Adv C g Mater.vo.8.No.1,pp.55-76(1999 O VSP I9o Standards and codes for ceramic matrix composites MICHAEL G. JENKINS University of Washington, Department of Mechanical Engineering. Box 352600. Stevens Way. Seattle, 98/95-2600, Washington, USA Abstract--Ceramic matrix composites(CMCs) and, in particular, continuous fibre ceramic compos ites( CFCCs)are targeted for industrial, aerospace and other high-technology applications that require the high-temperature properties and the wear/corrosion resistance of advanced ceramics while provid ing inherent damage tolerance (i.e. increased"toughness')without the volume /surface area-dependent strengths of monolithic ceramics. To utilize CFCCs designers need reliable and comprehensive data bases(and the design codes that contain them). Generating reproducible information for these data bases requires standards. Presently, there are relatively few (compared to metals)national (e. g. ASTM. CEN, JIS etc. )or international standards(e. g. ISO)for testing CFCCs In this paper. the various stan- dards for CFCCs are reviewed and additional areas requiring normalization are discussed (e.g.me- chanical, thermal, electrical, electro-magnetic, optical, and biological testing ). "Design codes such as he AsME Boiler and Pressure Vessel Code discussed here, are widely accepted, general rules for the construction of components or systems (for performance, efticiency, usability, or manufacturability) with emphasis on safety. Wide-ranging codes incorporate figurative links between materials, general design, fabrication techniques, inspection, testing, certification, and finally quality control to insure that the code has been followed. Implicit in design codes are many of the standards for materials testing, characterization, and quality control Logical outcomes of design codes are data bases of ma- terial properties and performance qualified for inclusion in the code. As discussed in this paper, data ases(such as those contained in the Mil-Hdbk-17 CMC effort)may be in print, electronic or world- wide web-based formats and may include primary summary data (e. g. mean, standard deviation, and number of tests) along with secondary data (e. g. graphical information such as stress-strain curves) Keywords: Ceramic matrix composites: standards; design codes; data bases 1 INTRODUCTION Thermo-mechanical behaviour(and its subsequent characterization) of ceramic ma- trix composites(CMCs) along with the CMC subset, continuous fiber-reinforce ceramic-matrix composites(CFCCs) is currently the subject of extensive investiga- tion worldwide. In particular, determination of the properties and performance(me- chanical, thermal, thermo-mechanical, physical, environmental, etc. of CMCs and CFCCs is required for a number of reasons: (1)to provide basic characterization for
M.G. Jenkins purposes of materials development. quality control and comparative studies:(2)to provide a research tool for revealing the underlying mechanisms of thermal and me- chanical performance; and (3)to provide engineering performance-prediction data for engineering applications and components design [1]. As CFCC prototype and trial products begin to reach the marketplace, the paucity of standards (i.e. test meth ods, classification systems, unified terminology, and reference materials) for these materials and the lack of CfCC design codes and their related data bases are limit- ing factors in commercial diffusion and industrial acceptance [2] of these advanced Standards. The term 'standards has many implications. To the researcher and the technical community it may be fundamental test methodologies and units of measure. To the manufacturer or end-product user it may be materials pecifications and tests to meet requirements. Commercial standards equate to the rules and terms of information transfer among designers, manufacturers and product users [2 There are even fundamental differences between levels of standards company(internal use with only internal consensus): industry(trade/ project use with limited organizational consensus); government ( wide usage and varying levels of consensus); full-consensus(broadest usage and greatest consensus) At present, there are few-nationally or internationally- full-consensus stan- dards [3-5] for testing not only advanced ceramics but especially CFCCs. This limited ability to test on a common-denominator basis hampers further mater- al development [2]. Specific areas where standardization(or consensus)are re- quired include: test fixtures, test specimen geometries, specimen preparation, ma- hining procedures and allowable tolerances, test specimen alignment, optimal straining/stressing rates, metrology (temperature and strain), testing environment and identification of fracture and failure modes. These needs are particularly acute at elevated temperatures or in aggressive environments where test equipment and measurement techniques are often being developed simultaneously with the test material. Although considerable development may be required for standards for CMCs and CFCCs, rather than adopting entirely new or unconventional methods and techniques, test methods developed originally for the room temperature charac terization of polymer matrix composites(PMCs) are a good starting point to develop test methodologies for CMCs and CFCCs in particular [6] Design codes and data bases. The meaning of the term 'design code is not generally well understood. As used in the following discussion ' design codeis not a design manual (i.e. a'cookbook' design procedure resulting in a desired component or system). Instead, 'design codes' are widely-accepted but general rules for the construction of components or systems with emphasis on safety. A primary objective is the reasonably certain protection of life and property for a and inspectors are recognized, the safety of the design can never be compromised reasonably long safe-life of the design. Although needs of the users, manufacturer By not imposing specific rules for design, codes allow flexibility for introducing new designs as required for performance, efficiency, usability, or manufacturability
Standards and codes for CMC while still providing constraints for safety. Codes must be wide ranging, incorpo- rating figurative links between materials, general design( formulas, loads, allowable stress, permitted details ), fabrication techniques, inspection, testing, certification by stamping and data reports, and finally, quality control to insure that the code has been followed. Thus, implicit in design codes may be many of the standards previously discussed for materials testing, characterization, and quality control. In addition, unlike standards which provide no rules for compliance or accountability, codes require compliance through documentation, and certification accountability through inspection and quality control A logical outcome of design codes is the incorporation of data bases of material properties and performance qualified for inclusion in the codes. These data are'qualified because they have been attained through testing per the statistical requirements of the codes as well as per the standards indicated in the codes Qualified data bases often require a minimum numbers of tests for(I) a particular batch of material and (2)multiple batches of material. In addition, data bases may nclude primary summary data(e. g. mean, standard deviation, and numbers of tests) along with secondary data from the individual tests. Some data bases may contain only numerical information while others may include graphical information (e.g stress-strain curves, temperature profiles, or test specimen geometries). Data bases are increasingly in electronic form to speed data retrieval and many are even world wide web-based to provide instant access and frequent updatability Design codes and their data bases may be backed as legal requirements for implementing an engineering design (e.g. certification and compliance with the American Society of Mechanical Engineers(ASME) Boiler and Pressure Vessel Code is a legal requirement in 48 of the 50 United States). At present there are no national or international design codes allowing CFCCs in any type of design This situation may be hampering material utilization since designers cannot use a material directly in new designs, but instead must(1) show evidence that the material meets the requirements of the code and (2)obtain special permission to use the material in the code design. In addition, material development is impaired since without a demand for a new material. there is no incentive for further refinement This paper concentrates on standards and codes for CFCCS. First, current standards for CFCCs are briefly reviewed keying on important mechanical, thermal, and physical aspects of testing. Next, a similar brief review of current design codes and evolving data bases for CFCCs is presented. Finally, the summary and onclusion section recaps successes and lessons and indicates future directions for tandards and codes for CCCS 2. STANDARD Although the number of standards(compared to metals and metal alloys)for CFCCs is limited. this number increases with each year as this relatively new material sys tem matures (serious concerted research dates to the mid-1970s). Table I shows
M.G.Jenkins Table I Current full-consensus standard test methods for CFCCS tion title Compl ASTM Committee C28 on'Advanced Ceramics Approved C1275-95 Standard Test Method for Monotonic Tensile Strength Testing 1995 of Continuous Fiber- Reinforced Advanced Ceramics with Solid Rectangular Cross Sections at Ambient Temperatures Cl292-95 Standard Test Method for Shear Strength of Continuous Fiber- 1995 Reinforced Advanced Ceramics at Ambient Temperatures C1337-96 Standard Test Method for Creep and Creep Rupture of Continuous 1996 Fiber- Reinforced Advanced Ceramics under Tensile Loading C1341-96 Standard Test Method for Flexural Properties of Continuous Fiber-. 1996 Reinforced Advanced Ceramics C1358-97 Standard Test Method for Monotonic Compressive Strength Testing 1997 of Continuous Fiber-Reinforced Advanced Ceramics with Solid Rectangular Cross Sections at Ambient Temperatures C1359-97 Standard Test Method for Monotonic Tensile Strength Testing 1997 of Continuous Fiber-Reinforced Advanced Ceramics with Solid Rectangular Cross Sections at Elevated Temperatures C1360-97 Standard Practice for Constant-Amplitude, Axial, Tension-Tension 1997 Cyclic Fatigue of Continuous Fiber-Reinforced Advanced Ceramics at Ambient Temperatures D3379-75 Standard Test Method For Tensile Strength And Young's Modulus 1997 For High-Modulus Single-Filament Materials (C28 jurisdiction) CXXXX Standard Test Method for Tensile Hoop Strength of Continuous 1998(expected) Fiber-Reinforced Advanced Ceramic Tubular Specimens at Ambi ent Temperature CxxXX Standard Test Method for Interlaminar Shear Strength of Continu- 1998(expected ous Fiber-Reinforced Advanced Ceramics at Elevated Temperatures Standard Test Method for Transthickness Tensile Strength of Con- 1999(expected) tinuous Fiber-Reinforced Advanced Ceramics Ambient Tempera ture CxXXX Standard Test Method for Tensile Strength and Youngs Modulus 1999(expected) for High-Modulus Single Filament Advanced Ceramics Xxxx Standard Test Method for Asymmetric Four-Point Shear Strength of 1999(expected) Ceramic Joints in Continuous Fibre-Reinforced Advanced Ceram Committee TC184 'Advanced Technical Ceramics ENV 6.58-1 Tensile Strength of Continuous Fibre Reinforced Ceramic Compos- ENV 6.58-2 Compressive Strength of Continuous Fibre Reinforced Ceramic ENV 658-3 Flexural Strength of Continuous Fibre Reinforced Ceramic Com- ENV 6584 Shear Strength(compression) of Continuous Fibre Reinforced Ce ramic Composites
Standards and codes for CMCs Table 1. Continued V658-5 hear Strength (3-point) of Continuous Fibre Reinforced Ceramic Compos ENV658-6 Shear Strength(double punch)of Continuous Fibre Reinforced Ceramie Composites ENv 1159-1 Thermal Expansion of Continuous Fibre Reinforced Ceramic Composites ENv 1159-2 Thermal Diffusivity of Continuous Fibre Reinforced Ceramic Composites ENV 1159-3 Specific Heat of Continuous Fibre Reinforced Ceramic Composites ENV 1389 Density of Continuous Fibre Reinforced Ceramic Composites ENV 1007- Size Level of Fibres for Continuous Fibre Reinforced Ceramic Composites ENv 1007-2 Linear Mass of Fibres for Continuous Fibre Reinforced Ceramic Compos- ENV 1007-3 Filament Diameter of Fibres for Continuous Fibre Reinforced Ceramic ENV 1007-4 Filament Strength of Fibres for Continuous Fibre Reinforced Ceramic Composit ISO Committee TC206'Fine(Advanced Technical) Ceramics CD 15733 Test Method for Tensile Stress-strain Behaviour of Continuous Fibre-reinforced Composites at Room Temperature ASTM: American Society for Testing and Materials: CEN: Committee for European tion: ISO: International Organization for Standardization Draft standard in the ballot process current full-consensus standards for CFCCs( American Society for Testing and Ma- erials(ASTM) Committee for European Normalization(CEN), International Orga- nization for Standardization(ISO). Table 2 lists industry/ government standards for CFCCs(Enabling Propulsion Materials(EPM), Petroleum Energy Center(PEC) The following subsections provide brief overviews of some of the key issues in stan- dards for tension, compression, shear, flexure, constituents, physical, and other ar- eas. It is important to realize that although no component fabricated from CFCC will always be subjected to uniaxial, uniform stress states, unambiguous interpretation test results for characterization of the mechanical response of CFCCs requires well developed and understood stress fields such as uniaxial tension or compression In particular, CFCCs often show different responses in tension and compression(see Fig. I)which complicate analysis of results from the tests with non-uniform stresses uch as flexure [7, 8]. In addition, although many of the standards developed to date have been for ambient conditions. CFCCs are targeted for elevated temperatures in aggressive environments which compound already stringent testing demands
M. G elkins NOminal 15 mmgs ≤100 000-4000-20000 00040006000 LONGITUDINAL STRAIN E (HE) Figure 1. Differences in response of a CFCC in tension and compression [81 2 /. Tension Uniaxial tensile testing is the most fundamental means of measuring mechanical response of most engineering materials and interest in it for CFCCs is driven primarily by limitations of the flexure tests widely used to characterize monolithic ceramics. However, because of the difficulty of deconvoluting tensile response from the flexure results, a uniaxial tensile test is the preferred tensile test for the response of CFCCs in the plane of the reinforcement. This preference has led to a wide range of test specimen geometries(straight-sided to contoured) and gripping systems(face, pin or edge 'loaded')as illustrated in Fig. 2. Another issue which must be addressed for CFCCs is the material,s sensitivity to non-uniform stress (i.e bending during the tensile test)(see Fig 3)[9]. The mode(force, displacement, or strain control) and rate(rapid or slow) of testing as well as means of strain measurement(optical, contacting, etc. are important concerns as well, especially at elevated temperature at which creep or other time-dependent mechanisms may be operatIve. Because the uniaxial tension test is the most fundamental test for mechanical characterization, the greatest number of in-plane monotonic tensile test standards (see Tables I and 2)exist for it(two ASTM, three CEN(standards or advanced drafts), one ISO, one EPM, one PEC). Once developed, the monotonic tensile test also becomes the basis for other types of tests, such as cyclic fatigue or creep/creep rupture. Recent efforts in both ASTM and CEN are moving toward developing plane, elevated-temperature tensile tests for various environments other than mbient air. Concerns here are the type of grip(hot, warm, or cold)and the design of the test specimen so as to minimize the imposition of undesirable thermal and imposed stresses(see Fig 4)[8] More recently, demand has increased to determine the tensile response perpen- dicular to the in-plane reinforcements(see Fig. 5). In two-directionally reinforced CFCCs, this direction can be the weakest since only matrix material with some interphase and no reinforcing fibres are present. An AStM document on'trans-
Standards and codes for CMCs LEWE DPONT FACE LOADED GEOME TRIES (mm) 能是re PIN FACE LOADED GEOMETRES tmmi EDGE LOADED GEOME TRES mmi Figure 2. Examples of various tensile test specimen geometries 150 OF5 u苏-=5 58添 PERCENT BENDING PERCENT BENDING, a)Proportional limit stress b)Ultimate tensile strength Figure 3. Strength as a function of percent bending in room temperature monotonic tensile tests (a)Proportional limit stress and (b) Ultimate tensile strength [81
M. G. Jenkins Table 2 Current industry/ government standard test methods for CFCC Title EPM"(USA) Approve HSR/EPM-D-O01-93 Monotonic Tensile Testing of Ceramic Matrix, Intermetallic 1993 Matrix and Metal Matrix Composite Materials HSR/EPM-D-002-93 Tension-tension Load Controlled Fatigue Testing of Ceramic 199 Matrix, Intermetallic Matrix and Metal Matrix Composite Materials HSR/EPM-D-003-93 Four Point Flexure Testing of Ceramic Matrix, Intermetallic 1993 Matrix and Metal Matrix Composite Material HSR/EPM-D-004-93 Creep-Rupture and Stepped Creep Rupture of Ceramic Ma- 1993 Matrix Compo tIs HSR/EPM-TSS-001-93 Measurement of Test System Alignment Under Tensile Load- 1993 HSR/EPM-NDE-001-93 Measurement of the Bow and Warp of Continuous Fiber 1993 Reinforced Test Specimens Task A 54/A.5.5 CMC Pre-Cracking Standard PEC (Japan) A PEC-TS CMCOI uous Fibre Reinforced Ceramic Matrix Composites at Rom +pproved Test Method for Tensile Stress-Strain Behaviour of Contin- 1997 and Elevated Temperatures PEC-TS CMC04 Test Method for Flexural Strength of Continuous Fibre Re- 1997 inforced Ceramic Matrix Composites at Room and Elevated emperatures PEC-TS CMCO6 Test Method for Shear Strength of Continuous Fibre Rein- 1997 forced Ceramic Matrix Composites at Room and Elevated PEC-TS CMCO8 Test Method for Fracture Toughness of Continuous Fibre 1997 reinforced Ceramic Matrix Composites PEC.'TS CMCO9 Test Method for Fracture Energy of Continuous Fibre Rein- 1997 PEC-TS CMCO1O Test Method for Tensile-Tensile Cyclic Fatigue of Continu- 1997 ous Fibre Reinforced Ceramic Matrix Composites at Room and Elevated Temperatures PEC-TS CMCO Test Method for Tensile Creep of Continuous Fibre Rein- 1997 forced Ceramic Matrix Composites at Elevated Temperatures EC-TS CMCO13 Test Method for Elastic Modulus of Ceramic Matrix Com- 1997 posites at Room and Elevated Temperatures PEC-TS CMCO14 Test Method for Oxidation Resistance of Non- Oxide Ceramic 1997 Matrix Composites at Elevated Temperatures EPM Enabling Propolusion Materials program(NASA, GE. Pratt and Whitney consortium) USA). PEC=Petroleum Energy Center (Japan)
Standands and codes for CMCs Ambient Air.T=1260°c L=200 mm, t=W=6 mm 9S o70FEL Grip length a 25 m 0503 Warm Grips, Tarp=600C 0230 Cold Gnps.Ty。=20 Normalized distance from center of the specimen, y /(L2) Figure 4. Temperature distributions in a contoured tensile test specimen for various types of grip cooling arrangements [71 Front 欧平 Figure 5. In-plane and trans thickness directions for a 2D CFCC. thickness' tensile strength is currently undergoing the balloting process to address this concern 2. 2. Compre While it is accepted that elastic moduli for CFCCs measured from compressive tests are independent of the test method, compressive strength results depend on the mode of failure and hence, the compression test method employed. Regardless of the method used, compressive performance is the most difficult to evaluate because of test scatter greater than that of any other test. The major source of scatter is differences in failure modes [101. while the major experimental difficulties for compression tests are buckling instabilities and achieving good alignment to avoid load eccentricities that induce bending To avoid brooming and bending, robust tabs are usually bonded to the uniaxially loaded specimens. Buckling resistance can be increased if clamped end conditions are achieved and the bending stiffness near the ends is increased [101. Finite
M. G. Jenk element analyses( FEA)have been employed to optimize the loading configuration of compression test specimens which are based upon: preventing overall buckling preventing debonding of end tabs and preventing failure at the gage region-tab juncture while insuring stress uniformity in the central region [10]. Standardized compression tests for CFCCs have tended toward uniaxially-loaded, unsupported test specimens (i.e. no antibuckling guides) in anticipation of elevated temperature tests. Results(see Fig. 6)from these compression tests have been compared to tension and flexure test results to illustrate similarities(elastic modulus) and differences(strengths) between the different types of tests Another test available to determine the compressive performance of composites is the bending of sandwich beams. If the face in tension is deliberately made ger than the face in the compression side, a compressive failure will be induced. This test provides effective load transfer, ideal compressive strengths and data cor mparable to uniaxially loaded compression tests making the bending of composite sandwich beams highly recommended for design allowables. The major disadvantages for this test are that the test specimen is quite large and that the bonded core may artificially enhance the performance of the face in compression, although round-robin tests with graphite/epoxy composites have alleviated the latter concern [II]. Limited data are available in the open literature [8, 12]on compressive strength of CFCCs, although the standards currently in place(see Tables I and 2) should provide reliable and reproducible test results 2. 3. Shear The three primary testing modes are tension, compression and shear. There are three components of tensile and compressive stresses: axial, in-plane transverse and hrough the thickness transverse. Correspondingly, there are also three components of shear stresses: in-plane and two through-thickness components. Most current shear tests methods measure in-plane properties and only a few measure the interlaminar or through-thickness shear. Correspondingly, there are tests that only provide data on strength, or stiffness or both [11l Ta20C. AMBIENT AIR E=667X10 FLEXURE Figure 6. Bar chart comparisons of elastic modulus, ultimate strength, and proportional limit at 20 C for tension, compression and flexure for a 3D braided CFCC [71