M.G. Jenkins/Composites: Part A 30(1999)561-567 two-dimensionally reinforced(2-D)CFCCs it recommends As indicated, test standard C1275 became the template the use of specimens with contoured gauge sections. This for all other ASTM test standards for CFCCs involving document also addresses the need to test specimens having tensile testing. These standards, particularly those addres- dimensions(e.g, volume)that are consistent with the ulti- sing tests at elevated temperatures, have special provisions mate use of the tensile data. Fig. 2 shows examples of in addition to the requirements outlined in C1275. These recommended test specimen geometries in C1275 include, for example, issues such as heating methods and Applied force is transferred to the specimens through temperature measurements, and address the importance of gripping devices. Gripping devices can have either active mechanical testing rates, since many CFCCs may exhibit (e.g, hydraulically-actuated grips)or passive(e.g, edge- time-dependent deformation at elevated temperatures pin-loaded arrangements)interfaces, and the availability of all of the existing tensile-based astm standard test a given type of testing arrangement will dictate the type of methods for CFCCs address conducting tests in ambient pecimen geometry and vice versa. air, but given the importance of environmental effects on Gripping devices are typically attached to the test the mechanical behavior of CFCCs, ongoing work has been system through couplers, which can be classified either focused on the drafting of test methods to conduct tests in as fixed or non-fixed. However, regardless of the type of controlled simulated environments coupler used, C1275 mandates verification of the align- ment of the test system either prior to each test, or before 2. 2. Shear testing and after a series of tests. Analytical and empirical studies have concluded that, for negligible effects on the estimates Except for the torsion testing of thin-walled tubes, there is of the strength distribution parameters for monolithic cera- no singlegood test to measure the shear strength ofCFCCs mics,allowable percentage bending as defined in ASTM [9]. Because the preparation and testing of tubular speci- Practice E1012 should not exceed 5%[5, 6]. Although mens to measure the shear strength of CFCCs would be recent studies have revealed that the ultimate tensile prohibitively expensive, standard test method C1292 for strength of some CFCCs is relatively insensitive to perce Shear Strength of Continuous Fiber-Reinforced Advanced ge bending [51 2 in the absence of more complete studies Ceramics at Ambient Temperatures addresses two popular, but less than perfect, test methods for determining the for CFCCs C1275 adopted the same percentage bending shear strengths of uni-directionally and two-directionally In contrast to the apparent insensitivity of the ultimate specimen to determine interlaminar shear strength, and the tensile strength of CFCCs to percentage bending, the so- losipescu test to determine both in-plane and interlaminar called proportional stress limit, which is associated with shear strength. Schematics of the specimen geometries and the onset of matrix cracking, is very sensitive to percentage test configurations for these two tests are shown in Fig. 3 bending [7]. This is important because, in the absence of Since both of these tests have been widely used for the environmentally stable fibers and fiber coatings, it appears evaluation of polymer matrix composites, much of the that the proportional stress limit will be considered an uppe groundwork for the drafting of C1292 had already been imit for design stresses for many CFCCs. Therefore, to laid down accurately determine the proportional stress limit of Although both the compression of double-notched speci- hens and the losipescu test have the advantage of requiring FCCs, it is essential to meet the allowable bending strain relatively small specimens and being simple to conduct, requirements prescribed in C1275 Test standard C1275 also addresses different techniques their main disadvantage is that both rely on the stress concentration at the root of notches to initiate shear failure for strain measurements, such as optical methods using As a consequence, in the case of the compression of double- lasers and flags, or contact methods such as adhesively notched specimens, for example, the shear stress in the bonded strain gauges and extensometers. However, regard- less of the type of extensometer used, this shall satisfy Class gauge section( the region between notches)is not uniform, I requirements as outlined in Practice E83 [8]. Additional and furthermore the apparent interlaminar shear strength requirements for contact-type extensometers are that the depends on the distance between the notches [10]. Never- extensometer should not damage the specimen, and to be theless, these test methods provide conservative shear externally supported so that its weight does not introduce trength values bending strains greater than those allowed in C1275 2.3. Flexure testing One of the motivations for developing a standard test This apparently results from loading large aspect ratio fibers when these method to evaluate CFCCs in tension was attributed to the bridge matrices crack in the composite. discrepancies between, and the wide range of, strength 3 Actually, matrix microcracking occurs at stresses lower than the values reported when CFCCs were evaluated in flexure proportional stress limit, but environmental sensitivity, which results from ingress of the nment into the composite, is mostly associated The problem is that, when testing CFCCs in flexure, it is with this macroscopic stress. difficult to relate the forces and displacements measuredtwo-dimensionally reinforced (2-D) CFCCs it recommends the use of specimens with contoured gauge sections. This document also addresses the need to test specimens having dimensions (e.g., volume) that are consistent with the ulti￾mate use of the tensile data. Fig. 2 shows examples of recommended test specimen geometries in C1275. Applied force is transferred to the specimens through gripping devices. Gripping devices can have either active (e.g., hydraulically-actuated grips) or passive (e.g., edge- or pin-loaded arrangements) interfaces, and the availability of a given type of testing arrangement will dictate the type of specimen geometry and vice versa. Gripping devices are typically attached to the test system through couplers, which can be classified either as fixed or non-fixed. However, regardless of the type of coupler used, C1275 mandates verification of the align￾ment of the test system either prior to each test, or before and after a series of tests. Analytical and empirical studies have concluded that, for negligible effects on the estimates of the strength distribution parameters for monolithic cera￾mics, allowable percentage bending as defined in ASTM Practice E1012 should not exceed 5% [5,6]. Although recent studies have revealed that the ultimate tensile strength of some CFCCs is relatively insensitive to percen￾tage bending [5],2 in the absence of more complete studies for CFCCs C1275 adopted the same percentage bending requirements as for tensile testing of advanced monolithic ceramics. In contrast to the apparent insensitivity of the ultimate tensile strength of CFCCs to percentage bending, the so￾called proportional stress limit, which is associated with the onset of matrix cracking,3 is very sensitive to percentage bending [7]. This is important because, in the absence of environmentally stable fibers and fiber coatings, it appears that the proportional stress limit will be considered an upper limit for design stresses for many CFCCs. Therefore, to accurately determine the proportional stress limit of CFCCs, it is essential to meet the allowable bending strain requirements prescribed in C1275. Test standard C1275 also addresses different techniques for strain measurements, such as optical methods using lasers and flags, or contact methods such as adhesively bonded strain gauges and extensometers. However, regard￾less of the type of extensometer used, this shall satisfy Class B-1 requirements as outlined in Practice E83 [8]. Additional requirements for contact-type extensometers are that the extensometer should not damage the specimen, and to be externally supported so that its weight does not introduce bending strains greater than those allowed in C1275. As indicated, test standard C1275 became the template for all other ASTM test standards for CFCCs involving tensile testing. These standards, particularly those addres￾sing tests at elevated temperatures, have special provisions in addition to the requirements outlined in C1275. These include, for example, issues such as heating methods and temperature measurements, and address the importance of mechanical testing rates, since many CFCCs may exhibit time-dependent deformation at elevated temperatures. All of the existing tensile-based ASTM standard test methods for CFCCs address conducting tests in ambient air, but given the importance of environmental effects on the mechanical behavior of CFCCs, ongoing work has been focused on the drafting of test methods to conduct tests in controlled simulated environments. 2.2. Shear testing Except for the torsion testing of thin-walled tubes, there is no single ‘good’ test to measure the shear strength of CFCCs [9]. Because the preparation and testing of tubular speci￾mens to measure the shear strength of CFCCs would be prohibitively expensive, standard test method C1292 for Shear Strength of Continuous Fiber-Reinforced Advanced Ceramics at Ambient Temperatures addresses two popular, but less than perfect, test methods for determining the shear strengths of uni-directionally and two-directionally reinforced CFCCs: the compression of a double-notched specimen to determine interlaminar shear strength, and the Iosipescu test to determine both in-plane and interlaminar shear strength. Schematics of the specimen geometries and test configurations for these two tests are shown in Fig. 3. Since both of these tests have been widely used for the evaluation of polymer matrix composites, much of the groundwork for the drafting of C1292 had already been laid down. Although both the compression of double-notched speci￾mens and the Iosipescu test have the advantage of requiring relatively small specimens and being simple to conduct, their main disadvantage is that both rely on the stress concentration at the root of notches to initiate shear failure. As a consequence, in the case of the compression of double￾notched specimens, for example, the shear stress in the gauge section (the region between notches) is not uniform, and furthermore the apparent interlaminar shear strength depends on the distance between the notches [10]. Never￾theless, these test methods provide conservative shear strength values. 2.3. Flexure testing One of the motivations for developing a standard test method to evaluate CFCCs in tension was attributed to the discrepancies between, and the wide range of, ‘strength’ values reported when CFCCs were evaluated in flexure. The problem is that, when testing CFCCs in flexure, it is difficult to relate the forces and displacements measured E. Lara-Curzio, M.G. Jenkins / Composites: Part A 30 (1999) 561–567 563 2 This apparently results from loading large aspect ratio fibers when these bridge matrices crack in the composite. 3 Actually, matrix microcracking occurs at stresses lower than the proportional stress limit, but environmental sensitivity, which results from ingress of the environment into the composite, is mostly associated with this macroscopic stress