An experimental investigation of the sequence effect in block amplitude loading of cross-ply composite laminates |
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| Affiliation: | 1. Materials Research Department, Risø National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark;2. Structures Department, The Aeronautical Research Institute of Sweden, PO Box 11021, SE-16111 Bromma, Sweden;1. Structural Technologies Division, CSIR-National Aerospace Laboratories, Bangalore 560017, India;2. Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560012, India;1. School of Aeronautical Sciences, Hindustan Institute of Technology and Science, Chennai 603103, India;2. Dept. of Civil Engineering, Hindustan Institute of Technology and Science, Chennai 603103, India;1. College of Applied Technical Sciences, Department of Civil Engineering, Aleksandra Medvedeva 20, 18000 Ni?, Serbia;2. University of Novi Sad, Faculty of Technical Sciences, Department of Civil Engineering, Trg Dositeja Obradovi?a 6, 21000 Novi Sad, Serbia;3. Brandenburg University of Technology, Chair of Steel and Timber Structures, Konrad-Wachsmann-Allee 2, 03046 Cottbus, Germany;1. AMADE, Mechanical Engineering and Industrial Construction Department, Universitat de Girona, Carrer Universitat de Girona 4, E-17003 Girona, Spain;2. AERNNOVA Engineering Division, Structural Integrity Department, 20 Manoteras Avenue – Building B, E-28050 Madrid, Spain |
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| Abstract: | The Palmgren–Miner rule has been shown to be inexact in many cases for various composite materials. Several empirical models have been conceived to account for this discrepancy, as well as the effect of block sequence. The approach taken here is based on the underlying mechanisms. A cross-ply laminate was used as a model material. In general, composites show both initiatory and progressive mechanisms under fatigue loading. The former is active at high static stresses, whereas the latter predominates at lower stress amplitudes where they are given sufficient time to propagate. Initiatory mechanisms give rise to damage from which the progressive mechanisms can start, and conversely the progressive mechanisms continually alter the local stress state which results in further damage accumulation caused by the initiation controlled mechanisms. In a cross-ply laminate, the initiatory mechanism is the formation of transverse cracks, and the progressive mechanism is mainly delamination growth initiated from the transverse cracks. In an experimental investigation of carbon fiber/epoxy cross-ply laminates, the interaction of these mechanisms has shown why a sequence of high–low amplitude levels results in shorter lifetimes than a low–high order. Such a sequence effect seems to be a common behavior for many other composite materials, and can be mechanistically explained by a similar kind of interaction. Advantages and drawbacks of the mechanistic approach compared with empirical rules are also discussed. |
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