MIATERIALS MENE& ENGEERNG ELSEVIER Materials Science and Engineering A250(1998)194-208 The effect of fibre surface roughness on the mechanical behaviour of ceramic matrix composites J. Shi*.c. Kumar methods Group, Aero and Technology Products, Mechanical Engineering Centre, GEC-ALSTHOM, Cambridge Road, Whetstone, Leicester LE8 6LH. UK Abstract The interface between the matrix and the fibre plays an important role in controlling the strength and toughness of ceramic matrix composites. It has been found experimentally that depending on manufacturing process, the interface may show substantial surface roughness, which has been modelled analytically with certain degree of success. The analytical models, however, do not take into account the interface geometry. Instead, only the magnitude of the surface undulation is included. In this paper, a direct simulation of fibre-matrix interface roughness by the finite element method is performed on an axisymmetric unit cell with a fully debonded interface. The simulation is employed to account for the three dimensional stress state, surface roughness and interface friction, which are normally simplified or idealised in theoretical studies. The model gives the highly non-uniform interface shear and pressure, which have direct implications on the interface damage and composite behaviour. Under the approximation made in the model, the positive transverse strains does not show up in the simulation despite the fact that two different surface roughness are used. o 1998 Elsevier Science S.A. All rights reserved Keywords: Interface roughness; Simulation 1. Introduction experimental characterisation, macro-mechanics and micro-mechanics analysis. Although it is believed that a For aeroengines to achieve a thrust to weight ratio of comprehensive understanding of CMCs is only possible 15: I by the turn of this century, materials of high by combining all these approaches, the micro-mechan- specific stiffness and strength, which also retain these cs is important in interpreting experimental findings, properties in a highly corrosive, erosive and oxidiser building macro-mechanics models and guiding material operating environment, are required. Such materials are designs. Consequently a large number of micro-me- also needed to increase engine thermal efficiency and to chanics models have been put forward to appreciate the decrease CO and NOx emissions in view of the tighter fundamental deformation and failure mechanisms of nd tighter regulations on environmental impact. Ce CMCs. Some of these studies have been concentrated ramic matrix composites(CMCs) have potential for on the damage and energy dissipation mechanisms, almost all of these stringent requirements and thus hich have direct impact on strength and toughness stand out as strong candidates for future engine compo- The interface between fibre and matrix plays an nents. In consequence, CMCs are widely recognised as important role in controlling the toughness, ultimate key materials for the next generation of gas turbine tensile strength and fatigue life of CMCs. In conse- engines. quence, there has been extensive study on various prop. Before large scale application of these relatively new erties of the interface and their effect on composite materials is possible, a clear understanding of thei performance. Experimentally, a number of tests have thermal and mechanical properties is essential. This has been devised to determine the interface shear strength been approached from a number of angles in the past: and slide stress [l]. Among these, the single fibre push in/push-out test seems to be the most popular. Various Corresponding author. Tel +44 116 2750750 fax: +44 116 theoretical models exist in order to model the tests and 750768 to interpret the results e.g. Ref [2]. One interesting 0921-5093/98/S1900c 1998 Elsevier Science S.A. All rights reserved PIs0921-5093(98)00592-9Materials Science and Engineering A250 (1998) 194–208 The effect of fibre surface roughness on the mechanical behaviour of ceramic matrix composites J. Shi *, C. Kumar Methods Group, Aero and Technology Products, Mechanical Engineering Centre, GEC-ALSTHOM, Cambridge Road, Whetstone, Leicester LE8 6LH, UK Abstract The interface between the matrix and the fibre plays an important role in controlling the strength and toughness of ceramic matrix composites. It has been found experimentally that depending on manufacturing process, the interface may show substantial surface roughness, which has been modelled analytically with certain degree of success. The analytical models, however, do not take into account the interface geometry. Instead, only the magnitude of the surface undulation is included. In this paper, a direct simulation of fibre–matrix interface roughness by the finite element method is performed on an axisymmetric unit cell with a fully debonded interface. The simulation is employed to account for the three dimensional stress state, surface roughness and interface friction, which are normally simplified or idealised in theoretical studies. The model gives the highly non-uniform interface shear and pressure, which have direct implications on the interface damage and composite behaviour. Under the approximation made in the model, the positive transverse strains does not show up in the simulation despite the fact that two different surface roughness are used. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Interface roughness; Simulation 1. Introduction For aeroengines to achieve a thrust to weight ratio of 15:1 by the turn of this century, materials of high specific stiffness and strength, which also retain these properties in a highly corrosive, erosive and oxidising operating environment, are required. Such materials are also needed to increase engine thermal efficiency and to decrease CO and NOX emissions in view of the tighter and tighter regulations on environmental impact. Ce￾ramic matrix composites (CMCs) have potential for almost all of these stringent requirements and thus stand out as strong candidates for future engine compo￾nents. In consequence, CMCs are widely recognised as key materials for the next generation of gas turbine engines. Before large scale application of these relatively new materials is possible, a clear understanding of their thermal and mechanical properties is essential. This has been approached from a number of angles in the past: experimental characterisation, macro-mechanics and micro-mechanics analysis. Although it is believed that a comprehensive understanding of CMCs is only possible by combining all these approaches, the micro-mechan￾ics is important in interpreting experimental findings, building macro-mechanics models and guiding material designs. Consequently a large number of micro-me￾chanics models have been put forward to appreciate the fundamental deformation and failure mechanisms of CMCs. Some of these studies have been concentrated on the damage and energy dissipation mechanisms, which have direct impact on strength and toughness. The interface between fibre and matrix plays an important role in controlling the toughness, ultimate tensile strength and fatigue life of CMCs. In conse￾quence, there has been extensive study on various prop￾erties of the interface and their effect on composite performance. Experimentally, a number of tests have been devised to determine the interface shear strength and slide stress [1]. Among these, the single fibre push￾in/push-out test seems to be the most popular. Various theoretical models exist in order to model the tests and to interpret the results e.g. Ref [2]. One interesting * Corresponding author. Tel.: +44 116 2750750; fax: +44 116 2750768 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S09 21- 5093(98)0059 2 - 9