Mechanics of Fracture, Fatigue and Damage

The present study is aimed to understand the influence of dynamic precipitation on the low cycle fatigue (LCF) behavior of an under-aged (UA) AA6063 Al–Mg–Si alloy. This was accomplished by the estimation of plastic strain energy density (PSED) at varied isolated cycles during LCF of the UA alloy with subsequent comparison of these results with those of peak-aged (PA) and over-aged (OA) ones. The LCF tests of the UA alloy were carried out in the range of strain amplitudes of 0.2–1.0 % together with the evolution of hardness and tensile properties. The UA alloy shows Masing behavior, evaluated in terms of the variation of Bauschinger strain with plastic strain amplitude, and exhibits continuous hardening till failure unlike the PA and OA alloys. Higher average PSED value for the UA alloy in comparison to that for the PA and the OA alloys indicates dynamic precipitation during cycling; the magnitudes of average PSED were calculated using a proposed method. In addition, pronounced increase in the post LCF hardness values substantiate the dynamic precipitation.

Based on very early occurring ruptures found in the first stage high pressure turbine blades of a turbo reactor in a local aviation company, this study has the aim to determine their safe life. The first stage blades are subjected to simultaneous action of gas pressure coming from the combustion chamber, centrifugal forces in the case of the rotor blades and to important temperatures transients, which progress in a very aggressive environment due to hot gases. These combined parameters cause a high state of stress involving several complex mechanisms of damage, such as: fatigue caused by mechanical stress fluctuations, thermo-mechanical fatigue caused by temperature variations and corrosion caused on the stressed elements. Life cycle determination asks for stress evaluation of blades regarding several variables which are approached deterministically in the study. Heat exchange between combustion gases and metal blades is considered. The total stress on two kinds of blades is calculated by the addition of the thermal effect and the mechanical loading. The stress cycle is then calculated for different steps of the engine function during the operation by considering the variation of the thermal and the mechanical properties of the system. Safe life determination is done by two different approaches: the safe life approach by the initiation model and the damage tolerance approach considering the defect growth mechanics and considering the pitting corrosion effect. The calculation is applied for stator and rotor blades of an aero engine high pressure turbine made of NI 738. Since these parts are high risk components from the point of view of potential failure consequences, the risk is assessed as well. The results obtained are studied to determine the solution to the problem, and to propose a safe decision to be taken about the design or maintenance procedures.

The common failures mechanisms of boiler components at elevated temperatures and monitoring techniques used in power plants to detect damaging or failures are generally well described in literature. Despite of different classification concerning the principal failure mechanisms of boiler thermal power plant components, it is necessary to emphasize that essential difference between these classification does not exist. Concerning the different failures mechanisms it is obvious that the most of them are completely explained and clear in physical sense of process but some of them are not still so explicit. Therefore, until today there is a great interest to investigate the failures of power plant boiler components especially having in mind the all peculiarities of particular events. This efforts are necessary while a full assessment of the failure mechanism(s) is required to prevent further failures due to the same causes. The numerous case studies presented in this paper were done as a contribution to this efforts.

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions. In this paper structural, low carbon steel, for pressure purposes, grade 20 - St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX. Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials.

The present work deals with the concept of post-initiation softening applied to ductile crack initiation and propagation problems in thick structural components such as pre-cracked pipes and fracture specimens. The methodology includes a set of calibration steps to determine model parameters of crack initiation and propagation under monotonic quasi static loading condition. The phenomenological Modified Mohr-Coulomb (MMC) fracture criterion is used for fracture initiation along with post-initiation softening for crack extension. A comprehensive experimental/numerical program provides access to the load histories of each specimen geometry at the critical point, where fracture is assumed to occur. This first step enables the construction of the limits of ductility of the material in terms of a weighted von Mises equivalent plastic strain, which strongly dependent on stress tri-axiality and Lode angle evolution. Post-initiation softening parameters are then calibrated from tearing analysis built upon resistance curves of single edge notch tension (SENT) specimens. The calibrated parameters model along with detailed FE analyses shows that the present fracture criterion accurately predicts the onset of stable crack growth and propagation along the uncracked ligament of low constraint specimens.

Damage of boiler tubes is a very serious problem in many utilities worldwide, including even in the most advanced countries. Preserve of boiler tube integrity present major part in maintenance programs centered to achievement of high levels of availability and reliability of thermal power plants. This is due to a very specific and complex interconnection of design and operating characteristics of the boiler unit. Based on the results of numerous boiler tube damages investigation performed on domestic thermal power plants it is concluded that the initiation of different damage mechanism was strongly influenced by tube weld geometry defects. In this paper particular attention was dedicated to this not well documented problem. Structural integrity of the boiler tubes are greatly affected by the weld excessive root penetration (melt-through) on the tube inner surface. This type of weld geometry defect, which occurred often on boiler tubing system, present local flow disruptor and also very strong stress riser. In the vicinity and downstream of excessive root penetration type disruptor initiation of different damage mechanisms (erosive and corrosive wear, under-deposit corrosion, stress corrosion, corrosion fatigue) occurred due to working fluid nucleate boiling destabilization (evaporator), or due to turbulent flow (superheater and reheater), followed by intensive suspended solids deposition.

Due to environmental issues, the efforts for light weighting in automobile design and manufacturing have become the main focus for automotive players to reduce fuel consumption and CO2 emission through intensive use of aluminium alloys. However, since aluminium alloys are known to be difficult to weld, joining of similar and dissimilar aluminium alloys in automotive are crucial to be solved. In this study, the friction stir welding known as solid state joining process is extensively used in joining of similar and dissimilar 5mm thickness aluminium alloys. The butt-joint types of similar joint (Al5083-Al5083) and dissimilar joint (Al5083-Al6061) were carried out under the same welding parameters, in which the rotation speed was 1000rpm and transverse speed was 100 mm/minute. Macro and microstructure observations were acquired at the transverse cross section of the weld regions respectively by stereo and optical microscope. The microstructural study indicated the spread out of materials mixing between two materials at both sides of welding part. In the similar welding the formation an ‘onion ring’ structure has been detected in the nugget zone, while wavy and distorted patterns appeared in dissimilar joints. The tensile test results showed similar fracture patterns in all tensile specimens for similar welding joint where all fractures occurred in thermo-mechanically affected zone (TMAZ). However, tensile specimens of dissimilar welding joint break up at TMAZ region as well as at Al6061 base metal in retreating side. In comparison the tensile strength of similar joints Al5083-Al5083 and dissimilar joint Al5083-Al6061 were 22% and 19% lower compared to base metal of Al5083 and Al6061, respectively.

In the present study we consider an adaptation of Nguyen’s model [G.D. Nguyen, A thermodynamic approach to constitutive modelling of concrete using damage mechanics and plasticity theory, D.Phil. Thesis, Dept. of Engng. Sci., University of Oxford, 2005] for brittle materials to the case of ductile rupture. We focus our attention on the formulation of a consistent tangent stiffness matrix which is essential for the efficiency and convergence of the implementation within a finite element framework. We demonstrate the capability of the model to generate mesh-independent results and to follow the rupture process very close to failure. Rupture analysis of thin plates of different geometry is presented (symmetric double notch-edged specimen, asymmetric double notch-edged specimen, double notch-edged specimen containing a hole). Further modifications are outlined required to simulate crack growth and paths under the conditions of creep and fatigue.

Gallium nitride (GaN)nanos tructures have applications inopt oelectronics and other allied nanoelectronic devices like nanosensors and nanogenerators.The stability and reliabil ity ofaGaN nanosheet isrelated to its mechanical behavior under external loads.The presence ofdefects influencesthe mechanical behavior of such structures.Inthis article,classical molecular dynamics simulations were performed by employing the Stillinger–Weberpotential onasystem ofsingle layer nanosheet ofGaN containing single and double atomic vacancy defects.The stress–strainresponses show that,the fracture limit decreases due to the presence ofsuch defects when compared with pristine GaN nanosheets.The fracture process of suc hnanosheet with single atomic vacancy has been illustrated.The effect ofseparation dis-
tance ofthe divacancies onthe fracture stress,fracture strain and the Young’s modulus were investigated. In addition,the effect ofstrain rates onthe failure stress and strain ofpristine GaN nanos heet has also been studied.

Hydrogen embrittlement of austenitic steels is of high interest because of the potential use of these materials in hydrogen-energy related infrastructures. In order to elucidate the associated hydrogen embrittlement mechanisms, the mapping of heterogeneities in strain, damage (crack/void), and hydrogen and their relation to the underlying microstructures is a key assignment in this field.
Specifically mapping the connection between microstructure heterogeneity
and the associated hydrogen trapping at similar spatial resolution opens a novel pathway to identify hydrogen embrittlement mechanisms in complex alloys.
One of the materials classes that is expected to be applied for energy-related structure parts are austenitic steels with high Mn content. In particular, twinning-induced plasticity (TWIP) high Mn austenitic steels are well known for an exceptional balance of ductility and strength with less hydrogen susceptibility compared to ferritic steels with a similar strength.
The hydrogen embrittlement phenomenon has been observed under severe mechanical deformation and hydrogen charging conditions such as delayed fracture testing in a deep drawn cup or tensile testing during hydrogen charging at a high current density. Fracture was in such cases caused by various metallurgical factors e.g. by the formation and failure of deformation twins. The importance of deformation twins on hydrogen embrittlement of austenitic steels has been recently studied. It was found that they can act as crack initiation sites and enable crack propagation. Microstructure-sensitive hydrogen mapping in a TWIP steel has been conducted by using microprinting experiments, which enable visualization of hydrogen emission from a sample through reduction of silver ions in gelatin-based AgBr emulsion. The microprinting technique demonstrated that hydrogen is indeed localized at deformation twins, hence, promoting the initiation of hydrogen embrittlement. Also, from our previous experiments we suggested that the hydrogen localization at/near deformation twins requires local plastic straining at/before deformation twins. These facts indicate that the hydrogen-assisted twin boundary cracking of TWIP steels is a complex phenomenon including both, local plastic straining and local hydrogenation. To better understand the influence of deformation twins on the hydrogen embrittlement phenomenon, it is thus essential to map the spatial hydrogen distribution through a high resolution detection approach that is sufficiently microstructure and concentration sensitive. It should be noted here that the microprinting technique is only sensitive to hydrogen at relatively
high concentration levels, or more precisely at relatively high emission rates. It has, therefore, to be applied directly after a sample is charged with hydrogen. It will be shown here that hydrogen is critical for prolonged times after charging. Hence, the findings obtained by microprinting needed to be confirmed also for longer times after charging by a more sensitive technique. In the last years several promising novel approaches have been reported for the localized resolved and sensitive detection of hydrogen.
One is a direct electrochemical detection via a capillary cell, developed by Suter et al. which, however, does not provide the resolution required here. Another approach is the use of Kelvin probe techniques, which allow detection of hydrogen in a material by means of the change ofwork function caused by the hydrogen entering the oxide at the surface. Studies at quite high resolution have been carried out with Scanning Kelvin Probe ForceMicroscopy (SKPFM), where diffusion profiles of hydrogen have been mapped successfully at relatively high
resolution at cross section of samples after hydrogen charging.
However, a direct quantification is not possible by this method, due to the complex dependence of the work function of oxide on different defect states in the oxides. For the same reason this approach is not suitable to provide reliable information on hydrogen at different features of the microstructure, because different oxides show a different dependence on hydrogen. A new approach by applying a thin palladium layer eliminates this problem. More specifically, the SKPFM with a thin palladium layer is a novel and efficient method to very sensitively analyze local hydrogen concentrations down to levels well below 0.01 atom ppm at spatial resolutions as small as several tens of nanometers. The SKPFM can hence be used for detecting the hydrogen distribution with high sensitivity, since the potential measured by the SKPFM on the palladium that has been deposited as a thin layer on the hydrogen charged sample surface correlates logarithmically with the hydrogen content in the palladium. Because of this Nernstian-like behavior, which is similar to a hydrogen electrode, as also reported for immersed palladium metal, we also refer to the work function as electrode potential. In fact, as discussed previously, the surface of the palladium is, even in dry nitrogen atmosphere, still covered by an ultrathin water layer, which together with the hydrogen in the palladium leads to the formation of an hydrogen electrode “in the dry”. By exploiting this effect, SKPFM was successfully applied to the spatially resolved detection of hydrogen in ferrite/austenite duplex stainless steels. More specific in this project the hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1–2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations
of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption.