Showing papers on "Creep published in 2007"

Fundamentals of Materials Science and Engineering : An Integrated Approach

Abstract: Chapter 1 - Introduction. 1.1 Historical Perspective 1.2 Materials Science and Engineering 1.3 Why Study Materials Science and Engineering? 1.4 Classification of Materials Materials of Importance-Carbonated Beverage Containers 1.5 Advanced Materials 1.6 Modern Materials Needs 1.7 Processing/Structure/Properties/Performance Correlations Chapter 2 - Atomic Structure and Interatomic Bonding. 2.1 Introduction 2.2 Fundamental Concepts 2.3 Electrons in Atoms 2.4 The Periodic Table 2.5 Bonding Forces and Energies 2.6 Primary Interatomic Bonds 2.7 Secondary Bonding or van der Waals Bonding Materials of Importance-Water (Its Volume Expansion Upon Freezing) 2.8 Molecules Chapter 3 - Structures of Metals and Ceramics 3.1 Introduction 3.2 Fundamental Concepts 3.3 Unit Cells 3.4 Metallic Crystal Structures 3.5 Density Computations-Metals 3.6 Ceramic Crystal Structures 3.7 Density Computations-Ceramics 3.8 Silicate Ceramics 3.9 Carbon Materials of Importance-Carbon Nanotubes 3.10 Polymorphism and Allotropy Material of Importance-Tin (Its Allotropic Transformation) 3.11 Crystal Systems 3.12 Point Coordinates 3.13 Crystallographic Directions 3.14 Crystallographic Planes 3.15 Linear and Planar Densities 3.16 Close-Packed Crystal Structures 3.17 Single Crystals 3.18 Polycrystalline Materials 3.19 Anisotropy 3.20 X-Ray Diffraction: Determination of Crystal Structures 3.21 Noncrystalline Solids Chapter 4 - Polymer Structures 4.1 Introduction 4.2 Hydrocarbon Molecules 4.3 Polymer Molecules 4.4 The Chemistry of Polymer Molecules 4.5 Molecular Weight 4.6 Molecular Shape 4.7 Molecular Structure 4.8 Molecular Configurations 4.9 Thermoplastic and Thermosetting Polymers 4.10 Copolymers 4.11 Polymer Crystallinity 4.12 Polymer Crystals Chapter 5 - Imperfections in Solids 5.1 Introduction 5.2 Point Defects in Metals 5.3 Point Defects in Ceramics 5.4 Impurities in Solids 5.5 Point Defects in Polymers 5.6 Specification of Composition 5.7 Dislocations-Linear Defects 5.8 Interfacial Defects Materials of Importance-Catalysts (and Surface Defects) 5.9 Bulk or Volume Defects 5.10 Atomic Vibrations 5.11 Basic Concepts of Microscopy 5.12 Microscopic Techniques 5.13 Grain Size Determination Chapter 6 - Diffusion 6.1 Introduction 6.2 Diffusion Mechanisms 6.3 Steady-State Diffusion 6.4 Nonsteady-State Diffusion 6.5 Factors That Influence Diffusion 6.6 Diffusion in Semiconducting Materials Material of Importance-Aluminum for Integrated Circuit Interconnects 6.7 Other Diffusion Paths 6.8 Diffusion in Ionic and Polymeric Materials Chapter 7 - Mechanical Properties 7.1 Introduction 7.2 Concepts of Stress and Strain 7.3 Stress-Strain Behavior 7.4 Anelasticity 7.5 Elastic Properties of Materials 7.6 Tensile Properties 7.7 True Stress and Strain 7.8 Elastic Recovery after Plastic Deformation 7.9 Compressive, Shear, and Torsional Deformation 7.10 Flexural Strength 7.11 Elastic Behavior 7.12 Influence of Porosity on the Mechanical Properties of Ceramics 7.13 Stress-Strain Behavior 7.14 Macroscopic Deformation 7.15 Viscoelastic Deformation 7.16 Hardness 7.17 Hardness of Ceramic Materials 7.18 Tear Strength and Hardness of Polymers 7.19 Variability of Material Properties 7.20 Design/Safety Factors Chapter 8 - Deformation and Strengthening Mechanisms 8.1 Introduction 8.2 Historical 8.3 Basic Concepts of Dislocations 8.4 Characteristics of Dislocations 8.5 Slip Systems 8.6 Slip in Single Crystals 8.7 Plastic Deformation of Polycrystalline Metals 8.8 Deformation by Twinning 8.9 Strengthening by Grain Size Reduction 8.10 Solid-Solution Strengthening 8.11 Strain Hardening 8.12 Recovery 8.13 Recrystallization 8.14 Grain Growth 8.15 Crystalline Ceramics 8.16 Noncrystalline Ceramics 8.17 Deformation of Semicrystalline Polymers 8.18 Factors That Influence the Mechanical Properties of Semicrystalline Polymers Materials of Importance-Shrink-Wrap Polymer Films 8.19 Deformation of Elastomers Chapter 9 - Failure 9.1 Introduction 9.2 Fundamentals of Fracture 9.3 Ductile Fracture 9.4 Brittle Fracture 9.5 Principles of Fracture Mechanics 9.6 Brittle Fracture of Ceramics 9.7 Fracture of Polymers 9.8 Fracture Toughness Testing 9.9 Cyclic Stresses 9.10 The S-N Curve 9.11 Fatigue in Polymeric Materials 9.12 Crack Initiation and Propagation 9.13 Factors That Affect Fatigue Life 9.14 Environmental Effects 9.15 Generalized Creep Behavior 9.16 Stress and Temperature Effects 9.17 Data Extrapolation Methods 9.18 Alloys for High-Temperature Use 9.19 Creep in Ceramic and Polymeric Materials Chapter 10 - Phase Diagrams 10.1 Introduction 10.2 Solubility Limit 10.3 Phases 10.4 Microstructure 10.5 Phase Equilibria 10.6 One-Component (or Unary) Phase Diagrams 10.7 Binary Isomorphous Systems 10.8 Interpretation of Phase Diagrams 10.9 Development of Microstructure in Isomorphous Alloys 10.10 Mechanical Properties of Isomorphous Alloys 10.11 Binary Eutectic Systems Materials of Importance-Lead-Free Solders 10.12 Development of Microstructure in Eutectic Alloys 10.13 Equilibrium Diagrams Having Intermediate Phases or Compounds 10.14 Eutectoid and Peritectic Reactions 10.15 Congruent Phase Transformations 10.16 Ceramic Phase Diagrams 10.17 Ternary Phase Diagrams 10.18 The Gibbs Phase Rule 10.19 The Iron-Iron Carbide (Fe-Fe3C) Phase Diagram 10.20 Development of Microstructure in Iron-Carbon Alloys 10.21 The Influence of Other Alloying Elements Chapter 11 - Phase Transformations 11.1 Introduction 11.2 Basic Concepts 11.3 The Kinetics of Phase Transformations 11.4 Metastable versus Equilibrium States 11.5 Isothermal Transformation Diagrams 11.6 Continuous-Cooling Transformation Diagrams 11.7 Mechanical Behavior of Iron-Carbon Alloys 11.8 Tempered Martensite 11.9 Review of Phase Transformations and Mechanical Properties for Iron-Carbon Alloys Materials of Importance-Shape-Memory Alloys 11.10 Heat Treatments 11.11 Mechanism of Hardening 11.12 Miscellaneous Considerations 11.13 Crystallization 11.14 Melting 11.15 The Glass Transition 11.16 Melting and Glass Transition Temperatures 11.17 Factors That Influence Melting and Glass Transition Temperatures Chapter 12 - Electrical Properties 12.1 Introduction 12.2 Ohm's Law 12.3 Electrical Conductivity 12.4 Electronic and Ionic Conduction 12.5 Energy Band Structures in Solids 12.6 Conduction in Terms of Band and Atomic Bonding Models 12.7 Electron Mobility 12.8 Electrical Resistivity of Metals 12.9 Electrical Characteristics of Commercial Alloys Materials of Importance-Aluminum Electrical Wires 12.10 Intrinsic Semiconduction 12.11 Extrinsic Semiconduction 12.12 The Temperature Dependence of Carrier Concentration 12.13 Factors that Affect Carrier Mobility 12.14 The Hall Effect 12.15 Semiconductor Devices 12.16 Conduction in Ionic Materials 12.17 Electrical Properties of Polymer 12.18 Capacitance 12.19 Field Vectors and Polarization 12.20 Types of Polarization 12.21 Frequency Dependence of the Dielectric Constant 12.22 Dielectric Strength 12.23 Dielectric Materials 12.24 Ferroelectricity 12.25 Piezoelectricity Chapter 13 - Types and Applications of Materials 13.1 Introduction 13.2 Ferrous Alloys 13.3 Nonferrous Alloys Materials of Importance-Metal Alloys Used for Euro Coins 13.4 Glasses 13.5 Glass-Ceramics.

Creep-resistant, Al2O3-forming austenitic stainless steels.

Abstract: A family of inexpensive, Al2O3-forming, high-creep strength austenitic stainless steels has been developed. The alloys are based on Fe-20Ni-14Cr-2.5Al weight percent, with strengthening achieved through nanodispersions of NbC. These alloys offer the potential to substantially increase the operating temperatures of structural components and can be used under the aggressive oxidizing conditions encountered in energy-conversion systems. Protective Al2O3 scale formation was achieved with smaller amounts of aluminum in austenitic alloys than previously used, provided that the titanium and vanadium alloying additions frequently used for strengthening were eliminated. The smaller amounts of aluminum permitted stabilization of the austenitic matrix structure and made it possible to obtain excellent creep resistance. Creep-rupture lifetime exceeding 2000 hours at 750 degrees C and 100 megapascals in air, and resistance to oxidation in air with 10% water vapor at 650 degrees and 800 degrees C, were demonstrated.

A periodic shear-heating mechanism for intermediate-depth earthquakes in the mantle

Abstract: Intermediate-depth earthquakes, at depths of 50-300 km in subduction zones, occur below the brittle-ductile transition, where high pressures render frictional failure unlikely. Their location approximately coincides with 600 to 800 degrees C isotherms in thermal models, suggesting a thermally activated mechanism for their origin. Some earthquakes may occur by frictional failure owing to high pore pressure that might result from metamorphic dehydration. Because some intermediate-depth earthquakes occur approximately 30 to 50 km below the palaeo-sea floor, however, the hydrous minerals required for the dehydration mechanism may not be present. Here we present an alternative mechanism to explain such earthquakes, involving the onset of highly localized viscous creep in pre-existing, fine-grained shear zones. Our numerical model uses olivine flow laws for a fine-grained, viscous shear zone in a coarse-grained, elastic half space, with initial temperatures from 600-800 degrees C and background strain rates of 10(-12) to 10(-15) s(-1). When shear heating becomes important, strain rate and temperature increase rapidly to over 1 s(-1) and 1,400 degrees C. The stress then drops dramatically, followed by low strain rates and cooling. Continued far-field deformation produces a quasi-periodic series of such instabilities.

Revision of the Superpave High Temperature Binder Specification: The Multiple Stress Creep Recovery Test (With Discussion)

Abstract: The inadequacy of the Superpave high temperature specification parameter, G*/sin δ, to correctly grade the superior field performance of modified asphalt binders has been demonstrated by several researchers. A new parameter that is blind to modification type and is performance based is now needed. As a replacement for the existing high temperature binder test (G*/sin δ), the FHWA has developed an easy to use Multiple Stress Creep and Recovery Test (MSCR) that measures fundamental characteristics of asphalt binders. In this study, several binder parameters proposed to replace the existing Superpave rutting parameter were validated using hotmix testing. Several different binder tests were evaluated to determine which would provide a replacement for the Superpave high temperature binder criteria. The new test and criteria will have to be performance related and blind to modification. The results from these binder tests were compared against hot-mix rutting results from the Asphalt Pavement Analyzer, the Hamburg Wheel Tracking, the ALF test sections and actual roadway sites. The results from the mixture rut testing showed that different rut testers will provide completely different ranking of binders. This difference is related to the stress level applied by the different testers. This hot-mix testing indicates that the different binders, specifically the polymer modified binders, have different stress dependencies. The binder criteria currently used to specify the high temperature properties are specifically intended to be run in the linear viscoelastic range and therefore can not determine the stress dependency on binder materials. The multi step creep and recovery test can be run at multiple stress levels and can characterize the stress dependency of polymer modified binders. The MSCR test was developed as a result of these findings and other results from various internal studies conducted by FHWA. A separate sub-study was also conducted in this research to understand the effect of stress and strain on the microstructure of polymer modified binders. It was found that in MSCR data there is a clear relationship between %recovery and %strain in the creep portion of the test. In some cases, at least, this is the dominant relationship. Very high strain causes yield behavior in polymer modified asphalt binders (PMA). After high strain, PMAs still exhibit recovery but the rate of recovery is reduced. At high strain, binder morphology, tensile and shear properties change. A test procedure was developed to run creep and recovery testing on one sample at multiple stress levels (MSCR). This test procedure makes it easy to evaluate how the binder response will change under different stress conditions. A property called non-recoverable compliance Jnr was developed based on the non-recovered strain at the end of the recovery portion of the test divided by the initial stress applied during the creep portion of the test. The Jnr value normalizes the strain response of the binder to stress which clearly shows the differences between different polymer-modified binders.

Creep and dynamic mechanical behavior of PP–jute composites: Effect of the interfacial adhesion

Abstract: The dynamic mechanical response and the short term creep-recovery behavior of composites made from bi-directional jute fabrics and polypropylene were studied. In order to improve the compatibility of the polar fibers and the non-polar matrix, two alternatives were compared: the addition of coupling agents and the chemical modification of the fibers. In the first case, two commercial maleated polypropylenes and lignin, a natural polymer, were used. In the second approach, the fibers were esterified using a commercial alkenyl succinic anhydride. The degree of interfacial adhesion was inferred from the measured properties and confirmed by the observation of the composite fractured surface. The maleated polypropylenes acted as compatibilizers since they were able to join the fibers to the neat PP, locating themselves in the interphase region. On the other hand, a clear separation between fibers and matrix could be observed when lignin was used as compatibilizing agent and when the chemically modified fibers were used to prepare the composite. The creep deformation could be directly related to the interfacial properties. Burgers model parameters were calculated from the creep part of the curves, and the recovery part was modeled using these values. A very good agreement between experimental data and theoretical curves were obtained in the creep region, although small discrepancies were found in the recovery part. The feasibility of the construction of a master curve (using the time–temperature principle) to predict long term creep behavior of the composites was investigated.

Effect of Cement on Emulsified Asphalt Mixtures

Abstract: Emulsified asphalt mixtures have environmental, economical, and logistical advantages over hot mixtures. However, they have attracted little attention as structural layers due to their inadequate performance and susceptibility to early life damage by rainfall. The objective of this article is to provide an improved insight into how the mechanical properties of emulsion mixtures may be improved and to determine the influence of cement on emulsified asphalt mixtures. Laboratory tests on strength, temperature susceptibility, water damage, creep and permanent deformation were implemented to evaluate the mechanical properties of emulsified asphalt mixtures. The test results showed that mechanical properties of emulsified asphalt mixtures have significantly improved with Portland cement addition. This experimental study suggested that cement modified asphalt emulsion mixtures might be an alternate way of a structural layer material in pavement.

Prediction of Creep Stiffness of Asphalt Mixture with Micromechanical Finite-Element and Discrete-Element Models

Abstract: This study presents micromechanical finite-element (FE) and discrete-element (DE) models for the prediction of viscoelastic creep stiffness of asphalt mixture. Asphalt mixture is composed of graded aggregates bound with mastic (asphalt mixed with fines and fine aggregates) and air voids. The two-dimensional (2D) microstructure of asphalt mixture was obtained by optically scanning the smoothly sawn surface of superpave gyratory compacted asphalt mixture specimens. For the FE method, the micromechanical model of asphalt mixture uses an equivalent lattice network structure whereby interparticle load transfer is simulated through an effective asphalt mastic zone. The ABAQUS FE model integrates a user material subroutine that combines continuum elements with viscoelastic properties for the effective asphalt mastic and rigid body elements for each aggregate. An incremental FE algorithm was employed in an ABAQUS user material model for the asphalt mastic to predict global viscoelastic behavior of asphalt mixture. In regard to the DE model, the outlines of aggregates were converted into polygons based on a 2D scanned mixture microstructure. The polygons were then mapped onto a sheet of uniformly sized disks, and the intrinsic and interface properties of the aggregates and mastic were assigned for the simulation. An experimental program was developed to measure the properties of sand mastic for simulation inputs. The laboratory measurements of the mixture creep stiffness were compared with FE and DE model predictions over a reduced time. The results indicated both methods were applicable for mixture creep stiffness prediction.

Mechanical Behaviour of Engineering Materials: Metals, Ceramics, Polymers, and Composites

Abstract: The structure of materials.- Elasticity.- Plasticity and failure.- Notches.- Fracture mechanics.- Mechanical behaviour of metals.- Mechanical behaviour of ceramics.- Mechanical behaviour of polymers.- Mechanical behaviour of fibre reinforced composites.- Fatigue.- Creep.- Exercises.- Solutions.

Characterization of Microstructures across the Heat-Affected Zone of the Modified 9Cr-1Mo Weld Joint to Understand Its Role in Promoting Type IV Cracking

Abstract: In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac1 to above Ac3 and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure.

Modeling of creep for structural analysis

Abstract: Constitutive Models of Creep.- Examples of Constitutive Equations for Various Materials.- Modeling of Creep in Structures.

Damage accumulation during creep deformation of a single crystal superalloy at 1150 °C

Abstract: Microstructural degradation in the CMSX-4 single crystal superalloy during creep deformation at 1150 °C and 100 MPa is studied. Tensile deformation in the 〈0 0 1〉 direction is considered due to its technological importance. Under these conditions, rafting of the γ′ structure is completed quickly and within the first 10 h. It is demonstrated that the creep rupture event is highly localised, the instability being associated with a critical and well-defined strain being reached with failure occurring within a further few tens of hours. It is shown that the high strain rates and shear stresses associated with the rupture process are sufficient to cause realignment of the rafted γ′ structure with respect to the γ matrix. Creep cavitation damage near to the rupture surface is prevalent, at microporosity inherited from the casting process but more significantly, at topologically close-packed (TCP) phases and associated pores and voids formed in their vicinity which have formed via vacancy condensation. Hot isostatic pressing (HIPing) prior to creep testing reduces the incidence of casting microporosity, but the creep rupture life is not improved significantly. It is suggested that it is the formation of TCP phases which limits creep rupture life.

Alumina-Forming Austenitic Stainless Steels Strengthened by Laves Phase and MC Carbide Precipitates

Abstract: Creep strengthening of Al-modified austenitic stainless steels by MC carbides or Fe2Nb Laves phase was explored. Fe-20Cr-15Ni-(0–8)Al and Fe-15Cr-20Ni-5Al base alloys (at. pct) with small additions of Nb, Mo, W, Ti, V, C, and B were cast, thermally-processed, and aged. On exposure from 650 °C to 800 °C in air and in air with 10 pct water vapor, the alloys exhibited continuous protective Al2O3 scale formation at an Al level of only 5 at. pct (2.4 wt pct). Matrices of the Fe-20Cr-15Ni-5Al base alloys consisted of γ (fcc) + α (bcc) dual phase due to the strong α-Fe stabilizing effect of the Al addition and exhibited poor creep resistance. However, adjustment of composition to the Fe-15Cr-20Ni-5Al base resulted in alloys that were single-phase γ-Fe and still capable of alumina scale formation. Alloys that relied solely on Fe2Nb Laves phase precipitates for strengthening exhibited relatively low creep resistance, while alloys that also contained MC carbide precipitates exhibited creep resistance comparable to that of commercially available heat-resistant austenitic stainless steels. Phase equilibria studies indicated that NbC precipitates in combination with Fe2Nb were of limited benefit to creep resistance due to the solution limit of NbC within the γ-Fe matrix of the alloys studied. However, when combined with other MC-type strengtheners, such as V4C3 or TiC, higher levels of creep resistance were obtained.

Creep and creep fracture of polycrystalline copper

Abstract: Normal creep curves are recorded over extended stress ranges at 686–823 K for fine-grain copper. Analyses of the curve shape variations, together with the results of stress change experiments, do not support the view that a transition from dislocation to diffusional creep mechanisms occurs with decreasing stress. Instead, the observed behaviour patterns suggest that dislocation processes are dominant at all stress levels. However, strain accumulation within the grains becomes progressively less important as deformation is increasingly confined to the grain boundary zones when the stress is reduced below the yield stress at the creep temperature. New approaches are then introduced for rationalization of creep rate and creep life measurements, which account for the data trends taken as evidence for major mechanism changes when the creep properties are described using power law relationships.

Thermoporomechanics of creeping landslides: The 1963 Vaiont slide, northern Italy

Abstract: [1] The catastrophic Vaiont landslide (Southern Alps, Italy) of 9 October 1963 moved 2.7 × 108 m3 of rock that collapsed in an artificial lake, causing a giant wave that killed 1917 people. The landslide was preceded by 2–3 years of creep that ended with the final collapse of the rock mass slipping at about 30 m s−1. Assuming that creep was localized in a clay-rich water-saturated layer, in this study we propose shear heating as the primary mechanism for the long-term phase of accelerating creep. We study only the creeping phase of the slide, and we model this phase using a rigid block moving over a thin zone of high shear strain rates. Introducing a thermal softening and velocity strengthening law for the basal material, we reformulate the governing equations of a water-saturated porous material, obtaining an estimate for the collapse time of the slide. Our model is calibrated upon real velocity measurements from the Vaiont landslide and provides an estimation of the critical time of failure up to 169 days before the collapse. We also show that the slide became critical ∼21 days before the collapse, when shear heating started localizing in the clay-rich layer, inducing a tendency for slip localization and thermal runaway instability in a plane. The total loss of strength in the slipping zone during the last minutes prior to the slide is explained by the onset of thermal pressurization, triggered by the temperature rise within the clay-rich layer.

Long term creep properties and microstructure of SUPER304H, TP347HFG and HR3C for A-USC boilers

Abstract: SUPER304H (18Cr–9Ni–3Cu–Nb–N; ASME CC2328) and TP347HFG (18Cr–12Ni–Nb; ASME SA213) have been developed for high strength oxidation resistant steel tubes to operate at high steam temperatures and pressures. The longest creep rupture tests performed to date (600°C for 85 426 h for SUPER304H; 700°C for 55 858 h for TP347HFG) showed that the stable strength and microstructure were retained, with very little formation of σ-phase compared with conventional austenitic stainless steels and no other brittle phases. The alloy HR3C (25Cr–20Ni–Nb–N; ASME CC2115) has been developed for the high strength and high corrosion resistant steel tubes used in recent ultrasupercritical (USC) boilers with steam temperatures of ∼600°C. The longest creep test conducted to date (700°C, 69 MPa for 88 362 h) confirmed a stable creep strength and microstructure at 600–800°C. Superheater and reheater tubes of these alloys installed in the Eddystone No.1 USC power plant since 1991 have been removed and investigated. Updated lon...

The relation between collagen fibril kinematics and mechanical properties in the mitral valve anterior leaflet.

Abstract: We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315-325; Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509-1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (epsilonD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90 N/m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, epsilonD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using epsilonD, the tangent modulus of collagen fibrils was estimated to be 95.5+/-25.5 MPa, which was approximately 27 times higher than the tissue tensile tangent modulus of 3.58+/-1.83 MPa. In creep tests performed at 90 N/m equibiaxial tension for 60 min, both tissue strain and epsilonD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90 min epsilonD was found to rapidly decrease in the first 10 min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3 min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9 min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a "load-locking" behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function.

Diffusion creep of dry, melt-free olivine

Abstract: [1] Deformation experiments were conducted on fine-grained (3–6 μm), fully synthetic Fo90 olivine aggregates in a gas-medium apparatus at 300 MPa confining pressure and temperatures of 1150–1360°C. The strain rates of the solution-gelation-derived and therefore genuinely melt-free, dry samples are about two orders of magnitude lower than the strain rates for nominally melt-free aggregates at the same pressure and temperature conditions and grain size. Benchmark deformation tests with Anita Bay dunite and mild steel reproduce published data. The creep strength of melt-added sol-gel olivine is similar to the published creep strength of dry, melt-bearing olivine derived from natural rocks. Nonlinear least-squares fits to the melt-free deformation data give an activation energy of 484 kJ/mol, a stress exponent of 1.4, and a grain-size exponent of 3 over a range of stresses from 15 to 210 MPa. These results suggest that small amounts of melt may be similarly effective in reducing the creep strength of upper mantle rocks as small amounts of water. However, a possible contribution of grain boundary composition to the observed differences in rheology in the absence of melt cannot be conclusively ruled out by the current experiments.