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Fatigue fracture of fiber-reinforced polymer composites (FRP) occurs when microcracks are induced by debonding, pull-out and delamination at the interface between the matrix and fiber. This microcrack area increases with increase in fatigue cycles and a damage region is formed. In our previous paper, fatigue life of a short fiber-reinforced polymer composite consisting of glass fibers and polycarbonate matrix was found to be related not to the main crack growth behavior but to the progression behavior of the damage region. In this paper, using our proposed real time observational system, we performed detailed observations on the behavior of fatigue damage and clarified the mechanism of damage progression. Furthermore, mechanical considerations were performed by finite-element elastic-plastic stress analysis. The results mentioned above indicate that control of short fiber alignment makes it possible to release the stress concentration caused in the matrix, and disperse fatigue damage. This results in an enormous improvement in fracture toughness. [ABSTRACT FROM AUTHOR]

In the field of cultural objects conservation efficient stabilisation of fragile and failing, multilayered decorative coatings is a complex and challenging task. This paper introduces a new application of the standardised double-cantilever beam (DCB) test method to improve the understanding of the mechanical properties of failing material and to determine the effect of polymeric agents (consolidants) added for their stabilisation. The adapted DCB method was used to measure the fracture energy, G, and the fracture behaviour of brittle, protein-bound (gesso-type) foundation layers on wooden substrates that typically suffer from delamination and flaking. Wooden DCB specimens containing a brittle layer of protein glue mixed with finely ground clay powder were prepared, fractured, then consolidated with a range of commonly used polymer formulations and finally re-fractured to provide measurements for direct comparison. Consolidants tested included gelatine-based glues (bovine hide glue, isinglass; both pre-stained with Fast Green dye), acrylics (Lascaux Medium for Consolidation, Paraloid B-72/B-48N), poly(vinyl acetates) (Mowilith 50, Mowilith DMC2) and poly(vinyl alcohol) (Mowiol 3-83). Before second-phase fracture cross-sections were taken from the DCB specimens for determining penetration depth and gap-filling ability. For better visibility, the specimens containing acrylics were stained with Solvent blue G dye; iodine-potassium iodide was used for staining the other synthetic consolidants. The resulting data showed that the test method could determine measurable differences between initial G (47 ± 22 J/m) and post-consolidation G values. Also, penetration behaviour could be well characterised and valuable, and detailed information on the type and location of crack path propagation was gained. [ABSTRACT FROM AUTHOR]

A high fidelity assessment of accumulative damage of woven fabric composite structures subjected to aggressive loadings is strongly reliant on the accurate characterization of the inherent multi-scale microstructures and the underlying deformation phenomena. Damage in composite sandwich and joint structures is characterized by the coexistence of discrete (delamination) and continuum damage (matrix cracking and intralaminar damage). A purely fracture mechanics-based or a purely continuum damage mechanics-based tool alone cannot effectively characterize the interaction between the discrete and continuum damage and their compounding effect that leads to the final rupture. In this paper, a hybrid discrete and continuum damage model is developed and numerically implemented within the LS-DYNA environment via a user-defined material model. The continuum damage progression and its associated stiffness degradation are predicted based on the constituent stress/strain and their associated failure criteria while the discrete delamination damage is captured via a cohesive interface model. A multi-scale computational framework is established to bridge the response and failure predictions at constituent, ply, and laminated composite level. The calculated constituent stress and strain are used in a mechanism-driven failure criterion to predict the failure mode, failure sequence, and the synergistic interaction that leads to global stiffness degradation and the final rupture. The use of the cohesive interface model can capture the complicated delamination zone without posing the self-similar crack growth condition. The unified depiction of the continuum and discrete damage via the damage mechanics theory provides a rational way to study the coupling effects between the in-plane and the out-of-plane failure modes. The applicability and accuracy of the damage models used in the hybrid dynamic failure prediction tool are demonstrated via its application to a circular plate and a composite hat stiffener subjected to shock and low velocity impact loading. The synergistic interaction between the continuum and discrete damage is explored via its application to a sandwich beam subjected to a low velocity impact. [ABSTRACT FROM AUTHOR]

This paper describes an investigation into the low velocity impact response on complex geometry sections taken from a pultruded glass/polyester product. The empirical impact behaviour of the system was evaluated using instrumented falling weight impact testing (IFWI) in conjunction with ultrasonic C-Scan, optical microscopy and thermal deply techniques to detect delamination, matrix cracking, and fibre breakage. [ABSTRACT FROM AUTHOR]