Acoustic Emission

The use of acoustic emission (AE) technique for detecting and monitoring damages and the progress on damages in different structures is widely used and has earned a reputation as one of the most reliable and well-established technique in non-destructive testing (NDT). Acoustic Emission is a very efficient and effective technology used for fracture behavior and fatigue detection in metals, fiberglass, wood, composites, ceramics, concrete and plastics. It can also be used for detecting faults and pressure leaks in vessels, tanks, pipes, as well as for monitoring the progression of corrosion in welding. This paper reviews major research developments over the past few years in application of acoustic emission in numerous engineering fields, including manufacturing, civil, aerospace and material engineering.

The Joint Transportation Research Program serves as a vehicle for INDOT collaboration with higher education institutions and industry in Indiana to facilitate innovation that results in continuous improvement in the planning, design, construction, operation, management and economic efficiency of the Indiana transportation infrastructure.

Maintenance is a set of organised activities that are carried out in order to keep an item in its best operational condition with minimum cost acquired. Predictive maintenance (PdM) is one of the maintenance program that recommends maintenance decisions based on the information collected through condition monitoring techniques, statistical process control or equipment performance for the purpose of early detection and elimination of equipment defects that could lead to unplanned downtime of machinery or unnecessary expenditures. Particularly Gears and rolling element bearings are critical elements in rotating machinery, so predictive maintenance is often applied to them. Fault signals of gearboxes or rolling-element bearings are nonstationary. This paper concludes with a brief discussion on current practices of PDM methodologies such as vibration analysis and Acoustic Emission analysis, which are widely used as they offers a complimentary tool for health monitoring or assessment of gears in rotating machineries.

Shaft angular misalignment (SAM) is a common and crucial problem in rotating machinery. Misalignment can produce several shortcomings such as premature bearing failure, increase in energy consumption, excessive seal lubricant leakage and coupling failure. Vibration analysis has been traditionally used to detect SAM; however, it presents some drawbacks i.e. high influence of machine operational conditions and strong impact of the coupling type and stiffness on vibration spectra. This paper presents an extensive experimental investigation in order to evaluate the possibility of detecting SAM, using acoustic emission (AE) technique. The test rig was operated at under different operational conditions of load and speed in order to evaluate the impact on the AE and vibration signature under normal operating conditions. To the best of the author's knowledge, this is the first attempt to use AE for the detection of SAM under varying operational conditions. A comparative study of vibration and AE was carried out to demonstrate the potentially better performance of AE. The experimental results show that AE technique can be used as a reliable technique for SAM detection, providing enhancements over vibration analysis.

Acoustic emission (AE) and microseismic (MS) events are indicators of rock fracturing or damage as the rock is brought to failure at high stress. By capturing the AE/MS events, underground excavation induced rock mass degradation or damage can be located and evaluated. A better understanding of the extent and shape of the excavation damaged zone (EDZ) or yield zone around caverns helps to arrive at safe and economic design and construction of the caverns. For this purpose, one needs to understand the AE mechanism associated with the excavation process.

... b) Source locations of induced microseismic events and compos-ite focal mechanisms for cluster events. ... towards a better understanding of the physical processes related to induced microseismicity at reservoir or ... Passive Seismic Monitoring of Natural and Induced Earthquakes ...

This study concerns with fault diagnosis of low speed bearing using multi-class relevance vector machine (RVM) and support vector machine (SVM). A low speed test rig was developed to simulate various types of bearing defects associated with shaft speeds as low as 10 rpm under several loading conditions. The data was acquired from the low speed bearing test rig using acoustic emission (AE) and accelerometer sensors under a constant load with different speeds. The aim of this study is to address the problem of detecting an incipient bearing fault and to find reliable methods for low speed machine fault diagnosis. In this paper, two methods of multi-class classification techniques for fault diagnosis through RVM and SVM are presented and the effectiveness of using AE and vibration signals due to low impact rate for low speed diagnosis. In the present study, component analysis was performed initially to extract the features and to reduce the dimensionality of original data features. The classification for fault diagnosis was also conducted using original data feature and without feature extraction. The result shows that multi-class RVM produces promising results and has the potential for use in fault diagnosis of low speed machine.

This paper investigates failure in Carbon Fibre Reinforced Plastics (CFRP) using Acoustic Emission (AE). Signals have been collected and post-processed for various test configurations: tension, Compact Tension (CT), Compact Compression (CC), Double Cantilever Beam (DCB) and four-point bend End Notched Flexure (4-ENF).

The Internet of Things (IoT) concept facilitates our life in many areas. One of the facilities provided in this area is undoubtedly condition monitoring. Unlike regular maintenance, IoT systems that perform continuous control operations can provide great advantages to the company with a warning that a serious failure will occur. It is a vital importance to determine defective bearings during the rotation of the power generating and power consuming machines without reaching the critical level. In this study; an experimental setup was created, and a Condition Monitoring Application (CMA) was performed in the laboratory environment which could monitor vibration, temperature, acoustic emission (AE), current and revolution measurements for ORS 6203 type defective bearing detection. In the designed test setup, a microcontroller platform used as a system brain, a direct current machine with a shaft mounted bearing, and miscellaneous sensors used such as 3-axis accelerometer that senses vibrations, a hall sensor module for revolution measure, a microphone module for AE detection, current and voltage sensor modules for power consumption measurement, contactless temperature sensor module for temperature measurement. CMA has been designed for monitoring of system via Bluetooth module. With the help of the microcontroller platform, the data collected from the sensors is transmitted wirelessly to the Android smartphone to be plotted and recorded. After the data exceeds certain thresholds, the control system is able to provide SMS and/or e-mail notification to the related maintenance team with the Android smartphone. Linear Discriminant Analysis (LDA), Random Forest (RF), Support Vector Machine (SVM), k-Nearest Neighbor (kNN), and Decision Tree (DT) algorithms applied to the collected data and models were created. As a result of accuracy analysis over %96 success was achieved for all models.

In condition monitoring of ball bearings, traditional techniques involving vibration, acceleration may not be able to detect a growing fault due to the low impact energy generated by the relative motion of the components. This study presents an experimental evaluation for incipient fault detection of lightly loaded ball bearings by using acoustic emission method. A table top bearing test rig is fabricated to simulate bearing faults such as inner race & outer race defect etc., with rotating shaft carrying low loads. The effectiveness of the acoustic analysis technique is demonstrated through signal processing techniques; use of Empirical Mode Decomposition (EMD) method to derive the characteristics features related to defective bearings from time domain and enveloped spectra in the frequency domain. The results reveal that the acoustic technique is more effective in detecting bearings' failure than that of the vibration measurement.

The monitoring of progressive wear in gears using various non-destructive technologies as well as the use of advanced signal processing techniques upon the acquired recordings to the direction of more effective diagnostic schemes, is the scope of the present work. For this reason multi-hour tests were performed in healthy gears in a single-stage lab scale gearbox until they were seriously damaged. Three on-line monitoring techniques are implemented in the tests. Vibration and acoustic emission recordings in combination with data coming from oil debris monitoring (ODM) of the lubricating oil are utilized in order to assess the condition of the gears. A plethora of parameters/features were extracted from the acquired waveforms via conventional (in time and frequency domain) and nonconventional (wavelet-based) signal processing techniques. Data fusion was accomplished in the level of integration of the most representative among the extracted features from all three measurement technologies in a single data matrix. Principal component analysis (PCA) was utilized to reduce the dimensionality of the data matrix whereas independent component analysis (ICA) was further applied to identify the independent components among the data and correlate them to different damage modes of the gearbox. Finally heuristic rules based on characteristic values of the resulted independent components were set, realizing thus a health monitoring scheme for gearboxes.

Moisture absorption and durability in water environment are major concerns for natural fibres as reinforcement in composites. This paper presents a study on the influence of water ageing on mechanical properties and damage events of flax-fibre composites, compared with glass-fibre composites. The effects of the immersion treatment on the tensile characteristics, water absorption and acoustic emission (AE) recording were investigated. The water absorption results for the flax-fibre composites show that the evolution appears to be Fickian and the saturated weight gain is 12 times as high that the glass-fibre composites. Decreasing continuously with increasing water immersion time, the tensile modulus and the failure strain of flax-fibre composites are hardly affected by water ageing whereas only the tensile stress is reduced regarding the glass-fibre composites. AE indicate that matrix-fibres interface weakening is the main damage mechanism induced by water ageing for both composites.

Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests. Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In granite samples initial P-wave velocity is 5 km/s and increases with pressure by < 20%. The pressure-induced changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of P-wave velocities was significantly higher for basalt than for granite, with vertical velocity components being faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small increase for basalt but a significant decrease for granite. Using first motion polarity we determined AE source types generated during triaxial loading of the samples. With increasing differential stress AE activity in granite and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of double-couple events indicating shear, suggests shear cracks linking previously formed tensile cracks.

Quality control during fabrication and failure monitoring during service have gained importance in the field of metal matrix composites because of their growing uses, especially for structural applications. This review aims to provide a survey of quality control and nondestructive testing relevant to metal matrix composites. The first part presents the determination of the reinforcement parameters which play a very important role in controlling the performance of metal matrix composites. The second part describes the main relevant nondestructive techniques used to identify the defects produced during fabrication or in service. It also shows that the validation of nondestructive techniques and the relationship between their results and component performance should be particularly emphasized in future research work on metal matrix composites. ; Fax: ϩ32 16 32 1992.

This paper gives a review of acoustic and ultrasonic optical fiber sensors (OFSs). The review covers optical fiber sensing methods for detecting dynamic strain signals, including general sound and acoustic signals, high-frequency signals, i.e., ultrasonic/ ultrasound, and other signals such as acoustic emissions, and impact induced dynamic strain. Several optical fiber sensing methods are included, in an attempted to summarize the majority of optical fiber sensing methods used to date. The OFS include single fiber sensors and optical fiber devices, fiber-optic interferometers, and fiber Bragg gratings (FBGs). The single fiber and fiber device sensors include optical fiber couplers, microbend sensors, refraction-based sensors, and other extrinsic intensity sensors. The optical fiber interferometers include Michelson, Mach-Zehnder, Fabry-Perot, Sagnac interferometers, as well as polarization and model interference. The specific applications addressed in this review include optical fiber hydrophones, biomedical sensors, and sensors for nondestructive evaluation and structural health monitoring. Future directions are outlined and proposed for acousto-ultrasonic OFS.

The mechanical removal of materials using miniature tools, known as micro-mechanical milling processes, has unique advantages in creating miniature 3D components using a variety of engineering materials, when compared with photolithographic processes. Since the diameter of miniature tools is very small, excessive forces and vibrations significantly affect the overall quality of the part. In order to improve the part quality and longevity of tools, the monitoring of micro-milling processes is imperative. This paper examines factors affecting tool wear and a tool wear monitoring method using various sensors, such as accelerometers, force and acoustic emission sensors in micro-milling. The signals are fused through the neuro-fuzzy method, which then determines whether the tool is in good shape or is worn. An optical microscope is used to observe the actual tool condition, based upon the edge radius of the tool, during the experiment without disengaging the tool from the machine. The effectiveness of tool wear monitoring, based on a number of different sensors, is also investigated. Several cutting tests are performed to verify the monitoring scheme for the miniature micro-end mills.

This paper discusses the work towards the development of a sound methodology for the application of acoustic emission techniques for detecting and monitoring crack growth in rail steels. At the moment rails that are found to be damaged on the network are not immediately replaced. Instead a speed restriction is imposed and affected rails are clamped with special fish-plates until they are replaced, usually after a period of a few days. Unfortunately damaged rails prior to their replacement cannot be reliably assessed for further crack growth with any of the conventional non-destructive evaluation techniques (e.g. ultrasonic) and this is the reason that a speed restriction is imposed. Acoustic emission techniques could potentially be applied for the detection as well as continuous monitoring of further crack growth therefore removing the need of imposing strict speed restrictions which can be as low as 20MPH (32km/h). In order to validate the potential of the AE technique tests are carried out under laboratory conditions on three and four-point bending samples. Various signal processing methods will be examined using AEWin and Noesis software supplied by PAC and Envirocoustics respectively.

The micromechanics of failure in Berea sandstone were investigated by characterizing quantitatively the evolution of damage under the optical and scanning electron microscopes. Three series of triaxial compression experiments were conducted at the fixed pore pressure of 10 MPa and confining pressures of 20, 50 and 260 MPa, respectively, corresponding to three different failure modes: shear localization with positive dilatancy, shear localization with relatively little dilatancy and distributed cataclastic flow. To distinguish the effect of non-hydrostatic stress from that of hydrostatic pressure, a fourth suite of hydrostatically loaded samples was also studied. Using stereological procedures, we characterized quantitatively the following damage parameters: microcrack density and its anisotropy, pore-size distribution, comminuted volume fraction and mineral damage index. In the brittle regime, shear localization did not develop until the post-failure stage, after the peak stress had been attained. The microcrack density data show that very little intragranular cracking occurred before the peak stress was attained. We infer that dilatancy and acoustic emission activity in the prefailure stage are primarily due to intergranular cracking, probably related to the shear rupture of lithified and cemented grain contacts. Near the peak stress, intragranular cracking initiates from grain contacts and this type of Hertzian fracture first develops in isolated clusters, and their subsequent coalescence results in shear localization in the post-failure stage. The very high density of intragranular microcracking and pronounced stress-induced anisotropy in the post-failure samples are the consequence of shear localization and compactive processes operative inside the shear band. In contrast, Hertzian fracture was a primary cause for shear-enhanced compaction and strain hardening throughout the cataclastic flow regime. Grain crushing and pore collapse seem to be most intense in weakly cemented regions. Finite element simulations show that the presence of cement at grain contacts alleviates the tensile stress concentration, thus inhibiting the onset of Hertzian fracture and grain crushing.

This study investigates the geopolymerization process of fly ash–based geopolymer paste specimens using acoustic emissions; the activating solution is a combination of silica fume and sodium hydroxide, made with two different water:solid weight ratios (w/s), 0.30 and 0.35. The specimens are cured in ambient conditions (22 AE 3°C). The acoustic emission data are processed through pattern recognition (PR), and two clusters are identified and assigned to a specific mechanism depending on their characteristics. Results show that there is a predominant difference in the acquisition data between the two water:binder weight ratios. Pattern recognition shows that the greatest number of geopolymerization mechanisms, including dissolution of Si and Al atoms, forming bubbles, and microcrack initiation, occur at roughly the same time for the 0.3 w/s samples, whereas in 0.35 w/s samples the mechanisms occur sequentially. Final setting time of Vicat penetration tests, which assume a starting point of microcrack initiations, is 60 min after the test onset for 0.3 w/s ratio samples, whereas it occurs after approximately 600 min for 0.35 w/s samples. Microcrack initiation predicted by the pattern recognition technique coincides with the final setting times. Scanning electron microscope observations also coincide with the pattern recognition findings.

— The bubble cavitation phenomenon in two phase gas/liquid systems happens in several hydraulic components, for example, valves and centrifugal pumps. This is a common occurrence, leading to a drop in hydraulic performance, reduction of equipment efficiency, possible damage to the structure of pump and valve components. In addition, it causes high vibration and noise and solid surface erosion. This study identifies the feasibility of the use of the Acoustic Emission technology to detect and monitor bubble formation throughout boiling processes. To undertake this task a special purpose test-rig was employed. It was concluded that bubble formation is detectable with AE technology and there is a clear correlation between increasing AE levels and the bubble formation during the boiling process.

Along the east coast of the United States, by-catches of bottlenose dolphins, Tursiops truncatus, in gillnet fisheries exceed removal levels set under the US Marine Mammal Protection Act. One measure proposed to reduce this mortality is the use of acoustic alarms, or pingers, which have proven effective in reducing by-catches of other small cetaceans, but have not been tested with bottlenose dolphins. We examined the responses of bottlenose dolphins to a commercial gillnet equipped with functional (active) and non-functional (control) Dukane NetMark 1 1000 alarms near Fort Macon, NC. Between 5 April and 10 May 2001 we used a theodolite to track 59 groups of dolphins around the net. Choice of treatment was random each day and the two shore-based observers were unaware of whether alarms were active (13 days) or controls (9 days). There were no significant differences in the number of groups observed (P=0.315; 1À=0.835) or in the closest observed approach to the net (P=0.307; 1À=0.828) between treatments. However, dolphins entered a circular buffer approximately 100 m around the net more frequently with control than active alarms (P=0.015). We conclude that some dolphins responded to the alarms by avoiding the net, but caution that the potential efficacy of alarms is confounded by dolphin behavior. Most dolphins were aware of the net, regardless of the status of alarms, and some dolphins fed on fish in the net or discarded by the fishing vessel. We believe that it would be unwise to use pingers in these fisheries because of the limited behavioral responses we observed in our experiment. Furthermore, the responses we observed are likely to diminish or change over time as dolphins habituate or sensitize to these alarms. Further research is required to understand the behavior responsible for entanglement. #

Background: Selective mutism is a psychiatric disorder of childhood characterized by consistent inability to speak in specific situations despite the ability to speak normally in others. The objective of this study was to test whether reduced auditory efferent activity, which may have direct bearings on speaking behavior, is compromised in selectively mute children. Methods: Participants were 16 children with selective mutism and 16 normally developing control children matched for age and gender. All children were tested for pure-tone audiometry, speech reception thresholds, speech discrimination, middle-ear acoustic reflex thresholds and decay function, transient evoked otoacoustic emission, suppression of transient evoked otoacoustic emission, and auditory brainstem response. Results: Compared with control children, selectively mute children displayed specific deficiencies in auditory efferent activity. These aberrations in efferent activity appear along with normal pure-tone and speech audiometry and normal brainstem transmission as indicated by auditory brainstem response latencies. Conclusions: The diminished auditory efferent activity detected in some children with SM may result in desensitization of their auditory pathways by self-vocalization and in reduced control of masking and distortion of incoming speech sounds. These children may gradually learn to restrict vocalization to the minimal amount possible in contexts that require complex auditory processing.

Bearings are critical components of rotating machinery and monitoring their condition is important to avoid catastrophic failures and reduce the machinery down-time. Acoustic emission (AE) is gaining ground as a complementary condition monitoring technique as it offers earlier fault detection compared with other more established techniques, such as vibration analysis or oil analysis. However, AE signals always include a significant level of noise reducing the potential of defect detection at early stage. For this reason, this paper proposes a novel envelope analysis method for bearing incipient defect detection. This method is able to identify localized defects in an incipient stage, in which the signal-to-noise ratio (SNR) is extremely low. This method combines Wavelet packet, for AE signal denoising, the Hilbert Transform (HT) for envelope extraction and autocorrelation function, to find patterns in the AE signal. An extensive experimental investigation was carried out in order to evaluate the performance of the proposed method under extremely low SNR, adding high level of noise to the signals. The results indicate that the proposed enhanced envelope method is able to detect incipient defects with 9 dB lower SNR than traditional envelope analysis.

We investigated the face-stabilized Open-Hole Compression (OHC) test method for evaluating
the effects of fiber waviness on the compression strength of continuous carbon fiber
reinforced polymer composites. Temporal evaluations of the load-deformation response,
acoustic emissions and optical microscopy are used to understand the failure modes and
damage progression in the OHC specimen. The failure modes observed are structurally
correlated to matrix failure and kink zone formation leading to fiber fracture. The results
show how the resin pocket plays a more critical role than the layup in influencing the
initiation of damage in the composite specimens.

A new method for separate identification and determination of the spatial distribution of the two components of the energy intensity in an ultrasound bath (due to the ultrasound waves and cavitation activity) uses two media—cavitating (water) and noncavitating (silicon oil)—under the conditions of the acoustic field in the ultrasound bath. The variation of cavitation intensity in the frequency domain was obtained by subtracting the acoustic emission spectrum of silicon oil from that of water. Measurements at various locations in the bath revealed significant spatial variations in the cavitation intensity in the bath. The local cavitation phenomena in the bath (stable or transient cavitation) were explained based on the spectral characteristics of acoustic emission. The radial dynamics of the bubbles at the location of cavitation intensity measurements was determined using the Gilmore model of bubble dynamics. The bubbles in the region of highest cavitation intensity underwent a transient motion, while the bubbles in the region of lowest cavitation intensity underwent stable/oscillatory motion. The transient collapse of the bubbles that gives rise to local temperature and pressure maxima is at the root of the observed effects of ultrasound on chemical systems. The more violent the collapse of the bubbles, the higher the local cavitation intensity. It was verified using the spectral characteristics of the acoustic emission and simulation of the radial motion of the bubbles.

Composites fabricated by VARTM technology with the use of single-ply non-crimp 3D orthogonal woven preforms 3WEAVE Ò find fast growing research interest and industrial applications. It is now well understood and appreciated that this type of advanced composites provides efficient delamination suppression, enhanced damage tolerance, and superior impact, ballistic and blast performance characteristics over 2D fabric laminates. At the same time, this type of composites, having practically straight in-plane fibers, show significantly better in-plane stiffness and strength properties than respective properties of a ''conventional" type 3D interlock weave composites. One primarily important question, which has not been addressed yet, is how the in-plane elastic and strength characteristics of this type of composites compare with respective in-plane properties of ''equivalent" laminates made of 2D woven fabrics. This 2-part paper presents a comprehensive experimental study of the comparison of in-plane tensile properties of two single-ply non-crimp 3D orthogonal weave E-glass fiber composites on one side and a laminate reinforced with four plies of plain weave E-glass fabric on the other. Results obtained from mechanical testing are supplemented by acoustic emission data providing damage initiation thresholds, progressive cracks observation, full-field surface strain mapping and cracks observation on micrographs. The obtained results demonstrate that the studied 3D non-crimp orthogonal woven composites have considerably higher in-plane ultimate failure stresses and strains, as well as damage initiation strain thresholds than their 2D woven laminated composite counterpart. Part 1 presents the description of materials used, experimental techniques applied, principal results and their mutual comparisons for the three tested composites. Part 2 describes in detail the experimentally observed effects and trends with the main focus on the progressive damage: detailed results of AE registration, full-field strain measurements and progressive damage observations, highlighting peculiarities of local damage patterns and explaining the succession of local damage events, which leads to the differences in strength values between 2D and 3D composites.

The mechanical removal of materials using miniature tools, known as micro-mechanical milling processes, has unique advantages in creating miniature 3D components using a variety of engineering materials, when compared with photolithographic processes. Since the diameter of miniature tools is very small, excessive forces and vibrations significantly affect the overall quality of the part. In order to improve the part quality and longevity of tools, the monitoring of micro-milling processes is imperative. This paper examines factors affecting tool wear and a tool wear monitoring method using various sensors, such as accelerometers, force and acoustic emission sensors in micro-milling. The signals are fused through the neuro-fuzzy method, which then determines whether the tool is in good shape or is worn. An optical microscope is used to observe the actual tool condition, based upon the edge radius of the tool, during the experiment without disengaging the tool from the machine. The effectiveness of tool wear monitoring, based on a number of different sensors, is also investigated. Several cutting tests are performed to verify the monitoring scheme for the miniature micro-end mills.

The present work investigates tensile and flexural behavior of untreated New Zealand flax (Phormium tenax) fiber reinforced epoxy composites. Two series of laminates were produced using the same reinforcement content (20 wt.%), arranged either as short fibers or quasi-unidirectional ones. Composites reinforced using quasi-unidirectional fibers showed higher modulus and strength both in tensile and flexural loading, when compared to neat epoxy resin. Short fiber composites, although still superior to epoxy resin both for tensile and flexural moduli, proved inferior in strength, especially as concerns tensile strength. These results have been supported by scanning electron microscopy (SEM), which allowed characterizing fiber-matrix interface, and by acoustic emission (AE) analysis, which enabled investigating failure mechanisms. In addition, thermal behavior of both untreated phormium fibers and composites has been studied by thermogravimetric analysis (TGA), revealing the thermal stability of composites to be higher than for phormium fibers and epoxy matrix alone.

In elastohydrodynamic lubrication (EHL), an extreme smoothness of the contacting surfaces is essential for the formation of a thin lubricant film, which separates the moving surfaces from each other. Sharp asperities, in particular, are very detrimental for the sliding surfaces. Because of the limited thickness of the lubricating film, the contact zone is very vulnerable if the lubricant contains contaminants, especially if hard particles are present in the lubricant.

The main focus of this study is to identify the most influential and common sensory features for the process quality characteristics in CNC milling operations-dimensional accuracy (bore size tolerance) and surface roughness-using three different material types (6061-T6 aluminum, 7075-T6 aluminum, and ANSI-4140 steel). The materials were machined on a vertical CNC mill, retrofitted with multiple sensors and data acquisition systems, to investigate the effects of variations in material types and machining parameters. The sensor data include cutting force measurements, spindle quill vibration, and acoustic emission, each of which further divided into measurable components, such as x, y, and z components in cutting force, x and y spindle quill vibration, DC, AC, and Count Rate for acoustic emission signals. Those components were filtered and analyzed to determine the sensory features that best correlate with process quality characteristics. Tool wear rate and machining characteristics appeared differently, depending on the material types, yet some components of the sensory data were found to be significant with relation to the variations in bore size and surface roughness for all three types of materials. This suggests that even under the varying cutting conditions involving different materials, the identified sensory features can be used for the reliable and accurate control of milling operations. 

In the aim to study of the effects of process-induced stresses upon some mechanical properties of simple stacking sequence composite laminates, three different cure cycles were designed. This was achieved on the basis of a cure kinetics modelling. A minimum number of curing parameters were modified in order to restrain changes in material physical properties. Those latter were determined from the molecular network of the thermosetting matrix to the fibres volume fraction and thermal expansion of the laminated plates. The level of process-induced stresses generated by the three cure cycles was assessed by measuring the out-of-plane deflection on unsymmetrical laminated strips. Manufactured laminates were submitted to a set of tensile tests in order to determine the changes in their mechanical properties due to the different curing conditions. All the mechanical tests were followed by acoustic emission. This has enabled to distinguish the occurrence of the various damaging processes with respect to curing conditions and therefore process-induced stresses. Lastly, process-induced stresses on a micromechanical level were theoretically determined. This determination has enabled a first analysis of the effects of process-induced stresses upon mechanical properties to be settled.

This study focuses on the description of damage and failure behaviour of triaxial braided carbon/epoxy composites under tension. The tensile tests were instrumented with optical surface strain and acoustic emission measurements. Damage was observed using X-ray and microscopy. The damage develops in two stages: (1) intra-yarn cracking: increase of the crack density and crack length, (2) local inter-yarn delamination and conjunction of the intra-yarn cracks. Statistics of crack sizes at both stages were collected and the 3D geometry of cracks was reconstructed. A finite element model of the unit cell of the textile reinforcement is used to predict damage initiation and crack orientation using Puck's criterion. Progressive damage and stiffness deterioration is modelled using the degradation scheme of Murakami-Ohno and the damage evolution law of Ladeveze, applied to the average stress state of the yarns. Good agreement with experimental damage initiation threshold and non-linear tensile diagrams is found both for loading in fibre and off-axis directions.

This chapter examines metallurgical developments related to bridge construction. Because of availability and cost, iron and steel have played the central role. We examine their roles and applications starting from ancient chain bridges of wrought iron, cast iron, and steel arch bridges, iron and steel truss bridges, wires and cables, and finally to spectacular long-span suspension and dazzling cable-stayed bridges. Introduction This chapter considers metallurgical developments that are related to bridge design and construction. Bridges are naturally of large scale, and iron and steels have been used as the principal components since antiquity. Common knowledge for engineers and historians includes the famous Iron Bridge in England, completed in 1779 (see Figure 1, reproduced from Nicholson 1829). It seems certain that this is the oldest iron arch bridge (Kostof 1985). Some argue that the completion of this bridge was the critical event for the Industrial Revolution, while others contend that Watt's steam engine in 1784 was the main impetus. Perhaps both contributed in tandem, multiplying their influence synergistically. From the metallurgical side, Abraham Darby's use of coke for pig iron production in 1709 represents an important step forward in allowing the construction of the Iron Bridge and for the rapid rise of the British steel industry (Cossons and Trinder 1979). The British Isles did not have an adequate supply of charcoal even in the 18 th century; for example, 70 wt % (from now on %) of its iron was imported from Russia and Sweden in 1770 (Ågren 1998). Yet, British iron production started to accelerate around 1780 (blue arrow), as shown in Figure 2 (data from Birch 1967 and Swank 1888). The history of iron bridges, however, takes us first to ancient times, long before the Industrial Revolution. A natural first place to examine the ancient metallurgy that enabled iron bridge building is in Asia, with a long tradition of constructing bridges. More recently, wires of ultra-high strength steels have been developed by the 1990s, making it possible to build long-span bridges. Advanced microstructural design at the nanoscale and modern fabrication methods are the keys to discover how we progressed from the Brooklyn Bridge to the Akashi Kaikyo Bridge.

The characterisation of the damage state of composite structures is often performed using the acoustic behaviour of the composite system. This behaviour is expected to change significantly as the damage is accumulating in the composite. It is indisputable that different damage mechanisms are activated within the composite laminate during loading scenario. These ''damage entities'' are acting in different space and time scales within the service life of the structure and may be interdependent. It has been argued that different damage mechanisms attribute distinct acoustic behaviour to the composite system. Loading of cross-ply laminates in particular leads to the accumulation of distinct damage mechanisms, such as matrix cracking, delamination between successive plies and fibre rupture at the final stage of loading. As highlighted in this work, the acoustic emission activity is directly linked to the structural health state of the laminate. At the same time, significant changes on the wave propagation characteristics are reported and correlated to damage accumulation in the composite laminate. In the case of cross ply laminates, experimental tests and numerical simulations indicate that, typical to the presence of transverse cracking and/or delamination, is the increase of the pulse velocity and the transmission efficiency of a propagated ultrasonic wave, an indication that the intact longitudinal plies act as wave guides, as the transverse ply deteriorates. Further to transverse cracking and delamination, the accumulation of longitudinal fibre breaks becomes dominant causing the catastrophic failure of the composite and is expected to be directly linked to the acoustic behaviour of the composite, as the stiffness loss results to the velocity decrease of the propagated wave. In view of the above, the scope of the current work is to assess the efficiency of acoustic emission and ultrasonic transmission as a combined methodology for the assessment of the introduced damage and furthermore as a structural health monitoring tool.

Nickel-based superalloys are widely used in aircraft industry as they are exceptionally thermal resistant retaining mechanical properties up to 700°C. On the other hand, they are very difficult to machine, so tool life is typically short and can finish abruptly. As catastrophic tool failure can destroy usually expensive workpiece, automatic tool condition monitoring (TCM) become particularly critical. Paper presents an application of Wavelet Packet Transform (WPT) for extracting features from cutting forces and AE signals, useful for TCM, while rough turning of Inconel 625. New, improved methods of signal feature (SF) relevancy evaluation were proposed based on determination and correlation coefficients. Out of several SFs calculated from band pass signals, the most useful for TCM were automatically selected. The selected features were used for tool condition monitoring.

Disorder and long-range interactions are two of the key components that make material failure an interesting playfield for the application of statistical mechanics. The cornerstone in this respect has been lattice models of the fracture in which a network of elastic beams, bonds or electrical fuses with random failure thresholds are subject to an increasing external load. These models describe on a qualitative level the failure processes of real, brittle or quasi-brittle materials. This has been particularly important in solving the classical engineering problems of material strength: the size dependence of maximum stress and its sample to sample statistical fluctuations. At the same time, lattice models pose many new fundamental questions in statistical physics, such as the relation between fracture and phase transitions. Experimental results point out to the existence of an intriguing crackling noise in the acoustic emission and of self-affine fractals in the crack surface morphology. Recent advances in computer power have enabled considerable progress in the understanding of such models. Among these partly still controversial issues, are the scaling and size effects in material strength and accumulated damage, the statistics of avalanches or bursts of microfailures, and the morphology of the crack surface. Here we present an overview of the results obtained with lattice models for fracture, highlighting the relations with statistical physics theories and more conventional fracture mechanics approaches.

Statistics show that fatigue crack development comes ®rst and foremost as a damage source in aerospace metallic structures. Currently, widespread methods are available to inspect these structures, but they are quite time-consuming, costly and require the structural system to be idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectric transducers coupled onto the plates. The detected damage is cracks in joint resulting from fatigue loading. The collected signals on the piezoelectric transducers are analysed using Hilbert transform and time±frequency analysis. It is shown that the ®nal interpretation of Lamb wave analysis may provide a means of sizing the defects and following the crack development. In addition to that, an acoustic emission system is used jointly with the Lamb wave analysis in order to discuss results and damage development. Finally, it is demonstrated that both methods can work together and the results obtained are in good agreement. q