Physics of delamination onset in unidirectional composite laminates under mixed-mode I/II loading

Abstract

A B S T R A C TIn order to understand the physics of the delamination onset in laminated composites, an experimental investigation of delamination growth is presented. The main objective of this paper is to demonstrate that delamination onset occurs at lower values than G c defined by the ASTM standards, and that the strain energy release rate level at which the crack growth onset occurs under any mixed mode I/II loading is governed by the critical strain energy density (SED) approach. Quasi-static delamination experiments have been performed under mode I, mode II and mixed mode I/II loadings. The value of the strain energy release rate at the observed crack onset and the angle of the initial crack growth were correlated with the SED theory to test the validity. The acoustic emission was also used to provide more insight into the physics of the delamination growth. The investigation shows that the onset of delamination growth occurs at the strain energy release rate levels predicted by the critical SED approach, and well before reaching the critical strain energy release rate determined via delamination tests following the ASTM standards. Moreover, results indicated that the predicted angle of the initial crack for delamination onset was in a good agreement with the experimental data. linearity (NL), the point at which the delamination growth is visually observed (VIS) or the intersection of a 5% offset line (5% Max).

Mode I and mode II delamination of carbon/glass/epoxy hybrid composite: A statistics‐based analysis

“…Several experimental data have been added from the literature. [12,[46][47][48][52][53][54] The Weibull model generated is viable due to the low variation between the experimental and theoretical data, as well as the possibility of measuring any reliability value for the NCF (0/90) carbon and glass fiber composite using the Weibull model or a trend chart, as presented herein. The selection of the reliability level is fundamentally dependent on the planning and the material applied.…”

“…[8][9][10] For this reason, recent studies have been focused on the physical identification and interpretation of delamination fracture mechanisms, seeking realistic values for the strain energy release ratio (SERR) and crack propagation behavior. [11][12][13] However, the anisotropic characteristic of composites presents a challenge in failure analysis and prediction in mechanical tests, generating considerable data dispersion in the results. [13,14] Considering that in most structural applications failure is due to cyclic loading, [15] in mechanical tests the reliability level of the measurement and analysis of variance are fundamental for determining the fatigue delamination parameters.…”

Loading rate effects on mode-I delamination in glass/epoxy and glass/CNF/epoxy laminated composites

“…In ASTM D 5528, three methods are described to determine the onset of delamination growth initiation, i.e., NL point, VIS point, and 5% / MAX point. The NL point method normally represents a lower bound for G IC , while in some physics-based studies it is claimed that this value does not necessarily refer to the actual onset [34,35]. In the present study, the initiation value of G IC at quasistatic and low-rate tests has been determined based on the NL point method.…”

Development of a physics-based theory for mixed mode I/II delamination onset in orthotropic laminates

“…Additional experiments are needed to verify that this onset indeed happens. This aspect is studied and reported by the authors in another publication [54].…”