Active Thermography for Automated Testing of Composite Structural Components
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While the use of active thermography for non-destructive testing supplies valuable information on component quality, the measurement results are still very often evaluated manually. Ottronic now presents a method that automatically classifies and interprets measured data. Active thermography is thus paving the way for intelligent, self-optimised production lines.
Lightweight Design Requires a Systematic Approach
Mechanics Determine Quality
The special mechanical properties of CFRPs and other FRCs originate in the interaction between fibres and the polymer in which they are embedded. The layered structure of FRC components allows the flexible design of stiffness properties that can be highly anisotropic and arranged differently depending on location [1, 2, 3]. However, these advantages over more homogeneous materials depend on the quality of the composite structure. Deviations from the planned orientation of the fibres, dry areas resulting from missing polymer, delamination between the individual layers and variations in the fibre content are only a few examples of potential quality variations that can have drastic effects on the mechanical properties of the component. Many quality attributes are determined by the production process . For this reason, a large number of quality-determining measures accompany the formation of FRC components — from development through to production. Non-destructive testing in particular is gaining in importance in this area.
Active Thermography for Extensive Testing
Modern testing methods, such as active thermography, supply comprehensive quality information on components like the CFRP rear wall. Active thermography is a test method in which the elements to be tested are subjected to brief thermal variations using sources of heat. Areas in the test object with different thermophysical properties will also have an effect on the way heat is transferred locally. These differences are picked up by corresponding infrared (IR) detectors and visualised with the help of appropriate algorithms. The resulting images show variations in the thermal properties of the test objects and can be interpreted and evaluated by the operator on the basis of experience and correlation.
Comprehensive studies on a range of quality attributes and material combinations of FRC show that the method is not only suitable for testing the quality of FRCs but is actually sometimes preferable to other non-destructive testing methods [5, 6, 7, 8]. While economic considerations support this view, the method has other advantages that can be highlighted. First, active thermography is based on measuring the thermophysical properties of test objects. It is precisely these properties that are influenced by the quality attributes and variations that are sought. Second, the method does not emit any ionising radiation.
Testing Large Aircraft Parts
The fact that aircraft manufacturer Boeing has authorised its supplier FACC to use active thermography as a test method — showing that the aerospace industry is adopting this method on an industrial scale in parallel with the automotive industry — can be attributed to advanced developments in sensors, data interfaces and electronics . An advantage of active thermography is the speed at which large-format test objects can be measured. This is the reason why the rapid development of infrared cameras (greater temporal, geometric and thermal resolution) and the inexpensive availability of large computing capacities are contributing to the deployment of commercial applications in a range of industries.
Divergent fibre orientation, dry areas or delamination can have drastic effects on the mechanical properties of the component.
Optimised Test Results for Visual Assessment
Differences in thermophysical properties are picked up by infrared detectors and visualised.
Self-optimisation of Production Chains
The character of the testing method enables it to be used for other industrial purposes. This non-invasive and non-destructive approach that can measure various test areas quickly and flexibly makes it ideal for interim and final testing in manufacturing processes. Making the test results available to the processing systems for intervention in the manufacturing process creates a closed control loop consisting of the manufacturing process and quality testing, and this forms the basis for self-optimising production chains.
For this purpose, it must be possible to automatically classify the data from active thermography. Every class contains values of the measured parameter that can be linked to a test object attribute. Two classes can be used, for example, to differentiate between expected and divergent value ranges. Further classes allow other variations to be differentiated. While humans are able to classify and evaluate complex visual impressions using their knowledge and experience, automated classification is based on clear criteria and rules. These need to be determined and defined by humans before being “taught” to the machine in a further step.
There are two goals associated with the approach to this task. First, the measurement setup for changing environmental conditions needs to be thermodynamically independent. Second, measurement information is required to which clear and robust criteria and rules can be applied. Ottronic Regeltechnik has introduced successful concepts based on its system for active thermography.
Automated Results Evaluation
Thermodynamics as the Basis
The second goal is concerned with finding a thermodynamic measurement parameter that allows simple assignment of thermophysical properties for every point on the test object. If the required assignment can be based on direct physical correlations, it is possible to trace clear and solid relationships between the value of the measured parameter and the cause of the variation in the thermophysical property.
This example shows that it is relatively easy to classify different areas based on physical relationships. This makes it easier to specify rules for automated classification, often without the need for time-consuming empirical studies.
Automatic Evaluation of Quality
Additional rules need to be set up for an interpretation in terms of a good/bad evaluation of the variations classified automatically by the test system. Direct assignment of test object properties to a physical measurement parameter also helps in this case. Following initial classification of the good/bad decision and after the system has “learned” the necessary rules for this, interpretation can be performed automatically following classification.
A closed control loop consisting of the manufacturing process and quality testing permits self-optimising production chains.
The example of the automated interpretation of results can also be used to illustrate how correlation is easier to achieve via the direct relationship of the measured parameter to the properties of the test object than when the measurement results are available in an abstract form, such as differences in contrast and relative variations in information.
This automated classification and interpretation of test information, which has been made possible for the first time with the new test system from Ottronic, represents a milestone for the application of active thermography integrated into the process chain.
Electric Vehicles with Greater Range
Further possible uses for the new system are demonstrated by an application in the field of e-mobility. Energy efficiency plays a very special role for electric-powered vehicles, in particular through its correlation with range. As discussed at the beginning of this article, lightweight design is one of the possibilities to exert a positive influence on this. If powerful batteries are used, heat management in their proximity becomes an important parameter in energy optimisation. Since active cooling consumes large amounts of energy, the ability to dissipate heat via the structure surrounding the batteries plays a vital role. The possibility of using large-format active thermography to determine these structures allows an individual — and hence optimised control loop — to be developed to dissipate heat from the battery. This enables thermal management to be optimised and makes a positive contribution towards the range of the overall vehicle. In addition, taking account of individual and locally different values for α allows highly efficient quality assurance to be performed.
This and other applications in the automotive and aerospace industries show how the combination of established processes and methods and new ways of thinking can turn a robust and simple principle of measurement, such as active thermography, into a versatile testing method with many possible uses. Improved test equipment and new analytical algorithms can contribute to new and better results in many areas, ranging from classic manual test applications to high-end applications in automated intelligent production lines. Applications for optimising weight and in the thermal management of electric-powered vehicles show the wide range of possible applications for active thermography. Directly measured thermophysical parameters as well as information about the mechanical characteristics of components obtained through interpretation contribute to knowledge about the status and quality of individual structures and entire systems and thereby to their optimisation.
- Fischlschweiger, M.: Integrated defect classification in manufacturing of carbon fibre reinforced thermoplastic polymer matrix composites. International Conference on Processing and Manufacturing of Advanced Materials, Graz, 2016Google Scholar
- Fischlschweiger, M.; Stock, A. Thurmeier, M.: Integrated Defect Classification in Manufacturing of Carbon Fibre Reinforced Thermoplastic Polymer Matrix Composites. In: Materials Science Forum, 879 (2017)Google Scholar
- Maierhofer, C.; Röllig, M.; Ehrig, K.; Meinel, D.: Validierung der aktiven Thermografie mittels CT zur Charakterisierung von Inhomogenitäten und Fehlstellen in CFK. In: DGZfP-DACH-Jahrestagung Mi.3.C.1. (2015)Google Scholar
- Schachinger, A.: Prüfkompetenz bei Leichtbauteilen: FACC erhält Boeing Qualifikation für aktive Thermografie. Online: www.facc.com/Aktuelles/News-Presse/Pruefkompetenz-bei-Leichtbauteilen-FACC-erhaelt-Boeing-Qualifikation-fuer-aktive- Thermografie, access: 31.08.2017.
- Zauner, G.; Mayr, G. Hendorfer, G.: Application of wavelet analysis in active thermography for nondestructive testing of CFRP composites. In: Proceedings of SPIE, 6383 (2006)Google Scholar
- Thurmeier, M.; Fischlschweiger, M.; Stock, A.: Defying Hostile Environments. In: Kunststoffe international 6-7 (2016), pp. 32–34Google Scholar