Mixed Mode (I-Ii) Delamination Fracture Criteria For Im7-G/8552 Intermediate Modulus Carbon/Epoxy Composite Laminate

Jr. Samuel Williams, North Carolina Agricultural and Technical State University


In aerospace vehicles such as rotorcraft, structural components are subjected to bending and stretching loads that introduce peel and shear stress between the composite plies. Delamination is a primary failure mode for composite laminate. Delamination is caused by interlaminar stresses that act in the matrix, which is the weaker part of the composite laminate. Damage tolerant design of structures requires two types of material/laminate data: 1. Mixed mode delamination criteria to predict failure and 2. Delamination growth criteria to predict the life of a structural component. This research focuses on developing mixed-mode (a combination of mode I and II) fracture criteria for IM7-G/8552 composites, which are widely used in the rotorcraft industry. IM7-G/8552 prepreg was procured from the HEXCEL Corporation and 24 ply unidirectional laminate with a 3 in width Fluorinated ethylene propylene film was fabricated. Standard split beam specimens were prepared. The specimens were tested in double cantilever beam and end notched flexure beam modes to measure mode I (GIC) and II (GIIC) fracture toughness, respectively. Split beam specimens were tested using the modified mixed-mode test apparatus for values of GIIm/GC = 0.18, 0.37, 0.57 and 0.78, where GC is the total fracture at GIm and GIIm loading. From the data, a mixed mode fracture equation was developed in the form: The equation agreed well with the literature for GIC, and GII m/GC = 0.33 but it differs at GII m/GC = 0.66 and GIIC. 2 115 550               c II c II IC G G G G G G