Advanced monolithic ceramics like ceramic matrix composites (CMC's) and continuous-fiber-reinforced ceramic matrix composites (CFCC's) are candidate materials for high-temperature structural applications in heat engines. However, the process of slow crack growth often limits the service life of structural monolithic ceramic components. Hence, it is important to develop an appropriate test methodology to determine accurately the slow crack growth parameters required for component life prediction. This methodology also should be useful in determining the influences of component processing and composition variables on the slow crack growth behavior of newly developed or existing materials, thereby allowing the component processing and composition to be tailored and optimized to specific needs.
In 1998, the authors initiated the development of a test method to determine the life prediction parameters of advanced structural monolithic ceramics at elevated temperatures. Conducted at the NASA Glenn Research Center, the work was done for the C28 Advanced Ceramics Committee of the American Society for Testing and Materials (ASTM). The draft standard written by the authors had gone through all the required balloting and was established in 2000 as a new ASTM test method, ASTM C1465-00, "Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Elevated Temperatures." This new standard is to be published in the year 2001 Annual Book of ASTM Standards, Vol. 15.01.
Briefly, the test method employs constant stress-rate (or dynamic fatigue) testing to determine strengths as a function of the applied stress rate at elevated temperatures. The merit of this test method lies in its simplicity: Strengths are measured in a routine manner at four or more applied stress rates by applying a constant displacement rate or constant load rates. The slow crack growth parameters necessary for life prediction are then determined from a simple relationship between the strength and the applied stress rate. Some of the limiting factors such as creep, material deterioration, and crack-tip morphological change that are often encountered at higher test temperatures have been also taken into account to minimize their effects on the determination of slow crack growth parameters.
Currently, this test method is being extended to various continuous-fiber-reinforced ceramic matrix composites in tension at elevated temperatures to see if the test method is applicable to those composite materials (refs. 1 and 2). It has been found that a good relationship between ultimate strength and applied test rate existed and that reasonable agreement was observed within the experimental range between constant stress-rate and constant stress testing. This is a promising result since life prediction parameters of even composite materials would be determined by this simple, fast test method, allowing one to achieve a significant test-time saving. Furthermore, this method would be very useful in screening composite materials in terms of slow crack growth or damage accumulation behavior within a limited time frame.
Glenn has maintained an active leadership role in the standardization of the life prediction testing of advanced monolithic ceramics within ASTM. The authors also wrote a companion ambient-temperature standard, ASTM C 1368-97, "Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics at Ambient Temperature."
Ohio Aerospace Institute contact: Dr. Sung R. Choi, 216-433-8366, Sung.R.Choi@grc.nasa.gov
Glenn contact: Dr. John P. Gyekenyesi, 216-433-3210, John.P.Gyekenyesi@grc.nasa.gov
Authors: Dr. Sung R. Choi and Dr. John P. Gyekenyesi
Headquarters program office: OAT
Programs/Projects: ZCET, HOTPC, UEET
Last updated June 6, 2001, by Nancy.L.Obryan@nasa.gov
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
