Christian Boller

Next generation structural health monitoring and its integration into aircraft design

Structural health monitoring (SHM) has become increasingly important with regard to ageing aircraft, required enhanced performance and the need to reduce aircraft operational cost. Affordable advanced miniaturized sensors and continuous improvement in data processing technology combined with powerful software algorithms has allowed non-destructive testing (NDT) to become an integral part of structural materials and has thus given structures a self-sensing functionality. This paper describes where to integrate SHM into the aircraft design process and how to validate the established loads monitoring process in comparison to an emerging damage monitoring solution, before more precisely describing a technology selection regarding damage monitoring. Acousto-ultrasonics is a technique being especially emphasized. Results from laboratory experiments will be shown and commented and a perspective of future trends will be given.

Ways and options for aircraft structural health management

The number of aircraft is increasing worldwide as well as their age. This has led to an increasing market of aging aircraft. The older an aircraft structure becomes, the more difficult it may be to predict under which conditions it will be flown and this especially when it is influenced by continuous upgrades in avionics, flight control systems or even engines, thus resulting in a possible change of flight envelopes. It is under these conditions that the aircraft structure needs increased care regarding monitoring and subsequent life management. This paper addresses the different means of how this can be done, starting with conventional inspection and gradually moving over to state-of-the-art loads and finally damage monitoring, where the latter is very much driven by ongoing smart materials and structures initiatives. Benefits either obtained or still to be expected from the different ways of monitoring are described.

Fatigue in aerostructures—where structural health monitoring can contribute to a complex subject

An overview of the aircraft design and maintenance process is given with specific emphasis on the fatigue design as well as the phenomenon of the ageing aircraft observed over the life cycle. The different measures taken to guarantee structural integrity along the maintenance process are addressed. The impact of structural health monitoring as a means of possibly revolutionizing the current aircraft structural monitoring and design process is emphasized and comparison is made to jet engines and helicopters, where health monitoring has already found the respective breakthrough.

Analysis of Controlled Beam Deflections Using SMA Wires

In this paper the active control of beam deflection through heating and cooling of Shape Memory Alloy (SMA) wires is examined. A phenomenological constitutive law for SMA wires is coupled with beam theory to provide predictions of beam shape upon temperature change in the SMA actuator. Both the linear and nonlinear beam theory are presented, enabling calculation of large deflections. Examples for a single wire attached at the tip of a uniform beam are given, but the procedure can easily be generalized for other configurations and utilized in control algorithms. Issues of design constraints for shape control with shape memory wires are addressed and the model is qualitatively verified by experiments.

Crack Detection in Metallic Structures Using Broadband Excitation of Acousto-Ultrasonics:

This paper reports the application of piezoceramic sensors for crack detection in metallic structures. Rectangular aluminium plates with a crack initiated by spark erosion were used in a simple fatigue experiment. The plates were instrumented with piezoceramic devices bonded in a symmetrical configuration on both sides of the crack. One of the piezoceramics was used as an actuator and excited by a sine sweep and Gaussian white noise signals in order to exploit broadband excitation. The plates were subjected to static and dynamic tensile loading. The growing crack was monitored by the remaining piezoceramic sensors. The experimental data were analyzed using time, frequency and wavelet domain statistical parameters. The results show the potential of the low-frequency broadband excitation for structure-integrated damage detection systems based on acousto-ultrasonics in metallics.

New technological development of passive and active vibration control: analysis and test

We present a brief summary of new technical developments of passive and active vibration controls which we have performed for the last several years partly as an international collaborative R&D project on Smart Materials and Structural Systems sponsored by the Japanese Ministry of Economy, Trade and Industry. In connection with the passive damping control, shape memory alloys (SMAs) were used as damping elements. To examine the effect of damping enhancement, beams with SMA films bonded onto them or SMA wires embedded into them were made, and free damped oscillations were measured. The damping coefficient increased by more than 100% compared with the beams without SMAs. Thermodynamic behaviors of an SMA wire and film were also investigated experimentally and numerically. In active vibration control, a new concept of smart material systems was proposed. That is a partially magnetized alloy, which is stiff and strong enough as a structural element and responds sufficiently quickly as an actuator due to an electromagnetic force. A simplified experiment and numerical simulation were performed and the results showed the feasibility of the proposed smart material system using the electromagnetic force.

Design and performance of a shape memory alloy-reinforced composite aerodynamic profile

Based on a shape memory alloy (SMA)-reinforced composite developed separately, the applicability of the composite has been demonstrated through realization of a realistically scaled aerodynamic profile of around 0.5 m span by 0.5 m root chord whose skins had been made from this composite. The design, manufacturing and assembly of the profile are described. The curved skins were manufactured with two layers of SMA wires integrated into the layup of aramid fibre prepregs. All SMA wires were connected such that they can be operated as individual sets of wires and at low voltages, similar to the conditions for electrical energy generation in a real aircraft. The profile was then mounted on a vibration test rig and excited by a shaker at its tip which allowed the dynamic performance of the profile to be validated under internal actuation conditions generated through the SMA wires.

Minimalistic Devices and Sensors for Micromagnetic Materials Characterization

Micromagnetic materials characterization requires sensors which essentially consist of two critical elements: an electromagnet which introduces a well-defined magnetic field to the material, and a sensor system which detects the materials response to the applied magnetic field. The devices developed at Fraunhofer IZFP obtain a multiparametric “magnetic fingerprint” with these sensors by means of several methods. The magnetic fingerprints of calibration samples are used as input for pattern recognition or regression analysis, thus allowing the prediction of mechanical-technological material characteristics (hardness, yield strength, etc.) or residual stress. This approach is called micromagnetic multiparameter microstructure and stress analysis (3MA). The long-term stability and reproducibility of the sensor and device characteristics are crucial for the reliability of the measured results. Therefore, the measuring hardware should follow a minimalistic approach. In this paper, we propose a way of simplifying the measuring hardware by multiple use of sensor elements, reducing the analog signal processing chain and transferring most signal processing tasks to the PC.

Performance of SMA-reinforced composites in an aerodynamic profile

Within the European collaborative applied fundamental research project ADAPT, fundamentals of SMA-reinforced composites were evaluated and the specific manufacturing techniques for these composites developed and realised. The involved partners are listed at the end. To demonstrate applicability of these composites a realistically scaled aerodynamic profile of around 0.5m span by 0.5m root chord was designed, manufactured and assembled. The curved skins were manufactured as SMA composites with two layers of SMA-wires integrated into the layup of aramid fibre prepregs. All SMA wires were connected such that they can be operated as individual sets of wires and at low voltages, similar to the conditions for electrical energy generation in a real aircraft. The profile was then mounted on a vibration test rig and activated and excited by a shaker at its tip which allowed to test the dynamic performance of the profile under different external loading conditions with various internal actuation conditions through the SMA wires. The paper includes some background of the design and manufacturing of the aerodynamic profile and will discuss some of the results determined recently on the test rig. A view with regard to future wind tunnel testing will be given as well.

Nonlinear ultrasonic testing of carbon fibre reinforced plastics in the very high cycle fatigue regime

Carbon fibre reinforced plastics (CFRP) are applied as a light-weight construction material. In their lifespan in service CFRP components are exposed to variable loads, which can amount to 1011 load cycles. It is therefore important to examine the fatigue of CFRP materials in the very high cycle regime (more than 107 loading cycles). In a joint project a three point bending ultrasonic fatigue testing system was developed at WKK. A high amplitude sinusoidal vibration is added to a static load. The operating frequency of the system f = 20 kHz is used for sample load case and at the same time as input for online ultrasonic monitoring of the fatigue process. CFRP plate material with a thickness of 4 mm was investigated. During the fatigue process the vibrations of CFRP samples were measured with a laser vibrometer and a microphone. The analysis methods fast Fourier, short time Fourier, and Hilbert Huang transformation were used to evaluate the linear and nonlinear spectral content of the signals. Increase of ...