Wolfgang Hillger

Experiments to Detect Damage Progression in Axially Compressed CFRP Panels under Cyclic Loading

The aircraft industry strives for significantly reduced development and operating costs. Reduction of structural weight at safe design is one possibility to reach this objective which is aimed by the running EU project COCOMAT. The main objective of COCOMAT is a future design scenario for composite curved stiffened panels which are understood as parts of real aircraft structures. This design scenario exploits considerable reserve carrying capacities in fibre composite fuselage structures by accurate simulation of collapse. The project results will comprise an experimental data base, improved slow and fast computational tools as well as design guidelines. A reliable simulation of the collapse load requires also taking degradation into account. For the validation of the tools a sound database of experiments are needed which give information about the progress of damage during the loading process. This paper focuses on experimental results of four nominally identical CFRP panels tested within the COCOMAT project at the buckling test facility of the Institute of Composite Structures and Adaptive Systems (DLR). In a first step, three of the four panels were loaded several thousand times. Each time the panel was loaded beyond global buckling and was unloaded to zero. Finally, all panels were tested until collapse. During the tests, advanced measurement systems such as High-Speed-ARAMIS, thermography and Lamb-waves were applied. The test results given in this paper may be used as benchmarks.

Visualisation of guided wave propagation by ultrasonic imaging methods

Structural health monitoring (SHM) with Lamb waves (LWs) principally enables an in-service damage detection of large aircraft components without time-consuming scanning. Piezoelectric elements can be effectively used as senders and receivers for LWs. However, the complex interaction between defects and Lamb waves – especially for composites, the presence of two wave modes in minimum and its dispersive behaviour and reflections from all structure elements contribute to receiver signals which are very difficult to predict and to evaluate. Ulltrasonic imaging technique (UIT) can also be used for the visualisation of LW propagation in composites especially using air-coupled techniques. Out of the recorded full-wave data calculations of Lamb wave A-, B-, C- and D-scans and video animations can be carried out. Examples of visualisation in composites such as monolithic specimens, a stringer stiffened laminate and a sandwich tail boom which impressively show the Lamb wave propagation and also their interaction with defects.

Structural Health Monitoring Based on Guided Waves

Structural Health Monitoring (SHM) based on ultrasonic guided waves, so-called Lamb waves, is a promising method for in-service inspection of composite structures without time consuming scanning like conventional ultrasonic techniques. Lamb waves are able to propagate over large distances and can be easily excited and received by a network of piezoelectric actuators and sensors. In principle different kinds of structural defects can be detected and located by analyzing the sensor signals. The chapter describes recent research activities at DLR on Structural Health Monitoring. The research are focused on the visualisation of Lamb wave propagation fields based on air-coupled ultrasonic technique, the simulation of virtual sensors, mode selective actuators as well as manufacturing of actuator and sensor networks. Additionally, the chapter present the development of a SHM system for impact detection in a helicopter tailboom (Eurocopter—EC 135).

Results of Nondestructive Inspection of Layered Composites Using IR Thermography and Ultrasonics

Polymeric layered composites exhibit a variety of damages following in service loading conditions, like delamination, matrix cracking or even fibre breaking. Detection of such damages and assessing their extension and severity is vital during maintenance cycle, in view of keeping the normal operational reliability. For local inspections, IR thermography and ultrasonic scanning are among the best valued NDT methods. The paper describes the inspections performed by IR active thermography, in different variants, and pulse-echo ultrasonic scanning on GFRP. A variety of layered composites and defects/damages were inspected and the results are evaluated independently, in some cases being compared each other, with valuable conclusions for the users of the mentioned NDI techniques.

Characterization of mode selective actuator and sensor systems for Lamb wave excitation

Structural Health Monitoring (SHM) based on Lamb waves, a type of ultrasonic guided waves, is a promising technique for in-service inspection of composite structures. This study presents the development of mode selective actuator and sensor systems based on interdigital transducer (IDT) design. Various parameters such as wavelengths, number and apodization of electrodes as well as eigenfrequencies of the transducer are characterized. Therefore, an analytical model based on the theory of surface acoustic wave (SAW) filters is investigated in order to evaluate the acoustic response of the transducer. Furthermore, experimental tests on composite plates are performed.