Invisible Damage Research Articles

Acoustic emission localization based on FBG sensing network and SVR algorithm

In practical application, carbon fiber reinforced plastics (CFRP) structures are easy to appear all sorts of invisible damages. So the damages should be timely located and detected for the safety of CFPR structures. In this paper, an acoustic emission (AE) localization system based on fiber Bragg grating (FBG) sensing network and support vector regression (SVR) is proposed for damage localization. AE signals, which are caused by damage, are acquired by high speed FBG interrogation. According to the Shannon wavelet transform, time differences between AE signals are extracted for localization algorithm based on SVR. According to the SVR model, the coordinate of AE source can be accurately predicted without wave velocity. The FBG system and localization algorithm are verified on a 500 mm×500 mm×2 mm CFRP plate. The experimental results show that the average error of localization system is 2.8 mm and the training time is 0.07 s.

Damage Detection of Fibre-Reinforced Composite Structures Using Experimental Modal Analysis

The dynamic properties of structures change due to the damages produced during the manufacturing process or operation conditions. This is more critical for structures made of fiber-reinforced composite (FRC) materials owing the fact that they possess more complicated structural components which are under invisible delaminations. If the relationship between the properties and the damage is not known, experimental investigation of the changes can be employed to detect and quantify the damages to some extent. In this paper, the structural response of several FRC components are investigated by using non-contact experimental modal analysis employing Laser Vibrometer. To this end, the components with and without artificial invisible damages are tested to identify the changes in the natural frequencies and mode shapes. The method merits and its limitations are discussed.

Study of Impact Damage Behavior in Woven Carbon Fiber Plates

Composites are being used extensively in aerospace structures as their properties offer significant advantages such as better strength to weight capacity. However, failure in composite structures due to low-velocity impact raises a maintenance concern because it can lead to an invisible damage. Hence, the objective of this study is to investigate impact damage behavior in woven composite plates via microscopic examinations. Several specimens were fabricated and were subjected to different levels of impact energy. Scanning Electron Microscopy (SEM) and X-ray imaging were used to examine the impacted area. The results showed that the impact damage is dependent on the plate thickness and also the impact energy. The outcome of this research will help to establish an efficient modelling technique for non-destructive evaluation of composites.

Propagation and scattering of guided waves in composite plates with defects

Failure in aerospace composites owing to low-velocity impact raises a significant maintenance concern because it can lead to invisible damage. For the aerospace industry, such defects pose a potential danger to the structural integrity of aircraft. This in turn jeopardises passenger safety and incurs high repair costs. Hence, it is important to efficiently monitor composite structures during the service life. In this study, the potential of low-frequency guided ultrasonic waves for health monitoring in laminated composite plates is investigated. This study focuses on the use of the first antisymmetric guided wave mode (A0). The first part of this study is to investigate the propagation of the A0 mode in three different undamaged composite plates experimentally. The dispersive and anisotropic behaviour are in agreement with the results of finite element simulations and semi-analytical analysis. The final part of this study presents the scattering of guided waves at the impact damage using a non-contact laser interferometer. Significant scattering activities were observed and the impact damage size can be estimated to be about 10 × 25 mm. In conclusion, these results demonstrate the potential of guided ultrasonic waves for the inspection of aerospace composite structures.

Application of air-coupled ultrasonics for the characterization of polymer and polymer-matrix composite samples

This study investigates the application of air-coupled ultrasonics for nondestructive, non-contact characterization of polymer and polymer-matrix composite samples. A lot of progress has been made with this technique which does not require any liquid couplant, hence water sensitive samples can be studied without deterioration. We demonstrate the power of highly focused transducers for topography imaging of structured polymer surfaces such as a craquelure, fingerprint or an acoustic lens with a lateral resolution up to 0.1 mm and an elevation resolution of 2 μm. With a further technique, transmission resonance, the thickness of plates can be imaged with a resolution of 3 μm. Air-coupling is also used for checking the integrity of a polymer-matrix composite plate, revealing invisible internal impact damage. Finally, a single-sided Lamb-wave technique is used to detect sub-wavelength artificial defects such as notches and through-holes in plates.

Impact and post impact behavior of fabric reinforced geopolymer composite

Impact and post impact behavior of fabric reinforced geopolymer composites is investigated in this study. Carbon, E-glass and basalt fabrics are incorporated in the geopolymer matrix. The geopolymer composites are tested for impact resistance to an out-of-plane low velocity impact test, followed by post impact performance testing using residual strength measurement on the sample. Impact properties such as impact energy, energy absorption capacity, and damaged area of the composites were examined for fabric reinforced geopolymer composite samples. The drop weight impact tests were performed on composite samples of 15×15cm. The quality of the samples was examined using C-scan in ultrasonic vibration mode and μCT before and after impact test. The post impact behavior of the composite samples was characterized using four point bending tests. Fiber orientation and alignment was observed using SEM. The results obtained showed that E-glass fabric reinforced composites show slightly better performance than carbon reinforced composites. Especially the damaged area is smaller for E glass composites. Basalt fabric reinforced composites have low bending strength and very low impact resistance. The energy absorption in E-glass is due to post-debond fiber sliding phenomena and in carbon fiber to fiber pull out from the composite. The smaller damaged area is attributed to the higher elastic energy absorption of E-glass fibers. The residual strength of carbon and E-glass composites decreased in relation to the size of the visually damaged area, meaning that no invisible damage took place.

Impact load identification for composite structures using Bayesian regularization and unscented Kalman filter

In structural health monitoring of composite structures, one important task is to detect and identify the low-velocity impact events, which may cause invisible internal damages. This paper presents a novel approach for simultaneously identifying the impact location and reconstructing the impact force time history acting on a composite structure using dynamic measurements recorded by a sensor network. The proposed approach consists of two parts: (1) an inner loop to reconstruct the impact force time history and (2) an outer loop to search for the impact location. In the inner loop, a newly developed inverse analysis method with Bayesian inference regularization is employed to solve the ill-posed impact force reconstruction problem using a state-space model. In the outer loop, a nonlinear unscented Kalman filter (UKF) method is used to recursively estimate the impact location by minimizing the error between the measurements and the predicted responses. The newly proposed impact load identification approach is illustrated by numerical examples performed on a composite plate. Results have demonstrated the effectiveness and applicability of the proposed approach to impact load identification. Copyright © 2016 John Wiley & Sons, Ltd.

Rapid Multi-Damage Identification for Health Monitoring of Laminated Composites Using Piezoelectric Wafer Sensor Arrays.

Through the use of the wave reflection from any damage in a structure, a Hilbert spectral analysis-based rapid multi-damage identification (HSA-RMDI) technique with piezoelectric wafer sensor arrays (PWSA) is developed to monitor and identify the presence, location and severity of damage in carbon fiber composite structures. The capability of the rapid multi-damage identification technique to extract and estimate hidden significant information from the collected data and to provide a high-resolution energy-time spectrum can be employed to successfully interpret the Lamb waves interactions with single/multiple damage. Nevertheless, to accomplish the precise positioning and effective quantification of multiple damage in a composite structure, two functional metrics from the RMDI technique are proposed and used in damage identification, which are the energy density metric and the energy time-phase shift metric. In the designed damage experimental tests, invisible damage to the naked eyes, especially delaminations, were detected in the leftward propagating waves as well as in the selected sensor responses, where the time-phase shift spectra could locate the multiple damage whereas the energy density spectra were used to quantify the multiple damage. The increasing damage was shown to follow a linear trend calculated by the RMDI technique. All damage cases considered showed completely the developed RMDI technique potential as an effective online damage inspection and assessment tool.

Monitoring Network and Interfacial Healing Processes by Broadband Dielectric Spectroscopy: A Case Study on Natural Rubber

Broadband dielectric spectroscopy (BDS) is introduced as a new and powerful technique to monitor network and macroscale damage healing in an elastomer. For the proof of concept, a partially cured sulfur-cured natural rubber (NR) containing reversible disulfides as the healing moiety was employed. The forms of damage healed and monitored were an invisible damage in the rubber network due to multiple straining and an imposed macroscopic crack. The relaxation times of pristine, damaged, and healed samples were determined and fitted to the Havriliak-Negami equation to obtain the characteristic polymer parameters. It is shown that seemingly full mechanical healing occurred regardless the type of damage, while BDS demonstrates that the polymer architecture in the healed material differs from that in the original one. These results represent a step forward in the understanding of damage and healing processes in intrinsic self-healing polymer systems with prospective applications such as coatings, tires, seals, and gaskets.

Monitoring bolt tightness of rail joints using axle box acceleration measurements

Rail joints are a weak component in railway tracks because of the large impact and wheel-rail contact forces. Every train passage contributes to the deterioration of rail joints, causing visible (e.g. battered rails) and invisible (e.g. loose bolts) damages. The invisible damage cannot be detected by the commonly performed visual inspection, which is labor intensive, unreliable, intrusive and unsafe. In this paper, a vehicle-borne monitoring system is used to automatically detect and assess the tightness condition of bolts at rail joints. The monitoring method is developed based on field Axle Box Acceleration (ABA) measurements using different bolt tightness conditions. The suitability of the method is assessed by bolt tightness prediction and verification of a set of rail joints in the tram network of Sheffield, United Kingdom. The results show that ABA system can be employed to monitor bolt tightness conditions at rail joints.With this information better planning for selective preventive maintenance actions can be taken over rail joints.

Low Velocity Impact Localization on CFRP Based on FBG Sensors and ELM Algorithm

Carbon fiber reinforced plastics (CFRPs) structures are very susceptible to invisible damage induced by the low velocity impact. The impact area localization can be useful information for detecting this damage. In this paper, the low velocity impact area localization system using fiber Bragg grating (FBG) sensors and area localization algorithm based on extreme learning machine (ELM) were investigated. The FBG sensors were used to detect the impact signal. Fourier transform, ReliefF algorithm, and principal component analysis technology were used to extract the impact signal characteristic. The ELM technology was used to realize the impact area localization. Finally, the impact area localization system was established and verified on a CFRP plate with 240-mm $\times 240$ -mm experiment area. The experimental results showed that the proposed system made accurate identification 35 times for 36 times experiments. The area localization accuracy was 96.9%. In addition, the precision of the area localization system was 40-mm $\times 40$ -mm area. This paper provided a reliable method for CFRP low velocity impact area localization.

Finite element simulation of buckling-induced failure of carbon fibre-reinforced laminated composite panels embedded with damage zones

This work is concerned with the buckling-induced failure prediction of aerospace grade carbon fibre-reinforced laminated composite panels embedded with pre-assumed damage. Fibrous composites are being broadly used in aircraft and aerospace vehicle components. However, a major drawback in widespread use is their susceptibility to drop tool impacts and ground equipment collisions. Such incidents could inflict invisible internal damage and delamination that divides laminate into sub-laminates of lower bending stiffness/resistance to buckling load that might result in catastrophic consequences during later operations. This situation is a major concern for aircraft/aerospace industry and necessitates thorough understanding of damage tolerance capabilities of structural members for safety of human lives and structural assets. Extensive studies, based on experimental testing, were conducted on the topic. Since physical testing consumes time and resources, generate limited data, and lack in ply level effect of mixed-modes buckling hence development of a computational model is required. The present study consists of simulation models developed using ABAQUS™ software. Impact-induced damage was introduced as an area of reduced stiffness (soft-inclusion) or hole. Overall damage zones of known size and shape were inserted at various locations in the volumes of various laminates to predict critical buckling load based on global and local buckling with emphasis on mixed-mode buckling. Simulated cases of single and multiple damage zones/holes, switching and coupling of local–global buckling modes were investigated. Damage size, location, type, and penetration depth against critical buckling load and mode shape were investigated. The critical buckling load is found to correlate well corresponding to soft-inclusions to predict ply-level failure. Selected results of eight-, sixteen, and twenty-four ply laminates were compared against the data available in the literature and found to be in agreement up to 90%. The model could also be useful to efficiently study various lay-ups, loading, and material properties for the similar cases.

Beyond the wounds: The SCAR Project as space for examining breast cancer and mastectomy experiences

Most people never see the impact of breast cancer removal on a body. Even breast cancer survivors rarely see its physical effects on others. Gender-laden symbols underwrite breast cancer narratives, with mass media romanticizing the female body through pink ribbon imagery and youthful fashion model-perfect representations. The real body which has experienced breast cancer surgery through lumpectomy or mastectomy mostly is invisible, passive, and features scar tissue damage considered un-gaze-worthy. Yet, the Pulitzer Prize nominated Surviving Cancer: Absolute Reality (SCAR) Project uses Facebook to enable those who experience breast cancer to see post-mastectomy images and to share their own stories. Actual use of the SCAR Project Facebook page and to what end is the current study's focus. Four themes emerged among narratives: (1) Inspiration and Support, (2) Cancer Scar Meanings, (3) Authenticity, and (4) Redefined Beauty and Femininity. By examining women's experiences with breast cancer as shared on Facebook, we discuss slippages that emerge between pink ribbon imagery and processes of rebuilding female gender identity.

Real-Time Impact Monitoring of Composites for Ship Applications

Composites are getting more and more attention in the usage of major structures on ships owing to many advantages such as high specific strength and rigidity. However, composites are sensitive to impact, which indicates that the load-carrying capacity and stability of composite structures will decrease dramatically even for an invisible impact damage. Hence, a real-time impact monitoring system which has already been applied in aviation industry is important for reliability of ships, which inevitably encounter impacts during their lifetime due to different sources. This paper deals with the realization of an operational processing scheme to monitor the impact on composite structures, and estimate the impact location and load history by power distribution approach and system identification method. A PZT (piezoelectric lead zirconate titanate) sensor network was designed and employed to collect sensing signals for monitoring the impact. In the monitoring process, stress wave signals caused by impact were captured, and used to determine the impacted location and load history. Results demonstrate that the proposed method can be successfully applied in identifying the location and load history of impact on composite structures. The impact identification approach is of great significance for marine applications.

Remote inspection system for impact damage in large composite structure

A significant opportunity for reducing the weight of composite aircraft is through the development of an economically efficient method to detect barely visible or invisible impact damage sustained in service. In this paper, a structurally integrated, inert, wireless system for rapid, large-area impact damage detection in composite is demonstrated. Large-area inspection from single sensors using ultrasonic-guided waves is achieved with a baseline-subtraction technique. The wireless interface uses electromagnetic coupling between coils in the embedded sensor and inspection wand. Compact encapsulated sensor units are built and successfully embedded into composite panels at manufacture. Chirp-based excitation is used to enable single-shot measurements with high signal-to-random-noise ratio to be obtained. Signal processing to compensate for variability in inspection wand alignment is developed and shown to be necessary to obtain adequate baseline subtraction performance for damage detection. Results from sensors embedded in both glass fibre and carbon fibre-reinforced composite panels are presented. Successful detection of a 10 J impact damage in the former is demonstrated at a range of 125 mm. Quantitative extrapolation of this result suggests that the same level of impact damage would be detectable at a range of up to 1000 mm with an inspection wand alignment tolerance of 4 mm.

γH2AX foci on apparently intact mitotic chromosomes: Not signatures of misrejoining events but signals of unresolved DNA damage

The presence of γH2AX foci on apparently intact mitotic chromosomes is controversial because they challenge the assumed relationship between γH2AX foci and DNA double-strand breaks (DSBs). In this work, we show that after irradiation during interphase, a variety of γH2AX foci are scored in mitotic cells. Surprisingly, approximately 80% of the γH2AX foci spread over apparently undamaged chromatin at Terminal or Interstitial positions and they can display variable sizes, thus being classified as Small, Medium and Big foci. Chromosome and chromatid breaks that reach mitosis are spotted with Big (60%) and Medium (30%) Terminal γH2AX foci, but very rarely are they signaled with Small γH2AX foci. To evaluate if Interstitial γH2AX foci might be signatures of misrejoining, an mFISH analysis was performed on the same slides. The results show that Interstitial γH2AX foci lying on apparently intact chromatin do not mark sites of misrejoining, and that misrejoined events were never signaled by a γH2AX foci during mitosis. Finally, when analyzing the presence of other DNA-damage response (DDR) factors we found that all γH2AX foci—regardless their coincidence with a visible break—always colocalized with MRE11, but not with 53BP1. This pattern suggests that these γH2AX foci may be hallmarks of both microscopically visible and invisible DNA damage, in which an active, although incomplete or halted DDR is taking place.

Effect of diclazuril on intestinal morphology and SIgA expression in chicken infected with Eimeria tenella.

Secretory immunoglobulin A (SIgA), as a vital actor involving in the mucosal immunity, plays a key role in defending a variety of pathogenic infections, such as bacteria, viruses and parasites. Eimeria tenella is an obligate intracellular apicomplexan parasite contacting with the digestive tract mucosa and specially parasitizes chicken caecum, causing a severe form of coccidiosis. Coccidiosis is currently mainly controlled using chemotherapeutic agents. Diclazuril, a classic coccidiostat, was used widely in the poultry industry. Because of the rising problem of drug resistance, it is therefore crucial to understand the pattern of the SIgA expression in the action of diclazuril against E. tenella. In this study, the intestinal morphology in the caecum was analyzed by haematoxylin-eosin (HE) staining, and the SIgA expression was examined by immunohistochemical technique. At the same time, the duodenum, jejunum and ileum tissues have also been evaluated. HE staining results showed that E. tenella infection caused severe damage characterized by structural disorder, haemorrhage, inflammatory cell infiltration, serous and fibrinous exudation in chicken caecum and invisible damage in the duodenum, jejunum and ileum. With the treatment of diclazuril, the damage in the caecum was alleviated obviously. Immunohistochemical analysis demonstrated that the SIgA level in the infected group was increased in the duodenum (p < 0.05), jejunum and ileum, respectively, but decreased (p < 0.01) in the caecum, compared with the control group. Interestingly, the SIgA level was decreased in the duodenum (p < 0.05), jejunum and ileum but increased (p < 0.05) in the caecum in the infected/diclazuril group in comparison to the infected group. The results showed that diclazuril effectively alleviated the damage in the caecum induced by E. tenella and provided a cure for coccidiosis by improving the immune function in chickens.

Experimental Modal Analysis of the Footbridge across Vltava River in Prague Damaged by Flood in June 2013

The investigated footbridge across Vltava River in Prague was built in 1984. Almost the entire footbridge structure was overflowed by the extreme flood in August 2002 and the substantial part of the footbridge deck was overflowed afterwards by the great flood in June 2013 again. A significant damage on the footbridge deck was found out by a visual inspection of the load-bearing structure of the footbridge after the second flood. The basic objective of the performed experimental modal analysis was the initial experimental reliability verification of the obviously damaged footbridge structure. The measured natural frequencies and modes were compared with results of the several dynamic tests which were performed regularly during the lifetime of the footbridge. The localization of potential invisible damages was carried out. The recommendations for the range and the arrangement of the static load test, which was performed afterwards for fundamental reliability assessment of a footbridge structure, were made on the basis of the evaluated results of the experimental modal analysis.

Real-time optoacoustic monitoring of the temperature of the retina during laser therapy

Laser coagulation of the retina carries risk of damaging the neuroretina because of variation of the beam power and of the local pigmentation of the retinal pigmented epithelium and the choroid. The degree of coagulation depends on the induced temperature rise and the exposure time. Laser retinotherapy with optoacoustic monitoring of the temperature provides reproducible ophthalmologically invisible (subthreshold) damage of the retina in a wide range of laser power, regardless of local changes of the tissue structure. Optoacoustic temperature measurements are based on the excitation of a pressure wave in the retinal tissue by repeated low-energy probe laser pulses. The amplitude of the pressure signal, which can be detected by means of an acoustic transducer incorporated in a laser contact lens, correlates with the temperature of the tissue. This paper discusses promising directions of the use in clinical practice of real-time optoacoustic temperature monitoring of the retina during laser therapy in various irradiation regimes.

Early detection of mechanical damage in mango using NIR hyperspectral images and machine learning

Mango fruit are sensitive and can easily develop brown spots after suffering mechanical stress during postharvest handling, transport and marketing. The manual inspection of this fruit used today cannot detect the damage in very early stages of maturity and to date no automatic tool capable of such detection has been developed, since current systems based on machine vision only detect very visible damage. The application of hyperspectral imaging to the postharvest quality inspection of fruit is relatively recent and research is still underway to find a method of estimating internal properties or detecting invisible damage. This work describes a new system to evaluate mechanically induced damage in the pericarp of ‘Manila’ mangos at different stages of ripeness based on the analysis of hyperspectral images. Images of damaged and intact areas of mangos were acquired in the range 650–1100 nm using a hyperspectral computer vision system and then analysed to select the most discriminating wavelengths for distinguishing and classifying the two zones. Eleven feature-selection methods were used and compared to determine the wavelengths, while another five classification methods were used to segment the resulting multispectral images and classify the skin of the mangos as sound or damaged. A 97.9% rate of correct classification of pixels was achieved on the third day after the damage had been caused using k-Nearest Neighbours and the whole spectra and the figure dropped to 91.4% when only the most discriminant bands were used.