J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 used as loading pins to accommodate the inherent composite strength in bending without interlaminar roughness of the woven fibers on the specimen surface shear type failures. Strain was calculated based on and to reduce contact loading stress. No permanent measurements of the bottom beam displacement. In deformation of the loading rods was observed after addition, based on a technique used by Heathcote et testing, which indicates that they remained elastic under al., 22 notched bending tests were performed in a similar the stresses encountered in testing. A servo-electric test- manner. Two diferent bar specimens(3.5x7x90 mm ing machine (Instron, Inc. model 8562) with a high and 3. 5x7x45 mm nominal dimensions)were tested in stifness frame was used to load the specimens at a cross 3-point flexural load with an outer span of 35 mm and head speed of 0. 1 mm/ min 89 mm for the shorter and longer specimens respec The shear stress at the mid-plane of a flexural bar tively. The cross head displacement rate was 0. 1 mm/ pecimen can be calculated from beam theory as min. two different fiber with0°/90° and one with+/-45° fiber directions rela r=3/4L/(W*1) (2) tive to the direction of the bar lengtH Notches were cut in the center of bars with a diamond where L=load. w=width, and t= thickness The mea wheel with a resulting notch thickness of 0.55 mm. The surement of delamination stress for CMCs usually pre- notch depth was nominally one half of the sample scribes a span(S)to thickness ratio(S/ n)of greater than height, a/W=0.485+0.015. The high stifness of the 10 to help insure that the specimen fails by delamination testing machine/load cell, and the non-catastrophic fail- (shear)rather than a tensile failure on the surface, given ure of the specimens allowed for careful measurement of the energy required to break the notched specimens The measured projected surface area of the sample was 3/2PS (3) used to calculate energy per unit area to produce frac- ture The stress-strain response of the un-notched compo- The midplane shear stress to maximum tensile stress sites was nearly linear elastic with some significant ratio(t/o)is given by deviations in the +/-45 fiber direction tests. Due to the low loads encountered in testing specimens of this r/a=1/2(/S) (4) length, no damage was observed at the loading po The specimens were tested in the same high-stifiness Therefore, for a given specimen thickness, the shorter universal testing machine used for the interlaminar the span, the greater the probability that failure will shear strength tests take place by a delamination of the cloth layers, rather than crack extension through the layers. Flexural testing with small values of S/t is called short beam bend test esults ing and is used to characterize the interlaminar shear trength 3. 1. Interlaminar shear strength 2.3. In-plane flexure testing of notched and un-notched Fig. 3 reports the apparent interlaminar shear strength as a function of span to thickness ratio(S/n) for individual CMC specimens heat treated in HCI for dif- Attempts to perform tensile tests on 100 mm long ferent time periods and temperatures. As shown, the specimens(same material as above) with a reduced delamination stress was 10+2 MPa for all heat treat gauge section(5. 1 mm wide, 40 mm long, produced with ments, and that specimens produced from one heat a 152 mm diameter diamond grinding wheel) were not treatment(1250C/5 h) failed in tension and did not successful with our limited amount of material. Despite delaminate. Fig. 3 also reports the delamination stress the use of fiberglass tabs that were epoxied to the ends reported by Levi et al. for a CMC with a porous of the specimen and double knife-edge universal joints matrix, but fabricated by the older method(pressure within the tensile train, most specimens failed either in filtration, multiple precursor infiltration and pyrolysis the non-reduced gauge section or adjacent to the cycles ). Its delamination strength of 8 MPa is a little clamping grip lower than most of the values reported for our newer Because of the limited amount of fiber cloth available method but Levi et al. used harder, steel loading pins, for fabricating specimens, the implementation of an which could have produced a stress concentration and a improved tensile test was not possible. The testing mode lower delamination stress. was changed to an in-plane flexural test, subjected to 3- Figs. 4 and 5 illustrate typical stress versus strain plot point flexural loading as shown in Fig. 2(b). This con- for specimens that delaminated prior to tensile failure figuration and loading mode allowed for testing of the In general, one or two load drops were observed similarused as loading pins to accommodate the inherent roughness of the woven ®bers on the specimen surface and to reduce contact loading stress. No permanent deformation of the loading rods was observed after testing, which indicates that they remained elastic under the stresses encountered in testing. A servo-electric test￾ing machine (Instron, Inc. model 8562) with a high sti€ness frame was used to load the specimens at a cross head speed of 0.1 mm/min. The shear stress at the mid-plane of a ¯exural bar specimen can be calculated from beam theory as:  ˆ 3=4 L=…Wt†; …2† where L=load, w=width, and t=thickness. The mea￾surement of delamination stress for CMCs usually pre￾scribes a span (S) to thickness ratio (S/t) of greater than 10 to help insure that the specimen fails by delamination (shear) rather than a tensile failure on the surface, given by  ˆ 3=2PS …bt2† : …3† The midplane shear stress to maximum tensile stress ratio (t/s) is given by = ˆ 1=2…t=S†: …4† Therefore, for a given specimen thickness, the shorter the span, the greater the probability that failure will take place by a delamination of the cloth layers, rather than crack extension through the layers. Flexural testing with small values of S/t is called short beam bend test￾ing and is used to characterize the interlaminar shear strength. 2.3. In-plane ¯exure testing of notched and un-notched specimens Attempts to perform tensile tests on 100 mm long specimens (same material as above) with a reduced gauge section (5.1 mm wide, 40 mm long, produced with a 152 mm diameter diamond grinding wheel) were not successful with our limited amount of material. Despite the use of ®berglass tabs that were epoxied to the ends of the specimen and double knife-edge universal joints within the tensile train, most specimens failed either in the non-reduced gauge section or adjacent to the clamping grip. Because of the limited amount of ®ber cloth available for fabricating specimens, the implementation of an improved tensile test was not possible. The testing mode was changed to an in-plane ¯exural test, subjected to 3- point ¯exural loading as shown in Fig. 2(b). This con- ®guration and loading mode allowed for testing of the composite strength in bending without interlaminar shear type failures. Strain was calculated based on measurements of the bottom beam displacement. In addition, based on a technique used by Heathcote et al.,22 notched bending tests were performed in a similar manner. Two di€erent bar specimens (3.5790 mm and 3.5745 mm nominal dimensions) were tested in 3-point ¯exural load with an outer span of 35 mm and 89 mm for the shorter and longer specimens respec￾tively. The cross head displacement rate was 0.1 mm/ min. Two di€erent ®ber alignments were tested, one with 0/90 and one with +/ÿ45  ®ber directions rela￾tive to the direction of the bar length. Notches were cut in the center of bars with a diamond wheel with a resulting notch thickness of 0.55 mm. The notch depth was nominally one half of the sample height, a/W=0.485‹0.015. The high sti€ness of the testing machine/load cell, and the non-catastrophic fail￾ure of the specimens allowed for careful measurement of the energy required to break the notched specimens. The measured projected surface area of the sample was used to calculate energy per unit area to produce frac￾ture. The stress±strain response of the un-notched compo￾sites was nearly linear elastic with some signi®cant deviations in the +/ÿ45  ®ber direction tests. Due to the low loads encountered in testing specimens of this length, no damage was observed at the loading points. The specimens were tested in the same high-sti€ness universal testing machine used for the interlaminar shear strength tests. 3. Results 3.1. Interlaminar shear strength Fig. 3 reports the apparent interlaminar shear strength as a function of span to thickness ratio (S/t) for individual CMC specimens heat treated in HCI for dif￾ferent time periods and temperatures. As shown, the delamination stress was 10‹2 MPa for all heat treat￾ments, and that specimens produced from one heat treatment (1250C/5 h) failed in tension and did not delaminate. Fig. 3 also reports the delamination stress reported by Levi et al.5 for a CMC with a porous matrix, but fabricated by the older method (pressure ®ltration, multiple precursor in®ltration and pyrolysis cycles). Its delamination strength of 8 MPa is a little lower than most of the values reported for our newer method but Levi et al. used harder, steel loading pins, which could have produced a stress concentration and a lower delamination stress.5 Figs. 4 and 5 illustrate typical stress versus strain plot for specimens that delaminated prior to tensile failure. In general, one or two load drops were observed similar J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618 611