Papers by Author: Francesco Caputo

Abstract: It is well known that composite materials are founding increasing applications in the transport field thanks to their high strength to mass ratio. However, their use in primary structures is very challenging because of their high sensitivity to in-service damages and manufacturing defects. As a result, the current adopted damage tolerance approach leads to the oversizing of such structures. Structural health monitoring systems, aimed to the real time damage detection, can provide several benefits in terms of lightweight of the structures, maintenance operations and inspection costs. This paper deals with the use of the Probability Ellipse (PE) method, based on the propagation of ultrasonic guided waves on a composite winglet of a small aircraft. The PE method estimates the probability of the presence of the damage in the monitored area, starting from the knowledge of selected damage indexes for each sensors-path. The winglet, equipped with piezoelectric sensors, usable as both actuating and receiving devices, has been numerically and experimentally investigated under several configurations, varying the actuator location. Sensitivity analysis has been performed to assess the effectiveness of the PE method. The accuracy of the PE method in detecting both location and damaged area is herein discussed.

Abstract: Thanks to their high damage detection sensitivity and low requested power consumption, guided-waves (Lamb waves) have been increasingly used in the last years to monitor the structural integrity in primary and secondary composite structures. The monitoring of the structural health through the propagation of Lamb waves in composite structures is notoriously complex and, for this reason, the development of a prediction model can be a helpful tool for the improvement of Structural Health Monitoring (SHM) systems. Finite Element Method (FE) appears to be the best candidate for such type of simulation. However, since Lamb waves propagation depends strictly on the local material properties of the medium they propagate through, their numerical characterization is a thorny phase. Real composite components are usually affected by the presence of a large number of voids and defects, which cannot be reproduced in numerical models; this leads to a variability of the mechanical properties of materials, with particular reference to elastic moduli and density. These aspects get really ambitious the development of a well-established FE model. In this paper, a design of experiment (DOE) has been carried out to numerically investigate on the effects of the material properties variability on guided-waves time of flight.

Abstract: The contribution collects and illustrates "creative" methodological and design paths in the sectors of mobility and transport where design, biology, material technologies and bioengineering cooperate in a synergistic and proactive way for configurating new systems for fixing vehicles’ interior components. The aim has been to reduce the tolerance of human error during assembly by using innovative technological and material solutions and the conscious study of biological and biomechanical processes present in nature. In order to articulate the research activities, parameters have been identified and defined according to the requirements of the workers during the preparation and fixing of the components, according to the principles of safety, good design and material optimization. From the identification and literature survey of the patent documents and from the structural, material, feasibility and reliability assessments of the current installation procedures, critical issues related to stress, safety, times of installation and assembly and disassembly of the components have been carried out. Therefore, innovative biomimetic concept-solutions have been developed for the definition of architectures and fixing systems through an in-depth observation of the biological heritage, based on the organic integration of form, function, and processes. The path allowed the acquisition of specific knowledge and interdisciplinary skills to plan tangible results useful for optimizing, innovating and strengthening the design process from various industrial sectors such as engineering, aerospace, automotive, medical and pharmaceutical packaging.

Abstract: Laminated composite plates are widely used in the aerospace field, the prediction of their residual life is a interesting challenge for research communities. Their structural behaviour could be affected by several rupture mechanisms due to exercise loading conditions. One of the most critical is the low velocities impacts with different impact energies. This paper deals with an experimental test program performed in order to validate a numerical model developed by using finite element method. All experimental tests were carried out under international standard ASTM D7136 while all numerical tests were carried out by use of a worldwide numerical code Abaqus®. Inter-laminar and intra-laminar rupture mechanisms were taken into account and special-purpose elements were used. Rupture criteria were implemented in the numerical models thanks to their functional ease; results of numerical-experimental comparison were presented and discussed.

Abstract: The paper deals with dynamic tensile tests on Carbon Fibre Reinforced Polymer – CFRP laminates. As a result of their viscoelastic nature, plastics exhibit rate dependence in their stress-strain response. The need to develop failure criteria to determine the dynamic failure stress for composite material under dynamic loading conditions is a current challenge for the research community. The main goal of such paper is to assess the efficiency of the analytical models provided by literature to predict the strain-rate effects on composite coupons tensile strength. Moreover, experimental tests have been performed in order to evaluate the mechanical behaviour of different stacking sequences at different strain rate.

Abstract: The study proposed within this paper deals with an application of finite element techniques to the thermo-structural analysis of a dissimilar butt-welded joint. Residual stresses induced by the fusion arc-welding of steel joints in power generation plants are a concern to the industry. Nowadays, the application of finite element method appears to be a very efficient method for the prediction and the investigation of the weld-induced residual stresses, nevertheless the detailed modelling of all phenomena involved in such process is still challenging. The structural integrity assessment of welded structures strongly requires a deep investigation of weld-induced residual stresses in order to be compliant with safety requirement of power plant. The longitudinal and transversal residual stresses in dissimilar material butt joints of 8 mm thick for V-groove shape were studied. The developed thermo-mechanical FE model as well as the simulation procedures are detailed and results are discussed. As a result of such work, it has been found out that residual stresses in the two dissimilar plates are characterized by very different magnitudes and distribution.

Abstract: The aim of this paper is to understand the effects of the damage criteria modelling on the training phase (performed by means of Finite Element simulations) of an artificial neural network (ANN) enabled to locate impacts onto a CFRP laminate. The developed FE models have been also used to investigate the intra-laminar damage mode, which, among different ones, has the most effects on the residual strength of the panel.

Abstract: In the recent years, the study of the behaviour of damaged structures has been focused on cracked components in presence of an extensive material yielding at the crack tip; under this condition, linear elastic fracture mechanics theory (LEFM) is not able to describe the real plastic zone shape and size. Within this work, an extensive numerical analysis, based on elastic plastic fracture mechanics theory (EPFM), of the plastic zone size at the tip of a Mode I pre-crack at the notch edge in a plate is presented.

Abstract: This paper concerns the experimental characterization of static and fatigue strength of a flat panel stiffened by bonded pad made of aluminum alloy. The panels were full scale and tested under both static and fatigue loads, applied by means of an in house designed and built multi-axial static and fatigue machine. The fatigue crack propagation life of the stiffened panel has been compared to that of a simple flat panel and to that of a flat panel with chemical milling pad-up, getting that the fatigue crack growth life can be significantly improved.

Abstract: During the last few years various experimental destructive and non-destructive methods were developed to evaluate residual stresses. However it is impossible to obtain a full residual stress distribution in welded structures by means of experimental methods. This disadvantage can be solved by means of computational analysis which allows to determine the whole stress and strain fields in complex structures. In this paper the temperature distribution and residual stresses were determined in a single-pass butt joint welded by GMAW (Gas Metal Arc Welding) process by finite element model (FEM). A 3D finite parametric element model has been carried out to analyze temperature distribution in butt weld joints and thermo-mechanical analyses were performed to evaluate resulting residual stresses. Temperature fields have been investigated by varying an initial preheating treatment. Moreover the technique of “element birth and death” was adopted to simulate the process of filler metal addition The high stresses were evaluated, with particular regard to fusion zone and heat affected zone. The influence of preheating and post-heating treatment on residual stresses was investigated. The residual stresses decrease when preheating temperature increases. The maximum value of longitudinal residual stresses without pre-heating can be reduced about 12% and 38% by using the preheating and post-heating process respectively.