Martin Bach

Damage Assessment in Composite Structures Based on Acousto-Ultrasonics - Evaluation of Performance

This work focuses on the damage detection and assessment of barely visible impact damages that occur after impacting a composite stiffened structure through the acousto-ultrasonics technique. Delaminations and debondings have been introduced in two stiffened panels and afterwards interrogated with an integrated structural health monitoring (SHM) network within the project. In order to perform the damage assessment, a methodology for the identification of damages along with a graphical user interface is developed. The performance of the developed methodology is evaluated and a damage assessment based on several damage indices and actuation frequencies is carried out. The most suitable parameters for the given structure and damage types are finally reported.

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.

Guided wave propagation in porous unidirectional carbon fiber reinforced plastic

Networks of piezoelectric transducers mounted on aircraft structures for Acousto Ultrasonics (AU) purposes are designed to be applied during the service life of the aircraft. The approach to integrate these sensor networks already during the manufacture of carbon fiber reinforced plastic (CFRP) host structures prompts ideas to achieve an additional benefit by their application for cure monitoring, thus extending their use to the manufacturing chain. This benefit could be extended even further if guided waves generated by AU sensor networks could be used for porosity testing extensively applied for CFRP aircraft structures. In light of this, an experimental study was conducted to investigate effects of porosity on the propagation of guided waves in a basic configuration of unidirectional CFRP. Several samples were manufactured at different porosity levels by variation of the processing pressure. Wave fields were acquired using an ultrasonic scanning device. In the present work, phase velocities are chosen ...

Damage introduction, detection and assessment at CFRP door surrounding panel

Acousto-ultrasonics (AU) is a Structural Health Monitoring (SHM) technique based on a permanently installed piezoelectric transducer network, which actuates and receives ultrasonic guided waves to provide information concerning the structure integrity. In order to verify AU approach, a full-scale CFRP panel is manufactured including a co-bonded transducer network. In this paper, the verification approach is described, starting with the test procedure including damage introduction and reference NDI. The verification procedure is followed by data interrogation of the transducer network using a commercial interrogation unit and respective data evaluation by probability-based imaging methodology, which has been integrated into a Graphical User Interface (GUI) in the SARISTU project. In this paper, the methodology is verified against requirements concerning the minimum damage size to be detected, the accuracy of damage size assessment, and the accuracy of localization of damage. Damage-type differentiation, like delamination or debonding is not aimed in the course of the methodology.

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