Impact Energy Data Research Articles

Further development of the gamma exponent model for assessment of flaws in oil and gas pipelines

The Gamma Exponent Model (“GEM”) is a recently developed methodology for failure pressure prediction of flaws in oil and gas pipelines. The model has previously demonstrated applicability for the assessment of axial surface breaking (or “part wall”) cracks and localized metal loss/corrosion. This work extends the applicability of the model to circumferential surface cracks. In addition, the GEM is shown to be consistent and complementary to the log-secant model for assessment of axial through wall cracks, developed by the NG18 Committee of the Battelle Institute in the early 1970s. The ongoing development of the GEM raises some important theoretical considerations related to understanding of the stress state at the flaw tip during the failure process. The current industry models are not explicit regarding the plane stress or plane strain conditions at the flaw tip, though this has a significant effect on model development. Analysts often assume plane stress conditions, due to oil and gas pipelines typically being considered “thin wall” pipe. However, this work confirms the theory that surface cracks are subject to plane strain conditions, and the selection of fracture toughness correlations from impact energy data becomes very important. The model is validated against available laboratory, hydrotest and in-service failure data. Statistical analyses demonstrate the mathematical form of the new model is valid. Failure pressure predictions are shown to be improved over current industry models. This can have a significant effect on the reliability pipeline integrity programs and improved efficiency in the form of cost savings.

Charpy impact energy correlation with fracture toughness for low alloy structural steel welds

This work addresses an experimental investigation of the cleavage fracture behavior for ASTM A572 Grade 50 steel welds fabricated by different welding processes. A primary purpose of this study is to examine a correlation approach to evaluate fracture toughness values of steel welds based on Charpy V-notch (CVN) impact energy data thereby enlarging existing relationships between fracture toughness and CVN energy. Fracture toughness testing conducted on three-point bend SE(B) specimens having a weld centerline notch provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc, for the steel welds, including the baseplate material. Conventional Charpy tests produce further experimental data which serve to characterize the ductile-to-brittle transition of the tested steel welds and the CVN energy correlation with fracture toughness data derived from conventional correlation formulas. A statistical analysis based on the Master Curve methodology conducted on the experimentally measured fracture toughness data provides the reference temperature for the tested materials from which estimates of fracture toughness based on CVN energy can be made. The results suggest that current correlations between T0 and the CVN energy may provide nonconservative fracture toughness estimates and, thus, should be modified when the primary purpose is to ensure adequate safety levels, as usually required in flaw assessment procedures.

A study of accidental impact scenarios for composite wing damage tolerance evaluation

ABSTRACTThe field data characterising aircraft accidental in-service damage was collected, sorted and processed. By means of probabilistic analysis, the wing damageability statistical parameters were determined. The scenarios of wing accidental impacts were described and the qualitative distribution of impact intensity over the wing surfaces was obtained. By means of original analytical method, the metal dent depth data were converted into impact energy data and energy probabilistic distributions were established. It was shown that the functional relationships generated on domestic data are generally consistent with similar foreign results obtained on other types of aircraft with serious differences in operating conditions. Along with realistic impact damage scenarios, the high energy impact events were considered. It was noted that in some cases severe damage events should not be addressed as extremely improbable and should be included into design and certification process.

Viscoelastically prestressed polymeric matrix composites – Effects of delayed moulding on Charpy impact properties

Viscoelastically prestressed polymeric matrix composites (VPPMCs) are produced by subjecting fibres to creep, then releasing the creep load before moulding. Previous work has demonstrated mechanical property improvements up to ∼50% from nylon 6,6 fibre-polyester resin VPPMCs, compared with control (unstressed) counterparts. Since fibre stretching and moulding processes are decoupled, the time interval between releasing the fibre stretching load and moulding (delayed moulding) offers considerable production flexibility. This paper investigates delayed moulding over 0–1272 h, using fibres stored at 20 °C and −25.4 °C. Charpy impact tests demonstrated increased energy absorption from all VPPMC samples compared with control counterparts, this increase reducing with delayed moulding time. A 1272 h delay gave an increase of ∼23% for fibre storage at 20 °C, and ∼40% at −25.4 °C, the latter demonstrating “decelerated” ageing. For all samples, the magnitude of fibre-matrix debonding (the principal energy absorption mechanism) increased linearly with impact energy data.

The Role of the Atmosphere on Boron-Activated Sintering of Ferrous Powder Compacts

AbstractBoron has been known to activate densification during sintering of ferrous powder compacts, though with risk of embrittlement. In the present study, specimens Fe-B and Fe-C-B prepared from standard atomized iron powder with addition of ferroboron Fe-21%B were sintered in different atmospheres, and the resulting microstructures and properties were studied. It showed that the activating effect of boron is observed during sintering in argon and in hydrogen while sintering in N2containing atmospheres results in rapid deactivation of boron, through formation of stable BN. In hydrogen atmosphere, surface deboronizing was observed to considerable depth. Ar is chemically inert, but Ar trapped inside closed pores tends to inhibit further densification. The impact energy data indicated that the embrittling effect of boron is enhanced significantly by presence of carbon. In the fracture surfaces, transgranular cleavage fracture can be observed both at very low and high impact energy values.

Phosphorus in Sintered Steels: Effect of Phosphorus Content and P Carrier in Sintered Steel Fe-C-P

Abstract Phosphorus as an alloy element is quite common in powder metallurgy, the contents industrially used being markedly higher than those present in wrought steels. In this study, the influence of phosphorus addition through different P carriers was investigated. PM steels of the type Fe-0.7%C-x%P (x = 0.0 … 0.8%) were manufactured by pressing and sintering in H2. It showed that Fe3P is the best phosphorus carrier, resulting in fine and regular microstructure and in high impact energy data at 0.3 … 0.45%P while red P and also Fe2P showed a tendency to agglomeration, with resulting secondary porosity. At high P levels the mechanical properties tend to drop, for the tensile strength at P > 0.60%P while for the impact energy the threshold is 0.45%P. The dimensional behaviour of Fe-C-P can be related to PM aluminium alloys, expansion by transient liquid phase being followed by shrinkage by persistent liquid phase, at least at higher temperatures. In contrast to the dimensional behaviour, degassing and reduction is hardly affected by the phosphorus content.

Comparison of Material Properties and Creep Behavior of 20MnCr5 and S275JR Steels

t is well known that the optimization procedure has an important role in the structure design. Furthermore, experimentally obtained data of material behavior is known to serve as the most relevant data in the mentioned design procedure. Therefore, this paper sets out to examine some experimentally determined data of the material subjected to certain environmental conditions. Based on these parameters, some analyses of material behavior can be made. In addition, data are of such nature that it is possible to make an appropriate comparison between the two investigated materials. Materials under considerations were 20MnCr5 steel and S275JR steel. Both of materials have been subjected to the same environmental conditions and the following properties can be singled out: ultimate tensile strength, 0.2 offset yield strength, the modulus of elasticity, elongation, creep behavior and Charpy impact energy. Each of the mentioned details are determined by the corresponding test, e.g. data related to strengths and to creep behavior are determined by tensile tests while impact energy is determined by Charpy impact test. In this way, the obtained values are presented in the form of the engineering stress-strain diagrams, creep curves and impact energy data.

Application of extra‐low impact energy SIMS and data reduction algorithm to USJ profiling

Secondary ion mass spectrometry analysis of ultra‐shallow (sub‐200 eV) B implants is complicated by the presence of native oxide on the surface of Si. Knowledge of the type of oxide present on the surface as well as accurate oxide thickness is very important for correct data reduction in extra‐low energy depth profiling. Sputter rate (SR) variation approach based on the accurate SR measurement in Si and SiO2 can be successfully applied to B implants done through thermal oxides of various thicknesses. Lowest profiling energy should be used to achieve the least profile distortion in the case of de‐channeled B implants: the rule of half of the implant energy used for profiling energy is confirmed. Profiling energy that is equal to the implant energy can be used if implantation is done through native oxide or into the stripped Si. If no thermal oxide is present on the surface, the most consistent result will be obtained when implants are done into the pre‐stripped Si thus eliminating uncontrollable and variable native oxide‐related issues. Copyright © 2012 John Wiley & Sons, Ltd.

Comparison of ductile-to-brittle transition curve fitting approaches

Ductile-to-brittle transition (DBT) curve fitting approaches are compared over the transition temperature range for reactor pressure vessel steels with different kinds of data, including Charpy-V notch impact energy data and fracture toughness data. Three DBT curve fitting methods have been frequently used in the past, including the Burr S-Weibull and tanh distributions. In general there is greater scatter associated with test data obtained within the transition region. Therefore these methods give results with different accuracies, especially when fitting to small quantities of data. The comparison shows that the Burr distribution and tanh distribution can almost equally fit well distributed and large data sets extending across the test temperature range to include the upper and lower shelves. The S-Weibull distribution fit is poor for the lower shelf of the DBT curve. Overall for both large and small quantities of measured data the Burr distribution provides the best description.

Probabilistic characterisation of model error using Gaussian mixture model—With application to Charpy impact energy prediction for alloy steel

A novel approach to characterise the model prediction errors using a Gaussian mixture model is proposed. The motivation for this work lies behind many data models that are developed through prediction error minimisation with the assumption of a normal noise distribution. When the noise is non-normal, which may often be the case in complicated data modelling scenarios, the model prediction errors may contain rich information, which can be further exploited for model refinement and improvement. The key contents presented in this paper include: choosing the relevant variables to form the error data, optimising the number of Gaussian components required for the error data modelling, and fitting the Gaussian mixture parameters using an expectation-maximisation algorithm. Application of the proposed method for further model improvement, within the framework of hybrid deterministic/stochastic modelling, is also discussed. Preliminary results on the real industrial Charpy impact energy data for heat-treated steels show its effectiveness for model error characterisation, and the potential for model performance improvement in terms of prediction accuracy as well as providing accurate prediction confidence intervals.

Microwave window breakdown experiments and simulations on the UM/L-3 relativistic magnetron

Experiments have been performed on the UM/L-3 (6-vane, L-band) relativistic magnetron to test a new microwave window configuration designed to limit vacuum side breakdown. In the baseline case, acrylic microwave windows were mounted between three of the waveguide coupling cavities in the anode block vacuum housing and the output waveguides. Each of the six 3 cm deep coupling cavities is separated from its corresponding anode cavity by a 1.75 cm wide aperture. In the baseline case, vacuum side window breakdown was observed to initiate at single waveguide output powers close to 20 MW. In the new window configuration, three Air Force Research Laboratory-designed, vacuum-rated directional coupler waveguide segments were mounted between the coupling cavities and the microwave windows. The inclusion of the vacuum side power couplers moved the microwave windows an additional 30 cm away from the anode apertures. Additionally, the Lucite microwave windows were replaced with polycarbonate windows and the microwave window mounts were redesigned to better maintain waveguide continuity in the region around the microwave windows. No vacuum side window breakdown was observed in the new window configuration at single waveguide output powers of 120+MW (a factor of 3 increase in measured microwave pulse duration and factor of 3 increase in measured peak power over the baseline case). Simulations were performed to investigate likely causes for the window breakdown in the original configuration. Results from these simulations have shown that in the original configuration, at typical operating voltage and magnetic field ranges, electrons emitted from the anode block microwave apertures strike the windows with a mean kinetic energy of 33 keV with a standard deviation of 14 keV. Calculations performed using electron impact angle and energy data predict a first generation secondary electron yield of 65% of the primary electron population. The effects of the primary aperture electron impacts, combined with multiplication of the secondary populations, were determined to be the likely causes of the poor microwave window performance in the original configuration.

A CVN-K(K<SUB align=right>IC) correlation for H13 tool steels

Fracture is the most common cause of die failure in hot extrusion. Prediction of fracture life requires a reasonably accurate determination of plane-strain fracture toughness (KIC). Standard K(KIC) testing is difficult, time-consuming, and costly. Charpy impact energy (CVN) is often used as an indirect measure of fracture toughness. Published CVN-K(KIC) correlations do not generally apply to tool steels, and are not suitable for high temperature predictions. This paper is part of a study aimed at the development of a reasonable CVN-K(KIC) correlation for the hot work tool steel H13, subjected to various tempering cycles and tested at different elevated temperatures, based on hardness and impact energy data.

Upper shelf energy normalisation for sub-sized Charpy-V specimens

When the plate thickness is less than 10 mm, testing with standard sized Charpy-V notch specimens is impossible. In such cases the testing must be based on sub-sized specimens. Sometimes, standard size specimens are impractical also for thicker sections. The use of sub-sized specimens requires extrapolation of the result from the sub-sized specimen to correspond to the result from a standard sized specimen. In this work, literature data combined with newly generated data were collected and examined in order to find the correct normalisation procedure for sub-sized specimen, upper shelf impact energy data. The resulting normalisation procedure, combined with a new Charpy-V-J–R curve correlation, was shown to produce realistic estimates of the materials upper shelf fracture toughness. In the case of materials showing pronounced splitting in the Charpy-V test, the estimates may be over-conservative.

Effect of temper embrittlement and specimen size on Charpy impact testing of a Cr–Mo–V rotor steel

Full and sub­size Charpy V notch specimens from several locations of a high pressure–intermediate pressure Cr–Mo–V turbine rotor were tested. A comparison between full and sub­size impact energy data showed that the smaller specimens exhibited qualitatively similar behaviour, with a systematic reduction in fracture appearance transition temperatures (FATTs). The full and sub­size impact energy data were normalised against the specimen area and volume. The latter normalisation produced the closest match and the offset between the two data sets was described by a simple linear equation. The sensitivity of impact energy and FATT to specimen size was examined in samples possessing different degrees of temper embrittlement. It was found that the difference in FATT between full and sub­size specimens for embrittled samples was at least double that of de­embrittled samples. It is proposed that the observed specimen size/impact energy/FATT variations with degree of embrittlement arise from sensitivity of intergranular fracture to lineal specimen thickness, since fracture occurs predominantly through a two­dimensional network of grain boundaries.

Measurements of Ballistic Impact Response of Novel Coextruded PC/PMMA Multilayered-Composites

Ballistic impact behavior of coextruded composites consisting of alternate layers of ductile polycarbonate (PC) and brittle poly(methyl methacrylate) (PMMA) were investigated. These PC/PMMA composites were 256, 1024, 2048 and 4096 layers; each consisted of various component compositions and their corresponding layer thickness was on the order of micron or sub-micron scale. Individual layer thickness of PMMA was determined to be critical for the impact response of these ductile/brittle hybrids. A brittle mode of failure was encountered as the thickness of PMMA layers reached 0.5 mm or higher, regardless of their composition or layer configuration. Damage zone size increased significantly as the PMMA layer thickness reduced to approximately 0.36 mm; a mixed mode of failure resulted in these PC/PMMA composites. With the PMMA layer thickness further reduced to approximately 0.14 mm, ductile deformation, which was predominant in the pure PC, occurred in these coextruded composites. Ballistic impact energy data showed that the extent of energy absorption or dissipation increased with an increase in either the PC content or the total number of layers. These results further revealed that the individual layer thickness of PMMA was the critical material parameter, and the determined impact energy values were consistently higher for the composites with PMMA layer thickness being 0.14 mm or thinner. Adhesion between the layers appeared to be very good since delamination did not occur whether these PC/PMMA composites failed in a brittle or a ductile mode.

An overview of the principles of modeling charpy impact energy data using statistical analyses

Integrity assessments of Magnox nuclear reactors with steel pressure vessels quantify the temperature margins between the operating temperature of the plant, at any given location, and the onset of upper-shelf temperature. The onset of upper-shelf temperature can be estimated from the fracture toughness properties of each material used in the construction of the pressure vessels. Although start-of-life fracture toughness properties of the materials have been measured, such properties are not available for the neutron-irradiated and thermally aged condition. One of the main effects of neutron irradiation and temperature experienced during service is to increase the ductile-to-brittle transition temperature (DBTT), which can be represented in terms of temperature shifts. In the irradiation surveillance schemes for the Magnox reactors, these temperature shifts can be inferred from Charpy impact energy data which have been measured regularly during the service life. Since Charpy impact energy data are inherently scattered, it is necessary to optimize the interpretation of the data by statistical processing. A recent analysis undertaken by Moskovic et al. concluded that Bayesian analyses are best suited to address the problem. In this overview, we consider the requirements of such analyses and the various options available. We then consider the method proposed by Moskovic et al. with respect to the requirements of the inputs to the integrity assessment and the validity of this approach. In this method of analysis, the distribution of all possible values of model coefficients is established by judging the various possible combinations of these model coefficients in relation to the likelihood of the observed data. Analysis of artificially generated data has been used to compare the effectiveness of Bayesian analyses with those used traditionally.

Charpy Impact Energy Data: A Markov Chain Monte Carlo Analysis

SUMMARY To assess radiation damage in steel for reactor pressure vessels in the nuclear industry, specimens are subjected to the Charpy test, which measures how much energy a specimen can absorb at a given test temperature before cracking. The resulting Charpy impact energy data are well represented by a three-parameter Burr curve as a function of test temperature, in which the parameters of the Burr curve are themselves dependent on irradiation dose. The resulting non-linear model function, combined with heteroscedastic random errors, gives rise to complicated likelihood surfaces that make conventional statistical techniques difficult to implement. To compute estimates of parameters of practical interest, Markov chain Monte Carlo sampling-based techniques are implemented. The approach is applied to 40 data sets from specimens subjected to no irradiation or one or two doses of irradiation. The influence of irradiation dose on the amount of energy absorbed is investigated.

Influence of inclusion characteristics on the formability and toughness properties of a hot-rolled deep-drawing quality steel

In industrial practice, variations in the steelmaking process may cause significant change in inclusion characteristics. During hot rolling of flat steel products, manganese sulfides, which are plastic at elevated temperatures, are elongated in the rolling direction. These elongated inclusions affect the formability properties, such as ductility, strain hardening exponent, average plastic strain ratio, critical strain represented by the forming limit diagram, and Charpy V- notch (CVN) impact energy as well as fracture behavior. The inclusion characteristics and microstructural features of three commercially produced hot- rolled deep- drawing quality steels were evaluated and their effects on formability and impact properties were investigated. All three heats were made in a basic oxygen furnace. Two heats were teemed into ingots while the other heat was argon purged and continuous cast. These heats were then processed into 3.10 mm thick strips with identical processing parameters. Manganese sulfide stringers were found to reduce the transverse ductility, whereas yield and tensile strengths remained virtually the same in all directions. The formability parameters were not significantly affected by small variations in inclusion characteristics. However, CVN impact energy and impact transition temperature data were observed to improve with steel cleanliness. The sulfide stringers were also found to adversely affect the impact energy, transition temperature, and fracture behavior in the transverse direction.

Toughness, fracture markings, and losses in bisphenol-A polycarbonate at high strainrate

As-received and heat-treated specimens of bisphenol-A polycarbonate were impacted at temperatures ranging from −196 to +100° C. The critical stress intensity has been calculated for the as-received case from the impact energy data by making a strain-rate correction. To make this correction, the time-temperature superposition principle has been applied to existing dynamic mechanical measurements of the storage modulus and loss modulus (tan δ). Critical stress intensity values of 2.2 to 3.9 MPa m1/2 were found to be comparable with those obtained from instrumented impact and low strain-rate test techniques. Resulting fracture surfaces of the specimens were studied with the scanning electron microscope. Specifically, the morphology of the regions in which sharp striations were present was investigated and the width of the striations have been reported as a function of testing temperature for both as-received and heat-treated cases. There appears to be a direct correlation between the strain-energy release rate, the stress intensity, the striation spacing and the loss curve (tan δ) for the as-received case.