Showing papers in "Insight in 2004"
TL;DR: A review of the literature documenting NDE methods for porosity measurement in fiber-reinforced polymer composites can be found in this paper, where the majority of the papers concentrated on ultrasonic methods to determine porosity levels, mainly for quality control purposes.
Abstract: Low porosity levels are essential for ensuring the performance of carbon fibre-reinforced composite structures. Despite many attempts to measure absolute porosity levels as a process-control tool, the currently accepted methods often rely on relative measurements in comparison with a reference standard. However, newer low-cost manufacturing methods such as resin-transfer moulding (RTM) and resin film infusion (RFI) would benefit from a reliable absolute 'porosity meter.' This paper reviews the literature documenting NDE methods for porosity measurement in fibre-reinforced polymer composites. The majority of the papers have concentrated on ultrasonic methods to determine porosity levels, mainly for quality control purposes. A few workers have reported on the application of other NDE methods. The review concentrates on methods that have potential for use during composite manufacture. To date, no one NDE method has been able to provide a porosity measurement independent of both the pore morphology and the fibre and resin matrix materials.
123 citations
TL;DR: An alternative approach in which the complete raw data set offline domain signals from every transmitter-receiver pair is collected, stored and post-processed is described, which allows novel inspections to be performed which would be impossible with single-element transducers and impractical using the traditional phased array controller methodology.
Abstract: The use of ultrasonic arrays for NDT has increased significantly in recent years. This is due to the flexibility of array systems which can be electronically configured to produce plane, focused, steered and steer-focusedbeams. In this way, one array transducer can do the job of many standard single-element transducers. Currently, the approach is to have independently controlled parallel pulser-receiver channels connected to a number of array elements. These channels allow timing delays to be applied to each element on both transmission and reception. In this way a specific set of transmit-receive delays are applied to an array to create a specific beam profile. This paper describes an alternative approach in which the complete raw data set offline domain signals from every transmitter-receiver pair is collected, stored and post-processed. Theoretically, the time taken to acquire this data is approximately the same as that taken to perform a B-scan with the array. The key advantage is that post-processing of the complete raw data set enables any beam profile to be recreated. Additionally, this approach allows novel inspections to be performed which would be impossible with single-element transducers and impractical using the traditional phased array controller methodology. This paper concerns one such post-processing algorithm, the Total Focusing Method (TFM). In the TFM, an image is created in which the beam has been focused on every point within the field of view. This optimises the focusing performance of a given array. The use of the TFM is then demonstrated on a number of test structures and is proposed as a superior alternative to conventional array test techniques.
87 citations
72 citations
67 citations
TL;DR: In this paper, an automated defect sizing algorithm using the Embedded Signal Identification Technique (ESIT) was developed for separating partially superimposed signals often encountered in thin sections and the results were compared with the manual sizing method.
Abstract: The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well developed technique for sizing defects in thick sections (thickness >10 mm). Attempt has been made here to extend this technique for thin sections (6-10mm). An automated defect sizing algorithm using the Embedded Signal Identification Technique (ESIT) was developed for separating partially superimposed signals often encountered in thin sections and the results were compared with the manual sizing method. Both EDM notches and more realistic fatigue cracks in thin section were used to evaluate the proposed technique.
37 citations
TL;DR: In this article, the authors presented the results from a pilot study of a "smart system" used for defect detection in railroad rails, particularly the critical transverse-type defects.
Abstract: This paper presents the results from a pilot study of a 'smart system' used for defect detection in railroad rails, particularly the critical transverse-type defects. The experimental data used to train the pattern recognition 'smart system' were extracted from experiments conducted during a previous long-range ultrasonic guided wave study conducted at the University of California, San Diego. Reflection coefficient plots corresponding to a variety of transverse and oblique defects were shown to provide features that were successfully used to train a 'smart system' to identify the defects automatically. This paper presents a brief introduction to support vector machines, followed by a description of the procedure used to determine the best data to be used to train the 'smart system', and concludes with lessons learned during this study.
35 citations
TL;DR: The Qualistir project as discussed by the authors developed an online quality control of friction stir welds (FSW) in aluminium, which can detect conventional defects and provide a method for determining whether the weld has been correctly forged, in turn, safeguards against joint line remnant defects.
Abstract: This paper is based upon a project 'Qualistir' for the on-line quality control of friction stir welds (FSW) in aluminium ( 1 ) . Qualistir is a collaborative project lead by R/D-Tech with co-industrial partners Vermont, Isotest, Neos Robotics and Gatwick Fusion, and research partners TWI, GKSS and TUS. The objective of the project is to develop an on-line method for determining the quality of FSW in a butt-welded configuration. The inspection technique described in this paper detects conventional defects and supplies a method for determining whether the weld has been correctly forged which, in turn, safeguards against joint line remnant defects.
35 citations
34 citations
TL;DR: In this article, a pitch-catch low-frequency wideband Rayleigh wave EMAT system has been developed that has been used for gauge corner cracking defect detection on the head of samples of rail track.
Abstract: A 'pitch-catch' low-frequency wideband Rayleigh wave EMAT system has been developed that has been used for gauge corner cracking defect detection on the head of samples of rail track. Strictly speaking, the generated waves are a type of guided wave mode as the propagation surface is not a flat halfspace, and these waves propagate along the surface of the rail penetrating down to a depth of several millimetres. Crack depth can be estimated by measuring the relative amount of the Rayleigh wave at a particular frequency that passes underneath the crack. The EMAT system also has the potential to assess the condition of the combined microstructure and stress state around the rail head by measuring accurately the velocity of the surface waves.
26 citations
TL;DR: In this paper, an experimental test-rig was designed such that defects of varying sizes could be seeded onto the outer race of a test bearing, and it was concluded that the acoustic emission technique can provide an indication of the defect size.
Abstract: The investigation reported in this paper was centred on the application of the acoustic emission technique for identifying the size of a defect on a radially loaded bearing. An experimental test-rig was designed such thatdefects of varying sizes could be seeded onto the outer race of a test bearing. It is concluded that the AE technique can provide an indication of the defect size; this has been unachievable with vibration analysis.
25 citations
TL;DR: In this article, the authors report the investigations of air coupled ultrasonic imaging techniques which are part of the German research project MaTech, and the development and the construction of a noncontact ultrasound imaging system (Demonstrator III) for basic researches.
Abstract: This paper reports the investigations of air coupled ultrasonic imaging techniques which are part of the German research project MaTech. Project partners are Airbus, Bremen, Ingenieurburo Dr. Hillger, Braunschweig, and intelligeNDT, Erlangen, with DLR, Braunschweig, as subcontractor. One task is the development and the construction of a non-contact ultrasonic imaging system (Demonstrator III) for basic researches. This system provides fast scanning with a single-shot data acquisition (without signal averaging). The dynamic range in one shot reaches up to 50 dB. The demonstrator allows the application of different types of air coupled transducers and therefore it allows the comparison of their performances. The modular design of the system provides further developments and adaptations to special inspection tasks. A power transmitter with burst-signals ( peak power of 1.2 kW) and a free programmable transmitter can be used for the excitation of the transducers. The user-friendly software records C-scans and full wave scans and provides C-scan evaluations. The development and construction of special transducers for air coupling and the development of a system for echo-technique are further research tasks of the MaTech project. Usually, the air coupled ultrasound is used in through-transmission technique with separate receiver and transmitter transducers on opposite sides of the component under test. In order to get a high acoustic power and a low noise in the receiver amplifier usually the bandwidth of the system is smaller than 10% of the centre (test) frequency. Therefore, the effects of (*/2)-resonances in the components are investigated. Also the application of broadband transducers will be investigated.
TL;DR: In this paper, a model for transient ultrasonic wave generation by Electromagnetic Acoustic Transducers (EMATs) is presented, which combines analytical solutions and numerical computation to predict the ultrasonic field generated by arbitrary sources.
Abstract: A model for transient ultrasonic wave generation by Electromagnetic Acoustic Transducers (EMATs) is presented. Analytical solutions are currently available only for a few kinds of sources and the model combines these analytical solutions and numerical computation to predict the ultrasonic field generated by arbitrary sources. This model can be used to calculate bulk waves within samples as well as surface waves with the advantages of explicit physical meaning and quick processing speed over pure numerical calculations such as the Finite Element Method (FEM). The model is used to explain how static and dynamic magnetic fields generate ultrasonic waves in a sample. It is desirable to characterise the EMAT source in detail in order to tailor sources for optimal configuration for specific NDE applications. A Michelson laser interferometer is used to measure the out-of-plane surface displacement of a sample, and results agree well with the modelling simulation. The modelling can be used for sources with arbitrary shape and temporal profile, but works most efficiently for sources with circular symmetry or line shape forces.
TL;DR: This work presents a new methodology for efficient training with imbalances in classes by showing that if there are sufficient cases of the smaller class, then it is possible to reduce the size of the larger class by using the correlation between cases of this latter class, with a minimum information loss.
Abstract: The automatic detection of flaws through non-destructive testing uses pattern recognition methodology with binary classification. In this problem a decision is made about whether or not an initially segmented hypotheticalflaw in an image is in fact a flaw. Neural classifiers are one among a number of different classifiers used in the recognition of patterns. Unfortunately, in real automatic flaw detection problems there are a reduced number of flaws in comparison with the large number of non-flaws. This seriously limits the application of classification techniques such as artificial neural networks due to the imbalance between classes. This work presents a new methodology for efficient training with imbalances in classes. The premise of the present work is that if there are sufficient cases of the smaller class, then it is possible to reduce the size of the larger class by using the correlation between cases of this latter class, with a minimum information loss. It is then possible to create a training set for a neural model that allows good classification. To test this hypothesis a problem of great interest to the automotive industry is used, which is the radioscopic inspection of cast aluminium pieces. The experiments resulted in perfect classification of 22936 hypothetical flaws, of which only 60 were real flaws and the rest were false alarms.
TL;DR: A number of statistical methods used to quantify the reliability of flaw detection and sizing by NDT from experimental data are reviewed, and specific applications carried out recently by TWI to support the inspection reliability are given.
Abstract: This paper reviews a number of statistical methods used to quantify the reliability of flaw detection and sizing by NDT from experimental data. For flaw detection, different methods are applicable according to whether the outcome of a trial is recorded as: a) a binary variable, i.e. hit/miss data (typical of enhanced visual techniques such as MPI), or b) a continuous variable, i.e. a signal amplitude relative to a given threshold (typical of ultrasonics or eddy current testing). In the former case, the method of analysis is further divided into methods that group the data, and methods that treat it as a whole to calculate a probability of detection (POD) curve. In any event there are restrictions on the data set that can make this method cumbersome for experimental application. The second method (called 'response versus size' or â versus a) requires the signal amplitude and a threshold for detection. The POD is then produced from a set of data that contains more information than the hit/miss method, and this can allow a smaller number of flaws to be used. For flaw sizing, attention is usually focussed on the amount of undersizing that can be allowed, which can then be used to set realistic acceptance criteria. However, it may also be important to quantify the amount of any oversizing, since this can cause unnecessary repairs or plant shutdown. Examples of the use of each method are given, and specific applications carried out recently by TWI to support the inspection reliability. The paper also discusses the meaning of what is meant by a ‘reliable’ inspection.
TL;DR: In this article, a broadband vibration-based Structural Irregularity and Damage Evaluation Routine (SIDER) was developed for the inspection of large-scale composite structures not amenable to more conventional inspection methods.
Abstract: This paper presents a broadband vibration-based Structural Irregularity and Damage Evaluation Routine (SIDER) that uses features in complex curvature operating shapes to locate damage and other areas with structural stiffness variations. SIDER was developed for the inspection of large-scale composite structures not amenable to more conventional inspection methods. This paper extends the procedure such that it can be used on general, multi-dimensional engineering structures. Experimental results from a composite road bridge and a composite ship hull show that even when an a priori model of the structure is not available, structural variability and some manufacturing defects can be located. Damage caused over time can be detected by comparing results of an inspection with the results obtained from the structure before it is put into service. Underwater explosion testing of the ship hull deliberately caused damage, which was located using this method. The results are reported here.
TL;DR: The use of an embedded waveguide to monitor the setting and early hardening of concrete has been investigated in this paper, where measurements of the attenuation of a guided wave propagating along the waveguide were used to obtain the shear velocity, and hence shear modulus, of the concrete.
Abstract: The use of an embedded waveguide to monitor the setting and early hardening of concrete has been investigated. The technique uses measurements of the attenuation of a guided wave propagating along the waveguide to obtain the shear velocity, and hence shear modulus, of the concrete. Tests have been carried out on mortar and concrete specimens monitored continuously over 48 hours. Over the first few hours, while the mortar/concrete was still fluid, the absolute rate of increase in shear modulus was low. From around 5 hours to around 20 hours, shear modulus increased linearly. Thereafter, shear modulus increased, but at an ever-decreasing rate. The shear velocity of the mortar was calculated to be 1720 m/s after two days, corresponding to a shear modulus of 5.6 GPa, while the 48 hour-old concrete had a shear modulus of 16.5 GPa. Repeated tests showed similar end states, but some differences in hardening rate; these were attributed either to temperature variations or to differences in mould geometry. The technique worked equally well on mortar and concrete, indicating that the presence of the large aggregate particles in concrete does not adversely affect the results.
TL;DR: In this paper, the effect of scanning at different angles to the crack (not parallel), scanning not directly over the crack, scanning with lift-off, and also evaluating the accuracy for surface crack depth sizing is discussed.
Abstract: ACFM is often used to search for surface longitudinal cracks in weld seams, for which the technique will have maximum sensitivity to. Attempts are always made where possible to scan with the probe centre over the crack and along the crack for maximum sensitivity to the crack. However, inevitably, this is not always possible with cracks deviating from expected paths. This report details results of the effect of scanning at different angles to the crack (not parallel), scanning not directly over the crack, scanning with lift-off, the effect of probe angulation (ie probe wobble) and also evaluating the accuracy for surface crack depth sizing. Most of the specimens were fabricated from the commonly utilised ASTM-A36 structural steel with accurately fabricated EDM and containing real defects. It was shown that the ACFM technique is generally reliable for detection and accurate sizing of surface defects in air or in water. The essentials of an algorithm which will automate crack detection and sizing is also described.
TL;DR: In this article, the authors give an overview of the way certain operations have decided to replace film by computed radiography, and what the major benefits for them have been, including the advantages of the manipulation of digital images, but also the digital data that is associated with it.
Abstract: Computed radiography, or digital radiography by use of reusable storage phosphor screens, offers a convenient and reliable way to replace film. In addition to the reduced cost on consumables, the return on investment of CR systems is strongly determined by savings in exposure time, processing times and archival times. Furthermore, intangible costs like plant shutdown, environment safety and longer usability of isotopes are increasingly important when considering replacing film by storage phosphor systems. But more than in traditional radiography, the use of digital images is a trade-off between the speed and the required quality. Better image quality is obtained by longer exposure times, slower phosphor screens and higher scan resolutions. Therefore, different kinds of storage phosphor screens are needed in order to cover every application. Most operations have the data, associated with the tests to be performed, centrally stored in a database. Using a digital radiography system gives not only the advantages of the manipulation of digital images, but also the digital data that is associated with it. Smart methods to associate cassettes and storage screens with exposed images enhance the workflow of the NDT processes, and avoid human error. Automated measurements tools increase the throughput in different kinds of operations. This paper gives an overview of the way certain operations have decided to replace film by computed radiography, and what the major benefits for them have been.
TL;DR: In this article, a 3D NDT software package is presented to aid planning of NDT data acquisition via 3D simulation, and to aid defect visualisation and measurement in 3D by superimposing 2D nDT images on their 3D CAD models.
Abstract: When performing non-destructive testing of aerostructures with complex geometrical shapes and curved surfaces, two problems arise: one is generation of an appropriate sequence of 3D coordinates and orientations for the inspection probe to follow complex component contours; the other is interpretation and characterιsation of defects based on 2D NDT images. By utilising the information from CAD models, this paper presents a 3D NDT software package to aid planning of NDT data acquisition via 3D simulation, and to aid defect visualisation and measurement in 3D by superimposing 2D NDT images on their 3D CAD models. The particular noteworthy and unique characteristics of the software are (a) a versatile 3D CAD model editing tool for extraction of CAD features and construction of simplified CAD models; (b) 3D scanning simulation for automatic generation of probe coordinates and orientations with respect to curved component surfaces; (c) tools for aligning and projecting a 3D CAD model on to its 2D NDT image; and (d) texture mapping to project a NDT image on to its 3D CAD model surface. Using real aircraft components with their radiographic, ultrasonic and thermographic images as examples, the software is shown to offer a unique, versatile and powerful CAD-based 3D NDT environment for fast and reliable structure integrity assessment of complex aerostructures.
Abstract: Pulsed Eddy Current (PEC) techniques have been investigated and applied for field applications. On-line inspection is essential for many manufacturing processes and other NDE applications. This paper investigates the dynamic behaviour of pulsed eddy current techniques and the feasibility of their use in an on-line inspection system. After discussing the PEC principles and their signal signatures, an approach on improving the dynamic response of PEC is introduced and tested. The results have illustrated that the proposed PEC system and their feature extraction can be used for real-time inspection such as NDT for manufacturing processes
TL;DR: In this article, an inspection robot system was developed to test boiler tube effectively based on the magnetic flux leakage (MFL) technique and ultrasonic method using the outer diameter (OD) surface can be inspected.
Abstract: An inspection robot system has been developed to test boiler tube effectively based on the magnetic flux leakage (MFL) technique and ultrasonic method. Using the magnetic flux leakage technique, the corrosion and erosion on the outer diameter (OD) surface can be inspected. The wall loss can be tested with the magnetic bridge principle firstly, and inspected by ultrasonic probe quantitatively. A magnetic sensor was designed by the above-mentioned principle and developed with Hall element arrays. Experiments demonstrate that the NDT robot, which includes furnace wall cleaning apparatus, adhesion and crawling and safety mechanisms, the inspection device, the control system and the signal processing software, can be driven up and down the tube automatically, and the apparatus was able to identify a through hole (bigger than 1.6 mm diameter) defect and wall loss.
TL;DR: In this paper, the advances that have been made in non-destructive examination by utilizing phased array technology for power station applications to improve accuracy, repeatability and integrity whilst reducing the overall inspection time.
Abstract: This paper describes the advances that have been made in non-destructive examination by utilising phased array technology for power station applications to improve accuracy, repeatability and integrity whilst reducing theoverall inspection time. It details the technical challenges we have overcome during our five years experience of working with ultrasonic phased array technology by presenting a number of examples which include steam turbine disc head inspections, curved blade root and the inspection of thick-walled components such as steam chests. Phased array has rapidly become a useful tool for the inspection of critical plant, however there are still challenges ahead for industry to accept the new technique and to recognise the need for certified training.
TL;DR: In this article, the authors presented a comparison of results of induction motor broken rotor bar fault detection using vibration and stator current methods, which were acquired under different speed and load conditions.
Abstract: Induction motors are widely used in industry as prime electromechanical energy conversion devices. Consequently, the condition monitoring and fault diagnosis of induction motors have received significant attention recently and become an integrated part of various maintenance strategies (for example preventive, condition-based and reliability-based maintenance). This paper presents a comparison of results of induction motor broken rotor bar fault detection using vibration and stator current methods. A broken rotor bar fault was induced into in a variable speed three-phase induction motor. Both the vibration and stator current signatures were acquired under different speed and load conditions. The fault detection sensitivities of vibration and stator current methods are evaluated. This paper also addresses the relationship between current and vibration signatures under normal and faulty motor conditions using correlation and frequency response methods. This relationship is desirable in order to determine the fault signature transmission mechanism and to exclude the irrelevant vibration sources so as to enhance fault detection accuracy. The relationship, studied during steady-state operation and start-up, enabled the identification of the vibrations from other sources.
TL;DR: In this paper, a new method using time and frequency related transformations has been developed for evaluation of response signals obtained by rail fastening analysis, which can also be used for designing new railway fastening systems and their parts.
Abstract: For evaluation of response signals obtained by rail fastening analysis a new method using time and frequency related transformations has been developed. In the paper the laboratory measurements and dynamic parameter analyses of a flexible fastening of the Vossloh SKL14 type have been described. The method can also be used for designing new rail fastening systems and their parts.
TL;DR: In this article, analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, have been developed for the first time and their validity proved by comparison with experiments on a well-defined fatigue crack in aluminium alloy.
Abstract: Cracks in solids can be detected by ultrasound if they are open. However their detection is not easy when they are closed with a closure stress. This is a fundamental problem in ultrasonic testing. Subharmonics ultrasoundwith half the input frequency is potentially useful in the detection and evaluation of such cracks. Analytical and numerical theories accounting for the crack parameters, such as closure stress and crack surface conditions, have been developed for the first time and their validity proved by comparison with experiments on a well-defined fatigue crack in aluminium alloy. Based on these theories, a novel method is proposed to estimate size of partially closed cracks. which solves the fundamental problem in ultrasonic testing.
TL;DR: In this paper, the authors evaluate the characterisation of failure mechanisms that occur in fibreglass reinforced polymeric matrix composites when subjected to tensile and flexural loads using the Fourier transform and wavelet transform.
Abstract: The applications for composite materials, especially in the automotive, chemical, petroleum and aeronautical industries, have been growing steadily in recent years due to their excellent properties, such as high specific mechanical properties and good corrosion resistance. Consequently, there has been a considerable increase in research toward the development of their structural properties, as well as the development of non-destructive inspection techniques for these materials. This paper aims to evaluate the characterisation of failure mechanisms that occur in fibreglass reinforced polymeric matrix composites when subjected to tensile and flexural loads. The acoustic emission signals of failure mechanisms, emitted at the moment of breaking of test samples, are analysed in the frequency domain by the Fourier transform technique (spectral analysis), and in the time-frequency domain by the wavelet transform. Both techniques are evaluated in terms of the characterisation of failure mechanisms, such as: transverse matrix cracking (resin cracking), fibre/matrix debonding and delamination. The results obtained attest the efficiency of both techniques and are an encouragement for publication.
TL;DR: In this article, the main piezoelectric properties of PZN-PT are predicted and the composite fabrication method is outfitted and the properties for four different single-crystal volume fractions are reported.
Abstract: Piezoceramic-polymer composite materials are finding increasing acceptance in ultrasonic transducers for NDT. However, new piezocrystal materials are also becoming available, such as lead zinc niobate doped with lead titanate (PZN-PT). Made into piezocomposite, they have significant theoretical performance advantages. Although the new materials are presently more expensive than piezoceramic, their prices have recently fallen dramatically, making them viable for high-end NDT. In this paper, the authors first review the background to the new piezocrystals then report their experience in piezocomposites made with them. The authors describe how they have predicted the main piezoelectric properties of these piezocomposites, establishing a robust design process. The composite fabrication method is outfitted and the properties for four different single-crystal volume fractions are reported. Prototype elements have been realised in the form of coupons approximately 5 mm x 10 mm suitable for dual-element probes and these probes have been evaluated on test objects of various materials, including a steel block, an aluminium block and a carbon fibre reinforced composite (CFRC) panel. The results confirm that the theoretical performance advantages can be achieved in practice, including doubling the gain-bandwidth product compared with conventional piezoceramic-polymer composite. It is concluded that applications such as ultrasonic spectroscopy, inspection of difficult materials and multi-element imaging arrays may benefit significantly.
TL;DR: In this paper, two kinds of Artificial Neural Networks (ANN): feedforward networks and self-organizing maps (SOM) were used for electric motor fault diagnosis using measurement data from stator currents and mechanical vibration signals.
Abstract: This paper presents electric motor fault diagnosis using two kinds of Artificial Neural Networks (ANN): feedforward networks and self organising maps (SOM). Major faults such as bearing faults, stator winding fault, unbalanced rotor and broken rotor bars are considered. The ANNs were trained and tested using measurement data from stator currents and mechanical vibration signals. The effects of different network structures and the training set sizes on the performance of the ANNs are discussed. This study shows that the feedforward ANN with a very simple internal structure can give satisfactory results, while SOMs can classify the type of motor faults during steady state working conditions. The experiment results also show that the feedforward ANN is the more promising scheme in this case where fault data from electric motors is available.
TL;DR: In this paper, the authors investigated the impact of fatigue and creep damage on the performance of a 500 MW coal-fired power plant with high-energy steam pipework and high temperature rotating plant, and developed procedures for both prevention of creep cavitation cracking and assessment of bore high strain fatigue life.
Abstract: The de-regulated electricity market which has existed in the UK for around 15 years has led to energy companies operating their power plant flexibly to maintain profitability in a very competitive commercial environment To do this, the company maintains a very strong engineering capability in the area of structural integrity of boiler components and high temperature rotating plant The key issues for the older 500 MW boiler plant, which is daily two-shifted and operates at temperatures well into the creep range for low-alloy steels, are fatigue and creep damage, the latter being influenced greatly by the operating hours seen by the older coal-fired units which have exceeded their design lives Major components of the boiler which are subject to rigorous and thorough examination are headers, drums and high-energy steam pipework Methodologies include on-line plant damage analysis for controlling degradation and optimising start-up procedures, plant modifications, innovative repair and retro-fitting with improved materials Steam and gas turbine rotors operate at high speeds at elevated temperatures Maintaining rotors in a good commercial and safe operational condition in compliance with any regulatory requirements can also bring increased efficiency and reduced costs, for example by incorporating proactive measures into planned overhauls Procedures have been developed for both prevention of creep cavitation cracking and assessment of bore high strain fatigue life (for hollow rotors) These use material data, temperature and stress information along with the results of NDE inspections to determine any operational constraints (starts, temperatures, and running hours) which may be necessary Vibration, which can be an indication of cracking in an operating rotor is monitored remotely and on-line