Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Cellular Metals: Fabrication, Properties and Applications

Cellular solids and porous metals have become some of the most promising lightweight multifunctional materials due to their superior combination of advanced properties mainly derived from their base material and cellular structure. They are used in a wide range of commercial, biomedical, industrial, and military applications. In contrast to other cellular materials, cellular metals are non-flammable, recyclable, extremely tough, and chemically stable and are excellent energy absorbers. The manuscripts of this Special Issue provide a representative insight into the recent developments in this field, covering topics related to manufacturing, characterization, properties, specific challenges in transportation, and the description of structural features. For example, a presented strategy for the strengthening of Al-alloy foams is the addition of alloying elements (e.g., magnesium) into the metal bulk matrix to promote the formation of intermetallics (e.g., precipitation hardening). The incorporation of micro-sized and nano-sized reinforcement elements (e.g., carbon nanotubes and graphene oxide) into the metal bulk matrix to enhance the performance of the ductile metal is presented. New bioinspired cellular materials, such as nanocomposite foams, lattice materials, and hybrid foams and structures are also discussed (e.g., filled hollow structures, metal-polymer hybrid cellular structures)

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901

Lightweight energy absorbing structures for crashworthy design

PhD ThesisThe application of lightweight composite materials into the rail industry requires a stepwise approach to ensure rail vehicle designs can make optimal use of the inherent properties of each material. Traditionally, materials such as steel and aluminium have been used in railway rolling stock to achieve the energy absorption and structural resistance demanded by European rail standards. Adopting composite materials in primary structural roles requires an innovative design approach which makes the best use of the available space within the rolling stock design such that impact energies and loads are accommodated in a managed and predictable manner. This thesis describes the innovative design of a rail driver’s cab to meet crashworthiness and structural requirements using lightweight, cost-effective composite materials. This takes the application of composite materials in the rail industry beyond the current state-of-the-art and delivers design solutions which are readily applicable across rolling stock categories. An overview of crashworthiness with respect to the rail industry is presented, suitable composite materials for incorporation into rolling stock designs are identified and a methodology to reconfigure and enhance the space available within rail vehicles to meet energy absorption requirements is provided. To realise the application of composite materials, this body of work describes the pioneering application of aluminium honeycomb to deliver unique solutions for rail vehicle energy absorbers, as well as detailing the use of lightweight composite materials to react the structural loads into the cab and carbody. To prove the capability of the design it is supported by finite element analysis and the construction of a full-scale prototype cab which culminated in the successful filing of two patents to protect the intellectual property of the resulting design.The European Commission whose Framework 6 funded project “De-Light” (Contract Number 031483) forms the basis of this work

Energy absorbing characteristics of hybrid composite pipe systems

The aim of this research is to investigate the structural response of carbon fibre reinforced plastic (CFRP) tubes and their hybrid systems subjected to quasi-static and dynamic loading conditions. The work also includes the investigation of the mechanical properties and energy-absorbing characteristics of other novel composite structures for the potential use in aerospace and a wide range of engineering applications. Firstly, a series of experimental tests have been carried out to obtain the mechanical properties of all constituent materials and structural behaviour of the composite structures, which are used to validate numerical models. The material tests undertaken include (1) quasi-static and dynamic crushing of individual CFRP tubes and the related hybrid systems (2) compression of PU foams. The corresponding failure modes are obtained. In addition, specific energy absorption of the individual tubes and the hybrid systems investigated is evaluated. Then, finite element (FE) models are developed using the commercial code ABAQUS/Explicit to simulate the structural response of CFRP tubes, the related hybrid systems and syntactic foam core based sandwich beams. The agreement between the numerical predictions and experimental results is very good across the range of the structures and configurations investigated. The FE models have produced accurate predictions of the static and dynamic load-displacement responses, the specific energy absorption and failure characteristics recorded for CFRP tube structures. The modelling has been further undertaken on the low velocity impact response of the sandwich beams, with reasonably good correlation to the corresponding experimental results. The dynamic characteristics of the fibre reinforced composite pipe structures through a series experimental tests and numerical predictions investigated in this project can be used in assisting the design of lightweight composite structures for energy-absorbing applications

An investigation on the vibroacoustic behavior of systems in similitude

Similitude theory allows engineers to establish the necessary conditions to design a scaled - up or down - model of a full-scale prototype structure. In recent years, the research on similitude methods, which allow to design the models and establish similitude conditions and scaling laws, has grown so that many obstacles associated with full-scale testing, such as cost and setup, may be overcome. This thesis aims at, on the one hand, expanding the possibilities of similitude methods by means of their application to new structural configurations; on the other hand, at the investigation of new approaches. Therefore, similitude conditions and scaling laws of thin aluminium plates with clamped-free-clamped-free boundary conditions, first, and aluminium foam sandwich plates with simply supported and free-free boundary conditions, then, are derived. Particularly, two sets of conditions are derived for the sandwich plates: the first by expliciting all the geometrical and material properties, the second by combining some parameters into just one with physical meaning, that is, the bending stiffness. These conditions and laws are successively validated by means of dynamic experimental tests, in which reconstructions of the natural frequencies and the velocity response of the prototype are attempted. Also the prediction of the radiated acoustic power is performed for the sandwich plates. All the tests highlight that these laws do not work fine when the models are distorted, i.e., when the similitude conditions are not satisfied. Therefore, the potentialities of machine learning are investigated and used to establish degrees of correlation between similar systems, without invoking governing equations and/or solution schemes. In particular, artificial neural networks are used in order to predict the dynamic characteristics, first, and the scaling parameters, then, of beams, as test (since they do not exhibit distorted models), and plates. In the latter case, the predictions of the artificial neural networks are validated by the results provided by the experimental tests. The networks prove to be robust to noise, very helpful in predicting the response characteristics, and identifying the model type. Finally, the similitude methods are used as a tool for supporting, and eventually validating, noisy experimental measurements, not for predicting the prototype behavior. In this way, they can help to understand if a set of measurements is reliable or not. Therefore, the sandwich plates are analysed with digital image correlation cameras. Then, with the help of an algorithm for blind source separation, the force spectra and velocity responses are reconstructed. It is demonstrated that the similitude results are coherent with the quality of the experimental measurements, since the curves overlap when the spatial patterns are recognizable. Instead, when the displacement field is too polluted by noise, the reconstruction exhibits discrepancies. This proves that the application of similitude methods should not be underestimated, especially in the light of the expanding range of approaches which can extract important information from noisy observations