Delamination | EPFL Graph Search

Delamination is a mode of failure where a material fractures into layers. A variety of materials including laminate composites and concrete can fail by delamination. Processing can create layers in materials such as steel formed by rolling and plastics and metals from 3D printing which can fail from layer separation. Also, surface coatings such as paints and films can delaminate from the coated substrate. In laminated composites, the adhesion between layers often fails first causing the layers to separate. For example, in fiber-reinforced plastics, sheets of high strength reinforcement (e.g., carbon fiber, fiberglass) are bound together by a much weaker polymer matrix (e.g., epoxy). In particular, loads applied perpendicular to the high strength layers, and shear loads can cause the polymer matrix to fracture or the fiber reinforcement to debond from the polymer. Delamination also occurs in reinforced concrete when metal reinforcements near the surface corrode. The oxidized metal ha

Two-dimensional crack growth in FRP laminates and sandwich panels

Aida Cameselle Molares

Fiber-reinforced polymer (FRP) composite materials are currently being selected for the design of lightweight and efficient structural members in a wide number of engineering applications. Sandwich panels with glass fiber-reinforced polymer (GFRP) face sheets and low-density cores are notably one of the most common applications of FRP materials in the civil engineering field. The load-bearing capacity of FRP structures can be significantly reduced by delamination and debonding damage which, in actual structural members, may extend all around its perimeter, thus constantly changing the size of the crack front. However, most research efforts concerning the fracture characterization of delamination and debonding damage have focused on one dimensional (1D) fracture specimens where cracks propagate longitudinally with an approximately constant crack width, thus resulting in fracture properties that may lead to inaccurate predictions of fracture behavior in real structures. With the aim of establishing a more realistic fracture approach to delamination and debonding damage in FRP structures and thus improving their fracture characterization, the two dimensional (2D) crack growth in GFRP laminates and GFRP/balsa sandwich panels is investigated in this research. The 2D quasi-static delamination behavior in GFRP plates with embedded defects subjected to out-of-plane tensile loads was experimentally investigated. Increasing loads were obtained due to the disproportionate increase of the crack area throughout the propagation of the 2D crack. Analysis of the load-bearing and compliance responses indicated that a stiffening of the opened part of the plates occurred as a result of in plane stretching stresses that developed due to the boundary conditions associated with an embedded 2D crack. To investigate the effect of the stress stiffening and associated fracture mechanisms, the 2D delamination behavior was numerically studied. Results confirmed a 50% increase, compared to 1D fracture, in the total strain energy release rate (SERR) required to propagate the crack. This was correlated to the increase of the fiber-bridging area as a result of the increase in the flexural stiffness (from beam to plate) and the stress-stiffening effect. A numerically-based method, suitable for determining the mean total SERR involved in the Mode I-dominated 2D delamination of FRP laminates was further developed and validated. This method permits a reduction of experimental and computational efforts. The 2D fracture investigation was extended to GFRP/balsa sandwich panels whose 2D debonding behavior was experimentally studied under quasi-static and fatigue out of-plane tensile loads. Circular embedded disbonds were introduced at the face sheet/core interface. Stretching strains again developed, thereby activating shear fracture modes which, depending on the face sheet layup, triggered different crack propagation behaviors. The fatigue load-displacement hysteresis loops exhibited an increase in the cyclic stiffness due to the stiffening caused by the stretching stresses. The introduction of plies prone to develop fiber-bridging at the face sheet/core interface resulted in enhanced quasi-static debonding resistance, while under fatigue their fracture efficiency was highly dependent on the fatigue amplitude. High R-ratios improved fatigue performance due to a reduction in fiber-bridging crushing.

EPFL2019

Crack tip identification with long FBG sensors in mixed-mode delamination

John Botsis, Daniel Sans Canovas, Samuel Stutz

The uncertainty of the crack tip position inside composite coupons during a delamination test under variable fracture mode conditions reduces the accuracy of the experimental results. In this work a method is presented where the crack tip position is located using long embedded Fibre Bragg Grating (FBG) sensors together with the Optical Low Coherence Reflectometry (OLCR) technique. With this technique the local Bragg wavelength is measured which then enables to calculate the axial strain profile within the grating. Carbon/epoxy samples are tested under different mode ratios through the standard Mixed-Mode Bending (MMB) test. Embedded long FBG sensors are used to measure the axial strain profile along the whole grating at different stages of the test. The crack tip can be precisely identified with the long embedded FBGs and then checked by means of a visual inspection after complete delamination of the sample. The data indicate that the long FBG sensor detects the crack tip inside the sample at least as precisely as the traditionally performed visual inspections carried out during the test by means of the lateral markings. Numerical simulations using cohesive elements are in good agreement with the experimental data. (c) 2012 Elsevier Ltd. All rights reserved.

2012

Delamination in cross-ply laminates: Identification of traction-separation relations and cohesive zone modeling

David Enrique Alanis Rojas, John Botsis, Joël Cugnoni, Ebrahim Farmand Ashtiani

Identification of traction separation relations is at the forefront of damage and fracture mechanics of composite laminates. Such relations are of particular interest in fracture of laminates consisting of layers of different fiber orientations. In this study, an iterative method based on internal strain measurements and parametric numerical modeling is employed to identify a traction-separation relation in quasi-static mode I dominated delamination of a cross-ply carbon fiber epoxy laminate. The results demonstrate that crack propagation is accompanied by a large bridging zone which is smaller than the zone in a uniaxial composite specimen of the same materials and linear dimensions. While the initiation value of the energy release rate (ERR) is the same, the ERR at steady state propagation and the maximum stress in the bridging zone are respectively 1.7 and 3.1 times larger than the corresponding values of the unidirectional specimen. The obtained traction-separation relation is employed in a cohesive zone model to predict the loading response and crack growth. The adopted approach is a step towards a better understanding of delamination in cross-ply composites. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier2015

Two-dimensional crack growth in FRP laminates and sandwich panels

Aida Cameselle Molares

EPFL2019