Non-recoverable Strain Research Articles

Inelastic deformation accrued over multiple seismic cycles: Insights from an elastic-plastic slider-and-springboard model

We study a toy model designed to build physical insight into the problem of slow accumulation of non-recoverable strain in fault blocks over multiple earthquake cycles. The model consists of a thin, horizontal elastic-plastic plate (springboard) in frictional contact with a vertical, rigid wall moving downward at a steady speed. Our model produces stick-slip cycles consisting of interseismic plate downwarping and coseismic plate upwarping as long as the moment of the frictional force at the contact does not exceed the maximum (purely plastic) bending moment the plate can sustain. We show that the duration of individual earthquake cycles and the spatial pattern of interseismic deflection are controlled by two stress ratios involving the peak yield stress of the plate, the frictional strength of the fault and the coseismic stress drop. We show that non-recoverable plate deflection accumulates over successive earthquake cycles if the plate’s yield strength decreases through time, causing a progressive decrease of the aforementioned stress ratios. We derive scaling relations between the rate of accumulation of inelastic deformation, the relative tectonic plate velocity, and the rate of lithospheric weakening. Our results are consistent with observations of long-term permanent deformation of natural fault regions.

On the Schapery nonlinear viscoelastic model: A review

Among the various models presented so far to describe the nonlinear viscoelastic behavior of materials, the model proposed by Schapery in the 1960s stands out for its wide acceptance and significant contributions to the field. Through a detailed inspection of the main articles presented by Schapery and other related studies, the present paper reviews the Schapery nonlinear viscoelastic model. One-dimensional and three-dimensional formulations of the model for both isotropic and anisotropic materials, along with model accuracy, are discussed. The present article highlights the challenges of material characterization methods, such as creep-recovery testing, stress relaxation testing, and dynamic mechanical analysis. The article also addresses the issues of characterizing the material's behavior under non-recoverable strain and varying temperatures. Finally, the future research needs in this field are outlined, emphasizing the potential for further advancements in our understanding of nonlinear viscoelasticity.

Mechanical behaviour of ductile polymer cellular model structures manufactured by FDM

In this work, Acrylonitrile-Butadiene-Styrene model structures were manufactured by FDM, and their mechanical behaviour investigated under compression, both at small and at large strains. The structure design strategy adopted, based on the use of circular cross-section beam-like elements formed under controlled conditions, led to obtain open-cell structures (with a porosity degree of ≈ 65%) composed of unit cells with different shapes and dimensions assembled to form regularly repeating patterns. The stress-strain behaviour, from cube- and prism-shaped specimens with different sizes and loaded along different directions, was discussed in the light of the outcomes from (i) cyclic compression experiments and (ii) morphological analyses of cryogenic fracture surfaces of specimens compressed at high strain levels. The response along the 3D-stacking direction was traced back to the elastic-plastic case, with non-recoverable strain starting to accumulate between 3% and 5% strain and structure densification starting below 20%. The specimen size effects turned out to be little pronounced. Slightly higher levels of stiffness and strength were measured for the largest cube. This result was discussed on the basis of the peculiar morphology of the structure examined.

Direction of non-recoverable strain in the glenohumeral capsule following multiple anterior dislocations: Implications for anatomic Bankart repair.

The study aimed to analyze the direction of non-recoverable strain and determine the optimal direction for anatomic capsular plication within four sub-regions of the inferior glenohumeral capsule following multiple dislocations. Seven fresh-frozen cadaveric shoulders were dissected. A grid of strain markers was affixed to the inferior glenohumeral capsule. Each joint was mounted in a 6-degree-of-freedom robotic testing system and repeatedly dislocated in the anterior direction 10 times at 60° of abduction and 60° of external rotation of the glenohumeral joint. The 3D positions of the strain markers were compared before and after dislocations to define the non-recoverable strain. The strain map was divided into four sub-regions. The angles of deviation between each maximum principle strain vector and the anterior band of the inferior glenohumeral ligament (AB-IGHL) or posterior band of the IGHL (PB-IGHL) for the anterior and posterior regions of the capsule were determined. The mean direction of all strain vectors in each sub-region was categorized. The direction of the non-recoverable strain in the anterior-band and anterior-axillary-pouch sub-regions was categorized as parallel to the AB-IGHL, whereas the posterior-axillary-pouch and posterior-band sub-regions were mostly perpendicular to the PB-IGHL. Clinical Significance: Plication of the anteroinferior capsule parallel to the AB-IGHL may be preferred during arthroscopic Bankart repair to restore anatomy; posteroinferior capsular plication may also be necessary and best performed perpendicular to the PB-IGHL.The direction of the capsular injury remains the same irrespective of the number of dislocations.This study provides the scientific and quantitative rationale for an anatomic approach to capsular plication.

Mechanistic understanding of the performance of personalized 3D-printed cardiovascular polypills: A case study of patient-centered therapy

The 3D printing has become important in drug development for patient-centric therapy by combining multiple drugs with different release characteristics in a single polypill. This study explores the critical formulation and geometric variables for tailoring the release of Atorvastatin and Metoprolol as model drugs in a polypill when manufactured via pressure-assisted-microextrusion 3D printing technology. The effects of these variables on the extrudability of printing materials, drug release and other quality characteristics of polypills were studied employing a definitive screening design. The extrudability of printing materials was evaluated in terms of flow pressure, non-recoverable strain, compression rate, and elastic/plastic flow. The extrudability results helped in defining an operating space free of printing defects. The Atorvastatin compartment of polypill consisted of mesh-shaped layers while Metoprolol compartment consisted of a core surrounded by a release controlling shell with a hydrophobic septum between the two compartments.The results indicated that both the formulation and geometric variables govern the drug release of the polypill. Specifically, the use of HPMC E3 matrix, and a 2 mm distance between the strands at a weaving angle of 90° were critical in achieving the desired immediate-release profile of Atorvastatin. The core and shell design primarily determined the desired extended-release profile of Metoprolol. The carbopol and HPMC K100 concentration of 1% in the core and 10% in the shell and the number of shell layers in Metoprolol compartment were critical for achieving the desired Metoprolol dissolution. Polymer and Metoprolol content of the shell and shell-thickness affected the mechanical strength of the polypills. In conclusion, the 3D printing provides the flexibility for independently tailoring the release of different drugs in the same dosage form for patient centric therapy, and both the formulation and geometric parameters need to be optimized to achieve desired drug release.

Mechanical Response of Castlegate Sandstone under Hydrostatic Cyclic Loading

The stress history of rocks in the subsurface affects their mechanical and petrophysical properties. Rocks can often experience repeated cycles of loading and unloading due to fluid pressure fluctuations, which will lead to different mechanical behavior from static conditions. This is of importance for several geophysical and industrial applications, for example, wastewater injection and reservoir storage wells, which generate repeated stress perturbations. Laboratory experiments were conducted with Castlegate sandstone to observe the effects of different cyclic pressure loading conditions on a common reservoir analogue. Each sample was hydrostatically loaded in a triaxial cell to a low effective confining pressure, and either pore pressure or confining pressure was cycled at different rates over the course of a few weeks. Fluid permeability was measured during initial loading and periodically between stress cycles. Samples that undergo cyclic loading experience significantly more inelastic (nonrecoverable) strain compared to samples tested without cyclic hydrostatic loading. Permeability decreases rapidly for all tests during the first few days of testing, but the decrease and variability of permeability after this depend upon the loading conditions of each test. Cycling conditions do affect the mechanical behavior; the elastic moduli decrease with the increasing loading rate and stress cycling. The degree of volumetric strain induced by stress cycles is the major control on permeability change in the sandstones, with less compaction leading to more variation from measurement to measurement. The data indicate that cyclic loading degrades permeability and porosity more than static conditions over a similar period, but the petrophysical properties are dictated more by the hydrostatic loading rate rather than the total length of time stress cycling is imposed.

Deformation mechanisms in a superelastic NiTi alloy: An in-situ high resolution digital image correlation study

An in-situ high resolution digital image correlation investigation during uniaxial tensile deformation reveals the recoverable and the non-recoverable strain mechanisms in a Ni51Ti49 alloy with a mean grain size of 35 μm. Recoverable strain is due to the martensitic transformation, for which more than one variant per grain can be activated. The majority of the activated variants exhibit high Schmid factor. The variant selection can be influenced by shear transmission across grain boundaries, when the geometrical compatibility between the neighboring habit plane variants is favourable; in these cases variants that do not have the highest Schmid factor, with respect to the macroscopically applied load, are activated. The experimentally determined transformation strains agree well with theoretical calculations for single crystals. The non-recoverable strain is due to deformation slip in austenite, twinning in martensite and residual martensite. The results are discussed in view of possible twinning modes that can occur in austenite resulting in significant non-recoverable strain.

Direction of Capsular Strain Implies Surgical Repair Following Recurrent Anterior Shoulder Dislocation

Objectives:Capsular plication is often performed in addition to arthroscopic Bankart repair. However, little is known regarding the direction of capsular injury making the direction of plication fairly arbitrary. This study aimed to determine the optimal direction for capsular plication within four sub-regions of the inferior glenohumeral capsule following multiple dislocations.Methods:Seven fresh-frozen cadaveric shoulders (age range 48-66 yrs) were dissected free of all soft tissue except the glenohumeral capsule. A grid of strain markers was affixed to the anterior and posterior band (A/PB) of the inferior glenohumeral ligament (IGHL), and the axillary pouch. The position of the markers while the capsule was inflated with minimal pressure served as the reference state. The humerus and scapula were then mounted in a 6 degree-of-freedom robotic testing system. At 60 degrees of abduction and 60 degrees of external rotation of the glenohumeral joint, an anterior load was applied to reach an anterior translation of one half the maximum AP width of the glenoid plus 10 mm. This definition of dislocation resulted in non-recoverable strain and a reproducible Bankart lesion. Following 1, 2, 3, 4, 5 and 10 dislocations, the positions of the strain markers were again recorded with the capsule inflated. The difference in these positions compared to the reference state defined the non-recoverable strain. The strain map was split into four sub-regions, the anterior band of IGHL (AB), anterior axillary pouch (AA), posterior axillary pouch (PA), and the posterior band of IGHL (PB) (Fig. 1). The angle of deviation between each of the maximum principle strain vectors and the AB-IGHL or PB-IGHL for the anterior and posterior regions of the capsule were determined using ImageJ. Circular statistics were employed to calculate mean direction of each sub-region and a Watson-Williams test was performed to compare mean direction among each dislocation with significance set at p < 0.05. The mean direction of all strain vectors in each sub-region was categorized as parallel or perpendicular to the AB-IGHL or PB-IGHL serving as the clinical reference. Direction ranging from 0 to 45 or 135 to 180 degrees was categorized as parallel. Direction ranging between 45 and 135 degrees was categorized as perpendicular.Results:The direction of 81.8% of the AB sub-regions was categorized as parallel and 18.2% categorized as perpendicular to the AB-IGHL. Direction of 61.3% of the AA sub-region was categorized as parallel (Table 1) and 38.7% categorized as perpendicular to AB-IGHL. The direction of 33.3% of the PA sub-region was categorized as parallel and 66.7% categorized as perpendicular to the PB-IGHL. The direction of 21.4% of PB sub-region was categorized as parallel and 78.6% categorized as perpendicular to PB-IGHL. A Watson-Williams test demonstrated that the direction of 81.3% of the sub-regions were not significantly different (p > 0.05) among dislocations for each specimen (Table 1).Conclusion:The non-recoverable strain in most of the AB and AP sub-regions were categorized as parallel to the AB-IGHL while for the PA and PB sub-regions mostly perpendicular to the PB-IGHL. These findings imply that it may be more optimal to plicate the anteroinferior capsule parallel to the AB-IGHL while posteroinferior capsular plication, which is often not classically considered for plication in the setting of anterior instability, may also be necessary and best performed perpendicular to the PB-IGHL.Figure 1.Fringe plot of non-recoverable strain for anterior half of inferior capsule in a typical shoulder following 5th dislocation. The borderline of AA and posterior axillary pouch sub-regions was the column of markers placed at the 6 o'clock position of the glenoid. AB-IGHL, anterior band of inferior glenohumeral ligament; AA, anterior axillary pouch sub-region; AB, anterior band of IGHL sub-region.Table 1.Direction of non-recoverable strain in the anterior axillary pouch sub-region following 1st to 10th dislocations for a typical shoulder (mean ± SD)Anterior Axillary Pouch sub-regionDirection of non-recoverable strainCategorizationDislocation 1168±5Parallel to AB-IGHLDislocation 2168±8Parallel to AB-IGHLDislocation 3166±5Parallel to AB-IGHLDislocation 4165±5Parallel to AB-IGHLDislocation 5175±13Parallel to AB-IGHLDislocation 10158±20Parallel to AB-IGHLWatson-Williams test showed directions were not significantly different among dislocation (p=0.33). Direction through 6 dislocation states were categorized as parallel (between 135 to 180 degrees) to anterior-band (AB) of IGHL

Prediction of pelleting outcomes based on moisture versus strain hysteresis during the loading of individual pea seeds

The pelleting of pea is related to some of the mechanical features of the seeds. Varying in moisture, intact seeds were assigned to unconfined loading–unloading with the help of a universal testing machine. Elastic and permanent strain energy and associated deformations were evaluated on the basis of load–displacement curves. In parallel, samples of pea at the same moisture levels were pelleted. The pelleting energy usage and pellet quality features, that is strength, durability, and bulk density, were tested for correlations with strain hysteresis data. Seed moisture and energy loss during the loading of a single seed showed a strong relevance to the processing and pellet properties. An observed increase in the proportion of non-recoverable strain energy, from 15.5 to 176.1 mJ, resulted in a significant decrease in all analysed pelleting features. The specific energy consumption decreased from 296.5 to 240.1 kJ kg−1. Bulk density and pellet strength SI decreased from 773.6 to 652.7 kg m−3 and 16.61 to 13.60 kN m−1, respectively. An empirical model was introduced for the estimation of pelleting effects as a function of seed plasticity. It was based on a hyperbolic formula, which proved to be accurate for the prediction of energy requirements for pelleting and the properties of pellets.

Capsule function following anterior dislocation: Implications for diagnosis of shoulder instability

During shoulder dislocation, the glenohumeral capsule undergoes non-recoverable strain, leading to joint instability. Clinicians use physical exams to diagnose injury and direct repair procedures; however, they are subjective and do not provide quantitative information. Our objectives were to: (1) determine the relationship between capsule function following anterior dislocation and non-recoverable strain; and (2) identify joint positions at which physical exams can be used to detect non-recoverable strain in specific capsule regions. Physical exams were simulated at three joint positions including external rotation (ER) using robotic technology before and after anterior dislocation. The resulting joint kinematics, strain distribution in the capsule, and non-recoverable strain were determined. Following dislocation, anterior translation increased by as much as 48% (0° ER: p = 0.03; 30° ER: p = 0.03; 60° ER: p < 0.01). Capsule sub-regions with less non-recoverable strain required more ER to detect differences in the strain ratios between the intact and injured joint. Strain ratio changes on the humeral side of the posterior axillary pouch (0.31 ± 0.32) were significant at all joint positions (0° ER: p = 0.03; 30° ER: p = 0.048; 60° ER: p = 0.04), whereas strain ratio differences on the humeral side of the anterior axillary pouch (0.18 ± 0.21) were significant only at 60° of ER (p = 0.03). Therefore, standardizing physical exams for joint position could help surgeons identify specific locations of non-recoverable strain that may have been ignored.

Injury to the anteroinferior glenohumeral capsule during anterior dislocation

BackgroundGlenohumeral dislocation commonly results in permanent deformation of the glenohumeral capsule. Knowing the location and extent of tissue damage may aid in improving diagnostic and repair procedures for shoulder dislocations. Therefore, the objectives of this study were to determine: (1) the strain in the anteroinferior capsule at dislocation and (2) the location and extent of injury to the anteroinferior capsule due to dislocation by quantifying the resulting non-recoverable strain. MethodsA robotic/universal force–moment sensor testing system was used to anteriorly dislocate six cadaveric shoulders. The magnitude of the maximum principle strain at dislocation and the resulting non-recoverable strain due to dislocation in the anteroinferior capsule were measured by tracking the change in the location of a grid of strain markers from a reference position. FindingsThe glenoid side of the capsule experienced higher strains at dislocation than the humeral side. The greatest strains at dislocation were found on the glenoid side of the anterior band (strain ratio of 0.60), but the greatest non-recoverable strains were found in the posterior axillary pouch (strain ratio of 0.34 on the glenoid side and 0.31 on the humeral side). InterpretationThese findings suggest that even though the glenoid side of the anterior band undergoes more deformation during anterior dislocation, the most permanent deformation occurs in the posterior axillary pouch, and surgeons should consider also plicating the posterior axillary pouch when performing repair procedures following anterior dislocation. In the future, the mechanical properties of the normal and injured glenohumeral capsules will be compared.

Effects of simulated injury on the anteroinferior glenohumeral capsule

Glenohumeral dislocation results in permanent deformation (nonrecoverable strain) of the glenohumeral capsule which leads to increased range of motion and recurrent instability. Minimal research has examined the effects of injury on the biomechanical properties of the capsule which may contribute to poor patient outcome following repair procedures. The objective of this study was to determine the effect of simulated injury on the stiffness and material properties of the AB-IGHL during tensile deformation. Using a combined experimental and computational methodology, the stiffness and material properties of six AB-IGHL samples during tensile elongation were determined before and after simulated injury. The AB-IGHL was subjected to 12.7 ± 3.2 % maximum principal strain which resulted in 2.5 ± 0.9 % nonrecoverable strain. The linear region stiffness and modulus of stress-stretch curves between the normal (52.4 ± 30.0 N/mm, 39.1 ± 26.6 MPa) and injured (64.7 ± 21.3 N/mm, 73.5 ± 53.8 MPa) AB-IGHL increased significantly (p = 0.03, p = 0.04). These increases suggest that changes in the tissue microstructure exist following simulated injury. The injured tissue could contain more aligned collagen fibers and may not be able to support a normal range of joint motion. Collagen fiber kinematics during simulated injury will be examined in the future.

Silks cope with stress by tuning their mechanical properties under load

We compare the nonlinear mechanical properties of silks under load with the quasi-static and isothermal dynamic mechanical properties of nylon as well as human hair. For silk and nylon, the dynamic storage modulus increases with increasing static load, while the quasi-static modulus decreases considerably through yield. However, the modulus of hair decreases irreversibly for both loading conditions. For silk, the increase in storage modulus is only partially reversible after high-loading and is accompanied by a non-recoverable strain. For nylon, the dynamic modulus increase is largely reversible after increased static loading up to a second high stress yield point, where modulus then decreases. Taken together, our data suggest that the dynamic modulus increases with increasing order in the silk and nylon structures under static load, whereas the morphology of hair is gradually degraded under load.

Characterization of creep and recovery curve of polymer modified binder

The Multiple Stress Creep and Recovery (MSCR) curve was developed recently to better characterize polymer modified binders. The two parameters that are measured from the MSCR curve are the non-recoverable compliance (Jnr) and the percent recovery. The purpose of this study was to determine if the parameters of the entire creep and recovery curve rather than just the two parameters that are currently used would help to better understand the behavior of polymer modified binders. Eight in-house binders and six industry binders were tested in order to evaluate MSCR parameters. The creep and recovery calculated from the methodology developed in this study was within 15% of the measured creep and recovery curve. The linear, non-linear, and non-recoverable compliance was similar between cycles. The non-recoverable compliance was sensitive to base binder, Elvaloy, PPA, and SBS polymer modification. The permanent strain calculated from this procedure is significantly lower than the non-recoverable strain measured at the end of 10s. The sensitivity of the different parameters to polymer modification should be evaluated to provide a better understanding on how a given polymer influences the mechanical behavior of a given binder.

Direct measurement of strain behavior of compression loaded plasma-sprayed yttria-stabilized zirconia

Direct measurement of the stress–strain behavior of stand-alone plasma-sprayed 7 wt.% Y 2O 3–ZrO 2 (YSZ) coatings was made at room temperature. YSZ coatings were evaluated in the as-sprayed condition, and after heat-treatments for 10, 50, and 100 h at 1200 °C using both monotonic and two different cyclic uniaxial compression loading profiles. Heat-treatments were used to change the primarily mechanically interlocked system of lamella to a chemical interlocked system. Monotonic loading of the coatings indicated three different moduli. The first modulus was lower (~ 10 GPa) than observed typically in a plasma sprayed coating. The relatively easy deformation associated with the low modulus was attributed to minor sample crushing near its ends. The second modulus observed was ~ 24 GPa and is due to closure of cracks oriented perpendicular to the stress and lamella sliding and compaction. The third modulus measured ranged from 40 to 48 GPa and represents the elasticity of the material after the easy crack closure and lamella sliding events have occurred. In repeated cyclic loading to a compressive stress of 60 MPa, the coatings were observed to demonstrate large amounts of non-recoverable strain during the first several load/unload cycles, presumably due to permanent sliding and compaction of lamella. Permanent strain tended to a minimum amount after ~ 20 load/unload cycles. Increased cyclic loading from 20 to 320 MPa in 20 MPa increments revealed significant anelastic behavior as evidenced by stress–strain hysteresis loops, particularly in the as-sprayed coating. The elastic moduli of the coatings upon reloading were observed to increase after each subsequent 20 MPa increase in stress applied to the coating to values as high as ~ 80 GPa. This was attributed to an increase in the volume fraction of coating sustaining the load. The amount of non-recoverable strain was highest in the heat-treated coatings indicating a loss of strain tolerance of YSZ coatings during simulated service conditions.

Development of Nonlinear Railway Track Model Applying Modified Plane Strain Technique

The contribution of ballast and subgrade layers behavior to railway track performance is given insufficient consideration in the current railway track models. In particular, stress- and traffic-dependent characteristics need to be incorporated into the analysis. In this research, a new theoretical model is developed to include nonlinear characteristics of the track substructure such as resilient modulus and nonrecoverable strain, incorporating for the first time stress- and traffic-dependent properties of track sublayers. The new model uses a modified finite element plane strain technique which allows simulation of a three-dimensional load spread with a two-stage two-dimensional model. This technique keeps calculation time and cost to a minimum. Comprehensive field tests were conducted to evaluate the accuracy and reliability of the model. Challenging the current assumptions in the analysis and design of railway track systems, the impact of the consideration of the nonlinear properties of the ballast and subgrade materials on the accuracy of the theoretical results is discussed. The proposed technique was found to be accurate and easily applicable to railway track analysis.

Relationships Between Total and Non-Recoverable Strain Fields in Glenohumeral Capsule During Shoulder Subluxation

Non-recoverable strain in the glenohumeral capsule is of prime clinical significance, but the factors that contribute to non-recoverable strain are largely unknown. This study examined the relationship between total and non-recoverable strain in the antero-inferior glenohumeral capsule using an experimental model. Maximum principal total strain alone explained up to 35% of the variance in non-recoverable strain. A multiple regression model, including variables for lateral position and specimen, explained 50% of the variance in non-recoverable strain. Both linear and quadratic terms for maximum principal total strain were significant predictors of non-recoverable strain. The correlation of total and non-recoverable strain directions exhibited a slope of nearly 1:1. The regression model showed that non-recoverable strain is likely to be low for small levels of total strain, and increase non-linearly with total strain. Non-recoverable strain tended to be higher closer to the glenoid, even when controlling for total strain. Minimum principal total strain was not a significant predictor of non-recoverable strain for the cases examined, indicating that the glenohumeral capsule may demonstrate uniaxial failure behavior even when loaded biaxially. These results are important toward prediction of non-recoverable strain in computational models of glenohumeral subluxation, as well as for theoretical models of ligament failure.

Block kinematics of the Pacific–North America plate boundary in the southwestern United States from inversion of GPS, seismological, and geologic data

The active deformation of the southwestern United States (30°–41°N) is represented by a finite number of rotating, elastic‐plastic spherical caps. GPS‐derived horizontal velocities, geologic fault slip rates, transform fault azimuths, and earthquake‐derived fault slip vector azimuths are inverted for block angular velocities, creep on block‐bounding faults, permanent strain rates within the blocks, and the rotations of 11 published GPS velocity fields into to a common North American reference frame. GPS velocities are considered to be a combination of rigid block rotations, recoverable elastic strain rates resulting from friction on block‐bounding faults, and nonrecoverable strain rates resulting from slip on faults within the blocks. The resulting Pacific–North America angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the Gulf of California, earthquake slip vectors in the Gulf of California and Alaska, and GPS velocities along coastal California and within the Pacific Basin. Published fault slip rates are satisfied except in the southern Mojave Desert where the motion of the Mohave block relative to North America is faster than can be explained by mapped faults. The largest blocks, the Sierra Nevada–Great Valley and the eastern Basin and Range, show permanent strain rates, after removing elastic strain, of only a few nanostrain per year, demonstrating approximately rigid behavior. Observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short‐term strains evident in GPS velocities are largely elastic. In only about 20% of the region is distributed deformation needed to match the data, indicating that a plate tectonic style description of the deformation of the western United States is plausible. Most blocks rotate about vertical axes at approximately the same rate as the Pacific (relative to North America), suggesting that locally, spin rates are communicated from block to block, arguing against both floating block and ball‐bearing mechanisms of block rotation. The similarities of the blocks' spin rates to that of the Pacific suggests that the Pacific strongly influences their motions through edge tractions. However, it is shown that the blocks cannot rotate about the Pacific–North America pole without spinning counter to the sense of Pacific–North America shear. Unlike some other broad plate boundaries, in the western United States, vertical axis rotations take up very little of the slip rate budget across the region.

Pseudoelastic cycling of ultra-fine-grained NiTi shape-memory wires

Abstract In the present study, we investigate pseudoelastic pull – pull cycling of ultra-fine-grained (40 nm) Ni-rich (50.9 at.% Ni) NiTi shape-memory wires at temperatures ranging from 301 to 323 K. Strain-controlled experiments were performed using incremental strain steps and different constant maximum strains. Pull-pull cycling results in decreasing/increasing plateau stresses characterizing the forward/reverse transformations and an accumulation of non-recoverable strain. Saturation is reached after 30 cycles. We interpret our results in terms of a microstructural scenario where dislocations, which are introduced during the martensitic transformation (lattice invariant shear) and during pull – pull cycling (dislocation plasticity), interact with the stress-induced formation of martensite. We show that the slopes of stress– strain curves naturally depend on the total strain imposed in strain-controlled testing. We also provide a dislocation-based explanation for the evolving stress levels of the loadi...

The effects of ambient humidity on the mechanical properties and surface chemistry of hygroscopic silica aerogel

The stress vs. strain relation of hygroscopic aerogels was tested in a controlled humidity chamber to study how the Young’s modulus and non-recoverable strain are affected by the adsorption of water in the gel. Mass gains over desiccated conditions varied from 4% at 32% relative humidity (RH) to 12% at 70% RH. Above 70% RH, the samples failed catastrophically. The Young’s modulus increased from 0.51 to 0.70 MPa and non-recoverable strain increased from 0.047 to 0.059 over the same humidity range. Yield stress did not show significant changes. The samples were also studied with near-infrared transmission measurements to determine if the adsorbed water was chemically as well as physically bound to the surface. Several absorption lines indicative of hydrogen bonding between water and silica were seen to increase with increasing humidity while surface silanol lines decreased.