Free-Edge Effects in Composite Laminates—A Review of Recent Developments 2005–2020

Abstract

The design and analysis of composite structures in the form of layered plates or shells is often driven by stress concentration phenomena that occur due to geometric or material discontinuities. One prominent example is the so-called free-edge effect that manifests itself in the form of significant localized interlaminar stress fields in the vicinity of free laminate edges and that is given rise to due to the mismatch of the elastic properties of the individual laminate layers. The free-edge effect has been under scientific investigation for more than five decades, and this paper aims at providing an overview of recent developments and scientific advances in this specific field wherein an emphasis is placed on investigations that were published in the time range between the years 2005 and 2020. This paper reviews closed-form analytical methods as well as semi-analytical and numerical analysis approaches and summarizes the recent state of the art concerning the investigation of stress singularities and experimental characterization of free-edge effects. This paper also reviews advanced problems such as free-edge effects in curved laminated structures and in piezoelectric laminates as well as in the vicinity of holes and other geometric discontinuities, and two new aspects in the field of free-edge effects, namely, the development and application of a new semi-analytical method (the so-called scaled boundary finite element method (SBFEM)) and the fracture mechanical strength assessment, also by novel approaches such as finite fracture mechanics, are also discussed. This paper closes with a summary and an outlook on future investigations.

Very high cycle fatigue of fiber‐reinforced polymer composites: Uniaxial ultrasonic fatigue

“…40,41 For instance, cross-ply laminates manifest two-dimensional stress within the plane but three-dimensional stress in the vicinity of the free edges. 42,43 The progression and nature of damages, including matrix fractures, delamination, and fiber-matrix de-bonding, are contingent on a material system, fabrication configuration, and stress levels. 42,44 When tension-tension cyclic loading is applied to crossply and quasi-isotropic laminates, the initial damage observed is a transverse fracture.…”

“…Various layup configurations display distinct fatigue responses 40,41 . For instance, cross‐ply laminates manifest two‐dimensional stress within the plane but three‐dimensional stress in the vicinity of the free edges 42,43 . The progression and nature of damages, including matrix fractures, delamination, and fiber‐matrix de‐bonding, are contingent on a material system, fabrication configuration, and stress levels 42,44 .…”

Computational model for the semi‐analytical assessment of the free‐edge effect in composite laminated shells

“…It is for this reason that in the last decades, fiber-reinforced plastics have attracted more and more attention due to their favourable specific material properties and their ability to arbitrarily tailor the stacking sequence of composite laminates in order to meet given design aims. Several experimental investigations of composite laminated shells undergoing different loading conditions revealed [1], however, that those safety-critical structures exhibit complex failure modes with an interaction of matrix cracking and delamination mainly called forth due to threedimensional stress concentrations at the traction-free edges between adjacent unidirectional reinforced laminate layers. This phenomenon, commonly referred to as the free-edge effect, is omnipresent since it is a consequence of the varying deformation properties of composite structures and thus, has to be taken into consideration throughout the whole development process.…”

Free-edge effects at circular holes in composite laminates under hygrothermomechanical load

“…In their subsequent paper a simple closed-form method for the analysis of the stress fields in the vicinity of free laminate corners with arbitrary layup was presented. 19 Furthermore, Mittelstedt et al 20,21 published two review papers investigating free-edge effects in composite laminates in 2007 20 and 2020 21 to analyze and discuss various studies published on the current topic during more than 50 years.…”