A study on fiber-arrangement close to the root of a sharp notch, for short fiber-reinforced thermoplastics

An approach, which aims at the morphological characterization near the sharp notch of specimens, has been developed for Short Fiber-Reinforced Thermoplastics. This work is directly related to the fa- tigue behavior of such materials, since the early stages of the cyclic damage are strictly influenced by the local microstructure at the stress concentration sites. Therefore, a comprehensive description of fi- bers\u2019 arrangement is needed in order to proceed with a modeling activity for the lifetime duration esti- mation. To this end, a semi-automatic tool has been developed, which is capable of evaluating fiber- arrangements through statistical descriptors, after submitting 2D pictures of the notch-tip area. Particu- larly, the attention was focused onto the nearest neighbor distance distribution function and onto a new formulation, which gives information about the level of the fiber-clustering phenomenon. On this basis, the repeatability of results has been evaluated with the goal of stating whether such information can be inherited by lifetime estimating models

fatigue damage and stiffness evolution in composite laminates a damage based framework

Abstract A damage-based design procedure has been developed by the authors to predict the damage evolution and the stiffness degradation in polymeric composite laminates under fatigue loading. For a safe and reliable design against fatigue degradation and failure, the initiation of the main damage mechanisms (off-axis cracks, delamination and fiber failure) as well as their evolution are considered and suitable models are proposed for the quantitative assessment of the lifetime associated to each mechanism. In parallel, the stiffness degradation deriving from the damage evolution over the fatigue life is properly described. After the illustration of the overall damage-based strategy, the paper discusses in details the analysis and modelling of the off-axis crack initiation and propagation. The initiation of cracks in the off axis plies has been proved to be the consequence of a damage process occurring at the microscopic scale since the early stages of fatigue. On this basis, crack initiation prediction is based on the use of local stress parameters: Local Hydrostatic Stress, LHS, and Local Maximum Principal Stress, LMPS, depending on the local degree of multiaxiality of the stress state and accounting for the statistical distribution of the local laminate strength. The propagation phase is then quantified by using a conventional fracture mechanics approach. The model has been implemented in a Matlab procedure for the quantitative evaluation of the crack density in each ply of a laminate during its entire fatigue life. The knowledge of the crack density trend allows the description of the laminate stiffness evolution taking advantage of another model recently developed by the authors, valid for a generic laminate configuration and accounting for the interaction between cracks in the neighbouring plies

Low- to high-velocity frictional properties of the clay-rich gouges from the slipping zone of the 1963 Vaiont slide, northern Italy

The final slip of about 450 m at about 30 m/s of the 1963 Vaiont landslide (Italy) was preceded by >3 year long creeping phase which was localized in centimeter-thick clay-rich layers (60–70% smectites, 20–30% calcite and quartz). Here we investigate the frictional properties of the clay-rich layers under similar deformation conditions as during the landslide: 1–5 MPa normal stress, 2 × 10^(−7) to 1.31 m/s slip rate and displacements up to 34 m. Experiments were performed at room humidity and wet conditions with biaxial, torsion and rotary shear apparatus. The clay-rich gouge was velocity-independent to velocity-weakening in both room humidity and wet conditions. In room humidity experiments, the coefficient of friction decreased from 0.47 at v 0.70 m/s: full lubrication results from the formation of a continuous water film in the gouge. The Vaiont landslide occurred under wet to saturated conditions. The unstable behavior of the landslide is explained by the velocity-weakening behavior of the Vaiont clay-rich gouges. The formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero, even without invoking the activation of thermal pressurization. This explains the extraordinary high velocity achieved by the slide during the final collapse

MIXED MODES INTERLAMINAR FRACTURE TOUGHNESS OF CFRP LAMINATES TOUGHENED WITH CNF INTERLAYER

In the present paper, the influence of carbon nanofiber on interlaminar fracture toughness of CFRP investigated using MMB(Mixed Mode Bending) tests. Vapor grown carbon fiber VGCF and VGCF-S, and multi-walled carbon nanotube MWNT-7 has been employed for the toughener of the interlayer on the CFRP laminates. In order to evaluate the fracture toughness and mixed mode ratio of it, double cantilever beam (DCB) tests, end notched fracture (ENF) tests and mixed mode bending (MMB) tests have been carried out. Boundary element analysis was applied to the CFRP model to compute the interlaminar fracture toughness, where extrapolation method was used to determine the fracture toughness and mixed mode ratio. The interlaminar fracture toughness and mixed mode ratio can be extrapolated by stress distribution in the vicinity of the crack tip of the CFRP laminate. It was found that the interlaminar fracture toughness of the CFRP laminates was improved inserting the interlayer made by carbon nanofiber especially in the region where shear mode deformation is dominant.ArticleACTA MECHANICA SOLIDA SINICA. 25(3):321-330 (2012)journal articl

Modeling of Polymer Clay Nanocomposite for a Multiscale Approach

The mechanical property enhancement of polymer reinforced with nano-thin clay platelets (of high aspect ratio) is associated with a high polymer-filler interfacial area per unit volume. The ideal case of fully separated (exfoliated) platelets is generally difficult to achieve in practice: a typical nanocomposite also contains multilayer stacks of intercalated platelets. Here we use numerical modelling to investigate how the platelet properties affect the overall mechanical properties. The configuration of platelets is modelled using a statistical interpretation of the Representative Volume Element (RVE) approach, in which an ensemble of "sample" heterogeneous material is generated (with periodic boundary conditions). A simple Monte Carlo algorithm is used to place non-intersecting platelets in the RVE according to a specified set of statistical distributions. The effective stiffness of the platelet-matrix system is determined by measuring the stress (using standard Finite Element analysis) produced as a result of applying a small deformation to the boundaries, and averaging over the entire statistical ensemble. In this work we determine the way in which the platelet properties (curvature, filling fraction, stiffness, aspect ratio) and the number of layers in the stack affect the overall stiffness enhancement of the nanocomposite. Thus, we bridge the gap between behaviour on the macroscopic scale with that on the scale of the nano-reinforcement, forming part of a multi-scale modelling framework.Comment: 39 pages, 19 figure

Special Issue: DIRECTIONS IN DAMAGE AND DURABILITY OF COMPOSITE MATERIALS

This Special Issue contains a selection of the papers presented at the symposium Directions in Damage and Durability of Composite Materials, held at Palazzetto Foscari (Venice) in May 2006. The symposium was organised with the aim to gather together, by invitation, a representative group of international researchers, both from industry and academia, to exchange ideas on the current status and future directions of the composites durability field, in an informal setting surrounded by the beauty of Venice. The symposium was the right occasion to celebrate the 60th birthday of a great friend, Prof. Ramesh Talreja (Texas A&M University), and for properly recognising his achievements and invaluable contribution to the research in the field of fatigue and durability of composite structures. The three days of symposium was very effective, with more than 40 participants and 30 talks. The papers selected for publication provide a good indication of the topics discussed during the symposium, with particular reference to the analysis, characterization and modelling of damage mechanics, both at macroscopic and microscopic scale, fatigue, fracture, impact, and environmental effects. I take the occasion of this guest editorial to express my sincere gratitude to my co-chairmen, Prof. David Allen, (University of Nebraska Lincoln), Prof. J.N. Reddy (Texas A&M University) and Prof. Enzo Siviero (University IUAV of Venice) for their help in organising the event, to all the authors for their highly qualified contribution to this Special Issue and to all the colleagues who agreed to serve as reviewers for their help and efforts in ensuring a strong collection of papers. Special thanks are also due to Dr. Ing. Ugo Dibennardo (head of Veneto regional Department of ANAS SpA) for the logistic support to the event and the wonderful venue of Palazzetto Foscari. To conclude, I warmly thank DIAB and the European Office of Aerospace Research and Development, Air Force Office of Scientific Research, United States Air Force Research Laboratory. Their support to the symposium is greatly acknowledged