Constant Displacement Rate Research Articles

Magnitude Clustering During Stick‐Slip Dynamics on Laboratory Faults

AbstractWe present an analysis of magnitude clustering of microfractures inferred from acoustic emissions (AEs) during stick‐slip (SS) dynamics of faulted Westerly granite samples in frictional sliding experiments, with and without fluids, under triaxial loading with constant displacement rate. We investigate magnitude clustering in time across periods during, preceding and after macroscopic slip events on laboratory faults. Our findings reveal that magnitude clustering exists such that subsequent AEs tend to have more similar magnitudes than expected. Yet, this clustering only exists during macroscopic slip events and is strongest during major slip events in fluid‐saturated and dry samples. We demonstrate that robust magnitude clustering arises from variations in frequency‐magnitude distributions of AE events during macroscopic slip events. These temporal variations indicate a prevalence of larger AE events right after (0.3–3 s) the SS onset. Hence, magnitude clustering is a consequence of non‐stationarities.

Mechanical Impact of Heterogeneously Distributed H2O on Quartz Deformation

AbstractIn order to identify relations between mechanical behavior, deformation mechanisms, microstructural properties, and H2O distribution, Tana‐quartzite samples with added H2O ranging from 0 to 0.5 wt.% were deformed by axial shortening at constant displacement rates, at 900°C and 1 GPa, reaching up to ∼30% bulk strain. Samples with lower quantities of added H2O (0.1 and 0.2 wt.%) were in average ∼30 MPa weaker than the as‐is samples with no added H2O. In contrast, samples with more than 0.2 wt.% added H2O revealed more variable mechanical behavior, showing either weaker or stronger trend. The weaker samples showed strain localization in their central parts in the vicinity of the thermocouple, that is, the hottest parts of the samples, whereas the stronger samples showed localization in their upper, slightly colder parts. Bulk deformation is accommodated by crystal plasticity and dissolution‐precipitation processes. Distribution of H2O in our samples revealed systematic decrease of H2O content in the interiors of original grains, caused by increasing strain and H2O draining into grain boundary regions. With increasing content of added H2O, the quartz recrystallization gradually changes from subgrain‐rotation‐dominated to crack‐induced nucleation, along with increasing quantity of melt/fluid pockets. The unexpected strain localization in the upper parts of stronger samples most likely results from mode‐1‐cracking that led to drainage of grain boundaries (GB) due to the crack dilatancy effect, and inhibited dissolution‐precipitation in the hottest part of the samples next to the thermocouple. The locus of deformation is then shifted to colder regions where more H2O is available along GB.

Influence of test paradigm on loading dynamics during proximal femur fracture tests simulating sideways falls

Fall-related hip fractures are a serious public health issue in older adults. As most mechanistic hip fracture risk prediction models incorporate tissue tolerance, test methods that can accurately characterize the fracture force of the femur (and factors that influence it) are imperative. While bone possesses viscoelastic properties, experimental characterization of rate-dependencies has been inconsistent in the whole-femur literature. The goal of this study was to investigate the influence of experimental paradigm on loading rate and fracture force (both means and variability) during mechanical tests simulating lateral fall loadings on the proximal femur. Six pairs of matched femurs were split randomly between two test paradigms: a ‘lower rate’ materials testing system (MTS) with a constant displacement rate of 60 mm/s, and a hip impact test system (HIT) comprised of a custom-built vertical drop tower utilizing an impact velocity of 4 m/s. The loading rate was 88-fold higher for the HIT (mean (SD) = 2465.49 (807.38) kN/s) compared to the MTS (27.78 (10.03) kN/s) paradigm. However, no difference in fracture force was observed between test paradigms (mean (SD) = 4096.4 (1272.6) N for HIT, and 3641.3 (1285.8) N for MTS). Within-paradigm variability was not significantly different across paradigms for either loading rate or fracture force (coefficients of variation ranging from 0.311 to 0.361). Within each test paradigm, significant positive relationships were observed between loading rate and fracture force (HIT adjusted R2 = 0.833, p = 0.007; MTS adjusted R2 = 0.983, p < 0.0001). Overall, this study provides evidence that energy-based impact simulators can be a valid method to measure femoral bone strength in the context of fall-related hip fractures. This study motivates future research to characterize potential non-linear relationships between loading rate and fracture threshold at both macro and microscales.

Intermittent Criticality Multi‐Scale Processes Leading to Large Slip Events on Rough Laboratory Faults

AbstractWe discuss data of three laboratory stick‐slip experiments on Westerly Granite samples performed at elevated confining pressure and constant displacement rate on rough fracture surfaces. The experiments produced complex slip patterns including fast and slow ruptures with large and small fault slips, as well as failure events on the fault surface producing acoustic emission bursts without externally‐detectable stress drop. Preparatory processes leading to large slips were tracked with an ensemble of ten seismo‐mechanical and statistical parameters characterizing local and global damage and stress evolution, localization and clustering processes, as well as event interactions. We decompose complex spatio‐temporal trends in the lab‐quake characteristics and identify persistent effects of evolving fault roughness and damage at different length scales, and local stress evolution approaching large events. The observed trends highlight labquake localization processes on different spatial and temporal scales. The preparatory process of large slip events includes smaller events marked by confined bursts of acoustic emission activity that collectively prepare the fault surface for a system‐wide failure by conditioning the large‐scale stress field. Our results are consistent overall with an evolving process of intermittent criticality leading to large failure events, and may contribute to improved forecasting of large natural earthquakes.

Biomechanical Behavior of a 3D-Printed Denture Base Material.

To evaluate relevant material properties (flexural strength [σf], elastic modulus [E], water sorption [Wsp] and solubility [Wsl], and biocompatibility) of an additive manufacturing (AM) polymer vs a heat-curing acrylic resin (AR; control) for the manufacture of complete dentures, testing the hypothesis that fabrications from both materials would present acceptable material properties for clinical use. The σf, E, Wsp, and Wsl were evaluated according to the ISO 20795-1:2013 standard, and the biocompatibility was evaluated using MTT and SRB assays. Disk-shaped specimens were fabricated and used for Wsp (n = 5), Wsl (n = 5), and biocompatibility (n = 3) testing. For assessment of σf and E, bar-shaped specimens (n = 30) were fabricated and stored in 37°C distilled water for 48 hours or 6 months before flexural testing in a universal testing machine with a constant displacement rate (5 ± 1 mm/minute). Data from σf, E, Wsp, Wsl, and biocompatibility tests were statistically analyzed using Student t test (α = .05). Weibull analysis was also used for σf and E data. Significant differences between the two materials were found for the evaluated material properties. Water storage for 6 months did not affect the flexural strength of the AM polymer, but this material showed inadequate σf and Wsl values. Despite adequate biocompatibility and strength stability after 6 months of water storage, the AM polymer recommended for complete dentures needs further development to improve the material properties evaluated in this study.

Large-scale heterogeneities can alter the characteristics of compressive failure and accelerated seismic release.

Externally stressed brittle rocks fail once the stress is sufficiently high. This failure is typically preceded by a pronounced increase in the total energy of acoustic emission (AE) events, the so-called accelerated seismic release. Yet, other characteristics of approaching the failure point such as the presence or absence of variations in the AE size distribution and, similarly, whether the failure point can be interpreted as a critical point in a statistical physics sense differs across experiments. Here, we show that large-scale stress heterogeneities induced by a notch fundamentally change the characteristics of the failure point in triaxial compression experiments under a constant displacement rate on Westerly granite samples. Specifically, we observe accelerated seismic release without a critical point and no change in power-law exponent ε of the AE size distribution. This is in contrast to intact samples, which exhibit a significant decrease in ε before failure. Our findings imply that the presence or absence of large-scale heterogeneities play a significant role in our ability to predict compressive failure in rock.

Facile mechanochemical cycloreversion of polymer cross-linkers enhances tear resistance.

The mechanical properties of covalent polymer networks often arise from the permanent end-linking or cross-linking of polymer strands, and molecular linkers that break more easily would likely produce materials that require less energy to tear. We report that cyclobutane-based mechanophore cross-linkers that break through force-triggered cycloreversion lead to networks that are up to nine times as tough as conventional analogs. The response is attributed to a combination of long, strong primary polymer strands and cross-linker scission forces that are approximately fivefold smaller than control cross-linkers at the same timescales. The enhanced toughness comes without the hysteresis associated with noncovalent cross-linking, and it is observed in two different acrylate elastomers, in fatigue as well as constant displacement rate tension, and in a gel as well as elastomers.

Sintering and deformation properties of forsterite + diopside aggregates in an electrical field

A new technique called “flash sintering” has been proposed in the field of materials science. When a compact powder of oxide material is sintered in an electrical field, densification accelerates at a certain temperature, and the sintering is completed in a shorter time at a lower temperature as compared with conventional sintering experiments. In addition, tensile deformation experiments conducted on a dense oxide body in an electrical field have shown that plastic deformation can be enhanced at lower temperatures with lower stresses, as compared with conventional deformation experiments. Fluctuations in the magnetic and electrical fields derived from outside Earth, such as solar activity, induce an electrical field inside Earth. The strength of the electrical field in Earth's mantle is still debated, but it could be up to 10−6 V/cm. Although electrical fields inside Earth may affect the deformation behavior of rocks, no experimental studies have been conducted on geomaterials to assess this effect. Therefore, based on these recent findings in materials science, we conducted sintering and deformation experiments to examine the effects of an electrical field on high-temperature mass transport. We used forsterite + diopside crystal aggregates, which are good analogs for Earth's mantle. The sintering experiments demonstrated that the relative density increases with an increasing electrical field at a constant temperature. In the deformation experiment with a constant displacement rate, the samples in an electrical field of 1000 V/cm were deformed by 4.4× lower stress than for samples not in an electrical field. These features suggest that diffusional mass transport is enhanced in an electrical field. We propose that the presence of an electrical field within Earth may accelerate the deformation of mantle materials.

Biomechanical properties of a 3D printing polymer for provisional restorations and artificial teeth.

To characterize a resin-based polymer used for 3D printing (3D) provisional restorations and artificial teeth by evaluating relevant material's properties (flexural strength (σf), elastic modulus (E), water sorption (Wsp) and solubility (Wsl)) and biocompatibility, and comparing to a bis-acryl composite resin (BA) and a heat-cured acrylic resin (AR). Structures were fabricated from 3D, BA and AR. Bar-shaped specimens (n=30) were submitted to three-point flexure (in 37ºC water and constant displacement rate: 1±0.3mm/min) until fracture to calculate σf and E. Additional specimens (n=30) were aged in 37ºC distilled water for six months before testing for σf. Disc-shaped specimens (n=5) were dried in desiccators and oven until weight stability was reached, then they were immersed in distilled water for seven days, weighed and submitted to the drying process to obtain Wsp and Wsl. SRB and MTT assays were used to evaluate biocompatibility. Data were statistically analyzed using Kruskal Wallis, Student-Newman-Keuls (α=.05), and Weibull distribution. ANOVA and Tukey (α=.05) were used to evaluate the biocompatibility data. 3D structures showed higher σf than AR after aging. The BA showed the lowest values for σf and E, at baseline and after aging. All materials showed Wsp and Wsl values within the recommended standard values. AR structures showed lower cell viability (71.9%) than 3D (92.9%) and BA (90.8%) when using the SRB test. No difference was found when using MTT (p>.05). The evaluated polymer-based 3D printing material showed adequate biomechanical behavior for using as a provisional restoration and artificial teeth.

The geodynamic and limnological evolution of Balkan Lake Ohrid, possibly the oldest extant lake in Europe

Studies of the upper 447 m of the DEEP site sediment succession from central Lake Ohrid, Balkan Peninsula, North Macedonia and Albania provided important insights into the regional climate history and evolutionary dynamics since permanent lacustrine conditions established at 1.36 million years ago (Ma). This paper focuses on the entire 584‐m‐long DEEP sediment succession and a comparison to a 197‐m‐long sediment succession from the Pestani site ~5 km to the east in the lake, where drilling ended close to the bedrock, to unravel the earliest history of Lake Ohrid and its basin development. 26Al/10Be dating of clasts from the base of the DEEP sediment succession implies that the sedimentation in the modern basin started at c. 2 Ma. Geophysical, sedimentological and micropalaeontological data allow for chronological information to be transposed from the DEEP to the Pestani succession. Fluvial conditions, slack water conditions, peat formation and/or complete desiccation prevailed at the DEEP and Pestani sites until 1.36 and 1.21 Ma, respectively, before a larger lake extended over both sites. Activation of karst aquifers to the east probably by tectonic activity and a potential existence of neighbouring Lake Prespa supported filling of Lake Ohrid. The lake deepened gradually, with a relatively constant vertical displacement rate of ~0.2 mm a−1 between the central and the eastern lateral basin and with greater water depth presumably during interglacial periods. Although the dynamic environment characterized by local processes and the fragmentary chronology of the basal sediment successions from both sites hamper palaeoclimatic significance prior to the existence of a larger lake, the new data provide an unprecedented and detailed picture of the geodynamic evolution of the basin and lake that is Europe’s presumed oldest extant freshwater lake.

Short-term axial loading of continuous-flight pile segment in frozen soil

Research on the performance of continuous-flight piles for the use in permafrost is rare. In the present laboratory research, continuous-flight pile segments with 89 mm diameter and 300 mm length were installed in frozen soils and loaded under a constant displacement rate. The objectives were to investigate the load-transfer mechanism and the failure modes associated with the shaft resistance of the piles. Pile segments were screwed into a chamber filled with frozen soil and then loaded in the axial direction; installation torques and pile load resistance were measured, respectively, during the installation and loading stages. The effects of soil temperature, salinity, and water content on the short-term performance of these piles were examined. Results show that the pile capacities decreased with warming temperatures. The installation torque increased significantly as the soil temperature and salinity decreased. Cylindrical shearing mode (CSM) and individual bearing mode (IBM) of pile failure were observed at soil temperatures of −1 and −5 °C, respectively. Adfreeze and plate bearing resistance were determined under these failure modes. The global failure surface occurred at the edge of the threads under the CSM, and the shear resistance was increased along this surface. The existence of plate (i.e., flights) bearing resistance under the IBM suggests a potential advantage of continuous-flight piles over conventional smooth piles, because the plate bearing resistance is mobilized during loading.

The Influence of Static Load and Sideways Impact Fall on Extramedullary Bone Plates Used to Treat Intertrochanteric Femoral Fracture: A Preclinical Strength Assessment.

Hip fracture accounts for a large number of hospitalizations, thereby causing substantial economic burden. Majority (> 90%) of all hip fractures are associated to sideways fall. Studies on sideways fall usually involve loading at quasi-static or at constant displacement rate, which neglects the physics of actual fall. Understanding femur resonance frequency and associated mode shapes excited by dynamic loads is also critical. Two commercial extramedullary implants, proximal femoral locking plate (PFLP) and variable angle dynamic hip screw (VA-DHS), were chosen to carry out the preclinical assessments on a simulated Evans-I type intertrochanteric fracture. In this study, we hypothesized that the behavior of the implant depends on the loading types-axial static and transverse impact-and a rigid implanted construct will absorb less impact energy for sideways fall. The in silico models were validated using experimental measurements of full-field strain data obtained from a 2D digital image correlation (DIC) study. Under peak axial load of 3 kN, PFLP construct predicted greater axial stiffness (1.07 kN/mm) as opposed to VA-DHS (0.85 kN/mm), although the former predicted slightly higher proximal stress shielding. Further, with greater mode 2 frequency, PFLP predicted improved performance in resisting bending due to sideways fall as compared to the other implant. Overall, the PFLP implanted femur predicted the least propensity to adverse stress intensities, suggesting better structural rigidity and higher capacity in protecting the fractured femur against fall.

Effects of surface curvature and dislocation dynamics: Dynamical deformation mechanisms for uniaxial compression tests at the nanoscale

The understanding of size effects in micro-crystal plasticity has been in-part based on controlled uniaxial mechanical testing of crystalline micropillars that may be monitored in-situ, using modern microscopy approaches. Nevertheless, it has always been clear that mechanics and materials science are not ideally decoupled in uniaxial micropillar compression, thus agreement between experiments and theory remains challenging. We present a theoretical analysis of the uniaxial compression of micropillars with curved top free surfaces, in consistency with modern experimental thresholds. By using coupled Finite Element and Discrete Dislocation Dynamics simulations we investigate the effect of the small curvature to dislocation microstructure evolution at constant displacement rate. The uniaxial compression of flat micropillars is shown to be consistent with existing literature, with homogeneous stress build up and random activation of sources inside the volume. However, in the presence of a small top-surface micropillar curvature, there are significant dynamical effects on dislocation mechanisms and an overestimate of strain at yielding that leads to large errors on capturing elastic compression moduli and avalanche noise characteristics. Characteristically, for 10 nm-high isotropic curvature, large (>100MPa) stress drops emerge in the average stress, that become larger as the initial dislocation density increases, in direct contrast to expectations and findings for ideally flat micropillars.

Strain localisation in sensitive clays: can rate dependency provide mesh independent results?

Finite element modelling involving strain-softening materials becomes mesh dependent if not regularised by a mesh independent length scale. The objective of this paper is to investigate whether the inherent strain rate dependency of a sensitive clay can result in mesh independent results in the post-peak strain-softening regime. It describes first a constitutive model that accurately reproduces the rate-dependent behaviour obtained from laboratory tests on block samples of a natural sensitive clay. Then finite element analyses of soil columns under constant shear displacement rate are conducted. The boundaries are closed but internal flow of pore water is allowed. Strain localisation is triggered using a zone with lower undrained shear strength. The results show an interval of regularised strain-softening behaviour before the solution becomes mesh dependent. Pore water flow is found to delay the onset of mesh dependency. In conclusion, this paper achieves a regularised force displacement response for a strain-softening clay, without the need to introduce an artificial internal length scale.

Debonding of a soft adhesive fibril in contact with an elastomeric pillar.

The debonding criterion of fibrils of soft adhesive materials is a key element regarding the quantitative modelisation of pressure sensitive adhesive tapes' peeling energy. We present in this article an experimental study of the detachment of a commercial acrylic adhesive tape from the top surface of a single micrometric pillar of PDMS elastomer. During an experiment, the pillar and the adhesive, after being put in contact, are separated at a constant displacement rate, resulting in the formation, the elongation and the final detachment of a fibril of adhesive material. A systematic study allows us to uncover power laws for the maximum force and the critical elongation of the fibril at debonding as a function of the diameter of the cylindrical pillar which controls the diameter of the fibril. The scaling law evidenced for the critical elongation appears as a first step toward the understanding of the debonding criterion of fibrils of soft adhesive materials. In addition, viscoelastic digitation at the triple debonding line is observed during detachment for large pillar diameters. The wavelength and penetration length of the fingers that we report appear to be consistent with existing models based on pure elastic mechanical response.

Multiscale Modeling and Homogenization Analysis of FW-CFRP for FCV Hydrogen Tanks

In this study, elastic-viscoplastic properties of filament-winding carbon fiber-reinforced plastics (FW-CFRPs) are investigated using a three-scale homogenization method developed by the authors. For this, the three-scale homogenization method for FW-CFRPs is described. Then, uniaxial tensile tests of FW-CFRPs with five kinds of laminate configurations are conducted at constant displacement rates. The experimental results obtained show that the laminate configurations have significant effects on the elastic-viscoplastic properties of FW-CFRPs. Such experimental results are analyzed using the three-scale homogenization method, and both results are in excellent agreement, showing the validity of the present method. Moreover, a fiber bundle undulation model is created and analyzed using the three-scale homogenization method to investigate the effects of undulation of fiber bundles on the elastic-viscoplastic properties of FW-CFRPs. It is shown that the undulation clearly affects the elastic stiffness and flow stress of FW-CFRPs.

A single experiment with variable low-pressure air flow through a packed bed leads to two methods for determining particle size

Two piston-in-cylinders, charged with air at ambient temperature and pressure, are moved from rest to a constant displacement rate in a syringe pump. The air is pumped to a packed column and an elevated steady state pressure, measured just upstream of the bed, is quickly attained. Upon stopping the pistons' motion, the pressure decays back to its ambient level. The growth and decay phases are described by material balances and the Blake–Kozeny laminar-flow model. The unknown parameters in solutions to the resulting first-order differential equations are determined by fitting the pressure–time data. For the growth phase, the fit leads to the determination of the initial system volume and the steady state pressure, whereas for the decay phase the fit gives a pseudo-time constant. This provides sufficient information to calculate the permeabilities and mean particle sizes—the Sauter mean diameters—for both phases. Packed columns of Aldrich Sand, sieve-size range 0.211–0.297 mm, and glass beads, nominal size 1 mm, give the following means: for Aldrich sand, 0.26 ± 0.02 and 0.27 ± 0.02 mm; for the glass beads, 1.11 ± 0.06 and 1.09 ± 0.06 mm, for the growth and decay phases, respectively. Analogous experiments with a capillary tube, internal diameter 0.485 ± 0.001 mm, give the following internal diameters: 0.492 ± 0.007 and 0.501 ± 0.007 mm for the growth and decay phases, respectively.

Biomechanical comparison of two medial patellofemoral ligament reconstruction techniques: Quadriceps tendon fixation versus single-tunnel patella fixation with gracilis autograft did not differ in load to failure and stiffness

BackgroundThe purpose of this study was to evaluate the ultimate failure load and stiffness of two patellar fixation techniques for medial patellofemoral ligament (MPFL) reconstruction: (1) quadriceps tendon fixation (QT), (2) single tunnel (STG) patella fixation with gracilis autograft. MethodsA total of 16 fresh-frozen cadaveric knees (eight matched pairs) were randomized into two groups (QT vs. STG). The MPFL reconstructions were subjected to cyclic loading for 10 cycles to 30 N and then tested to failure at a constant displacement rate of 15 mm/min using a materials-testing machine (MTS 810 Universal Testing System). Failure mode, ultimate failure load and stiffness were recorded for each cadaveric specimen. ResultsThere was no significant difference in mean ultimate failure load among groups (P = 0.35). The STG group failed at a mean ultimate load of 190.04 N [standard deviation (SD) 23.18] and the QT group failed at 206.24 N (SD 37.99).The STG group had a mean stiffness of 21.38 N/mm (SD 1.44). This was not significantly higher than the mean stiffness value achieved for the QT group at 20.36 N/mm (SD 1.3) (P = 0.19). In the QT group all reconstructions failed due to tendon rupture at the patella attachment. The reason for failure in the STG group was the graft–suture connection. ConclusionsThis cadaver study showed no statistically significant difference in biomechanical performance of the evaluated patella fixation techniques, in terms of maximum load to failure and stiffness. Both techniques are reliable in terms of biomechanical properties and could offer additional surgical solutions.

Polystyrene/TiO2 nanocomposite coating for strength and toughness enhancement of aluminum alloy 2024–T3 in accelerated stress corrosion cracking

The polystyrene/TiO2 nanocomposite coating is evaluated for reducing the susceptibility of aluminum alloy AA2024-T3 to stress corrosion cracking (SCC). The as-synthesized titania nanoparticles of aspect ratio (AR) 1, 2, and 4 are individually loaded at 25 wt% to the acetone-dissolved polystyrene (PS) to make the nanocomposites. Coated compact tension specimens are tested under accelerated SCC testing that combines the constant displacement rate equivalent to 10−7 s−1 and 3.5 wt% NaCl solution dripped at the crack tip. The load and crack-mouth opening displacement of each specimen are recorded and analyzed to determine the indices to the mechanical ductility, strength, toughness, and fracture toughness. The PS/TiO2 AR1 coated specimens exhibit the least SCC susceptibility, with a relatively high fracture toughness of 55 MPa-m1/2 and more than 60 h of lifetime without apparent cracking under the SCC testing. Intriguingly, the PS-coated specimens perform similarly well. The results are explained by an SCC failure mechanism that considers the complex interaction among the agglomerated nanoparticles, polystyrene matrix, and the substrate in various aspects. The mechanism suggests that the nanocomposite coating with a higher yield strength and less permeability performs better. The highly loaded nanoparticles such as 25 wt% used herein make the nanocomposite only more marginally yieldable than polystyrene, but undesirably introduce morphological and electrochemical defects that count against the SCC susceptibility improvement.

Deformation of upper mantle rocks with contrasting initial fabrics in axial extension

To explore the role of the olivine grain size and crystal preferred orientation (CPO), on the evolution of the microstructure and the mechanical behavior of upper mantle rocks up to large strains, we performed axial extension experiments at 1200 °C, 300 MPa confining pressure, and constant displacement rate leading to strain rates ~10−5 s−1 on three natural peridotites: a fine-grained mylonitic harzburgite with a weak olivine CPO and two coarse-grained well-equilibrated dunites with olivine CPO of variable intensity. Despite the contrasting initial microstructures, initial flow stresses show a limited range of variation (115–165 ± 5 MPa), with the fine-grained harzburgite displaying the highest initial strength. However, the evolution of both mechanical behavior and microstructure differs markedly between fine and coarse-grained peridotites. In the fine-grained harzburgite, necking is associated with decrease in the apparent differential stress. Focusing of strain and stress resulted in increase of the olivine CPO intensity and recrystallized fraction and decrease of the recrystallized grain size in the neck. Analysis of the final stress and strain in the neck indicates softening in response to the evolution of the microstructure and CPO. In contrast, necking of the coarse-grained dunite samples is associated with either a weaker or no decrease in the apparent differential stress. This implies hardening, consistently with (1) the increase in bulk intragranular misorientation with increasing strain observed in these samples and (2) final stresses in the neck similar or higher than the initial ones. All coarse-grained dunites displayed a highly heterogeneous deformation. Crystals well oriented to deform by dislocation glide became elongated and developed marked undulose extinction, whereas crystals in hard orientations remained almost undeformed. In the neck, stress and strain concentration (local stresses and strains attained up to 365 ± 15 MPa and 240%, respectively) resulted in formation of kinks in “hard” crystals, dynamic recrystallization in “soft” crystals and, where the axial stress overcame the confining pressure, development of extensional fractures. We interpret the more effective strain-induced softening of the fine-grained peridotite as due to easier dynamic recrystallization, probably due to the higher proportion of grain boundaries acting as nucleation sites.