Spectrum fatigue lifetime and residual strength for fiberglass laminates in tension

Abstract

This paper addresses the effects of spectrum loading on lifetime and residual strength. Over 900 tests have been run on a typical fiberglass laminate configuration under a variety of load sequences. Repeated block loading at two or more load levels as well as a modified standard spectrum have been studied. Data have been obtained for residual strength at various stages of the lifetime. Several lifetime prediction theories have been applied to the results. The repeated block loading data show lifetimes that are usually shorter than predicted by the most widely used linear damage accumulation theory, Miner's sum. Actual lifetimes are in the range of 10-20% of predicted lifetime in many cases. Linear and nonlinear residual strength models tend to fit the data better than Miner's sum, with the nonlinear providing the better fit of the two. Direct tests of residual strength at various fractions of the lifetime are consistent with the residual strength damage models for several cases. Load sequencing effects are not found to be significant. The more a spectrum deviates from constant amplitude, the more sensitive predictions are to the damage law used. The nonlinear model provided improved correlation with test data for a modified standard wind turbine spectrum. When a single, relatively high load cycle was removed, all models provided adequate correlation with the experimental results. The residual strength models may provide a more accurate estimate of blade lifetime than Miner's rule for some loads spectra. They have the added advantage of providing an estimate of current blade strength throughout the service life. Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. and the American Society of Mechanical Engineers. All rights reserved. INTRODUCTION An investigation of the relationship between spectrum loading and fatigue lifetimes of a typical wind turbine blade fiberglass material has been undertaken for the development of refined design tools. Present design tools for estimating lifetimes of fiberglass materials produce results that may be significantly nonconservative for some loads spectra. These tools or prediction models range from the simple Miner's sum and the various deviations to more complicated ones based upon residual strength.' Many require testing of the materials to establish fitting parameters to obtain the best performance of the model. The objective of this study is to identify cumulative damage laws which provide improved accuracy in predicting lifetime under a variety of loads spectra for wind turbine blade materials.. This program of spectrum loading for fatigue of fiberglass materials was conducted in a logical progression from simple to complicated spectra; that is, from constant amplitude loading, to multi-amplitude loading, to a modified standard spectrum. Small dogbone coupons were manufactured, tested and favorably compared to standard tensile test coupons. Baseline data for the development of stress-cycle (S-N) trends was obtained by constant amplitude fatigue of coupons by using computer controlled hydraulic testing equipment. Results of this preliminary testing are consistent with earlier work. Tests were then run using repeated blocks at two stress levels. Initially this two-block testing was such that the first block contained ten cycles of the higher stress load, followed by a varying number of cycles at a lower stress level, repeated until failure. Two-block testing by some investigators has been limited to the application of a fixed number of cycles of the first stress level, followed by an undetermined number of cycles at a second level, until failure. This reportedly results in either Miner's sums greater than unity for high stress levels followed by low stress (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. levels or Miner's sums less than unity for low stress levels followed by high stress levels. ] The present work used the more general case of repeated application of two-blocks until failure, and also explored load sequencing effects. Testing of multi-block spectra was then performed with blocks of three and six stress levels. Finally, coupons were subjected to a modified WISPERX' spectrum which has been reported to produce Miner's sums less than unity. NOMENCLATURE AND DEFINITIONS The linear damage accumulation rule of Miner's sum is frequently applied to fatigue test results and is here defined as: