P.N.B. Reis

Analysis of fatigue and damage in glass-fibre-reinforced polypropylene composite materials

Abstract This paper concerns fatigue studies of polypropylene/glass-fibre thermoplastic composites produced from a bi-directional woven cloth of co-mingled E-glass fibres and polypropylene fibres with a fibre volume fraction V f of 0.338. The effect of lay-up design and load conditions on fatigue performance were investigated. The S-N curves, the rise in the temperature of the specimens, and the loss of stiffness during the tests, are discussed. Fatigue tests were performed in controlled displacement mode and in an imposed stress range. Similar results were obtained for both load conditions. The loss of stiffness was used as a damage parameter and related to the rise of temperature. The results show that the damage parameter E present a nearly linear relationship with the rise in temperature. A small deviation was probably caused by the stress release observed in the first period of fatigue life. ©

Residual Strength after Low Velocity Impact in Carbon-Epoxy Laminates

The aim of present work is to study the influence of low energy impacts on residual strength of carbon-epoxy laminates. Experimental tests were performed on [0,90,0,90]2s and [0,90]8 laminates using a drop weight-testing machine. The influence of the laminate stacking sequence is analysed under 1.5 J, 2 J, 2.5 J and 3 J impact energies, corresponding to a 0.91 ms-1, 1.05 ms-1, 1.18 ms-1 and 1.29 ms-1 of impact velocity, respectively. The impacted plates were inspected by CScan to evaluate the size, shape and position of the delaminations through the thickness of the plate. The same plates were inspected by C-Scan before the impact, to evaluate the eventual presence of defects produced during the manufacturing process. The residual flexural strength showed that the [0,90,0,90]2s laminates have better performance than the [0,90]8 ones. The explanation is related with the lower flexural stiffness of the antisymmetric lay-up relatively to the symmetric one.

Effect of Interlayer Delamination on Mechanical Behavior of Carbon/Epoxy Laminates

This article presents the results of a current study on the influence of interlayer delaminations on the static and fatigue behavior of composite laminates. The composite was manufactured by a vacuum molding method using 12 balanced bi-directional carbon fiber layers and epoxy resin. Delaminations with different length were artificially introduced. The specimens with dog bone shape were cut from the original plates having 3 mm thickness and fiber weight fraction of 0.66. Static tests were performed in order to study the influence of delamination size on the laminate stiffness and strength. Complementary finite element analysis was carried out showing the influence of angular misalignments of fiber/matrix delaminations on the laminate stiffness. Fatigue tests were performed in load control for R = 0.05 and R = —1, with a loading frequency of 10 Hz, at room temperature. The artificial interlayer delaminations have a negligible influence on the fatigue strength for tensile cycle loadings, but produce significant decreases in strength for R = —1 fatigue loadings.

Static and fatigue behaviour of glass-fibre-reinforced polypropylene composites

Abstract This paper is concerned with fatigue of polypropylene/glass-fibre thermoplastic composites produced from a bi-directional woven cloth mixture of E glass fibres and polypropylene fibres. The latter becomes the matrix after the application of heat and pressure. This composite was manufactured with a fibre volume fraction Vf of 0.338. The effect of layer design on the static and fatigue performance was investigated. The S–N curves, the rise in the temperature of the specimens during the tests and the loss of stiffness, were obtained and discussed. The loss of stiffness was related to the rise of temperature and stress release observed in the material. The effect of load rate on the static properties was also studied and discussed accordingly.

Edite - A Natural Language Interface to Databases A new dimension for an old approach

This article presents the Edite system, a Natural Language Interface for Databases (NLIDB), that tries to explore the advantages of joining natural language processing with the expressiveness of graphical interfaces. In order to guarantee a permanent adaptation of this type of solution to a dynamic domain one should consider two critical fundamental factors: extensibility and portability.

Impact response of sandwich composites with nano-enhanced epoxy resin

Present work intends to study the improvement of impact performance on sandwich composites by the addition of nanoclays. For this purpose, nanoclays Cloisite 30B were previously subjected to a silane treatment in order to improve their dispersion and interface adhesion. Different incident impact energy levels were used and, for both sandwiches, the maximum load, displacement or elastic recuperation shows to be very dependent of the impact energy. Mathematical relationships are proposed to estimate the maximum impact force and displacement, based on the total impact energy and impact bending stiffness. Finally, sandwiches enhanced by nanoclays presented higher maximum impact loads, lower displacements, the best performance in terms of elastic recuperation and maximum residual flexural strength.

Effect of the Surface Preparation on PP Reinforced Glass Fiber Adhesive Lap Joints Strength

This article is concerned with the influence of surface treatment on the strength of single lap joints. The tensile tests were carried out in order to find the surface treatments which maximize joint strength using polypropylene reinforced glass fiber as adherents. The maximum shear strength was obtained with the trichlorethylene plus primer treatment when using a cyanoacrylate adhesive. The pretreatment effectiveness was evaluated in terms of environmental durability and an important decrease in static strength of adhesive joints occurred when they were immersed in water.

Influence of the specimen thickness on low velocity impact behavior of composites

Abstract In this work, the influence of specimen thickness on low velocity impact behavior of carbon-epoxy composite laminates is studied. Plates with different thicknesses were tested under low velocity impact using a hemispherical impactor. The internal damage was mainly constituted by delaminations which were evaluated through the inspection of the impacted plates by the ultrasonic C-scan technique. It was observed that delaminations increase with plate thickness. In order to better understand the physical phenomenon explaining this result, a progressive damage model was used to simulate composites behavior under low velocity impact. In this context, a three-dimensional numerical analysis considering interface finite elements, including a cohesive mixed-mode damage model, which allows simulating delaminations onset and growth between layers, was performed. Good agreement was obtained between experimental and numerical analysis, which validated the proposed procedure. In addition, the proposed numerical methodology allowed identification of physical phenomena related to the influence of plate thickness on delamination size.

Determination of Elastic Properties by Resonant Technique: A Sensitivity Analysis

The in-plane elastic properties of materials can be determined using an experimental-numerical procedure based on the resonant frequencies of thin beams and plates. The objective of this paper is to study the accuracy of the material constants obtained with this technique. The procedure is presented and the parameters affecting its accuracy are identified. Specimens of epoxy reinforced with carbon fibers and 6082-T6 aluminum alloy were produced and experimental work was developed to obtain resonant frequencies in free-free boundary conditions. A numerical procedure based on FEM was developed replicating the experimental procedure and was used for a sensitivity analysis on the numerical and physical parameters. A great sensitivity relative to geometry was found, which emphasizes the need for ideally shaped specimens and accurate measurements. The influence of elastic properties on resonant frequencies is comparatively lower and varies quite considerably with geometry. The accuracy of experimental frequencies and specific mass was found to have a great impact on material constants.

Fatigue and creep in titanium grade 2

The titanium grade 2 is currently used in high-pressure heat exchanges, piping systems in sea water desalination plants, offshore technology and chemical plants. In many of the applications fatigue and/or creep loading conditions occur. In this paper are presented the results of the mechanical behaviour, low cycle fatigue parameters and fatigue crack propagation at room temperature and at 400°C. The influence of temperature, frequency and the shape of wave loading was studied. The creep behaviour was also studied. Curves of the strain and the stress against the time were obtained to 400°C and 500°C. Also the Larson-Miller parameter was determined. Tests to the creep crack propagation study were carried out. Crack propagation in the creep tests conditions used was not verified.