Uniaxial Conditions Research Articles

Experimental study of the dynamic mechanical responses and failure characteristics of coal under true triaxial confinements

Investigations on the dynamic mechanical properties and failure mechanisms of coal under in-situ stress is essential for the prevention of dynamic disasters in deep coal mines. Thus, a modified true triaxial Hopkinson bar was employed to explore the dynamic mechanical behaviors of coal at different confining pressures (0–20 MPa) and strain rates (40–220 s−1). The results show that the dynamic peak stress is positively correlated with lateral static pre-stress σy and σz, but negatively correlated with axial static pre-stress σx. At approximate strain rates, increasing the lateral static pre-stress facilitates increasing the dynamic peak stress, but the minimum lateral static pre-stress is the primary factor limiting a significant increase in dynamic peak stress of coal. Furthermore, the dynamic differential stress is linearly related to the logarithm of strain rate, and the peak strain varies linearly with strain rate. However, there is no significant correlation between confining pressure and peak strain. Moreover, X-ray CT images and photographic fracture observations of coal samples show the failure patterns under uniaxial and triaxial conditions are splitting failure and shear failure, respectively. The device provides a viable approach for fully comprehending the dynamic mechanical behaviors of rock-like material in complex stress conditions.

To the scrutiny of the experiential and computational elicitation of factors of the Williams multi-parameter expansion

The article defines stress fields near the tips of mathematical cracks in an isotropic linearly elastic plate with two horizontal collinear cracks lying on a straight line of different lengths under the uniaxial tensile condition, using two approaches - experimental, based on the method of digital photomechanics, and numerical, based on finite element calculations. To represent the stress field at the tip of the section, the Williams polynomial series is used - the canonical representation of the field at the top of the mathematical section of a two-dimensional problem of elasticity theory for isotropic media. The main idea of the current study is to take into consideration the regular (non-singular) terms of the series and analyze their impact on the holistic description of the stress field in the immediate vicinity of the top of the section. The first fifteen coefficients of the Max Williams series were preserved and determined in accordance with experimental patterns of isochromatic bands and finite element modeling. To extract the coefficients of the Williams series used a redefined method designed to solve systems of algebraic equations, the number of which is significantly greater than the unknown - amplitude multipliers. The influence of the non-singular terms of the Williams series on the processing of the experimental pattern of interference fringes is demonstrated. It is validated that the preservation of the terms of a high order of smallness makes it possible to expand the area adjacent to the tip of the crack, from which experimental points can be selected. The finite element study was carried out in the SIMULIA Abaqus engineering analysis system, in which experimental samples tested in a full-scale experiment were reproduced. It is revealed that the results obtained by the two methods are in good agreement with each other.

A Molecular Dynamics Simulation for Thermal Activation Process in Covalent Bond Dissociation of a Crosslinked Thermosetting Polymer

A novel algorithm for covalent bond dissociation is developed to accurately predict fracture behavior of thermosetting polymers via molecular dynamics simulation. This algorithm is based on the Monte Carlo method that considers the difference in local strain and bond-dissociation energies to reproduce a thermally activated process in a covalent bond dissociation. This study demonstrates the effectiveness of this algorithm in predicting the stress-strain relationship of fully crosslinked thermosetting polymers under uniaxial tensile conditions. Our results indicate that the bond-dissociation energy plays an important role in reproducing the brittle fracture behavior of a thermosetting polymer by affecting the number of covalent bonds that are dissociated simultaneously.

A new stress-based model to predict the fatigue life of 880-grade pearlitic rail steel under uniaxial ratcheting conditions

This work explores a comparative analysis to verify the applicability of the existing stress-based model to predict life under ratcheting for pearlitic rail steel. The walker model is the most accurate of the available models, but its application is limited owing to the intricacy involved and the experimental reliance on its constant (γ) assessment. As a consequence, a novel stress-based life prediction model is proposed in this study. The applicability of the different models, and the proposed one, is described using equivalent stress amplitude vs life plot, stress deviation plot, error distribution box plots, and probability density function plots.

Damage amplification during repetitive seismic waves in mechanically loaded rocks

Cycles of stress build-up and release are inherent to tectonically active planets. Such stress oscillations impart strain and damage, prompting mechanically loaded rocks and materials to fail. Here, we investigate, under uniaxial conditions, damage accumulation and weakening caused by time-dependent creep (at 60, 65, and 70% of the rocks’ expected failure stress) and repeating stress oscillations (of ± 2.5, 5.0 or 7.5% of the creep load), simulating earthquakes at a shaking frequency of ~ 1.3 Hz in volcanic rocks. The results show that stress oscillations impart more damage than constant loads, occasionally prompting sample failure. The magnitudes of the creep stresses and stress oscillations correlate with the mechanical responses of our porphyritic andesites, implicating progressive microcracking as the cause of permanent inelastic strain. Microstructural investigation reveals longer fractures and higher fracture density in the post-experimental rock. We deconvolve the inelastic strain signal caused by creep deformation to quantify the amount of damage imparted by each individual oscillation event, showing that the magnitude of strain is generally largest with the first few oscillations; in instances where pre-existing damage and/or the oscillations’ amplitude favour the coalescence of micro-cracks towards system scale failure, the strain signal recorded shows a sharp increase as the number of oscillations increases, regardless of the creep condition. We conclude that repetitive stress oscillations during earthquakes can amplify the amount of damage in otherwise mechanically loaded materials, thus accentuating their weakening, a process that may affect natural or engineered structures. We specifically discuss volcanic scenarios without wholesale failure, where stress oscillations may generate damage, which could, for example, alter pore fluid pathways, modify stress distribution and affect future vulnerability to rupture and associated hazards.

Atomic investigation on optimal interfacial bonding for enhanced fracture properties in polymer nanocomposites

Functionalization of nanofillers is known to have stiffening and strengthening effects on nanocomposites through improved interfacial bonding. However, the optimality in the interfacial bonding is generally not reported in the literature. This issue is addressed here through a series of reactive molecular dynamics simulations on carbon nanotube reinforced polymer nanocomposite. For various degrees of functionalization, uniaxial tension conditions are simulated to study the stress–strain behavior, crack propagation, and fracture toughness. The J-integral is used to quantify the fracture toughness. Through these simulations we demonstrate the existence of an optimal degree of functionalization for maximum enhancement in elastic property, tensile strength, ductility, and fracture toughness. The underlying mechanics behind this optimality is identified through careful studies on crack propagation mechanisms, including crack arresting and formation of new crack surfaces. This optimality, which might also be expected in other material systems, will help in designing efficient nanocomposites.

Experimental study on mechanical properties and fracture characteristics of shale layered samples with different mineral components under cyclic loading

In shale plays, the in-situ effective stress is continuously being perturbed because of the production which impacts field operations. Therefore, mechanical properties of the formation and its response to stress field changes, with respect to mineral variations, is of great significance for the proper design of horizontal drilling and hydraulic fracturing. In this paper, clay-rich, calcareous and siliceous black shales are classified based on their XRD analysis, from the Wufeng and Longmaxi Formations in the Sichuan Basin of the Upper Yangtze Plate. Uniaxial quasi-static and cyclic loading tests, acoustic emissions, and CT scanning are combined to study mechanical behavior and fracturing mechanisms of the shale layer samples that own different mineral components. The results show that the strength and deformation response of the samples is a function of their mineral composition. The damage from cyclic loading is gradually accumulated in the samples which ultimately leads to fatigue and sample failure. The compressive strength of the samples under cyclic loading is lower than in the uniaxial quasi-static conditions. Furthermore, the fracturing of rock under uniaxial quasi-static loading mainly occurs after the peak strength is achieved while in cyclic loading, when a certain stress level is reached, fracturing begins, the damage accumulates up to the peak stress and the rock suddenly fails. Based on CT images from the samples after rupture, failure modes of the clay-rich, calcareous and siliceous black shales mainly constitute extensional and shear fractures, with some conjugate shearing. In comparison with uniaxial quasi-static loading, the fracture volume of the clay-rich, calcareous and siliceous black shales after cyclic loading increases by 12.2%, 25.9% and 32.9%, respectively. Overall, this study explains how cyclic loading that can be applied in the field via changing the injection pressure and other stimulation parameters, can effectively improve the complexity of the shale fracture network and lead to higher productivity of the formation.

Relationship between the heterogeneity of low-permeability reservoirs and the dynamic evolution of fractures under uniaxial compression conditions by CT scanning: A case study in the jiyang depression of Bohai Bay Basin, China

Based on an analysis of core fractures and computed tomography scanning by X-ray under uniaxial compression tests with loading and unloading, this study investigated the three-dimensional evolution of fracture initiation and propagation in different heterogeneity conditions of low-permeability reservoirs. Reservoir heterogeneity affected the 3D dynamic development of fractures. With increasing loading value, fractures with strong heterogeneity in low-permeability reservoirs followed the development sequence from shallow to deep depth in the profile. When the fracture tip reached the incompetent layer, propagation stopped. Subsequently, when the deformation continued to increase, the fracture crossed the incompetent layer and initiated a new fracture at the front end. Therefore, the across-bedding and through-going macro fractures formed in conjugate sets or were Y-shaped, with roughly equal spacing. In the plane, the fractures initiated along the pre-existing bedding parallel fracture and further propagated into large fractures. Consequently, they formed a net-shaped fracture from the edge to the center, with a long development period. These fractures intersected to form a complex, interconnected, and high-fracture-density system in three dimensions. Under the same loading value, owing to uniform lithology, the formation of micro-fractures in the weak heterogeneity of low-permeability reservoirs is difficult. Furthermore, long shear fractures were only produced in a relatively short formation period with a low fracture density. In the plane, the fractures stepped in an en-énchelon and terminated with horse-tailing microfractures. Moreover, the strong heterogeneity of low-permeability reservoirs was more favorable for producing fractures. The fracture volume was 13.17% of the total sample volume compared to 0.67% in the weak-heterogeneity reservoir under the same loading compression. Furthermore, a better fracturing effect was achieved by fracturing along the direction >50° with the natural fracture and under the condition of small differential horizontal stresses.

A model for rock dissipated energy estimation based on acoustic emission measurements

The temporal domain of dissipated energy during rock damage and failure is commonly quantified using loading and unloading tests or elastic mechanics-based theoretical calculation methods. However, these approaches cannot be applied to obtain the spatial distribution of rock dissipated energy. This paper presents a novel model to estimate rock dissipated energy based on acoustic emission measurements. The proposed model is used to estimate the temporal and spatial distribution of dissipated energy in a sandstone specimen under uniaxial compression conditions. The results indicate that the model well describes the energy dissipation evolution trend in the temporal domain with an error of 38.63% compared with results calculated using the traditional method. The dissipated energy concentration area estimated by the model is located near the macroscopic fractures, which indicates that the model can describe the evolution process of rock energy dissipation in the spatial domain.

Experimental study on delamination and strength characteristics of cemented waste rock backfill

To analyze the relation of segregation and strength of cemented waste rock backfill (CWRB), the aggregate distribution, uniaxial compressive strength (UCS), and failure law of the CWRB with different mass concentrations and accelerator contents were investigated by scanning the backfill section and acoustic emission test under uniaxial compression conditions. The test results show that: (1) The higher the mass concentration and accelerator content, the more uniform the aggregate distribution of the specimens; (2) The higher the mass concentration and accelerator content, the better the skeleton structure of the specimens, and the UCS of the specimens was significantly increased from 3.53 MPa to 21.59 MPa; (3) The higher the mass concentration and accelerator content, the material properties of the specimens changed from plastic to elastic, and the integrity of the specimens was better after destruction (4) The relationship between the uniformity coefficient and UCS was constructed by defining the aggregate uniformity coefficient according to the degree of specimen delamination, and the strength increased exponentially with the increase of the aggregate uniformity coefficient.

A determination method for the shear strength of soil-rock mixture considering the size effect and its application

The shear strength of the soil-rock mixture (S-RM) is the primary mechanical factor influencing the stability of the deposit slope. The particle size range inside the test sample, however, is not the same as that of the S-RM in the deposit slope due to the limitations of the test device, which results in a variation in the mechanical properties. In this study, computer simulation tests were conducted to examine the impact of particle size on mechanical behaviors under the uniaxial compressive condition, and the size effect was analyzed from both macroscopic and mesoscopic perspectives. Results indicate that when the particle size increases, the anisotropy levels of the S-RM sample also rise, causing the particle displacement difference inside the sample to increase and the rounding rock effect to become more pronounced. The quantitative relationship between the uniaxial compressive strength (UCS) and the maximum particle size (dmax) was proposed, and the accuracy of the prediction formula for the UCS value of S-RM that takes the size effect into account has been confirmed. The improvement of the shear strength determination method was achieved by unifying the particle size range corresponding to the uniaxial compressive and shear test results used in the approach, and the determination method was used to evaluate the stability of a typical deposit slope. The study results can be used as a guide for estimating strength parameters of S-RM throughout the design and construction process of the deposit slope.

A Method to Study the Correlation Between Local Collagen Structure and Mechanical Properties of Atherosclerotic Plaque Fibrous Tissue.

The rupture of atherosclerotic plaques in coronary and carotid arteries is the primary cause of fatal cardiovascular events. However, the rupture mechanics of the heterogeneous, highly collagenous plaque tissue, and how this is related to the tissue's fibrous structure, are not known yet. Existing pipelines to study plaque mechanics are limited to obtaining only gross mechanical characteristics of the plaque tissue, based on the assumption of structural homogeneity of the tissue. However, fibrous plaque tissue is structurally heterogeneous, arguably mainly due to local variation in the collagen fiber architecture. The mechano-imaging pipeline described here has been developed to study the heterogeneous structural and mechanical plaque properties. In this pipeline, the tissue's local collagen architecture is characterized using multiphoton microscopy (MPM) with second-harmonic generation (SHG), and the tissue's failure behavior is characterized under uniaxial tensile testing conditions using digital image correlation (DIC) analysis. This experimental pipeline enables correlation of the local predominant angle and dispersion of collagen fiber orientation, the rupture behavior, and the strain fingerprints of the fibrous plaque tissue. The obtained knowledge is key to better understand, predict, and prevent atherosclerotic plaque rupture events.

Analysis on the buckling performance of embedded co-cured composite sandwich plates under uniaxial compression condition

The research aims at exploring the buckling behavior of embedded co-cured composite sandwich structure (ECCSS) under static load. In accordance with the research objective, an analytical model, which is based on the interlayer stress continuous theory to formulate governing differential equations for predicting the critical buckling loads of ECCSS, has been established. The theoretical solution that satisfies the boundary conditions is obtained using the principle of minimum potential energy. The results obtained by ANSYS software have been used to corroborate the correctness of the analytical model. In addition, effects of various structural parameters (such as ply angles and skin-to-core thickness ratio) on buckling behavior of ECCSS have been investigated.

Expansion characteristics of concrete with free lime based expansive additives under uniaxial restraint conditions

Expansive additives in higher dosages induce chemical prestress in concrete by imparting higher early-age expansion. However, induced stress and concrete expansion can vary significantly, depending on different restraint conditions. In addition, concrete expansion can be highly anisotropic depending on the nature of external restraints and boundary conditions. Experimentations have been performed to study the effect of uniaxial (anisotropic) restraint on the concrete expansion (in both restraint and stress-free directions) and induced prestress. A higher degree of restraint resulted in a substantial decrement in the longitudinal expansion. In contrast, expansion in the stress-free (lateral) direction was not much affected by the restraint in other direction. As a result, an overall decrement in the volumetric expansion was observed with an increase in the reinforcement ratios. Stress induced in concrete due to the restraint expansion in the longitudinal direction was found to be higher for a higher reinforcement ratio. Data provided from the current work can be used for the comprehensive modelling of expansive concrete under external restraints.

Stretchability above glass transition of mechanically recycled poly(ethylene terephtalate), PET

Poly(ethylene terephthalate) (PET) is nowadays an interesting polymer, largely used for food packaging applications. It has the ability to develop a pseudo-crystalline structure under stretching above its glass transition temperature. Environmental concerns have granted an increasing interest for recycled materials and thus recycled PET (rPET). Mechanical recycling of PET can affect the chains length as well as the contaminant concentration that will change is formability. Two rPET and two virgin PET (vPET) having dissimilar intrinsic viscosity were then studied by first analyzing the initial properties of the extruded sheets. The four grades were then stretched in uniaxial conditions, based on the time/temperature equivalence principle identified in the low deformations domain. A relevant parameter, the natural draw ratio (NDR) appears to be very efficient so as to discriminate the relative mechanical responses of the four grades. This parameter whose use has been comforted in industrial conditions allows to optimize the forming range of rPET, compared to vPET. • Mechanically recycled and virgin PET stretch ability above glass transition is explored, and their mechanical behavior do obey to time/temperature equivalence principle. • For all the stretching settings applied, the natural draw ratio (NDR) measured is a relevant parameter able to discriminate mechanical behavior of virgin or recycled polymers. • An optimization of the forming range is proposed where enhanced induced microstructures are obtained.

High throughput determination of creep behavior of additively manufactured AlSi10Mg alloy

Power-law creep behavior of laser powder bed fusion (LPBF) AlSi10Mg alloy is evaluated at 250 °C under uniaxial and bending test conditions. Uniaxial tests are performed in both tension and compression, and the cantilever samples are tested using digital image correlation (DIC)-augmented-bending test methodology, which is a way of establishing creep parameters in high throughput fashion. In each test condition, the applied normal stress is along the transverse direction relative to the build direction. The creep response of the alloy exhibits a tension-compression symmetry in terms of the steady-state and minimum creep rates. Correlations of uniaxial and high throughput bending creep tests reveal an excellent correspondence, thereby establishing the viability of evaluating creep behavior via bending, as a first of its kind, for additively manufactured Al alloys. Detailed electron microscopy of the crept specimens revealed the stability of microstructure during the prolonged exposures up to 500 h (during uniaxial creep) and the pinning of dislocations by intra-granular Si (20–50 nm) particles. A detailed discussion on the structure-property relationships is provided.

Concussion Prone Scenarios: A Multi-Dimensional Exploration in Impact Directions, Brain Morphology, and Network Architectures Using Computational Models.

While individual susceptibility to traumatic brain injury (TBI) has been speculated, past work does not provide an analysis considering how physical features of an individual's brain (e.g., brain size, shape), impact direction, and brain network features can holistically contribute to the risk of suffering a TBI from an impact. This work investigated each of these features simultaneously using computational modeling and analyses of simulated functional connectivity. Unlike the past studies that assess the severity of TBI based on the quantification of brain tissue damage (e.g., principal strain), we approached the brain as a complex network in which neuronal oscillations orchestrate to produce normal brain function (estimated by functional connectivity) and, to this end, both the anatomical damage location and its topological characteristics within the brain network contribute to the severity of brain function disruption and injury. To represent the variations in the population, we analyzed a publicly available database of brain imaging data and selected five distinct network architectures, seven different brain sizes, and three uniaxial head rotational conditions to study the consequences of 74 virtual impact scenarios. Results show impact direction produces the most significant change in connections across brain areas (structural connectome) and the functional coupling of activity across these brain areas (functional connectivity). Axial rotations were more injurious than those with sagittal and coronal rotations when the head kinematics were the same for each condition. When the impact direction was held constant, brain network architecture showed a significantly different vulnerability across axial and sagittal, but not coronal rotations. As expected, brain size significantly affected the expected change in structural and functional connectivity after impact. Together, these results provided groupings of predicted vulnerability to impact-a subgroup of male brain architectures exposed to axial impacts were most vulnerable, while a subgroup of female brain architectures was the most tolerant to the sagittal impacts studied. These findings lay essential groundwork for subject-specific analyses of concussion and provide invaluable guidance for designing personalized protection equipment.

Cyclic constitutive equations of rock with coupled damage induced by compaction and cracking

In this paper, the cyclic constitutive equations were proposed to describe the constitutive behavior of cyclic loading and unloading. Firstly, a coupled damage variable was derived, which contains two parts, i.e., the compaction-induced damage and the cracking-induced damage. The compaction-induced damage variable was derived from a nonlinear stress–strain relation of the initial compaction stage, and the cracking-induced damage variable was established based on the statistical damage theory. Secondly, based on the total damage variable, a damage constitutive equation was proposed to describe the constitutive relation of rock under the monotonic uniaxial compression conditions, whereafter, the application of this model is extended to cyclic loading and unloading conditions. To validate the proposed monotonic and cyclic constitutive equations, a series of mechanical tests for marble specimens were carried out, which contained the monotonic uniaxial compression (MUC) experiment, cyclic uniaxial compression experiments under the variable amplitude (CUC-VA) and constant amplitude (CUC-CA) conditions. The results show that the proposed total damage variable comprehensively reflects the damage evolution characteristic, i.e., the damage variable firstly decreases, then increases no matter under the conditions of MUC, CUC-VA or CUC-CA. Then a reasonable consistency is observed between the experimental and theoretical curves. The proposed cyclic constitutive equations can simulate the whole cyclic loading and unloading behaviors, such as the initial compaction, the strain hardening and the strain softening. Furthermore, the shapes of the theoretical curves are controlled by the modified coefficient, compaction sensitivity coefficient and two Weibull distributed parameters.

Investigation of deformation behaviour with yield point phenomenon in cold-rolled medium-Mn steel under hot stamping conditions

Medium manganese (MMn) steels are a promising class of advanced high-strength steels (AHSS) with potential for application as vehicle panels in the automotive industry. In this study, the deformation behaviour of a representative cold-rolled MMn steel and its dependence on processing parameters are studied using uniaxial tensile testing under low-temperature hot stamping (LTHS) conditions, covering austenitisation soaking times of 60–600 s, deformation temperatures of 500–700 °C and strain rates of 0.01–5 s −1 . A yield point phenomenon is observed for the first time in the early stage of deformation at such temperatures in this MMn steel, which has a submicron grain size. The extent of the yield point phenomenon is reduced, and strain hardening capability and total elongation are enhanced, with longer austenitisation soaking times, which give coarser-grained microstructures. The yield point phenomenon also tends to be weaker at higher deformation temperatures and lower strain rates; under these conditions, both the flow stress and the degree of strain hardening decrease, while the total elongation is insensitive to differences in deformation conditions. The mechanisms for the deformation behaviour and its dependence on test conditions are discussed. In addition, a set of unified constitutive equations is established and calibrated using the experimental data to predict the deformation behaviour of the MMn steel under uniaxial LTHS conditions, and close agreement with experiment, including the yield point phenomenon, is obtained.

High elastic modulus polyethylene – A two-stage multi-axial rolling and orientation process

In this study, we propose a novel two-stage solid-state processing technique to fabricate thin-oriented high-density polyethylene (HDPE) rods with enhanced mechanical properties. This unique solvent-free technology involves biaxial rolling (Stage 1) followed by a uniaxial orientation procedure (Stage 2). The oriented thin HDPE rods exhibit an impressive modulus of 37 GPa. This is a result of a complex, oriented composite structure containing crazes with a network of interlocking fibers. Surprisingly, the high-modulus thin rods were created under relative low uniaxial tensile orientation conditions having draw ratio of eleven. Gel-spun ultra-high-molecular-weight fiber processing requires orientations greater than thirty. A corresponding scaled hierarchical model was proposed to summarize the process-structure-property relationships. This study demonstrates a strategy for creating high-modulus polymeric materials via controlled solid-state processing.