Stress corrosion cracking

The theory of stress corrosion for slow crack propagation is reviewed in the light of classical Griffith theory of fracture. Experimental data for stress corrosion cracking for glasses, ceramics, and metals are reviewed. We suggest that stress corrosion cracking plays an important role in the intrusion of magmas and in the transport of magmas upward through the lithosphere. It is shown that the effect of decreasing temperature (at progressively shallower levels along the geotherm) would be to decrease the crack velocity by several orders of magnitude if other factors were equal. We also propose that stress corrosion may be an important process in time-dependent earthquake phenomena such as premonitory behavior and earthquake aftershocks. We suggest that slow cracking in the earth is not seismically detectable but may nevertheless precede the terminal (catastrophic) phase of the fracture that is discerned as an earthquake.

Supercritical water (SCW) has attracted increasing attention since SCW boiler power plants were implemented to increase the efficiency of fossil-based power plants. The SCW reactor (SCWR) design has been selected as one of the Generation IV reactor concepts because of its higher thermal efficiency and plant simplification as compared to current light water reactors (LWRs). Reactor operating conditions call for a core coolant temperature between 280°C and 620°C at a pressure of 25 MPa and maximum expected neutron damage levels to any replaceable or permanent core component of 15 dpa (thermal reactor design) and 100 dpa (fast reactor design). Irradiation-induced changes in microstructure (swelling, radiation-induced segregation (RIS), hardening, phase stability) and mechanical properties (strength, thermal and irradiation-induced creep, fatigue) are also major concerns. Throughout the core, corrosion, stress corrosion cracking, and the effect of irradiation on these degradation modes are critical issues. This paper reviews the current understanding of the response of candidate materials for SCWR systems, focusing on the corrosion and stress corrosion cracking response, and highlights the design trade-offs associated with certain alloy systems. Ferritic-martensitic steels generally have the best resistance to stress corrosion cracking, but suffer from the worst oxidation. Austenitic stainless steels and Ni-base alloys have better oxidation resistance but are more susceptible to stress corrosion cracking. The promise of grain boundary engineering and surface modification in addressing corrosion and stress corrosion cracking performance is discussed.

Rapid, giant landslides, or sturzstroms, are among the most powerful natural hazards on Earth. They have minimum volumes of f 10 6-10 7 m 3 and, normally preceded by prolonged intervals of accelerating creep, are produced by catastrophic and deepseated slope collapse (loads f 1-10 MPa). Conventional analyses attribute rapid collapse to unusual mechanisms, such as the vaporization of ground water during sliding. Here, catastrophic collapse is related to self-accelerating rock fracture, common in crustal rocks at loads f 1-10 MPa and readily catalysed by circulating fluids. Fracturing produces an abrupt drop in resisting stress. Measured stress drops in crustal rock account for minimum sturzstrom volumes and rapid collapse accelerations. Fracturing also provides a physical basis for quantitatively forecasting catastrophic slope failure.

This paper is a review of the texture development in zirconium alloys (in the form of thick walled tube reduced extrusion or TREX, thin-walled tubing and sheets) of importance to light and heavy water nuclear reactor technology along with the resultant anisotropic mechanical properties. Quantitative characterization of texture and mechanical anisotropy are emphasized leading to procedures useful to fabricators in optimizing textures for good formability as well as for acceptable in-service performance. A brief history of the development of zirconium alloys is presented followed by texture development and characterization. Mechanical anisotropy is discussed in terms of transverse contractile strain ratios from which the formability (B parameter) is derived. Results on the effect of annealing temperature as well as test temperature on anisotropy parameters are presented. The review concludes with a brief summary of texture effects on creep, stress corrosion cracking and hydride formation. Recent advances in fuel cladding bring out the challenges in characterizing the texture and anisotropy due to Nb additions and microstructural gradients in the new Zircaloyse , 1 such as Zirloe , 2 , Duplexe , 3 and Triclad e , 4 .

This paper presents an updated review of the external corrosion and failure mechanisms of buried natural gas and oil pipelines. Various forms of external corrosion and failure mechanisms such as hydrogen-induced cracking (HIC), hydrogen embrittlement (HE), corrosion fatigue (CF), stress corrosion cracking (SCC) and microbiologically influenced corrosion (MIC) for oil and gas pipelines are thoroughly reviewed. The factors influencing external corrosion and possible forms of environment-assisted cracking (EAC) of pipeline steels in the soil are also reviewed and analyzed in depth. In addition, the existing monitoring tools for the external corrosion assessment and the models for corrosion prevention and prediction, failure occurrence, and remaining life of oil and gas pipelines, are analyzed. Moreover, the articles on external corrosion management, reliability-based models, risk-based models, and integrity assessment including machine learning and fuzzy logic approaches, are also reviewed. The conclusions and recommendations for future research in the prevention and prediction of external corrosion are presented at the end.

Hydrogen evolution and permeation occur during electroplating, corrosion, and cathodic protection. Hydrogen accumulates in areas of high stress and may reach a critical concentration, potentially causing fractures and catastrophic damage. This chapter describes hydrogen permeation and hydrogen-induced damage in metals and alloys. It also discusses hydrogen evolution kinetics, theoretical diffusion solutions, and basic hydrogen permeation models. Models are used as a diagnostic tool to determine the effectiveness of various metals and alloys as hydrogen permeation inhibitors. It then explains experimental atomic hydrogen permeation transient determination and hydrogen absorption rate constants and diffusivity evaluation into metals using case studies. Hydrogen embrittlement, hydrogen-induced cracking, hydrogen blistering, and hydrogen stress cracking are also discussed to show the relationship to hydrogen permeation and hydrogen-induced cracking mechanisms. Finally, various techniques used to prevent and control hydrogen damage of metals and alloys are described.

We present a model of hydrogen embrittlement based upon: (i) a cohesive law dependent on impurity coverage that is calculated from first principles; (ii) a stress-assisted diffusion equation with appropriate boundary conditions accounting for the environment; (iii) a static continuum analysis of crack growth including plasticity; and (iv) the Langmuir relation determining the impurity coverage from its bulk concentration. We consider the effect of the following parameters: yield strength, stress intensity factor, hydrogen concentration in the environment, and temperature. The calculations reproduce the following experimental trends: (i) time to initiation and its dependence on yield strength and stress intensity factor; (ii) finite crack jump at initiation; (iii) intermittent crack growth; (iv) stages I and II of crack growth and their dependence on yield strength; (v) the effect of the environmental impurity concentration on the threshold stress intensity factor; and (vi) the effect of temperature on stage II crack velocity in the low-temperature range. In addition, the theoretically and experimentally observed intermittent cracking may be understood as being due to a time lag in the diffusion of hydrogen towards the cohesive zone, since a buildup of hydrogen is necessary in order for the crack to advance. The predictions of the model are in good quantitative agreement with available measurements, suggesting that hydrogen-induced degradation of cohesion is a likely mechanism for hydrogen-assisted cracking.

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel. #

The effect of microstructural changes in 304 austenitic stainless steel induced by the processes of gas tungsten arc welding (GTAW) and laser-beam welding (LBW) on the pitting and stress corrosion cracking (SCC) behaviors was investigated. According to the in situ observations with scanning reference electrode technique (SRET) and the breakdown potentials of the test material with various microstructures, the GTAW process made the weld metal (WM) and heat-affected zone (HAZ) more sensitive to pitting corrosion than base metal (BM), but the LBW process improved the pitting resistance of the WM. In the initiation stage of SCC, the cracks in the BM and HAZ propagated in a transgranular mode. Then, the crack growth mechanism changed gradually into a mixed transgranular + intergranular mode. The cracks in the WM were likely to propagate along the dendritic boundaries. The crack initiation rate, crack initiation lifetime and crack propagation rate indicated that the high-to-low order of SCC resistance is almost the same as that for pitting resistance. High heat-input (and low cooling rate) was likely to induce the segregation of alloying elements and formation of Cr-depleted zones, resulting in the degradation in the corrosion resistance.

Time-dependent brittle deformation is a fundamental and pervasive process operating in the Earth's upper crust. Its characterization is a pre-requisite to understanding and unraveling the complexities of crustal evolution and dynamics. The preferential chemical interaction between pore fluids and strained atomic bonds at crack tips, a mechanism known as stress corrosion, allows rock to fail under a constant stress that is well below its short-term strength over an extended period of time; a process known as brittle creep. Here we present the first experimental measurements of brittle creep in a basic igneous rock (a basalt from Mt. Etna volcano) under triaxial stress conditions. Results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress (and can be equally well fit by a power law or an exponential law); with a 20% increase in stress producing close to three orders of magnitude increase in creep strain rate. Results from stress-stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure. We show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure, with the remainder interpreted as due to a reduction in stress corrosion reactions; the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips. Overall, our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure, regardless of the level of applied stress and the time taken to reach this point. The experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law, and compared with the observed deformation history at Mt. Etna volcano.

In order to appraise the in~uence of s!phase on the behaviour of duplex stainless steels\ two tests are performed[ The _rst one is the double loop electrochemical potentiodynamic reac! tivation "DLEPR# test that indicates the degree of sensitisation to intergranular corrosion[ The second one is the slow strain rate test "SSRT# that enables us to relate the degree of sensitisation to stress corrosion cracking[ A metallurgical study is also performed on two duplex stainless steels of grade UNS S20792[ Sensitisation is carried out by several heat treatments at 564>C or 899>C[ The in~uence of s!phase is clearly shown on the mechanical properties and the corrosion resistance[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

In the present study alloy 600 was tested in simulated pressurised water reactor (PWR) primary water, at 360 °C, under an hydrogen partial pressure of 30 kPa. These testing conditions correspond to the maximum sensitivity of alloy 600 to crack initiation. The resulting oxidised structures (corrosion scale and underlying metal) were characterised. A chromium rich oxide layer was revealed, the underlying metal being chromium depleted. In addition, analysis of the chemical composition of the metal close to the oxide scale had allowed to detect oxygen under the oxide scale and particularly in a triple grain boundary. Implication of such a finding on the crack initiation of alloy 600 is discussed. Significant diminution of the crack initiation time was observed for sample oxidised before stress corrosion tests. In view of these results, a mechanism for stress corrosion crack initiation of alloy 600 in PWR primary water was proposed.

In the absence of a corrosive environment, brittle materials such as silicon should be immune to cyclic fatigue. However, fatigue effects are well known in micrometer-sized polycrystalline silicon (polysilicon) samples tested in air. To investigate the origins of this phenomenon in polysilicon, we developed a fixed-grip fracture mechanics microspecimen but could find no evidence of static stress corrosion cracking. The environmental sensitivity of the fatigue resistance was also investigated under cyclic loading. For low-cycle fatigue, the behavior is independent of the ambient conditions, whether air or vacuum, but is strongly influenced by the ratio of compressive to tensile stresses experienced during each cycle. The fatigue damage most likely originates from contact stresses at processing-related surface asperities; subcritical crack growth then ensues during further cyclic loading. The lower far-field stresses involved in high-cycle fatigue induce reduced levels of fatigue dama...

The effect of retrogression and re-aging (RRA) on the strength and stress corrosion cracking (SCC) performance was evaluated in AA7050 and AA7150 aluminum alloys. Samples from extruded profiles, at their original condition (T77511 for AA7150 and T76511 for AA7050), were solution heat treated and aged at 120 • C for 1440 min (T6 condition). After that, retrogression was performed at 200 • C for times of 5, 10, 20 and 40 min, followed by a re-aging at 120 • C for 1440 min. Hardness and electrical conductivity measurements were used to characterize the samples after different retrogression times and during re-aging. Maximum hardness after re-aging was obtained in the samples retrogressed for 20 and 40 min. Tensile and SCC testing were performed on samples retrogressed for 20 and 40 min and re-aged, and the results were compared with the alloys at their original conditions. After RRA treatment it was observed an increase of 30% in the yield strength of the AA7050 alloy, while for the AA7150 alloy the mechanical properties were similar to the original condition.

Stress corrosion cracking (SCC) of the high performance rare-earth (RE) containing magnesium alloy
Elektron 21 (ASTM - EV31A) was studied using slow strain rate test (SSRT). Glycerol, distilled water,
and 0.01 N and 0.1 N NaCl solutions saturated with Mg(OH)2 were employed. For comparison a wellknown
AZ91E alloy was also studied. SSRT studies indicate that both the alloys were susceptible to
SCC to some extent in 0.1 N, 0.01 N NaCl solutions and distilled water. However, EV31A showed
higher SCC resistance than AZ91E. The fractography of SSRT tested AZ91E and EV31A specimens
were largely found to be transgranular (TGSCC) in distilled water. However in 0.1 N and 0.01 N NaCl
solutions AZ91E exhibited transgranular, while EV31A showed mixed TGSCC and intergranular
(IGSCC) fracture involving hydrogen embrittlement. EV31A showed lower susceptibility to SCC in all
the three solutions than AZ91E, that is there is not much loss in mechanical properties of EV31A as
compared to that of AZ91E in these environments.

The present review is intended to revisit the advances and debates in the comprehension of the mechanisms of subcritical crack propagation in silicate glasses almost a century after its initial developments. Glass has inspired the initial insights of Griffith into the origin of brittleness and the ensuing development of modern fracture mechanics. Yet, through the decades the real nature of the fundamental mechanisms of crack propagation in glass has escaped a clear comprehension which could gather general agreement on subtle problems such as the role of plasticity, the role of the glass composition, the environmental condition at the crack tip and its relation to the complex mechanisms of corrosion and leaching. The different processes are analysed here with a special focus on their relevant space and time scales in order to question their domain of action and their contribution in both the kinetic laws and the energetic aspects.

Breakout of magmatic activity at Soufflere Hills volcano, Montserrat, was preceded by a tenfold increase in rate of earthquake occurrence. A new model of subcritical rock failure shows that this increase is consistent with the growth, possibly episodic, of the magma conduit at a rate controlled by progressive weakening of the host country rock. The preferred weakening mechanism is stress corrosion, by which circulating juvenile and hydrothermal fluids chemically attack the country rock and promote failure at stresses smaller than the rock's theoretical strength. The results illuminate the potential for slow-cracking models to enhance eruption forecasts using the inverse-rate technique combined with traditional monitoring methods.

Influence of retrogression and re-aging treatment on the microstructure, strength, exfoliation corrosion, inter-granular corrosion and stress corrosion cracking of an Al-Zn-Mg-Cu alloy has been investigated by means of optical microscope (OM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The results show that retrogression and re-aging treatment can increase the size and the distribution discontinuity of the grain boundary precipitates, and lead to the increase of the corrosion resistance without the loss of strength and ductility. In addition, the analysis of electrochemical impedance spectroscopy shows that retrogression and re-aging treatment can enhance the resistance to exfoliation corrosion.

At Mi~ville, in the Aiguilles-Rouges Massif, granitic rocks of the basement are deformed into mylonites within a major subvertical shear zone. The ambient temperature during translation is estimated at 250 ~ C + 30 ~ C from fluid inclusion filling temperatures in syntectonic microveins, from Also quartzilmenite of + 15 ~ and from mineralogical criteria. Porphyroclasts of both oligoclase and orthoclase feldspar decrease from initial diameters of 20 mm and assume elliptical shapes during progressive deformation, due to recrystallisation of the margins to ultrafine polygonal grains which extend out from the porphyroclasts in thin trails: the final stable grain size is < 5 gin. The recrystallised feldspar has a composition of the parent porphyroclast, +albite, requiring relative gains of Na and losses of K+Ca compared to the precursor, and implying short range redistribution of the components during deformation. Decrease of free energy associated with the deformation catalysed change in feldspar composition, coupled with stored strain energy in the porphyroclasts may account for recrystallisation to a stable aggregate of ultrafine grain size. The long trails imply exceptionally high ductility, which, coupled with microstructural criteria, and admixture of quartz from neighbouring pure quartz aggregates by grain boundary sliding, is interpreted in terms of superplastic flow. Estimated temperatures of T/Tm ~ 0.2 for the inferred superplastic deformation is lower by a factor of 2 than previously recorded for this flow michanism in silicates. The feldspar and quartz probably accomodated grain boundary sliding by intercrystalline diffusion.

Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.

It is essential that a metallic implant material possesses adequate resistance to cracking/fracture under the synergistic action of a corrosive physiological environment and mechanical loading (i.e. stress corrosion cracking (SCC)), before the implant can be put to actual use. This paper presents a critique of the fundamental issues with an assessment of SCC of a rapidly corroding material such as magnesium alloys, and describes an investigation into the mechanism of SCC of a magnesium alloy in a physiological environment. The SCC susceptibility of the alloy in a simulated human body fluid was established by slow strain rate tensile (SSRT) testing using smooth specimens under different electrochemical conditions for understanding the mechanism of SCC. However, to assess the life of the implant devices that often possess fine micro-cracks, SCC susceptibility of notched specimens was investigated by circumferential notch tensile (CNT) testing. CNT tests also produced important design data, i.e. threshold stress intensity for SCC (K ISCC) and SCC crack growth rate. Fractographic features of SCC were examined using scanning electron microscopy. The SSRT and CNT results, together with fractographic evidence, confirmed the SCC susceptibility of both smooth and notched specimens of a magnesium alloy in the physiological environment.

This study aims to understand the mechanism of increased SCC susceptibility of machined 304L stainless steel in chloride environment. Austenitic stainless steel grade 304L was surface machined up to a depth of 0.5 mm from the surface. In depth characterization was carried out by optical, scanning electron microscopic technique, hardness measurement and by EBSD and XRD studies. The stress corrosion cracking (SCC) susceptibility was estimated by exposing constant strained samples made up of machined and unmachined stainless steel to 5 M H2SO4 + 0.5 M NaCl solution at room temperature (28 °C) until cracking. In addition strips of machined and unmachined stainless steel were exposed to boiling MgCl2 solution as per ASTM G36 to understand the effect of residual stress and strain generated due to machining on the SCC susceptibility. The study reveals that surface machining results in extensive grain refinement, strain induced martensite transformation and high magnitude of plastic deformation near the surface.

Cyclic voltammetry, hydrogen permeation tests and electrochemical impedance spectroscopy measurements were combined to study the mechanism for hydrogen evolution reaction on X-70 pipe steel in near-neutral pH solution. It is found that hydrogen evolution reaction is dominated by the reduction of water molecules, followed by either an electrochemical hydrogen recombination reaction or a hydrogen absorption reaction. The near-neutral pH environment is capable of generating catalytic surface effect on hydrogen evolution on the pipe steel. The increasing dissolution of the cathodically pre-polarized steel could be due to the enhanced activation of the steel, rather than the increasing amount of hydrogen atoms in the steel. These results provide mechanistic information to understand the nearneutral pH stress corrosion cracking of pipelines.

Effect of residual stress generated during tube fabrication, roll expansion and machining of stainless steel on the stress corrosion cracking (SCC) susceptibility was studied by testing fabricated tubes, tube-tube sheet joint and heavily machined plate of austenitic stainless steel (SS) in boiling MgCl 2 . U bend samples of machined plate were exposed to acidified SO 4 + Cl À environment at room temperature to study its ambient temperature SCC behavior. The results correlate the SCC behavior of the SS tubes and roll expanded joints to the nature and magnitude of residual stresses present. The study also highlights the distinct difference in ambient temperature SCC behavior of machined vs. nonmachined surfaces.

Failure strengths of granite and andesite have been measured under various conditions of strain rate and con®ning pressure both in the dry and wet states. Constant-stress creep tests for granitic samples have been also conducted. All specimens show that the failure strength decreased linearly as the logarithm of the strain rate decreased. The strain rate dependence of the failure strength is increased at higher con®ning pressures. The strain rate effect is more apparent on the failure strengths of wet samples than dry samples in lower con®ning pressure ranges. In the constant-stress creep experiments, the creep failure strength decreased as the logarithm of the time to failure increased. Time-dependent failure strength of all rocks observed in the regime of the present study may be interpreted by subcritical crack growth assisted by the stress corrosion mechanism. q

Until a few years ago there had been no record of stress corrosion cracking (SCC) as a main cause of failures in Argentine pipelines, but as the pipeline system became of a certain age this mechanism started to have an important impact on reliability. This study analyzes three blowouts attributed to high pH SCC in different oil and natural gas transmission pipelines, which occurred by the sudden propagation of longitudinal cracks at the outer surface of the pipes. #

The interfacial shear (IS) force of copper ball onto aluminium-based bond pad in microelectronics packaging depends on the formation and growth of Cu±Al intermetallic. This paper reports the study on the behaviour of the IS force of the Cu±Al bonds that were subjected to pressure cooker test up to 576 h. Initially, the IS force increases with test readout point until 192 h, due to Cu±Al intermetallic growth that has strengthened the bonding. However, IS force decreases signi®cantly from 157.4 gf at 192 h to 97.6 gf at ®nal test readout point, 576 h. The number of shear-induced cratering shows similar reduction trend after 288 h. Result of scanning electron microscopy (SEM) on bonding morphology shows the evidence of crack at the aluminium bond periphery or outer Cu±Al interface and the cracking trend continues at higher time point. Surface analysis of ball-peeled bond pad using X-ray photoelectron spectroscopy (XPS) indicated that the cracks were due to stress corrosion cracking at aluminium that has been stimulated by copper. The concentration of CuO at the surface bonded area was found to be increased at higher readout point and reached 100% at 576 h. These results indicated that the Cu±Al bond had been weakened by stress corrosion cracking at outer bond interface and reduced the IS force.

Tensile strengths of differently sized E-glass fibres have been characterised using a bimodal Weibull two parameters cumulative distribution function. By comparing unsized fibres, pure silanes, different film formers, and silane/film former combinations, a comprehensive summary on the healing effect for surface flaws in relation to the type of sizing emulsion has been obtained. The great influence of the film former, which is the main component of the sizing by weight, was shown to affect both the healing of initially occuring flaws in the unsized fibre and the possibility of creating new defects. Besides the single influence of the film former, the synergetic effect of silane and film former polymer has been shown. In fact, the presence of sizing influences both the population of flaws on the fibre surface and the structure of the interphase, which will be created from the impregnation with a polymer matrix. Data from statistics of fracture as a function of the nature of sizings were discussed according to the literature on stress corrosion of E-glass filaments. C 2001 Kluwer Academic Publishers

Although bulk silicon is not known to exhibit susceptibility to cyclic fatigue, micron-scale structures made from silicon films are known to be vulnerable to degradation by fatigue in ambient air environments, a phenomenon that has been recently modeled in terms of a mechanism of sequential oxidation and stress-corrosion cracking of the native oxide layer. To date, most stress-life (S=N ) fatigue tests on such silicon films have been conducted using resonant-loaded specimens. Consequently, there is a need to establish the interaction between the dynamic loading and the driving force for fatigue-crack growth. In this paper, finite element models are used to establish the relationship between natural frequency, specimen compliance, and linear-elastic stress-intensity factor for a commonly used micron-scale, micromechanical fatigue characterization structure. These results are then incorporated into a general, lumped parameter model to evaluate the stability of fatigue cracks in resonant-loaded structures. It is well known that the applied stress amplitude and corresponding driving force for crack advance depend on the system damping, as well as sample geometry. Consequently, changes in damping caused by cycling in different environments can have a significant mechanical effect on the stability of fatigue cracks. In the case of the fatigue characterization structure used by the authors, the models show that tests conducted at atmospheric pressure subject cracks to a monotonically increasing driving force for crack advance. However, when the damping in the system is reduced (e.g., by testing in vacuo) fatigue cracks may arrest, independent of environmental effects on crack growth. Therefore, testing of structures loaded in resonance at a fixed natural frequency in vacuum should not be considered equivalent to an ''inert'' atmosphere. Finally, the finite element models are applied to polycrystalline silicon structural films to determine the critical crack lengths ($5.5-66 nm) and an average fracture toughness ($0.85 MPa p m) from specimens subjected to fatigue cycling at stress amplitudes ranging from $2.2 to 3.5 GPa.

The finite element method is presented to attain the numerical simulation of the residual stresses field in the material treated by laser shock processing. The distribution of residual stresses generated by a single laser shock with square and round laser spot is predicted and validated by experimental results. With the Finite Element Method (FEM) model, effects of different overlapping rates and impact sequences on the distribution of residual stresses are simulated. The results indicate that: (1) Overlapping laser shock can increase the compressive residual stresses. However, it is not effective on the growth of plastically affected depth;

The concept of Voronoi tessellation has recently been extensively used in materials science, especially to model the geometrical features of random microstructures like aggregates of grains in polycrystals, patterns of intergranular cracks and composites. Solution of the underlying field equations usually requires use of numerical methods such as finite elements.

There is a need to destroy both military and civilian hazardous waste and an urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). By capitalizing on the properties of water above its critical point (374°C and 22.4 MPa for pure water), this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While Ni and Ni-based alloys are generally considered important for severe service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking and dealloying, are seen in some environments. Although exotic liners such as platinum are currently promoted as a solution to aggressive conditions, some evidence suggests the potential for corrosion control by judicious feed modification. Various alloys were exposed in a SCWO system at 600°C for 66.2 h. After exposure, samples were coated with a thick outer salt layer and an inner oxide layer. It is considered likely that, at the high supercritical temperature employed during this test, the salt was molten and contained a substantial quantity of gas. The inner oxide layer revealed the presence of numerous defects and a thickness that is proportional to the corrosion rate determined by mass loss, suggesting the oxide layer is nonprotective. Of the alloys tested, G-30 exhibited the highest corrosion resistance. Experiments in which a C-276 tube was instrumented with thermocouples and exposed to a HCl feed indicate for this simple non-saltforming influent that there is a strong correlation between temperature and the extent and form of corrosion, with the most pronounced degradation being at high subcritical temperatures. These experiments corroborate previous results from a failure analysis for C-276, suggesting a corrosion maximum in the subcritical region.

Static fatigue of a polycrystalline porous rock (iron ore) was studied by performing multistep uniaxial creep tests under partially saturated conditions, and the impact of water saturation on creep was analyzed. Recorded strains and acoustic emissions (AE) show that water saturation induces a strong increase in AE activity and dilatant inelastic volumetric strain and a decrease in Young's modulus and in the b value of the Gutenberg-Richter law (i.e., the relative number of large-amplitude events increases) as the rock approaches failure, indicating that microfracturing plays an important role in the creep process. Water saturation accelerates static fatigue through hydromechanical coupling and subcritical stress corrosion cracking. The chemical reactions involved in the corrosion of iron ore and leading to a decrease in its intrinsic mechanical properties are described. These reactions play a major role in the static fatigue of iron ore, which on a large scale is probably the main mechanism explaining certain collapses in underground iron mines. It is also shown that creep straining of iron ore is reversible after stress removal, indicating that it results also from a time-dependent viscoplastic mechanism (i.e., dislocation creep).

The stress corrosion cracking (SCC) susceptibility of two aluminum-lithium alloys, a binary AI-Li and a ternary A1-Li-Cu alloy, in 0.5 M NaCI solution was investigated using the constant elongation rate technique (CERT). Susceptibility increased with decreasing strain rate and with aging. The alloys were susceptible under both anodic and cathodic applied potentials. The susceptibility dependence of the alloys as a function of applied potential correlates well with published hydrogen permeability data. The susceptibility increased dramatically when hydrogen was charged into the specimen using a hydrogen re-combination "poison" during CERT testing. These experiments suggest that hydrogen plays a major role in the SCC of these alloys. A brittle hydride having the composition LiAIH4 forms in the AI-Li system under conditions of severe SCC susceptibility. The brittleness of the hydride is explained. The formation of the hydride is a sufficient condition for SCC of AI-Li alloys. A process of SCC in AI-Li alloys is proposed wherein hydrogen causes damage by the formation of a hydride. Rtsumt-On 6tudie la susceptibilit6 d la fissuration par corrosion sous contrainte de deux alliages aluminium-lithium, un alliage binaire A1-Li et un ternaire AI-Li-Cu, dans une solution d 0,5 M de NaC1 en utilisant la technique de la vitesse d'61ongation constante (TVEC). La susceptibilit6 augmente lorsque la vitesse de drformation drcroit et avec la vieillissement. Les alliages sont sensibles d la fois sous des potentiels anodique et cathodique. La d~pendanee de la susceptibilit6 des alliages vis d vis du potentiel appliqu6 est en bon accord avec les donnres de permrabilit6 publires pour l'hydrogrne. La susceptibilit6 augmente dramatiquement lorsque l'hydrogen est charg6 dans l'rchantillon en utilisant un hydrogrne "poison" de recombinaison pendant les essais de TVEC. Ces exprriences suggrrent que l'hydrogrne joue le rrle principal dans la fissuration par corrosion sous contrainte de ces alliages. Un hydrure fragile, ayant la composition LiAIH4, se forme dans le systrme AI-Li Iorsque cette fissuration est trrs importante. La fragilit6 de l'hydrure est expliqure. La formation de l'hydrure est une condition suffisante pour la fissuration des alliages AI-Li. On propose un mrcanisme de fissuration dans les alliages AI-Li, dans lequel l'hydrogrne cause des drgfits par la formation d'un hydrure. Zusammenfassung-Die Neigung zweier Aluminium-Lithium-Legierungen, einer bin~iren AI-Li-und einer tern/iren AI-Li-Cu-Legierung, zur SpannungsriBkorrosion in einer 0,5-molaren NaCI-Lrsung wurde mittels des Verfahrens der konstanten Verlfingerungsrate untersucht. Diese Neigung nimmt zu mit abnehmender Dehnungsrate und mit der Alterung. Spannungsriflkorrosion tritt sowohl unter anodischem als auch kathodischem Potential auf. Die Abh~ingigkeit dieser Neigung vom angelegten Potential korreliert gut mit den publizierten Daten fiber die Wasserstoffpermeabilit~it. Die SopannungsriBkorrosion nimmt dramatisch zu, wenn die Probe mit Wasserstoff mittels eines "'Giftes '° gegen Wasserstoff-Rekombination w/ihrend des Versuches mit konstanter Dehnungsrate beladen wird. Diese Experimente legen nahe, dab der Wasserstoff eine wesentliche Rolle bei der Spannungsril3korrosion dieser Legierungen spielt. Unter Bedingungen starker SpoannungsriBkorrosion bildet sich in diesem AI-Li-Systern ein sprrdes Hydrid mit der Zusammensetzung LiAIH4. Die Sprrdigkeit des Hydrids wird erklirt. Die Bildung des Hydrids ist eine hinreichende Bedingung f'fir die Spannungsril3korrosion der AI-Li-Legierungen. Ein Mechanismus der SpannungsriBkorrosion in AI-Li-Legierungen wird vorgeschlagen, bei dem der Wasserstoff durch Hydridbildung zur Sch~idigung fiihrt.

Room temperature cathodic hydrogen embrittlement in alloy 718 was investigated by means of slow strain rate tensile tests conducted on specimens charged either prior to or during deformation. Tensile tests performed on precharged specimens at strain rates of 5 ×10 − 7 , 5×10 − 5 and 5×10 − 3 s − 1 suggest that hydrogen embrittlement is correlated with hydrogen segregation to moving dislocation and transport by these dislocations. Observations of 1 mm planar cleavage microfacets on fracture surfaces of specimens charged either prior to or during deformation support the idea that embrittlement occurs by strong hydrogen-deformation interactions. In light of these results, the role of the cathodic hydrogen produced by the corrosion process inside a crack during stress corrosion cracking is discussed in conjunction with the corrosion enhanced plasticity model, proposed some years ago by one of the authors. It is suggested that hydrogen transport by dislocations and localisation along active slip planes may be the controlling stage of the cracking process.

In order to reduce the costs related to corrosion damage in aircraft structures, it is vital to develop new robust, accurate and reliable damage detection methods. A possible answer to this problem is offered by newly developed nonlinear ultrasonic techniques, which monitors the nonlinear elastic wave propagation behaviour introduced by damage, to detect its presence and location.In this paper, a new nonlinear time reversal technique is presented for the detection and localization of a scattered zone (damage) in a multi-material medium. In particular, numerical findings on a friction stir-welded aluminium plate-like structure are reported. Damage was introduced in the heat affected zone and modelled using a multi-scale material constitutive model (Preisach–Mayergoyz space).Studies were conducted for two different transducer configurations. Particular attention was devoted to find the optimum time-reversed window to be re-emitted in the structures. The methodology was compared with traditional time-reversal acoustics (TRA), showing significant improvements. While the traditional TRA was not able to clearly localise the damage, the developed technique identified in a clear manner the faulted zone, showing its robustness to locate and characterize nonlinear sources, in presence of a multi-material medium.

Localized corrosion continues to be a major cause of degradation failure in a wide variety of technological applications. The propagation stage of failure is no longer a fundamentally difficult job to characterize for pits and stress corrosion cracks, for example. It is the initiation stage that remains difficult to characterize, arising in part because of the difficulty in being able to predict where and when localized attack will occur. Recent developments in scanned probe techniques have created renewed interest in this problem. The present paper will describe some of the recent advances in optical, electrochemical and photoelectrochemical techniques that are directed at providing local information on precursor sites and vulnerable areas on metal and semiconductor surfaces.

The motivation of the study is the development of a coupled approach able to account for the interaction between environment and plasticity in a polycrystalline material. The paper recalls first the constitutive equations used to describe the behaviour of the grain core and of the grain boundary (GB). The procedure that is applied to generate synthetic polycrystalline aggregates with an explicit representation of the grain boundary area by 2D or 3D finite elements is then described. The approach is applied to the modeling of iodine assisted stress corrosion cracking (IASCC) in Zircaloy tubes used in nuclear power plants.

Earthquake swarms occur mostly in regions with CO 2 -enriched pore fluids. It is generally accepted that both geodynamic stress accumulation and critical pore fluid pressures act as a triggering mechanism for most seismic events. The new thesis presented here is that hydrothermal alteration processes in fault zones help facilitate the shear failure propagation due to mechanical weakening and dissolution of the wall rock, in addition to the normal shear stress and fluid overpressure. The basic idea that stress corrosion cracking results from chemical weakening and comminution has been discussed for many years. However, it has not yet been applied to explain the earthquake swarm phenomenon. Studies of extensive alteration as well as the latest investigations of CO 2 sequestration give evidence that these high dissolution rates of wall rock in contact with an acid fluid phase exist in seismogenic fault zones. Several indications support the assumption that in the Vogtland/NW Bohemia region, the weakening of stressed fault zones by hydrothermal alteration could take place at seismogenic depths and could generate earthquake swarms. Investigations of quartz samples from the fracture zones by means of cathodoluminescence as well as spatiotemporal analysis of seismicity and numerical modelling of alteration-induced earthquake swarms support this hypothesis.

Using transmission electron microscopy (TEM), scanning electron microscopy, X-ray diffraction (XRD), and optical microscopy, the effects of microalloying elements of Sc, Ni, and Ce on the microstructure of a new super-high-strength ingot metallurgy (IM)/Al-Zn-Mg-Cu alloy (C912) have been correlated with mechanical properties and stress corrosion cracking (SCC) behavior. Using microalloying with Sc, Ni, and Ce, the C912 alloy can exhibit very high strength and good SCC resistance. Compared to the baseline C912 alloy, Sc refines the microstructure and retards recrystallization, Ni promotes the development of matrix precipitates, which enhance the strength and SCC resistance, and Ce has little effect on alloy strengthening in the three microalloying additions studied. The Sc-containing alloy (C912S) is the most attractive and even exhibits higher strength (ultimate tensile strength (UTS) greather than 660MPa) than the new Alcoa aluminum alloy 7055 and the Russian alloy B96, which have the highest strengths of the commercial IM/Al-Zn-Mg-Cu alloys.

This paper considers our recent research on rock bolt stress corrosion cracking (SCC) which studied the influence of metallurgy using a range of (1) existing rock bolt steels and (2) commercial steels. The chemical composition, mechanical properties and microstructures of these steels varied considerably. The aim is to understand this failure mechanism and in particular to apply the knowledge gained from the study of SCC and hydrogen embrittlement (HE) of steels. The resistance of 1008, X65, X70 and 4145H to SCC/HE was similar, an effect which could be hardly expected as these steels have different strengths, microstructures and H-trap distributions. Different steel microstructures (pearlitic-ferritic and tempered martensite) had similar resistance to SCC and HE in the most aggressive conditions (pH 2.1 and À1500 mV SCE ) and had the same dimple rupture fracture surface.