s.m tahami - Composite Department, Malek Ashtar University of Technology, Tehran, Iran.
m farhadinia - Composite Department, Malek Ashtar University of Technology, Tehran, Iran.
y aljasem - Composite Department, Malek Ashtar University of Technology, Tehran, Iran.
j Eskandari Jam - Composite Department, Malek Ashtar University of Technology, Tehran, Iran.

Fiber reinforced composite materials are well known for their high stiffness to weight and strength to weight ratios. Low interlaminar strength of composite laminates is one of majordisadvantages, which delayed the widespread use of composite laminates. Delamination becomes a particularlydangerous damage mode of high performance laminated composites[1]. Therefore interlaminar fracture toughness play important rule in design process. The four end notchedflexural (4ENF) used for the measurement of the mode II interlaminar critical stress energy release rate because of thesimple setup[2]. Recent studies of the four end notched flexure test specimens have shown that these test methods can yieldsignificantly different results because of the combined influence of crack face friction. Paul Robinson & Stephen Message used an experimental technique (based on amodification of the 4ENF test) for directly assessing the influence of friction on mode II fracture toughness [3]. In the present paper, effects of the crack face friction between delamination surfaces on Mode II interlaminar fracture toughness have been experimentally measured