Damage Growth Monitoring Research Articles

Damage Growth Monitoring in Cementitious Materials by Nonlinear Ultrasonic and Acoustic Emission Techniques

Damage Growth Monitoring in Cementitious Materials by Nonlinear Ultrasonic and Acoustic Emission Techniques

Real-time sinusoidal parameter estimation for damage growth monitoring during ultrasonic very high cycle fatigue tests

Ultrasonic fatigue tests (UFT) are used to study the fatigue life behavior of metallic components undergoing a very high number of cycles (typically ≥107−109 cycles) under relatively low mechanical loads. By soliciting fatigue specimens at 20 kHz, ultrasonic fatigue machines are indispensable for monitoring damage growth and fatigue failures in a reasonable amount of time. As fatigue damage accumulates in the specimen, the specimen’s free-end exhibits a nonlinear dynamic response. The resulting quasi-stationary, harmonic signals have sinusoidal parameters (frequency and amplitude) which are slowly time-varying with respect to the excitation frequency. The discrete Fourier transform (DFT) is typically used to extract these evolving sinusoidal parameters from a window of finite data of the vibration signal. Alternative spectral estimation methods, specifically line spectra estimators (LSEs), exploit a priori information of the signal via their modeling basis and overcome limitations seen by the DFT. Many LSEs are known to have state-of-the-art results when benchmarked on purely stationary signals with unit amplitudes. However, their performances are unknown in the context of slowly time-varying signals typical of UFT, leading to a widespread use of the DFT. Thus, this paper benchmarks classical and modern LSEs against specific synthetic signals which arise in UFTs. Adequate algorithms are then recommended and made publicly available to process experimental data coming from ultrasonic fatigue tests depending on performance metrics and experimental restraints.

Vibration-based damage growth monitoring in beam-like structures

Damage growth monitoring plays an important role in providing early warning of structural failure. The existing methods for damage growth monitoring are mainly local inspection methods, such as acoustic emission. These methods need a priori knowledge of accessible damage vicinity, which may not be realized in practice. Hence, vibration-based global approach is adopted to overcome these difficulties. Natural frequency, as a global modal parameter, can be measured easily and is used for vibration-based damage growth monitoring in this study. A concept of damage-induced relative natural frequency change (RNFC) curve is defined first and its relation with mode shape is then derived analytically, giving a good way to approximate RNFC curves. For monitoring damage growth, a damage growth indicator is proposed based on RNFCs between two damaged stages of a beam. The effectiveness of the indicator for damage growth monitoring is proved by both numerical and experimental cases in beam-like structures.

Fatigue damage growth monitoring for composite structures with holes

The problem discussed in the present paper deals with theoretical (numerical) and experimental analysis of static and fatigue problems for composite open holes laminated panels. The novelty of our approach depends on the application of the hybrid experimental methods and the comparison of their effectiveness in the description of complicated fatigue problems arising in the analysis of the behavior of laminated panels with open holes and subjected to tensile loading. Three experimental methods are used herein: the infrared thermography (passive), the structural health monitoring (active) and the digital image correlation. The experimental investigations are supplemented by the finite element description of the problem dealing mainly with the static behavior, monitoring the development and final fracture of composites. The considerations concern the laminated panels oriented at ±45° with different types of holes, i.e. vertical elliptical, horizontal elliptical and circular.

An experimental study of acoustic emission methodology for in service condition monitoring of wind turbine blades

A laboratory study is reported regarding fatigue damage growth monitoring in a complete 45.7 m long wind turbine blade typically designed for a 2 MW generator. The main purpose of this study was to investigate the feasibility of in-service monitoring of the structural health of blades by acoustic emission (AE). Cyclic loading by compact resonant masses was performed to accurately simulate in-service load conditions and 187 kcs of fatigue were performed over periods which totalled 21 days, during which AE monitoring was performed with a 4 sensor array. Before the final 8 days of fatigue testing a simulated rectangular defect of dimensions 1 m × 0.05 m × 0.01 m was introduced into the blade material. The growth of fatigue damage from this source defect was successfully detected from AE monitoring. The AE signals were correlated with the growth of delamination up to 0.3 m in length and channel cracking in the final two days of fatigue testing. A high detection threshold of 40 dB was employed to suppress AE noise generated by the fatigue loading, which was a realistic simulation of the noise that would be generated in service from wind impact and acoustic coupling to the tower and nacelle. In order to decrease the probability of false alarm, a threshold of 45 dB was selected for further data processing. The crack propagation related AE signals discovered by counting only received pulse signals (bursts) from 4 sensors whose arrival times lay within the maximum variation of travel times from the damage source to the different sensors in the array. Analysis of the relative arrival times at the sensors by triangulation method successfully determined the location of damage growth, which was confirmed by photographic evidence. In view of the small scale of the damage growth relative to the blade size that was successfully detected, the developed AE monitoring methodology shows excellent promise as an in-service blade integrity monitoring technique capable of providing early warnings of developing damage before it becomes too expensive to repair.

An On-Line Multiway Approach to In-Situ NDI Looking at the PZL-130TCII

Abstract Providing a reliable and universal Structural Health Monitoring (SHM) system allowing for remote aircraft inspections and a reduction of maintenance costs is a major challenge confronting the aerospace industry today. SHM based on guided Lamb waves is one of the approaches capable of addressing the issue while satisfying all the associated requirements. This paper presents a holistic approach to the continuous real time damage growth monitoring and early damage detection in aircraft structure. The main component of the system is a piezoelectric transducers (PZT) network. It is complemented by other SHM methods: Comparative Vacuum Monitoring (CVMTM) and Resistance Gauges at selected aircraft hot spots. The paper offers the description of damage detection capabilities including the analysis of data collected from the PZL-130 Orlik aircraft full-scale fatigue test.

Active self-healing mechanisms for discrete dynamic structures

SUMMARY In this paper, vibration database for damage growth monitoring and mitigation (active self-healing) in discrete structures operating with dynamic loads is considered. SHM in this context assumes a known damage location present in the structure and then determines its size at which it is present. The variations in the linear spring stiffness across the structure are considered as an indication of the damage growth. A transient response of the vibrating structure is assumed available to monitor the damage size using an extended Kalman filter algorithm. The growth of this damage is considered mitigated when the potential energy due to mode shapes that take tensile and compressive loads is minimized. These mode shapes are determined by a state feedback controller for which an actuator combination (more than one actuator) is assumed available. A linear spring–mass–damper system is considered to illustrate the active self-healing mechanisms presented in this paper. Copyright © 2013 John Wiley & Sons, Ltd.

An Approach to Damage Detection in the Aircraft Structure with the Use of Integrated Sensors – The Symost Project

Abstract This paper presents an approach to damage growth monitoring and early damage detection in the structure of PZL - 130 ORLIK TC II turbo-prop military trainer aft using the statistical models elaborated by the Polish Air Force Institute of Technology (AFIT) and the network of the sensors attached to the structure. Drawing on the previous experiences of the AFIT and AGH in structural health monitoring, the present research will deploy an array of the PZT sensors in the structure of the PZL -130 Orlik TC II aircraft. The aircraft has just started Full Scale Fatigue Test (FSFT) that will continue up to 2013. The FSFT of the structure is necessary as a consequence of the structure modification and the change of the maintenance system - the transition to Condition Based Maintenance. In this paper, a novel approach to the monitoring of the aircraft hot-spots will be presented. Special attention will be paid to the preliminary results of the statistical models that provide an automated tool to infer about the presence of damage and its size. In particular, the effectiveness of the selected signal characteristics will be assessed using dimensional reduction methods (PCA) and the so-called averaged damage indices will be delivered. Moreover, the results of the signal classification based on the neural network will be presented alongside the numerical model of the wave propagation. The work contains selected information about the project scope and the results achieved at the preliminary stage of the project

The Development of the Non-Parametric Classification Models for the Damage Monitoring on the Example of the ORLIK Aircraft Structure

This paper presents approach for the damage growth monitoring and early damage detection in the PZL-130 ORLIK TC II turbo propeller military trainer based on the array of the PZT sensors which will be deployed in the structure of the aircraft. Special attention will be paid to the preliminary results of the statistical models which provide an automated tool to infer about the damage presence and its size. In particular the effectiveness of the selected signal characteristics will be assessed using dimensional reduction methods (PCA) and the so called averaged damage indices will be described. Verification of the several classification models based on the emerged damage indices will be presented using cross validation techniques. The preliminary results of the data collected from the subcomponents tests with the model description, as well as approach for the SHM system design will be delivered. The verification of the models results will be presented on the example of the aerospace structures.

Structural Health Monitoring Approach to the Aerospace Structures

In the article only selected information regarding the SHM applications has been presented. These applications are associated with damage detection and damage growth monitoring. That approach has got crucial influence on the aircraft maintenance procedures. SHM ensures higher reliability and safety of the structures as well as repair monitoring. Right now in the AFIT, there is being conducted research work focused on the application of the SHM system for the integration with the aerospace structures for periodical and on-line monitoring.

Pulsed thermography for non-destructive evaluation and damage growth monitoring of bonded repairs

Military and civilian aircraft fleets require effective, robust, quick and reliable means of repairing aircraft structures that have experienced battle, environmental or operational damage (e.g., fatigue and corrosion). Common repair schemes use metallic or composite patches that are either mechanically fastened or adhesively bonded to the area of concern. The advantages of adhesive bonding over mechanical fastening of patch repair include the reduction of stress concentration associated with over-sizing of existing holes or machining of additional holes in the structure and improved aerodynamic profiles. However, the primary disadvantage of an adhesively bonded repair is the difficulty in identifying if the patch is properly bonded to the structure and whether it is still capable of carrying the required loads. Depending on the application, a loss of load-bearing capability of the bonded repair could lead to a reduction in the fatigue life or even a premature catastrophic failure of the structure. In this paper, pulsed thermography, an infrared non-destructive evaluation (NDE) technique, is proposed for the detection of disbond and monitoring of disbond growth in bonded graphite repairs. Correlated results with ultrasonic pulse-echo c-scan inspections and destructive testing show good disbond detection capability with an accuracy similar to that of ultrasonic inspection.

Evaluation of impact damage mechanism of multi-axial stitched CFRP laminate

This study focuses on multi-axial stitched fabric, which is a thick, high performance reinforcement for large-scale composite structures. The effects of impact damage on multi-axial stitched CFRP laminates molded by vacuum-assisted resin transfer molding (VARTM) method were evaluated. Impact damage within material was evaluated by ultrasonic scanning device and optical cross-sectional observations. Probed images obtained by both non-destructive and destructive methods were compared, and internal damage distributions of multi-axial stitched CFRP laminates were clarified. In addition, residual compressive strength and fatigue property of impact-damaged CFRP laminates were evaluated by in situ damage growth monitoring using the thermo-elastic stress analyzer (TESA). Three-dimensional damage distribution of impacted CFRP laminate was obtained from ultrasonic C-scan images and cross-sectional photographs. Damage progress behavior was observed on a destructive and non-destructive basis by post-impact fatigue (PIF) test.

Initiation and growth of delamination in glass/epoxy composites subjected to static and dynamic loading by acoustic emission monitoring

Delamination is one of the most common failure modes of composite materials. It may result from imperfections in the production process or may be caused by service life conditions, such as impact by foreign objects. The presence of delamination in the composite material may reduce the overall stiffness. The geometrical parameters, material proprieties, and loading conditions are important factors which affect the initiation, development and growth of delamination (Chen HP, David L. Comp. Sci. and Technol. 1993; 46:325–33). Experimental results obtained from monotonic tests and instrumented drop tests have shown good agreement between monotonic and dynamic critical load and crack-growth data. Previous studies made on different composite materials (Meraghni F, Benmedakhane S, Benzeggagh ML. In: Poursartip A, Street K, editors. Proc. ICCM 10, vol. I Vancouver, Canada: Woodhead Publishing, 1995:359–66; Barre S, Benzeggagh ML. Comp. Sci. and Technol. 1994; 54:369–76; Benzeggagh ML, Benmedakhene S. Comp. Sci. and Technol. 1995; 55:1–11) have allowed us to set up a schematic model of acoustic emission. In this model the different levels of amplitude signals emitted by materials under different types of loading are assigned to different damage mechanisms. This experimental methodology has been applied to this work and has allowed continuous monitoring of damage growth through delamination tests. This type of analysis has also allowed us to highlight the complementary aspect of acoustic-emission amplitude distributions with microscopic observations.