Shear Loading Rate Research Articles

Effects of shear loading rate on tetrahydrofuran Hydrate Adhesion strength for enhanced flow assurance

Hydrate formation and accumulation pose significant challenges in oil and gas pipelines, leading to flow assurance issues and safety hazards. It is crucial to understand hydrate adhesion to prevent the accretion of hydrate in petro-pipelines. In this study, the adhesion strength (AS) of tetrahydrofuran (THF) hydrate on various solid surfaces with different elastic moduli subjected to shear loading rates ranging from 10 to 10000 μm/s is comprehensively investigated by shear force experimental measurements and finite element (FE) simulations. The results reveal that the hydrate AS is primarily influenced by the elastic modulus of the substrates, and the shear-induced detachment properties are strongly dependent on the shear loading rate. Specifically, when the hydrate is synthesized on a soft polydimethylsiloxane (PDMS) substrate, the AS exhibits minimal changes at low and high shear loading rates. However, under intermediate shear loading rates, the AS rapidly increases by approximately 500 %. In contrast, when the hydrate is formed on stiff polytetrafluoroethylene (PTFE) and polyethylene (PE) substrates, the AS is increased by approximately 200 % and 230 %, respectively as shear loading rate increases. Interestingly, for the hydrate on a stiff hydrophobic coated glass slide (CG) substrate, the AS anomalously decreases with increasing shear loading rates. These findings provide valuable insights into the effects of substrate properties and shear loading rate on hydrate adhesion. The results are of significance for the design of optimal coatings that ensure flow assurance of oil and gas in pressure-variant petro-pipelines.

Numerical simulation of hybrid CFRP/Al hole-clinched joints at different tensile shear loading rates

In this study, the tensile-shear damage mechanisms and mechanical responses of carbon fiber reinforced plastic (CFRP)/Al clinched joints were investigated at different loading rates. A continuous damage model was developed that incorporated strain rate effects and nonlinear in-plane shear behaviors. The nonlinear in-plane shear behaviors included two stages: the plastic deformation stage and the strengthening by fiber rotation stage. The finite element algorithm was implemented by applying the VUMAT user subroutine in Abaqus to analyze the responses of the CFRP/Al clinched joints with three layups (orthogonal, diagonal, and hybrid). The experimental and simulation results were highly consistent in terms of the joint failure modes, damage evolution predictions and load–displacement responses. The strain rate effects on CFRP damage were determined. Under high-strain-rate conditions, the average reductions in deformation required for the various damage modes were 51.5%, 13.2% and 13.1% for the orthogonal, diagonal and hybrid layup joints, respectively. Furthermore, the corresponding average area reductions in the damage modes were 16.0%, 41.7% and 70.6%, respectively. Then, the strain rate effects on joint mechanical performance were evaluated. The mechanical properties of the orthogonal layup joints did not exhibit strain rate sensitivity, whereas the energy absorption levels decreased by 21.1% and 36.6% for the diagonal and hybrid layup joints, respectively. For the diagonal layup joints, the maximum load increased by 22.4% under dynamic loading.

Frictional behavior of quartz gouge during slide-hold-slide considering normal stress oscillation

Slide-hold-slide (SHS) test is an essential experimental approach for studying the frictional stability of faults. The origin SHS framework was established based on a consistent constant normal stress, which cannot truly reflect the stress disturbance around fault zones. In this paper, we conducted a series of ‘dynamic SHS tests’, which includes normal stress oscillations in the relaxation stage with different oscillation amplitudes and frequencies on synthetic quartz gouge using a double direct shear assembly. The experimental results reveal that the amplitude of the normal load oscillation has a remarkable effect on the frictional relaxation and healing patterns. However, the frequency of the normal load oscillation has a minor effect. Additionally, the shear loading rate is proportional to the normal loading rate during the relaxation stage, and the normal stiffness of the quartz layer remains nearly constant under various loading conditions. The creep rate during the hold phase is not obviously affected by the normal load oscillation, while the precursory slip is also sensitive to the oscillation amplitude. This study provides insights into the evolution of frictional stability in discontinuities and is beneficial for controlling relative disasters in fault zones.

Visualization of stick-slip shear failure process of granite by 3D reconstruction technique and DEM

Understanding the mechanism of stick-slip instability of faults is of great significance for earthquake forecast and seismic fortification. In this paper, combined with the 3D reconstruction technology and discrete element method (DEM), the stick-slip shear failure process is visualized and the evolution mechanism of each stage of stick-slip process is studied. First, the 3D morphology scanning tests are carried out to define the 3D topography of the sample interface, which is imported into PFC3D to generate particle assembly, and micro-parameters of the particle assembly are calibrated by the experimental results. Second, the loading conditions are applied to the particle assembly, the stick-slip instability of the simulated fault occurs spontaneously, and the evolution of fault displacement and velocity distribution during the whole stick-slip period are studied. Third, the effects of normal stress, shear loading rate and interface roughness on stick-slip instability are investigated. Finally, nine parameters related to the steady-state friction coefficient, namely normal stress, natural density, Young's modulus, Poisson's ratio, maximum static friction coefficient, reference slip velocity, reference friction coefficient, a-b and slip velocity, are selected to establish the steady-state friction coefficient forecast model based on back propagation neural network (BPNN), and the relative importance analysis of relevant parameters is also implemented. Our research is helpful to reveal the seismogenic mechanism.

The evolution of rock friction is more sensitive to slip than elapsed time, even at near-zero slip rates

Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions have led to constitutive models that assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact-area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during "load-point hold" experiments, wherein a sliding interface is allowed to gradually self-relax down to subnanometric slip rates. In contrast, following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease-a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenological regime traditionally inferred to demonstrate time-dependent frictional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 .[Formula: see text]. to less than [Formula: see text], we found that constitutive models where low slip-rate friction evolution mimics log-time contact-area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model, in which friction evolves only with interfacial slip, fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip-dependent, even at subnanometric slip rates.

Experimental investigation of the effects of loading rate, contact roughness, and normal stress on the stick-slip behavior of faults

The tectonic movement rate, in situ stresses (e.g., normal stress) and fault roughness may change during the long evolutionary histories of seismogenic faults. To investigate the effects of these three factors on fault slip behaviors, we conducted direct shear tests by sequentially changing the shear loading rate and normal stress on small-scale granite specimens under three roughness conditions. We found that for a given fault system, the shear loading rate is a primary controlling parameter of the recurrence intervals of stick-slip events, and the correlation between them can be empirically expressed by a power law. Stick-slip recurrence intervals can influence the stress drops of stick-slip events via two different tendencies. The shear stress drops increase quasilinearly with the recurrence intervals at a given shear loading rate, while the combined data from all shear rate tests show that the shear stress drops increase with the logarithm of the recurrence time. In the investigated range of fault roughness, we observed that surface roughness conditions strikingly affect the sliding mode and recurrence frequency of stick-slip events. In detail, the two smooth fault systems slide with regular stick-slip events, and the roughest fault system exhibits bimodal behavior. Namely, at lower normal stresses, stick-slip events cannot be stably generated, a few were occasionally generated, and regular stick-slip events were interrupted by local stick-slip. When the applied normal stress is sufficiently high, the fault slides with continuous regular stick-slip events in the same way as those in the smoother fault systems. We further noticed that the recurrence frequency of stick-slip and friction drop are correlated at a fixed sliding distance, and this relationship can be described by an inverse proportional function. Our results suggest that all three factors, namely, shear loading rate, fault surface roughness and normal stress, can influence the behaviors of fault slip and hence are important for evaluating earthquake hazards.

On the Effect of Shear Loading Rate on Contact Area Shrinking in Adhesive Soft Contacts

Adhesion and, its interplay with friction, is central in several engineering applications involving soft contacts. Recently, there has been an incredible push towards a better understanding on how the apparent contact area evolves when a shear load is applied to an adhesive soft contact, both experimentally and theoretically. Although soft materials are well-known to exhibit rate-dependent properties, there is still a lack of understanding in how the loading rate could affect the contact area shrinking. Indeed, most of the experiments involving a sphere-flat contact have been conducted at a fixed loading rate, and, so far, analytical models have assumed a constant work of adhesion, independent on the peeling velocity. Here, by using linear elastic fracture mechanics, an analytical model is derived for the contact of a rigid sphere on a soft adhesive substrate, which is aimed at elucidating the effect that a rate-dependent work of adhesion has on the contact area shrinking. The model results show that contact area reduction is very sensitive to the loading rate, with slower loading rates promoting a stronger shrinking, which seems in agreement with Literature results. Furthermore it is shown that rate effects enhance the apparent interfacial toughness, i.e. more energy is needed to drive the system from full stick up to gross sliding.

Direct shear experiment on salt rock incorporating loading rate effect

To investigate the shear behavior of salt rock under different loading rates, direct shear testing was conducted on high-purity salt rock obtained from Pakistan using a YZW-100 apparatus. The experimental results of the shear stress, deformation, and failure characteristics of the salt rock showed that the loading rate is an essential factor influencing the shear strength parameters of the rock. When the shear loading rate was increased, the cohesion of the salt rock significantly increased; however, the internal friction angle slightly decreased, nearly remaining unchanged. The shear failure mode of the salt rock manifested by shear stress-displacement curves tended to be ductile failure with increased normal force. The residual stress of the salt rock was large, indicating its high friction-bearing capacity. The dilatancy deformation was also subjected to shear loading. The fracture section of the salt rock exhibited visible local scratch marks. These experimental results are helpful for further understanding the failure mechanism of salt rock and the stability of underground gas storage under different gas injection and production scenarios.

Comparative assessment of the interlayer shear-bond strength of geogrid reinforcements in hot-mix asphalt

With the increasing use of geogrid reinforcements to mitigate reflective cracking in hot-mix asphalt (HMA) overlays, interlayer (interface) bonding has become an even more critical aspect of HMA placement/construction to mitigate delamination and debonding of the HMA overlay. To comparatively evaluate the interlayer bond strength due to the effects of the geogrid reinforcements, the shear bond strength test was conducted in this laboratory study, using unreinforced control HMA samples as the reference datum. Cylindrical HMA samples (150 mm diameter) gyratory compacted in two 75-mm lift thicknesses, with the geogrid reinforcement in-between the two lifts, were used for testing at room temperature under a monotonically shear loading rate of 5 mm/min. Emulsified asphalt was used as the interlayer tack coat and six different geogrid materials, which are polyester-based (FA) and fiberglass-based (FG), were comparatively evaluated. As theoretically expected, the control (unreinforced) HMA samples exhibited superiority followed closely by samples reinforced with polyester-based geogrids. Although comparable to the values reported in the literature, HMA samples reinforced with fiberglass-based geogrids performed the poorest with the lowest interlayer bond strengths – that is the polyester-based outperformed the fiberglass-based geogrids. Overall, the interlayer bond strength exhibited a general decreasing trend with a decrease in the geogrid mesh size (open area), increase in the geogrid strand thickness, and material grade. Thus, in as much as reflective crack mitigation is structurally desired, due diligence must be cautiously exercised when selecting the geogrid type/grade for use in HMA reinforcement to ensure sufficient interlayer bonding and minimize any potential delamination/debonding problems in service.

The effect of salt in dilatant faults on rates and magnitudes of induced seismicity – first results building on the geological setting of the Groningen Rotliegend reservoirs

AbstractThe presence of salt in dilatant normal faults may have a strong influence on fault mechanics in the Groningen field and on the related induced seismicity. At present, little is known of the structure of these fault zones. This study starts with the geological evolution of the Groningen area, where, during tectonic faulting, rock salt may have migrated downwards into dilatant faults. These fault zones therefore may contain inclusions of rock salt. Because of its rate-dependent mechanical properties, the presence of salt in a fault may introduce a loading-rate dependency into fault movement and affect the distribution of magnitudes of seismic events. We present a first-look study showing how these processes can be investigated using a combination of analogue and numerical modelling. Full scaling of the models and quantification of implications for induced seismicity in Groningen require further, more detailed studies: an understanding of fault zone structure in the Groningen field is required for improved predictions of induced seismicity. The analogue experiments are based on a simplified stratigraphy of the Groningen area, where it is generally thought that most of the Rotliegend faulting has taken place in the Jurassic, after deposition of the Zechstein. This suggests that, at the time of faulting, the sulphates were already transformed into brittle anhydrite. If these layers were sufficiently brittle to fault in a dilatant fashion, rock salt was able to flow downwards into the dilatant fractures. To test this hypothesis, we use sandbox experiments where we combine cohesive powder as analogue for brittle anhydrites and carbonates with viscous salt analogues to explore the developing fault geometry and the resulting distribution of salt in the faults. Using the observations from analogue models as input, numerical models investigate the stick-slip behaviour of fault zones containing ductile material qualitatively with the discrete element method (DEM). Results show that the DEM approach is suitable for modelling the seismicity of faults containing salt. The stick-slip motion of the fault becomes dependent on shear loading rate with a modification of the frequency–magnitude distribution of the generated seismic events.

Evaluation of the effective parameters on shear resistance of interface in a geocomposite-reinforced pavement

Application of geosynthetics is known as an important way to reduce and delay emergence of reflective cracks. However, reliance on these materials can have some disadvantages, including reduction of shear resistance in the interface between two HMA layers, along with emergence of some defects in areas on which vehicles brake, accelerate or turn. A large number of parameters affect the interlayer shear resistance. In this research activity, a double-shear device has been developed and evaluated in order to measure shear resistance between layers in the interface zone. The device is equipped by a system for application and control of normal compressive and tensile loading. In this research, using the L16 (2 × 2–4 × 4) orthogonal array, the factors that are most influential on the shear resistance of the interlayer zone reinforced by geocomposite are evaluated. Temperature, with effectiveness 2.4 times greater than that of the other parameters, the effect of which is relatively uniform, is found to be the most relevant parameter, followed by the tack-coat application rate. The findings indicate that the shear loading rate and geocomposite grid size have the next most significant contribution to the maximum shear resistance, both with and without application of normal load.

An experimental investigation on the shear mechanism of fatigue damage in rock joints under pre-peak cyclic loading condition

In this study, the shear mechanism of fatigue damage in rock joints with first-order and second-order triangular asperities under pre-peak cyclic loading conditions is investigated in laboratory. A monotonic shear test is firstly conducted to determine the shear strength in rock samples containing joints with different orders of asperities. Secondly, influence of the pre-peak cyclic loading conditions with various numbers of cycles on the shear mechanism of fatigue damage in rock joints is studied at constant normal stress. In the cyclic loading conditions, two consecutive steps, such as load-controlled and displacement-controlled, are applied in tests. The load-controlled step is used to achieve the cycles of pre-peak loading in shear tests, and the displacement-controlled step is then applied to realize the final failure of rock samples with constant rate of 0.5mm/min. Moreover, the effects of shear loading rates, numbers of cycle and cyclic shear loading magnitude on fatigue damage, peak shear strength and residual shear strength of rock joints are researched. We found that fatigue damage occurs at the second-order asperities in the upper and lower blocks within low number of cycles, but the fatigue cracks initiated with initiation angle of 90° with respect to the first-order asperities in the upper and lower blocks coalesce with each other (or rock joints) within the high number of cycles. Meanwhile, the variations of peak shear strength and residual shear strength of rock joints within low number of cycles are also different from ones within the high number of cycles.

Rearfoot and Midfoot or Forefoot Impacts in Habitually Shod Runners

Shear loading rates (LR) have not been investigated in runners with a mid- or forefoot strike (FFS) versus rearfoot strike (RFS). The purpose of this study was to compare three-dimensional ground reaction forces (GRF) and LR during impact in habitual rearfoot strikers (hRF) and habitual forefoot strikers (hFF) strikers. Thirty competitive runners performed 10 overground running trials with both foot strike styles. Peak three-dimensional and resultant GRF and instantaneous LR during impact were compared. Vertical LR significantly decreased for hRF using an FFS (RFS = 148 ± 36 body weight [BW]·s(-1), FFS = 98 ± 31 BW·s(-1)) but was similar for hFF running with either foot strike (FFS = 136 ± 35 BW·s(-1), RFS = 135 ± 28 BW·s(-1)). Posterior impact forces were present during FFS but not during RFS, and posterior LR was significantly greater for both groups during FFS (-58 ± 17 vs -19 ± 6 BW·s(-1)). Medial impact forces were also present during FFS but not during RFS, and medial LR was significantly larger for both groups during FFS (-21 ± 7 vs -6 ± 6 BW·s(-1)). Interestingly, hFF had greater impact peaks and LR in all directions compared with hRF during FFS. This may be explained by hFF using a smaller strike index (hFF = 62% ± 9%, hRF = 67% ± 9%; P = 0.02), which was significantly inversely related to vertical LR and impact peak. Peak resultant and vertical LR are not ubiquitously lower when using a shod FFS versus RFS despite an absence of resultant and vertical impact peaks. Furthermore, there were impact peaks in the posterior and medial directions, leading also to greater LR in these directions during FFS. Therefore, transitioning from RFS to FFS in traditional running shoes may not offer long-term protection against impact-related running injuries because hFF running with an FFS demonstrated many GRF and LR similar to or greater than RFS.

Dynamic Geometrically Nonlinear Behavior of FRP-Strengthened Walls with Debonded Regions

AbstractThis paper studies the dynamic geometrically nonlinear behavior of walls that are strengthened with fiber-reinforced polymer (FRP) composite materials but include preexisting debonded regions. For that purpose, two specially tailored finite elements corresponding to the perfectly bonded regions and debonded regions within the layered wall are formulated under the umbrella of a dynamic analysis. The two finite elements are based on a high-order multilayered plate theory. The geometrical nonlinearity is introduced by means of the Von Karman nonlinear strains. The convergence and validity of the geometrically nonlinear dynamic model are studied for the case of a locally debonded FRP-strengthened wall under in-plane shear loads applied at a constant rate. The unified model of the strengthened wall with a local delamination is then used for studying the dynamic nonlinear behavior under different levels of shear loading rate. The dynamic results and the comparison with static analyses reveal the impact ...

Evaluation of Shear-Induced Platelet Activation Models Under Constant and Dynamic Shear Stress Loading Conditions Relevant to Devices

The advent of implantable blood-recirculating devices such as left ventricular assist devices and prosthetic heart valves provides a viable therapy for patients with end-stage heart failure and valvular disease. However, device-generated pathological flow patterns result in thromboembolic complications that require complex and lifelong anticoagulant therapy, which entails hemorrhagic risks and is not appropriate for certain patients. Optimizing the thrombogenic performance of such devices utilizing numerical simulations requires the development of predictive platelet activation models that account for variations in shear-loading rates characterizing blood flow through such devices. Platelets were exposed in vitro to both dynamic and constant shear stress conditions emulating those found in blood-recirculating devices in order to determine their shear-induced activation and sensitization response. Both these behaviors were found to be dependent on the shear loading rates, in addition to shear stress magnitude and exposure time. We then critically examined several current models and evaluated their predictive capabilities using these results. Shear loading rate terms were then included to account for dynamic aspects that are either ignored or partially considered by these models, and model parameters were optimized. Independent optimization for each of the two types of shear stress exposure conditions tested resulted in different sets of best-fit constants, indicating that universal optimization may not be possible. Inherent limitations of the current models require a paradigm shift from these integral-based discretized power law models to better address the dynamic conditions encountered in blood-recirculating devices.

Simulating Ballast Shear Strength from Large-Scale Triaxial Tests

The railroad ballast layer consists of discrete aggregate particles, and the discrete element method (DEM) is the most widely adopted numerical method to simulate the particulate nature of ballast materials and their particle interactions. Large-scale triaxial tests performed in the laboratory under controlled monotonic and repeated loading conditions are commonly considered the best means to measure macroscopic mechanical properties of ballast materials, such as strength, modulus, and deformation characteristics, directly related to load-carrying and drainage functions of the ballast layer in the field. A DEM modeling approach is described for railroad ballast with realistic particle shapes developed from image analysis to simulate large-scale triaxial compression tests on a limestone ballast material. The ballast DEM model captures the strength behavior from both the traditional slow and the rapid shear loading rate types of monotonic triaxial compression tests. The results of the experimental study indicated that the shearing rate had insignificant influence on the results of the triaxial compression tests. The results also showed that the incremental displacement approach captured the measured shearing response, yet could save significant computational resources and time. This study shows that the DEM simulation approach combined with image analysis has the potential to be a quantitative tool to predict ballast performance.

Brittle Versus Ductile Failure of a Lead-Free Single Solder Joint Specimen Under Intermediate Strain Rate

The failure mode of Sn-4.0Ag-0.5Cu solder joints under the intermediate strain rates is explored. Tensile and shear tests were performed on miniature solder joints at two different displacement rates, i.e., 5 and 50 mm/s. For the uniaxial tensile test, brittle failure occurred at the intermetallic compound (IMC)/Cu pad interface on the board side under both loading rates. However, for shear loading at 5 mm/s, ductile failure along the solder matrix was detected. With the increase of the shear loading rate up to 50 mm/s, the failure mode is converted into the (ductile+brittle) mixed mode. Numerical simulation results based on the extracted elastic-plastic solder properties indicate that the maximum stress concentration is located at the interfaces of the board side, which indicates the weak position of the crack initiation. The plastic deformation of the solder alloy is significantly suppressed at 50 mm/s loading rate. The amplitude of the solder plastic deformation under shear loading is much greater than that for the tensile test. Furthermore, most plastic deformation under shear loading is in the solder alloy adjacent to solder/IMC interface of the board side and no visible plastic deformation is detected in the middle of solder ball, which results in cohesive failure at the rate of 5 mm/s and the (ductile+brittle) mixed failure mode at the rate of 50 mm/s.

Potential for earthquake triggering from transient deformations

We report on laboratory experiments in which stick‐slipping shear surfaces are subject to transient stressing to simulate earthquake triggering by seismic waves. Granular layers and bare granite surfaces were sheared in a servo‐controlled deformation apparatus in double‐direct shear. The seismic waves from an earthquake and tectonic load were simulated by superimposing a loading rate sinusoid on a constant shear loading rate. The dependence of triggered stick‐slip failure on fault stress state and architecture was analyzed. Fault architecture was evaluated by varying gouge layer thickness (2–6 mm) and studying bare granite surfaces. We compare the shortened recurrence times for faults under transient loading conditions to the consistent recurrence intervals under constant loading rate. Our results imply that triggering depends on oscillation amplitude and frequency, as well as properties of the fault. Larger‐amplitude dynamic stresses reduce stick‐slip recurrence intervals for granular layers, whereas failure times for granite surfaces are uncorrelated with oscillation amplitude. Granular layers have shorter recurrence rates at higher frequency, whereas the recurrence intervals of granite surfaces are lengthened or unaffected by high‐frequency oscillations. Higher frequencies can inhibit failure when fault slip exceeds a critical distance, Dc, prior to peak velocity and encourages failure if Dc is achieved postpeak velocity. Increasing velocity temporarily strengthens faults, whereas velocity reduction further weakens and promotes failure, as predicted by the rate‐and‐state friction laws. Our results may explain variations in earthquake triggering thresholds and imply that high‐frequency thresholds may not be constant, as has been previously proposed.

Friction of sheared granular layers: Role of particle dimensionality, surface roughness, and material properties

We report on laboratory experiments designed to investigate three fundamental deformation mechanisms for frictional shear of granular fault gouge: sliding, rolling, and dilation. Mechanisms were isolated by shearing layers composed of rods in geometric configurations that resulted in one‐dimensional, two‐dimensional, and rolling‐only particle interactions. Results of digital video are presented with measurements of friction and strain to illuminate the distribution of shear and the relationship between particle motions and friction. The double‐direct‐shear configuration was used with boundary conditions of constant layer normal stress (1 MPa) and controlled shear loading rate (10 μm/s) with initial layer thickness of 6 mm. Layers were sheared in a servo‐hydraulic testing machine at room temperature (22°C) and relative humidity (5 to 10%). Three materials were studied: alloy 260 brass, dried semolina pasta, and hardwood dowels, with particle diameters of 1.59 mm, 1.86 mm, and 2.06 mm, respectively. Pasta layers had mean sliding friction coefficients of 0.24, 0.11, and 0.02 in 2‐D, 1‐D, and rolling configurations, respectively. Layers of brass rods had average friction coefficients of 0.23, 0.15, and 0.01, respectively, in 2‐D, 1‐D, and rolling configurations; and the wood samples exhibited friction values of 0.18, 0.19, and 0.09, respectively. Evolution of strength during shear correlated strongly with the displacement derivative of layer thickness. SEM images document the role of surface finish on frictional properties. Rapid reorientations of particles correspond to stick‐slip stress drops and may be related to the collapse and reformation of granular force chains. We find a systematic relationship between the strength of granular layers and (1) the surface roughness of particles and (2) the number of particle contact dimensions. Our data provide important insights on the mechanics of granular fault gouge and constraints on the fundamental parameters used in numerical models of tectonic faulting.

Effects of the intralaminar shear loading rate on the damage of multi-ply composites

It is well known that the specific absorption energy of structural elements based on composite materials is higher than those made of steel. However, this capacity may be limited due to localized damage mechanisms, inducing premature failure. Intralaminar shear is one of such mechanisms and is investigated here in a broad range of strain rates using tensile specimens made of E Glass/Epoxy (±45) s crossply laminate. Several interrupted dynamic tensile tests were performed thanks to modifications in a servo-hydraulic testing machine and in a tensile split Hopkinson bar. The damage caused by shear is investigated under the damage mechanics frame, permitting access to the influence of the strain rate on damage threshold and damage propagation rate.