Applied Mechanics and Materials Vol. 146

Abstract: Thin-walled textile-reinforced composite parts possess excellent properties, including lightweight, high specific strength, internal torque and moment resistance which offer opportunities for applications in mass transit and ground transportation. In particular, the composite material is widely used in aerospace and aircraft structure. In order to estimate accurately the parameters of the constitutive law of woven fabric composite, it is recommended to canvass multi-scale modeling approaches: meso, micro and macro. In the present investigation, based on the experimental results established by carrying out observations by Scanning electron microscope (SEM), we developed a micro-scale FEM model of carbon-fiber reinforced thermoplastic using a commercial software ABAQUS. From the SEM cartography, one identified two types of representative volume elementary (RVE): periodic and random distribution of micro-fibers in the yarn. Referring to homogenization method and by applying the limits conditions to the RVE, we have extracted the coefficients of the rigidity matrix of the studied composites. In the last part of this work, we compare the results obtained by random and periodic RVE model of carbon/PPS and we compute the relative error assuming that random model gives the right value.

Abstract: In this study, we present the manufacturing process of two new composites materials in the form of long fibers of polylactic-acid (PLA) or polypropylene (PP), reinforced by cellulose whiskers micro-fibers loads. In order to evaluate the mechanical properties of these advanced materials, a several uniaxial tensile tests were carried out. The PP and the PLA have initially been spinning without the addition of cellulose whiskers micro-fibers. In order to study the effects of cellulose whiskers micro-fibers reinforcements in the Mechanical behavior of the PLA and PP filaments, we determinate the proprieties of these advanced material from the tensile results. For the PP composite filaments material case, the whiskers reinforcement increases Young's modulus and failure resistance, but it reduces the limit strength failure. For the PLA composites the addition of 1% wt of cellulose whiskers from the total volume fraction of the material, increase the Young’s modulus more than 50% and a decrease of the failure resistance and the limit strength of composite. The obtained composites fibers are very rigid and brittle. What follows, that the addition of cellulose whiskers micro fibers in PP matrix, provides mechanical properties more convenient compared to the PLA matrix.

Abstract: The specific objectives of this study are: verifying the applicability of the proposed method of reinforcement of the beams by oblique connecting rods confined by a metallic embedded grid material to improve the behavior of concrete from the point of view strength to shear force, and confronting the experimental results acquired with empirical formulas developed by other researches. In this study, experimental investigations were performed to evaluate performance characteristics such as flexural ductility, resistance to shear force and load capacity. The experimental and numerical studies in the present work represent a promising revelation regarding the effectiveness of the proposed reinforcement process by an oblique connecting rods confined by a embedded metallic grid material laid out in the zone of influence of the shear force tilted to 45°. The confrontation of the experimental and theoretical results shows a satisfactory agreement.

Abstract: The use of the lightweight concrete in the so wide field of construction currently makes important great strides considering their threshold of mechanical performances. The use of the lightweight materials such as the hemp, the sawdust, the modified polymers increase their applications. This study which wants to be an innovation in the field them construction materials consists in proposing an new approach of realization of polypropylene grains coated to thwart the lightweight phenomenon of increase of the grains in the matrix concrete of the cylinder. This technique makes it possible in addition to ensure the homogeneity of the matrix concrete and to thus increase the mechanical resistances. This study comprises a packed experimental investigation and a mathematical modeling of the thermal phenomena. The obtained results put forward the advantages of this technique which fulfil the requirements of sustainable development.

Abstract: The aim of this work is the study of the long term behaviour of a laminated composite made in glass/polyester and aged in seawater at various temperatures. The results obtained show that the immersion in seawater causes ageing of the composite material. This was reflected first by a change in color of the material depending on the temperature of the immersion and the gel-coat loses its brightness. The infrared spectroscopy has confirmed activation of chemical degradation (hydrolysis of the resin and of the fibre/matrix interface, fibre degradation). This chemical degradation could be the cause of decrease in strength and ductility detected by analysing the behaviour of immersed testing specimens. It could also be the cause of the increase of absorption rate recorded for immersions exceeding 22 days.

Abstract: In this experimental study, we are interested in local fiber wavy chips derived from waste machining steel parts. This work has focused on studying the mechanical behavior of reinforced concrete, with this type of fiber, in direct tensile. Direct tensile tests were carried out on samples in free weights section and square (100x100) mm². This test involves the design and the implementation of special mounting specimens on the tensile machine type Ibertest. Five (05) fibers percentages were retained in (W = 0.5%. W = 0.8%, W = 1%, W = 1.2%, W = 1.5% with W: volume fraction of added fiber) and two (02) concrete witness whose report on gravel sand is equal to: S / G = 0.8 and S / G = 1. The fibers have been characterized to the strength and tear by the tensile test. The interest lies in optimizing the fiber length and the number of undulations to use in a cement matrix, which will improve the mechanical properties especially tensile strength and post-cracking behavior. The comparison of different results obtained in direct tension on different percentages of fiber, as well as two reports showed that the fibers have conferred a significant ductility to the material after cracking of concrete for different percentages of fiber and a strength for improving the S / G = 0.8.

Abstract: The aim of this work is to study the influence of artificial defect and mean stress on fatigue strength under torsion loading. Spherical artificial defects have been machined at the surface of gauge length of fatigue samples. Experimental investigations conducted on both defective and defect free materials. The crack initiation mechanisms have been identified based on several observations on Scanning Electron Microscope (SEM) at different stage of fatigue life. It is observed that the defect free material subjected to torsion loading allows relatively earlier initiation. Experimental results show that mean shear stress has no effect on fatigue behaviour under torsion loading. It is also concluded that defects are much more damaging in high cycle fatigue regime than in low cycle fatigue one.

Abstract: The control of residual stress is crucial in ensuring the integrity of engineering components and Laser Shock Peening (LSP) process can be used to good effect to introduce the beneficial compressive residual stress levels required. It is, however, difficult to use normal laser peening control systems to establish the ideal peening conditions that will result in the best component performance. This paper presents results from a study to optimise the laser peening parameters for a typical titanium super alloy used in high performance turbine blade by investigating how the main peening process parameters influence residual stress profiles resulted by numerical simulations. Statistical Design of Experiments (DoE) was used to limit the number of experiments required for optimisation to be possible. Using this technique and numerical depth profiling methods for residual stress analysis, the maximum compressive residual stresses in Ti-6Al-4V were measured for a range of peening conditions. The results of the detailed process characterisation investigations have shown that, by using careful DoE, it is possible to fully optimise the laser shock peening process to obtain greater benefits than would be possible with traditional control processes.

Abstract: t is well-established that fatigue crack growth process is one of the main process which can produce failure of structures and mechanical components. The aim of this work is to develop a model predicting an updating inspection time model for structural fatigue crack growth life based on updating reliability analysis taking into account the additional information generated by the previous inspection results. First order reliability method (FORM) and Surface response method are used to evaluate the reliability. The uncertainties such as material parameters and geometrical parameters which affect the lifespan of the structure were regarded as random variables. Updating reliability assessment based on Bayesian approach was introduced to determine the updating inspection time for target reliability. The method was illustrated through an application to a plate with an emergent rectilinear crack loaded with a constant amplitude cyclic stress. The results of the application are in a good agreement with the physical results and show that the proposed method is proved to be feasible and applicable in the general complex fatigue loading and able to give accurate updating framework for scheduling inspections. Furthermore the proposed approach leads to determine the optimal Inspection time strategy based upon cost-minimization by considering a single type or multiple types of inspections.