C. Navarro

Modelling of the adhesive layer in mixed ceramic/metal armours subjected to impact

Abstract The effect of the adhesive layer, used to bond ceramic tiles to a metallic plate, on the ballistic behaviour of ceramic/metal mixed armours is analysed mathematically and experimentally. Two types of adhesives, polyurethane (soft adhesive) and rubber-modified epoxy resin (rigid adhesive), and different thicknesses, are considered in the study. Numerical simulations were made of low calibre projectiles impacting on alumina tiles backed by an aluminium plate, using a commercial finite difference code. Full-scale tests were carried out to check the influence of the adhesive. An engineering model was also developed to provide a preliminary design tool taking account of the influence of the adhesive.

The effect of low temperatures on the intermediate and high velocity impact response of CFRPs

Abstract The influence of low temperature on the damage produced on CFRPs by intermediate and high velocity impacts is analyzed. Spherical projectiles were launched against different carbon fiber/epoxy laminates (tape and woven). Experimental tests were done at temperatures ranging from 25 to −150xa0°C. The extension of the damage was measured by C-Scan. Results show a clear dependence of damage on temperature, impact velocity and the type of the laminate.

Statistical analysis of the mechanical properties of composite materials

The Weibull statistic is currently used in designing mechanical components made of composite materials. This work presents useful formulae to describe the behaviour of the Weibull modulus estimator, which in turn may be described by means of a three-parameter Weibull distribution. Expressions for the parameters of this latter distribution, dependent on the sample size, are also given in the paper, so, the percentage points, published until now in tabular form, may be directly calculated. Empirical expressions are derived for determining the A-basis and the B-basis material properties as a function of the sample size.

Numerical modeling of the impact behavior of new particulate-loaded composite materials

Ballistic efficiency and cost are the main considerations in the design of lightweight armors. Metallic materials have the drawback of their high density. Mixed armors, of ceramic tiles backed by a metallic plate, are an efficient shield against low and medium caliber projectiles since they combine the light weight and high resistance of a ceramic with the ductility of a metal. The drawback is their high cost. The authors developed a new material composed of ceramic particles and a polymeric matrix. It fills the gap between metallic and ceramic materials and could be interesting for applications in which weight is not the primary concern and cost benefits are welcome. A model of the mechanical behavior of this composite is presented in this paper, implemented in a numerical code and validated by experimental results.

Analytical modelling of metallic circular plates subjected to impulsive loads

This paper presents an analytical approach to the dynamic response of metallic circular plates subjected to impulsive loads. It is based on the plate energy balance equation and assumes that the plate material behaves viscoplastically. The proposed method permits a consideration of the influence of the different terms of the kinetic energy and the plastic work of the plate. A yield criterion is proposed, which involves the coupled effect of the radial and circumferential internal force resultants. By applying the normality rule, the distribution of the bending moments and membrane forces inside the plate are computed. For model validation, its analytical predictions are compared with experimental results.

Static behavior of CFRPs at low temperatures

Abstract This paper summarizes the results of the tests to determine the effect of the low temperature on the mechanical behavior of carbon fiber reinforced epoxy laminates. Tensile and bending static tests were carried out on two laminate lay-ups (quasi-isotropic and cross-ply laminates), determining properties such as the mechanical strength, stiffness and strain to failure. The results show the changes in the mechanical behavior of this material at different test temperatures (20, −60 and −150xa0°C).

Statistical distribution of the estimator of Weibull modulus

where F is the probability of rupture under uniaxial tensile stress σ , m the shape parameter or Weibull modulus and σ0 the scale parameter. From the results of a limited number of tests and by applying standard statistical techniques (maximum likelihood, generalized regression, moments method, etc.), estimations of the parameters m and σ0 can be obtained. Obviously these estimation values are subject to uncertainties, so, for design purposes, it is necessary to calculate the appropriated confidence intervals of the estimators. The confidence interval of the Weibull modulus estimation, m, can be obtained from the percentage points, lα , of the variable m/m, defined as:

On the estimation of percentiles of the Weibull distribution

Advanced ceramics exhibit brittle behavior. The lack of ductility and the presence of flaws and defects of different sizes and orientations lead to scatter in failure strength. This variability depends also on the specimen size, stress distribution and stress state. The Weibull theory explains correctly this dependence [1], so the fracture strength of ceramic materials has been studied using the Weibull statistic [2–5] as recommended by ASTM Standards for reporting uniaxial strength data of these materials [6]. The Weibull statistic is also applicable to describe the scatter of the fracture toughness of steels in the ductile-brittle transition region, where failure occurs by cleavage [7–10]. The two-parameter Weibull distribution function is given by:

An analysis of the static and dynamic fracture behaviour of a pipeline steel

This work deals with the dependence of fracture behaviour on the strain rate of a commercial pipeline steel. Low-blow impact tests, using a Charpy pendulum setup, and conventional static fracture tests were carried out with this material. Experimental results showed that the material fracture toughness increases slightly with strain rate. Numerical analyses of all the experiments were also performed, using a micromechanical damage model that explains the influence of the strain rate on the fracture toughness. Particular attention was paid to the blunting process at the crack tip under dynamic conditions.

Bittella: a novel content distribution overlay based on bittorrent and social groups

This paper presents Bittella: a newsocial network for content distribution based on Peer-to-Peer technologies. It exploits the common interests of the users in order to create social groups based on an algorithm called Ranking Algorithm. On the other hand, Bittella is deployed over a semantic-search based and unstructured p2p network, in spite of this it uses Bittorrent-like download techniques in order to improve the download time. For this purpose, a new Bittorrent trackerless scheme is proposed.