Energy criteria for FRP delamination

Delamination (along ply interfaces) and splitting (along the fiber-matrix interfaces) are the main modes of FRP fracture. This modes follow all types of fracture and they lead to bear capacity lose for composite elements of construction. Traditional strength criteria in term of maximum stresses or maximum strains cannot describe exactly the conditions of delamination or splitting onset, start and propagation. The energy approach in this paper, uses as the necessary condition of delamination (in according with Griffith-type criterion) the excess of the initial value of the elastic energy over the value of final energy (stored after fracture) by the value of the work of delamination which is proportional to square of crack and to specific work of delamination. Some simple examples for bending, torsion and compression of composites plates demonstrate an important consequent of energy criterion: scale effect, critical stresses (strength) dependence on absolute sizes of structure elements. Some new results are connected with analysis of composite plates delamination under the combination of bending and torsion moments and also with the conditions of equi-strength beams delamination under bending and torsion. Some experimental methods of specific work of delamination estimation are reviewed.