Dennis J. Buchanan

Determination of Crack Bridging Stresses from Crack Opening Displacement Profiles

This paper discusses the development of an optimization procedure to deduce the bridging stress from the crack opening displacements (COD) measured during fatigue crack growth. Finite element analysis was conducted using the center-cracked geometry to verify the optimization procedure. The proposed procedure successfully predicted the bridging stress distributions with zero stresses at the crack tip and the bridging stress distributions with non-zero stresses at the crack tip. The results also showed that COD measurements spaced at ≈ 0.8-1.0 mm are sufficient for reliable prediction of bridging stresses. Accurate prediction of bridging stresses near the crack tip required COD data within ≈ 0.1 mm from the crack tip. The application of the proposed procedure to a metal matrix composite (SCS-6/TIMETAL®21S) is also discussed.

Relaxation of Shot-Peened Residual Stresses Under Creep Loading

Abstract : Shot peening is a commonly used surface treatment process that imparts compressive residual stresses into the surface of metal components. Compressive residual stresses retard initiation and growth of fatigue cracks. During the component loading history, loading, or during elevated temperature static loading, such as thermal exposure and creep. In these instances, taking full credit for compressive residual stresses would result in a methodical approach for characterizing and modeling residual stress relaxation under elevated temperature loading, near and above the monotonic yield strength of IN100. The model incorporates the dominant creep deformation mechanism, coupling between the creep and plasticity models, and effects of prior plastic strain. The initial room temperature residual stress and plastic strain profiles provide the initial conditions for relaxation predictions using the coupled creep-plasticity model. Model predictions correlate well with experimental results on shot-peened dogbone specimens subject to single cycle and creep loading conditions at elevated temperature.

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.

Retained residual stress profiles in a laser shock‐peened and shot‐peened nickel base superalloy subject to thermal exposure

Purpose – The purpose of this paper is to analyze the residual stress relaxation in laser shock‐peened and shot‐peened IN100 subject to thermal exposure.Design/methodology/approach – Shot peening (SP) is a commonly used surface treatment that imparts compressive residual stress into the surface of components. The shallow depth of compressive residual stresses, and the extensive plastic deformation associated with SP, has been overcome by modern approaches such as laser shock peening (LSP). LSP surface treatment produces compressive residual stress magnitudes that are similar to SP that extend four to five times deeper, and with less plastic deformation. Retention of compressive surface residual stresses is necessary to retard initiation and growth of fatigue cracks under elevated temperature loading conditions.Findings – Results indicated that the LSP processing retains a higher percentage of the initial residual stress profile over that of SP.Originality/value – The retained residual stresses after therm...

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.

Test System for Elevated Temperature Characterization of Thin Metallic Sheets

Thin gage Ni-based superalloy materials are being targeted for hypersonic applications up to 1100 °C. To achieve an optimized system design, standard mechanical behavior data on these materials are needed under a range of loading conditions such as tensile, creep, and fatigue at representative temperatures. These tests require direct measurements of displacements on specimens. In order to meet this need, a test system has been developed as part of a comprehensive in-house program to advance state-of-the-art testing capabilities for thin foils and very thin sheets. The test system was developed using a conventional hydraulic load frame outfitted with specialized capabilities, and was designed for determining material properties on a macro-scale. Specimen thicknesses used in this study range from 127-508 μm, with specimen lengths on the order of 150 mm. This paper outlines the developmental process, including unique challenges, along with the system validation and some preliminary data.

Prediction of transverse fatigue behavior of unidirectionally reinforced metal matrix composites

Unidirectionally reinforced metal matrix composites (MMC) are targeted for use in many aerospace applications which require high specific strength and stiffness at elevated temperatures. Such applications include blings and disks. The primary weakness of a component made of unidirectionally reinforced MMC is its susceptibility to transverse loads. The strength of the component in the transverse direction is significantly lower than that in the longitudinal direction under monotonic, sustained and fatigue loading conditions. Hence, replacement of monolithic components with MMC components requires that the transverse strength of the MMC should be predicted accurately. This paper discusses the applicability of a net-section based model to predict the fatigue behavior of [909] MMC under transverse loading.

Ultrasonic longitudinal and surface wave methods for in situ monitoring of damage in metal matrix and ceramic matrix composites

This paper discusses the development of a unique set of integrated Nondestructive Evaluation (NDE)/Mechanical test techniques for characterization of ceramic matrix composites (CMC) and metal matrix composites (MMC). These techniques are sensitive to the initiation and accumulation of damage in CMC and MMC. These techniques use longitudinal (bulk) wave and surface wave ultrasonic energy to interrogate the composite specimens, in situ, during mechanical testing. Changes in surface and longitudinal wave characteristics are compared with stiffness changes for extensometer measurements during tension, creep and fatigue testing of SiC/BMAS (barium-magnesium alumino-silicate) CMC. Surface waves were used to identify the onset of surface and near-surface cracks in CMC. Changes in the surface wave characteristics provide a better indicator of impending failure during fatigue testing of Sigma/Ti-6242 MMC than extensometer data. Longitudinal wave propagation was successfully used to monitor the changes in the stiffness of the composite during fatigue loading conditions of CMC and MMC. The changes in the peak-to-peak amplitude of the transmitted wave signal appears to be an equal or better indicator of impending failure of the composite than the extensometer data. 21 refs., 10 figs.

Constitutive Model Calibration via Autonomous Multiaxial Experimentation

Modern plasticity models contain numerous parameters that can be difficult and time consuming to fit using current methods. Additional experiments are seldom conducted to validate the model for experimental conditions outside those used in the fitting procedure. To increase the accuracy and validity of these advanced constitutive models, software and testing methodology have been developed to seamlessly integrate experimentation, parameter identification, and model validation in real-time over a range of multiaxial stress conditions, using an axial/torsional test machine. Experimental data is reduced and finite element simulations are conducted in parallel with the test based on experimental strain conditions. Optimization methods reconcile the experiment and simulation through changes to the plasticity model parameters. Excursions into less-traveled portions of the multiaxial stress space can be predicted, and then executed experimentally, to identify deficiencies in the model. Most notably, the software can autonomously redirect the experiment to increase the robustness of the plasticity model where further deficiencies are identified, thus providing closed loop control of the experiment. This novel process yields a calibrated plasticity model upon test completion that has been fit and more importantly validated, and can be used directly in finite element simulations of more complex geometries.

STUDYING THE EFFECT OF RESIDUAL STRESSES ON FATIGUE CRACKS THROUGH FULL-FIELD METHODS (PREPRINT)

In this effort, a corner crack was grown from a notch in a nickel-based superalloy specimen with a shot-peened surface treatment to induce residual stresses. The crack length was less than 200 microns, and the full displacement field near the crack was determined using advanced digital image correlation. The specimen was then annealed at elevated temperatures to reduce or eliminate the residual stresses, and the full displacement field near the crack was again determined. The displacements after annealing were indeed significantly larger than those previous to annealing, demonstrating the reduction in residual stresses. Modeling has been done to determine the approximate level of residual stresses induced and then reduced through annealing. Through this method, the effect of residual stresses on short fatigue cracks can be directly studied. Further work will be discussed on the effect of temperature on residual stresses, an area of great concern in predicting fatigue lives of components. The degree to which these residual stresses are reduced under service conditions is not well understood, and the approach described here is expected to be extremely useful in determining and predicting this residual-stress reduction, leading to greatly enhanced life prediction where residual stresses are involved.