Type Flaw Research Articles

Estimating the True Sensitivity of Acceptance Ultrasonic Inspection of Rails

The acoustic path has been analyzed when carrying out the acceptance pulse echo inspection of the rail head. The influence of the parameters of the employed piezoelectric transducers on the sensitivity distribution for the sounding circuit used in the existing installations has been assessed, and the true sensitivity of detecting “nonmetallic inclusion” type flaws has been evaluated.

A SAT-Based Planning Approach for Finding Logical Attacks on Cryptographic Protocols

Cryptographic protocols form the backbone of digital society. They are concurrent multiparty communication protocols that use cryptography to achieve security goals such as confidentiality, authenticity, integrity, etc., in the presence of adversaries. Unfortunately, protocol verification still represents a critical task and a major cost to engineer attack-free security protocols. Model checking and SAT-based techniques proved quite effective in this context. This article proposes an efficient automatic model checking approach that exemplifies a security property violation. In this approach, a protocol verification is abstracted as a compact planning problem, which is efficiently solved by a state-of-the-art SAT solver. The experiments performed on some real-world cryptographic protocols succeeded in detecting new logical attacks, violating some security properties. Those attacks encompass both “type flaw” and “replay” attacks, which are difficult to tackle with the existing planning-based approaches.

Butt welded joints assessment after fire exposure

The need of in service life of a steel structure after being exposed to fire raises the problem of strengthening or replacement of some structural elements. In many cases, after structural safety assessment, fire exposed steel structures can be reused. Establishing the post-fire residual capacity of the structure has a major importance in overall assessment process. The paper presents a study for fire behavior of the butt-welded joints subjected to low cycle fatigue loading, taken into account the post fire conditions and the possibility of welding strengthening. There is presented a case study – a building structural element subjected to dynamic loading after fire exposure. For the in case study, post fire investigations revealed several welding flaws including welding surface type flaws. An assessment was needed in order to determine the structural integrity and life assessment of some structural elements. From the fracture mechanics point of view, a Failure Assessment Diagrams FAD - level 2 procedure was applied in order to determine the in service safety of the structure. Several flaws types were assessed and conclusions were taken. The results can be used for damage assessment and strengthening technique of post fire steel structures.

Structural integrity of butt welded connection after fire exposure

The paper presents a study for fire behavior of the butt-welded joints subjected to low cycle fatigue loading, taken into account the post fire conditions and the possibility of welding strengthening. The need of in service life of a steel structure after being exposed to fire raises the problem of strengthening or replacement of some structural elements. There is presented a case study – a building structural element subjected to dynamic loading after fire exposure [1]. For the in case study, post fire investigations revealed several welding flaws including crack type flaws. An assessment was needed in order to determine the structural integrity and life assessment of some structural elements. From the fracture mechanics point of view, a Failure Assessment Diagrams level 2 – FAD 2 procedure was applied in order to determine the in service safety of the structure. Several flaws types were assessed and conclusions were taken. The results can be used for damage assessment and strengthening technique of post fire steel structures.

Technical Problem Identification for the Failures of the Liberty Ships

The U.S. Liberty Ship Building Program in World War II set a record—a total of 2700 Liberty Ships were built in 6 years, in order to support the battle against Nazi-Germany. However, numerous vessels suffered sudden fracture, some of them being split in half. This paper demonstrates and investigation of the Liberty Ships failure and problems, which reveals that the failures are caused by a combination of three factors. The welds produced by largely unskilled work force contain crack type flaws. Beyond these cracks, another important reason for failure associated with welding is the hydrogen embitterment; most of the fractures initiate at deck square hatch corners where there is a stress concentration; and the ship steel has fairly poor Charpy-Impact tested fracture toughness. It has been admitted that, although the numerous catastrophic failures were a painful experience, the failures of the Liberty Ships caused significant progress in the study of fracture mechanics. Considering their effect, the Liberty Ships are still a success.

Fabrication of imitative cracks by 3D printing for electromagnetic nondestructive testing and evaluations

This study demonstrates that 3D printing technology offers a simple, easy, and cost-effective method to fabricate artificial flaws simulating real cracks from the viewpoint of eddy current testing. The method does not attempt to produce a flaw whose morphology mirrors that of a real crack but instead produces a relatively simple artificial flaw. The parameters of this flaw that have dominant effects on eddy current signals can be quantitatively controlled. Three artificial flaws in type 316L austenitic stainless steel plates were fabricated using a powderbed-based laser metal additive manufacturing machine. The three artificial flaws were designed to have the same length, depth, and opening but different branching and electrical contacts between flaw surfaces. The flaws were measured by eddy current testing using an absolute type pancake probe. The signals due to the three flaws clearly differed from each other although the flaws had the same length and depth. These results were supported by subsequent destructive tests and finite element analyses.

Detection, characterization and sizing of hydrogen induced cracking in pressure vessels using phased array ultrasonic data processing

Pressure vessels operating in sour service conditions in refinery environments can be subject to the risk of H₂S cracking resulting from the hydrogen entering into the material. This risk, which is related to the specific working conditions and to the quality of the steel used, shall be properly managed in order to maintain the highest safety at a cost-effective level.Nowadays the typical management strategy is based on a risk based inspection (RBI) evaluation to define the inspection plan used in conjunction with a fitness for service (FFS) approach in defining if the vessel, although presenting dangerous defects such as cracks, can still be considered “fit for purpose” for a given time window based on specific fracture mechanics analysis.These vessels are periodically subject to non-destructive evaluation, typically ultrasonic testing. Phased Array (PA) ultrasonic is the latest technology more and more used for this type of application.This paper presents the design and development of an optimized Phased Array ultrasonic inspection technique for the detection and sizing of hydrogen induced cracking (HIC) type flaws used as reference for comparison. Materials used, containing natural operational defects, were inspected in “as-service” conditions.Samples have then been inspected by means of a “full matrix capture” (FMC) acquisition process followed by “total focusing method” (TFM) data post processing. FCM-TFM data have been further post-processed and then used to create a 3D geometrical reconstruction of the volume inspected. Results obtained show the significant improvement that FMC/TFM has over traditional PA inspection techniques both in terms of sensitivity and resolution for this specific type of defect. Moreover, since the FMC allows for the complete time domain signal to be captured from every element of a linear array probe, the full set of data is available for post-processing.Finally, the possibility to reconstruct the geometry of the component from the scans, including the defects present in its volume, represents the ideal solution for a reliable data transferring process to the engineering function for the subsequent FFS analysis.

Detection of Localized Damage in Water Wall Tubes of Thermal Power Plants Using GMR Sensor Array Based Magnetic Flux Leakage Technique

A flexible giant magneto-resistive (GMR) sensor array based magnetic flux leakage technique has been developed for nondestructive inspection of carbon steel water wall tubes (outer diameter 64.0 mm and wall thickness 6.3 mm) used in a thermal power plant. Two saddle coils are used for magnetisation of the tube and a flexible GMR sensor array is used for detection of leakage fields from localized damage in the tube. 3D-nonlinear finite element modeling has been performed to optimize the magnetizing current, inter-coil spacing of the saddle coils and location of the GMR sensor array within the saddle coils. The performance of the technique has been evaluated by measuring the axial component of leakage flux from outer diameter (OD) erosion and inner diameter (ID) corrosion type flaws artificially machined on the water wall tube. Studies reveal that the proposed technique detects both OD erosion and ID corrosion damage in the water wall tubes.

Thermoelectric and mechanical properties of melt spun and spark plasma sintered n-type Yb- and Ba-filled skutterudites

• N-type double filled skutterudites can be formed by a combination of melt spinning and spark plasma sintering, obviating annealing steps. • The consolidated billets have thermoelectric properties that are comparable to those produced using more time and energy intensive powder metallurgical methods. • We demonstrate that the here described preparation route is scalable to 80 g billets as evidenced by the comparable nature of the transport properties of specimens cuts from large samples to those of smaller lab-scale specimens. Here we present thermoelectric and mechanical properties of n-type filled-skutterudites produced by a combination of melt spinning of pre-melted charges with subsequent consolidation by spark plasma sintering, a process we refer to as MS-SPS. This combination of processing steps leads to phase-pure n-type filled-skutterudites and obviates more energy and time intensive annealing steps. We show that both the thermoelectric properties and the tensile fracture strength compare favorably to materials made by traditional methods. The process is scalable to at least 80 g billets, such that the transport properties measured on test bars harvested from these larger billets compare favorably to those measured on lab-scale billets (5 g total billet mass). ZT values approaching 1.1 at 750 K were observed in materials made by MS-SPS. In addition, the tensile fracture strength of test bars cut from an 80 g billet is ∼128 MPa at room temperature and decreases with increasing temperature. Fractography of the test bars reveals that the majority failed due to surface and edge flaws with few failures due to volume type flaws. This indicates that the powder metallurgical methods employed to produce these samples is mature.

Three-dimensional singular stress fields near the circumferential junction corner line of an island/substrate system either free-standing or fully/partially bonded to a rigid block

The primary motivation of the present investigation stems from the need for understanding the three-dimensional singular stress fields in the vicinity of the re-entrant circumferential bimaterial junction corner line between a thin film island and the underlying substrate, either free-standing or fully/partially bonded to a super-rigid block. A three-dimensional eigenfunction expansion approach is employed for determination of the singular stress field in the vicinity of the re-entrant circumferential bimaterial junction corner line. The island-substrate bimaterial system is subjected to extension/bending (mode I), shear/twisting (mode II) and torsional (mode III) far field loadings. Both the island and substrate materials are assumed to be isotropic and elastic. The boundary conditions, prescribed on either side of the interfacial bond line between the island and substrate materials (free–free, fixed–fixed, free–fixed and fixed–free), are exactly satisfied. Numerical results include the dependence of the lowest eigenvalue(s) (or most severe order(s) of stress singularity) on the wedge aperture angle of the island material (looking at a axisymmetric cross-section). Variation of the same with respect to the shear moduli ratio of the constituent island and substrate materials is also an important part of the present investigation. Hitherto unobserved interesting and physically meaningful conclusions in regards to the mixed free–fixed type singularity and delamination type flaw sensitivity of an island-substrate bimaterial system are also presented.

Reference loads for plates with semi-elliptical surface cracks subjected to tension and bending for application within R6 type flaw assessment

Two of the authors of the present paper proposed a definition of a reference load F0 which can be used as an alternative option to the common limit load for cases where the definition of the latter might be problematic in the frame of flaw assessment procedures such as R6, BS 7910, SINTAP or FITNET. The reference load is defined as that load at which the ligament yielding parameter Lr approaches one. F0, in general, varies along the crack front. It has to be obtained by finite element analyses but the results can be approximated by analytical expressions. In a previous study of the authors such a solution was presented for tension loaded plates with semi-elliptical surface cracks. In the present paper it is extended to pure bending and to combined bending and tension.

Leveraged Municipal Bond Arbitrage: What Went Wrong?

Substantial losses suffered by several multibillion dollar fixedincome hedge funds have brought attention to risks involved in what was advertised as a leveraged municipal bond arbitrage strategy. Brokerage firms marketed these hedge funds to investors as higher-yielding alternatives to conventional municipal bond portfolios with little, if any, additional risk. In this article, the authors explain what the strategy really was, why it was not an arbitrage, and why it failed. Sponsors of these hedge funds portrayed the failures as the result of unprecedented and unforeseeable market events operating on a fundamentally sound strategy. In contrast, the authors show the funds failed because of the confluence of flaws in the strategy:ineffective hedges, high leverage, poor incentives created by management fees, and leverage limits based on portfolio acquisition costs (not market values), and the mis measurement of within horizon risk, which differentiates strategies operated on proprietary trading desks and in retail hedge funds. Fundamentally, the strategy was simply a highly leveraged bet on the value of short call options, interest rates, and liquidity and credit risk. Events in late 2007 and early 2008 revealed the fundamental flaws in this hedge fund type that is now effectively extinct. <b>TOPICS:</b>Real assets/alternative investments/private equity, fixed income and structured finance, derivatives, risk management

A three-dimensional eigenfunction expansion approach for singular stress field near an adhesively-bonded scarf joint interface in a rigidly-encased plate

A three-dimensional eigenfunction expansion approach for determination of the singular stress field in the vicinity of an adhesively-bonded scarf joint interface in a plate, with its top and bottom surfaces being encased, fully or partially, between infinitely rigid blocks is presented. The plate is subjected to extension/bending (mode I) and in-plane shear/twisting (mode II) far-field loading. Both the adhesive layer and plate materials are assumed to be isotropic and elastic. The boundary conditions that are prescribed on the end-faces (free, fixed and lubricated) of the plate as well as those, prescribed at the bottom or top surface of the scarf-bonded plate on either side of the interface between the plate and adhesive layer materials (fixed-fixed, free-fixed and fixed-free), are exactly satisfied. Numerical results include the dependence of the lowest eigenvalue (or most severe stress singularity) on the wedge aperture angle of the plate material. Variation of the same with respect to the shear moduli ratio of the constituent plate and adhesive layer materials is also an important part of the present investigation. Hitherto unobserved interesting and physically meaningful conclusions in regards to the fixed edge singularity and delamination type flaw sensitivity of an adhesively bonded plate surface are also presented. Finally, hitherto unavailable results, pertaining to the through-width variations of stress intensity factors corresponding to symmetric and skew-symmetric sinusoidal loads that also satisfy the boundary conditions on the end-faces of the adhesively bonded plate, in the vicinity of the scarf joint interface, under investigation, bridge a longstanding gap in the bonded joint stress singularity/fracture mechanics literature.

The HOIS recommended practice for in-service computed radiography of pipes

The comparatively new technology of computed radiography (CR) for NDT imaging has a number of advantages over traditional film-based radiography, including reduced exposure times and greater dynamic range. In addition, it does not require facilities for the developmentof radiographic film, which can be difficult to install and maintain given the often restricted deck facilities space in an offshore environment. However, the technology associated with CR is substantially more complex than film and, in the absence of any internationally recognised standards for in-service radiographic inspection, the quality of the resulting CR images can be variable. To improve the quality control of this form of inspection, the HOIS JIP has developed a recommended practice covering CR for the in-service inspection of pipes for wall loss type flaws (corrosion, erosion). This project involved a series of experimental trials to first develop and then validate the recommendations, and also quantitatively compare the characteristics of different commercially available CR systems. The results of the CR trials undertaken during this project and the main recommendations of the recommended practice are described.

A novel method for detection and quantification of incomplete penetration type flaws in weldments

This paper presents an image processing approach to process incomplete penetration type flaws in radiographic images of the weld specimens considering morphological aspects of the image. In the present approach, radiographic weld image is processed to get the image with good contrast and reduced noise using image enhancement techniques like histogram equalization and noise filtering. This is followed by an effe ctive edge detector such as Canny operator to find the edges of flaws, which are further used to segment the image to fix the bou ndaries. Image segmentation is performed using morphological approach implemented through dilation of similar boundaries and erosion of dissimilar and irrelevant boundaries decided on the basis of pixel characteristics. The incomplete penetra tion type flaws are clearly identifiable in the finally process ed image, which is obtained by superimposing the segmented image over the original enhanced image. The experimental results proved that proposed approach can be used as a reliable method for automatic detection of incomplete penetration types of flaws i n radiographic weld images.

Detecting and preventing type flaws at static time*

A type flaw attack on a security protocol is an attack where an honest principal is cheated on interpreting a field in a message as the one with a type other than the intended one. In this paper, we shall present an extension of the LYSA calculus to cope with types, by using tags to represent the i ntended types of terms. We develop a Control Flow Analysis for this calculus which soundly over-approximates all the possible behaviour of a protocol and, in particular, is able to capture any type confusion that may occur during the protocol execution. The analysis acts in a descriptive way: it describes which violations may occur. In the same setting, our approach also offers a prescriptive usage: we can impose a type discipline, by forcing some data to be of the expected types. At this point, the analysis may statically check that type violations are not possible any longer. In other words, we instrument the code with the only checks necessary to enforce type security. Finally, we apply our framework to a multi-protocol setting, where the risk of having type flaw attacks is higher. Our analysis has been implemented and successfully applied to a number of security protocols, showing it is able to capture type flaw attacks. The implementation complexity of the analysis is low polynomial.

Model checking wireless sensor network security protocols: TinySec + LEAP + TinyPK

In this paper, a formal analysis of security protocols in the field of wireless sensor networks is presented. Three complementary protocols, TinySec, LEAP and TinyPK, are modelled using the high-level formal language HLPSL, and verified using the model checking tool AVISPA, where two main security properties are checked: authenticity and confidentiality of messages. As a result of this analysis, two attacks have been found: a man-in-the-middle-attack and a type flaw attack. In both cases confidentiality is compromised and an intruder may obtain confidential data from a node in the network. Two solutions to these attacks are proposed in the paper.

Detecting and Preventing Type flaws: a Control Flow Analysis with Tags

A type flaw attack on a security protocol is an attack where an honest principal is cheated on interpreting a field in a message as the one with a type other than the intended one. In this paper, we shall present an extension of the LySa calculus with tags attached to each field, indicating the intended types. We developed a control flow analysis for analysing the extended LySa, which over-approximates all the possible behaviour of a protocol and hence is able to capture any type confusion that may happen during the protocol execution. The control flow analysis has been applied to a number of security protocols, either subject to type flaw attacks or not. The results show that it is able to capture type flaw attacks on those security protocols.

The effect of flaw shape on fracture initiation at a blunt flaw

The author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. The approach is based on the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture. An earlier paper has derived a blunt flaw fracture initiation relation which gives the critical elastic flaw-tip peak stress σpcr (a “signifier” of a critical condition in the process zone) in terms of the process zone material parameters, subject to the proviso that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius ρ. The relation has been derived using a “two-extremes” procedure, whereby the separate σpcr solutions for small and large s/ρ are blended together to give an all-embracing relation that is valid for all s/ρ. A key feature of the relation is that σpcr essentially depends on only one geometrical parameter: the flaw root radius ρ. Though the relation has evolved from a consideration of the characteristics of one model, i.e. that of an elliptical flaw in an infinite solid that is subjected to an applied tensile stress, it is anticipated that the relation can be applied equally well for a wide range of geometrical configurations involving different flaw shapes. It is against this background that the present paper demonstrates that the relation also applies to the behaviour of an intrusion type flaw in the surface of a semi-infinite solid subjected to an applied tensile stress.

On preventing type flaw attacks on security protocols with a simplified tagging scheme

A type flaw attack on a security protocol is an attack in which a field in a message that was originally intended to have one type is subsequently interpreted as having another type. Heather et al. proved that type flaw attacks can be prevented by tagging each field with the information that indicates its intended type. We simplify Heather et al.'s tagging scheme by combining all the tags inside each encrypted component into a single tag and by omitting the tags on the outermost level. The simplification process reduces the sizes of messages in the security protocol. We also formally prove that our simplified tagging scheme is as secure as Heather et al.' with the strand space method.