Durability of Marine Composites: A Study of the Effects of Fatigue on Fiberglass in the Marine Environment

Abstract

Abstract : The growing use of marine composite materials has led to many technical challenges. One is predicting lifetime durability. This step has a large uncertainty as the methods used to design metal ships have never been verified at full-scale for composite vessels. This project systematically extended the isotropic method to the application of fiberglass vessels in the marine environment. The research compared two identical fiberglass vessels having significantly different fatigue histories with undamaged laminates representing a new vessel. Analytical models based on classical lamination theory, finite element analysis, ship motions, probability and wind and wave mechanics were used to predict hull laminate strains. Material properties used in the finite element analysis were based on tested wet and dry conditions. Fatigue tests were used to determine S-N residual stiffness properties. These were compared against strains measured while underway and good correlation was achieved. Fatigue damage indicators were identified which could be used in vessel inspection procedures. Endurance limits were found to be near 25% of static load, indicating a fatigue design factor of 4 is required for these materials. This exceeds ABS and US Navy design guides. Comparisons were made of standard moisture experiments using boiling water versus long-term exposure. Results indicated the boiling water test yielded significantly conservative values and was not a reliable means of predicting long-term effects. Panel tests were compared to a combined coupon and finite element procedure. Results indicated the proposed procedure was a viable substitute, at least for the materials studied. A rational explanation for using thicker outer laminates in marine composites was verified through single-sided moisture flex tests. These showed that the reduced strength and stiffness due to moisture of