Threshold stress in aggressive environments is usually determined by tests under constant load, which are very time-consuming, so the incremental loading technique published in the ASTM F1624 standard was created to solve this issue. This approach has been recently applied to Small Punch tests, but it requires an optimization of the incremental step times, which is carried out in this work. Three medium- and high-strength quenched and tempered steels of 35, 50 and 60 HRC are exposed to three different cathodic polarization environments of 1, 5 and 10 mA/cm2 in 1N H2SO4 acid electrolyte with a Platinum anode, studying in each case three different step durations of one-quarter, one-sixth and one-eighth of the ones indicated in ASTM F1624. Optimal step times for Small Punch tests are derived from this work as one-sixth of the ones recommended in ASTM F1624 for tensile specimens, which are 20 min and 40 min for steps 1–10 and 11–20, respectively, in the case of 33 ≤ HRC < 45 steels, 10 min and 20 min for steps 1–10, 11–20 in the case of 45 ≤ HRC < 55 steels, and 10 min for steps 1–20 in the case of HRC ≥ 55 steels.

Medium-term load forecasting is a useful tool for the maintenance planning of grids and as a market research of electric energy. In this work medium-term load forecasting methods are developed, the most forgotten time scaling process in the load forecasting bibliography. These methods will be applied to the peninsular Spanish monthly energy consumption. Methods traditionally employed with this objective are based on regression, statistical techniques (mainly Box-Jenkins ARIMA), and also with neural networks, fuzzy logic or expert systems. Most of them need the use of nonelectric variables, mainly climatic or economic ones, which strongly influence electric energy demand. These variables, of cyclic nature, provide a periodic behaviour to the energy consumption time series. This work presents a study of this periodic behaviour by means of spectral analysis, with the identification and interpretation of the dominant frequencies. A forecasting method for future values of electric energy demand will be then presented, which is based on a simple regression technique combined with neural networks. It does not take into account any climatic or economic variables, because only periodic behaviour of the time series is considered. Acceptable results are reached, with percentage errors lower than 5 % in most cases.

This work shows the need to implement new methodologies for assessing environmentally assisted cracking processes in notched components. When structural integrity assessments under aggressive conditions are performed, defects are often considered to behave as cracks, something that leads to overconservative results when these defects exhibit a finite radius at their tip (notches). Here, as a novel approach, the theory of critical distances (TCD), through the point method (PM) and the line method (LM), has been reformulated to address the environmentally assisted cracking phenomenon applied to conventional fracture mechanics compact tension (C[T]) specimens and also to circumferential notched tensile (CNT) ones. In order to validate this methodology, the stress intensity factor above which crack propagation initiates in cracked and notched specimens (KIEAC and KINEAC, respectively) has been obtained in 80 C(T) specimens and 8 CNT specimens. The tests cover two steels (API X80 and S420), five different notch radii from 0.00 mm (crack-like defect) up to 2.00 mm, two cathodic polarization levels (1 mA/cm2 and 5 mA/cm2), and two slow displacement rates: 6 × 10−8 m/s and 6 × 10−9 m/s for C(T) specimens and their equivalents for CNT specimens. Finite element simulations have been carried out to calibrate the TCD parameters. The results reveal the potential of the TCD to analyze environmentally assisted cracking processes.

The Small Punch Test is a recently standardized technique successfully employed for estimating the tensile, creep and fracture properties of metallic materials, and is considered as one of the most suitable options for evaluating mechanical properties when materials are in shortage. Several advances have been made in order to apply this methodology for mechanical characterizations in aggressive environments. The first tests were carried out pre-embrittling the samples in environment and then testing them in air. Subsequent methodologies proposed tests in environment after pre-embrittling the samples in order not to lose part of the embrittling capacity while testing; in these scenarios the punch rate has an enormous influence on environmental characterizations. The most recent works propose to implement in the Small Punch Test the step loading methodology collected in ASTM F1624 standard, which solves these problems by applying steps at a constant load which gradually increases until the sample fails. In this work, guidelines for the application of Small Punch Tests to determine the threshold load in aggressive environment are given, based on tests results under hydrogen embrittlement scenarios. A range of punch rates for constant punch displacement is provided, together with the suitable step times when applying the step loading technique, and a correlation to estimate the threshold stress based on Small Punch samples tested with this novel technique.

This paper evaluates the fracture of notched epoxy matrix composites using the Brazilian disk (BD) test from both numerical and experimental points of view. The study began with a comprehensive experimental program covering three different composite lay-ups (quasi-isotropic, unidirectional, and cross-ply) and various geometries of U and V notches. Specifically, the BD samples combined the three layouts, four different notch angles, and three notch radii with three specimens per combination, leading to an overall number of 108 fracture tests. The experiments showed the appropriateness of the BD test for the study of the fracture behavior of composite materials and provided a good pool of data for further investigations. Subsequently, the virtual isotropic material concept (VIMC) was applied in combination with two fracture criteria to theoretically predict the experimentally acquired fracture loads. This study demonstrated that using the VIMC approach can provide robust predictions while incurring much lower computational costs compared to the conventional approaches found in the literature.

The small punch tests consists of punching a plane small specimen until it breaks. This technique is very interesting in situations where there is a shortage of material. In recent works, it has been used with steel employed in aggressive environments, to estimate the threshold stress under which subcritical cracking will never occur. It has been presented in previous papers in combination with standard ASTM 1624, applying gradually increasing constant loads until the sample fails, to reduce the duration of the test and the results dispersion. In the present paper, a further optimization is performed on the steps durations for SPT, simplifying the test and therefore helping to reduce the lab workload while at the same time saving costs and resources and increasing productivity. The present work is carried out on an X80 medium-strength rolled steel in hydrogen embrittlement environments under three different levels of cathodic polarization in an acid electrolyte; the chosen steel belongs to the lowest hardness range (33 <= HRC < 45) according to the ASTM 1624 standard. Different steps duration from 10 to 60 mintures had been analyzied, cocluding that 20-40 min for 1st to 10th and 11th to 20th steps respectivley are proposed as the minimum ones to reach accurate results. The proposed optimization allows to reduce the total test duration, being for sure of great interest for the metal industry as well as for the scientific comunity.

Medium and high-strength steels working in harsh environments are susceptible of suffering of mechanical properties loss due to hydrogen presence. In order to avoid catastrophic failures, the estimation of the fracture toughness in environment is necessary, but sometimes it is not convenient the use of standard tests; an alternative to the aforementioned traditional fracture mechanics specimens are Circumferential Notched Tensile specimens (CNT), which consist of cylindrical tensile specimens that incorporate a circumferential notch. In this work, CNT specimens subjected to acid aggressive environments, generated by cathodic polarization, are used to estimate the fracture toughness in environment, K-IEAC. For this purpose, two methodologies, one proposed by Singh Raman et al. in the 2000's and another by Cayon et al. in the 2010's, are compared. Both formulations rely on similar classic fracture mechanics bases, differing mainly in the way to correct the effects of non-centering of the ligament, caused by asymmetries in the growth of annular cracks in CNT specimens. Both are finally validated to estimate K-IEAC, showing good accuracy within the +/- 15%.

In this work, two microalloyed steels, one used for oil&gas pipelines and the other one for structural components in hydrotreating reactors in petrochemical industries, have been studied under hydrogen embrittlement (HE) conditions. They have been cathodically charged with hydrogen under different aggressive conditions, its hydrogen content being determined afterwards. In order to know the effect of triaxiality on the HE behaviour of this steel, two different types of axisymmetric round notched tensile specimens have been tested under continuous hydrogen charging processes, whilst varying cathodic charging conditions and loading rate. The effects on the global mechanical characterization have been analysed, as well as the correlation between the observed fracture micromechanisms with both local environmental (as hydrogen concentration) and mechanical conditions (as hydrostatic stress and plastic strain determined with a finite elements analysis). Also, subcritical cracking due to embrittlement has been analysed and compared with those obtained by fracture mechanics standardised specimens.

The main objective of this work is the study of the hillock and zinc whisker evolution of five different commercial zinc coatings applied on the same base steel wires of the patented EASYCONNECT system cable trays manufactured by VALDINOX Ltd.: white zinc alkaline electrolyte, yellow zinc trivalent electrolyte, acid zinc electrolyte, hot dip galvanized, and zinc nickel coating. The limited literature on the subject is summarized, and then the coating thickness, chemical composition, hardness and surface rugosity are characterized. The hillock and whisker density evolution are evaluated over a period of 12 months, considering the presence of compression bending stresses. It is concluded that the white alkaline and yellow trivalent coatings are the most affected, while the zinc-nickel shows the best behavior with no presence of whiskers; the acid zinc electrolyte also shows good results despite the delayed appearance of whiskers from the ninth month; the hot-dip galvanized coating does not show any presence of zinc whiskers or hillocks.

This paper provides a methodology for the structural integrity assessment of tubular beams containing U-notches, and particularises the analysis to the case of cantilever beams containing through thickness U-notches. The methodology is based on the combined use of Failure Assessment Diagrams and the Theory of Critical Distances, with the BS7910 as the reference fracture assessment document. The results, obtained in Al6060 and PVC (Polyvinyl chloride) tubular cantilever beams, demonstrates that the proposed approach provides accurate predictions of failure loads.

This paper gathers experimental and theoretical investigations about both the geometry-dependent fracture initiation angle and the fracture strength in VO-notched polymethyl methacrylate (PMMA) specimens under mode I loading conditions. The numerical analyses revealed that despite the application of pure mode I loading on the geometrically symmetric VO-notched samples, the maximum tangential stress occurs at two points symmetrically placed on either side of the notch bisector line. The experimental tests performed on some specimens showed that a crack does not necessarily propagate along the notch bisector line. Stress-based theoretical studies were then carried out to justify the experimental findings. The conventional maximum tangential stress (MTS) criterion gave weak predictions of the fracture. Therefore, the predictions were checked with the generalized MTS (GMTS) criterion by taking into consideration the higher-order stress terms. It was demonstrated that the GMTS criterion predictions have satisfactory consistency with the experimental results of the crack initiation angle and the fracture strength.

This paper validates a methodology for the estimation of critical loads in tubular beams containing notch-type defects. The methodology is particularized for the case of Al6060-T66 tubular cantilever beams containing U-shaped notches. It consists in obtaining the stress field at the notch tip using finite element analysis (FEA) and the subsequent application of the theory of critical distances (TCD) to derive the corresponding critical load (or load-bearing capacity). The results demonstrate that this methodology provides satisfactory predictions of fracture loads.

The small punch test consists on punching a plane small specimen until it breaks. This technique, born in the 80's, should be considered when evaluating mechanical properties in situations where materials are in shortage. In recent works, it has been used to estimate the mechanical properties of steels in aggressive environments, where characterizations usually consist on the determination of the threshold stress to avoid subcritical cracking by means of constant loading tests, which is a slow technique, and sometimes presents a considerable dispersion in the results. The standard ASTM F1624 solves these problems, by applying constant loads gradually increased, called loading steps, until the sample fails. In the present work, it is proposed to apply the incremental step loading technique from ASTM F1624 adapted to the Small Punch Test (SPT). As a novel approach, modifications on the steps durations for SPT are proposed according with the sample thickness, allowing to obtain the threshold stress in aggressive environments within a few days, by using at least 3 samples. The proposed methodology is applied to a set of two steels, of medium and high-strength, in hydrogen embrittlement environments under three different levels of cathodic polarization in an acid electrolyte. As a reference, cylindrical tensile specimens were subjected to conventional standard tests in accordance with ASTM F1624. The correlation between the threshold stresses, obtained according to ASTM F1624, and the threshold loads, obtained by the Small Punch proposal, is presented and analyzed. Finally, from the aforementioned correlation, a threshold stress estimation based just on Small Punch tests is proposed.

The Small punch test, which consists on punching a small plane specimen up to failure, is a technique to be taken into account for the estimation of mechanical properties when there is shortage of material. In recent works it has been applied to the estimation of mechanical properties steels in aggressive environments. In aggressive environments, tests under a constant load are usually employed for the threshold stress determination, but this a slow and sometimes inaccurate technique. The standard ASTM F1624 solves these issues; it consists on applying steps of constant loads subsequently increased up to the specimen’s failure. In a previous work, it was indicated how to implement this technique for Small Punch testing of steels in hydrogen embrittlement scenarios, adapting the steps duration. This proposal allows to obtain a threshold load by using at least 3 specimens in a total time of around a week. In the present work, the incremental step loading technique from ASTM F1624 standard is applied to the Small Punch test in order to estimate tensile threshold stress of a X80 high strength steel in hydrogen embrittlement environments by cathodic polarization in an acid electrolyte. Regular standard tests on cylindrical tensile specimens were carried out following the ASTM F1624 standard, in order to validate the methodology proposed.

There are multiple references to sample cleaning methods prior to hydrogen content determination, or hydrogen spectroscopy analysis, but there is still no unified criteria; different authors use their own “know-how” to perform this task. The aim of this paper is to solve, or at least clarify, this issue. In this work, the most commonly used sample cleaning methods are compared. Then, five different methodologies are applied on certified hydrogen content calibration pins and on high strength steel concrete-prestressing strands and the three main situations regarding hydrogen content in the microstructural net (non-charged, charged, and charged and uncharged) are studied. It was concluded that the HCl solution C-3.5 cleaning method recommended by ASTM G1 introduces large amounts of hydrogen in the samples; but can be useful for eliminating superficial oxides if necessary. The rest of the methods had similar results; but the more complete ones that involve ultrasounds and last longer than 8 min are not appropriated when important diffusion may occur on the samples during their application. Simple methods that involve acetone or trichloroethylene and last around 1 min are preferable for almost all situations as these are faster, easier, and cheaper. As a final recommendation, as trichloroethylene is toxic, the simple acetone method is, in general, the most convenient one for regular hydrogen content analysis.

The Small Punch test has been recently used to estimate mechanical properties of steels in aggressive environments. This technique, very interesting when there is shortage of material, consists in using a small plane specimen and punch it until it fails. The type of tests normally used are under a constant load in an aggressive environment, with the target to determine the threshold stress. However, this is an inaccurate technique which takes time, as the tests are quite slow. In this paper, the Small Punch tests are combined with the step loading technique collected in the standard ASTM F1624 [1] to obtain the value of threshold stress of an S420 steel in a total time of approximately one week. The ASTM F1624 indicates how to apply constant load steps in hydrogen embrittlement environments, increasing them subsequently and adapting their duration until the specimen fails. The environment is created by means of cathodic polarization of cylindrical tensile specimens in an acid electrolyte. A batch of standard tests are performed to validate the methodology.

This article presents the analysis of environmentally assisted cracking (EAC) processes in two steels containing U-shaped notches through the point method (PM). This methodology belongs to the theory of critical distances (TCD) and has been widely validated in fracture and fatigue analysis in many materials. However, it has never been used in EAC process assessment. The resulting methodology is validated by testing C(T) notched specimens of X80 and S420 steels subjected to aggressive environments under hydrogen embrittlement conditions. The parameters of the TCD have been obtained by finite element simulations, and the results reveal that the PM accurately predicts the crack propagation onset condition (KNIEAC), as well as the evolution of the apparent EAC crack propagation threshold of the material. The study has been completed with an exhaustive analysis of the fracture surfaces to determine the failure mechanisms using scanning electron microscopy (SEM).

The complex interaction between physiological stresses and corrosive human fluids can lead to the premature failure of metallic biomaterials due to the development of Environmental Assisted Cracking (EAC) processes. In this paper, the EAC phenomenon is analysed through a Theory of Critical Distances based methodology, which has been validated in other materials and aggressive environments, and the apparent crack propagation threshold in notched conditions is estimated. Notch-like defects, which are frequently found in aggressive environments, may present higher values of crack propagation thresholds than those exhibited in cracked components. The knowledge of this higher material performance makes it possible to address the problem avoiding oversizing or unnecessary replacements in biomaterials, which leads to an improvement in the quality of life of the people carrying these materials. In this study, the susceptibility of AZ31 magnesium alloy to EAC and the evolution of the apparent crack propagation threshold have been analysed. The aggressive environment used was Simulated Body Fluid (SBF). The main conclusion is that the Theory of Critical Distances predicts the behaviour of this biomaterial in notched conditions and subjected to the aggressive environment being studied. (C) 2020 The Authors. Published by Elsevier B.V.

Fracture, fatigue, and other subcritical processes, such as creep crack growth or stress corrosion cracking, present numerous open issues from both scientific and industrial points of view [...]