Tensile Stress Zone Research Articles

Effect of aging on the onset of cracks due to redistribution of residual stresses in functionally graded environmental barrier coatings of mullite/ZrO2

Environmental barrier coatings (EBCs) are proposed as an option to reduce the high temperature water vapour corrosion in gas turbines ceramic components made of Si3N4 or SiC/SiCf, which are projected to achieve further energy efficient gas turbines. These coating are commonly designed as multilayer systems firmly attached to the ceramic substrate with the aim of retarding or avoiding its degradation after exposure to environmental conditions close to those in gas turbines. Therefore, to fulfil this function crack formation/propagation in the coatings must be controlled. In present work, three types of environmental barrier coatings fabricated by air plasma spray and containing a Si layer attached to SiC substrate plus 2 to 5 layers of different mullite/Y2O3 stabilized–ZrO2 mixtures are examined. To determine the level of residual stresses in the as-sprayed coating/substrate systems a three dimensional finite element model is developed and also tested for same coatings but aged under, high temperature and rich water vapour atmosphere. The model calculates the zones of maximum tensile stresses in the coatings which agree with experimental observation identifying the type, number and location of cracks. This model could be extended to similar EBC systems, and more importantly, could be use as a powerful designing tool for these complex structures.

Structure Health Monitoring in Natural Environments: Pre-failure Event Location and Full-waveform Characterization by Nanoseismic Monitoring

Nanoseismic monitoring (NM) is a section of experimental seismology that focuses on the detection, location and characterization of extremely low-energy (0.0 ≥ ML ≥ −4.0) source processes recorded by portable sparse arrays ( Wust-Bloch and Joswig, 2006 ; Joswig, 2008 ; Wust-Bloch, 2010 ; Walter et al., 2012 ). The structural health monitoring technique presented here down-scales NM to suit engineering applications in shallow geomaterials, whereby pre-failure microcracking is monitored within the 1–200 Hz band and fully characterized at unusually short slant distances (102–10−1 m). Here, NM techniques are validated in lab by monitoring incipient microcracking generated by unreinforced concrete beams and limestone plates undergoing four-point bending tests. These ground truth (GT) tests show that progressive loading triggers a wide range of impulsive signals whose frequency and rate patterns evolve until complete material failure occurs. Following this, NM techniques are applied to monitor microcracking generated by unstable archeological caverns that were excavated in natural chalk. Although signals can be detected in unfavorable signal-to-noise ratio conditions by a single array at slant-distances beyond 102 m, reliable locations can only be obtained when several mini-arrays are deployed in the vicinity of caverns suspected to be unstable. Epicentral locations of microcracking events tend to cluster near free boundaries and in zones of high tensile stress, as predicted by numerical models computed for these caverns. These results confirm the operative capacities of low-cost NM techniques in locating and analyzing pre-failure microcracking processes occurring within weak natural media. Complex source processes, which occur at distances that exceed the detection range of standard acoustic emission and ultrasonic monitoring, can be fully evaluated using portable equipment that can be deployed within minutes with no prior infrastructure.

Seismic Affection Analysis of Weir Dam Based on 3-D FEM

The vibrant-type-disintegration method is applied to calculate structure seismic action of a weir dam. Based on calculation and analysis of the weir dam, special attention is paid to the stress state of key parts. The results show that the displacement and stress of weir dam are affected by earthquake excitation direction. Since large principal tensile stress appears in the joint of girder and pier, and the zone of principal tensile stress is large because of the thrust of radial gate and steep changes of geometry shape, we should pay special attention when calculating anti-seismic checking and reinforcement arrangement.

Residual stress and softening in welded high-strength low-alloy steel with a buffering layer

Welding residual stresses, Vickers hardness and microstructure of welded high-strength low-allow steel, known for high strength and low carbon content, were studied under the following conditions: as-received high-strength low-alloy steel, welded high-strength low-alloy steel without a buffering layer, and welded high-strength low-allow steel with various thickness buffering layer. A soft buffering layer with a modest thickness between the parent metal and the weld metal could be incorporated to welded high-strength low-alloy steel to effectively reduce the widths of tensile residual stress zone and welding softening zone, to change the residual stress (in y-direction) nature at the weld root from tensile to compressive, and to refine the grains of the welded high-strength low-alloy steel. The width of the tensile residual stress (in x-direction) zone was approximately equivalently to that of the welding softening zone for those welded high-strength low-alloy steel with and without buffering layers.

Integration of finite element simulation and intelligent methods for evaluation of thermo-mechanical loads during hard turning process

The machined surfaces are mainly affected by thermo-mechanical loads during machining processes. In this regard, thermal loads increase tensile residual stress and heat-affected zone; however, mechanical loads increase fatigue strength and compressive residual stress on the machined workpiece during the process. Since experimental investigation is difficult, the problem becomes more difficult if the aim is minimizing thermal loads, while maximizing mechanical loads during the hard turning process. This article presents a hybrid method based on the artificial neural networks, multiobjective optimization, and finite element analysis for evaluation of thermo-mechanical loads during the orthogonal turning of AISI H13-hardened die steel (52HRC). First, using an iterative procedure, controllable parameters of simulation (including contact conditions and flow stress) are determined by comparison between finite element and experimental results from the literature. Then, the results of finite element simulation at the different cutting conditions and tool geometries were employed for training neural networks by genetic algorithm. Finally, the functions implemented by neural networks were considered as objective functions of nondominated genetic algorithm and optimal nondominated solution set were determined at the different states of thermal loads (workpiece temperature) and mechanical loads (workpiece effective strain). Comparison between the obtained results of nondominated genetic algorithm and predicted results of finite element simulation showed that the hybrid technique of finite element method–artificial neural networks–multiobjective optimization provides a robust framework for machining simulation of AISI H13.

Stress and Strain Evolution in Electroless Copper Films Evaluated with X-ray Diffraction and Substrate Curvature

The scope of electroless copper plating technology is evolving to applications with smaller structures and smoother substrates, where adhesion of the copper film can be hard to achieve. The extent to which different factors, such as film stress and hydrogen evolution, contribute to delamination and blister formation on smooth organic substrates is a topic of active discussion. Stress and strain evolution is reported here for a nickel- and formaldehyde-free formulation for a range of plating bath temperatures. X-ray diffraction, with the sin 2ψ method, was used to monitor the strain within the copper crystallites during and after their growth. Substrate bending measurements were used to detect the average stress of the film. This technique indicates a 60 nm wide zone of tensile stress at the interface between substrate and film. During and immediately following deposition, the average substrate stress is more compressive than implied by the strain of the Cu crystallites. After deposition, the additional compressive stress seen by substrate bending relaxes until it matches the Cu crystallite strain. Furthermore, the thermal substrate contraction effect upon termination of plating is considered, since it may influence the film adhesion.

Mechanical Properties Analysis of Heat Insulation Structure in High Temperature Circular Roadway

Roadway insulation and active cooling supporting structure is a complex system, and the mechanical properties play an important roal in its safety. Analyse the shotcrete layer pattern heat insulation structure by Midas-GTS software, calculation results shows that tensile stress zone appears in the top and bottom of heat insulation layer, while the will compressive stress area appears in two sides of the roadway insulation layer; in the case of 500m buried depth, heat insulation layer material, such as vitreous micro-bead, can well meet the engineering mechanical strength requirements.

Mechanical Properties Analysis of Heat Insulation Structure in High Temperature Circular Roadway

Roadway insulation and active cooling supporting structure is a complex system, and the mechanical properties play an important roal in its safety. Analyse the shotcrete layer pattern heat insulation structure by Midas-GTS software, calculation results shows that tensile stress zone appears in the top and bottom of heat insulation layer, while the will compressive stress area appears in two sides of the roadway insulation layer; in the case of 500m buried depth, heat insulation layer material, such as vitreous micro-bead, can well meet the engineering mechanical strength requirements.

Microstructural changes in ground 3Y-TZP and their effect on mechanical properties

In the present work we studied the changes in the microstructure and mechanical properties that are produced by grinding of tetragonal polycrystalline zirconia doped with 3 mol.% yttria (3Y-TZP). It is shown that the X-ray diffraction spectrum of ground 3Y-TZP presents asymmetric broadening of the (1 1 1) tetragonal peak, reversal of the intensity of the (0 0 2) and (2 0 0) tetragonal peaks, and tetragonal to monoclinic (t–m) phase transformation. The in-depth monoclinic phase distribution obtained by micro-Raman spectroscopy has been related with the compression residual stresses profile generated during grinding. These compressive residual stresses are also responsible for the observed increase in mechanical properties (strength and apparent fracture toughness). The subsurface microstructure of ground specimens has been analyzed by transmission electron microscopy and three different regions were found from the interior to the surface: (1) a t–m phase transformation zone in the deeper region, (2) a plastically deformed zone, with dynamically recovered dislocation cells, and (3) a surface recrystallized zone with grains of ∼20 nm in diameter, resulting from in situ recrystallization. In addition, microcracking is concentrated on the sides of the grinding grooves, corresponding to the zone of maximum tensile stress during contact with an abrasive particle.

Nonlinearity and scaling behavior in a soft lead zirconate titanate piezoceramic

Lead oxide-based ferroelectrics, represented by lead zirconate titanate [Pb(Zr,Ti)O3] or PZT), are the most widely used materials for piezoelectric actuators, sensors, and transducers due to their excellent piezoelectric properties. Most of these piezoelectric materials are employed under a variety of strains (stress, electrical field, and temperature). It would thus be interesting to predict their behaviors under different excitations without having to perform too much experimental work, i.e., just carry out a single experiment and still be able to provide the other experimental values. The purpose of this paper has thus been to propose several behavioral laws linking the electrical field, temperature and mechanical stress. The first law rendered it possible to express the mechanical stress by an equivalent electric field [ΔE≡αΔT×P(E,T0)]. Subsequently, a law linking the electrical field and temperature {ΔE≡[2β×P(E,θ0)]×Δθ} was proposed. From these two laws, a third law was identified reflecting the mechanical stress as an equivalent temperature; each expressed by a temperature equivalent stress (T≡δ×Δθ). After experimental validation of these laws, the mapping could be extended in order to predict the polarization behavior in the tensile stress zone as well as that as a function of the negative temperature.

The Study of the Relationships between Material Properties and Welding Residual Stresses

The objective of this study is to explore the influence of the steel’s material properties on the distribution of welding residual stresses of the butt-welded steel plate. First, S15C carbon steel’s material properties are varied in this study to calculate the corresponding welding residual stress by carrying out 3D finite element analyses. Results show the maximum longitudinal tensile residual stresses increase linearly with increasing thermal expansion coefficient and increasing yield stress. The length of the tensile residual stress zone decreases linearly with increasing thermal conductivity. Finally, based on the relationship between material properties and welding residual stresses, a useful prediction equation is developed and verified in this study.

Electromigration-induced stress in a confined bamboo interconnect with randomly distributed grain sizes

Microfailure of thin film interconnects by electromigration (EM)-induced stress is one of primary causes of degradation of the reliability of microelectronic circuits. We describe a two-dimensional model to simulate EM-induced stress in a confined bamboo interconnect line with randomly distributed grain sizes. We find that the steady-state stress distribution along the line is linear, the stress gradient being solely dependent upon the applied electric field, independent of the grain distribution and the diffusion condition at the line ends. However, the grain boundary concentration produces a significant shift in the steady-state stress along the line. When the grain boundaries concentrate in the tensile (compressive) stress zone, the steady-state stress lines shift up (down). Accordingly, the maximum tensile stress increases (decreases). The more the grain boundaries are in the tensile (compressive) stress zone, the more the maximum tensile stress increases (decreases). Thus, the maximum stress achieved at steady state is not only dependent on the line length for given applied electric field, but is also dependent on the grain boundary distribution along the line. A line microstructure with more grains in the compressive stress zone is of benefit to reduce the maximum tensile stress in the line. The preset results not only interpret why the EM-induced failure observed in experiments is sensitive to the variation in the line microstructures, but also imply that designing a line with fine grains in the compressive stress zone (near anode end) may significantly reduce the maximum tensile stress in the line, which can cause inhibition of failure due to EM-induced microcracking and void nucleation.

Stress/electrical scaling in ferroelectrics

The present paper introduces a scaling law correlating three factors of ferroelectric materials in quite simple functions: the polarization (P), the electrical field (E), and the stress (T). The law is based on the physical symmetries of the problem and renders it possible to express the mechanical stress (T) as an electric field equivalent {Eeq=Th[P(E=0,T)]} and, as a consequence, also the relationship between strain (S) and polarization (P). Three materials with various phase structure (tetragonal, rhombohedral, and morphotropic) were used for the verification. It was found that such an approach permitted the prediction of the maximal stress using only purely electrical measurements [i.e., measurements of S(E) and P(E)]. Once this law was validated for the compressive stresses, the mapping could be extended in order to predict the polarization behavior in the tensile stress zone. It was shown that the polarization behaved differently as a function of the compressive and tensile stresses. The scaling law could also predict the piezoelectric constant (d33) under stress using only purely electrical measurements. Reciprocally, the dielectric constant (ε33) under an electrical field can be predicted using only purely mechanical measurements. Comparisons of hysteresis loops were made and a good agreement was found for the polarization versus electric field, compressive stresses, and, albeit to a lesser extent, for the coefficients (ε33) and d33.

Fracture of Thin Walled Translucent Polycrystalline Alumina (PCA) Tubes

Room temperature fracture behavior of short thin walled translucent coarse-grained polycrystalline alumina (PCA) tubes was investigated under compression loading in radial direction using conventional mechanical tests combined with the finite element modelling (FEM). FEM indicated strongly inhomogeneous stress distribution with the maximum tangential tensile stresses in the centre of the inner surface of a tube and opposite to the line of loading. Another zone of tangential tensile stresses form on the outer surface of the tube and oriented perpendicularly to the loading direction. However, these tensile stresses are lower compared to those under contact lines. Radial stresses were negligible compared to tangential stresses which were concluded to control tube fracture. Fracture occurs in two steps: firstly along the loading lines under the lines of contacts and secondly in the direction perpendicular to loading. It results in four regular pieces along the lines with the highest tangential tensile stresses. The results were analysed using Weibull statistics and standard fractography methods. The characteristic strength of the PCA tubes was 186 MPa and the Weibull parameter m‘corr = 16.9 ± 0.5. High m values and the absence of the obvious fracture origins were attributed to homogeneous microstructure and well developed PCA technology.

Influence of Edge Rolling Reduction on Plate-Edge Stress Distribution During Finish Rolling

Dimensions of one kind of stainless steel plate before finish rolling were obtained through analysis of the rough rolling processes by finite element method and updated geometrical method. The FE models of finish rolling process with a front edge roll were built, and influences of the edge rolling reduction on the stress change in the plate edge during finish rolling were analyzed. The results show that when the edge rolling reduction is increased from 0 mm to 2 mm, the compressive stress in plate corner clearly increases in edge rolling process, and the zone of tensile stress during whole rolling decreases; when the edge rolling reduction is increased from 2 mm to 5 mm, the compressive stress in the plate corner seldom changes, and the compressive stress decreases after the horizontal rolling.

On the nature of plastic strain localization in solids

Theoretical and experimental investigation is performed into the relation between plastic strain localizations of different scale in solids and the respective stress concentrators arising in the surface layer and at internal interfaces. It is found that localized plastic flow of any kind may form and propagate only under strongly nonequilibrium conditions in the zones of normal tensile stress. In the presence of excessive atomic volume, virtual nodes of a structure with higher energy emerge in the space of interstitials and a local structural transformation occurs via collective atom-vacancy configuration excitations. It is concluded that the nature of the plastic flow localization should be described on the basis of representation of strained solid as a multilevel system.

Fragmentation of liquid and liquid-plastic media under unsteady strains

Based on a qualitative analysis of results of experimental studies, the main mechanisms of fragmentation of polar liquids and liquid-plastic media under dynamic loading are determined. In the case of low-viscosity liquids, such mechanisms are thermodynamic instability of foam, hydrodynamic instability of initial disturbances of the free surface, and the action of capillary forces. In the case of a polar high-viscosity liquid, the main mechanisms are shear instability of the structure, responsible for stratification of the medium along the lines of local failure of structural viscosity, and the action of capillary forces. The main mechanisms acting in liquid-plastic structured media (gels) are “spalling” in the zone of tensile stresses if the time of their formation is smaller than the time needed for the gel to transform to the sol state, as well as thermodynamic instability of foam and the action of capillary forces after the medium transition to the sol state.

Physical mesomechanics of a deformed solid as a multilevel system. III. Inelastic precursor of plastic shear generation

As a continuation of our previous papers, we consider the problem of plastic shear generation in a deformed solid based on a multilevel approach of physical mesomechanics. We develop a method of stochastic excitable cellular automata, which allows taking into account the self-organization of configurational perturbations of various scales in the system “surface layer – its interface with the substrate”. It is shown that any plastic shear can be generated only in the zone of normal tensile stresses that give rise to inelastic configurational disturbances in a crystal as a plastic shear precursor.

Fundamental role of nanoscale structural level of plastic strain in solids

A conceptual substantiation of the necessity to consider the nanoscale structural level of plastic strain in the physics of plasticity and strength of solids is presented. It is inferred that the fundamental mechanism of plastic strain is represented by local structural transformations (of the type of rearrangement of atomic clusters of various configurations) that occur in a loaded solid in local zones of tensile normal stresses. This mechanism determines generality of the nature of all possible kinds of plastic deformation of solids.

Residual stresses in selective laser sintering and selective laser melting

PurposeThis paper presents an investigation into residual stresses in selective laser sintering (SLS) and selective laser melting (SLM), aiming at a better understanding of this phenomenon.Design/methodology/approachFirst, the origin of residual stresses is explored and a simple theoretical model is developed to predict residual stress distributions. Next, experimental methods are used to measure the residual stress profiles in a set of test samples produced with different process parameters.FindingsResidual stresses are found to be very large in SLM parts. In general, the residual stress profile consists of two zones of large tensile stresses at the top and bottom of the part, and a large zone of intermediate compressive stress in between. The most important parameters determining the magnitude and shape of the residual stress profiles are the material properties, the sample and substrate height, the laser scanning strategy and the heating conditions.Research limitations/implicationsAll experiments were conducted on parts produced from stainless steel powder (316L) and quantitative results cannot be simply extrapolated to other materials. However, most qualitative results can still be generalized.Originality/valueThis paper can serve as an aid in understanding the importance of residual stresses in SLS/SLM and other additive manufacturing processes involving a localized heat input. Some of the conclusions can be used to avoid problems associated with residual stresses.