Three-dimensional static and dynamic analysis of a composite cruciform structure subjected to biaxial loading: a discontinuum approach

A three-dimensional structural integrity analysis using the eXtended Finite Element Method (XFEM) is considered for simulating the crack behaviour of a chopped fibre-glass-reinforced polyester (CGRP) cruciform specimen subjected to a quasi-static tensile biaxial loading. This is the first time this problem is accomplished for computing the stress intensity factors (SIFs) produced in the biaxially loaded area of the cruciform specimen. A static crack analysis for the calculation of the mixed-mode SIFs is carried out. SIFs are calculated for infinite plates under biaxial loading as well as for the CGRP cruciform specimens in order to review the possible edge effects. A ratio relating the side of the central zone of the cruciform and the crack length is proposed. Additionally, the initiation and evolution of a three-dimensional crack are successfully simulated. Specific challenges such as the 3D crack initiation, based on a principal stress criterion, and its front propagation, in perpendicular to the principal stress direction, are conveniently addressed. No initial crack location is pre-defined and an unique crack is developed. Finally, computational outputs are compared with theoretical and experimental results validating the analysis

Mixed-mode damage into a CGRP cruciform subjected to biaxial loading

In this paper, a three-dimensional progressive damage model (PDM) is implemented within a chopped glass-reinforced polyester (CGRP) cruciform structure for modelling its damage under loading. Three different cruciform specimens subjected to biaxial tensile loading are studied. In order to simulate the computational behaviour of the composite, the constitutive model considers an initial elastic behaviour followed by strain-softening. The initiation criterion defined is based on the maximum principal stress of the composite and once this criterion is satisfied, stiffness degradation starts. For the computation of damage, the influence of the fibre and the matrix are taken into account within the damage rule. Realistic values of the energy dissipated during damage are computed. The computational results obtained by means of an explicit time marching solver are compared with experimental outcomes for validation purposes. Finally, it is concluded that the PDM is able to localise the damage effectively as well as predicting its initiation. In the best of authors' knowledge, this is the first time a three-dimensional PDM is implemented into a composite cruciform structure subjected to biaxial loading

An approach for dynamic analysis of stationary cracks using XFEM

A numerical implementation of the eXtended Finite Element Method (XFEM) is presented. The proposed approach solves the system of discrete equations using an explicit integration scheme and it is capable of addressing dynamic and static fracture mechanics problems. Special attention to the mass matrix construction is required in order to avoid instability issues such a null stable time increment. Hence, different mass lumping strategies are adopted for enriched elements. The in-house implementation of this approach, so-called X in-house FE platform called MULE. Numerical tests demonstrate that the proposed approach is able to provide an accurate calculation of static and dynamic stress intensity factors (SIFs) for different geometries and loading scenarios. Finally, in order to extend our point of view, an experimental analysis of a 10◦ off-axis carbon fibre laminate is carried out using Digital Image Correlation (DIC)

All-sky search for long-duration gravitational wave transients with initial LIGO

We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10-500 s in a frequency band of 40-1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10-5 and 9.4×10-4 Mpc-3 yr-1 at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves. © 2016 American Physical Society

All-sky search for long-duration gravitational wave transients with initial LIGO

We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10-500 s in a frequency band of 40-1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10-5 and 9.4×10-4 Mpc-3 yr-1 at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves. © 2016 American Physical Society

Crack propagation in a chopped glass-reinforced composite under biaxial testing by means of XFEM

Crack initiation and propagation are analysed numerically and experimentally for a cruciform specimen subjected to quasi-static biaxial tensile loading. The material tested is a chopped glass-reinforced composite with a quasi-isotropic homogeneous elastic behaviour. The eXtended Finite Element Method (XFEM) is used for studying the linear elastic fracture problem within the finite element software ABAQUS™. Simulations are developed without the need of defining an initial imperfection or re-meshing as the discontinuity evolves. Crack emerges and evolutes as a natural outcome without any ad hoc assumption. The numerical results are in good agreement with the corresponding experimental observations obtained on cruciform specimens biaxially loaded. This demonstrates the benefits of the XFEM technique as a predictive methodology under multiaxial loading without any initial supposition of the crack location and its posterior path direction