David A. Stubbs

An ultrasonic sensor for high-temperature materials processing

A sensor has been developed and tested that is capable of emitting and receiving ultrasonic energy at temperatures exceeding 900°C and pressures above 150 MPa. The sensor works with standard ultrasonic pulser-receivers and has demonstrated the capability of measuring workpiece deformation during hot isostatic pressing. Details of the sensor design, performance, and coupling of the ultrasound to the workpiece are described. Ultrasonic data acquired by the sensor in-situ during hot-isostatic-pressing runs are presented.

Considerations for Using Phased Array Ultrasonics in a Fully Automated Inspection System

The University of Dayton Research Institute (UDRI) under contract by the US Air Force has designed and constructed a fully automated ultrasonic inspection system for the detection of embedded defects in rotating gas turbine engine components. The system performs automated inspections using the “scan plan” concept developed for the Air Force sponsored “Retirement For Cause” (RFC) automated eddy current system. Execution of the scan plan results in a fully automated inspection process producing engine component accept/reject decisions based on probability of detection (POD) information. Use of the phased‐array ultrasonic instrument and probes allows for optimization of both the sensitivity and resolution for each inspection through electronic beamforming, scanning, and focusing processes. However, issues such as alignment of the array probe, calibration of individual elements and overall beam response prior to the inspection have not been addressed for an automated system. This paper will discuss current pr...

High-temperature ultrasonic sensor for in-situ monitoring of hot isostatic processing

A sensor has been developed and tested that is capable of emitting and receiving ultrasonic energy at temperatures exceeding 900 degrees C and pressures above 150 MPa. The sensor is based on a unique form of aluminum nitride that retains tits piezoelectric properties at high temperatures. The sensor works with standard ultrasonic pulse-receivers and has demonstrated the capability of measuring workpiece deformation during hot isostatic pressing (HIP). Details of the sensor design, performance, and coupling of the ultrasound to the workpiece are described. Ultrasonic data acquired by the sensor, in situ, during HIP runs and at elevated temperatures in air are presented.

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.

Index Size Determination with Varying Coil Diameters for Improved Automated Eddy Current Inspections on Aerospace Engine Components

The effect of index size and coil size on eddy‐current inspection throughput rate, and resulting Probability of Detection (POD), are being examined using the USAF’s Eddy Current Inspection System (ECIS). Original estimates of potential maximum index sizes were made by applying a sampling and shifting algorithm to the experimental data. Better estimates of the new maximum index sizes were obtained by fitting Gaussian functions to the experimentally determined line‐spread functions.

Characterization of Damage Progression in Ceramic Matrix Composites Using an Integrated NDE/Mechanical Testing System

This paper discusses the use of an unique integrated Nondestructive Evaluation (NDE)/Mechanical test system to characterize the damage progression in a ceramic matrix composite (CMC) under tensile and fatigue loading conditions. During the tests, the effects of damage were characterized in situ using conventional extensometry, and ultrasonic surface and longitudinal wave transducers. The surface wave technique clearly identified the proportional limit of the composite, while the longitudinal wave technique captured the overall deformation behavior during the tension tests. Both ultrasonic NDE techniques showed equal or greater sensitivity to damage progression in the CMC compared to conventional extensometer-based modulus data.

Evaluation of Damage Evolution and Material Behavior in a Sigma/Ti-6242 Composite Using Nondestructive Methods

Correlations between damage, as it evolves under simulated service conditions, and the results produced from nondestructive evaluation (NDE) techniques are useful in establishing successful life prediction methodologies in metal-matrix composites. Traditional characterization techniques provide limited information on the failure mechanisms in metal-matrix composites because of the complexities caused by the inhomogeneous, anisotropic nature of these materials. In addition, the currently used destructive techniques yield only qualitative information on the internal damage of composites. Very little quantitative information exists correlating the internal damage with property changes in the material such as stiffness, elongation, and residual strength. This research effort correlated NDE results with the residual tensile strength of a six-ply, unidirectional BP Sigma-1240 SiC/Ti-6Al-2Sn-4Zr-2Mo composite after being isothermally fatigued. Baseline tension and fatigue curves were initially generated since minimal information on this particular metal-matrix composite was available in the literature. Information obtained from these tests was used to pinpoint load levels and interruption points for subsequent interrupted fatigue tests. The following nondestructive evaluation techniques were used to evaluate the test specimens before and after fatigue testing: (1) scanning acoustic microscopy, (2) oblique incidence shear wave scanning, (3) reflector plate ultrasonic scanning, (4) immersion surface wave scanning, (5) in situ surface and longitudinal waves and, (6) X-ray radiography. Following the interrupted fatigue tests, the composite specimens were nondestructively evaluated again prior to the residual tension tests to determine the residual strength. Scanning electron microscopy and metallography were used in the correlation and verification of fatigue damage. From these results, the immersion surface wave technique proved to be the most promising method for correlating damage with the residual tensile strength for this particular composite. This paper presents the results from each of the NDE techniques and examines the correlation among the techniques, other destructive methods, and the residual tensile strength.

An Update on Automatic Positioning, Inspection, and Signal Processing Techniques in the RFC/NDE Inspection System

This paper updates several techniques developed for the Retirement For Cause (RFC) Nondestructive Evaluation (NDE) Inspection System eddy current inspection module. Techniques to be discussed include: 1. Scallop Centering — development of an automatic scallop centering routine makes scallop inspections reliable. 2. Soft Survey Mode — improvements have been made for fast peak detection. 3. Method 2 Select Mode — a fine flaw detection technique based on the acquired waveform. 4. Antirotation Window Inspection — a frequency select mode has been established for detecting flaws in antirotation windows. 2. Scaling of Flaw Depth — a scaling factor has been developed, based on Phase I Reliability Test data, which converts flaw signal amplitude into estimated flaw depth.