Effect of hydrostatic pressure on the elastic work factor, h el and fracture toughness determination of unidirectional composites using h el K.Y. Rhee a, *, S.G. Lee a , J.H. Lee b , a Center for Media Transport System, School of Mechanical and Industrial System Engineering, Kyunghee University, Yongin 449-701, South Korea b Department of Polymer Science and Technology, Chonbuk National University, Cheonju 560-756, South Korea Received 2 April 2002; received in revised form 25 September 2002 Abstract Previously, fracture toughness of graphite/epoxy composites in the hydrostatic pressure environment was determined using the compliance method. However, it requires four to five test specimens of different crack lengths in order to determine fracture toughness. In this work, the elastic work factor approach was applied to determine fracture toughness of unidirectional (08) thick graphite/epoxy composites from a single fracture test in the hydrostatic pressure environment. First, the effect of hydrostatic pressure on the elastic work factor was studied. Second, the fracture toughness determined using the elastic work factor approach was compared with that determined from the compliance method at each hydrostatic pressure level. The results showed that the elastic work factor was not affected by hydrostatic pressure, and it decreased linearly with delamination length. The results also showed that the values of fracture toughness determined from both methods were comparable to each other at each hydrostatic pressure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fracture toughness; Hydrostatic pressure; Elastic work factor; Compliance method; Delamination 1. Introduction It is a well-known fact that the mechanical behavior of fiber-reinforced plastic composites in the hydrostatic pressure environment is different from that in air environment. Accordingly, extensive research has been carried out to investigate the effect of hydrostatic pressure on the tensile and compressive properties (elastic modulus, strength) of fiber-reinforced plastic composites [1  /6]. The effect of hydrostatic pressure on the fracture toughness of thick graphite/epoxy compo- sites was also investigated in recent years [7,8]. Presently, fracture toughness of fiber-reinforced composites is determined experimentally based on the compliance method. Although the compliance method is straight- forward, it requires at least four to five test specimens of different crack lengths to determine fracture toughness. Particularly, for the fracture test in the hydrostatic pressure environment, the method requires more efforts in determining fracture toughness because test samples should be prepared to prevent penetration of pressure medium. Therefore, it is required to develop a con- venient method to determine fracture toughness of fiber- reinforced composites in the hydrostatic pressure envir- onment with the smallest number of specimens. It was shown in the previous study that fracture toughness of fiber-reinforced composites could be determined from a single fracture test by using the work factor approach in an air environment [9]. In this work, elastic work factor was applied to determine fracture toughness of unidirectional (08) thick graphite/epoxy composites from a single fracture test in the hydrostatic pressure environment. For this purpose, the effect of hydrostatic pressure on the elastic work factor was studied. Also, the fracture toughness deter- mined using the elastic work factor was compared with * Corresponding author. Fax:  /82-31-202-6693. E-mail address: rheeky@khu.ac.kr (K.Y. Rhee). Materials Science and Engineering A349 (2003) 218  /223 www.elsevier.com/locate/msea 0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0921-5093(02)00810-9