Slow Loading Rates Research Articles

A foundational framework for the mesoscale modeling of dynamic elastomers and gels

Discrete mesoscale network models, in which explicitly modeled polymer chains are replaced by implicit pairwise potentials, are capable of predicting the macroscale mechanical response of polymeric materials such as elastomers and gels, while offering greater insight into microstructural phenomena than constitutive theory or macroscale experiments alone. However, whether such mesoscale models accurately represent the molecular structures of polymer networks requires investigation during their development, particularly in the case of dynamic polymers that restructure in time. We here introduce and compare the topological and mechanical predictions of an idealized, reduced-order mesoscale approach in which only tethered dynamic bonding sites and crosslinks in a polymer’s backbone are explicitly modeled, to those of molecular theory and a Kremer-Grest, coarse-grained molecular dynamics approach. We find that for short chain networks (∼12 Kuhn lengths per chain segment) at intermediate polymer packing fractions, undergoing relatively slow loading rates (compared to the monomer diffusion rate), the mesoscale approach reasonably reproduces the chain conformations, bond kinetic rates, and ensemble stress responses predicted by molecular theory and the bead–spring model. Further, it does so with a 90% reduction in computational cost. These savings grant the mesoscale model access to larger spatiotemporal domains than conventional molecular dynamics, enabling simulation of large deformations as well as durations approaching experimental timescales (e.g., those utilized in dynamic mechanical analysis). While the model investigated is for monodisperse polymer networks in theta-solvent, without entanglement, charge interactions, long-range dynamic bond interactions, or other confounding physical effects, this work highlights the utility of these models and lays a foundational groundwork for the incorporation of such phenomena moving forward.

Research on simulating coal crack development under high voltage electric pulse stress waves using zero thickness cohesive elements

High voltage electric pulse (HVEP) technology has demonstrated significant potential in enhancing coal seam permeability. However, there is a notable gap in numerical simulation studies that investigate the cracks development patterns in coal under the influence of this technology. In this study, a HVEP coal rock fracturing system was utilized to conduct a physical simulation experiment of coal sample electrical fragmentation. A finite-discrete element method (FDEM) model, incorporating zero-thickness cohesive elements, was developed to simulate the process of stress wave-induced crack propagation in coal, triggered by the plasma channel. The results indicate that over time, the stress wave typically exhibits an initial rapid rise followed by an oscillatory decline. The peak value of the stress wave decreases significantly with distance, stabilizing beyond a propagation distance of 10 mm. Furthermore, the fracturing efficacy of coal by HVEP stress waves is closely linked to specific characteristic parameters: within the range of 75–125 MPa, a positive correlation exists between the stress wave peak and the complexity of the coal crack network. Reducing the stress wave loading rate can increase the crack area to some extent, although excessively slow loading rates may lead to excessive energy dissipation during propagation, which is detrimental to crack expansion. Conversely, when the ratio of β to α ranges from 1 to 100, decelerating the attenuation rate of the stress wave aids in generating stress concentration within the crack zone, thereby facilitating crack propagation. The established numerical model aims to advance understanding of HVEP’s fracturing mechanisms and enhance its application in coalbed methane extraction.

Analysis of the Mechanical Characteristics of Human Pancreas through Indentation: Preliminary In Vitro Results on Surgical Samples.

Pancreatic surgery is extremely challenging and demands an extended learning curve to be executed with a low incidence of post-operative complications. The soft consistency of the human pancreas poses a primary challenge for pancreatic surgeons. This study aimed to analyze the preliminary mechanical characteristics of the human pancreas to develop a realistic synthetic phantom for surgical simulations in the near future. Pancreatic specimens, comprehensive of the pancreatic parenchyma and main pancreatic duct, were collected during pancreatic resections and analyzed through nano-bio-indentation (BioindenterTM UNHT3 Bio, Anton Paar GmbH, Graz, Austria) to measure the elastic modulus. Comparisons were made between slow and fast loading rates, immediate and post-freezing analyses, and multipoint indentations. The results demonstrated that a slow loading rate (30 μN/min), immediate analysis, and multipoint measurements are crucial for obtaining accurate values of the elastic modulus of the human pancreas (1.40 ± 0.47 kPa). In particular, the study revealed that analysis after freezing could impact the outcomes of the indentation. Moreover, the study suggested that both the pancreatic parenchyma and the main pancreatic duct should be analyzed to achieve a more precise and comprehensive definition of the. mechanical features of the pancreas. These preliminary findings represent the initial steps toward defining the consistency and mechanical characteristics of human pancreatic tissue with the goal of creating a realistic synthetic phantom.

High temperature in situ impact testing of hybrid thermoplastic polyether ether ketone laminates

AbstractIn order to evaluate the severity of thermal degradation on the impact response of carbon (C) and glass (G) fibers reinforced polyether ether ketone laminates, low velocity impact tests have been conducted at a temperature higher than the glass transition temperature (150°C). An instrumented Charpy pendulum (CP) equipped with a heating device was specifically designed to estimate its capability to perform low velocity impact tests at high temperature. The first important effect resulting from temperature increase is a reduction of the impact energy required to induce the barely visible impact damage (BVID), whose role is utmost important regarding the damage tolerance. The second effect is that temperature has also a tremendous influence on the internal damage as there is virtually no delamination at high temperature for impact energies. Finally, compared with the impact behaviors observed with the same laminates impacted with a drop tower and quasi‐static indentation, the impact velocity is about 2.5–3 times as low during CP impacts. As a result, slow loading rates are instrumental in increasing both the dissipated energies and the permanent indentation (PI) values (1.6–2 times as high) that are always higher than the BVID. Ultimately, as higher PI is usually correlated to higher impact damage tolerance.

Micromechanics and damage in slide-ring networks.

We explore the mechanics and damage of slide-ring gels by developing a discrete model for the mechanics of chain-ring polymer systems that accounts for both crosslink motion and internal chain sliding. The proposed framework utilizes an extendable Langevin chain model to describe the constitutive behavior of polymer chains undergoing large deformation and includes a rupture criterion to innately capture damage. Similarly, crosslinked rings are described as large molecules that also store enthalpic energy during deformation and thus have their own rupture criterion. Using this formalism, we show that the realized mode of damage in a slide-ring unit is a function of the loading rate, distribution of segments, and inclusion ratio (number of rings per chain). After analyzing an ensemble of representative units under different loading conditions, we find that failure is driven by damage to crosslinked rings at slow loading rates, but polymer chain scission at fast loading rates. Our results indicate that increasing the strength of the crosslinked rings may improve the toughness of the material.

Experimental test and PFC3D simulation of the effect of a hole on the tensile behavior of concrete: A comparative analysis of four different hole shapes

In this article, the effects of porosity and its geometry on tensile features of concrete were investigated, using the Brazilian test and three dimensions PFC model. In the first step, the PFC sample was calibrated by indirect tensile lab outcomes and uniaxial compression test outputs. In the next step, indirect tensile tests were done on the models consisting of various pore shapes. Cylindrical models consisted of internal pores with different shapes such as rectangular, circular, horizontal notch, and vertical notch. The diameter and or the length of these porosity changes in various values, i.e., 10 mm, 20 mm, and 30 mm. Twelve different configurations of samples were prepared by varying the porosity shapes. The mechanical behavior of samples has been provided in form of stress–strain curves at a constant and slow loading rate of 0.05 mm/min for ensuring the static condition based on Newton’s first law. Concurrent with numerical simulation, an experimental test was done on the concrete slabs containing different pores. The outcomes showed that porosity geometry plays an important role in the fracturing pattern. It was found that the porosity geometry has potent consequences on fracturing process stress and the value of the UCS for rocks. Numerical simulation shows that fracture energy decreased by increasing the dimension of the defect. Crack initiation stress was close to final stress in vertical notch configuration while the difference between the final stress and crack initiation stress has a high value for horizontal notch configuration. The sharper the hole tip, the faster the progressive failure occurred.

Tensile and Charpy impact properties of heat-treated high manganese steel at cryogenic temperatures

The structural materials for the superconductor in next generation DEMO fusion reactor are in great demanded with excellent nonmagnetic and mechanical properties such as strength, toughness, and ductility at the cryogenic temperature. High-Mn steels, newly developed for application to cryogenic industry, including DEMO fusion reactor, can change its deformation mechanism by twinning, martensite transformation and dislocation slip according to the range of stacking fault energy (SFE). Therefore, it is very important to investigate the mechanism of deformation behavior at cryogenic temperatures. In this study, tensile tests and Charpy impact tests were performed to investigate the effects of heat treatment on mechanical properties of high-Mn steels at cryogenic temperatures. In addition, the deformation mechanism of high-Mn steels at room temperature and cryogenic temperature was investigated through various analysis such as microstructure, XRD, EBSD and TEM. As a result, phase transformation from austenite to ε and α'-martensite was generated in the tensile test with a slow loading rate at cryogenic temperatures, while ε and α'-martensite was not observed in the Charpy impact test with a high loading rate.

Ductile-brittle transition at edge cracks (001)[110] in BCC iron under different loading rates in mode I: a 3D atomistic study

3D atomistic simulations via molecular dynamics (MD) at temperature of 0 K and 295 K (22 °C) with a high quasi-static loading rate dP/dt of 2.92 kN s−1 show that cleavage fracture is supported by surface emission of oblique dislocations and by their subsequent cross slip to {112} planes, which increases separation of the (001) cleavage planes inside the crystal. Under the slower loading rate by a factor 5, the crack growth is hindered by twin generation on oblique planes {112} and the fracture is ductile. The MD results explain the contribution of the crack itself to the ductile-brittle transition observed in our fracture experiments on Fe-3wt%Si single crystals of the same orientation and geometry, loaded at the same rates dP/dt as in MD. The loading rates are equivalent to the cross head speed of 5 mm min−1 and 1 mm min−1 used in the experiment. The MD results also agree with the stress analysis performed by the anisotropic LFM and comply with experimental observations.

Effects of embedment depth on the pullout capacity of bucket foundations in sand

A series of 1 g experimental tests was conducted to investigate the effects of embedment depth on the pullout capacity of bucket foundations in sand. Polycarbonate model buckets with a constant diameter of 150 mm and length-to-diameter (L/D) ratios of 0.5, 1.0 and 1.5 were employed varying loading rates from 0.002 N/s to 4.1 N/s. The slow loading rate of 0.002 N/s induced drained conditions characterised with substantially small suction force and high skirt resistance. The fast loading rate of 4.1 N/s induced nearly undrained conditions characterised by large suction force and almost full uplift of the soil plug. The pullout capacity increased almost linearly with the L/D ratio. A 50% increase in embedment depth resulted in an average increase in pullout capacity of 40%. The pullout capacity and suction force inside the bucket rapidly increased with the loading rate in the partially drained condition, and converged to nearly constant values in the undrained condition. In the nearly undrained condition, the suction force at the tip was approximately half of that under the lid. The simplified approach reasonably predicted the capacities of buckets with various L/D ratios.

Effect of Creep on Shear Tab Connections under Fire Temperatures

The effect of creep on the strength capacities, ductility, and failure modes of shear-bolted lap joints at elevated temperatures is investigated. Thirteen bolted lap joints (representing shear tab connections) are tested under different loading rates: fast rate (10 mm/min) and slow rates (1 mm/min and 0.3 mm/min) for temperatures ranging from 450°C to 700°C. The temperature- and the time-dependent behavior of the lap joints are examined. The results show a change in failure mode from tear-out failure when using fast loading rate to net section fracture when using slow loading rates at temperatures 450°C and 500°C. The results show that the failure mode of the bolted lap joints changed from plate failure to bolt shear failure at temperatures larger than 500°C. The slow loading rate tests resulted in a reduction in the bolt shear capacities when compared to the fast rate tests. Moreover, finite element models are developed to understand the change in failure mode of the connections from tear-out failure to net section fracture and to bolt shear failure. This study is considered as a step towards developing design guidelines for shear tab connections at elevated temperatures while considering the effect of creep.

Rate-dependent thermal behavior of fillet welds subjected to various load angles

The objective of this paper is to investigate the rate-dependent thermal behavior of fillet welds in welded lap joints subjected to different load angles. Performance of welds in fire is complex due to unexpected induced forces that are developed due to different fire-time exposures. To address this issue, an experimental program is performed to examine the rate-dependent behavior of fillet welds for three different welded lap joint configurations under steady-state thermal conditions. The lap joints are classified as longitudinal, inclined, and transverse where the angle between the axis of the fillet weld and the direction of the applied load is 0°, 45°, and 90°, respectively. Two displacement loading rates are used: 1.5 mm/min (fast) and 0.1 mm/min (slow). Retention factors for the weld material under different displacement loading rates and angles are presented and compared with those available in the literature. The results show that the effect of displacement loading rate is significant for temperatures larger than 400 °C and become severely significant at temperatures larger than 500 °C. The effect of the slow displacement loading rate (0.1 mm/min) at 700 °C reaches around 40%, 37%, and 29% decrease in strength capacities for transverse, inclined, and longitudinal welded lap joints, respectively.

Effect of Strain Rates on Failure of Mechanical Properties of Lumbar Intervertebral Disc Under Flexion.

ObjectiveTo evaluate the strain‐rate‐dependent viscoelastic properties of the intervertebral disc by in vitro experiments.MethodThe biomechanical experiments were conducted from September 2019 to December 2019. The lumbar spines of sheep were purchased within 4–6 hours from the local slaughterhouse, and the intervertebral disc samples were divided into three groups. In rupture group, the samples were used to test the mechanical behavior of the intervertebral disc rupture at different strain rates. In fatigue injury group, the samples were used to test the mechanical behavior of fatigue injury on the intervertebral disc under different strain rates. In internal displacement group, the samples were used to test the internal displacement distribution of the intervertebral disc at different strain rates by applying an optimized digital image correlation (DIC) technique.ResultsBoth the yielding and cracking phenomenon occurs at fast and medium loading rates, while only the yielding phenomenon occurs at a slow loading rate. The yield stress, compressive strength, and elastic modulus all increase with the increase of the strain rate, while the yield strain decreases with the increase of the strain rate. The logarithm of the elastic modulus in the intervertebral disc is approximately linear with the logarithm of the strain rate under different strain rates. Both before and after fatigue loading, the stiffness in the loading and unloading curves of the intervertebral disc is inconsistent, forming a hysteresis loop, which is caused by the viscoelastic effect. The strain rate has no significant effect on the internal displacement distribution of the intervertebral disc. Based on the experimental data, the constitutive relationship of the intervertebral disc at different strain rates is obtained. The fitting curves are well coupled with the experimental data, while the fitting parameters are approximately linear with the logarithm of the strain rate.ConclusionsThese experiments indicate that the strain rate has a significant effect on the mechanical behavior of the intervertebral disc rupture and fatigue injury, while the constitutive equation can predict the rate‐dependent mechanical behavior of lumbar intervertebral disc under flexion very well. These results have important theoretical guiding significance for preventing lumbar disc herniation in daily life.

Influence of impact velocity on impact behaviour of hybrid woven-fibers reinforced PEEK thermoplastic laminates

This study aims at examining the impact behavior of hybrid carbon and glass fibers woven-ply reinforced PolyEther Ether Ketone (PEEK) thermoplastic quasi-isotropic laminates. An instrumented Charpy pendulum is specifically designed to estimate its capability to perform low velocity impact tests. Through the comparison of different impact methods (Quasi-static indentation tests, Charpy and drop tower impacts), the influence of impact velocity on the impact behavior of this hybrid composite material is investigated. From the obtained results, it appears that the macroscopic impact response is similar in terms of force-displacement response. Indeed, the impact velocity is significantly higher (2.5 times higher) with falling weight impact testing. In PEEK-based laminates whose mechanical behaviour is time-dependent, slow loading rates (e.g. Charpy impact testing) are instrumental in ruling the dissipated energy (+20% at 35 and 40J) as well as in increasing the permanent indentation (1.6 times higher) that is always higher than the Barely Visible Impact Damage.

EXPERIMENTAL STUDY OF OBSERVABLE DEFORMATION PROCESS IN FAULT META-INSTABILITY STATE BEFORE EARTHQUAKE GENERATION

According to the steady state of fault and energy balance, we provided a new idea to observe the precursors for a stressed fault. The meta-instability (or sub-instability) state of a fault is defined as the transition phase from peak stress to critical stress of fast instability (earthquake generation) during a full period of slow loading and fast unloading. The accumulative deformation energy begins to release in this stage. Identifying its deformation before fast instability would be beneficial to obtain premonitory information, and to evaluate the seismic risks of tectonic regions. In this study, we emphasized to analyze deformation process of the meta-instable stage with stain tensor data from a straight precut fault in granite at a slow loading rate, and observed the tempo-spatial features during the full deformation process of the fault. Two types of tectonic zones and instabilities occur on the stick-slip fault. The low- and high-value segments in the volume strain component appear along the fault strike with a load increment. The former first weakens and then becomes initial energy release segments; the latter forms strong stress-interlocking areas and finally turns into the initial region of fast instability. And there are two stages in the entire instable process of the fault: the initial stage is associated with the release of the low volume strain segments, which means fault pre-slips, slow earthquakes or weak earthquakes. The second one characterizes a strong earthquake through the release of high volume strain parts. The rupture acceleration in the first stage promotes the generation of the second. Moreover, fault instability contains two types of strain adjustments along the fault: the front-like strain change along the transition segments from low- to high- strain portions with volume strain release, and the compressive strain pulse of fault instability after the volume strain release extends to a certain range with loading increment. In laboratory experiments, the front-type strain occurs about 12 seconds before fast fault instability; the compressive pulse initiates within less than 0.1 second, and then the fault turns quickly into a dynamic strain adjustment, which appears quasi-synchronously between different measurement points, and, finally, an earthquake is generated.

Effect of Asphalt Mixture Components on the Uncertainty in Dynamic Modulus Mastercurves

Practitioners and researchers in the paving industry have highlighted the importance of the adoption of reliability-based pavement design. The goal of developing reliable pavements with optimum performance over their design life has become a key factor to be considered during both pavement design and construction processes. This requires the adoption of statistical and probabilistic-based analyses for the formulation of the properties and behavior of pavement materials. Thus, many researchers worked on the quantification and modeling of the uncertainty caused by the inherent variability in pavement materials in general and that of asphalt concrete (AC) in particular. The dynamic modulus (| E*|), a fundamental property for mechanistic-empirical and purely mechanistic pavement designs, has been proven to have a significant level of uncertainty that is dependent on climatic and traffic loading conditions. The main objective of this study is to investigate the effect of the AC mixture properties and components on the uncertainty in the | E*| mastercurve. This objective is achieved by conducting an experimental program incorporating four different mixtures having the same material sources but different binder types and gradations. Monte Carlo simulations are used to model the uncertainty of | E*| for each of these mixtures. The paper shows that the uncertainty is dependent on mixture type, as the presence of larger nominal maximum aggregate size, modified binder, or additive can increase the uncertainty in the | E*| mastercurve, especially at high temperatures or slow loading rates. The uncertainty is proven to be material related and not imposed by the testing instrumentation.

Pure Mode I Fracture Toughness Determination in Rocks Using a Pseudo-Compact Tension (pCT) Test Approach

Mode I fracture toughness (KIC) quantifies the ability of a material to withstand crack initiation and propagation due to tensile loads. The International Society for Rock Mechanics (ISRM) has proposed four suggested methods for determining KIC. However, these methods present some drawbacks such as insufficient post-peak control, complex sample preparation and considerable material requirements. Here we present an alternative approach, called the pseudo-compact tension (pCT) method, to measure KIC in rocks using disc-shaped specimens loaded in pure tension. The pCT specimen has favourable features such as a simple geometry, small sample volume and minimal machining requirement. The tensile load is transmitted to the specimen through two high-strength, high-stiffness steel jaws that fit into a U-shaped groove cut in the specimen. An additional thin straight notch is introduced to act as a stress concentrator. The crack propagates from the notch tip along the ligament plane, splitting the specimen into two halves. The effects of specimen size and notch length on KIC are determined by testing specimens 100, 50 and 38 mm in diameter with different notch length ratios (0.1 ≤ a/b ≤ 0.4). Tests were performed under ambient conditions and a slow loading rate (0.1 mm/min). Our results show that the pCT method is convenient for the assessment of KIC of both fragile and ductile rocks. The method offers good control even beyond the maximum load, making it possible to study the post-peak behaviour of the material.

Rate-dependence of the tensile behaviour of fibre reinforced concrete in the quasi-static regime

The effect of crack rate on the tensile stress–crack width relation of fibre reinforced concrete is studied experimentally, using the three-point bending test, where the post-peak flexural response of the material is determined over a range of loading rates covering 5 orders of magnitude. The tetralinear stress–crack opening (σ–w) relations are subsequently obtained from inverse analyses. Concretes with hooked-ended steel fibres at different dosages and with polypropylene fibres have been studied. The results show that the tensile strength is reduced and the initial drop in cohesive stresses is less steep at slower loading rates, with the changes being more significant in concretes with lower toughness. Numerical analysis shows that the fracture process zone at crack initiation extends further as the loading becomes slower. The variation in the σ–w relation with crack rate has been modelled empirically based on the rate-dependence of the concrete matrix and the tensile strength of the FRC. The general variation of flexural toughness with the loading rate has also been identified, reflecting the moderating influence due to steel fibres, especially at higher dosages.

Modeling the transient behavior of the NiTi shape memory actuator using finite element analysis: Parametric study of the rate effects

Finite element simulation is presented to study the rate effects of the shape memory response of bent NiTi wire “C-shaped” exposure to load. A three-dimensional constitutive model for shape memory alloys proposed by Auricchio is implemented in the built-in library of Ansys Workbench 18.2 to capture the shape memory behavior of three different NiTi alloys that are subjected to different thermomechanical treatments. The quasi-static (slow) loading rate and the dynamic (fast) loading rate have been applied isothermally or at adiabatic conditions. Other models assumed the classical shape memory stress-strain behavior, but the realistic behavior of the NiTi SMA is different. The NiTi SMA is influenced by its microstructural changes, such as internal defects, grain size, chemical compositions, content of dislocations and density; all these effects should be added when modeling, which has been considered in this study. The results showed a significant effect of the dynamic loading of NiTi on decreasing the recoverable strain. As a result, it could be expected that the actuator lifetime could be reduced when a rapid, as opposed to a slow, loading rate is adopted. Inhomogeneity of stresses and deformations after loading can cause irreversible plastic deformation. The complete deformation recovery is noticed in the reorientation scenario, unlike in the SIM scenario, which exhibits unrecoverable deformation. Experimental validation is done by comparing the macroscopic displacement of the bent NiTi actuator with the model results, which shows good agreement.

Rate-dependent crack nucleation in cartilage under microindentation

This study investigates rate-dependent crack nucleation in cartilage under microindentation using a poroviscoelastic framework and nano/microscopic images. Localized crack failure was induced at known locations and at different loading rates via microindentation with an axisymmetric sphero-conical indenter. Finite element (FE) modeling was used to reproduce results of microindentation tests within a poroviscoelastic framework. Scanning electron microscopy (SEM) was used to examine nano- and microscale structural features of crack surfaces. Microindentation results showed rate-dependent crack nucleation in cartilage. In particular, critical total work required for crack nucleation was larger at the slow loading rate compared to the fast loading rate. FE results suggested that viscoelastic relaxation of cartilage was a major contributor to the rate dependency and that tensile stresses localized at the indenter tip was a governing factor in crack nucleation. SEM images combined with microindentation and FE results suggested that the solid matrix in the vicinity of the tip experienced relatively large relaxation and kinematic fiber rearrangement at the slow loading rate in comparison to the fast loading rate. These findings extend current understanding of rate-dependent failure mechanisms in cartilage.

Acoustic emission signatures prior to snow failure

ABSTRACTSnow slab avalanches are caused by cracks forming and propagating in a weak snow layer below a cohesive slab. The gradual damage process leading to the formation of the initial failure within the weak layer (WL) is still not entirely understood. To this end, we designed a novel test apparatus that allows performing loading experiments with large snow samples (0.25 m2) including a WL at different loading rates and simultaneously monitoring the acoustic emissions (AE) response. By analyzing the AE generated by micro-cracking, we studied the evolution of the damage process preceding snow failure. At fast loading rates, the exponent of the AE energy distribution (b-value) gradually changed, and both the energy rate and the inverse waiting time increased exponentially with increasing load. These changes in AE signature indicate a transition from small to large events and an acceleration of the damage processes leading to brittle failure. For the experiments at slow loading rate, these changes in the AE signature were not or only partially present, even if the sample failed, indicating a different evolution of the damage process. The observed characteristics in AE response provide new insights on how to model snow failure as a critical phenomenon.