CERAMICS INTERNATIONAL ELSEVIER Ceramics International 33(2007)279-284 www.elsevier.com/locate/ceramint Damage assessment of oxide fibre reinforced oxide ceramic matrix composites using acoustic emission Figen Kaya Department of Metal and Materials Engineering, Faculry of Engineering, Zonguldak Karaelmas University, Zonguldak, Tirkey Received 21 June 2005: received in revised form 9 July 2005; accepted 4 November 2005 Available online 21 February 2006 Abstract The damage mechanisms of woven mullite fibre(Nextel 720)reinforced alumina ceramic matrix composites( CMCs)were monitored during tensile tests using acoustic emission(AE)technique. The effects of microstructural characteristics, such as porosity and pore size as well as the nature of interfacial zone between fibre and matrix on the type of damage were also examined and correlated with the microstructural observations. It is shown that based on the energy level of acoustic events, during the tensile tests, delamination of the fibres from the matrix takes place first then multiple matrix cracks form and finally reinforcement fibres break before the composite's final failure. It is also shown that porosity content of the mposite samples influences the acoustic emission response during the tensile loading, as both amplitude and the energy values are higher for the composites with low porosity level and smaller pore diameter C 2006 Elsevier Ltd and Techna Group S.r. l. All rights reserved Keywords: B. Porosity: Acoustic emission; Ceramic composites: Tensile test 1. Introduction 20 KHz and 1 MHz, is generated by the rapid release of energy from the source within a material [8]. The elastic wave Continuous fibre reinforced ceramic matrix composites propagates through the solid to the surface, where it can be MCs) are prime candidate materials for use in advanced recorded by one or more sensors. The sensor is a transducer that ructural and high temperature applications due to their converts the mechanical wave into an electrical signal. In this excellent thermal stability, good thermal shock resistance and way information about the existence and location of possible high fractur SS-4 The fracture toughness and faw resistance of monolithic investigation of local damage in fibre-reinforced composite ceramics can be increased by the introduction of high strength materials. When a fibre-reinforced ceramic composites continuous ceramic fibres resulting in toughening mechanisms, subjected to tensile loading, a variety of damage mechanism such as fibre-debonding, pull-out, fibre bridging and crack including fibre debonding, matrix cracking and fibre fracture deflection which all contribute to a non-linear stress-strain can take place. As the damage progresses within the composite response and thus high energy dissipation before fracture [5,6]. energy is released in different forms such as acoustic waves. In service, CMC components, such as gas turbine airfoils, heat The energy and or the amplitude of the acoustic waves depends exchanger and combustors are subjected to tensile loading and on type of fracture event which can be detected by monitorin therefore under these conditions the damage mechanisms the AE. Each event can be separated from each other b within the composite should be analysed non-destructively for analysing AE parameters, such as amplitude, duration, energy long-term structural reliability as well as structural design [7]. and time. Several work had been carried out by researchers to Acoustic emission(AE) is the class of phenomena whereby investigate the damage accumulation in the ceramic matrix an elastic wave, in the range of ultrasound usually between composites [9-11] The main objective of the present work is to determine the main damage mechanisms in woven mullite fibre-reinforced 903722574023 alumina ceramic matrix composites with weak interface between the fibre and matrix under tensile loading at room x1 2-8842/$3200C 2006 Elsevier Ltd and Techna Group S.r.L. All rights reserved 05.11016Damage assessment of oxide fibre reinforced oxide ceramic matrix composites using acoustic emission Figen Kaya * Department of Metallurgical and Materials Engineering, Faculty of Engineering, Zonguldak Karaelmas University, Zonguldak, Turkey Received 21 June 2005; received in revised form 9 July 2005; accepted 4 November 2005 Available online 21 February 2006 Abstract The damage mechanisms of woven mullite fibre (Nextel 720TM) reinforced alumina ceramic matrix composites (CMCs) were monitored during tensile tests using acoustic emission (AE) technique. The effects of microstructural characteristics, such as porosity and pore size as well as the nature of interfacial zone between fibre and matrix on the type of damage were also examined and correlated with the microstructural observations. It is shown that based on the energy level of acoustic events, during the tensile tests, delamination of the fibres from the matrix takes place first then multiple matrix cracks form and finally reinforcement fibres break before the composite’s final failure. It is also shown that porosity content of the composite samples influences the acoustic emission response during the tensile loading, as both amplitude and the energy values are higher for the composites with low porosity level and smaller pore diameter. # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Porosity; Acoustic emission; Ceramic composites; Tensile test 1. Introduction Continuous fibre reinforced ceramic matrix composites (CMCs) are prime candidate materials for use in advanced structural and high temperature applications due to their excellent thermal stability, good thermal shock resistance and high fracture toughness [1–4]. The fracture toughness and flaw resistance of monolithic ceramics can be increased by the introduction of high strength continuous ceramic fibres resulting in toughening mechanisms, such as fibre-debonding, pull-out, fibre bridging and crack deflection which all contribute to a non-linear stress–strain response and thus high energy dissipation before fracture [5,6]. In service, CMC components, such as gas turbine airfoils, heat exchanger and combustors are subjected to tensile loading and therefore under these conditions the damage mechanisms within the composite should be analysed non-destructively for long-term structural reliability as well as structural design [7]. Acoustic emission (AE) is the class of phenomena whereby an elastic wave, in the range of ultrasound usually between 20 KHz and 1 MHz, is generated by the rapid release of energy from the source within a material [8]. The elastic wave propagates through the solid to the surface, where it can be recorded by one or more sensors. The sensor is a transducer that converts the mechanical wave into an electrical signal. In this way information about the existence and location of possible sources is obtained. AE analysis can be used for the investigation of local damage in fibre-reinforced composite materials. When a fibre-reinforced ceramic composites is subjected to tensile loading, a variety of damage mechanism including fibre debonding, matrix cracking and fibre fracture can take place. As the damage progresses within the composite energy is released in different forms such as acoustic waves. The energy and or the amplitude of the acoustic waves depends on type of fracture event which can be detected by monitoring the AE. Each event can be separated from each other by analysing AE parameters, such as amplitude, duration, energy and time. Several work had been carried out by researchers to investigate the damage accumulation in the ceramic matrix composites [9–11]. The main objective of the present work is to determine the main damage mechanisms in woven mullite fibre-reinforced alumina ceramic matrix composites with weak interface between the fibre and matrix under tensile loading at room www.elsevier.com/locate/ceramint Ceramics International 33 (2007) 279–284 * Fax: +90 3722574023. E-mail address: figenkaya81@hotmail.com. 0272-8842/$32.00 # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2005.11.016