Incipient Damage Research Articles

Robust cepstral-based features for anomaly detection in ball bearings

This paper proposes the linear frequency cepstral coefficients as highly discriminative features for anomaly detection in ball bearings using vibration sensor data. These features are based on cepstral analysis and are capable of encoding the patterns of a spectral magnitude profile. Incipient damages on bearings can grow rapidly under normal use resulting in vibration and harsh noise. If left undetected, this damage will worsen, leading to high maintenance costs or even injury. Multiple interferences in an industrial environment contaminate the signal, making it a challenge to correctly identify the bearings’ condition. Many studies have attempted to overcome this issue at the signal level. However, the discriminative capacity of the current vibration signal features is still vulnerable to interference, which motivates this work. In order to demonstrate the benefits of these features, we (1) show that they are computationally efficient and suitable for real-time incremental training; (2) conduct discriminative analysis by evaluating the separability performance and comparing it with the state of the art; and (3) test the robustness of the proposed features under noise interference, which is ideal for use in the harsh operating conditions of industrial machinery. The data was obtained from a laboratory workbench setting that reproduces bearing fault scenarios. Results show that the proposed features are fast, competitive when compared to state-of-the-art features, and resilient to high levels of interference. Despite the higher performance when using the quadratic model, the proposed features remain highly discriminative when used with several other discriminant function.

Internal load transfer in an interpenetrating metal/ceramic composite material studied using energy dispersive synchrotron X-ray diffraction

The mechanism of internal load transfer in a newly developed interpenetrating metal/ceramic composite is studied for the first time in this work using energy dispersive synchrotron X-ray diffraction. The composite was fabricated by infiltration of Al12Si melt in an open porous alumina preform, fabricated using a mixture of two different polymer waxes as pore formers, by gas pressure infiltration. Several diffraction planes of all three crystalline phases of the composite – alumina, silicon and aluminum solid solution are investigated. Lattice strains, both along and transverse to the loading direction, in each phase are calculated from the shifts in the measured diffraction peak positions. Results show that until about 100 MPa applied compressive stress, all three phases undergo elastic deformation. At higher applied stresses aluminum undergoes plastic deformation and correspondingly load is transferred to alumina. The load carrying capability of alumina is however limited by incipient damage in this phase. At applied compressive stresses higher than 187 MPa, load carried by the alumina phase continuously decreases and correspondingly more load is again carried by the aluminum solid solution. Throughout, the fraction of load carried by silicon remains almost constant, suggesting no significant load transfer within the metallic alloy itself.

Baseline-Free Hybrid Diagnostic Technique for Detection of Minor Incipient Damage in the Structure

AbstractOne of the practical difficulties in implementing the structural health monitoring schemes in the field is the challenge involved in detecting the incipient minor damage (subtle cracks) at ...

Reference-Free Breathing Crack Identification of Beam-Like Structures Using an Enhanced Spatial Fourier Power Spectrum with Exponential Weighting Functions

Detection of incipient damage of structures at the earliest possible stage is desirable for successful implementation of any health monitoring system. In this paper, we focus on breathing crack problem and present a new reference-free algorithm for fatigue crack detection, localization, and characterization for beam-like structures. We use the spatial curvature of the Fourier power spectrum as a damage sensitive feature for fatigue crack identification. An exponential weighting function that takes into account nonlinear dynamic signatures, such as sub- and superharmonics, is proposed in the Fourier power spectrum in order to enrich the damage-sensitive features of the structure. Both numerical and experimental studies have been carried out to test and verify the proposed algorithm.

Detection of Subtle Damage in Structures through Smart Signal Reconstruction

The primary function of structural health monitoring (SHM) is the process of extracting the damage features from the measured raw data, recorded using sensors on the structure of interest. The efficiency of SHM techniques lies in their capability to detect early damage, which alters the dynamic characteristics of only a few modal responses but in a feeble manner, in its incipient stage. Isolating these modal responses, hidden in the overall raw response, for damage diagnosis, is a real challenge to the SHM community. In order to handle this issue, an improved version of Empirical Mode Decomposition (EMD) is employed in this paper. EMD decomposes the measured response signals into mono-component signals, called intrinsic mode functions (IMFs). The mixed modes in EMD are handled using Intermittency criteria in the proposed EMD. Once the IMFs are extracted from the raw signal, the IMFs (signal components) which possess the valuable information of incipient damage called ‘critical IMFs’, are isolated. To determine the spatial location of damage, these critical IMFs are combined to reconstruct a new signal with enriched information on minor/incipient damage. ARMAX model is employed on the new signal with enriched damage information. A normalized distance measure of ARMAX models, in terms of subspace angles, is used as a damage indicator. The numerical and experimental investigations presented in this paper clearly reflect that the proposed output-only damage diagnostic technique using the smart reconstruction of the measured raw signal is capable of detecting the incipient subtle damages in the structures.

Detection for Incipient Damages of Wind Turbine Rolling Bearing Based on VMD-AMCKD Method

Incipient damages of wind turbine rolling bearing are very difficult to be detected because of the interference of multi-frequency components and strong ambient noise. To solve this problem, this paper proposes a new detected method named VMD-AMCKD, combining complementary advantages of variational mode decomposition (VMD) and adaptive maximum correlated kurtosis deconvolution (AMCKD). A novel index is proposed to screen out the most sensitive mode containing fault information after VMD decomposition. The mode also can determine a suspectable range for the fault frequency, based on which the optimized range of devolution period T in MCKD can be pre-determined. The Grasshopper optimization algorithm (GOA) is adapted to adaptively select the key parameters in MCKD. The proposed method can successfully diagnose the simulated signal mixed with strong white Gaussian noise. Its robustness is further proven by the diagnosis for three different types of experimental signal from CWRU bearing data center. Finally, the VMD-AMCKD is applied to detect incipient damages of rolling bearings in a laboratory wind turbine.

Incipient Damage Detection for Large Area Structures Monitored With a Network of Soft Elastomeric Capacitors Using Relative Entropy

Structural health monitoring of mesoscale structures is difficult due to their large sizes and often complex geometries. A solution to this challenge lies in the development of sensing skins. Sensing skins are an emerging technology that enables a broad range of sensors and their associated electronics to be integrated onto a single sheet, therefore, reducing the cost and complexity associated with deploying these dense sensor networks onto mesoscale structures. This paper presents a new algorithm for the detection and localization of incipient damage in structures. The algorithm is specialized for a sensing skin consisting of a large area electronic termed as soft elastomeric capacitor. The proposed algorithm utilizes relative entropy to quantify the dissimilarity between one sensor and every other sensor in the network, with more weight placed on the dissimilarities between the sensor of interest and those in its immediate vicinity. The algorithm is data-driven and does not require the healthy condition be known or historical data sets be available to generate damage sensitive indexes. Numerical simulations are used to demonstrate the effectiveness of the data-driven algorithm in both detecting and localizing incipient damage.

A hybrid structural health monitoring technique for detection of subtle structural damage

There is greater significance in identifying the incipient damages in structures at the time of their initiation as timely rectification of these minor incipient cracks can save huge maintenance cost. However, the change in the global dynamic characteristics of a structure due to these subtle damages are insignificant enough to detect using the majority of the current damage diagnostic techniques. Keeping this in view, we propose a hybrid damage diagnostic technique for detection of minor incipient damages in the structures. In the proposed automated hybrid algorithm, the raw dynamic signatures obtained from the structure are decomposed to uni-modal signals and the dynamic signature are reconstructed by identifying and combining only the uni-modal signals altered by the minor incipient damage. We use these reconstructed signals for damage diagnostics using ARMAX model. Numerical simulation studies are carried out to investigate and evaluate the proposed hybrid damage diagnostic algorithm and their capability in identifying minor/incipient damage with noisy measurements. Finally, experimental studies on a beam are also presented to compliment the numerical simulations in order to demonstrate the practical application of the proposed algorithm.

EMD-Shannon Entropy-Based Methodology to Detect Incipient Damages in a Truss Structure

Truss-type designs are widely used in civil structures. Despite the fact that they are robust and reliable structures, different kinds of damage can appear. In order to avoid human and economic losses, the development and application of damage-detection methodologies are paramount. In this work, a methodology based on the empirical mode decomposition (EMD) method and the Shannon Entropy Index (SEI) to detect incipient damages associated with corrosion in a 3D 9-bay truss-type bridge is presented. As different EMD methods are presented in literature, the most representative methods are investigated in order to evaluate their performance for this task. To this end, the vibration signals generated in the truss-type bridge at different conditions are analyzed. For the damage condition, four severity levels of simulated corrosion (1 mm, 3 mm, 5 mm, and 8 mm of diameter reduction) generated into the elements of truss-type bridge are considered. Results demonstrate the effectiveness of the proposal in terms of detecting corrosion in its very early stage (1 mm of reduction in the element).

Quantifying heterogeneous deformation in grain boundary regions on shock loaded tantalum using spherical and sharp tip nanoindentation

Grain boundaries play an important role in the overall mechanical performance of metals and alloys; however, isolating the effects of individual grain boundaries remains rather challenging experimentally. In this work, wire-feed, electron beam additively manufactured tantalum is studied under shock loading conditions generating incipient spall damage. Three grain boundaries aligned parallel to the shock direction were isolated inside a single sample. Postmortem metallography showed voids preferentially appeared on two of the three grain boundaries which had high misorientation angles > 30° compared to the third grain boundary with a relatively lower misorientation angle < 10°. Nanoindentation in grain boundary regions for both pyramidal and spherical tips showed characteristically different strain hardening trends for each grain boundary. Significant local strain hardening is present on both sides of one high angle grain boundary, while strain softening is captured at the other high angle grain boundary. A negligible trend of either hardening or softening is shown in the vicinity of the single low angle grain boundary. This leads to the conclusion that the trend of strain hardening/softening in grain boundary regions is not monotonically correlated to the susceptibility of specific grain boundaries to an increased propensity for void damage.

Design of nonlinear-Lamb-wave-based structural health monitoring systems with mitigated adhesive nonlinearity

Structural health monitoring (SHM) techniques with nonlinear Lamb waves offer the possibility of detecting incipient damages through the second harmonic generation. Particularly, the primary S0-secondary S0 Lamb wave mode pair in a weakly nonlinear plate is promising for SHM due to its appealing characteristics such as the cumulative second harmonic generation and flexible frequency selection. In a practical PZT-actuated SHM system, apart from the damage-related material nonlinearity of the plate (MNP), the adhesive nonlinearity (AN) is also proven to be a non-negligible nonlinear source, which may jeopardize damage diagnosis if not properly apprehended. By combining a nonlinear shear-lag model and the normal mode expansion method, a theoretical model is proposed in this work, which allows the investigation and the comparison of these two types of important nonlinearities in the system (AN and MNP). With the developed model, optimized nonlinear Lamb wave-based SHM systems can be designed with the mitigated influence of the undesired AN on the system. Finite element (FE) simulations are carried out to validate the proposed model and the effectiveness of the optimized systems through a tactical adjustment of the different nonlinear sources. The designed optimized system is verified by experiments, in comparison with an arbitrarily chosen one. The dominance level of the nonlinear sources in the two systems is evaluated through changing the MNP, which is realized by a thermal aging treatment. Both FE and experimental results demonstrate that the proposed model can effectively guide the design of SHM systems to mitigate AN so that the MNP can be well measured and further used as an indicator of the incipient structural damage.

Bispectral dynamics features for characterizing structural fatigue damage

Fatigue damage is a type of damage usually occurring to repeatedly loaded elements of structures in various engineering fields. Accumulation of fatigue damage may cause failure of structural elements. Identification of incipient fatigue damage is essential to ensure safety of structures. Fatigue crack under repeated loads commonly behaves in a nonlinear dynamic manner, typically manifested by both occurrence of higher harmonic components and interaction of harmonic components. Interrogation of nonlinear dynamic manner provides a promising way to characterize structural fatigue damage. This study aims at developing a new method to interrogate nonlinear dynamic manner for fatigue damage identification. This method is based on bispectral analysis of structural vibrational responses. This method portrays fatigue damage by inspecting the presence of higher harmonic components and quantifying the interaction of these harmonic components. The method can precisely locate and quantify a small-sized fatigue damage in a cantilever beam, presenting great accuracy in fatigue damage identification.

Viscoelastic lattice spring model for mechanical behavior of polymeric particle filled composites

In this work, viscoelasticity has been introduced into lattice spring model by utilizing van der Pol elements obeying the Boltzmann superposition principle to investigate the time-dependent mechanical behavior and damage evolution of viscoelastic particle filled composites (PFCs). Stress relaxation, crack mouth opening displacement test and uniaxial tension were simulated. Influence of filler content, filler dispersion state, strain rate and interfacial strength on the tensile response of PFCs were investigated. It is shown that (1) viscoelasticity of microscopic elements can be reflected to the global behavior of composites; (2) cluster effects exists when particles are closely packed and it will cause incipient damage under low extent of deformation; (3) increase in strain rate strengthens both the stiffness and strength of composites and the increment is proportional to the logarithm of strain rate; (4) well-dispersed PFCs can be severely weakened when adequate interfacial bonding is not provided.

Thermochromic Polymer Film Sensors for Detection of Incipient Thermal Damage in Carbon Fiber⁻Epoxy Composites.

Carbon fiber–epoxy composites have become prevalent in the aerospace industry where mechanical properties and light weight are at a premium. The significant non-destructive evaluation challenges of composites require new solutions, especially in detecting early-stage, or incipient, thermal damage. The initial stages of thermal damage are chemical rather than physical, and can cause significant reduction in mechanical properties well before physical damage becomes detectable in ultrasonic testing. Thermochromic fluorescent probe molecules have the potential to sense incipient thermal damage more accurately than traditional inspection methods. We have designed a molecule which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature–time combinations that can cause damage in composites. Moreover, this molecule can be dispersed in a polymer film and attached to composite parts as a removable sensor. This work presents an evaluation of the sensor performance of this thermochromic film in comparison to ultrasonic C-scan as a method to detect incipient thermal damage in one of the most widely used carbon fiber–epoxy composite systems. Composite samples exposed to varying thermal exposures were used to evaluate the fluorescent thermal sensor films, and the results are compared to the results of ultrasonic imaging and short-beam shear tests for interlaminar shear strength.

Investigations on effectiveness of embedded PZT patches at varying orientations for monitoring concrete hydration using EMI technique

The electro-mechanical impedance (EMI) technique has already been established to be an effective technique for detecting incipient damages in engineering structures including reinforced concrete (RC) using smart piezo-electric sensors. Its effectiveness in monitoring the hydration of concrete, using surface bonded and embedded lead zirconium titanate (PZT) patches has also been investigated well. However, the influence of patch orientation while embedding them in concrete is still unexplored. The present study investigates the effectiveness of the EMI technique using PZT patches embedded at different orientations (here, 0°, 45° and 90°) with the axis of the structure, while monitoring the hydration of a real-life prototype RC beam. Further, the effectiveness of the PZT patches in monitoring the strength gain in concrete is investigated. It is observed that the PZT patch placed in the inclined orientation (45°) least efficient in capturing the progressive changes of hydration in the concrete in comparison to the other two orientations. The EMI technique showed best results when PZT patch was placed in horizontal position (0°) with the longitudinal axis of the beam.

Prognosis of fatigue and impact induced damage in concrete using embedded piezo-transducers

Concrete structures are often subjected to a wide array of load conditions, such as fatigue and impact, and the consequent damages. Recently, smart materials, in particular the piezoelectric materials, have received high attention from the point of view of structural health monitoring (SHM). In this connection, the electro-mechanical impedance (EMI) is one of the latest and most effective techniques. The technique harnesses the piezoelectric property of lead-zirconate-titanate (PZT) patches to sense any incipient damage. In the research covered in the paper, embedded PZT patches are employed to monitor the accumulating damage in plain cement concrete subjected to fatigue and impact type loadings. Concrete specimens (150 mm cubes and 100φ × 200 mm cylinders) of grade M-25 are cast with embedded concrete vibration sensors (CVS). The impact loading is simulated by a free falling iron-ball of mass 5 kg dropped from variable heights of 2 m, 2.5 m and 3 m above the top surface of the test cubes. For fatigue tests, a 4000 kN cyclic compression testing machine is used to apply the fatigue load in a sinusoidal form. The admittance signatures from the embedded PZT patches for impact and fatigue tests are acquired using an LCR meter. These signatures are used to determine the PZT-identified equivalent system properties (damping, stiffness and mass) of the specimens at varying damage states. For impact tests, strain history during the impact process is acquired in addition to the admittance signatures. The results for impact tests show an increase in the maximum strain induced in the patch with the number of impacts, while the equivalent damping computed from the admittance signature is found to decrease with increasing damage. For fatigue tests, the equivalent damping is again found to decrease with increasing damage, finally undergoing a phenomenal increase near the failure. Based on these measurements, separate relationships have been developed for impact and fatigue induced damages linking the PZT-identified damping with remaining life.

Structural health monitoring of rotary aerospace structures based on electromechanical impedance of integrated piezoelectric transducers

Structural health monitoring of rotary aerospace structures is investigated in this research. A monitoring system is proposed based on the electromechanical impedance spectrum of piezoelectric transducers and a portable transceiver. To investigate the applicability and preliminary results of this method, a turbomachine prototype (laboratory device) is developed, and integrated composite piezoelectric films are deposited on the blades. Next, a self-diagnostic characterization is initially implemented to the piezo-films. Transceiver functionality and accuracy is verified using an Ivium impedance analyzer. The verified measuring path was used in structural health monitoring of pristine and damaged blades at rotational speed of 0 and 1000 r/min. The effects of damage formation and rotational speed on the impedance signature are discussed based on the variations in mechanical impedance using a two-dimensional model. Once damage occurs in a blade at each speed, it results in a frequency shift of the impedance signature at antiresonance peaks compared to the corresponding baseline. The results show a clear frequency shift of existing peaks and the appearance of new peaks as damage grows to a secure minimal detectable size. This achievement confirms the applicability of this method for incipient damage detection on rotary structures prior to any failure.

Design and Implementation of an Accurate, Portable, and Time-Efficient Impedance-Based Transceiver for Structural Health Monitoring

Reducing maintenance costs while increasing the safety and reliability, especially in moving structures, needs a reliable nondestructive analyzer. The aim of this research is to provide a practical solution for this problem based on high-frequency excitation of stationary and moving structures by propagating standing Lamb waves in the range of 1 to 1000 kHz. The proposed solution comprises of a controlled frequency swept signal source, a number of piezoelectric sensors, a portable analyzer, and a rotary mechanism. Measuring the accurate electromechanical impedance (EMI) is made possible by monitoring the applied voltages, currents, and phase differences accurately. In this study, design and implementation of a low-cost, compact, and portable transceiver is explored for periodic structural health monitoring of a proposed rotary structure using EMI technique. The compactness of the proposed system is an essential requirement for rotary structures as compared with bulky, heavy, and expensive impedance analyzers. Challenges in design and development of such a system are discussed in this paper, together with mitigations to make the system functional and practical. An experimental study is carried out in frequency domain to measure the real and imaginary parts of impedance spectrum of piezo-transducers. The results show that the portable transceiver has the capability to detect structural incipient damages before any catastrophic failure, thus avoiding undesirable shut down during the operation.

Focal Transplantation of Aberrant Glial Cells Carrying the SOD1G93A Mutation into Rat Spinal Cord Induces Extensive Gliosis

Objective: We aimed to determine the potential of aberrant glial cells (AbAs) isolated from the spinal cord of adult SOD1^G93A symptomatic rats to induce gliosis and neuronal damage following focal transplantation into the lumbar spinal cord of wild-type rats. Methods: AbAs were obtained from the spinal cords of SOD1^G93A symptomatic rats. One hundred thousand cells were injected using a glass micropipette into the lumbar spinal cords (L3-L5) of syngeneic wild-type adult rats. Equal volumes of culture medium or wild-type neonatal microglia were used as controls. Seven days after transplantation, immunohistochemistry analysis was carried out using astrocytic and microglia cell markers. Transplanted SOD1^G93A AbAs were recognized by specific antibodies to human SOD1 (hSOD1) or misfolded human SOD1. Results: Seven days after transplantation, AbAs were mainly detected in the medial region of the lumbar ventral horn as a well-limited cell cluster formed at the site of injection by their immunoreactivity to either misfolded SOD1 or normally folded hSOD1. Compared with controls, transplanted AbAs were surrounded by marked microgliosis and reactive astrocytes. Marked microgliosis was observed to extend bilaterally up to the cervical cord. Motor neurons close to AbA transplants were surrounded by activated glial cells and displayed ubiquitin aggregation. Conclusions: AbAs bearing mutant SOD1^G93A have the potential to induce neuroinflammation along the spinal cord and incipient damage to the motor neurons. The emergence of AbAs during amyotrophic lateral sclerosis pathogenesis may therefore be a mechanism to boost neuroinflammation and spread motor neuron damage along the neuroaxis.

Spall response and failure mechanisms associated with a hot-extruded AMX602 Mg alloy

Normal plate impact experiments were conducted to study the spall behavior of AMX602 magnesium alloy fabricated via Spinning Water Atomization Process (SWAP), which was followed by cold compaction and hot extrusion. Incipient spall damage in the specimens was measured at different shock conditions using 51mm and 105mm diameter bore gas guns. The Hugoniot Elastic Limit (HEL) was measured to be approximately 187 ± 11MPa. The spall strengths extracted from the free surface velocity profiles of the shocked specimens was found to increase by 8% for shock stresses ranging from 1.61 to 4.53GPa. The experimental data also revealed some inherent scatter in the spall strength data due to microstructural heterogeneity. Post-test fractographic analysis of the shock-recovered specimens suggests that the initiation of spall was most likely associated with intermetallic Al2Ca-bearing precipitates forming during the hot extrusion process. The subsequent spall failure propagated via cavitation events that involved the nucleation of nano- and micrometer-sized voids with very limited plastic growth.