Benjamin Eckstein

Model-based detection of sensor faults under changing temperature conditions:

Nowadays, a variety of civil, aeronautic or mechanical engineering structures need regular inspections. While non-destructive testing has become state of the art, there is a trend towards online testing on demand using built-in sensors. A vast amount of in-service monitoring, as part of structural health monitoring, uses piezoelectric elements. Their deployment requires a control of the sensor performance in order to prevent false alarm. Several kinds of damage can influence the sensor performance, for example, degradation of piezoceramic or adhesive, debonding or breakage of the element. Therefore, this paper aims to detect damage of circular piezoelectric elements and their bonding layers during in-service monitoring. For this purpose, a local method, using the coupled electro-mechanical admittance, is proposed. However, changing environmental and operational conditions such as temperature have an effect on the measured quantities, masking the damage. To cope with this, the dynamic behaviour of an attached piezoelectric element including temperature trends is modelled to enable physics-based temperature compensation. This novel combination of an improved analytical model, updated according to impedance measurements, realizes a method less dependent on experimental baseline data, than the sole comparison of experimental data, established in many monitoring systems. By comparison with experimentally obtained electro-mechanical susceptance the physical model of circular piezoelectric elements is validated. The feasibility of the proposed method is presented employing experimental data of piezoelectric elements mounted on aluminium coupons, partly damaged with degradation and sensor breakage. The application produces promising results, detecting the created defects.

Lamb wave interaction at debondings due to impact damage in complex stiffened CFRP structures

The increasing usage of Carbon Fiber Reinforced Plastics (CFRP) for primary aerospace structures involves dealing with the principal susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among the variety of impact sources is the so called blunt impact, which may cause primarily damage to the internal structure. Thus, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for Structural Health Monitoring by Guided Ultrasonic Waves. Wave propagation phenomena at impact damages as well as the utilized signal processing to extract a damage related feature (i.e. damage index) contribute to the sensitivity and thus to the reliability of SHM systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for use with imaging algorithms, such as the Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of debondings and delaminations as well as the implications on imaging algorithms are examined.

Lamb Wave Interaction at Delamination and Debondings Due to Impact Damage in Complex Stiffened CFRP Structures

The increased usage of carbon fiber reinforced plastics (CFRP) for primary aerospace structures involves dealing with the susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among impact sources is the so-called blunt impact, which may cause damage primarily to the internal structure. Therefore, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for structural health monitoring by guided ultrasonic waves. Wave propagation phenomena at impact damages as well as the signal processing utilized to extract a damage related feature (i.e., damage index (DI)) contribute to the sensitivity, and thus, to the reliability of structural health monitoring (SHM) systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of delaminations and stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for the use with imaging algorithms, such as the reconstruction algorithm for the probabilistic inspection of damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of delaminations and debondings as well as the implications on imaging algorithms is examined.

Damage Classification in Aeronautic Structures using Guided Waves

A composite aircraft door surround structure with an integrated SHM network has been manufactured. The structure has been impacted creating realistic damages such as skin delaminations and stringer debondings. The guided wave signal has been acquired using the SHM-network, allowing the subsequent damage identification. The presented work focuses on the classification of different types of damage using a data-driven approach. The method focuses on feature selection and a Support Vector Machine (SVM) algorithm for damage classification. The results show a SVM algorithm capable of distinguishing between three damage types.

Analysis of loading effects on Guided Ultrasonic Waves and Damage Assessment in a full-scale CFRP fuselage structure

Structural Health Monitoring by means of Guided Ultrasonic Waves for realistic applications relies on compensation of operational and environmental effects. Besides the dominant effect of temperature, the loading condition of the structure affects wave propagation and needs to be considered when designing a SHM system and verifying it’s damage assessment performance. Experimental data from a CFRP fuselage structure under different loading conditions are analyzed regarding the loading effect on the wave propagation and subsequently the influence on damage assessment performance. For the latter purpose, the responses from barely visible impact damages are regarded under the varying loading conditions as well. doi: 10.12783/SHM2015/5

Identification of Barely Visible Impact Damages on a Stiffened Composite Panel with a Probability-based Approach

A probability-based damage detection algorithm has been implemented in order to identify barely visible impact damages in two composite stiffened panels by means of Acousto Ultrasonics (AU). A modification of RAPID (reconstruction algorithm for probabilistic inspection of defects) has been implemented to adapt the algorithm to the current structures and transducer networks. An improvement of 40% in the localization accuracy is obtained with the new algorithm