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The recent development of laser scanners in environmental diagnostics made possible to implement their prototypes for remote characterization of cultural heritage (CH) surfaces. Specific data analysis algorithms have been developed to handle large data set acquired in high resolution operation. Non-invasive, or at least micro-invasive methodologies applied on real CH targets have shown the possibility to obtain information on surface appearance, morphology and composition. The added value of optical and spectroscopic integrated sensors is discussed for some significant case studies addressed both to a deep knowledge of the artifacts (materials and methods of realization) and to their structural characterization. [ABSTRACT FROM AUTHOR]

The goal of this study was to investigate whether a secondary plasma can be formed on a non-metallic target under water and to give insight into the related processes. The material of choice here was alumina, since its physical, thermal and mechanical properties are substantially different from those of pure Al, only for which secondary plasma formation was recently demonstrated. To achieve this, plasma and bubble formation on alumina under water after single pulse laser excitation were studied using fast photography, shadowgraphy, Schlieren and LIBS techniques. The results show that the secondary plasma caused by backward heating of the target and successive slow target evaporation into the growing vapour bubble also occurs for alumina. The secondary plasma formed on alumina involves only a narrow interaction region on the target resulting in an almost spherical plume shape. In contrast, on thermally conductive and easily melting/evaporating aluminium, the secondary plasma is intense, with a large volume which is flattened on the target surface. Inside the expanded bubble above the alumina target, glowing particles were not observed. Due to less efficient secondary plasma formation on alumina compared to aluminium, its optical emission only slightly increases at a delay of 400 ns from the laser pulse but emission persists during three bubble cycles with a total duration of about 650 μs. The LIBS spectra related to the secondary plasma are almost free from any continuum component and show narrow emission lines from low excited states. Here we discuss the observed differences in the plasma’s spatial, temporal and spectral evolution on the two considered target materials. The obtained results indicate that under water a secondary plasma might be formed on very different materials and that its detection produces a good quality LIBS signal from single pulse excitation using a commercial nanosecond laser source. [ABSTRACT FROM AUTHOR]

In this work we present studies of spatial and temporal plasma evolution after single pulse ablation of an aluminium target in water. The laser ablation was performed using 20 ns long pulses emitted at 1064 nm. The plasma characterization was performed by fast photography, the Schlieren technique, shadowgraphy and optical emission spectroscopy. The experimental results indicate the existence of two distinct plasma stages: the first stage has a duration of approximately 500 ns from the laser pulse, and is followed by a new plasma growth starting from the crater center. The secondary plasma slowly evolves inside the growing vapor bubble, and its optical emission lasts over several tens of microseconds. Later, the hot glowing particles, trapped inside the vapor cavity, were detected during the whole cycle of the bubble, where the first collapse occurs after 475 μs from the laser pulse. Differences in the plasma properties during the two evolution phases are discussed, with an accent on the optical emission since its detection is of primary importance for LIBS. Here we demonstrate that the LIBS signal quality in single pulse excitation underwater can be greatly enhanced by detecting only the secondary plasma emission, and also by applying long acquisition gates (in the order of 10–100 μs). The presented results are of great importance for LIBS measurements inside a liquid environment, since they prove that a good analytical signal can be obtained by using nanosecond pulses from a single commercial laser source and by employing cost effective, not gated detectors. [ABSTRACT FROM AUTHOR]

Experimental estimates of ecologically acceptable single-bath lubricant, are presented and compared to those of classical lubricants in this paper. A device was created for the realisation of the strip ironing test with double thinning and a appropriate definition of the friction coefficient is used. Strips of 2.5 mm-thick low carbon steel sheets were used in the single-phase process with a maximum thinning deformation of 25%. In addition to the single-bath ecological lubricant, a phosphate layer with mineral oil was applied, as was lithium lubricating grease with MoS2 and mineral oil with EP additives. The basic criterion for the estimates was the change in the friction coefficient and the secondary criterion was the level of surface microchanges due to sliding. The applied test procedure enables the clear differences between the lubricating properties of the investigated lubricants to be established. [ABSTRACT FROM AUTHOR]

In the study of wave propagation, bulk waves exist in infinite homogeneous bodies and propagate indefinitely without being interrupted by boundaries or interfaces. Such waves may be decomposed into finite plane waves propagating along arbitrary directions of the solid. The properties of these waves are determined by the dependence between the propagation direction and the constitutive properties of media. Three types of such waves may be distinguished in connection to the three displacement vectors which determine an acoustic polarization. These three polarization vectors are mutually orthogonal, but in most cases they are neither perpendicular nor parallel to the propagation direction. There are a number of papers and books describing wave front surfaces as a consequence of the study of the acoustic tensor, which, in fact, represents the propagation condition of such waves. The surfaces associated with wave front surfaces are slowness surfaces, with slowness defined by the inverse of the wave front speeds, and energy flow surfaces, also known as group velocity surfaces. Here slowness surfaces are studied analytically and numerically to obtain information about wave propagation in arbitrary directions. The degree of wave surface deviation depends upon degrees of anisotropy, and may give valuable information about dynamic deformations. The materials, used in the present analysis, are fibre-reinforced materials with two families of continuous elastic fibres. Since fibres are much stronger than a matrix, anisotropic properties are emphasized. Coordinate-free constitutive relations give the possibility of considering fibres embedded in such materials as extensible or inextensible. Such an approach gives the opportunity to follow behaviour directly considering both the geometry and the stiffness of the fibres. The material considered here is reinforced by two families of mechanically equivalent fibres and, therefore, behaves like orthotropic, having axes of symmetry along bisectors of the fibre directions and along the normal to the plane tangent to the fibres. Such a material has nine independent material constants. When constraints of inextensibility are imposed on the fibres, it leads to the so-called ideal fibre-reinforced material, which may be obtained from materials with extensible fibres in a limiting process. In order to follow the influence of fibre direction on the mechanical properties, constitutive relations are modelled as a function of the fibre directions in each point of the continuous media. An often-used representation of such materials is an epoxy resin-carbon fibre composite whose material constants, for materials reinforced by one family of fibres, are determined using ultrasound methods. Here we consider the dynamical behaviour of the material reinforced by two families of strong mechanically equivalent elastic fibres by comparing the behaviour of extensible fibres to the behaviour of inextensible fibres. Solids made of the above-described materials show some interesting dynamic properties in the propagation of elastic waves. Some recent results of the dynamic behaviour when the material has strong anisotropy shall be presented. [ABSTRACT FROM AUTHOR]

Light transmission through a laser formed bubble (LFB) following ablation of a metallic target inside water was studied. During the early expansion and late collapsing phases, the refraction index nb of the hot high-pressure vapor bubble is higher than 1.23 and close to that of the surrounding liquid. The cavity growth lowers nb down to 1.00 and causes strong defocusing of the incident laser beam with consequent enlargement of the ablation crater diameter, here overcoming factor two. Inhomogeneous water vapor clustering inside the cool expanded bubble further perturbs the light transmission and induces irregular ablation by the successive laser pulse. [ABSTRACT FROM AUTHOR]

In this work the influence of laser wavelength upon the analytical results obtained from applying LIBS diagnostics to bronzes was investigated theoretically and experimentally at 1,064 nm and 355 nm. The laser ablation process was modeled for a set of reference samples of quaternary Cu/Sn/Pb/Zn alloys and the difference between plume composition and known target stoichiometry was estimated for both of the wavelengths considered. LIBS measurements were performed on the same set of reference samples and under the same experimental conditions to validate the model at different wavelengths. Results from the application of the model to calculate sample optical properties during laser irradiation, absorption in the plasma and plasma temperature are also presented. [ABSTRACT FROM AUTHOR]

The use of laser ablation for cleaning stone is a tried-and-tested method for preserving outdoor artwork surfaces exposed to environmental stresses. However, it is of interest to spectroscopically characterize the sample surface before and during the laser ablation in order to implement automatic control of the cleaning process. To this aim, we have undertaken systematic LIBS analysis on various clean and dirty surfaces of marble fragments collected from ancient quarries in Mediterranean areas, without the characteristic patina that comes from the protective layers usually deposited on the final artwork. The effectiveness of the cleaning process was then monitored by following the disappearance from the LIBS spectra of the encrustation elements during successive laser shots. The LIBS analysis of the clean surfaces of the samples examined confirmed that main bulk composition is based on calcium and magnesium carbonates, with the addition of strontium and, to a minor extent, of manganese and copper substituents. On the other hand, the encrustations were rich in sodium, aluminum, iron, silicon, titanium, lithium, manganese, and chromium, probably coming from sand/soil deposition and, only to a minor extent, from atmospheric pollution. Although SEM imaging and SEM-EDX analyses performed on the same samples at different resolutions showed remarkable surface inhomogeneities from the crustal region deep into the bulk material, the work demonstrates the possibility of a quantitative, minimally invasive, LIBS stratigraphy. The results from the technique are suitable for monitoring cleaning processes by determining appropriate elemental markers present on the surface at trace level (of the order of 100 ppm). [ABSTRACT FROM AUTHOR]

The optimal experimental conditions for quantitative analyses of the Martian similar crust by Laser Induced Breakdown Spectroscopy (LIBS) were studied comparatively in the air at atmospheric pressure and under the simulated planet atmosphere. Different laser energies were applied and the plasma dynamic, in terms of the emission intensity, temperature and electron density, were studied. The choice of the best experimental parameters includes: acquisition delay from the laser pulse and gate where the conditions of Local Thermal Equilibrium (LTE) holds, as necessary for the quantitative analyses, and further optimisation of the same parameters, as well as of the shot number, in order to maximize signal-to-noise ratio (S/N). Preliminary results demonstrate that for the same laser energies in a rarefied CO[sub 2] atmosphere the acquisition window where LTE conditions hold is much shorter than in the air due to the low electron density, fast plasma cooling and decay, where the last is the cause for even shorter optimum interval for maximizing S/N. [ABSTRACT FROM AUTHOR]