Effects of Loading Rate and Thickness on Mixed-Mode I/II Fracture Toughness of Thermoset Epoxy Resin C. Kanchanomai, S. Rattananon Department of Mechanical Engineering, Faculty of Engineering, Thammasat University, Pathumthani 12120, Thailand Received 18 April 2007; accepted 18 February 2008 DOI 10.1002/app.28330 Published online 7 May 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The influences of loading rate and thick- ness on fracture behavior and mechanism of thermoset epoxy resin with polyamine hardener under mixed-mode (mode I/II) loading have been studied at low thickness and low loading rate (LTLL), as well as high thickness and high loading rate (HTHL). Under the variation of mixed-mode loading from mode I to mode II, fracture toughness of HTHL specimens were under plane-strain condition. For LTLL specimens, the fracture toughness at dominated mode I loading was under plane-stress condi- tion, whereas those at dominated mode II loading were under plane-strain condition. The stretched zone due to the principal stress in the normal direction to the crack plane as well as shear lips due to the Poisson contraction in the thickness direction were the main characteristic of the fracture surface of LTLL specimen tested at pure mode I loading. On the other hand, the mirror-like frac- ture surface was observed for the HTHL specimen tested at pure mode I loading. Under pure mode II loading, the aligned stretched zone due to the maximum shear stress was the main characteristic of the fracture surface of LTLL specimen, whereas irregular appearance of the stretched zone was observed for the HTHL spec- imen. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 2408–2416, 2008 Key words: fracture; mechanical properties; thermosets; toughness INTRODUCTION Epoxy resins generally have low shrinkage after cur- ing, low moisture absorption, and wide range of operating temperature (225 to 1508C). Moreover, the large number of compounds can react with the ep- oxy ring to form resin systems with a very wide range of properties, 1–3 therefore it has been used as a matrix in various polymer-matrix composites. Dur- ing services, the engineering polymers fail to per- form their structural function if they have excessive deformation or fracture. Without any significant dis- continuities within a part, the deformation distrib- utes uniformly on the load bearing area, and the part is likely to fail by excessive deformation. On the other hand, localized plastic deformation could occur around the discontinuities of a part. If the crit- ical condition is reached, the part is likely to fail by fracture. These discontinuities could be the defects during production, cracks during service, or complex geometry of product. Based on the concept of fracture mechanics, every linear elastic material has a fracture toughness called the critical stress intensity factor (K c ). Fracture of a part will occur if the stress intensity factor (K) equals or exceeds the K c of the material. The K is a fracture mechanics parameter around crack tip, i.e., a func- tion of crack size, geometry, and applied load. While the K c is a property of material and depends on vari- ous variables, e.g., loading rate, geometry, loading mode, temperature, environment, and stress system. During service, there are three types of loading mode that a crack can experience. Mode I loading is the principal load that applied normal to the crack plane, and can open the crack. Mode II loading is an in-plane shear loading that can slide one crack sur- face with respect to the other. Lastly, mode III load- ing is an out-of-plane shear loading that can tear one crack surface with respect to the other. The fracture toughness varies with the loading mode, i.e., K IIc and K IIIc are generally greater than K Ic . 4 The fracture toughness also depends on the stress system. Under mode I loading, the plane-strain dominated condi- tion around crack tip could occur for a thick part, which results in poor fracture resistance or low frac- ture toughness. While the plane-stress dominated condition around crack tip could occur for a thin part, which results in high fracture toughness. 5 The fracture toughness of modified diglycidyl ethers bisphenol-A resin with the modified aliphatic Correspondence to: C. Kanchanomai (kchao@engr.tu.ac.th). Contract grant sponsor: Thailand Research Fund, Nati- onal Research Council of Thailand, and Commission on Higher Education of Thailand. Journal of Applied Polymer Science, Vol. 109, 2408–2416 (2008) V V C 2008 Wiley Periodicals, Inc.