Noel E. Ashbaugh

Thermal Residual Stress Relaxation in Powder Metal IN100 Superalloy

Relaxation of shot peen induced compressive residual stresses due to thermal exposure was measured using X-ray diffraction. The material used in this study was a hot isostatically pressed (HIP) powder metal (PM) IN100 nickel base superalloy. A total of 14 IN100 samples were shot peened to an Almen intensity of 6A using MI-170-R shot with 125 % coverage. The sample dimensions were nominally 16×13×4-mm thick with an irradiated X-ray region of 8×5 mm. Residual stress measurements were made at the surface and at nominal depths of 12, 25, 50, 75, 125, 175, 250, and 350 microns. The shot peened samples were thermal exposed at two temperatures (650, 704°C) and a range of exposure times (0.5–300 h). Residual stress measurements on shot peened samples without thermal exposure were used as a basis for comparison. The relaxation of shot peened compressive residual stresses under purely thermal loading was examined. The residual stresses exhibited an initial rapid decrease on the surface and in the depth at both temperatures. However, continued thermal exposure produced little or no change in surface residual stresses while peak compressive stresses in the depth continued to relax with time at both temperatures. In all cases of this study the retained peak compressive residual stress after thermal exposure was greater than 50 % of the baseline value.

Stress-free edge effects on the transverse response of a unidirectional metal matrix composite

Abstract In using a representative volume element of a unidirectional composite, two out-of-plane boundary conditions represent edge and internal regions of a transversely loaded SCS-6/Timetal® 21S composite. A plane stress condition is proposed to represent a stress-free edge region of the composite, while a generalized plane (uniform longitudinal) strain condition is considered for internal regions. Numerical simulations with elastic-plastic matrix behavior and several different fiber-matrix interface strengths reveal a complex interaction of residual stress, fiber-matrix separation and matrix inelastic behavior which are all dependent on the out-of-plane boundary condition. Several permutations of plane stress and generalized plane strain solutions with several fiber-matrix interface strengths fail to accurately capture the non-linear behavior measured from experiments. The bounding of the experimentally determined transverse response between the plane stress and generalized plane strain solutions suggests that the transverse response may be a combination of both solutions. Strain measurements from the transversely loaded composite support speculation that the plane stress solution better represents edge regions of the composite than a generalized plane strain solution. Likewise, the generalized plane strain solution represents the internal strain state of the composite better than the plane stress case at low loads. Photomicrographs before and after transverse loading show fibers which have protruded from the edge of the specimen; thus, the strain state within the composite is transitioning to a less constrained condition.

Translaminar fracture toughness test methods and results from interlaboratory tests of carbon/epoxy laminates

Fracture tests were performed with carbon/polymer laminates and analyzed for the purpose of developing translaminar fracture toughness test and analysis procedures. Notched specimens were tested of two types of symmetrical layups--quasi-isotropic [0/45/90] and [0/90]; two carbon fiber/epoxy materials--a relatively brittle T300 fiber/976 epoxy and a tougher AS4 fiber/977-2 epoxy; two laminate thicknesses--2 mm and 4 mm; and three specimen configurations--the standard three-point bend and compact configurations, and an extended compact specimen with arm-height to specimen-width ratio of 1.9. Stress and displacement expressions were obtained for the extended compact specimen, including those for stress intensity factor, K, and crack mouth opening displacement, V, in terms of relative notch length, a/W, and for a/W in terms of V. Relationships for the bending stresses that control self-similar and off-axis cracking for the extended compact specimen were derived.

Crack-Bridging Effects in Notch Fatigue of SCS-6/TIMETAL 21S Composite Laminates

Fatigue tests of middle-hole tension specimens of SCS-6/TIMETAL 21S composite (silicon-carbide fibers reinforcing a matrix of Ti-15Mo-2.6Nb-3Al-0.2Si alloy) were performed on three laminate architectures: unidirectional, cross ply, and quasi-isotropic. Specimens were tested over a range of stress levels, and fatigue damage was documented in situ by macrophotography and direct-current electric potential drop measurements. Typically, failure evolved by the formation of a few dominant cracks at the notch that propagated into the composite matrix and, in many instances, were substantially affected by unbroken fibers bridging the cracks. Fractographic and failure mode characterization revealed key differences in the effectiveness of crack bridging in the three laminates. A shear-lag crack-bridging model was shown to correlate crack growth data in the laminates based on an empirical value of fiber/matrix interfacial shear stress. Crack-bridging stress distributions were predicted using the shear lag model and verified by comparing the predicted crack opening displacement profiles with measurements made using a laser interferometric displacement gage system. Implications of the results are discussed with respect to the potential use of these materials in practical structural applications.

Bridging fiber stress distribution during fatigue crack growth in [0]4 SCS-6/timetal®21S

Abstract A procedure was developed to determine the bridging fiber stress distribution from crack opening displacements measured during crack growth. The bridging fiber stress range is near-linear along the crack and non-zero stresses at the crack tip. The magnitude of the bridging fiber stress range is proportional to the applied stress range.

A Sampling of Mechanical Test Automation Methodologies Used in a Basic Research Laboratory

Basic research laboratories typically perform a variety of material tests and obtain the associated data to model material behavior phenomena and develop life prediction methodologies. In this research environment, a mechanical test automation system must meet challenges that are not always present in an industrial testing setting. For example, real-time crack closure load analyses, at the present time, are not widely performed in industrial crack propagation testing. In the research environment, however, on-line crack closure studies are used to make decisions in real-time about changes in test conditions. A previous paper described the overall system strategy and hardware and one of the crack propagation software modules from the fourth generation of the material analysis and testing environment (MATE) automation system. The present paper discusses selected methodologies that the current (fifth) generation of the MATE system uses to meet the challenges posed while automating research style mechanical tests. The methodologies addressed in this paper include waveform generation and synchronization for cyclic, monotonic, and thermomechanical (TMF) testing as well as specimen damage computation for self-similar cracked geometries.

Fatigue crack growth at high load ratios in the time-dependent regime

Experimental crack growth rates were determined in Inconel 718 middle cracked tension [M(T)] specimens at 649°C under conditons of high frequency (10 to 100 Hz) and high load ratio, R. Under these conditions, the materialexperiences cycle-dependent crack growth as well as time-dependent crack growth. At very high R values approaching unity, the observed growth rates are lower than those obtained under sustained load at the same mean load in the absence of the superimposed cyclic loading. Tests on compact tension [C(T)] specimens at lower frequencies were used to demonstrate the existence of three regions of behavior-cycle-dependent, mixed mode, and time-dependent. A linear cumulative damage model was used to predict the growth rates due to combined cycle-dependent and time-dependent mechanisms. The model was developed from 427°C data for the cyclic term and sustained load crack growth data for the time-dependent term. Although the model cannot predict the synergistic effect at high R, it provides a reasonable representation of much of the data. The authors conclude that the use of low-temperature data for the cyclic term is inadequate for representing the threshold values and growth rates at low AK values at the higher temperature.

Sustained load behavior of SCS-6/TIMETAL 21S composite

TIMETAL 21S matrix composite reinforced with silicon-carbide (SCS-6) fibers is a potential candidate material for the high temperature structural components of advanced aircraft. However, a considerable amount of mechanical property characterization is required before this material can be used for aerospace structures. This paper discusses the characterization of the sustained load behavior of SCS-6/TIMETAL 21S composite under simulated service conditions. TIMETAL 21S, reinforced with approximately 35% SCS-6 fibers (by volume) was used for this study. The tension creep specimens were fabricated from [0] 4 , [90] 4 , [0/90] s , and [0/′45/90] s laminates. Sustained load creep tests were conducted under cold grip conditions at 650, 760, and 815°C at various stress levels. The creep response for [90] 4 , [0/90] s , and [0/′45/90] s layups exhibited generic creep curves with three distinct stages: (1) an early region of primary creep, (2) a linear region similar to a steady stage, and (3) a relatively short region similar to a tertiary stage. In contrast, the creep response of the [0] 4 layup showed hardly any tertiary region and the failure was abrupt just after the secondary stage. The total strain to failure for all orientations, except the [90] 4 , was near the ultimate strain for fiber failure. Accelerated rate of damage accumulation was observed with increasing stress and temperature that resulted in higher creep strain under these conditions. A simple creep model based on the relaxation of matrix stress in the primary and secondary creep stages was developed to predict the creep response of the unidirectional SCS-6/ TIMETAL 21S composite at 650°C. The model predictions correlated well with the data for stresses less than 700 MPa. The optical and scanning electron microscopic (SEM) analyses of the failure surfaces showed multiple crack initiation sites near fiber/matrix interface. Two distinct zones of failures were observed in all cases. The flat zone, with little or no fiber pull-out, represented the stable crack growth region. The rugged zone, with significant fiber pull-out, was caused by overload failure. In all cases, the damage (crack) initiated from the edge. For [90] 4 and [0/90] s , the crack initiation sites were observed to be close to the 90° fiber/ matrix interface, while in the case of [0/′45/90] s ; the crack initiated predominantly from the 45° fiber matrix interface. The fibers that were damaged during the fabrication process provided the crack initiation site for the [0] 4 orientation. These edge cracks enhanced the environmental access to the fiber/matrix interface that resulted in progressive damage of the interface and the fiber. Data plotted on a Larson-Miller Parameter (LMP) chart enabled a direct comparison of the creep resistance of all the layups tested. The creep resistance of the [0] 4 layup was significantly greater than that of the other layups. The creep rupture life of specimens with 0° fibers was governed by the failure of unidirectional fibers. An empirical model was developed to predict the creep rupture life of all layups containing 0° fibers based on the LMP approach assuming that only the 0° fibers carry the load for the majority of the life.