Tensile Stress Evolution Research Articles

Tensile Stress Evolution Outside Deformation Zone of Cold Rolled Strip

To determine the exact distribution of tensile stress in a cold rolled strip, tensile stress evolution caused by lateral uneven velocity distribution outside the deformation zone of the strip is examined. The finite difference method is employed to solve the problem. Different factors, including the strip width and the form and amplitude of the initial stress distribution on the stress evolution, are analyzed. On the one hand, to improve the calculation speed, a “Gaussian” curve is proposed to fit the results calculated by the finite difference method. Simulation results show that the stress evolution calculated by the fitting equation is in good agreement with that obtained by the finite difference method. On the other hand, to verify the exactness of the model, an experiment is conducted, and the comparison between the calculated results and experimental values is discussed.

Barkhausen Noise Emission as a Function of Tensile Stress in Low-Alloyed Steels: Influence of Corrosion and Steel Strength

Components of civil buildings are frequently made of low-alloyed steel, which can suffer from corrosion damage. This damage reduces their bearing capacity and/or redistributes the load to neighboring regions with the potential risk of their collapse. For this reason, this study deals with the non-destructive monitoring of bodies made of these steels based on Barkhausen noise emission. The superimposing contribution of corrosion extent and exerted tensile load is investigated on samples of variable yield strength in the range from 235 to 1100 MPa. It is found that the presence of a corroded layer attenuates Barkhausen noise and makes the body harder from a magnetic point of view. The reduced effective thickness of the samples as a result of corrosion damage increases the true stress. Barkhausen noise grows along with the tensile stress in the direction of exerted stress at the expense of decreasing Barkhausen noise in the perpendicular direction. The evolution of Barkhausen noise versus tensile stress is mostly shifted to the lower values of Barkhausen noise, along with the increasing degree of corrosion damage. The evolution of Barkhausen noise versus tensile stress is also affected by the initial microstructure and the corresponding yield strength of the low-alloyed steel. Corrosion attack results in the growth of FWHM, which is compensated by the decreasing evolution along with the tensile stresses. The effective values drop down with the higher extent of corrosion damage. However, the response with respect to the tensile stress is asymmetric in RD and TD due to the realignment of DWs into RD. Finally, PP tends to increase with the corrosion attack as well as the tensile stress and this parameter only exhibits the systematic behavior in RD as well as TD. On the other hand, MBN-extracted parameters as well as their combination provide no exclusive values on which the pure contribution of corrosion and tensile stress can be distinguished.

Validation of two different analysis techniques to obtain dynamic mechanical properties of concrete using a modified Hopkinson Bar

In the present work, two different approaches for obtaining dynamic fracture properties of concrete are analyzed. Spalling tests over cylindrical samples using a modified Hopkinson Bar were carried to measure the tensile strength and the fracture energy of conventional concrete. Results for strain rates ranging from 60 to 130 s−1 are presented for both techniques and compared to the respective quasi-static values. Moreover, two different projectile shapes (cylindrical and conical) have also been evaluated. In this sense, a deep qualitative and quantitative analysis is also performed, regarding the variations in tensile stress evolution of the pulses. Validation of both methodologies is achieved by comparing the obtained results.

Tensile behavior of clayey soils during desiccation cracking process

Desiccation cracking is a common phenomenon on Earth's surface. It is a typical form of soils' tensile failure caused by drying-induced tensile stress. Understanding soil tensile behavior during the drying process is of great significance for revealing the soil desiccation cracking mechanism. In this investigation, fiber Bragg grating (FBG) technique was employed to develop a new restrained ring, which is able to continuously monitor the evolution characteristics of soil tensile strain as well as tensile stress during the desiccation cracking process. Desiccation tests were carried out on two clayey soils (a lean clay and a fat clay). Saturated slurry samples (inside radius of 25 mm, outside radius of 75 mm and height of 30 mm) were prepared with initial water contents of 58.1% for the tested lean clay and 106.6% the tested fat clay, and were subjected to continuous drying under constant environmental condition (25 ± 1 °C of room temperature, 50 ± 5% of relative humidity). Moreover, a camera was mounted above the soil sample to capture the surface changes as well as the initiation and propagation of desiccation cracks. The results show that the developed FBG-based restrained ring can effectively identify the tensile behavior of the soil during the desiccation cracking process. Both the evolution of tensile stress and graphic variation show three typical periodic features that are well matched with soil vertical deformation, lateral shrinkage and cracking. With decreasing water content, tensile stress gradually increases at first and then increases more rapidly. Once the tensile stress reaches the peak value, desiccation cracks initiate and a sudden reduction in tensile stress is observed. Finally, the tensile stress reaches a residual stage. We consider the maximal tensile stress during the drying process to correspond to the tensile strength of the tested material. It is found that clay content has a significant influence on the cracking tensile strength of soils. The fat clay with a higher clay content shows a larger cracking tensile strength (about 410 kPa) compared with the lean clay (about 90 kPa).

Effects of water adsorption on properties of electron-beam HfO2/SiO2 high-reflection coatings

The moisture-induced instability of electron-beam (e-beam) coating can even result in the performance instability of a large laser system. A dense plasma-ion-assisted-deposition SiO2 capping layer was employed to isolate the top surface of the coating from ambient humidity and extend the originally rapid water adsorption process, allowing a deeper understanding of the instability of e-beam coatings caused by moisture. Spectral measurements suggested that four water adsorption steps, including filling, stabilization, swelling and refilling, led to the overall redshift and short-term fluctuations in spectra. The stress and Fourier transform infrared analysis revealed that water adsorption and ice-like/ice structure water was responsible for the increasing compressive stress evolution at the first stage and subsequent transition from compressive to tensile stress evolution, respectively. A model based on the finite element method was proposed to calculate the water vapor transmission rate of e-beam coatings. Our study on the underlying mechanism of moisture-induced instability of e-beam coating is helpful to ensure the reliability and stability of large laser systems.

The role of service-induced residual stresses in initiating and propagating stress corrosion cracking (SCC) in a 316 stainless steel pressure-relief-valve nozzle set

A full failure analysis was applied to a set of failed stainless steel (SS) nozzle and sealing disk. The set was taken from a pressure relief valve (PRV) installed on a piping carrying hydrocarbon gas. The investigation included both experimental and numerical analysis. Experimental analysis confirmed the failure mode in the present case to be high-stress-regime transgranular stress corrosion cracking (TGSCC) promoted by susceptible microstructure and the presence of a chloride- and H2S-bearing working fluid. Finite element modelling (FEM) was used for the numerical analysis to model the evolution of tensile residual stress associated with the observed SCC. FEM results have shown that the tensile stresses, required for crack initiation and propagation, were associated with the service conditions, especially thermal history, of the nozzle set. FEM results have shown very close (almost exact) agreement with the experimental results obtained in terms of crack directions, relative stress levels and locations of dominance for each type of stress. The present results are thought to be very useful in designing/re-designing similar parts for better performance, under extreme thermal conditions, and for avoiding such failure in future.

On the influence of basic thermodynamics on thermal cracking resistance of asphalt mixtures in cooling tests

The cooling test or Thermal Stress Restrained Specimen Test (TSRST) simulates fully restrained pavements, as they occur in field for laboratory assessment of the thermal cracking resistance of asphalt mixtures. In the TSRST, cryogenic stress builds up due to cooling and prevented shrinkage until the tensile strength of the specimen is exceeded and the specimen fails by cracking. By carrying out TSRST various viscoelastic parameters, e.g. relaxation, evolution of tensile stresses, and tensile strength can be analyzed. Thus, a comprehensive view on the low temperature performance is possible. Standard TSRST is controlled by setting the cooling rate of the air within the chamber at a fixed value, e.g. -10°C/h. In thermodynamics, the actual cooling rate of objects is not only influenced by the cooling but also by external conditions like humidity, air velocity, radiation condition, etc. A current study investigates the impact of additional cooling parameters rather than just the air cooling rate. Two test machines of the same manufacturer that differ in the year of production and the setup of the climate chamber are compared. An initial wide scatter of test results from the two devices could be explained by taking thermodynamics into account and the reproducibility could be significantly enhanced.

FE temperature- and residual stress prediction in milling inserts and correlation with experimentally observed damage mechanisms

In milling applications thermal and mechanical loadings are affecting the damage behavior of milling inserts. There are several open questions regarding the influence of loading and tool temperature on inelastic deformations and damage mechanisms. The aim of the current work is to simulate industrial milling processes with the finite element method and generate knowledge about the acting damage mechanisms. The validation of the results is made by two recently reported experimental milling setups. The focus of the simulations is set on investigations of the evolution of tensile residual stresses orientated parallel to the cutting edge of a milling insert. These tensile residual stresses foster the formation and growth of so-called comb cracks growing in planes perpendicular to the cutting edge which are detrimental to the performance of the tool. The insert is made of WC-Co hard metal with 8 wt.% Co-binder and an average WC grain size of 1 μm. It is coated with a 7 μm thick TiAlN layer acting as a thermal shield. The workpiece material is 42CrMo4, described by a Johnson-Cook constitutive material model. The milling process is modeled with a 2D Arbitrary Lagrangian-Eulerian (ALE) approach. Results of the 2D simulations are used to generate the temperature and contact load imposed on a 3D solid-model of the milling insert that is in turn used to predict the evolution of stress and temperature over 50 milling cycles. The simulations reproduce a shift of residual stresses toward tension in the milling insert at the same location as observed in experiments from which one failed due to comb cracks and one due to wear.

Spalling behaviors of Pb induced by ramp-wave-loading: Effects of the loading rise time studied by molecular dynamics simulations

Spalling processes under ramp-wave loading are investigated by molecular dynamics simulations. The simulations reveal that spalling is greatly influenced by the loading rise time of the ramp-wave. It is discovered that, in spalling induced by ramp-wave loading, whether the surface layer is in solid state or in liquid state is tightly related to the rise time of the loading wave. In cases of shorter rise time, the surface layer may be in liquid state. In cases of longer rise time, the surface layer keeps in solid state even when the peak loading pressure exceeds the threshold of melting in compression. The formation of solid surface layer is due to that the maximum compression pressure experienced by materials near the free surface is smaller than the threshold of melting as the rise time increases to be long enough. The result provides a reasonable explanation for some controversy about the state (liquid or solid) of the surface layer when the peak pressure exceeds the threshold of melting. The rise time effects on the tensile stress evolution and the cavitation process are also discussed. Comparisons with experiments and other computer simulations are made where necessary.

Evolution of mechanical properties for a dual-phase steel subjected to different loading paths

The evolution of the mechanical properties of a dual-phase (DP590) steel sheet after being prestrained by uniaxial tension, plane strain and equal biaxial stretching was investigated. Specimens were first loaded using the three prestraining modes. Then, from the prestrained specimens, a few sub-sized samples were machined along the rolling direction and the transverse direction for further uniaxial tension testing. Six loading paths were provided. Equal biaxial stretching was performed using a cruciform specimen. The evolution of work hardening performance, elastic modulus, yield stress and tensile stress under the six loading paths were discussed in detail. The results indicate that loading paths can affect the latent work hardening performances, strain hardenability, yield stress and tensile stress evolution as well as the elastic modulus decrease during plastic deformation. The uniaxial tension–uniaxial tension path results in a cross-softening phenomenon, the largest yield stress enhancement and a mild maximum tensile stress increase. The equal biaxial stretching-uniaxial tension path leads to a cross-hardening phenomenon, the least yield stress enhancement and the largest tensile strength increase maximum tensile strength. The elastic modulus of DP590 steel not only changes with the accumulated plastic strain but also varies with the loading paths. The largest decrease of the elastic modulus equal biaxial stretching–uniaxial tension can reach 12.7% beyond 8% equivalent strain, which is 5.2% greater than that in the monotonic uniaxial tension path.

Nanoparticle generation and transport resulting from femtosecond laser ablation of ultrathin metal films: Time-resolved measurements and molecular dynamics simulations

The synthesis of metal nanoparticles by ultrafast laser ablation of nanometers-thick metal films has been studied experimentally and computationally. Near-threshold backside laser ablation of 2–20 nm-thick Pt films deposited on fused silica substrates was found to produce nanoparticles with size distributions that were bimodal for the thicker films, but collapsed into a single mode distribution for the thinnest film. Time-resolved imaging of blackbody emission from the Pt nanoparticles was used to reveal the nanoparticle propagation dynamics and estimate their temperatures. The observed nanoparticle plume was compact and highly forward-directed with a well-defined collective velocity that permitted multiple rebounds with substrates to be revealed. Large-scale molecular dynamics simulations were used to understand the evolution of compressive and tensile stresses in the thicker melted liquid films that lead to their breakup and ejection of two groups of nanoparticles with different velocity and size distributions. Ultrafast laser irradiation of ultrathin (few nm) metal films avoids the splitting of the film and appears to be a method well-suited to cleanly synthesize and deposit nanoparticles from semitransparent thin film targets in highly directed beams.

Stress evolution during growth of GaN (0001)/Al2O3(0001) by reactive dc magnetron sputter epitaxy

We study the real time stress evolution, by in situ curvature measurements, during magnetron sputter epitaxy of GaN (0 0 0 1) epilayers at different growth temperatures, directly on Al2O3(0 0 0 1) substrates. The epilayers are grown by sputtering from a liquid Ga target in a mixed N2/Ar discharge. For 600 °C, a tensile biaxial stress evolution is observed, while for 700 °C and 800 °C, compressive stress evolutions are observed. Structural characterization by cross-sectional transmission electron microscopy, and atomic force microscopy, revealed that films grew at 700 °C and 800 °C in a layer-by-layer mode while a growth temperature of 600 °C led to an island growth mode. High resolution x-ray diffraction data showed that edge and screw threading dislocation densities decreased with increasing growth temperature, with a total density of 5.5 × 1010 cm−2 at 800 °C. The observed stress evolution and growth modes are explained by a high surface mobility during magnetron sputter epitaxy at 700–800 °C. Other possible reasons for the different stress evolutions are also discussed.

Origins of saccharin-induced stress reduction based on observed fracture behavior of electrodeposited Ni films

This research presents experimental results of an investigation aimed at understanding grain size driven mechanical processes in electrodeposited Ni thin films where saccharine additions are commonly used to improve mechanical properties. Ni films were fabricated using salfamate-based electro chemical baths, where it is empirically known that mmol/l concentrations of saccharine will reduce the observed tensile stress in addition to lowering the grain size up to a few nanometer scales. Some previous observations and several theoretical models suggest that saccharine incorporation results in sulfur segregation at grain boundaries. Since grain boundary formation is also associated with tensile stress evolution, a plausible hypothesis is that saccharine additions are directly altering grain boundary energetics. This suggests that saccharine additions should also have an observable effect on intergranular fracture in these films. To test this prediction, in situ stress measurements during film growth and fracture testing of these same films were compared. Lithographically patterned substrates were used to produce films with ordered arrays of uniform islands, which demonstrated island size effects on stress evolution, and enabled a well-defined notch geometry along one of the island boundaries to facilitate fracture experiments. In situ uniaxial tensile testing under in a scanning electron microscope was then used to obtain the fracture strength of such specimens. This technique provided real time recording of microscopic deformation during uniaxial tensile loading. The observed relationships among residual stress, grain size, and fracture strength were then analyzed with detailed models of both film growth and fracture.

Flattened Brazilian Disc Method for Determining the Dynamic Tensile Stress-Strain Curve of Low Strength Brittle Solids

An indirect tensile testing method is proposed to measure the full dynamic tensile stress-strain curve of low strength brittle solids. In this method, the flattened-Brazilian disc (FBD) sample is loaded by modified split Hopkinson pressure bars (SHPB) system. Low amplitude dynamic forces were measured with a pair of piezoelectric force transducers embedded in the incident bar and the transmitted bar. The evolution of tensile stress at the center of the disc sample was determined through finite element analyses using the measured stress in SHPB as inputs. In a traditional Brazilian test, a strain gauge is mounted at the center of the specimen to measure the tensile strain, which is difficult to apply for low strength brittle materials. Thus, two types of non-contact methods, the Digital Image Correlation (DIC) technique and the Laser Gap Gauge (LGG), were used to measure the strain. The DIC method was used to monitor the displacement and the strain map of the specimen during the test, from which the strain at the center of the specimen can be obtained. The accuracy of the DIC results was assessed, and the displacement and strain uncertainties of our system were 0.003 mm and 0.003, respectively. LGG was used to monitor the expansion of the disc perpendicular to the loading axis, from which the average tensile strain is deduced. The numerical simulation revealed that the tensile strain at the center of the specimen is proportional to the average tensile strain and that the ratio is not sensitive to the material elastic parameters. The strain measured through LGG was compared with that measured by the DIC method using photos captured with a synchronized high-speed camera. The result of the LGG method was 20 % smaller than that of the DIC process. However, the latter was limited by the number of frames of the high-speed camera. The feasibility of this methodology was demonstrated using a polymer-bonded explosive (PBX).

Oxide growth stress measurements and relaxation mechanisms for alumina scales grown on FeCrAlY

AbstractEarly‐stage tensile stress evolution in α‐Al2O3 scales during oxidation of FeCrAlY at 1000, 1050, 1100, and 1200 °C was monitored in situ by use of synchrotron radiation. Tensile stress development as a function of oxidation temperature indicated a dynamic interplay between stress generation and relaxation. An analysis of the time dependence of the data indicated that the observed relaxation of the initial tensile stress in the oxide scales at 1100 and 1200 °C is dominated by creep in the α‐Al2O3. A thin layer of a (Fe,Cr,Al) oxide was observed at the oxide‐gas interface, consistent with a mechanism whereby the conversion of (Fe,Cr,Al)2O3 to α‐Al2O3 produces an initial tensile stress in the alumina scale.

Seismicity during lateral dike propagation: Insights from new data in the recent Manda Hararo–Dabbahu rifting episode (Afar, Ethiopia)

Seismicity released during lateral dike intrusions in the Manda Hararo–Dabbahu Rift (Afar, Ethiopia) provides indirect insight into the distribution and evolution of tensile stress along this magma‐assisted divergent plate boundary. In this paper, 5 dike intrusions among the 14 that form the 2005–present rifting episode are analyzed with local and regional seismic data. During dike intrusions, seismicity migrates over distances of 10–15 km at velocities of 0.5–3.0 km/h away from a single reservoir in the center of the rift segment, confirming the analogy with a slow spreading mid‐ocean ridge segment. Comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression. Dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward, suggesting an asymmetry of tension in the brittle‐elastic lithosphere. Seismicity during dike injections is concentrated near the propagating crack front. In contrast, faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions. The seismic energy released during dike intrusions in the Manda Hararo Rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion. The low level of seismic energy accompanying dike intrusions, despite their significant volumes, is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary.

First-principles study of void induced stresses at a diamond (100) grain boundary

Density functional theory was used to evaluate the impact of small voids on grain boundary densification in polycrystalline diamond films. The results provide atomic scale insight into tensile stress evolution during polycrystalline film growth, where prior modeling has been largely based on continuum descriptions. Diamond is an ideal material for these studies because the atomic mobility is extremely low and thus a variety of other mechanisms that influence stress evolution can be safely ignored. For the boundary configurations that were investigated, the results indicate that significantly more densification occurs when missing atoms at grain boundaries are clustered together to form nanovoids. Increased densification also occurs with a configuration where missing atoms are in close proximity, but not directly adjacent to each other. Calculations with hydrogen trapped in the nanovoids indicate that repulsive forces can induce compressive stresses instead.

Tensile Stress Evolution During the Early-Stage Constrained Sintering of Gadolinium-Doped Ceria Films

In situ measurements during the constrained sintering of Gd-doped ceria reveal tensile stresses up to ∼250 MPa. These large tensile stresses are likely to contribute to the reduced densification (compared with freely sintered material) typically observed during constrained sintering. While existing models postulate that the tensile stress in a densifying constrained film cannot exceed the “sintering stress,”σS, the observed tensile stresses are significantly larger than the estimated σS for these materials. To explain this observation, we propose that the formation and extension of interparticle grain boundaries induce substantial tensile stresses in constrained films. A model of this phenomenon shows that converting excess surface energy to elastic strain energy can produce stresses that are comparable to the measured values. Further, if these “cohesive” stresses exceed σS, grain-boundary diffusion should initially move material from the neck regions into the grain boundaries, not out of the grain boundaries as described by traditional sintering models.

Effect of Current Density on the Internal Stress Evolution during Galvanostatic Ti Thin Film Anodizing

A high resolution curvature measurement technique was used for investigating in situ the internal stresses developing during galvanostatic anodization of titanium thin films. The titanium electrodes were anodized in a 0.1 M electrolyte, with current densities ranging from 0.5 to . Two distinct stages were observed in the evolution of the cell voltage with time: a first, low efficiency growth stage and a second near 100% efficiency growth stage. The transition between both stages systematically occurred at 6.0 V, independent of current density. In terms of internal stresses, these two stages correspond to two distinct regimes as well. In the first stage, compressive internal stresses were observed on the order of several gigapascals, which increased with decreasing current density. In the second stage, a tensile instantaneous stress component was found with an average value of 237 MPa, independent of current density. For the first stage, oxygen evolution inside the oxide film was responsible for both the low efficiency growth and the large compressive stresses. The tensile instantaneous stress evolution in the second stage was rationalized in terms of the rate of net free volume generation at the metal/oxide interface.

Effect of heterogeneity of brittle rock on micromechanical extensile behavior during compression loading

A grain‐based Universal Distinct Element Code model was developed to generate a deformable polygonal grain‐like structure to simulate the microstructure of brittle rock. It takes into account grain‐scale heterogeneity including microgeometric heterogeneity, grain‐scale elastic heterogeneity, and microcontact heterogeneity. The microgeometric heterogeneity can be used to match the grain size distribution of the rock. The discrete element approach is able to simulate the microheterogeneity caused by elastic variation and contact stiffness anisotropy. The modeling approach was evaluated using Lac du Bonnet granite and Äspö Diorite. The microheterogeneity played an important role in controlling both the micromechanical behavior and the macroscopic response when subjected to uniaxial compression loading. The crack‐initiation stress was found to be controlled primarily by the microscale geometric heterogeneity, while the microcontact heterogeneity controlled the strength characteristics. The effect of heterogeneity on the distribution and evolution of tensile stresses and associated extension cracks was also examined.