Enrico Troiani

Reliability and accuracy of embedded fiber Bragg grating sensors for strain monitoring in advanced composite structures

This work investigated issues for an efficient and reliable embedding and use of Fiber Bragg Grating (FBG) sensors for strain monitoring of composite structures with particular regard to the manufacturing process of components in the nautical field by means of the vacuum bag technique in autoclave. CFRP material laminates with embedded FBGs were produced and the effect of the curing process parameters on the light transmission characteristics of the optical fibers was initially investigated. Two different types of coating, namely polyimide and acrylate, were tested by measuring the light attenuation by an Optical Time Domain Reflectometer. Tensile specimens were subsequently extracted from the laminas and instrumented also with a surface-mounted conventional electrical strain gage (SG). Comparison between the FBG and SG measurements during static tensile tests allowed the evaluation of the strain monitoring capability of the FBGs, in particular of their sensitivity (i.e., gage factor) when embedded.

An ultra-wideband radar approach to nondestructive testing

In this paper a new method for nondestructive testing (NDT), applied to carbon fiber composite materials, is investigated. The approach is based on the comparison between the electromagnetic signal reflected by the carbon fiber composite sheet under test, when an ultra-wideband (UWB) signal is incident on it, and the one reflected by a healthy sheet. The possible presence of a defect is then revealed by an appropriate metric measuring the mismatch between the two reflected waveforms. The performance of different metrics is investigated, for two defect types, based on real measurements in an indoor environment. As an outcome of our experimental activity, very promising results have been obtained for one of the tested metrics, which allows to accurately detect surface defects with a size comparable to the radar resolution on a carbon fiber composite sheet.

Characterisation of Fatigue and Crack Propagation in Laser Shock Peened Open Hole 7075-T73 Aluminium Specimens

The goal of this research activity is to evaluate the capability of Laser Shock Peening (LSP) technology to improve fatigue life in open-hole aluminium specimens. Thin, dog-bone specimens were LSP treated in direct ablation mode and subsequently tested. The obtained results have not proven the advantage of LSP technology over traditional residual stress insertion techniques around open-holes, such as cold working. Therefore, the focus of the activity was moved towards understanding the causes of the observed fatigue life reduction.

Damage Tolerance of Adhesive Bonded Stiffened Panels: Experimental and Analytical Investigation of the Fatigue Crack Propagation Underneath the Stringers

On the basis of well-known literature, an analytical model was developed by the authors to provide reliable predictions on the fatigue propagation of cracks growing through the skin underneath adhesively bonded stringers of aeronautical stiffened panel. In order to exploit the significant damage tolerance proprieties of adhesively bonded stiffened panels it is indeed fundamental to take into account the slow fatigue crack growth under the bonded stringers, currently completely neglected as a consequence of the limits of the employed prediction tools.

Numerical Analysis of Laser Shock Peening as a Process for Generation of Compressive Residual Stresses in Open Hole Specimens

A numerical analysis of Laser Shock Peening (LSP) process is illustrated, applied to an open hole specimen. This specimen is representative of a section of an aircraft fuselage lap joint, typically prone to fatigue crack nucleation at the rivet holes. The effect of the residual stress field induced by LSP on the fatigue life of open hole specimens is investigated. The results show that significant compressive residual stresses can be introduced in fatigue sensitive areas using LSP, postponing fatigue crack nucleation.

Design Optimization of a Laser Cutting Machine by Elastodynamic Modeling

This study deals with the elastodynamic modeling of a laser cutting machine and illustrates the guidelines followed for the design optimization of the machine’s basic structure from the dynamic behavior point of view. A finite element model was set up along with the conceptual design of the new machine, with the aim of performing dynamic simulations. The main purpose is to predict the vibrations arising in the structure that could significantly deteriorate the product quality in order to evaluate different design solutions. The vibrations can be excited by variable forces acting on the moving masses and by the oscillations affecting the machine basement due to external causes. The original modeling of the excitations is presented herein. Modal analysis and forced simulations were performed on the finite element model of the first conceptual design of the machine structure. The analysis of the results indicated some critical parts of the system to be stiffened in order to mitigate the vibrations, that is to improve the cutting quality. Structural modifications to the first conceptual design were therefore suggested and a new model of the machine was developed and simulated. The results of the simulations before and after the design modifications are reported and discussed.Copyright

Laser shock peening on a 6056-T4 aluminium alloy for airframe applications

Laser Shock Peening (LSP) is a material enhancement process used to introduce compressive residual stresses in metallic components. This investigation explored the effects of different combinations of LSP parameters, such as irradiance (GW/cm2) and laser pulse density (spots/mm2), on 3.2 mm thick AA6056-T4 samples, for integral airframe applications. The most significant effects that are introduced by LSP without a protective coating include residual stress and surface roughness, since each laser pulse vaporizes the surface layer of the target. Each of these effects was quantified, whereby residual stress analysis was performed using X-ray diffraction with synchrotron radiation. A series of fully reversed bending fatigue tests was conducted, in order to evaluate fatigue performance enhancements with the aim of identifying LSP parameter influence. Improvement in fatigue life was demonstrated, and failure of samples at the boundary of the LSP treatment was attributed to a balancing tensile residual stress.

Parametric damage tolerance design of metallic aeronautical stiffened panels

On the basis of well-known literature, an analytical tool named LEAF (linear elastic analysis of fracture) was developed by the authors to predict the damage tolerance (DT) proprieties of aeronautical stiffened panels. The tool is based on the linear elastic fracture mechanics and the displacement compatibility method. By means of LEAF, an extensive parametric analysis of stiffened panels, representative of typical aeronautical constructions, was performed to provide meaningful design guidelines. The effects of riveted, integral and adhesively bonded stringers on the fatigue crack propagation performances of stiffened panels were investigated, as well as the crack retarder contribution using metallic straps (named doublers) bonded in the middle of the stringers bays. The effect of both perfectly bonded and partially debonded doublers was investigated as well. Adhesively bonded stiffeners showed the best DT properties in comparison with riveted and integral ones. A great reduction of the skin crack growth propagation rate can be achieved with the adoption of additional doublers bonded between the stringers.

Analysis of Residual Stress Effect on Fatigue Crack Propagation in Bonded Aeronautical Stiffened Panels

The application of adhesively bonded straps made of high-static strength materials on aeronautical stiffened panels to retard the fatigue skin crack growth is currently a topical research subject. The detrimental effect of the residual stress fields induced as a consequence of the dissimilar coefficients of thermal expansion of the skin and strap materials on the fatigue skin crack propagation was investigated. The residual stresses induced in a stiffened panel representative of a pressurized fuselage shell with titanium doublers in the middle of the stringer bays is numerically quantified for two likely operational temperatures. Their effect on the fatigue crack propagation is analyzed by means of a linear elastic fracture mechanics approach. The results show that adhesively bonded straps on the cracked surface can significantly retard the fatigue crack propagation but, in order to achieve reliable and conservative predictions on their performances, the effect of the residual stress fields they introduce must be taken into account.