Ali Abdul-Aziz

Modeling of thermal residual stress in environmental barrier coated fiber reinforced ceramic matrix composites

For SiC/SiC composites to replace metallic materials in future turbine engines, prime reliant environmental barrier coatings (EBCs) are required. However, due to the mismatch in thermal expansion and elastic modulus between the substrate and the coating, thermal residual stresses are generated in the coating after processing as well as during exposure to turbine engine operating conditions. The nature and magnitude of the thermal stresses will have a profound effect on the durability and reliability of the EBC. To estimate the magnitude of in-plane (x- and y-directions) and through-the-thickness (z-direction) thermal residual stresses in the coating, a finite element model (FEM) was developed. Using FEM, the residual stresses were predicted for three multilayered EBC systems considered for the SiC/SiC composites: (1) barium strontium aluminum silicate, (2) ytterbium disilicate, and (3) ytterbium monosilicate. Influence of thickness and modulus of the coating layer on the thermal residual stress were modeled. Results indicate that thermal residual stresses in the SiC/SiC composite substrate are compressive and in all the three coatings tensile. Further examination indicates that in the z-direction, tensile stresses in all three systems are negligible, but in-plane tensile stresses can be significant depending on the composition of the constituent layer and the distance from the substrate. Comparison of predicted thermal residual stresses in the three systems shows that the ytterbium monosilicate system has the highest stress (~395 MPa), while the other two systems averaged about 80 MPa in one of the coating layers. A parametric analysis conducted indicates that lowering the modulus of the coating can lower the thermal residual stresses.

NDE using sensor based approach to propulsion health monitoring of a turbine engine disk

Rotor health monitoring and on-line damage detection have been increasingly gaining interest to manufacturers of aircraft engines, primarily to increase safety of operation and lower the high maintenance costs. But health monitoring in the presence of scatter in the loading conditions, crack size, disk geometry, and material property is rather challenging. However, detection factors that cause fractures and hidden internal cracks can be implemented via noninvasive types of health monitoring and or nondestructive evaluation techniques. These evaluations go further to inspect materials discontinuities and other anomalies that have grown to become critical defects that can lead to failure. To address the bulk of these concerning issues and understand the technical aspects leading to these outcomes, a combined analytical and experimental study is being thought. Results produced from the experiments such as blade tip displacement and other data collected from tests conducted at the NASA Glenn Research Centers Rotordynamics Laboratory, a high precision spin rig, are evaluated, discussed and compared with data predicted from finite element analysis simulating the engine rotor disk spinning at various rotational speeds. Further computations using the progressive failure analysis (PFA) approach with GENOA code [6] to additionally assess the structural response, damage initiation, propagation, and failure criterion are also performed. This study presents an inclusive evaluation of an on-line health monitoring of a rotating disk and an examination for the capability of the in-house spin system in support of ongoing research under the NASA Integrated Vehicle Health Management (IVHM) program.

Challenges in Integrating Nondestructive Evaluation and Finite Element Methods for Realistic Structural Analysis

Capabilities and expertise related to the development of links between nondestructive evaluation (NDE) and finite element analysis (FEA) at Glenn Research Center (GRC) are demonstrated. Current tools to analyze data produced by computed tomography (CT) scans are exercised to help assess the damage state in high temperature structural composite materials. A utility translator was written to convert velocity (an image processing software) STL data file to a suitable CAD-FEA type file. Finite element analyses are carried out with MARC, a commercial nonlinear finite element code, and the analytical results are discussed. Modeling was established by building MSC/Patran (a pre and post processing finite element package) generated model and comparing it to a model generated by Velocity2 in conjunction with MSC/Patran Graphics. Modeling issues and results are discussed in this paper. The entire process that outlines the tie between the data extracted via NDE and the finite element modeling and analysis is fully described.

Finite element design study of a bladed flat rotating disk to simulate cracking in a typical turbine disk: Part II

Developing health management and ultrasafe engine technologies are the primary goals of NASAs Aviation Safety Program. Besides improving safety, health monitoring can also reduce maintenance costs. A unique disk spin simulation system was assembled by the Nondestructive Evaluation (NDE) Group at NASA Glenn Research Center to verify and study a crack detection technique based upon observing center of mass changes of the rotor system using various sensing technologies. This paper describes the finite element analysis results of low cost, a 25.4 cm (10 in.) diameter, flat turbine disk used to evaluate the detection techniques by simulating typical cracks observed in turbine engine disks. Changes in radial tip displacement and center of mass are presented as a function of speed, crack size and location.

A Microwave Blade Tip Clearance Sensor for Propulsion Health Monitoring

The NASA Glenn Research Center has investigated a microwave blade tip clearance system for the structural health monitoring of gas turbine engines. This presentation describes the sensors and the experiments that have been conducted to evaluate their performance along with future plans for their use on an engine ground test.

Crack-Detection Experiments on Simulated Turbine Engine Disks in NASA Glenn Research Center's Rotordynamics Laboratory

The development of new health-monitoring techniques requires the use of theoretical and experimental tools to allow new concepts to be demonstrated and validated prior to use on more complicated and expensive engine hardware. In order to meet this need, significant upgrades were made to NASA Glenn Research Center’s Rotordynamics Laboratory and a series of tests were conducted on simulated turbine engine disks as a means of demonstrating potential crack-detection techniques. The Rotordynamics Laboratory consists of a highprecision spin rig that can rotate subscale engine disks at speeds up to 12 000 rpm. The crack-detection experiment involved introducing a notch on a subscale engine disk and measuring its vibration response using externally mounted blade-tip-clearance sensors as the disk was operated at speeds up to 12 000 rpm. Testing was accomplished on both a clean baseline disk and a disk with an artificial crack: a 50.8-mm- (2-in.-) long introduced notch. The disk’s vibration responses were compared and evaluated against theoretical models to investigate their applicability to and success of detecting cracks. This paper presents the capabilities of the Rotordynamics Laboratory, the baseline theory and experimental setup for the crack-detection experiments, and the associated results from the latest test campaign. I. Introduction HE development of fault-detection techniques for the in situ health monitoring of gas turbine engines is of high interest to NASA’s Aviation Safety Program (AVSP). The rotating components of modern gas turbine engines operate in severe environmental conditions and are exposed to high thermal and mechanical loads. The cumulative effects of these loads over time lead to high stresses, structural deformity, and eventual component failure. Current risk-mitigation practices involve periodic inspections and schedule-based maintenance of engine components to ensure their integrity over the lifetime of the engine. However, these methods have their limitations, and failures are experienced leading to unscheduled maintenance and unplanned engine shutdowns. To prevent these failures and enhance aviation safety, the NASA Integrated Vehicle Health Management (IVHM) Project, as part of the overall AVSP, is investigating new types of sensor technologies and methods for the in situ structural health monitoring and detection of flaws in gas turbine engines. The successful development and implementation of such technology and health-monitoring techniques requires the use of both theoretical and experimental tools to allow new concepts to be investigated and demonstrated prior to use on more complicated and expensive hardware. In order to meet this need, research has been conducted at the NASA Glenn Research Center to develop both global and local approaches for monitoring critical rotor components. 1-6

Nondestructive evaluation of ceramic matrix composites coupled with finite element analyses

Ceramic matrix composites (CMC) are engineered materials filled with manufacturing anomalies, such as voids, delamination, or fiber cracking. In this paper a non- destructive evaluation (NDE) of a CMC tensile specimen is coupled with a finite element analysis to locate the failure location prior to the actual testing. The tensile CMC specimen is scanned with computed tomography (CT) along various planes. The majority of the observed anomalies are porosities in the matrix. The CT images are then used to reconstruct a 3-D volume of the specimens gage section using velocity2 (an image processing software). Subsequently, a three dimensional finite element analysis (FEA) is carried out to include the scanned porosities. The stress variations along the scanned CT planes are determined, comparison of the FEA results with those extracted via NDS, and the test data are reported.

New Sensors and Techniques for the Structural Health Monitoring of Propulsion Systems

The ability to monitor the structural health of the rotating components, especially in the hot sections of turbine engines, is of major interest to aero community in improving engine safety and reliability. The use of instrumentation for these applications remains very challenging. It requires sensors and techniques that are highly accurate, are able to operate in a high temperature environment, and can detect minute changes and hidden flaws before catastrophic events occur. The National Aeronautics and Space Administration (NASA), through the Aviation Safety Program (AVSP), has taken a lead role in the development of new sensor technologies and techniques for the in situ structural health monitoring of gas turbine engines. This paper presents a summary of key results and findings obtained from three different structural health monitoring approaches that have been investigated. This includes evaluating the performance of a novel microwave blade tip clearance sensor; a vibration based crack detection technique using an externally mounted capacitive blade tip clearance sensor; and lastly the results of using data driven anomaly detection algorithms for detecting cracks in a rotating disk.

Rotor health monitoring combining spin tests and data-driven anomaly detection methods

Health monitoring is highly dependent on sensor systems that are capable of performing in various engine environmental conditions and able to transmit a signal upon a predetermined crack length, while acting in a neutral form upon the overall performance of the engine system. Efforts are under way at NASA Glenn Research Center through support of the Intelligent Vehicle Health Management Project (IVHM) to develop and implement such sensor technology for a wide variety of applications. These efforts are focused on developing high temperature, wireless, low cost, and durable products. In an effort to address technical issues concerning health monitoring, this article considers data collected from an experimental study using high frequency capacitive sensor technology to capture blade tip clearance and tip timing measurements in a rotating turbine engine-like-disk to detect the disk faults and assess its structural integrity. The experimental results composed at a range of rotational speeds from tests conducted at the NASA Glenn Research Center’s Rotordynamics Laboratory are evaluated and integrated into multiple data-driven anomaly detection techniques to identify faults and anomalies in the disk. In summary, this study presents a select evaluation of online health monitoring of a rotating disk using high caliber capacitive sensors and demonstrates the capability of the in-house spin system.

Development of a Flaw Detection/Health Monitoring Scheme for Turbine Engine Rotating Components

The hot section components of jet engines undergo severe environmental operating conditions where combined thermal and mechanical loading is the dominant factor in affecting their durability and performance. Minimizing the impact of these loads and inventing a flaw detection technology to monitor the health of these components is of utmost importance to the NASA Aviation Safety Program (AVSP). Work is underway through the AVSP’s Integrated Vehicle Health Management (IVHM) Project to conduct research on ways to improve the safety, structural durability of critical hot engine section components, reduce cost, and improve performance in every aircraft class. Therefore, it is NASA’s goal to develop a robust health monitoring technique via utilizing means of in-situ and wireless detection technology by expanding on sensor systems that are capable of functioning in severe environments, transmitting a signal upon detecting a predetermined crack length, and acting in an impartial fashion with respect to the overall performance of the engine system. Development and implementation of such sensor technology and diagnostic capabilities is possible only by conducting combined analytical and experimental studies (coupon and sub-scale levels) to determine their applicability and success. As a result, ongoing research at NASA GRC has aimed at investigating both global and local approaches for monitoring critical rotor components 1-4 . This paper is focused on presenting current ongoing research activities concerning health monitoringflaw detection systems of turbine engine rotating components and their relevance on meeting the IVHM program goals and millstones. Test data obtained under various operating conditions of a rotor disk with and without an artificially induced notch rotated at a rotational speed up to 12000 Rpm are presented, discussed and evaluated for health monitoring applications. Disk flaw observations and related assessments from the collected data are reported. Lastly, parallel analytical results of disk spinning at a range of rotational speeds showing the disk modal shapes and notch influence on damage initiation and crack propagation are also included.