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Laser-induced breakdown spectroscopy (LIBS) is a combination of laser ablation and optical emission spectroscopy. Due to the possibility of direct measurements on the sample surface with a minimum of sample preparation investigations of building materials can be conducted quite fast. In combination with a scanning technique (translation stage or scanning mirrors head) 2D element distributions of harmful species like chlorides and alkalis are evaluated with a sub-mm resolution. By scanning a surface of typically 100 mm x 100 mm the heterogeneity of the material may be considered. LIBS is capable to identify hot spots in element concentration with a resolution of 0.1 mm or even smaller. A LIBS-system for the on-site application on building materials is, in cooperation with system developers and companies which will use this technique for investigation on real structures, under test at BAM. We present an example. Further work will focus on providing guidelines to establish LIBS as a standard procedure for chemical investigations of building materials.
A laser induced breakdown spectroscopy (LIBS) system was combined with the Gas Tungsten Arc (TIG) Welding process for the in situ monitoring of chemical compositions in austenitic stainless steels during welding. The purpose of this project is to develop a prototype of an online chemical composition control system for welding applications. One of the use cases of this new LIBS-based measurement system can be the controlling of potentially dangerous emissions during the welding process in order to improve the working safety conditions.
For the damage assessment of reinforced concrete structures the quantified ingress profiles of harmful species like chlorides, sulfates and alkali need to be determined. In order to provide on-site analysis of concrete a fast and reliable method is necessary. Low transition probabilities as well as the high ionization energies for chlorine and sulfur in the near-infrared range makes the detection of Cl I and S I in low concentrations a difficult task. For the on-site analysis a mobile LIBS-system (k = 1064 nm, Epulse ≤ 3 mJ, t = 1.5 ns) with an automated scanner has been developed at BAM. Weak chlorine and sulfur signal intensities do not allow classical univariate analysis for process data derived from the mobile system. In order to improve the analytical performance multivariate analysis like PLS-R will be presented in this work. A comparison to standard univariate analysis will be carried out and results covering important parameters like detection and quantification limits (LOD, LOQ) as well as processing variances will be discussed (Allegrini and Olivieri, 2014 [1]; Ostra et al., 2008 [2]). It will be shown that for the first time a low cost mobile system is capable of providing reproducible chlorine and sulfur analysis on concrete by using a low sensitive system in combination with multivariate evaluation.
Time- and space-resolved in situ LIBS measurements of chemical compositions during TIG-welding
(2017)
An in situ monitoring of chemical compositions in the weld pool and the heat affected zone (HAZ) can enable the control of the welding process through the regulation of the welding parameters, and thus can prevent possible weld defects.
The most critical parameter for hot cracking -from a metallurgical point of view- is the chemical composition of the weld pool.
Chemical composition can be measured and quantified during the welding process with the LIBS technique having the appropriate calibration measurements.
2D evaluation of spectral LIBS data derived from heterogeneous materials using cluster algorithm
(2017)
Laser-induced Breakdown Spectroscopy (LIBS) is capable of providing spatially resolved element maps in regard to the chemical composition of the sample. The evaluation of heterogeneous materials is often a challenging task, especially in the case of phase boundaries. In order to determine information about a certain phase of a material, the need for a method that offers an objective evaluation is necessary. This paper will introduce a cluster algorithm in the case of heterogeneous building materials (concrete) to separate the spectral information of non-relevant aggregates and cement matrix. In civil engineering, the information about the quantitative ingress of harmful species like Cl−, Na+ and SO2−4 is of great interest in the evaluation of the remaining lifetime of structures (Millar et al., 2015; Wilsch et al., 2005). These species trigger different damage processes such as the alkali-silica reaction (ASR) or the chloride-induced corrosion of the reinforcement. Therefore, a discrimination between the different phases, mainly cement matrix and aggregates, is highly important (Weritz et al., 2006). For the 2D evaluation, the expectation-maximizationalgorithm (EM algorithm; Ester and Sander, 2000) has been tested for the application presented in this work. The method has been introduced and different figures of merit have been presented according to recommendations given in Haddad et al. (2014). Advantages of this method will be highlighted. After phase separation, non-relevant information can be excluded and only the wanted phase displayed. Using a set of samples with known and unknown composition, the EM-clustering method has been validated regarding to Gustavo González and Ángeles Herrador (2007).
The Tungsten Inert Gas (TIG) welding process was combined with a Laser-Induced Breakdown Spectroscopy (LIBS) system for the in situ measurement of chemical compositions in austenitic stainless steels. The main aim of this project was the development of a prototype for a real-time chemical composition analysis system for welding applications. In principle, the measurement of the weld pool composition allows governing the weld pool solidification behaviour by controlling the welding parameters. This control loop allows avoiding weld defects.
Chemical compositions of the weld seam is from a metallurgical point of view the most critical parameter for the occurrence of weld defects. Additionally, this new LIBS-based measurement system can be used for the monitoring and controlling of potentially dangerous emissions during the welding process in order to improve the working safety conditions. Preliminary investigations show that the chemical composition measurement of welds with LIBS is possible during and after welding.
For concrete production aggregates, cement and water are mixed together and after 28 days of hydration a porous and multiphase material is formed. To consider the heterogeneity of concrete a 2D scanning system is used. The coarse aggregates (limit > 2 X d ) can be excluded. laser spot The remaining content is a mixture of flour grains and cement particles (micro heterogeneity). Harmful species like chlorides may penetrate together with water through the capillary pore space. A quantification of Cl regarding to the cement content only (European standard EN 206) is necessary for the damage assessment. A LIBS system operating with a NdCr:YAG laser (pulse energy of 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz) and two Czerny-Turner spectrometer (UV and NIR range) have been used.
Concrete is a multiphase material made of cement, aggregates and water. The heterogeneity of concrete is a result of mixing aggregates (grain size between 0.125 mm to 32 mm) and cement (grain size < 0.04 mm) together. There are always aggregates with a grain size below 0.125 mm (flour grains) therefore the cement matrix is always a mixture of these small particles. Different grain size distributions by making concrete are important to ensure the needed density and com-pressive strength. In order to estimate the remaining lifetime of concrete structures the quantification of element concentra-tions of alkali and chlorides regarding to the cement matrix only (1/3 of the total mass), is a major concern in civil engineer-ing. Due to two-dimensional scanning with LIBS the coarse aggregates can be evaluated and excluded from the analysis. In the case of particles (flour grains and cement) smaller than the laser spot size, the microscopic hetero-geneity influences the laser-plasma interaction and has therefore an impact on the results. In this study the influence of the micro-heterogeneity and the impact on the laser-material interaction is examined. The effect of changing ratios between cement particles and aggregate particles in the laser-induced plasma has been analyzed (see figure 1, right). Therefore, different samples with defined grain sizes are prepared and the obtained distributions were analyzed with x-ray diffraction (XRD). For the LIBS measurements, an automated system that operates with a low energy NdCr:YAG laser (pulse energy of 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz) and a NIR Czerny-Turner spectrometer has been used.
A laser-induced breakdown spectroscopy (LIBS) system was combined with a bead-on-plate Tungsten Inert Gas (TIG) welding process for the in situ measurement of chemical compositions in austenitic stainless steels during welding.Monitoring the weld pool's chemical composition allows governing the weld pool solidification behavior, and thus enables the reduction of susceptibility to weld defects. Conventional inspection methods for weld seams (e.g. ultrasonic inspection) cannot be performed during the welding process. The analysis system also allows in situ study of the correlation between the occurrence ofweld defects and changes in the chemical composition in theweld pool or in the two-phase regionwhere solid and liquid phase coexist. First experiments showed that both the shielding Ar gas and the welding arc plasma have a significant effect on the selected Cr II, Ni II and MnII characteristicemissions, namely an artificial increase of intensity values via unspecific emission in the spectra.
In situ investigations showed that this artificial intensity increase reached a maximum in presence of weld plume. Moreover, an explicit decay has been observedwith the termination of thewelding plumedue to infrared radiation during sample cooling. Furthermore, LIBS can be used after welding to map element distribution. For austenitic stainless steels,Mnaccumulations on both sides of theweld could be detected between the heat affected zone (HAZ) and the base material.
19th and 20th centuries glass paint layers consist of a colour body and a colourless lead silicate flux, in which borax or boric acid was added as further component to improve the paint ability and to reduce the firing temperature for multiple layers of paint. Model glasses were used in laboratory tests to investigate the stability of glass paints with additions of boron oxide. To determine boron in paint layers, a LIBS-system with pulsed NdYAG-laser was used.