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CFRP materials are progressively used in areas where the weight as well as the strength of materials matter. Since they are used in safety relevant environments, like aircrafts and cars, an assessment of the components for defects is mandatory. Flash thermography has proven to be a valuable tool to resolve defects. However, a quantitative evaluation of the defect extent and depth within the specimen is extremely challenging, since the commonly used analytical models do not incorporate lateral heat flow around the defect areas. Hence a reconstruction of the samples by numerical simulations is a path to improve the results. This finally enables pass and fail tests for specimens based on quantitative results.
The properties of the anisotropic CFRP materials vary dependent on the matrix and fibre chosen. In addition the fabrication processes are distinct, hence it is strongly desirable to measure the properties of a sound sample in a fast and reliable way. The knowledge of diffusivity and heat capacity in three dimensions finally enables the simulation to improve modelling the specimen geometry.
The in-plane diffusivity is determined by heating the sample with a laser line (a flat top profiled focused in one dimension) and monitoring the transient temperature at the front surface of the sample. This method is compared to measurements of the diffusivity using a Gaussian laser spot. The results are compared to results obtained by flash thermography in transmission.
In addition the heating process of the sample on its surface as well the heat conduction is monitored using an IR-microscopic objective. Hence the transparency of the sample as well as the temperature conduction of the two components in microscopic scale can be studied. Finally the results are applied to provide precise parameters in the simulation of defects in a CFRP sample.
It is gratefully acknowledged that the present work is funded by the DFG within the framework of the project WE 2937/6-1.
So far, only destructive measurement techniques are available for thickness determination of polymer based surface protection systems for concrete surfaces. Pulse thermography appears to be well suited for non-destructive thickness evaluation in these systems. Here, we present first results of the development of a respective measurement and analysis procedure. Since surface protection systems consist of a number of layers, a model for the calculation of the surface temperature of a multi-layer structure on an infinite (concrete) substrate in pulse thermography setup was developed. It considers semitransparency of the upmost layer and thermal losses at the surface. It also supports the use of an arbitrary temporal shape of the heating pulse to properly describe the measurement conditions for different heat sources. First experimental results regarding the verification of the model are presented.
Funding by the Federal Ministry for Economic Affairs and Energy is gratefully acknowledged.
The detection and characterization of surface breaking cracks in steel prior to damage is a technologically as well as economically important task especially for safety-relevant structures. Detection of small cracks already during the steel production process might significantly reduce the risk of failure and reduce production costs due to an obsolete post-processing.
However, the hostile environmental conditions (high temperature specimens) together with very strict requirements in current steel production (production speeds, in-line testing and evaluation) are challenging and render many well-established NDE techniques hardly applicable.
We present an approach to use laser thermographic testing as a fast, remote and contactless NDE method, that addresses these challenges and might ultimately allow for online crack detection.
The basic idea of laser thermographic testing, introduced by Kubiak in 1968, is the monitoring of the heat flow as induced by local heating. Disturbances within the heat flow generated by the presence of surface cracks can then be analyzed by image processing algorithms, as we have shown in previous work.
The aim of the presented work is to advance laser thermographic testing to be applicable to the specific conditions of steel production environments. This purpose was met by the development of a laboratory setup that allows us to simulate production conditions, as rolling speed, specimen temperature, laser heating power and study their influence on crack detection performance. This parametric study enabled us to develop and improve data processing and crack detection algorithms with the final goal of providing optimized in-line crack detection. The studies were accompanied by comprehensive FEM simulations to intensify the understanding of the contrast formation as well as the crucial parameters influencing the performance of the method.
In this work, we describe the extension of the applicability of a point-shaped energy density detector formerly only applicable for flash lamp excitation to optical heat sources for step-heating thermography, such as IR lamps or halogen lamps. Such continuous heat sources are often used in active thermography testing, when materials with low thermal diffusivity and specimens with large heat capacity are examined and large amounts of energy have to be deposited. Here, the temperature transient at the sensor surface is recorded by the infrared camera that is already present. The energy (power) input into the investigated specimen can be determined during the following data evaluation by fitting the results of an analytical model to the experimental data.
Pulse and flash thermography are experimental techniques which are widely used in the field of non-destructive evaluation for materials characterization and defect detection. We recently showed that it is possible to quantitatively determine the thickness of semitransparent polymeric solids by fitting of results of an analytical model to experimental flash thermography data, for both transmission and reflection geometry. However, depending on the chosen experimental geometry, different effective optical absorption coefficients (i.e., penetration depths of the heating light) had to be utilized in the model in order to properly fit the experimental data. Here we show that this effect is caused by the wavelength dependency of the absorption coefficient of the sample material. We present an extension of the analytical model to incorporate this dispersion. Even accounting for only two different values of the absorption coefficient, the experimental results from both measurement geometries can be fitted by a single set of absorption coefficients. Additionally, the deviations between experimental data and fit are reduced compared to a single optimized effective absorption coefficient.
This work is part of the ZIM-project KF2201089AT4 and is funded by the German Federal Ministry for Economic Affairs and Energy due to an order of the German Bundestag.
Among the photothermal methods, full-field thermal imaging is used to characterize materials, to determine thicknesses of layers, or to find inhomogeneities such as voids or cracks. The use of classical light sources such as flash lamps (impulse heating) or halogen lamps (modulated heating) led to a variety of nondestructive testing methods, in particular lock-in and flash-thermography.
In vertical cavity surface emitting lasers (VCSELs), laser light is emitted perpendicular to the surface with a symmetrical beam profile. Due to the vertical structure, they can be arranged in large arrays of many thousands of individual lasers, which allows power scaling into the kilowatt range. Recently, a high-power yet very compact version of such a VCSEL-array became available that offers both the fast timing behavior of a laser as well as the large illumination area of a lamp.
Moreover, it allows a spatial and temporal control of the heating because individual parts of the array can be controlled arbitrarily in frequency, amplitude, and phase. In conjunction with a fast infrared camera, such structured heating opens up a field of novel thermal imaging and testing methods. As a first demonstration of this ansatz, we chose a testing problem very challenging to conventional thermal infrared testing: The detection of very thin subsurface defects perpendicularly oriented to the surface of metallic samples. First, we generate destructively interfering thermal wave fields which are then affected by the presence of defects within their reach. It turned out that this technique allows highly sensitive detection of subsurface defects down to depths in excess of the usual thermographic rule of thumb, with no need for a reference or surface preparation.
The detection of cracks before the failure is highly significant when it comes to safety-relevant structures. Crack detection in metallic samples at high surface temperature is one of the challenging situation in manufacturing industries. Laser thermography has already proved its detection capability of surface cracks in metallic samples at room temperature. In this work a continuous wave (CW) laser use to generate a laser, which is using to scan the metal surface with notch. The corresponding heat distribution on the surface monitored using infrared thermal (IR) camera. A simplified 3D model for laser thermography is developed and validated with experimental results. A dedicated image processing algorithm developed to improve the detectability of the cracks. To understand the dependency of surface temperature, laser power, laser scanning speed etc. in defect detection, we carried out parametric studies with our validated model. Here we report the capability of laser thermography in crack detection at elevated temperature.
Optical lock-in thermography is a completely contactless and very sensitive NDT technique. As an optical source of energy, incandescent lamps are most commonly used because they are relatively inexpensive and offer high irradiances at the test specimen. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration.
We present current activities with kilowatt-class high-power laser sources for advanced lock-in thermography and focus on the application of laser arrays that offer a very high irradiation strength over a large sample area beyond the mentioned advantages.
Optical lock-in thermography is a completely contactless and very sensitive NDT technique. As an optical source of energy, incandescent (i.e. halogen) lamps are most commonly used because they are relatively inexpensive and offer high irradiances at the test site. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz, see Fig.1. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration.
Using the one-dimensional solution to the thermal heat diffusion equation together with the absorptance of the material which is illuminated with a harmonically modulated light source, we can calculate the temperature oscillation at the surface of a solid. As a second step, we calculate the corresponding oscillation of the total thermal emission using Stefan-Boltzmann law as a first order approximation and taking into account the emissivity of the material. Within this framework we can calculate the minimal irradiance of a light source necessary to provoke a measurable signal within a thermographic camera at a noise equivalent temperature difference (NETD) of 30 mK. In Fig. 2 this relationship is displayed for a wide spectrum of modulation frequencies and for a number of different light sources scaled to the same electrical input power and illumination area. Using this figure, it is now easily possible to analyze the range of materials to be tested using lock-in thermography, since only the materials (dotted lines) below the irradiance-vs-frequency curves (solid lines) are heated in excess of the camera’s NETD. This figure clearly shows that laser sources considerably increase the application range of lock-in thermography, since especially for metals with a high reflectance and high thermal diffusivity a high irradiance is vitally important to allow for lock-in texting.
We present current activities with kilowatt-class high-power laser sources for advanced lock-in thermography and focus on the application of laser arrays that offer a very high irradiation strength over a large sample area beyond the mentioned advantages.
Quantification of impact damages in CFRP and GFRP structures with thermography and ultrasonics
(2018)
For the quantification of impact damage in CFRP and GFRP structures, first passive thermography measurements have been performed at the front and rear side of the test specimens during low velocity impact load. After loading, the damaged structures have been investigated non-destructively with active thermography with flash excitation and with step heating and with two different ultrasonic methods (phased array and air coupled ultrasonics). The damage sizes quantified with all methods are evaluated and compared. In addition, the depth of the greatest damage inside the different test specimens is analysed.