Strain Rate (strain + rate)

Distribution by Scientific Domains
Distribution within Polymers and Materials Science

Kinds of Strain Rate

  • constant strain rate
  • diastolic strain rate
  • different strain rate
  • high strain rate

  • Terms modified by Strain Rate

  • strain rate effects
  • strain rate imaging
  • strain rate sensitivity

  • Selected Abstracts


    Association of Left Atrial Strain and Strain Rate Assessed by Speckle Tracking Echocardiography with Paroxysmal Atrial Fibrillation

    ECHOCARDIOGRAPHY, Issue 10 2009
    Wei-Chuan Tsai M.D.
    Background: We hypothesized that contraction of the LA wall could be documented by speckle tracking and could be applied for assessment of LA function. This study tried to identify the association between LA longitudinal strain (LAS) and strain rate (LASR) measured by speckle tracking with paroxysmal atrial fibrillation (PAF). Methods: Fifty-two patients (61 ± 17 years old, 23 men) with sinus rhythm at baseline referred for the evaluation of episodic palpitation were included. Standard four-chamber and two-chamber views were acquired and analyzed off-line. Peak LAS and LASR were carefully identified as the peak negative inflection of speckle tracking waves after P-wave gated by electrocardiography. Results: Ten patients (19%) had PAF. LAS, LASR, age, left ventricular end-diastolic dimension, left ventricular mass, LA volume, and mitral early filling-to-annulus early velocity ratio were different between patients with and without PAF. After multivariate analysis, LASR was significantly independently associated with PAF (OR 8.56, 95% CI 1.14,64.02, P = 0.036). Conclusion: Speckle tracking echocardiography could be used in measurements of LAS and LASR. Decreased negative LASR was independently associated with PAF. [source]


    Horizontal Strain Rate in Relation to Vein Formation of the Hishikari Gold Deposits, Southern Kyushu, Japan

    RESOURCE GEOLOGY, Issue 1 2001
    Tadakazu UTO
    Abstract: The Hishikari deposits comprise the Honko (Main), Yamada, and Sanjin deposits. The horizontal strain in the direction (approx. N40°W normal to the general NE-SW strike of the Hishikari vein system was calculated, based on the measured total vein widths to the distance along three crosscuts. Veins were assumed to accompany no significant fault displacement in the calculation. Veins in the eastern and the middle parts of the Honko-Sanjin deposits spatially occupy 3. 2% and 1. 3%, respectively, and veins in the Yamada deposit occupy 6. 7%. Significant local variation of strain is observed in some areas. Reported K-Ar ages on adularia-quartz veins indicate the duration of vein opening to be 2. 6 × 105 yr in the Honko-Sanjin deposits and 5. 9 × 105 yr in the Yamada deposit. Horizontal strain rates were calculated to be 5. 0,12 × 10 -8 yr -1through the Hishikari deposits. The calculated strain rates at the Hishikari deposits are roughly comparable to the regional horizontal strain rate in the Recent. Widespread extensional movement in southern Kyushu seems to have been able to provide sufficient strain for the formation of the Hishikari deposits, rather than contribution of local movements. [source]


    Assessment of left ventricular systolic function using tissue Doppler imaging in children after successful repair of aortic coarctation

    CLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 1 2010
    Tomasz Florianczyk
    Summary Aim:, Assessment of left ventricular systolic function in children after the successful repair of aortic coarctation using tissue Doppler imaging (TDI). Methods:, The study group consisted of 32 patients (mean age 12·0 ± 4·2 years) after the aortic coarctation repair. The TDI parameters and the conventional echocardiographic endocardial and midwall indices of the left ventricular systolic function were analysed and compared with the results obtained from 34 healthy children. Results:, The systolic mitral annulus motion velocity, systolic myocardial velocity of the medial segment of the left ventricular septal wall, left ventricular strain and Strain Rate (SR) in the study group were significantly higher than in the control group, respectively: 6·92 ± 0·75 cm s,1 versus 6·45 ± 0·83 cm s,1; 5·82 ± 1·03 cm s,1 versus 5·08 ± 1·11 cm s,1; ,28·67 ± 6·04% versus ,22·53 ± 6·44% and ,3·20 ± 0·76 s,1 versus ,2·39 ± 0·49 s,1. Except midwall shortening fraction the conventional endocardial and midwall echocardiographic indices in the study group were significantly higher in comparison to the healthy controls. The left ventricular systolic meridional fibre stress and end-systolic circumferential wall stress did not differ between the examined groups. There were no differences of the TDI or conventional parameters between hypertensive and normotensive patients. Conclusions:, Left ventricular systolic performance in children after the surgical repair of aortic coarctation reveals tendency to rise in late follow-up despite a satisfactory result after surgery. Higher systolic strain and SR in children treated due to coarctation of the aorta may suggest the increased preserved left ventricular performance despite normalization of afterload. [source]


    Hochdynamische Materialeigenschaften von Ultrahochleistungsbeton (UHPC)

    BETON- UND STAHLBETONBAU, Issue 11 2009
    Markus Nöldgen Dipl.-Ing.
    Baustoffe; Versuche; Dynamische Einwirkungen Abstract Der vorliegende Bericht liefert einen Beitrag zur Werkstoffbeschreibung von Ultrahochleistungsbetonen unter hochdynamischer Belastung. Grundlage für die Ermittlung der Werkstoffeigenschaften ist eine Hopkinson-Bar Versuchsreihe, die Werte für die dynamische Zugfestigkeit, den dynamischen E-Modul und die dynamische Bruchenergie unter Dehnraten von 102 · s,1 liefert. Ein Vergleich mit den Ergebnissen dieser Parameter für Normalbeton und hochfesten Betonen ermöglicht eine Einordnung des Ultrahochleistungsbetons in etablierte Ansätze und Berechnungsvorschriften und liefert einen Bewertungsansatz für die maßgebenden mechanischen Vorgänge. Unter Einbeziehung der Bruchenergie und der Spannungs-Rissöffnungs-Beziehung für UHPC in das RHT-Betonmodell werden Hydrocode Simulationen durchgeführt, die den Versuch am Hopkinson-Bar adäquat abbilden können. Material Properties of Ultra High Performance Concrete (UHPC) at High Strain Rates The presented paper is a contribution to the material description of Ultra High Performance Concrete (UHPC) under high-speed dynamical loading conditions. Based on a series of Hopkinson-Bar experiments dynamical material parameters such as the Tensile Strength, Young's Modulus and Fracture Energy are derived at high strain rates of 102 · s,1. A comparison with the results of these parameters for normal and high strength concrete leads to a qualitative and quantitative evaluation of UHPC at high strain rates. With the extension of the established RHT material model for UHPC by the material's Fracture Energy and Stress-Crack-Opening-Relation the Hopkinson-Bar experiments are simulated appropriately. [source]


    Detection of Subclinical Cardiac Involvement in Systemic Sclerosis by Echocardiographic Strain Imaging

    ECHOCARDIOGRAPHY, Issue 2 2008
    Alper Kepez M.D.
    Background: Cardiac involvement is one of the major problems in systemic sclerosis (SSc). Subclinical cardiac involvement has a higher frequency than thought previously. In this study we investigated whether subclinical cardiac involvement can be detected by using echocardiographic strain imaging in SSc patients without pulmonary hypertension. Methods: Echocardiographic examinations were performed to 27 SSc patients and 26 healthy controls. Left ventricular strain parameters were obtained from apical views and average strain value was calculated from these measurements. Results: There were no significant differences between patients and controls regarding two-dimensional (2D), conventional Doppler and tissue Doppler velocity measurements. Strain was reduced in 6 of 12 segments of the left ventricle (LV) and in 1 of 2 segments of the right ventricle (RV). Strain rate (SR) was reduced in 2 of 12 segments of the LV and 1 of 2 segments of the RV in SSc patients as compared to controls (P < 0.05 for all). These involvements did not match any particular coronary artery distribution. More important differences were detected by average strain and SR values of the LV between patients and controls (19.78 ± 3.00% vs 23.41 ± 2.73%, P < 0.001; 2.01 ± 0.41 vs 2.23 ± 0.27/sec, P = 0.026, respectively). Furthermore, carbon monoxide diffusion capacity (DLCO) in scleroderma patients significantly correlated with LV average strain (r = 0.59; P = 0.001). Conclusion: Evaluation of ventricular function by using echocardiographic strain imaging appears to be useful to detect subclinical cardiac involvement in SSc patients with normal standard echocardiographic and tissue Doppler velocity findings. [source]


    Strain rate mediated microstructure evolution for extruded poly(vinylidene fluoride) polymer films under uniaxial tension

    JOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2007
    F. Fang
    Abstract The deformation and fracture mechanism during uniaxial tension under controlled strain rates are investigated for extruded poly(vinylidene fluoride) (PVDF) polymer films at room temperature. It was found that both the longitudinal and transversal film-samples exhibited pronounced strain rate effect, that is, the yield stress increases while the fracture strain decreases with the increasing of strain rates. For the longitudinal film samples, phase transformation from the nonpolar ,-phase to the polar ,-phase occurs during the uniaxial tension, and the extent of the phase transformation enhances when the strain rate decreases. For the transversal film samples, no phase transformation was detected in all tested strain rates. By combining the stress,strain behavior and the X-ray results, it can be inferred that the conformational change from , to , phase during uniaxial tension contributes to the higher fracture strain of the longitudinal films than that of the transversal films. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1786,1790, 2007 [source]


    Strain rates from snowball garnet

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2003
    C. Biermeier
    Abstract Spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals, during syn-metamorphic deformation. Thus, they contain information on the strain rate of the rock. Strain rates may be interpreted from such inclusion trails if two functions are known: (1) The relationship between rotation rate and shear strain rate; (2) the growth rate of the crystal. We have investigated details of both functions using a garnetiferous mica schist from the eastern European Alps as an example. The rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock. The growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals, thermodynamic pseudosections and information on the grain size distribution. For the largest porphyroblast size fraction (size L=12 mm) we constrain a growth interval between 540 and 590 °C during the prograde evolution of the rock. Assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of =6.6 × 10,14 s,1. These estimates are consistent with independent estimates for the strain rates during the evolution of this part of the Alpine orogen. [source]


    Origin of the in situ stress field in south-eastern Australia

    BASIN RESEARCH, Issue 3 2004
    Mike Sandiford
    The in situ stress field of south-eastern Australia inferred from earthquake focal mechanisms and bore-hole breakouts is unusual in that it is characterised by large obliquity between the maximum horizontal compressive stress orientation (SHmax) and the absolute plate motion azimuth. The evolution of the neotectonic strain field deduced from historical seismicity and both onshore and offshore faulting records is used to address the origin of this unusual stress field. Strain rates derived from estimates of the seismic moment release rate (up to ,10,16 s,1) are compatible with Quaternary fault,slip rates. The record of more or less continuous tectonic activity extends back to the terminal Miocene or early Pliocene (10,5 Ma). Terminal Miocene tectonic activity was characterised by regional-scale tilting and local uplift and erosion, now best preserved by unconformities in offshore basins. Plate-scale stress modelling suggests the in situ stress field reflects increased coupling of the Australian and Pacific Plate boundary in the late Miocene, associated with the formation of the Southern Alps in New Zealand. [source]


    Collapse of Reinforced Concrete Column by Vehicle Impact

    COMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 6 2008
    Hing-Ho Tsang
    The column slenderness ratio can be in the order of 6,9. Some of these buildings are right next to busy streets and hence continuously exposed to the potential hazard of a vehicle impacting on a column in an accident. In the early part of this study, the ultimate energy absorption capacity of a reinforced concrete column is compared to the kinetic energy embodied in the moving vehicle. The energy-absorption capacity is calculated from the force-displacement curve of the column as determined from a nonlinear static (push-over) analysis. The ultimate displacement of the column is defined at the point when the column fails to continue carrying the full gravitational loading. Results obtained from the nonlinear static analysis have been evaluated by computer simulations of the dynamic behavior of the column following the impact. Limitations in the static analysis procedure have been demonstrated. The effects of strain rate have been discussed and the sensitivity of the result to changes in the velocity function and stiffness of the impacting vehicle has also been studied. [source]


    Work-hardening characteristics of Zn-Ti alloy single crystals

    CRYSTAL RESEARCH AND TECHNOLOGY, Issue 2 2010
    G. Boczkal
    Abstract Shear stress , shear strain curves of 0.14 at.%Ti alloyed Zn single crystals were measured in compression at different temperatures and shear strain rates. The work-hardening coefficient for basal slip increases with decreasing temperature and increasing shear strain rate. The work-hardening characteristics are compared with those reported for Zn single crystals with different constituents and purities. It is discussed with respect to the interaction of dislocations with dislocations, vacancies, vacancy agglomerates and solute atoms. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Dynamic systems with high damping rubber: Nonlinear behaviour and linear approximation

    EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 13 2008
    Andrea Dall'Asta
    Abstract High damping rubber (HDR) shows a quite complex constitutive behaviour, which is nonlinear with respect to strain and is dependent on the strain rate. In addition, it exhibits a transient response during which the material properties change (scragging or more generally the Mullins effect). A number of recent works were dedicated to analysing and modelling material behaviour. This paper studies the nonlinear dynamics of systems with restoring force produced by HDR-based devices in order to propose a procedure to define equivalent linear models considering both transient and stationary behaviours. The reliability of these linear models is tested by evaluating the upper and lower bounds of the seismic response of a structural system equipped with HDR-based devices (structural system with dissipative bracings and isolated systems). Copyright © 2008 John Wiley & Sons, Ltd. [source]


    Relationship between Strain Rate Imaging and Coronary Flow Reserve in Assessing Myocardial Viability after Acute Myocardial Infarction

    ECHOCARDIOGRAPHY, Issue 8 2010
    Ph.D., Seong-Mi Park M.D.
    Objectives: To evaluate the relationship between strain rate (SR) imaging and coronary flow reserve (CFR) in assessing viability of akinetic myocardium after acute myocardial infarction (MI). Methods: Forty patients with acute first ST-elevation MI were analyzed. SR imaging and CFR by intracoronary flow measurement were obtained on the same day, 3,5 days after primary percutaneous coronary intervention. Viability of the akinetic myocardium was determined on 6-week echocardiography. Results: Systolic SR (SRs, ,0.42 ± 0.10 vs. ,0.35 ± 0.11 per second, P = 0.03), early diastolic SR (SRe, 0.68 ± 0.31 vs. 0.41 ± 0.22 per second, P = 0.003), and systolic strain (Ss, ,5.9 ± 3.4 vs. ,2.5 ± 4.0%, P = 0.04) were greater in akinetic, but viable myocardium of 21 patients than in akinetic and nonviable myocardium of 19 patients. CFR was also higher in patients with akinetic, but viable myocardium (2.0 ± 0.5 vs. 1.5 ± 0.5, P < 0.001). SRs, SRe, and Ss were significantly related to CFR (r =,0.50, r = 0.58, r =,0.56, respectively, all P , 0.001) and SRe was most related to CFR (P < 0.001). The sensitivity and specificity to predict myocardial viability were 85.7% and 68.4% for CFR (cutoff = 1.75), and 90.5% and 57.9% for SRe (cutoff = 0.37 per second), respectively. Conclusions: The degree of myocardial deformation determined by SR imaging was related to the degree of microvascular integrity determined by CFR, and can be used as a noninvasive method to predict myocardial viability after acute MI. (Echocardiography 2010;27:977-984) [source]


    Association of Left Atrial Strain and Strain Rate Assessed by Speckle Tracking Echocardiography with Paroxysmal Atrial Fibrillation

    ECHOCARDIOGRAPHY, Issue 10 2009
    Wei-Chuan Tsai M.D.
    Background: We hypothesized that contraction of the LA wall could be documented by speckle tracking and could be applied for assessment of LA function. This study tried to identify the association between LA longitudinal strain (LAS) and strain rate (LASR) measured by speckle tracking with paroxysmal atrial fibrillation (PAF). Methods: Fifty-two patients (61 ± 17 years old, 23 men) with sinus rhythm at baseline referred for the evaluation of episodic palpitation were included. Standard four-chamber and two-chamber views were acquired and analyzed off-line. Peak LAS and LASR were carefully identified as the peak negative inflection of speckle tracking waves after P-wave gated by electrocardiography. Results: Ten patients (19%) had PAF. LAS, LASR, age, left ventricular end-diastolic dimension, left ventricular mass, LA volume, and mitral early filling-to-annulus early velocity ratio were different between patients with and without PAF. After multivariate analysis, LASR was significantly independently associated with PAF (OR 8.56, 95% CI 1.14,64.02, P = 0.036). Conclusion: Speckle tracking echocardiography could be used in measurements of LAS and LASR. Decreased negative LASR was independently associated with PAF. [source]


    A Novel Approach to Assess Aortic Stiffness Related to Changes in Aging Using a Two-Dimensional Strain Imaging

    ECHOCARDIOGRAPHY, Issue 9 2008
    Yoshifumi Oishi M.D.
    Background: Recently, it has been demonstrated that aortic stiffness is associated with cardiovascular morbidity and mortality. The objective of the present study was to accurately evaluate the aortic stiffness relative to the changes in aging using two-dimensional (2D) strain imaging in 39 comparatively normal patients (15,85 years). Methods: We obtained short-axis images of the abdominal aorta (Ao) and determined the peak circumferential strain (Ao-S) and strain rate (Ao-SR) and the time from Q-wave of electrocardiogram to peak Ao-S using the 2D strain imaging. The stiffness parameters ,1 and ,2 of the abdominal aorta were measured using M-mode ultrasonography and 2D strain imaging, respectively. Results: The stiffness parameters ,1 and ,2 correlated significantly with age (r=0.51, P < 0.001 and r=0.69, P < 0.0001, respectively), particularly the latter parameter ,2. The peak circumferential Ao-S and Ao-SR correlated strongly with age (r=,0.79, P < 0.0001 and r=,0.87, P < 0.0001, respectively). The stiffness parameter ,1 was significantly greater in the old-aged group (>60 years) than in the young-aged group (<30 years). The peak circumferential Ao-S and Ao-SR were significantly lower in the middle-aged (30,60 years) and old-aged groups than in the young-aged group. Conclusion: The aortic circumferential strain and strain rate measured by 2D strain imaging allow simple and accurate determination of the aortic stiffness. [source]


    Cost-Affordable Technique Involving Equal Channel Angular Pressing for the Manufacturing of Ultrafine Grained Sheets of an Al,Li,Mg,Sc Alloy,

    ADVANCED ENGINEERING MATERIALS, Issue 8 2010
    Rustam Kaibyshev
    A two-step process consisting of modified equal channel angular pressing (ECAP) and subsequent isothermal rolling (IR) was developed to produce thin sheets of aluminum alloys with ultra-fine grained (UFG) structure. Significant increase in the efficiency of ECAP was attained by using flat billets and a back pressure system. The incorporation of final IR into technologic route provides a reduced strain which is necessary to impose for the fabrication of thin sheets with UFG structure. In addition, it allows producing relatively "long billets." In order to demonstrate the feasibility of this technique an Al,5.1Mg,2.1Li,0.17Sc,0.08Zr (wt %) alloy was subjected to ECAP at 325,°C to a total strain of ,8 using processing route CX. The operation time of this processing did not exceed 15,min. Subsequent IR at the same temperature with a total reduction of 88% was applied to produce thin sheets with a 1.8,mm thickness and an average size of recrystallized grains of ,1.6,µm. These sheets exhibit extraordinary high superplastic ductilities. In addition, this material demonstrated almost isotropic mechanical behavior at room temperature. The maximum elongation-to-failure of ,2700% was attained at a temperature of 450,°C and an initial strain rate of 1.4,×,10,2 s,1. Thus it was demonstrated that the two-step processing consisting of ECAP with a back pressure followed by IR was a simple technique providing potential capability for the fabrication of superplastic sheets from an Al,Mg,Li,Sc alloy on a commercial scale. [source]


    Strategies for Improving Tensile Ductility of Bulk Nanostructured Materials,

    ADVANCED ENGINEERING MATERIALS, Issue 8 2010
    Yonghao Zhao
    Abstract The low ductility that is consistently associated with bulk nanostructured (NS) materials has been identified as perhaps the single most critical issue that must be resolved before this novel class of materials can be used in a wide variety of applications. Not surprisingly, a number of published studies, published mostly after 2000, identify the issue of low ductility and describe strategies to improve ductility. Details of these strategies were discussed in review papers published by Koch and Ma in 2005 and 2006, respectively.15,16 In view of continued efforts and recent results, in this paper we describe progress in attempting to address the low ductility of NS materials, after 2006. We first analyze the fundamental reasons for the observed low ductility of bulk NS materials, and summarize early (prior to 2006) attempts to enhance the ductility of bulk NS materials, which often sacrificed the strength. Then, we review recent progress in developing strategies for improving the tensile ductility of bulk NS materials, which involve mainly microstructure modifications, after 2006. Different from early efforts, these new strategies strive to increase the tensile ductility while increasing/maintaining the strength simultaneously. In addition, the influence of tensile testing conditions, including temperature, strain rate, tensile specimen size and geometry, and strain measurement methods, on tensile ductility of NS materials will also be reviewed. Finally, we identify several issues that will require further, in depth analysis in the future. [source]


    Modeling of Hot Ductility During Solidification of Steel Grades in Continuous Casting , Part I,

    ADVANCED ENGINEERING MATERIALS, Issue 3 2010
    Dieter Senk
    The present paper gives an overview of the simultaneous research work carried out by RWTH Aachen University and ThyssenKrupp Steel Europe AG. With a combination of sophisticated simulation tools and experimental techniques it is possible to predict the relations between temperature distribution in the mould, solidification velocity, chemical steel composition and, furthermore, the mechanical properties of the steel shell. Simulation results as well as experimentally observed microstructure parameters are used as input data for hot tearing criteria. A critical choice of existing hot tearing criteria based on different approaches, like critical strain and critical strain rate, are applied and developed. The new "damage model" is going to replace a basic approach to determine hot cracking susceptibility in a mechanical FEM strand model for continuous slab casting of ThyssenKrupp Steel Europe AG. Critical strains for hot cracking in continuous casting were investigated by in situ tensile tests for four steel grades with carbon contents in the range of 0.036 and 0.76,wt%. Additionally to modeling, fractography of laboratory and industrial samples was carried out by SEM and EPMA and the results are discussed. [source]


    Hot Deformation Mechanisms and Microstructural Control in High-Temperature Extruded AZ31 Magnesium Alloy,

    ADVANCED ENGINEERING MATERIALS, Issue 7 2007

    A Processing map has been developed for Magnesium alloy AZ31 extruded at 450,°C which revealed that the extruded rods may be further processed into components industrially at 400,°C and at a strain rate of 10,s,1. If processed at lower strain rates in the vicinity of 0.1,s,1, unusual grain size variations with temperature and strain rate are observed, suggesting that grain size control will be difficult. [source]


    Negative Strain-rate Sensitivity in a Nanostructured Aluminum Alloy,

    ADVANCED ENGINEERING MATERIALS, Issue 10 2006
    Q. Han
    The influence of strain rate in the range of 10,1 to 10,5 s,1 on the compression behavior of a cryomilled 5083 Al alloy is studied. The compression flow stress remains constant after an initial short strain hardening and a small stress dip, which indicates a dynamic saturation of dislocations. It is found that the compression flow stress increases with decreasing strain rate, which is believed to be attributed to dynamic strain aging. [source]


    High-strain-rate Superplasticity in a Nanostructured Al-Mg Alloy

    ADVANCED ENGINEERING MATERIALS, Issue 4 2005
    B. Q. Han
    In this work, the authors report high-strain-rate superplasticity in a nanostructured Al-7.5%Mg alloy with a mean grain size of 90 nm processed via consolidation of cryomilled Al-Mg powders. Tensile ductility with an elongation of 291% was observed at a strain rate of 10-1 s-1 and at a temperature of 573 K. Noteworthy is the fact that the microstructure is essentially stable during testing at 573 K. Grain boundary sliding is suggested to be the dominant deformation mechanism in the superplastic deformation of the nanostructured Al-Mg alloy. [source]


    Effect of test frequency on fatigue strength of low carbon steel

    FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 6 2009
    N. TSUTSUMI
    ABSTRACT Ultrasonic fatigue tests (test frequency: 20 kHz) and conventional tension,compression fatigue tests (10 Hz) have been conducted on annealed and 10% pre-strained specimens of 0.13% carbon steel. Small holes were introduced on the specimen surface to investigate the effect of test frequency on small crack growth. The dynamic stress concentration factor and the stress intensity factor under ultrasonic fatigue tests were checked to be almost the same as those of conventional tension,compression fatigue tests. However, the fatigue properties were dependent on the test frequency. Ultrasonic fatigue tests showed longer fatigue life and lower fatigue crack growth rate for the annealed and 10% pre-strained specimens. Slip bands were scarce in the neighbourhood of cracks under ultrasonic fatigue tests, while many slip bands were observed in a wide area around the crack under conventional fatigue tests. In order to explain the effect of test frequency on fatigue strength, dynamic compression tests with Split Hopkinson bars were carried out. The stress level increases substantially with the strain rate. Thus, the increase in fatigue strength might be, to a large extent, due to a reduction in crack tip cyclic plasticity during ultrasonic fatigue tests. [source]


    A model of corrosion fatigue crack growth in ship and offshore steels

    FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 8 2007
    M. JAKUBOWSKI
    ABSTRACT A model describing corrosion fatigue crack growth rate da/dN has been proposed. The crack growth rate is assumed to be proportional to current flowing through the electrolyte within the crack during a loading cycle. The Shoji formula for the crack tip strain rate has been assumed in the model. The obtained formula for the corrosion fatigue crack growth rate is formally similar to the author's empirical formulae established previously. The different effects of ,K and the fatigue loading frequency f on da/dN, in region I as compared to region II of the corrosion fatigue crack growth rate characteristics can be described by a change of one parameter only: the crack tip repassivation rate exponent. [source]


    In situ Mechanical Testing Reveals Periodic Buckle Nucleation and Propagation in Carbon Nanotube Bundles

    ADVANCED FUNCTIONAL MATERIALS, Issue 14 2010
    Shelby B. Hutchens
    Abstract Uniaxial compression studies are performed on 50-µm-diameter bundles of nominally vertical, intertwined carbon nanotubes grown via chemical vapor deposition from a photolithographically defined catalyst. The inhomogeneous microstructure is examined, demonstrating density and tube orientation gradients, believed to play a role in the unique periodic buckling deformation mechanism. Through in situ uniaxial compression experiments it is discovered that the characteristic bottom-to-top sequential buckling proceeds by first nucleating on the bundle surface and subsequently propagating laterally through the bundle, gradually collapsing the entire structure. The effects of strain rate are explored, and storage and loss stiffnesses are analyzed in the context of energy dissipation. [source]


    Multiscale estimation of GPS velocity fields

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2009
    Carl Tape
    SUMMARY We present a spherical wavelet-based multiscale approach for estimating a spatial velocity field on the sphere from a set of irregularly spaced geodetic displacement observations. Because the adopted spherical wavelets are analytically differentiable, spatial gradient tensor quantities such as dilatation rate, strain rate and rotation rate can be directly computed using the same coefficients. In a series of synthetic and real examples, we illustrate the benefit of the multiscale approach, in particular, the inherent ability of the method to localize a given deformation field in space and scale as well as to detect outliers in the set of observations. This approach has the added benefit of being able to locally match the smallest resolved process to the local spatial density of observations, thereby both maximizing the amount of derived information while also allowing the comparison of derived quantities at the same scale but in different regions. We also consider the vertical component of the velocity field in our synthetic and real examples, showing that in some cases the spatial gradients of the vertical velocity field may constitute a significant part of the deformation. This formulation may be easily applied either regionally or globally and is ideally suited as the spatial parametrization used in any automatic time-dependent geodetic transient detector. [source]


    Viscoelastic,afterslip concurrence: a possible mechanism in the early post-seismic deformation of the Mw 7.6, 1999 Chi-Chi (Taiwan) earthquake

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004
    Shyh-Yang Sheu
    SUMMARY Observed coseismic data as well as 97 days of post-seismic GPS data for the Chi-Chi earthquake are used as constraints in the modelling of crustal evolution using the 3-D finite-element method. First, the coseismic GPS data are used to justify the use of the elastic earth model and the source rupture model. Subsequently, the most likely rheological model is determined by analysing several modelled time-dependent displacements for various viscosity structures. The range of viscosities of the lower crust in central Taiwan is determined in advance from laboratory measurements and the long-term strain rate. The estimated viscosity of 5.0 × 1017 Pa s seems to be very low and a relaxation time of 116 days seems very short, but the latter approximates the GPS measurement of 86 days. Since earlier studies have indicated that both the viscoelastic response model and the afterslip model may affect post-seismic deformation, we compare theoretical surface displacements for each of the two models that we evaluate. The results reveal that there is little doubt that while neither of these models alone is able to predict the GPS measurements well in a 97-day period, the combination of the two models improves the predictions considerably. We conclude that the afterslip mainly dominated Chi-Chi post-seismic deformation in the rupture area while the viscoelastic model did so elsewhere. [source]


    A damage mechanics model for power-law creep and earthquake aftershock and foreshock sequences

    GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000
    Ian G. Main
    It is common practice to refer to three independent stages of creep under static loading conditions in the laboratory: namely transient, steady-state, and accelerating. Here we suggest a simple damage mechanics model for the apparently trimodal behaviour of the strain and event rate dependence, by invoking two local mechanisms of positive and negative feedback applied to constitutive rules for time-dependent subcritical crack growth. In both phases, the individual constitutive rule for measured strain , takes the form ,(t),=,,0,[1,+,t/m,]m, where , is the ratio of initial crack length to rupture velocity. For a local hardening mechanism (negative feedback), we find that transient creep dominates, with 0,<,m,<,1. Crack growth in this stage is stable and decelerating. For a local softening mechanism (positive feedback), m,<,0, and crack growth is unstable and accelerating. In this case a quasi-static instability criterion , , , can be defined at a finite failure time, resulting in the localization of damage and the formation of a throughgoing fracture. In the hybrid model, transient creep dominates in the early stages of damage and accelerating creep in the latter stages. At intermediate times the linear superposition of the two mechanisms spontaneously produces an apparent steady-state phase of relatively constant strain rate, with a power-law rheology, as observed in laboratory creep test data. The predicted acoustic emission event rates in the transient and accelerating phases are identical to the modified Omori laws for aftershocks and foreshocks, respectively, and provide a physical meaning for the empirical constants measured. At intermediate times, the event rate tends to a relatively constant background rate. The requirement for a finite event rate at the time of the main shock can be satisfied by modifying the instability criterion to having a finite crack velocity at the dynamic failure time, dx/dt , VR,, where VR is the dynamic rupture velocity. The same hybrid model can be modified to account for dynamic loading (constant stress rate) boundary conditions, and predicts the observed loading rate dependence of the breaking strength. The resulting scaling exponents imply systematically more non-linear behaviour for dynamic loading. [source]


    SANISAND: Simple anisotropic sand plasticity model

    INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 8 2008
    Mahdi Taiebat
    Abstract SANISAND is the name used for a family of simple anisotropic sand constitutive models developed over the past few years within the framework of critical state soil mechanics and bounding surface plasticity. The existing SANISAND models use a narrow open cone-type yield surface with apex at the origin obeying rotational hardening, which implies that only changes of the stress ratio can cause plastic deformations, while constant stress-ratio loading induces only elastic response. In order to circumvent this limitation, the present member of the SANISAND family introduces a modified eight-curve equation as the analytical description of a narrow but closed cone-type yield surface that obeys rotational and isotropic hardening. This modification enables the prediction of plastic strains during any type of constant stress-ratio loading, a feature lacking from the previous SANISAND models, without losing their well-established predictive capability for all other loading conditions including the cyclic. In the process the plausible assumption is made that the plastic strain rate decomposes in two parts, one due to the change of stress ratio and a second due to loading under constant stress ratio, with isotropic hardening depending on the volumetric component of the latter part only. The model formulation is presented firstly in the triaxial stress space and subsequently its multiaxial generalization is developed following systematically the steps of the triaxial one. A detailed calibration procedure for the model constants is presented, while successful simulation of both drained and undrained behavior of sands under constant and variable stress-ratio loadings at various densities and confining pressures is obtained by the model. Copyright © 2007 John Wiley & Sons, Ltd. [source]


    Viscous behaviour of dry sand

    INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 15 2007
    D. Pham Van Bang
    Abstract Tests on air-dried Hostun sand were performed on a newly developed triaxial apparatus, which allows static and dynamic measurements at ,Département Génie Civil et Bâtiment' (DGCB) of ENTPE. The prototype was designed to perform tests in order to investigate the viscous effects of sand from small strain (some 10,5m/m) up to intermediate strain (some 10,3m/m). The accuracy of the measuring system is ensured by the use of local non-contact displacements transducers and by the use of an internal load cell. The viscous behaviour of sand is experimentally studied for loose and dense specimens for confining pressure values from 80 up to 400 kPa and considering different histories of loading. Three types of viscous loading: creep periods; stress relaxation periods; and paths with stepwise changes in the strain rate are specifically analysed. The proposed viscous modelling is issued from a three-component general framework and is able to reproduce the experimental observations. More specifically, the peculiar behaviour of sand observed during a stepwise increase or decrease in the strain rate is detailed and modelled by the viscous evanescent (VE) model, developed at DGCB/ENTPE. Based on the three types of viscous tests, a simple relation is finally proposed for the viscous parameters of the VE model. Comparisons between data and simulations validate the approach. Copyright © 2007 John Wiley & Sons, Ltd. [source]


    Formulation of a three-dimensional rate-dependent constitutive model for chalk and porous rocks

    INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2007
    R. J. Hickman
    Abstract Rate-dependent behaviour of chalk and other porous rocks has undergone widespread study in geomechanics due to its implications on the performance of engineering structures. We present a rate-dependent constitutive model for chalk and other porous rocks with several new features. The model formulation is based on a viscoplastic rate-lines approach in which the axial strain rate depends on the proximity of the stress point to an elliptical reference surface. A non-associated viscoplastic potential surface and an axial scaling algorithm are used to determine the viscoplastic strain components. The model predicts that axial yields stress varies as a power function of applied axial strain rate, as shown by published laboratory data. Comparisons with published experimental data indicate that the model is capable of reproducing observed rate-dependent behaviour of chalk under a variety of loading conditions. Copyright © 2006 John Wiley & Sons, Ltd. [source]


    Analysis of the steady-state flow of a compressible viscoplastic medium over a wedge

    INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 6 2006
    Oana Cazacu
    Abstract A new model for calculating the resistance to penetration into geological or geologically derived materials is proposed. We assume steady-state flow of the target material over the penetrator. The target medium is described by a rate dependent constitutive equation that accounts for combined effects of strain rate and compaction on yielding. The wedge-shaped penetrator is considered to be rigid. The influence of the characteristics of the penetrator/target interface, impact velocity, target mechanical properties and nose geometry on the resistance to penetration is investigated. It is found that for low to intermediate impact velocities, accounting for friction results in a blunter optimal wedge geometry for optimal penetration performance. Copyright © 2005 John Wiley & Sons, Ltd. [source]