Stress analysis in hydroxyapatite/poly-L -lactide composite biomaterials I. Bala c a, * , P.S. Uskokovi  c b , N. Ignjatovi  c c , R. Aleksi  c b , D. Uskokovi  c c a Department for Materials Mechanics, Faculty for Mechanical Engineering, 27 marta, 11000 Belgrade, Yugoslavia b Department for Materials Science, Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Yugoslavia c Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, 11000 Belgrade, Yugoslavia Received 5 July 2000; accepted 5 October 2000 Abstract A three-dimensional (3D) ®nite element (FE) analysis of the stress concentration factor (SCF) in biocomposite model cell has been performed. The model composite consisted of a hydroxyapatite hard particle (HAp) embedded in poly-L - lactide soft matrix (PLLA). Two cases were considered, namely the shape of the particle was held constant while the volume fraction of HAp was varied and the particle shape was changed whilst the volume fraction was constant. For block shaped embedded particles, it was found that the SCF decreases with an increase of the HAp particle volume fraction. It was also found that the shape of reinforcing particles had little eect on the mechanical behaviour of material. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Particulate composites have many engineering applications because they are easy to mould, they have low production costs and they have signi®- cantly improved stiness [1,2]. However, a draw- back is that fracture can occur in a particulate composite structure under service loading condi- tions or during its processing. Damage and cracks may occur in particles and the matrix and at the particle/matrix interface depending on the relative stiness and strength of the two constituent ma- terials and the interface strength. If the embedded particles are much stier and stronger than the matrix, which is the case in the present study, matrix cracking and particle/matrix interface debonding become the major damage modes. Recently, ceramic/polymer composites with hydroxyapatite (HAp) as ceramic and poly-L -lac- tide (PLLA) as the polymer phase have attracted great attention due to their ability to substitute the bone tissue [3±6]. Besides being biocompatible and nontoxic, this material exhibits unique osteoin- ductive properties. The methods used for their production so far were based on mixing of ce- ramics with monomers before polymerization [7]. Production of composites by dissolving PLLA prior to its mixing with HAp is the subject of re- cent studies [3,4]. Numerical and ®nite element methods (FEM) have been extensively used for evaluating the stress state inside composites reinforced with particulate inclusions [8]. At present, this level of re®nement for stress states in particulate composites is limited www.elsevier.com/locate/commatsci Computational Materials Science 20 (2001) 275±283 * Corresponding author. Fax: +381-11-185-263. E-mail address: uskok@itn.sanu.ac.yu (D. Uskokovi  c). 0927-0256/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 5 6 ( 0 0 ) 0 0 1 8 2 - 8