Abstract

Some composite morphologies are susceptible to loss of capacity due to the presence of interlaminar delaminations. The objective of this work is to experimentally evaluate the generation and propagation of these interlaminar cracks in a carbon-epoxy composite subjected to dynamic stress under mixed fracture mode I / II, using an ADCB type test methodology for this purpose. From a previous static characterization of the material the critical value of the energy release rate was obtained, from which the levels of the critical energy release rate to be applied in the fatigue tests were defined, the values ​​considered were: 45, 50, 55, 60, 65, 70 and 75% Gcr. In all the dynamic tests, an asymmetry coefficient of R = 0.1 was used. From these experimental data the fatigue initiation curves DGN (number of cycles necessary for the nucleation of a crack, for a given rate of energy release) were obtained, observing fatigue life limits of the order of 38% of Gcr . In addition, the crack propagation velocity curves da / dN-G were shown, which showed the trend of the Paris law. On the other hand, a fractographic analysis of the delaminated surfaces was carried out, showing the typical fracture typologies of both modes, I and II, both in the initiation and propagation of the crack, showing the presence of mixed mode I / II in the ADCB type assay.

Abstract

Composite materials manufactured by means of the superposition of layers and some specific morphologies, are susceptible to losing part of their resistant capacity due to the presence of delaminations between their  layers. The objective of this work is to evaluate experimentally the generation and propagation of these interlaminar cracks in a carbon-epoxy composite subjected to dynamic stress under mixed fracture mode I/II, using for this purpose two different test methodologies, on the one hand the standardized MMB (Mixed-Mode Bending) and on the other hand the ADCB (Asymmetric Double Cantilever Beam), with the intention of exploring the viability of the ADCB test as a simpler alternative of execution, especially in fatigue tests, to the MMB test. For this purpose and after a previous static characterization of the material in which the critical values of the energy release rate were determined in both test methods, the energy release rate levels to be applied in the fatigue tests were defined for two GII/Gc mixed mode grades of 0.2 and 0.4(0,33 ADCB)   and a ratio of stress levels to fatigue of R= Gmin/Gmax=0.1.DG-N fatigue initiation curves were obtained from these experimental data. The most relevant result of the work is that, in general, the MMB mixed mode provides different fatigue limits depending on the degree of mixed mode, whereas in the ADCB type test the fatigue limits tend to have the same value.

Abstract

This work presents an experimental method to determine the cohesive law of thin adhesive joints in mixed mode. The proposed method is based on the MMB (Mixed Mode Bending test). The Energy release Rate and Crack Tip Relative Displacement are determined based on a compliance variation data reduction method by processing the global load displacement curve, without monitoring the crack length and the crack tip relative displacement during the test. Representing the energy release rate versus relative displacement the cohesive law is determined by numerical differentiation. The method is validated numerically by virtual tests including the cohesive zone model. The results reveal that the proposed data reduction scheme is suitable to obtain the mixed mode cohesive law using only the load and displacement data obtained from the testing machine, without any external displacement measurement technique and without any assumption of the form of the cohesive law.