170,290 research outputs found
Enhancement of a new methodology based on the impulse excitation technique for the nondestructive determination of local material properties in composite laminates
No full textA new approach for the nondestructive determination of the elastic properties of composite laminates is presented. The approach represents an improvement of a recently published experimental methodology based on the Impulse Excitation Technique, which allows nondestructively assessing local elastic properties of composite laminates by isolating a region of interest through a proper clamping system. Different measures of the first resonant frequency are obtained by rotating the clamping system with respect to the material orientation. Here, in order to increase the robustness of the inverse problem, which determines the elastic properties from the measured resonant frequencies, information related to the modal shape is retained by considering the effect of an additional concentrated mass on the first resonant frequency. According to the modal shape and the position of the mass, different values of the first resonant frequency are obtained. Here, two positions of the additional mass, i.e., two values of the resonant frequency in addition to the unloaded frequency value, are considered for each material orientation. A Rayleigh–Ritz formulation based on higher order theory is adopted to compute the first resonant frequency of the clamped plate with concentrated mass. The elastic properties are finally determined through an optimization problem that minimizes the discrepancy on the frequency reference values. The proposed approach is validated on several materials taken from the literature. Finally, advantages and possible limitations are discussed
Residual elastic response in damaged woven laminates through local Impulse Excitation Technique
No full textIn this work, the assessment of the residual elastic response in damaged woven laminates is addressed through an innovative nondestructive technique. Based on the Impulse Excitation Technique (IET), the goal is to determine the local variation of the elastic properties through local vibrational tests. In particular, by non-destructively clamping its extremities, the vibrational response of the region of interest can be isolated and a vibrational mode can be excited, which depends only on the material properties of the inspected region. In presence of damage, the local degradation of the elastic response can be then correlated to the decrement of the first resonant frequency. The technique is applied to glass-fibre/epoxy laminates damaged by impact and its sensitivity to the size of the inspected region is investigated through three clamping devices of different dimensions. For validation, tensile tests are performed on specimens cut from the impacted plates, where the axial deformation is punctually measured through optic fibre glued along the specimen axis. Results show that the residual elastic properties assessed with the proposed technique are in very good agreement with those measured through the optic fibre, thus proving the effectiveness of the methodology
Experimental and Numerical Investigation of a Lattice Structure for Energy Absorption: Application to the Design of an Automotive Crash Absorber
No full textIn this work, an experimental and numerical analysis of a lattice structure for energy absorption was carried out. The goal was to identify the most influencing parameters of the unit cell on the crushing performances of the structure, thus guiding the design of energy absorbers. Two full factorial plans of compression tests on cubic specimens of carbon nylon produced by fused deposition modeling (FDM) were performed. The factors were the beam diameter and the number of unit cells. In the first factorial plan, the specimen volume is constant and the dimensions of the unit cell are varied, while the second factorial plan assumes a constant size of the unit cell and the volume changes in accordance with their number. The results showed that the specific energy absorption increases with the diameter of the beam and decreases with the size of the unit cell. Based on these results, a crash absorber for the segment C vehicle was designed and compared with the standard component of the vehicle made of steel. In addition to a mass reduction of 25%, the improved crushing performances of the lattice structure are shown by the very smooth force-displacement curve with limited peaks and valleys
Resonance Frequency as an Indicator of the Damage in Carbon Composite Plates: Analysis on Composites Prepared with Conventional and Sustainable Resins Subjected to Impact Tests
No full textThis paper experimentally investigates the impact response of composite laminates made with conventional and bio-based epoxy resin. Drop tower impact tests were conducted at varying energy levels, including repeated low-energy impacts, to evaluate perforation resistance. The laminates’ residual strength and damage tolerance were assessed using the Damage Index (DI) and by analysing the resonance frequency variations through the Impulse Excitation Technique (IET). The study demonstrates a strong correlation between the DI and the resonance frequencies of the specimens, suggesting that IET can effectively track damage progression in composite laminates. Bio-based resin laminates exhibited higher energy absorption at perforation and lower damage progression during repeated impacts due to the higher ductility of the resin. This method of using resonance frequencies to assess impact damage progression directly in composite laminates throughout the IET technique has not been previously reported in the literature
C. Thomasset et D. Boursier (sous la dir. de), Interpréter le droit : le sens, l'interprète, la machine
No full textC. Thomasset et D. Boursier (sous la dir. de), Interpréter le droit : le sens, l'interprète, la machine. In: Revue internationale de droit comparé. Vol. 50 N°3, Juillet-septembre 1998. pp. 986-988
C. Thomasset et D. Boursier (sous la dir. de), Interpréter le droit : le sens, l'interprète, la machine
No full textC. Thomasset et D. Boursier (sous la dir. de), Interpréter le droit : le sens, l'interprète, la machine. In: Revue internationale de droit comparé. Vol. 50 N°3, Juillet-septembre 1998. pp. 986-988
Residual properties in damaged laminated composites through nondestructive testing: A review
No full textThe development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation approaches, i.e., the residual strength in pres-ence of cracks, are not straightforwardly enforceable in composites. The nonhomogeneous nature of such materials gives rise to several failure mechanisms and, therefore, the definition of an ulti-mate load carrying capacity is ambiguous. Nondestructive techniques are thus increasingly re-quired, where the damage severity is quantified not only in terms of damage extension, but also in terms of material response of the damaged region. Based on different approaches, many nonde-structive techniques have been proposed in the literature, which are able to provide a quantitative description of the material state. In the present paper, a review of such nondestructive techniques for laminated composites is presented. The main objective is to analyze the damage indexes related to each method and to point out their significance with respect to the residual mechanical perfor-mances, as a result of the working principle of each retained technique. A possible guide for future research on this subject is thus outlined
Blunt notch effect on the fatigue response up to 109 cycles of selective laser melting Ti6Al4V specimens
No full textIn this paper, the influence of a blunt notch on the VHCF response of SLM Ti6Al4V specimens is investigated. Ultrasonic fully reversed tension–compression tests up to 109 cycles were carried out on unnotched specimens and specimens with a blunt notch. Unnotched specimens show a slightly larger fatigue response, with limited differences. All fatigue failures originated from defects, which are bigger in unnotched specimens, mainly due to the different risk volume of the tested specimens and the related size effect. Interactions between notch, stress gradient, and defect size distribution must be considered to properly assess the influence of notch on the VHCF response
Epoxy and Bio-Based Epoxy Carbon Fiber Twill Composites: Comparison of the Quasi-Static Properties
No full textIn recent years, interest in sustainability has significantly increased in many industrial sectors. Sustainability can be achieved with both lightweight design and eco-friendly manufacturing processes. For example, concerns on the use of thermoset composite materials, with a lightweight design and a high specific strength, have arisen, since thermoset resins are not fully recyclable and are mainly petrol based. A possible solution to this issue is the replacement of the thermoset matrix with a recyclable or renewable matrix, such as bio-based resin. However, the mechanical properties of composites made with bio-based resin should be carefully experimentally assessed to guarantee a safe design and the structural integrity of the components. In this work, the quasi-static mechanical properties of composite specimens (eight layers of carbon fiber fabric) made with commercially available epoxy and a bio-based epoxy resins (31% bio content) are compared. Tensile tests on the investigated resins and tensile, compression, shear and flexural tests have been carried out on composite laminates manufactured with the two investigated resins. A finite element model has been calibrated in the LS-Dyna environment using the experimentally assessed mechanical properties. The experimental results have proven that the two composites showed similar quasi-static properties, proving that bio-based composite materials can be reliably employed as a substitute for epoxy resins without affecting the structural integrity of the component but lowering their carbon footprint
Ultrasonic fully reversed axial tests for exploring the very high cycle fatigue of composite materials
No full textIn the present work, the feasibility of axial ultrasonic tests for exploring the fully reversed fatigue response of composite materials even in the Very High Cycle Fatigue (VHCF) regime is proved. VHCF tests are run on hourglass specimens made of twill 2x2 carbon woven fabric impregnated with epoxy resin with stacking sequences [0]8 and [0/90/+45/-45]s and designed through Finite Element (FE) modal analysis. The stress distribution within the specimen and the absence of buckling are first determined through an extensive strain gage campaign, which has validated the FE model. As the temperature is a main concern in ultrasonic tests, the temperature increment within the composite specimen is investigated by means of an embedded fiber optic sensor and controlled during the tests with an infrared sensor. With the proposed experimental setup, fully reversed ultrasonic tests have been carried out up to 109 cycles and the failure of the two investigated specimen types has been analyzed by comparing the failure origin location in relation to the stress distributions
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