In situ Thermographie zur Prognose des Ermüdungs-Risswachstums in intrinsischen Al-CFK Hybridbauteilen unter zugschwellender Ermüdung im High-Cycle Regime

Die Lebensdauer von intrinsischen Multimaterialhybriden wird maßgeblich von der Resistenz gegen ermüdungsbedingte Schädigung bestimmt. Eine Zustandsbewertung und Prognose dieser ist bis dato schwierig, da die zugrundeliegenden Prozesse an intrinsischen Grenzflächen in hybriden Strukturen nur wenig bekannt und die Anwendung der Bruchmechanik nur schwer möglich sind. Daher kommen den zerstörungsfreien Prüfverfahren in Methoden zur Zustandsbewertung und Lebensdauerprognose eine zentrale Rolle zu, da bei in situ Anwendung die Schädigung direkt lokalisiert und bestimmt werden kann. In dieser Arbeit findet zur Prognose der ermüdungsbedingten Schädigung in Aluminium-CFK Hybridverbindungen mit thermoplastischer Zwischenschicht die passive Thermographie in situ während quasi-statischer und ermüdender Zugbelastung Anwendung. Durch die thermische Dissipation wird die ermüdungsbedingte Schädigung an den intrinsischen Grenzflächen in T- und Lock-In-Amplituden-Bildern erfasst. Die gefundenen Schädigungsmechanismen, translaminares Risswachstum und Delaminationen, verhelfen zu einem besseren Verständnis der Schädigungsprozesse und verschiedener Einflussgrößen. Die Korrelation der Fehlergröße mit mechanischen Messgrößen führt zu einer Abschätzung des Schädigungsverlaufs. Zusätzlich geht die Korrelation der thermischen Dissipation mit der Energiefreisetzungsrate über den Stand der Forschung hinaus und realisiert die Verknüpfung von Thermographie und Bruchmechanik.The lifecycle performance of multi-material-hybrids is mainly determined by the structural resistance to fatigue damage propagation. The assessment and prognosis of the damage evolution come along with numerous difficulties, as the underlying damage processes at the intrinsic interfaces are hardly known. Also, the application of fracture mechanics is hard to pursue for non-self-similar damage growth. Hence, non-destructive testing methods became of major importance to methods of damage-evaluation and failure assessment, as they enable to locate and to evaluate the occurring damage during mechanical testing. In this work the damage propagation in aluminum-CFRP hybrid-joints with a thermoplastic interlayer is monitored with in situ passive thermography under quasi-static and fatigue tensile loading. Since any damage arouses thermal dissipation, it is evaluated using T- or Lock-In-amplitude images. The results reveal two competing damage mechanisms, translaminar cracking and delamination of the intrinsic interfaces and between the middle CFRP-plies, which helps to better understand the effect of influencing factors on the damage processes. The correlation of the damage-size with mechanical quantities leads to an estimation of the damage evolution up to failure. In a novel approach, the prognosis is achieved by correlating the thermal dissipation with the strain-energy-release-rate. Hence, the link between a quantitative thermography-measure with fracture mechanics is obtained.DF

Conception of an Eddy Current In-process Quality Control for the Production of Carbon Fibre Reinforced Components in the RTM Process Chain

The integration of quality control processes in immature production systems such as the resin transfer moulding (RTM) process in the production of carbon fibre reinforced plastics (CFRP) faces numerous challenges. Requirements towards the reliability and product design as well as the consideration of economic restrictions lead to challenging requirements for measurement systems. This paper presents the development of a process integrated quality control using eddy current inspection. The concept focuses on an eddy current sensor array that is integrated in a preforming tool and thus enables a 100% quality control of CRFP parts with minor effects on the production environment

Process Monitoring of a Vibration Dampening CFRP Drill Tube in BTA deep hole drilling using Fibre-Bragg-Grating Sensors

The large tool length in BTA deep hole drilling often leads to strong torsional vibrations of the tool system, leading to a reduced bore hole quality failures. When substituting steel drill tubes with tubes from composite material, the laminate structure dampens these vibrations. Secondly, the integration of sensors allow to monitor process vibrations. This contribution introduces a new sensor platform to measure process vibrations, feed force and drilling torque using Fibre-Bragg Grating Sensors. The presented experimental results focus on characteristic frequency spectra with natural torsional and compression frequencies of the CFRP drill tube, which show variations due to changed feed

Using Quantitative Passive Thermography and Modified Paris-Law for Probabilistic Calculation of the Fatigue Damage Development in a CFRP-Aluminum Hybrid Joint

Although metal to Carbon-fiber-reinforced-polymer (CFRP) hybrid-joints possess a high lightweight construction potential, their extensive application has to deal with interfacial stress concentrations promoting fatigue damage. Furthermore, the underlying damage processes and their influencing factors are still not completely understood. Besides interfacial property-gradients, generic shapes counteract a precise determination of local stresses or strains, respectively. Hence, new methods are required that combine non-destructive testing and fracture mechanics to account for the fatigue damage. In this work, data of mechanical fatigue testing of an aluminum-CFRP hybrid-structure is presented by means of the dynamic stiffness and the mechanical hysteresis. Additionally, in situ passive thermography allows for capturing the heat development due to delamination growth. Correlating the obtained data implies that faster delamination growth coincides with higher amplitude values of lock-in thermography and higher mechanical hysteresis. Supported by this observation, a model is formulated to calculate the local dissipation per loading cycle. Further integration into a Paris-law like formulation results in a calculation model to account for the mode-I fatigue delamination growth. Additional validation of the model parameters shows good agreement with the experimental data

Using Quantitative Passive Thermography and Modified Paris-Law for Probabilistic Calculation of the Fatigue Damage Development in a CFRP-Aluminum Hybrid Joint

Although metal to Carbon-fiber-reinforced-polymer (CFRP) hybrid-joints possess a high lightweight construction potential, their extensive application has to deal with interfacial stress concentrations promoting fatigue damage. Furthermore, the underlying damage processes and their influencing factors are still not completely understood. Besides interfacial property-gradients, generic shapes counteract a precise determination of local stresses or strains, respectively. Hence, new methods are required that combine non-destructive testing and fracture mechanics to account for the fatigue damage. In this work, data of mechanical fatigue testing of an aluminum-CFRP hybrid-structure is presented by means of the dynamic stiffness and the mechanical hysteresis. Additionally, in situ passive thermography allows for capturing the heat development due to delamination growth. Correlating the obtained data implies that faster delamination growth coincides with higher amplitude values of lock-in thermography and higher mechanical hysteresis. Supported by this observation, a model is formulated to calculate the local dissipation per loading cycle. Further integration into a Paris-law like formulation results in a calculation model to account for the mode-I fatigue delamination growth. Additional validation of the model parameters shows good agreement with the experimental data

Using nondestructive testing methods to characterise production-induced defects in a metal-CFRP hybrid structure

In this work different nondestructive testing methods, such as active and passive thermography as well as ultrasonic with special probes, so-called EMAT`s (ElectroMagnetic AcousticTransducer) will be applied for the characterisation of a metal-CFRP (Carbon Fibre Reinforced Polymer) hybrid structure. To validate these testing methods, different kinds of artificial defects are inserted into the hybrid structure. It is shown that active thermography is very suitable to detect the artificial inserted defects not only in the hybrid structure but also in the CFRP structure itself. Furthermore the bonding between metal and the polymer is characterised very well. Likewise CFRP samples are investigated by passive thermography while being mechanically loaded through tensile or fatigue tests to give a predication about the influence of the defects on the mechanical properties (effects of defects).Finally an outlook on an optimised geometry of the hybrid structure as well as on further excitation methods for active thermography is given

Non-destructive testing for evaluation of defects and interfaces in metal carbon fiber reinforced polymer hybrids

In this work, different non-destructive testing methods for the characterization of defects and interfaces are presented. It is shown that, by means of active thermography, defects in the interface and in the carbon fiber reinforced polymer (CFRP) itself can be detected and determined. The bonding of metal and thermoplastic can be characterized very well by ultrasonic testing with electromagnetic acoustic transducers (EMAT). Mechanical testing is combined with passive thermography to correlate mechanical values with the defect-size. There is also a comparison between active and passive thermography. Mechanical testing shows the influence of different defects. Furthermore, a correlation of defect-size and loading to rupture was performed

Characterization of Ancient Marquetry Using Different Non-Destructive Testing Techniques

Non-destructive testing of objects and structures is a valuable tool, especially in cultural heritage where the preservation of the inspected sample is of vital importance. In this paper, a decorative marquetry sample is inspected with three non-destructive testing (NDT) techniques: air-coupled ultrasound, X-ray micro-tomography, and infrared thermography. Results from the three techniques were compared and discussed. X-ray micro-tomography presented the most detailed results. On the other hand, infrared thermography provided interesting results with the advantage of being cheap and easy in the deployment of the NDT method