications. A wear tester was designed so that both the contact load level and the sliding velocity could be varied. Wear testing indicated that, following a short wear-in period, mass loss varied linearly with sliding distance for all contact loads and sliding velocities investigated. The steady state wear rates, expressed in terms of mass loss per unit sliding distance, were found to increase as the applied contact load was increased for all the sliding velocities investigated. Further, the wear rate was found to vary inversely with sliding velocity for the highest contact load level investigated. For the intermediate and lowest contact load levels, this inverse relationship between velocity and wear rate (i.e. reduced wear for higher velocities) was less pronounced and restricted to the lower range of velocities investigated. Several different grinding procedures were used to prepare the specimens prior to wear testing but were found to have no significant influence on the measured wear rates. The experimental results are qualitatively explained by considering combined wear mechanisms involving brittle fracture and plastic deformation, which depend on the relative magnitudes of contact load and sliding velocity. Brittle fracture is assumed to be the primary wear mechanism especially at high contact loads and low sliding velocities. Plastic deformation mechanisms are also active and intensify with increasing frictional heating and concomitant thermal softening.

Fatigue damage assessment using x-ray diffraction and life prediction methodology

Abstract X-ray diffraction line broadening was used to monitor surface damage due to deformation (distortion) that was induced by low cycle fatigue. The integral breadth of selected diffraction peaks was identified as a useful parameter with which to evaluate cumulative fatigue damage. Torsional fatigue tests were conducted on nickel-based Waspaloy material which exhibited planar slip at 1200° F (649°C). X-ray diffraction measurements were taken at 22, 41, 60, and 90% of the life. The data disclosed an increase in breadth with each increment of cycling. The results obtained from line broadening analysis were carefully correlated with observations made on the specimen surface using scanning electron microscopy which showed the progressive distortion occurring in the cycled specimen. The integral breadth, β, was successfully correlated with the applied shear strain to predict the expended fraction of life and hence the remaining cyclic life.

Thermal fatigue behavior of squeeze cast, discontinuous alumina-silicate fiber-reinforced aluminum alloy (A356) composite

Abstract Microstructural damage mechanisms owing to thermal cycling and isothermal exposure at elevated temperature are studied for a short alumina-silicate fiber-reinforced aluminum alloy (A356) composite produced by pressure casting. The tensile strength of the metal matrix composite is found to degrade considerably in each case. An X-ray double-crystal diffraction method is employed to study the mechanisms of recovery in the matrix. The fractal dimension of the X-ray “rocking curves” for individual grains in the composite reflects the substructure formation owing to the rearrangement of dislocations into subdomain walls. Recovery by polygonization is more pronounced in the case of thermal cycling than for equivalent isothermal exposure. The residual stresses in the matrix that provide the fiber clamping force undergo more relaxation in the case of isothermal exposure. The two competing damage mechanisms, thermally activated recovery by polygonization and relaxation of clamping stresses in the matrix, result in identical strength degradation (∼25%) for both thermal cycling and isothermal exposure.

Acoustic emission for in situ monitoring in metal-matrix composite processing

Abstract The attractive elevated-temperature properties of metal-matrix composities (MMCs) have not been exploited in commercial applications partly because of the high processing cost and lack of reliability in fabrication. In this exploratory study, the feasibility of using acoustic emission (AE) as an in-process, non-destructive quality control technique is examined. A variation of the squeeze casting technique is selected for investigation. Acoustic emission is employed with the intent of non-intrusively establishing whether complete infiltration has occurred during composite fabrication. The problems due to the background noise during AE monitoring are overcome by using transducers with different frequency responses. The acoustic signatures of machine noise, preform crushing and metal solidification are obtained by employing suitable transducers in a series of tests that systematically evaluate the individual processes that comprise infiltration casting. The results form a strong basis for the development of an in situ AE sensor for the infiltration process.

Surface features and plasticity induced by tension-tension fatigue of Inconel 718

The accumulation of fatigue damage in Inconel 718 has been investigated with a combination of X-ray diffractiOn techniques and scanning electron microscopy. X-ray line broadening analyses and computer -aided rocking curve measurements have been conducted for solutionsewed and age-hardened specimens, cycled in tension-tension. The line profile breadths were found to be very sensitive to the magnitude of the constant stress amplitude employed in the fatigue testing. After increasing sharply during the initial 15% of the fatigue life to a value which depends on the maximum stress level, the breadths are relatively invariant during the reminder of the life. The rocking curve breadths obtained using double crystal diffractometry, on the other hand, increase progressively with continued cycling from about 20% of the life to failure and are nearly independent of the stress amplitude used in the fatigue testing. These techniques can be applied effectively in combination to determine prior cyclic history by providing information on both the stress level and expended fraction of life. Therefore, application of the X-ray diffraction analyses to spectrum fatigue tests has been initiated, beginning with sequences involving regular decrements in the stress level. In order to evaluate differences in the response of the surface layer and the bulk material during fatigue, depth studies have also been carried out The development of topographical features, such as slip bands, and their relationship to crack initiation and propagation mechanisms have been examined by scanning electron microscopy and correlated to the X-ray diffraction data.

Fatigue Damage Assessment by X-Ray Diffraction and Nondestructive Life Assessment Methodology

X-Ray diffraction (XRD) has been employed to evaluate cumulative damage incurred under both high cycle fatigue (HCF) and low cycle fatigue (LCF) conditions. Additional objectives of the study were to correlate the x-ray diffraction data to the microstructural characteristics and deformation mechanisms activated by various fatigue regimes and to develop a methodology for nondestructive assessment of remaining life.