Design Criteria for Composite Masts

Abstract

Composite materials such as fiberglass have been used in recreational marine craft for over 30 years. Only within the last decade however have composites found applications in mast construction. This application to masts is due to the lighter weight possible by using composites such as carbon fiber/epoxy that have higher stiffness-to-weight ratios than the traditional aluminum and wood mast materials. Composites use in masts has not been without problems however as designers and manufacturers learn the techniques necessary to produce reliable structures. As with any maturing technology, empiric ii developments will eventually result in acceptably reliable structures. This paper uses a reliability-based design criterion to aid composite mast development. The basic approach characterizes the uncertainties and variabilities of mast design (loads, materials, analytical models, etc.) to predict a total uncertainty called "the probability of failure." The total probability of failure is then compared to "acceptable' probabilities of failure generated from existing aluminum mast designs to determine an appropriate factor of safety. This approach takes the guesswork out of factor of safety selection, giving the mast designer an important tool. By using reliability­based methods the influence of each part on the overall probability of failure can be determined. This allows for intelligent selection of design areas to improve that provide the biggest potential gain in reliability, cost or weight, and decreases the likelihood of either an overly­conservative, heavy mast, or an under-designed mast. The paper begins with the causes of mast failure and then presents the impact of load modeling, material properties and structural modeling on predicted mast reliability. An example mast design is used to illustrate the method, with the result that a composite mast can be designed at nearly the same factor of safety as an aluminum mast.