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Impulse-thermography is an active method for quantitative investigation of the near surface region of various structures which has recently been applied to civil engineering. It is well suited for the detection of voids and honeycombing in concrete up to concrete covers of 10 cm and more. For quantitative analysis, a computer program for numerical simulation of the heating up and cooling down processes was developed based on Finite Differences. With this program parameter studies have been performed for investigating the influence of environmental conditions, material parameters and geometry on the thermal behaviour. The comparison between experimental and simulated results enables the Inverse Solution.
The applicability of pulse phase thermography (PPT) for the investigation of structures is studied systematically on concrete test specimens and on a plastered sandstone column. In the test specimens, voids and delaminations are implemented in different depths and with different sizes, modelling real voids, honeycombing and debonding. Delaminations of plaster in concrete and masonry and behind tiles on concrete are investigated. PPT is based on the frequency analysis of the cooling down process of actively heated surfaces. Therefore, it is contactless and thus completely non-destructive (if overheating of the surface is prevented), fast and allows the inspection of large surface areas. The interpretation of amplitude and phase images gives semi-quantitative information about the observed defects. The phase images provide a deeper probing up to 1015 cm in relation to the interpretation of the thermograms and to the amplitude images. In addition, the influence of surface inhomogeneities and non-uniform heating is reduced.
New thermography systems enable the application of active investigation techniques, i.e. the observation of the cooling down process after heating the surface of a structure under investigation. Defects like voids in concrete having a different thermal diffusivity in comparison to the bulk material are visualised by different surface temperatures. The differences between temperature transient curves above sound regions and above inhomogeneities are expected to include information about the defect parameters.
Experimental investigations with impulse thermography and radar on a concrete test specimen containing voids were analysed. Radar was used to determine the depth of the voids. The influence of size and depth of the voids on the temperature transient curves was studied for different heating times. The transient curves were also fitted with a simple semi-empirical model.
Impulse-thermography has been established as a fast and reliable tool in many areas of non-destructive testing. In recent years several investigations have been done to apply active thermography to civil engineering. For quantitative investigations in this area of application, finite difference calculations have been performed for systematic studies on the influence of environmental conditions, heating power and time, defect depth and size and thermal properties of the bulk material (concrete). The comparison of simulated and experimental data enables the quantitative analysis of defects.
Impulse and lock-in thermography have been applied to detect delaminations of prototype solder joints, similar to those to be produced between Cu shunts and Cu busbar stabilisers at the Large Hadron Collider (LHC) at CERN. Two infrared cameras with different detector materials and with different spectral ranges and two excitation techniques have been tested and compared for their ability to detect delaminations behind 2 and 3 mm thick Cu shunts. We have analyzed the signal to noise ratio (SNR) for each detected defect and are able to detect defects down to a nominal edge length of 4 mm behind 2 mm thick Cu shunts by using fast impulse thermography and a camera with a microbolometer array. For the 3 mm thick Cu shunt, on the other hand, the nominal 4 mm defect is only visible in the lock-in thermography phase images and the highest SNR has been achieved with a cooled InSb-based camera. In addition, numerical simulations show the influence of the minimum detectable defect size on the shunt thickness and that the developed on-site testing technique is sufficient to find all defects that are detectable theoretically.
Thermal investigations using infrared (IR) thermometry and false colour thermography were carried out on flowering plants in the Botanical Garden of Berlin and in a private garden. Special interest went to phototropic plants that orient their blossoms towards a light source (mainly the sun) and within this group to solar trackers (heliotropism) that follow the path of the sun during the day. Best known among the latter is the sunflower. Bowl shaped flowers comparable to satellite disks focus the solar radiation effectively on their centre, resulting in a warming of the female organs at that point. Temperature differences above ambient can be as high as 10.7 K with a mean value of 6.0 K. Pollinating insects were often observed sun-basking in the bowls or on the flower disks attracted by energetic rewards in form of nectar and heat.
False colour infrared (IR) thermography was performed on a sunny summer day in the Berlin Botanic Garden. The main interest focused on blossoms of different size, colours and shapes: disk-, bowl- and funnel-like anatomy demonstrating various modes of light absorption and heating-up during the day. Some of the flowers were phototropic or even solar tracking. Blossom temperatures ranged from 18.0 to 33.3 °C (mean 26.3 ± 2.5 °C, S.D., n = 37) at 21 °C air temperature. Several thermograms showed honey- and bumblebees also. In some cases the results are compared with those of a contact-free IR thermometry. One winter- and one late-spring-flowering plant are included in the overview.
Impulse-thermography is an active method for quantitative investigations of the near surface region of various structures. It has recently been applied and optimised to applications in civil engineering. By using either an internal or external heat source, parts of the structure under investigation are heated up and the transient heat flux is observed by recording the temperature change at the surface as a function of time. This method is very well suited for the detection of voids and honeycombing in concrete, up to concrete covers of 10 cm as well as for the location of delaminations in multi-layered systems (e.g. plaster on concrete, CFRP-laminates on concrete, asphalt on concrete). Also safety relevant defects like voids in tendon ducts and cracks in concrete could be recognised.
Carbon fiber reinforced polymer (CFRP) specimens with artificial delaminations and with impact damage have been characterized using active thermography with flash excitation. Systematic investigations have been performed in four different experimental configurations of flash lamps and infrared (IR) camera in transmission as well as in reflection alignment. It is shown here that the diffusivities determined for the sound and for the damaged areas give a good measure for damage characterization. Although reflection measurements also give information about defect depth, reflection measurements from only one side are not sufficient for assessing the whole cross section of the specimens. Thus, depending on sample thickness the lateral size of damage could only be determined from reflections measurements from both sides or from transmission measurements. In this paper, measurement accuracy and limits of flash thermography for the investigation of CFRP specimens are presented in detail together with quantitative data concerning the defects.
Recurrent non-destructive testing inspections are necessary to prevent damages in wind turbine rotor blades, but so far, there is no established method that detects defects in blades from greater distances – although this becomes increasingly important in the context of hardly accessible offshore wind parks.
Thermography is a promising method for detecting subsurface defects, but various challenges arise when this method is applied on-site to turbine blades in operation. Disturbing influences from the environment easily lead to a misinterpretation of thermograms (i.e. thermographic images), such as thermal signatures caused by reflections, dirt and other superficial inhomogeneities.
This study explores several problems and effects that arise, when (rotating) blades are monitored with thermography.
It will then be demonstrated that a meaningful defect inspection in this scenario is essentially restricted to a procedure following three steps: Firstly, calculating the so-called difference thermograms of all blade pairs for eliminating disturbing reflections. Secondly, identifying potentially relevant signals, which are associated neither with structural features nor with dynamical effects, and the identification of these signals’ allocations (through comparison of all difference thermograms with each other). And thirdly, comparing these signals with (processed) photos for excluding incorrect indications by surface effects. Unlike common thermographic analysis methods, which typically only include an aspect of this procedure, the composition presented in this contribution constitutes an advanced technique for minimizing disturbing influences in thermograms.
The proposed thermographic technique enables the detection of potential subsurface defects within rotating rotor blades from greater distances – such as from the ground, air crafts or vessels.
Active thermography is an efficient non-destructive testing method for investigating the internal structure of larger carbon fiber reinforced plastic (CFRP) components as well as smaller CFRP components in mass customization. The method can be applied contactless and automated. This study contains systematic investigations of CFRP structures with typical defects and inhomogeneities occurring during production by means of flash thermography in reflection and transmission configuration and by computed tomography (CT). The latter one was used as a reference method, since also very small defects at larger depth can be visualized with high spatial resolution. The CFRP structures consist of plates which contain metallic and non-metallic inclusions, contaminations with glue or wax rests, areas with inhomogeneous re-injection of dry parts, fiber misalignments, and fiber damages. Further on, two specimens have been glued together with different artificial inhomogeneities of the four glue beads. The results of the applied methods are compared and the advantages and disadvantages of each configuration are discussed based on the detectability of the inhomogeneities. It is shown that although CT has led to best contrasts and spatial resolutions in displaying the inhomogeneities and inclusions, flash thermography is very well suited to detect most of these structures. Considering that flash thermography can be applied on-site and has a high potential for automation and for a fast and efficient testing, it can be highly recommended for quality assurance during and after production of CFRP structures.
Lockin thermography as well as flash thermography are very well suited for non-destructive testing and characterisation of inhomogeneities and damage in CFRP structures. Both methods are applied more and more in research and industry, but until know it is not clear which are the advantages of one method against the other. In
this paper, results of two research projects concerning the validation and standardisation of both methods are presented. Three different types of CFRP test specimens consisting of well-defined flat bottom holes, of artificial delaminations made of PTFE plates embedded in CFRP and of real impact damage have been investigated systematically with flash and lockin excitation. Here, the phase images of both methods are compared qualitatively as well as quantitatively.
Purpose – This purpose of this paper is to report about the temperature distribution in metal and ceramic powder beds during 3D printing. The differing powders are thoroughly characterized in terms of thermal conductivity, thermal diffusivity, emissivity spectra and density.
Design/methodology/approach – The temperature distribution was measured in a 3D printing appliance (Prometal R1) with the help of thin thermocouples (0.25 mm diameter) and thermographic imaging. Temperatures at the powder bed surface as well as at differing powder bed depths were determined. The thermal conductivity, thermal diffusivity and emissivity spectra of the powders were measured as well. Numerical simulation was used to verify the measured temperatures.
Findings – The ceramic powder heated up and cooled down more quickly. This finding corresponds well with numerical simulations based on measured values for thermal conductivity and thermal diffusivity as well as emissivity spectra. An observed color change at the metal powder has only little effect on emissivity in the relevant wavelength region.
Research limitations/implications – It was found that thermocouple-based temperature measurements at the powder bed surface are difficult and these results should be considered with caution.
Practical implications – The results give practitioners valuable information about the transient temperature evolution for two widely used but differing powder systems (metal, ceramic). The paramount importance of powder bed porosity for thermal conductivity was verified. Already small differences in thermal conductivity, thermal diffusivity and hence volumetric heat capacity lead to marked differences in the transient temperature evolution.
Originality/value – The paper combines several techniques such as temperature measurements, spectral emissivity measurements, measurements of thermal conductivity and diffusivity and density measurements. The obtained results are put into a numerical model to check the obtained temperature data and the other measured values for consistency. This approach illustrates that determinations of surface temperatures of the powder beds are difficult.
Applications of infrared thermography in civil engineering are not limited to the identification of heat losses in building envelopes. Active infrared thermography methods enable structural investigations of building elements with one-sided access up to a depth of about 10 cm. Masonry and especially historical masonry has a very heterogeneous structure containing several different materials (brick, stone, mortar, plaster, wood, metal, etc.) with various thermal properties. As many classes of damage originate from defects that are close to the surface, active thermography is in general very well suited to assessing different test problems in cultural heritage buildings. In this paper, the physical background, equipment, environmental influences and material properties are discussed. Several application results are presented. It is shown how active thermography can be integrated into a holistic approach for the assessment of historical structures.
Pulse thermography of concrete structures is used in civil engineering for detecting voids, honeycombing and delamination. Quantitatively realistic numerical 3D simulation is difficult due to the arising boundary layer at the heated surface and unreliable information about material parameters and environmental conditions. We address both issues by a semi-analytic reformulation of the heat transport problem and by parameter identification. Numerical results are compared with measurements of a test specimen.
Impulse thermography is an active method for quantitative investigations of the near
surface region of various structures. It has recently been applied and optimized to applications in civil
engineering. For quantitative analysis of data recorded on the building site, the problems are manifold.
Here, the influence of the different surface properties on the detection and characterization of voids
and honeycombing simulated by polystyrene cubes and cubes prepared of porous concrete are
demonstrated.
Methods for the combination (i. e. comparison and overlay) and data fusion (i. e. integration of all data in one data set, replacement of data) of active thermography and D laser scanner (light section method) have been developed. Systematic investigations for quantification of damage in historic structures are presented using both techniques. A case study shows that reproducible investigations at regular time intervals are very well suited for structural monitoring.
Comparison of quantitative defect characterization using pulse-phase and lock-in thermography
(2016)
Using optical excitation sources for active thermography enables a contactless, remote, and non-destructive testing of materials and structures. Currently, two kinds of temporal excitation techniques have been established: pulse or flash excitation, usingmostly flash lamps; and periodic or lock-in excitation, using halogen lamps, LED, or laser arrays. From the experimental point of view, both techniques have their advantages and disadvantages. Concerning the comparison of the testing results of both techniques, only very few studies have been performed in the past. In this contribution, the phase values obtained at flat bottom holes in steel and CFRP and the spatial resolution measured at crossed notches in steel using flash and lock-in excitation are compared quantitatively.
This study evaluates whether subsurface features in rotor blades, mainly made of Glass Fibre Reinforced Plastics (GFRP), can generally be detected with ‘‘solar thermography”. First, the suitability of the sun is tested for acting as a heat source for applying active thermography on a 30 mm thick GFRP test specimen. Second, a defective rotor blade segment is inspected outdoors under ideal natural conditions using the sun as excitation source. Additionally, numerical FEM-simulations are performed and the comparability between experiment and simulation is evaluated for outdoor measurements.
Quantification of impact damages in CFRP and GFRP structures with thermography and ultrasonics
(2018)
The extent of damage caused by impacts in fibre reinforced composites depends on the energy of the impacts, on the velocity and the shape of the impacting body, on the material and structure of the composite and on the geometry of the structure. Here, mainly the thickness of the component is essential. The non-destructive evaluation of these damages can be carried out using both ultrasound and active thermography methods. A comparison of the detection sensitivity of these methods for the different damages is carried out in this paper depending on the fibre composite material used (CFRP and GFRP), the thickness of the material and the impact energy. The NDT methods used after the damage are supplemented by thermographic measurements with high temporal resolution, which were already recorded during the impact.
We present an in situ triple coupling of synchrotron X-ray diffraction with Raman spectroscopy, and thermography to study milling reactions in real time. This combination of methods allows a correlation of the structural evolution with temperature information. The temperature information is crucial for understanding both the thermodynamics and reaction kinetics. The reaction mechanisms of three prototypical mechanochemical syntheses, a cocrystal formation, a C@C bond formation (Knoevenagel condensation), and the formation
of a manganese-phosphonate, were elucidated. Trends in the temperature development during milling are identified. The heat of reaction and latent heat of crystallization of the product contribute to the overall temperature increase. A decrease in temperature occurs via release of, for example, water as a byproduct.
Solid and liquid intermediates are detected. The influence of the mechanical impact could be separated from temperature effects caused by the reaction.
For assuring the safety and reliability of components and constructions in energy applications made of fiber-reinforced polymers (e.g., blades of wind turbines and tidal power plants, engine chassis, flexible oil and gas pipelines) innovative non-destructive testing methods are required. Within the European Project VITCEA complementary methods (shearography, microwave, ultrasonics and thermography) have been further developed and validated. Together with partners from the industry, test specimens have been constructed and selected on-site containing different artificial and natural defect artefacts. As base materials, carbon and glass fibers in different orientations and layering embedded in different matrix materials (epoxy, polyamide) have been considered. In this contribution, the validation of flash and lock-in thermography to these testing problems is presented. Data analysis is based on thermal contrasts and phase evaluation techniques. Experimental data are compared to analytical and numerical models. Among others, the influence of two different types of artificial defects (flat bottom holes and delaminations) with varying diameters and depths and of two different materials (CFRP and GFRP) with unidirectional and quasi-isotropic fiber alignment is discussed.
Larger high pressure die castings (HPDC) and decreasing wall thicknesses are raising the issue of casting defects like pores in aluminum structures. Properties of components are often strongly influenced by inner porosity. As these products are being established more and more in lightweight construction (e.g. automotive and other transport areas), non-destructive testing methods, which can be applied fast and on-site, are required for quality assurance. In this contribution, the application of active thermography for the direct detection of larger pores is demonstrated. The analysis of limits and accuracy of the method are completed by numerical simulation and the method is validated using computed
tomography.
Low-velocity impact damages were monitored in-situ using an infrared camera before, during and after Impact load. Thermal images were recorded as a function of time at the front side (impact) as well as at the rear side of the plates using a high frame rate. In CFRP and GFRP specimens with different thicknesses and made of various types of fibers and matrix materials, different kind of damages were observed. The sizes of the heated areas being related to the damages and the amount of energy dissipated into heat was determined quantitatively as a function of impact energy and are a measure of the resistance of the different materials against impact load.
We present a first direct measurement of the temperature during milling combined with in situ Raman spectroscopy monitoring. The data reveal a low temperature increase due to the mechanical impact and clear temperature increases as a consequence of the reaction heat. Based on the data, temperature rises as postulated in the magma plasma and hot spot theory can be excluded for soft matter milling syntheses.
The increased use of fibre-reinforced plastic (FRP) composites for improved efficiency and reliability in energy related applications e.g. wind and marine turbine blades, nacelles, oil and gas flexible risers, also increases the demand for innovative non-destructive testing technologies. Thus, in order to achieve increased acceptance of suited and optimized non-destructive testing (NDT) methods in industry, the European Metrology Research Programme (EMRP) project ENG57 Validated Inspection Techniques for Composites in Energy Applications (VITCEA) deals with the development and validation of innovative NDT technologies. In this contribution, results concerning thermographic investigations at test specimens during tensile loading and active thermography testing after tensile loading are presented. Additionally, the determination of the optical properties (relative transmittance and directional spectral emissivity) of CFRP and GFRP test specimens is described.
The increased use of fibre reinforced plastic (FRP) composites for improved efficiency and reliability in energy related applications, e.g. wind and marine turbine blades, nacelles, oil and gas flexible risers, also increases the demand for innovative non-destructive testing technologies. In this contribution, results concerning the characterisation of CFRP and GFRP during and after quasi-static tensile loading are presented. It includes the measurement of optical properties in the infrared spectral range, tensile loading tests with the observation of the temperature distribution at one or both sides of the specimens using an infrared camera for the preparation and monitoring of intended natural defects, and active thermography inspections after tensile loading. It is shown that the defect preparation was successful. Thermographic monitoring during and active thermography testing after tensile loading enable the detection of the lateral extend of the generated defects. Differences between CFRP and GFRP materials are discussed.
Fused filament fabrication (FFF) is a material extrusion-based technique often used in desktop 3D printers. Polymeric filaments are melted and are extruded through a heated nozzle to form a 3D object in layers. The extruder temperature is therefore a key parameter for a successful print job but also one of the main emission driving factors as harmful pollutants (e.g., ultrafine particles) are formed by thermal polymer degradation. The awareness of potential health risks has increased the number of emission studies in the past years. However, studies usually refer their calculated emission data to the printer set extruder temperature for comparison purposes. In this study, we used a thermocouple and an infrared camera to measure the actual extruder temperature and found significant temperature deviations to the displayed set temperature among printer models. Our result shows that printing the same filament feedstocks with three different printer models and with identical printer set temperature resulted in a variation in particle emission of around two orders of magnitude. A temperature adjustment has reduced the variation to approx. one order of magnitude. Thus, it is necessary to refer the measured emission data to the actual extruder temperature as it poses a more accurate comparison parameter for evaluation of the indoor air quality in user scenarios or for health risk assessments.