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The mechanism of the material removal in an oblique cutting process such as drilling, the variation of the forces with time and their influence on accompanying damage, the optimal drill point geometry for minimizing the drilling forces and the subsequent damage are some of the directions in which the work in the machining of composite material is progressing. Four facet, eight-facet, Jo-point, inverted cone and special geometry are some of the widely used tool designs in drilling composite materials.

The cutting process consists of removing a layer of material from blank to obtain a component of the required shape and dimensions and with specified quality of surface finish. During the process of material removal there is a relative motion between the workpiece and cutting tool. Such a relative motion is produced by a combination of rotary and translatory movement either of the work pieces or of cutting tool or both. The relative motion depends on the type of cutting operation. The cutting operation carried out on any machine tool is based on the theory, which is the same for all processes. The importance of the cutting process may be further appreciated by the observation that nearly every device in use in our complex society has one or more machined surfaces or holes.

High speed machining is now recognized as one of the key manufacturing technologies for higher productivity and throughput. It is well known that the most effective way of achieving good quality holes while drilling fibre reinforced plastics is by reducing the thrust and torque.

Delamination at the exit of the hole is considered the most costly damage in the composite structure. This defect leads to a significant loss of the tensile strength and the stiffness of the composite part when it is loaded in compression, shear or fatigue loading.

International audience; Carbon fiber reinforced polymers (CFRPs) have found wide-ranging applications in numerous industrial fields such as aerospace, automotive, and shipping industries due to their excellent mechanical properties that lead to enhanced functional performance. In this paper, an experimental study on edge trimming of CFRP was done with various cutting conditions and different geometry of tools such as helical-, fluted-, and burr-type tools. The investigation involves the measurement of cutting forces for the different machining conditions and its effect on the surface quality of the trimmed edges. The modern cutting tools (router tools or burr tools) selected for machining CFRPs, have complex geometries in cutting edges and surfaces, and therefore a traditional method of direct tool wear evaluation is not applicable. An acoustic emission (AE) sensing was employed for on-line monitoring of the performance of router tools to determine the relationship between AE signal and length of machining for different kinds of geometry of tools. The investigation showed that the router tool with a flat cutting edge has better performance by generating lower cutting force and better surface finish with no delamination on trimmed edges. The mathematical modeling for the prediction of cutting forces was also done using Artificial Neural Network and Regression Analysis.

Carbon fibre reinforced plastics (CFRP) and titanium (Ti) stacks have been steadily replacing metals as choice for engineering materials in aerospace applications. Although materials can be manufactured separately and stacked together to attain a near-net shape, it still involves post processing operations such as trimming and drilling. In order to drill holes efficiently without defects (delamination, circularity, variation in hole diameter) in the CFRP/Ti stacks, it is essential to understand the machining behavior of stacks. An experimental study on the drilling of CFRP/Ti stacks was conducted using K20 carbide drill. The drilling characteristics were evaluated for drilling force and torque, delamination in CFRP, drilled-hole quality (hole diameter and circularity) and exit burr height in Ti. This paper describes an attempt made to maximize the hole quality parameters by employing multi-objective optimization using weighted sum method.

National audience; Des éprouvettes carbone/époxy pour évaluer la qualité d’une réparation en escalier sont usinées, réparées puis testées mécaniquement. L’usinage préalable à la phase de réparation est effectué par Jet d’Eau abrasif, sous forme de marches régulières de ratio 1/30, et dont la profondeur correspond à l’épaisseur d’un pli. Le relâchement des contraintes résiduelles observé pendant la phase d’usinage est quantifié grâce à la technique de mesure de champs par stéréovision, et simulé par EF. La plaque usinée est ensuite reconstruite par remplacement des plis usinés, et cocuit en autoclave. Le schéma de stratification de la partie réparée est décalé d’un pli par rapport à la zone initiale, de manière à restituer le transfert de charges. Des éprouvettes sont découpées dans les plaques réparées, et sollicitées en traction. Les résultats montrent que les charges à rupture obtenues atteignent 70 % des charges à rupture d’éprouvettes «vierges». L’analyse des fragments des éprouvettes démontre que la rupture de la zone de collage dans les interfaces 0/0 est à l’origine de la rupture finale de toutes les éprouvettes. Le croisement essais/calcul conduit à l’évaluation d’un critère de rupture en contrainte de cisaillement admissible pour l’interface zone parent/patch.

International audience; The mode of development of sandwich panels with cork core by the method infusion cannot prevent the resin from penetrating inside cork. This study characterizes white cork agglomerated before and after the development of sandwich panels to see the influence of the resin on the mechanical and hygrothermic characteristics of cork. Two types of cork are considered, they are characterized by their density of 270 kg/m3 (C270) and of 190 kg/m3 (C190). One notes that the modulus of rigidity of C270 cork is 20% larger than that of C190.The presence of resin increases also the module of shearing 47% for C270, 64% for C190. The hygrothermic analysis out of drying oven at 100°C shows a fall of significant mass in the first 7 hours. The analysis thermogravimetry at 220°C, the fall of significant mass is recorded in the first 5 minutes.DOI: http://dx.doi.org/10.5755/j01.mech.18.1.1278

International audience; ling of composite material causes damages of different natures. To avoid machining in composite material, it is proposed to include holes while moulding. In the literature, there is few works concern the behaviour analysis of specimens with moulded holes. All the literature works concern a specimens with moulded holes made of woven fabric. Moreover no literature is available on local behaviour of specimens around the hole. In this paper, several mechanical tests are carried out on specimens with quasi-isotropic stacking sequence made from UD prepreg with drilled holes and with moulded holes. To analyse the behaviour of the specimens around the hole, the strain field is measured using three-dimensional Digital Image Correlation technique (DIC). The pictures analysed given by the CCD camera reveal that, the damage mechanisms are different between the plates with drilled holes and those with moulded holes. The SEM observation have shown that, the fibre content and porosity content near the hole is around approximate to 8% higher compared to the values obtained far the hole. Moreover the tensile strength of the plates with moulded holes is 28% higher than with drilled holes.

International audience; no abstract

International audience; Drilling of CFRP/Al or CFRP/Ti, or AI/CFRP/Ti is a challenge to manufacturing engineers. Drilling of CFRP is manageable but the minute drill hits the Al or Ti, those hot and continuous chips destroy the hole. Few studies are carried out in this aspect; CFRP/Al is not reported till date. It is important to select right process parameters when drilling multimaterial stack since each material in the stack requires a different set of machining parameters. Hence, Drilling trials have been carried out in carbon-fibre reinforced plastics (CFRP)/aluminium (Grade 2024) stack without coolant, with plain carbide (K20) drills of various diameters to choose optimum process parameters. The parametric influences on thrust force, torque as well as surface finish were experimentally evaluated. The experimental results show that the quality of holes can be improved by proper selection of cutting parameters. This is substantiated by monitoring thrust force, torque, surface finish, circularity and hole diameter. For the CFRP, the circularity is found to be around 6 mu m at low feed rates, when the feed is increased the circularity increases to 25 mu m. The wear tests carried out show that, during first 30 holes, thrust force in CFRP undergoes a more important increase (90%) than thrust force of aluminium (6%).

International audience; This article presents the development and the characterization of composite material (laminate) containing natural jute fiber reinforcement. Thermal characterization of jute fiber reinforcement shows the influence of the temperature on the mechanical behavior of fiber. At 180°C the jute fabric loses 50% of its mechanical characteristics. The laminate obtained by a process known as infusion is polymerized at a temperature lower than that which affects the mechanical properties of dry fabric. The digital image correlation carried out on laminated jute/epoxy (warp and weft direction) under tensile test shows the presence of a considerable gradient of deformation. This gradient is explained by the variability related to the local voluminal change of jute fibers of one place to the other and the nature of the weaving of the jute fiber. The three-point bending tests show a significant dispersion of rupture stress. The thermomechanical tests carried out on samples in the two principal directions, show that the thermal coefficient of expansion in warp direction is 48% larger compared to the weft direction. The thermogravimetric test shows that this laminate absorbs up to 4% water mass after 8 h in a climatic chamber with 70% moisture content.

International audience; This paper is a continuation of our previous study [Mulle M, Collombet F, Olivier P, Grunevald Y-H. Assessment of cure residual strains through the thickness of carbon-epoxy laminates using FBGs, part I: elementary specimen. Compos Part A 2008. doi:10.1016/j.compositesa.2008.10.008] pertaining to the assessment of autoclave cure-induced strains through the thickness of carbon-epoxy laminates. In this first part, postulates and measurement procedures were established for cure of elementary specimens. Based on these, this study undertakes investigation on what are called technological specimens. These specimens are of the beam type and contain geometrical specificities which represent typical structural issues. In-plane process-induced strains were studied through the thickness of a thick reinforced zone using several optical fibre Bragg gratings (FBGs) sensors embedded at different levels of the ply stack. A non-uniform distribution of residual strains was detected. Once cured, the technological specimen was subjected to a heating test whose cycle was comparable to the cure cycle. Thermally induced strains were measured with the embedded FBGs. The values recorded were compared with those of cure-induced residual strains and FEM simulation. Discrepancies were observed that strongly suggest the possible influence of environmental effects and the need for the calculation to take into account the through-the-thickness variability of thermal properties.

This study concerns the central reinforced zone of a composite structural specimen. In order to estimate the strain distribution during a series of three and four-point bending tests, several optical fiber Bragg grating sensors have been embedded in various levels of the ply stack. Simultaneously, surface strain field measurements by 3-D digital image correlation are undertaken. Both techniques show a general linear distribution of longitudinal strains through the thickness of the thick zone but values are slightly different. A numerical model is developed and a test-calculation dialogue is carried out. The complementary information given by the two optical techniques for in-core and surface measurements reveals the importance of considering structural and edge effects.

The present paper deals with the use of optical fibers with Bragg gratings for the identification of process-properties relations for composite structures. This study relates a 7 year-experience in the field of process and mechanical characterization of composite parts. It highlights some major topics in terms of technological issues such as sensor integration, information discrimination and measurement accessibility. Results of process monitoring concerning autoclave cure and filament winding are shown through which initial states are identified. Finally, structural tests of instrumented parts are presented, showing the efficiency of the embedded sensors to allow test-calculation dialogues and thus properties identification.

Advanced optical measurement techniques are used to analyze the mechanical behavior of a composite structure submitted to bending tests. This is carried out by considering a beam type specimen which presents design singularities such as important thicknesses, ply drop off zones and a reinforced zone. This study concerns the central reinforced zone of the specimen. In order to estimate the strain distribution during a series of 3 and 4-point bending tests, several optical fiber Bragg grating sensors have been embedded in various levels of the ply stack. Simultaneously, surface strain field measurements by three dimensional digital image correlations are undertaken. Both techniques show a general linear distribution of longitudinal strains through the thickness of the thick zone but values are slightly different. A numerical model is developed and a test-calculation dialogue is carried out. The complementary information gathered by the two optical techniques for in-core and surface measurements reveals the importance of considering structural and edge effects.

International audience; In this paper, high speed trimming of a multidirectional CFRP using unused and used burr tools is considered in order to investigate the influence of the machining parameters (e.g., feed speed, cutting speed and cutting distance) on the cutting forces, machining temperature, and the machined surface quality. To estimate the effect of the tools’ wear and cutting parameters on the cutting forces and surface roughness, a statistical method (ANOVA) has been used. When considering an unused tool, the recorded temperatures were below the glass transition temperature of the composite material (Tg). In the case of used tools, these temperatures were mostly higher than the Tg. Furthermore, SEM observations of the machined surface showed damaged areas. These areas were wider when the cutting distance increased. Statistical analyses have shown that the machining parameters have a significant influence on the variation of the machined surface quality and the cutting forces.

Titanium and its alloys are attractive materials due to their unique high strength-weight ratio that is maintained at elevated temperatures. Ti-6Al-4V alloy are notorious for their poor thermal properties and are classified as difficult-to-machine materials. Drilling is an important machining process since it is involved in nearly all titanium applications. It is desirable to develop optimised drilling processes for Ti and/or improve the hole characteristics such as hole diameter, circularity and exit burr of currently available processes. Due to the low machinability of the alloy under study, selecting the machining conditions and parameters is crucial. The hole quality, thrust force, torque and exit burr were evaluated at various spindle speeds, feed rates combinations. The optimised parameter is chosen using the multi-objective weighted sum optimisation technique. The tool life for the optimised parameter was studied and the life of the carbide drill was observed to be 140 holes in drilling of titanium.

Experimental and analytical studies were carried out regarding delamination at the hole exit during the drilling of composite structures made of a recent semi-preg called HexFIT® developed by Hexcel composites for resin film infusion (RFI). The plates studied were manufactured in autoclave and in oven. The experimental analysis showed that delamination at the hole exit is influenced by the machining parameters on the one hand and by the composite plate manufacturing process on the other. Following this, empirical models were developed to predict this delamination for each type of manufacturing process and according to the machining parameters. These models gave satisfactory results compared to the experimental values. The maximum relative variation obtained between the values given by the models and those measured by experiments was about 9%. From these empirical models, the critical machining parameters were determined in order to avoid delamination at the hole exit. Critical machining parameters were experimentally tested and their evaluation has shown acceptable results.

International audience; In recent days, automobile and construction industries are focus on the light weight, environmental friendly materials with good mechanical properties. Glass fiber reinforced composites have excellent specific properties and are widely used because of reduced mass. However the manufacturing of glass fibers and end of life disposal are the major problem to the environment. To overcome these problems, natural fibers are used to manufacture composites. Flax is the one of the naturally available fiber having good mechanical properties than other natural fibers. It needs to be pointed out that most of research effort about the flax fiber reinforced composites focuses on the manufacturing techniques and primary mechanical properties and not on the secondary properties like sound absorption and vibration damping. In this paper, sound absorption and vibration damping properties of flax fiber reinforced composites were characterized and compared with the glass fiber reinforced composites. It was experimentally observed that the sound absorption coefficient of flax fiber reinforced composites has 21.42% & 25% higher than that of glass fiber reinforced composites at higher frequency level (2000 Hz) and lower frequency level (100 Hz). From the vibration study it was observed that the flax fiber reinforced composites have 51.03% higher vibration damping than the glass fiber reinforced composites. The specific flexural strength and specific flexural modulus for flax fiber reinforced composites also good. These results suggest that the flax fiber reinforced composites could be a viable candidate for applications which need good sound and vibration properties. Keywords: flax; sound absorption; vibration damping

International audience; The main objective of this paper is the evaluation of the possibility of using a gypsum mortar reinforced with doum palm fibers as thermal insulators in building material. Several composite configurations with three sizes and five-weight ratios (from 0.5% to 2.5%) of doum palm fiber were prepared for mechanical and thermo-physical characterization. Generally, natural fibers are affected by the alkali environment of gypsum mortar. To overcome this problem, doum palm fibers were treated with a NaOH solution of 1% concentration to enhance their resistance against chemical degradation. Chemical treatment of fiber removes some hemicellulose and lignin and tends to make the fiber more homogenous, which enhances fiber–matrix interfacial properties. In addition, the mechanical properties of the specimens were tested after 7 days, 28 days and 1 year of curing in normal conditions. The obtained results show an improvement in the mechanical performance of composites reinforced with treated fibers. In fact, better results were obtained for gypsum mortar reinforced with treated reinforcement seive2 for the value of 1% of fibers. It was also noticed that NaOH treatment has an influence on the thermal conductivities. In fact, the use of treated doum palm fibers as reinforcement in gypsum mortar seems to be an alternative natural material to be used as a thermal insulator material.

International audience; no abstract