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- Impulse-thermography (3)
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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.
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.
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.