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Keyword : fatigue
Results 1 - 5 of 6
New Methodology of Bending Fatigue Test and Slamming Test on PVC Foam Core Sandwich with GFRP Faces
The purpose of this study is to investigate the influence of a spatially moving load and edge effects on the fatigue life of the foam-cored sandwich structures. A spatially moving load can be observed in structures subjected to slamming. A new geometry of specimen is developed to reduce the influence of edge effects in the test specimen. Numerical model results of the new geometry are presented. This study confirms that edge effects are leading to early failures and shear stress concentrations are significantly reduced near the edges, improving ASTM C393 standard.
Updated Fatigue Test Methods for Structural Foams and Sandwich Beams
Foam-cored sandwich yacht hulls are subjected to high core shear stresses during slamming events. As slamming is
repetitive by nature, failures observed on boats may be due to fatigue. This study aims to investigate possible improvements to
fatigue testing of both foam core materials and sandwich specimens. In general test set-up induces differences between the ways a
material behaves in a test coupon and in a real application. One such difference is “edge effect”, as the material behaviour can change
close to a free edge. For example, the micro-structure may have been affected by the specimen machining, which may influence
failure initiation. This is exacerbated if the test set-up induces stress concentrations close to the edge. Another difference is that in a
standard core shear fatigue testing by 4 point bending, the stress field is spatially static, when in a slamming event the stress field is
spatially variable. Does the material react differently to a static and moving stress field? This study aims to develop a core shear test
method replicating a moving stress field, free of edge effect. This paper presents the finding of the initial steps of this study: The
edge effect has been investigated using modified loading fixture. The moving stress field has been investigated with a modified 4-
point bending test using asynchronous loading. The differences in test results between the modified test methods and the relevant
standard test methods indicate that both aspects affect measured fatigue life.
In the field of aeroelasticity, flutter is a well-known instability phenomenon. Flutter is a synchronized vibration which takes place in a flexible structure moving through a fluid medium. It occurs when two regular, rhythmic motions coincide in such a way that one feeds the other, drawing additional energy from surrounding flow. A classic case of wing flutter might combine wing bending with either wing twisting. Flutter appeared for the first time on racing yacht keels with composite fins, so in water, in 2004, on both the IMOCA 60 POUJOULAT-ARMORLUX, which lost her keel, and SILL. Following these problems - particularly following the loss of the keel of Bernard STAMM sailboat, accident that could have dramatic consequences for the skipper - HDS company focused on the phenomenon. This paper will introduce the strategy of HDS faced to the problem and the analytical and numerical methods implemented to estimate the flutter critical speed. Our model is based on a truncated modal basis for the most energetic modes which are generally, for a bulb keel, the lateral bending predominant mode and the torsion predominant mode. One of our requirements was to make a simple model in order to integrate the calculation of the flutter critical speed in the first design loops of a composite or steel keel. Besides, an other requirement was to be able to calculate flutter critical speed on other type of appendages: hydrofoils, dagerfoils, daggerboards, rudders...This model has worked well for the two cases of flutter appeared on IMOCA sailboat keels. Besides, to verify the quality of the model and to complete our analysis of flutter phenomenon on racing yacht keels, a 3 dimensional multiphysic simulation has been developed using the software ADINA.
This page briefly describes a project that looked at fatigue effects on marine fiberglass. The project's goal was to correlate (and improve where possible) standard test methods, theory and computer-aided analysis tools to give designers more confidence when developing specifications for marine composite components.
The project included aspects of both theory and experimentation. Finite element analysis (FEA) (a numerical/graphical structural analysis computer tool) and various fatigue theories were compared and supported by coupon, panel and full-size testing. The "test case" was the J/24 class sailboat, a design that has enjoyed a long production run and has a reputation for durability. This six-year study on the durability of marine composites was conducted at the University of California, Berkeley and the U.S. Naval Academy and supported by the American Bureau of Shipping, TPI (builder of the J Boats), OCSC (a sailing school located in Berkeley, California) and Maricomp (a small structural analysis company in Costa Mesa, California).
Fatigue Prediction Verification of Fiberglass Hulls
The growing use of marine composite materials has led to many technical challenges and one is predicting lifetime durability. This analysis step has a large uncertainty due to the lack of data from in-service composite vessels. Analytical models based on classical lamination theory, finite-element analysis, ship motions, probability and wind and wave mechanics were used in this project to predict hull laminate strains, and fatigue tests were used to determine S-N residual stiffness properties of coupons. These predictions and test data were compared against two cored fiberglass sisterships having significantly different fatigue histories and undamaged laminates representing a new vessel. Strains were measured while underway and good correlation was achieved between predictions and measurements. Fatigue damage indicators were identified which could be used in vessel inspection procedures. Endurance limits were found to be near 25% of static failure load, indicating that a fatigue design factor of four is required for infinite service with this material. Standard moisture experiments using boiling water were compared with 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 with 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 skin laminates in marine composites was identified through single-sided moisture flex tests. These showed that the reduced strength and stiffness due to moisture of the outer hull skin laminate could be compensated by increased thickness. Although the resulting unbalanced laminate is not ideal from a warping standpoint, the approach leads to consistent tensile failure of the inner skin when subjected to normal loads. Permeability considerations make this desirable for hull laminates.