CRACK PROPAGATION IN SECONDARY BONDED FRP COMPOSITE JOINTS

CRACK PROPAGATION IN SECONDARY BONDED FRP COMPOSITE JOINTS

By Steven Paul Lewis Blake

Thesis Advisor:  Dr. Roberto Lopez-Anido, PE

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Civil Engineering)

May, 2010

Composites, which consist of more than one material, are becoming widely accepted as a structural material.  Fiber reinforced polymer (FRP) composites consist of a resin reinforced by fiberglass, carbon, or aramid fibers.  FRP offers significant material advantages over traditional materials used in marine construction such as steel; however, the design and production of high strength structural joints in composite structures remains difficult.  Most large marine structures must be fabricated in steps, which results in a number of bonded joints.  These bonded joints are referred to as secondary bonds.  Secondary bonds are relatively weak, and, as a result, composite structures commonly fail at secondary bonded joints.  Failure in secondary bonds commonly occurs as a result of the formation of a crack within the bond.  Two types of secondary bonded joints are examined in this thesis. 

The fatigue performance is investigated for a doubler joint, which is a stiffener that has been applied to a section to improve strength and stiffness.  Cracks form over the life of a structure, and this is commonly referred to as crack propagation.  Crack propagation in bonded doubler plate joints was investigated by applying fatigue loads based on typical vessel loads or service conditions.  The doubler plate joints were investigated with respect to lifespan and failure criteria typically used for marine composites.  The goal of the study is to characterize crack propagation in secondary bonded doubler plate joints under service conditions for marine structures.  The main contribution of the study to the marine industry is to improve current design methods for doubler plate joints.

The response of a tee joint, which serves to connect the hull and bulkhead in a marine structure, is also investigated.  The response of the tee joint is predicted using a numerical model of the joint and material properties.  The results of the model are considered with respect to the highly variable nature of the material.  A detailed sensitivity study is conducted to determine the effect of various geometric and material parameters on the performance of the tee joint.