Propagation Curve Research Articles

Corrosion fatigue crack growth modelling for subsea pipeline steels

Based on the recent development of research for environment-assisted cracking (EAC), a crack growth model for the corrosion fatigue (CF) of subsea pipeline steels is proposed in this paper. The model adopts a two-component structure in consistence with the physical fact that CF is a mixed process of stress corrosion (SC) and hydrogen-assisted cracking (HAC). Anodic dissolution (AD) and hydrogen embrittlement (HE) are believed to be responsible for SC and HAC and their models are integrated in the general model within a framework of fracture mechanics. The model is then applied to modelling the CF crack growth of X65 pipeline steel and the results show that the shape of the modelled crack propagation curve can be controlled well using the proposed model, and both the exacerbated cracking rate and behavior features can be captured with appropriate consideration of the effects of mechanical and environmental parameters, such as loading frequency, stress ratio, temperature, and hydrogen concentration, etc.

A new experimental method to study the influence of welding residual stresses on fatigue crack propagation

This paper presents a study on the influence of welding residual stresses (RS) on fatigue crack propagation rate (FCPR) in mode I. The objective of this work is to develop a novel methodology that allows a variation of a RS field in the studied specimen while keeping constant all other variables influencing FCPR. This led to the development of a novel specimen geometry, named CT-RES, in which RS are introduced by weld bead deposition far from the region in which fatigue crack propagation (FCP) occurs. As a consequence, the effect of factors influencing FCPR other than RS such as microstructural changes or plastic deformation, often introduced by welding processes, can be avoided. The welding RS introduced in the CT-RES specimen were determined by the contour method and the weight functions method was used to calculate the stress intensity factor (SIF), Kres, resulting from the RS as the fatigue crack propagates into the specimen. The evolution of cyclic stress ratio at the crack tip, Rlocal, was then computed from Kres to quantify the influence of RS on the cyclic stress ratio. The results show that for a given stress intensity range, K, the FCPR of the welded geometry with fixed externally low R ratio (R=0.1), but constantly increasing Rlocal, is the same as for the as-machined geometry without RS, solicited at high cyclic stress ratio (R=0.7). These observations partially validate the BS7910 standard philosophy in which the remaining life of a flawed structure in presence of tensile RS is calculated from a high cyclic stress ratio (R≥0.5) crack propagation curve to eliminate crack closure effects.

A model for high-cycle fatigue crack propagation

Abstract This paper deals with the prediction of high-cycle fatigue behavior for four different materials (7075-T6 alloy, Ti-6Al-4 V alloy, JIS S10C steel and 0.4 wt.-% C steel) using Chapetti's approach to estimate the fatigue crack propagation curve. In the first part of the paper, a single integral equation for studying the entire propagation process is determined using the recent results of Santus and Taylor, which consider a double regime of propagation (short and long cracks) characterized by the model of El Haddad. The second part of the paper includes a comparison of the crack propagation behavior model proposed by Navarro and de los Rios with the one mentioned in the first half of this work. The results allow us to conclude that the approach presented in this paper is a good and valid estimation of high-cycle fatigue crack propagation using a single equation to describe the entire fatigue crack regime.

Long fatigue crack growth in Inconel 718 produced by selective laser melting

Growth of long fatigue cracks is investigated in Inconel 718 superalloy produced by selective laser melting (SLM). The fatigue crack growth curve and the threshold value of the stress intensity factor are experimentally determined on compact-tension specimens fabricated using a RENISHAW A250 system and the recommended processing parameters.The crack propagation curve and the crack propagation threshold of this SLM material are compared with literature data describing the behavior of conventionally manufactured Inconel 718. The fatigue crack growth is discussed in terms of the specific microstructure and residual stresses produced by selective laser melting.

Short Interval Control for the Cost Estimate Baseline of Novel High Value Manufacturing Products – A Complexity Based Approach

Novel high value manufacturing products by default lack the minimum a priori data needed for forecasting cost variance over of time using regression based techniques. Forecasts which attempt to achieve this therefore suffer from significant variance which in turn places significant strain on budgetary assumptions and financial planning. The authors argue that for novel high value manufacturing products short interval control through continuous revision is necessary until the context of the baseline estimate stabilises sufficiently for extending the time intervals for revision. Case study data from the United States Department of Defence Scheduled Annual Summary Reports (1986-2013) is used to exemplify the approach. In this respect it must be remembered that the context of a baseline cost estimate is subject to a large number of assumptions regarding future plausible scenarios, the probability of such scenarios, and various requirements related to such. These assumptions change over time and the degree of their change is indicated by the extent that cost variance follows a forecast propagation curve that has been defined in advance. The presented approach determines the stability of this context by calculating the effort required to identify a propagation pattern for cost variance using the principles of Kolmogorov complexity. Only when that effort remains stable over a sufficient period of time can the revision periods for the cost estimate baseline be changed from continuous to discrete time intervals. The practical implication of the presented approach for novel high value manufacturing products is that attention is shifted from the bottom up or parametric estimation activity to the continuous management of the context for that cost estimate itself. This in turn enables a faster and more sustainable stabilisation of the estimating context which then creates the conditions for reducing cost estimate uncertainty in an actionable and timely manner.

Propagation Curve and Velocity of Swallowing Pressure in Healthy Young Adults.

The objective of this study is to construct a propagation curve and determine propagation velocities in young healthy adults examined using a 2.64-mm-diameter high-resolution manometry catheter with 36 circumferential sensors; to explore data reproducibility; and to determine whether the swallowing pressure (SP) propagation velocity correlated with bolus volume. Repeated measures with subjects serving as their own controls. Thirty healthy subjects (average age 25.3years) swallowed saliva and 2, 5, and 10mL of cold water to determine the maximum SP from the soft palate to the cervical esophagus. The SP propagation curve was obtained by plotting the duration to reach each SP peak. The SP propagation velocity was calculated for each region. These parameters were examined according to bolus size and gender. The intra-class correlation coefficient for estimating the SP propagation curves was >0.61 (i.e., highly consistent). The propagation velocity was maximal at the meso-hypopharynx and minimal at the UES and cervical esophagus. The SP propagation curve was very reproducible within any subject. Neither the water volume (with the exception of 2 and 5mL) nor gender exerted any apparent effect on velocity in any region. However, the velocity was quite variable at the cervical esophagus.

Prediction of delamination growth in carbon/epoxy composites using a novel acoustic emission-based approach

Delamination is the most common failure mode in laminated composites and it leads to loss of structural strength and stiffness. In this paper, acoustic emission monitoring was applied on the carbon/epoxy laminated composites when subjected to mode I, mode II, and mixed-mode I and II loading conditions. The main objective is to investigate delamination behavior and to predict propagation curve of the delamination in different GII/GT modal ratio values by the acoustic emission. First, a combination of acoustic emission and mechanical data (sentry function) is used to characterize the propagation stage of the delamination. Next, the crack tip location during propagation of the delamination in the specimens is identified using two methods. In the first method, by determining the velocity of the acoustic emission waves in the specimens, the position of the crack tip can be estimated throughout the tests. In the second method, the cumulative energy of the acoustic emission signals is utilized for localization of the crack tip. Agreement between the predicted crack length and the actual crack length verifies the presented procedures. It can be concluded from the results that the acoustic emission method is a powerful approach to investigate the delamination behavior and to estimate the crack tip position in the composites.

Adaptive Fitting Reference Frame for 2-D Indoor Localization Based on RFID

Indoor localization information is important to many different industries. Ever-rising demand from both custom and supplier pose great challenge for indoor localization technology. The emergence of rapid development of Radio Frequency Identification (RFID) technology enriches indoor positioning methodology. Traditional indoor localization algorithm mainly based on the signal propagation property. Among all of positioning algorithm, LANDMARC is most promising one but still has some limitation such as multipath effect, noise form environment and quantity constrain of tags. This paper identifies previous RFID-based indoor localization algorithm and proposes a new algorithm aiming at delivering above limitation to estimate objective position indoor environment. It use RSS propagation curve to obtain RSS value of virtual reference tag, and then estimate target tags position. To evaluate its performance, a comparison between LANDMARC and proposed algorithm was conducted. Experiment result show proposed algorithm is more accurate than LANDMARC.

Measuring the delamination length in static and fatigue mode I tests using video image processing

In order to measure the delamination length in mode I static and fatigue tests, different systems such as the visual methods, embedded fibre Bragg grating sensors and crack gauges are available. These methods have various disadvantages in terms of reproducibility, measurement range, fatigue sensitivity, and cost. The aim of this work is to describe a technique for measuring the delamination length in mode I tests, based on video image processing. Video image processing allows non-contact continuous crack length monitoring in static tests and for the entire fatigue crack propagation curve. A set of static and fatigue tests carried out with the developed method is included in the present work. The static fracture toughness GIC, the G–N fatigue onset curve and the fracture energy rate versus the crack growth rate da/dN for epoxy glass fibre reinforced composites are reported. Results show that visual image processing is a stable technique for facilitating the post-processing of static and fatigue mode I tests.

Characteristic of mode I fatigue crack propagation of CFRP laminates toughened with CNF interlayer

In the present study the effect of carbon nanofiber interlayers on the fatigue crack propagation of woven fabric CFRP laminates made by VaRTM is investigated. Double cantilever beam (DCB) tests are carried out to determine the mode I static fracture toughness and mode I fatigue crack propagation curve. The experimental results clearly show that the interlaminar fracture toughness and the fatigue crack growth resistance can be substantially improved by the addition of CNF interlayers to the CFRP laminates. In more details, as far as the mode I fatigue crack tests are concerned, thanks to the addition of MWNT-7 interlayers the number of cycles to failure becomes 1.5 or more times greater than that of base laminates. Furthermore, it is found that the fatigue fracture toughness GImax (the upper limit of the energy release rate in a fatigue test) is increased by about 300%.

Studies on Electric field strength distribution of uhf television signal propagation in Ekiti State. Nigeria.

Researchers have continued to carry-out work to determine the actual coverage area of some broadcasting stations and the level of signal strength variation with distance from the transmitter, which is useful for radio transmission designs.In Nigeria, few works exist on the frequency modulation (FM) signals propagation, and little work has been done on the television signals. The work determines the actual coverage of the Broadcasting Service of Ekiti State Television, in Ekiti - State, southwest Nigeria, with the expected coverage area being the entire land mass of the State. The method used is such that requires the physical presence of the investigator in all the towns and villages within the State with the necessary instrument to carry out the electric field strength measurements in all the locations.The variation of signal level with distance (i.e. Propagation curve) was analysed, to reveal areas of optimum coverage and areas that are poorly serviced.The coverage of the newly installed transmitter in the station was compared to the old transmitter (which had been in use for over 30 years), and a comprehensive statistical data base provided for the UHF television broadcasting Station. The results shows the field strength distribution of the broadcasting station in major towns and villages,its elevation pattern and the various grades of coverage enjoyed by its citizens in such locations. Recommendations were made on getting areas outside optimum coverage well serviced. Keywords: Coverage area, Reception, Signal strength variations, Transmission,Ultra high frequency.

Background-oriented schlieren with natural background for quantitative visualization of open-air explosions

This study describes an attempt of quantitative visualization of open-air explosions via the background-oriented schlieren method (BOS). The shock wave propagation curve and overpressure distribution were extracted from the obtained images and compared with the results of the numerical analysis. The potential of extracting the density distribution behind the shock front is also demonstrated. Two open-air explosions were conducted; one with a \(36\)-kg emulsion explosive and the other with a \(7.89\)-kg composition C4 explosive. A high-speed digital video camera was used with a frame rate of \(10{,}000\,\mathrm{Hz}\) and a pixel size of \(800 \times 600\). A natural background, including trees and grass, was used for BOS measurements instead of the random dots used in a laboratory. The overpressure distribution given by the passing shock was estimated from the visualized images. The estimated overpressures agreed with the values recorded by pressure transducers in the test field. The background displacement caused by light diffraction inside the spherical shock waves was in good agreement, except at the shock front. The results shown here suggest that the BOS method for open-air experiments could provide increasingly better quantitative and conventional visualization results with increasing spatial resolution of high-speed cameras.

Reduced-thickness CVN testing to represent slant failure of pipelines

To avoid longitudinal ductile crack propagation along a gas pipeline, the Batelle Two Curve method is used during pipeline design. This method states that a running crack will be arrested if the gas decompression velocity exceeds the crack propagation speed at the internal gas pressure. The crack propagation curve is scaled by impact energy values obtained through Charpy V-Notch (CVN) testing. However, for high-strength steel grades this scaling leads to unconservative predictions, because the experiment does not sufficiently represent the pipeline failure mode. The CVN specimen exhibits mainly mode I failure, without significant shear lips, while real failure is a combined mode often described as slant failure. In the present study, instrumented CVN tests are carried out on samples with different thickness reduction levels. To get a better insight in the crack initiation and propagation behaviour, the CVN test is simulated by finite element analysis. The dissipated energy and resulting fracture surfaces can be successfully represented. It is observed that slant failure is promoted by reducing the specimen thickness. In addition, the specific absorbed energy is decreased. However, most of the difference of absorbed energy is in crack initiation. This means that the fraction of the total energy dissipated in crack propagation is increased for reduced thickness specimens, making it a possible tool to assess the resistance of a material to crack propagation, provided that brittle fracture is avoided.

G031032 放射光ラミノグラフィによるFSW継手材の疲労き裂進展挙動調査([G031-03]材料力学部門,一般セッション : 画像,計測)

Synchrotron radiation computed laminography (SRCL) was carried out at SPring-8 to visualize a fatigue crack inside a friction stir welded plate and to investigate the propagation behavior. The surface fatigue crack shape and its size reproduced by SRCL agree well with the observation by an optical microscope. A fatigue crack propagation curve obtained by SRCL corresponded with that obtained by replication technique. There are two inflection points on a curve describing the relation between the crack propagation rate and the surface crack length. The cause of the inflection was discussed.

Influence of the Microstructure of Eutectoid Steel on the Cyclic Crack Propagation: Pearlite and Spheroidite

In this paper the influence of microstructure on fatigue crack growth was analysed in steel with slightly hypereutectoid composition. A material constituted of pearlite colonies and a thin layer of proeutectoid cementite (pearlitic steel) was studied in its initial condition (as received). In addition, the same material was analyzed after undergoing a spheroidization treatment obtained by an isothermal treatment on pearlitic steel at 700oC and heating time of 10h. Results indicate that fatigue crack propagation curve in the Paris region is not modified by the spheroidization process. Fractometallographic analysis showed a change in the micromechanism of fatigue, evolving from transcollonial (trying to break pearlite lamellae) in the pearlitic steel to intergranular in the spheroidized steel, where cracking takes place through the layer of proeutectoid cementite.

Crack-Tip Shielding Effect on Fatigue Crack Propagation in Porous Silicon Carbide

A sharply notched specimen of porous silicon carbide with porosity of 37% was fatigued under four-point bending. The opening displacement of a fatigue crack was measured at several positions along cracks by using scanning electron microscopy. The crack propagation curve was divided into stages I, II, and III. The crack propagation rate first decreased with crack extension in stage I and became constant in stage II. In stage III, the crack propagation rate increased again. The range of crack opening displacement measured in SEM was lower than that calculated from the applied load range by FEM, suggesting that the anomalous variation of the crack propagation rate with crack extension was caused by crack-tip shielding due to crack face contact. The crack-tip stress intensity factor was estimated as a true crack driving force from the relation between the crack opening displacement and the applied load. The amount of crack-tip shielding increased very quickly with crack extension, reducing the crack-tip stress intensity factor in stage I. In stage II, the increasing applied stress intensity factor is balanced by the increase in the crack-tip shielding. The crack-tip stress intensity factor increases with crack extension in stage III.

Fatigue performance of cold drawn prestressing steel: The effect of sudden load changes

This paper analyzes the effect produced by sudden load changes on the fatigue of cold drawn prestressing steel wires to be used in prestressed concrete. Fatigue tests were performed on cylindrical samples under load control, using fatigue steps with different stress, and observing the appearance of a transient state in the fatigue crack propagation curve (da/dN-ΔK) when the product KmaxΔK suddenly decreases, a sort of overload retardation effect that allowed the calculation of the plastic zone size. In addition, the fatigue crack growth rate in the Paris regime, da/dN, obeys mainly to the parameter ΔK and it is hardly ever affected by Kmax. The study of the fatigue fracture surface shows: (i) that the micro-roughness of ductile micro-tearing patterns is a function of Kmax; (ii) the existence, in some tests, of a small zone with fatigue initiation fractography (with a tearing topography surface), related to the decrease of crack tip opening displacement, a phenomenon similar to that of crack initiation and growth from a micro-notch.

Evaluating Parameters for Characterization of Cracking in Asphalt Concrete

Abstract This study evaluates parameters derived from laboratory tests to characterize crack initiation and propagation within the semi-circular asphalt concrete samples in the laboratory. Asphalt samples are compacted using a Superpave gyratory compactor, and then sliced and notched using a laboratory saw. The notch acts to facilitate crack initiation. Notched samples are subjected to a strain controlled compressive load in three point bending. Crack opening displacements (CODs) and loads are recorded in real time using four linear variable displacement transducers. Ultimate load, COD at ultimate load, cracking potential, fracture load, crack velocity, and slope of the crack propagation curve are determined from laboratory test results and evaluated for their suitability in characterizing crack initiation and crack propagation. In essence, these parameters are evaluated through examining the effects of notch tip location, moisture condition, and void ratio on crack initiation and propagation. It is shown that crack initiation can be best characterized by the ultimate load. Crack velocity and slope of the crack propagation curve show promise in characterizing crack propagation in the notched asphalt samples.

Modeling Propagation Dynamics of Bluetooth Worms (Extended Version)

In the last few years, the growing popularity of mobile devices has made them attractive to virus and worm writers. One communication channel often exploited by mobile malware is the Bluetooth interface. In this paper, we present a detailed analytical model that characterizes the propagation dynamics of Bluetooth worms. Our model captures not only the behavior of the Bluetooth protocol but also the impact of mobility patterns on the Bluetooth worm propagation. Validation experiments against a detailed discrete-event Bluetooth worm simulator reveal that our model predicts the propagation dynamics of Bluetooth worms with high accuracy. We further use our model to efficiently predict the propagation curve of Bluetooth worms in big cities such as Los Angeles. Our model not only sheds light on the propagation dynamics of Bluetooth worms, but also allows to predict spreading curves of Bluetooth worm propagation in large areas without the high computational cost of discrete-event simulation.

Analysis Approach to Durability Based on Material Initial Fatigue Quality and S-N Curve

Based on probabilistic fracture mechanics approach, a new concept of material initial fatigue quality (MIFQ) is developed. Then, the relation between S-N curve and crack propagation curve is studied. From the study, a new durability analysis method is presented. In this method, S-N curve is used to determine crack growth rate under constant amplitude loading and evaluate the effects of different factors on durability and then the structural durability is analyzed. The tests and analyses indicate that this method has lower dependence on testing, and higher accuracy, reliability and generality and is convenient for application.