L. Zhang, V.D. Krstic/ Theoretical and Applied fracture Mechanics 24(1995)13-19 o3focge Average Graphite Loyer Thickness (um) Fig. 2. The sequence of fracture of SiC laminates(four layers thout notch). The first drop in load (plot 1)is due to Fig. 3. Variation of strength with average graphite layer hickness in laminated sic ormation of first interfacial crack between first and second yers(from bottom). The second drop (plot 2)in load is due to fracture of the second layer, etc. the reduction of interfacial strength and the ease with which the cracks are initiated in the individ fracture, the laminated material showed a slow ual layers. Fig. 5 shows the path of a crack decrease of load after reaching a maximum level, through the laminated specimen during four-point along with large increase in roller displacement bend testing. Clearly, the position of crack initia prior to fracture. Another important difference tion at the surface of each individual Sic layer between the two materials is the shape of the varied from layer to layer, indicating that the load-displacement plots in the linear-elastic re- strength of these layers controls the strength of gion. An abrupt drop in load, followed by an the entire sample ncrease in load before the load reach a maxi- The relationship between the four-point bend mum value, was observed in all samples possess- fracture toughness and Sic and graphite layer ing high apparent fracture toughness(see Fig. 1). thickness is shown in Figs 6 and 7. In the case of A series of tests was conducted such that at the thinner graphite layers(5 um) there is a continu- moment when the first sign of drop in load was ous drop in toughness with the increase of Sic noticed the load was quickly relaxed to zero and layer thickness, whereas for thicker graphite lay he samples were examined microscopically(Fig. ers(10 um)a maximum in toughness was reached 2). It was found that all these samples developed t approximately 300 to 350 um thick SiC layers cracks along the interface between the first and the next adjacent layer. The ease at which crack- ing of the interface occurs appears to be con- trolled by the initial thickness of the carbon lay., soto- ers. Furthermore it was observed that both the carbon and sic layer thickness strongly influence he fracture response of the sample. Figs. 3 and 4 show the change of fracture strength with initial carbon and SiC layer thickness A maximum four- point bend strength was 2 Graphite Layer achieved with graphite layer thicknesses between 3 and 5 um and with Sic layer thicknesses be. 8 tween 300 and 600 um. a sharp drop in strength with samples having graphite layer thicknesses SiC Layer Thickness (urm) above 3 to 5 um is believed to be associated with g. 4. Variation of strength with average SiC layer thicknessL. Zhang, V.D. Krstic / Theoretical and Applied Fracture Mechanics 24 (1995) 13-19 15 4O 52 ' /i~ /' 12+1 i / i cl ~L _~_~._~_ F ~, _~ ~q,', I 2 3 4 5 h, elotive Disp,acernent (XO 1 rc/r~ ) Fig. 2. The sequence of fracture of SiC laminates (four layers without notch). The first drop in load (plot 1) is due to formation of first interracial crack between first and second layers (from bottom). The second drop (plot 2) in load is due to fracture of the second layer, etc. fracture, the laminated material showed a slow decrease of load after reaching a maximum level, along with large increase in roller displacement prior to fracture. Another important difference between the two materials is the shape of the load-displacement plots in the linear-elastic re￾gion. An abrupt drop in load, followed by an increase in load before the load reach a maxi￾mum value, was observed in all samples possess￾ing high apparent fracture toughness (see Fig. 1). A series of tests was conducted such that at the moment when the first sign of drop in load was noticed, the load was quickly relaxed to zero and the samples were examined microscopically (Fig. 2). It was found that all these samples developed cracks along the interface between the first and the next adjacent layer. The ease at which crack￾ing of the interface occurs appears to be con￾trolled by the initial thickness of the carbon lay￾ers. Furthermore, it was observed that both the carbon and SiC layer thickness strongly influence the fracture response of the sample. Figs. 3 and 4 show the change of fracture strength with initial carbon and SiC layer thickness. A maximum four-point bend strength was achieved with graphite layer thicknesses between 3 and 5 Ixm and with SiC layer thicknesses be￾tween 300 and 600 p~m. A sharp drop in strength with samples having graphite layer thicknesses above 3 to 5 p,m is believed to be associated with z= E. F 2 D o L 550 46O 370 SiC Lay --0-- 500~,m 0 -- £-- 120/zm lOG I I I I 0 5 6 9 li2 lt5 Average Graphite Layer Thickness {/~m) Fig. 3. Variation of strength with average graphite layer thickness in laminated SiC. the reduction of interfacial strength and the ease with which the cracks are initiated in the individ￾ual layers. Fig. 5 shows the path of a crack through the laminated specimen during four-point bend testing. Clearly, the position of crack initia￾tion at the surface of each individual SiC layer varied from layer to layer, indicating that the strength of these layers controls the strength of the entire sample. The relationship between the four-point bend fracture toughness and SiC and graphite layer thickness is shown in Figs. 6 and 7. In the ease of thinner graphite layers (5 p,m) there is a continu￾ous drop in toughness with the increase of SiC layer thickness, whereas for thicker graphite lay￾ers (10 I~m) a maximum in toughness was reached at approximately 300 to 350 Ixm thick SiC layers. o [L £ ~2 7 [r o 550 1 • 4so ,5j-//~- 9- I r / // / 250 ~// I 150- 50 -- q ----+- 100 200 Graphite Layer --0-- 5~,m • -- 10/~m 300 400 500 600 700 SiC Layer Thickness (,u,m) Fig. 4. Variation of strength with average SiC layer thickness