Time-dependent Mechanical Properties Research Articles

Comprehensive study on the hydration kinetics, mechanical properties and autogenous shrinkage of cement pastes during steam curing

Steam curing is a crucial stage for the strength development of steam-cured concrete. This study comprehensively investigated the hydration kinetics, mechanical properties, and autogenous shrinkage (AS) of cement pastes during steam curing. A hydration-dependent analytical method was proposed to analyze the mechanisms governing the development of mechanical properties and AS of steam-cured cement pastes (SCPs). Results revealed that in this investigated curing regimes, the peak hydration rate of SCPs was approximately 7 times higher than that of standard-cured pastes (NCPs). SCPs exhibited faster time-dependent mechanical property development but slower hydration-dependent mechanical property development than NCPs. This difference was primarily attributed to variations in initial structure establishment and free water content. Moreover, an elevated curing temperature reduced the growth rate of hydration-dependent AS due to large capillary pores and low surface tension, which suggested that AS should not be considered as a cause of heat damage to SCPs.

Beyond time: Enhancing corrosion resistance of geopolymer concrete and BFRP bars in seawater

To improve the durability of Basalt fiber reinforced polymer (BFRP) bars reinforced geopolymer concrete (GPC), it is important to study the time-dependent variation of the corrosion resistance ability of GPC and BFRP in a seawater environment. This paper presents an experimental investigation to study the time-dependent mechanical properties and durability of BFRP bars and geopolymer materials synthesized by granulated blast furnace slag (GGBFS), fly ash, and silica fume. The resulting GPC and Portland cement (PC) concrete were exposed to artificial seawater. The mechanical properties of GPC were evaluated by analyzing and comparing the volume expansion and strength loss rates of GPC and PC concrete in an artificial seawater environment. The corrosion resistance of geopolymer (GP) mortar and PC mortar was evaluated by studying the migration ability and pore structure in corrosive ions attack (Cl−, SO42−, Mg2+) in artificial seawater. Moreover, the time-dependent tensile strength of BFRP was comparatively investigated by immersing in different solutions (tap water, artificial seawater, and alkaline simulated seawater). In addition, the dual interface transition zones (ITZs) characteristics of BFRP reinforced GPC under artificial seawater were also investigated by SEM and BSE tests. The results showed that the volume expansion rate and strength loss rate of GPC decreased by 77.6% and 8.7%, respectively, after 360 days of seawater corrosion compared with PC concrete. This enabled the development of a time-dependent strength model of GPC in marine environments. The coefficient of ions diffusion in GP mortar is much lower than that of PC mortar, and GP mortar shows excellent resistance to ion migration. In addition, the effect of seawater corrosion on the tensile strength of BFRP bars increases with the increase of bars' diameter, and the ultimate strengths of BFRP bars with diameters of 6 mm and 8 mm were 695 MPa and 663 MPa, respectively. The tensile strength degradation model of BFRP bars in geopolymer concrete under seawater corrosion was established. After 360 days of seawater immersion, the average porosity of the ITZ between geopolymer and aggregates, and the average porosity of the ITZ between geopolymer and BFRP bars increased insignificantly compared to that of PC concrete. This research can provide a theoretical basis for the service life prediction of BFRP reinforced geopolymer concrete within marine environments.

Introducing Dynamicity: Engineering Stress Relaxation Into Hydrogels Via Thiol-Ene Modified Alginate for Mechanobiological in vitro Modeling of the Cornea.

Developing biomaterials for corneal repair and regeneration is crucial for maintaining clear vision. The cornea, a specialized tissue, relies on corneal keratocytes, that respond to their mechanical environment. Altering stiffness affects keratocyte behavior, but static stiffness alone cannot capture the dynamic properties of in vivo tissue. This study proposes that the cornea exhibits time-dependent mechanical properties, similar to other tissues, and aims to replicate these properties in potential therapeutic matrices. First, the cornea's stress relaxation properties are investigated using nanoindentation, revealing 15% relaxation within 10 seconds. Hydrogel dynamicity is then modulated using a specially formulated alginate-PEG and alginate-norbornene mixture. The tuning of the hydrogel's dynamicity is achieved through a photoinitiated norbornene-norbornene dimerization reaction, resulting in relaxation times ranging from 30 seconds to 10 minutes. Human primary corneal keratocytes are cultured on these hydrogels, demonstrating reduced αSMA (alpha smooth muscle actin) expression and increased filopodia formation on slower relaxing hydrogels, resembling their native phenotype. This in vitro model can enable the optimization of stress relaxation for various cell types, including corneal keratocytes, to control tissue formation. Combining stress relaxation optimization with stiffness assessment provides a more accurate tool for studying cell behavior and reduces mechanical mismatch with native tissues in implanted constructs.

Evaluation of time dependent mechanical properties through nanoindentation of different compositions of 2D Ruddlesden Popper and 3D lead halide perovskites

Hybrid organic-inorganic perovskite (HOIPs) films' simplicity in manufacturing, both in solution and at low temperatures, foresees its application in deformable technologies including flexible photovoltaics, sensors and displays. The next generation of flexible optoelectronic devices may be possible owing to the special characteristics of hybrid organic-inorganic perovskites (HOIPs). However, it is necessary to have a thorough knowledge of the mechanical response of hybrid organic-inorganic perovskites(HOIPs) to dynamic strain to successfully use them in deformable devices. In the current study, a range of perovskites: 3D, butylammonium(BA) based 2D and quasi-2D, phenylethylammonium(PEA) based 2D and quasi-2D perovskite have been fabricated with particle sizes ranging in 5–10 nm as found using TEM. The nanoindentation creep and stress relaxation experiments prove the time and rate-dependent mechanical properties of this 2D, quasi-2D, and 3D HOIPs crystal though varying in their magnitudes. We observe that the 3D as well as BA based perovskite show strain rate sensitivity whereas PEA based perovskite samples were relatively insensitive towards the rate of loading. Propagation and interaction of dislocation are much more difficult in PEA-based perovskite, which has a triclinic crystal structure and are less symmetrical as compared to the BA based and 3D perovskite orthorhombic and tetragonal structure respectively. The knowledge offered by this work is crucial for creating perovskite devices that can endure mechanical deformations.

Single and bi-compartment poro-elastic model of perfused biological soft tissues: FEniCSx implementation and tutorial

Soft biological tissues demonstrate strong time-dependent and strain-rate mechanical behavior, arising from their intrinsic visco-elasticity and fluid–solid interactions. The time-dependent mechanical properties of soft tissues influence their physiological functions and are related to several pathological processes. Poro-elastic modeling represents a promising approach because it allows the integration of multiscale/multiphysics data to probe biologically relevant phenomena at a smaller scale and embeds the relevant mechanisms at the larger scale. The implementation of multiphase flow poro-elastic models however is a complex undertaking, requiring extensive knowledge. The open-source software FEniCSx Project provides a novel tool for the automated solution of partial differential equations by the finite element method. This paper aims to provide the required tools to model the mixed formulation of poro-elasticity, from the theory to the implementation, within FEniCSx. Several benchmark cases are studied. A column under confined compression conditions is compared to the Terzaghi analytical solution, using the L2-norm. An implementation of poro-hyper-elasticity is proposed. A bi-compartment column is compared to previously published results (Cast3m implementation). For all cases, accurate results are obtained in terms of a normalized Root Mean Square Error (RMSE). Furthermore, the FEniCSx computation is found three times faster than the legacy FEniCS one. The benefits of parallel computation are also highlighted.

Step-changed strain rate effect on the mechanical properties of undisturbed expansive clay

Up to now, most of the research on rate effects has investigated the behavior of soft soils, much less is known for the stiff clays. In particular, the method of stepwise changing strain rate has rarely been used in studies of soil rate effects, but many studies have well demonstrated that this method is beneficial to overcome the problems of variability between samples and the time requirements associated with strain rates. Isotropic consolidated-undrained triaxial tests were conducted on specimens with the same void ratio at step-changed axial strain rates under four different confining pressures in this study. We carried out the effect of axial strain rates from 0.09 to 300%/hour on the mechanical properties of naturally expansive soils. Experimental results showed that the higher the strain rate, the higher the effective stress for a given strain. The shear strength increases from 5.9% to 11.4% per log cycle of axial strain rate for saturated undisturbed expansive soil specimens by introducing the strain-rate parameter ρ 0.09, which is similar to that of soft soils. The excess pore water pressure gradually decreases after reaching a peak at lower cell pressures and increasing at higher ones. In addition, the failure mode of the specimens is different from soft clay with both the main shear band and multi-fissures band. The new findings are helpful for better understanding the time-dependent mechanical properties of stiff clays.

A Rheological Model with Integer and Non-Integer Orders Nonlinearities for Predicting Stress Relaxation Behavior in Viscoelastic Materials

Viscoelastic materials are widely used as devices for vibration control in modern engineering applications. They exhibit both viscous and elastic characteristic when undergoing deformation. They are mainly characterized by three time-dependent mechanical properties such as creep, stress relaxation and hysteresis. Among them, stress relaxation is one of the most important features in the characterization of viscoelastic materials. This phenomenon is defined as a time-dependent decrease in stress under a constant strain. Due to the inherent nonlinearity shown by the material response over a certain range of strain when viscoelastic materials are subjected to external loads, nonlinear rheological models are needed to better describe the experimental data. In this study, a singlenonlinear differential constitutive equation is derived froma nonlinear rheological model composed of a generalized nonlinear Maxwell fluid model in parallel with a nonlinear spring obeying a power law for the prediction of the stress relaxation behavior in viscoelastic materials. Under a constant strain-history, the time-dependent stress is analytically derived in the cases where the positive power law exponent, α ˂ 1 and α ˃1. The Trust Region Method available in MATLAB Optimization Toolbox is used to identify the material parameters. Significant correlations are found between the experimental relaxation data taken from literature and exact analytical predictions. The obtained results show that the developed rheological model with integer and non-integer orders nonlinearities accurately describes the experimental relaxation data of some viscoelastic materials.

Experimental Study on Creep Characteristics of Unloaded Rock Masses for Excavation of Rock Slopes in Cold Areas

Seasonal freeze–thaw environments are one of the key factors that aggravate the mechanical strength decay of excavated and unloaded rock masses on reservoir banks in cold areas. To study the time-dependent mechanical properties of an excavated and unloaded rock mass on a bank slope under freeze–thaw action, triaxial unloading tests were carried out on sandstone, freeze–thaw tests simulating freezing strength were conducted, and triaxial creep tests were implemented with graded incremental loading on unloaded specimens subjected to freeze–thaw action. The test results showed that the total deformation of the unloaded specimens is significantly increased compared with the conventional specimens, and the lateral direction is more likely to produce creep behaviour than the axial direction. The level of confining pressure determines the level of creep deformation of unloaded specimens and affects the variation law of creep rate. The creep behaviour of the unloaded specimens is aggravated by freeze–thaw action and, the longer the freezing period, the larger the creep strain share, and the creep rate increases significantly. The creep damage pattern of the unloaded specimens subjected to freeze–thaw action is mainly manifested as shear damage, and the creep process intensifies the derivation of tension-type cracks in the specimens. The higher the confining pressure of the unloaded specimen, the more obvious the plastic characteristics and the weaker the brittle characteristics during creep failure. The freeze–thaw action significantly reduces the long-term strength of the unloaded specimen, which is approximately 50~55% of the instantaneous strength. The long-term strength decays significantly with an increasing freezing period, and the research results can provide a theoretical reference for the evaluation of the long-term stability of excavated and unloaded rock masses in cold areas.

Chemically Fueled Reinforcement of Polymer Hydrogels.

Carbodiimide-fueled anhydride bond formation has been used to enhance the mechanical properties of permanently crosslinked polymer networks, giving materials that exhibit transitions from soft gels to covalently reinforced gels, eventually returning to the original soft gels. Temporary changes in mechanical properties result from a transient network of anhydride crosslinks, which eventually dissipate by hydrolysis. Over an order of magnitude increase in the storage modulus is possible through carbodiimide fueling. The time-dependent mechanical properties can be modulated by the concentration of carbodiimide, temperature, and primary chain architecture. Because the materials remain rheological solids, new material functions such as temporally controlled adhesion and rewritable spatial patterns of mechanical properties have been realized.

Multi-criterion optimization of Low-Cost, Self-compacted and Eco-Friendly Micro-calcium-carbonate- and Waste-glass-flour-based Ultrahigh-Performance concrete

This paper provides a multi-objective optimization approach to designing and manufacturing ultra-high-performance concrete (UHPC) with low cement content. Micro calcium carbonate and ground waste glass were employed as partial substitutes for cement and microsilica (MS) to achieve this goal. A three-factor design of experiments was used to achieve the most desirable rheological and mechanical properties with minimal quantities of cement and MS. In this context, the cement dosage (factor A), water-to-binder ratio (factor B), and superplasticizer dosage (factor C) were taken into account. By utilizing Andreasen & Andersen's (MAA) particle packing theory, dense particle packing was achieved throughout the experimental design. Regression models were developed to simulate the time-dependent rheological and mechanical properties of the UHPC. It was observed that the mathematical optimization was valid experimentally with reduced absolute deviations between 0.22 % and 1.23 % for slump flow and 3.49 % to 4.62 % for compressive strength at 28 days. The study showed that approximately 590 kg/m3 of cement would be most effective for recycled glass UHPC mixes. The proposed design will likely produce a compressive strength exceeding 150 MPa with satisfactory rheological properties at a lower cost.

Collagen hydrogel viscoelasticity regulates MSC chondrogenesis in a ROCK-dependent manner.

Mesenchymal stem cell (MSC) chondrogenesis in three-dimensional (3D) culture involves dynamic changes in cytoskeleton architecture during mesenchymal condensation before morphogenesis. However, the mechanism linking dynamic mechanical properties of matrix to cytoskeletal changes during chondrogenesis remains unclear. Here, we investigated how viscoelasticity, a time-dependent mechanical property of collagen hydrogel, coordinates MSC cytoskeleton changes at different stages of chondrogenesis. The viscoelasticity of collagen hydrogel was modulated by controlling the gelling process without chemical cross-linking. In slower-relaxing hydrogels, although a disordered cortical actin promoted early chondrogenic differentiation, persistent myosin hyperactivation resulted in Rho-associated kinase (ROCK)-dependent apoptosis. Meanwhile, faster-relaxing hydrogels promoted cell-matrix interactions and eventually facilitated long-term chondrogenesis with mitigated myosin hyperactivation and cell apoptosis, similar to the effect of ROCK inhibitors. The current work not only reveals how matrix viscoelasticity coordinates MSC chondrogenesis and survival in a ROCK-dependent manner but also highlights viscoelasticity as a design parameter for biomaterials for chondrogenic 3D culture.

Investigation of Time-Dependent Electrical Resistivity and Mechanical Properties of Glass Powder and Fly Ash Additive Mortars

Mineral katkılar, çimento esaslı kompozitlerin bazı özelliklerini iyileştirmek ve çimento miktarını azaltmak için beton karışımında kullanılmaktadır. Kullanılan çimento miktarının azaltılması ile çevre kirliliğinin ve çimento üretimi sırasındaki yüksek maliyetin önüne geçilmesi sağlanmaktadır. Bu kapsamda çimento ile ağırlıkça %10, %20 ve %30 oranlarında yer değiştirilerek Cam Tozu (CT) ve Uçucu Kül (UK) karışıma ilave edilmiştir. Harç numunelerinin hazırlanmasında bağlayıcı olarak CEM I 42.5/R tipi Portland çimentosu, agrega olarak ise 0-4 mm boyutlarında kırma kum kullanılmıştır. Laboratuvar ortamında 40x40x160 mm boyutlarında üretilen numuneler kalıptan çıkarıldıktan sonra, standart kür havuzunda 7, 28, 56, 90 ve 180 gün boyunca 20±2 °C sıcaklıkta kür edilmiştir. Taze haldeki harç numunelerine yayılma, sertleşmiş harç numunelerine ise basınç ve eğilme dayanımı ile elektriksel özdirenç deneyleri yapılmıştır. Deneysel çalışmalar sonucu elde edilen veriler doğrultusunda erken yaşta CT ve UK ilavesinin elektriksel özdirenci düşürdüğünü fakat numune yaşı ilerledikçe artışlar meydana geldiği görülmektedir. 180 günlük elektriksel özdirenç değeri REF numunesinde 0.04223 kΩm iken %30UK numunesinde 0.04755 kΩm, %30 CT numunesinde ise 0,04621 kΩm değerine ulaşmıştır.

Influence of Stabilization Additive on Rheological, Thermal and Mechanical Properties of Recycled Polypropylene.

To decrease the amount of plastic waste, the use of recycling techniques become a necessity. However, numerous recycling cycles result in the mechanical, thermal, and chemical degradation of the polymer, which leads to an inefficient use of recycled polymers for the production of plastic products. In this study, the effects of recycling and the improvement of polymer performance with the incorporation of an additive into recycled polypropylene was studied by spectroscopic, rheological, optical, and mechanical characterization techniques. The results showed that after 20 recycling steps of mechanical processing of polypropylene, the main degradation processes of polypropylene are chain scission of polymer chains and oxidation, which can be improved by the addition of a stabilizing additive. It was shown that a small amount of an additive significantly improves the properties of the recycled polypropylene up to the 20th reprocessing cycle. The use of an additive improves the rheological properties of the recycled melt, surface properties, and time-dependent mechanical properties of solid polypropylene since it was shown that the additive acts as a hardener and additionally crosslinks the recycled polymer chains.

Analysis of Rheological Factors of Soft Rock Tunnel Based on Constitutive Model of Rock Parameters Attenuation with Equivalent Effect

Rock mass deformation is a time related process, especially for soft rock. Its deformation usually has a certain timeliness. Moreover, the deformation of surrounding rock is typically asymmetric. Therefore, it is of great significance to reasonably describe the time-dependent mechanical properties and behaviors of rock mass for practical engineering, especially when the actual engineering is a symmetric structure. Taking the chlorite schist section at the west end of the diversion tunnel of Jinping II Hydropower Station as the research object and introducing the characteristic that creep parameters attenuate with equal effect change into Burgers constitutive model used for numerical calculation, an improved Burgers model is proposed. Then, according to the actual situation of the project, the improved three-parameter H-K model, which is suitable for this study, is proposed by using the fractional calculus method. The effects of different factors on the rheological properties of soft rock tunnels are discussed. The results show that When K is equal to 1, creep is positively correlated with burial depth; When the burial depth H is 1500 m, the creep deformation is positively correlated with the horizontal geostress; The farther away from the working face, the greater the instantaneous elastic deformation release and the later creep displacement; After the tunnel excavation is stopped, the earlier the support is, the smaller the later creep deformation is; After excavation and support of the upper bench, the longer the stagnation time is, the more unfavorable the rheological deformation of the tunnel is.

Effects of Different Types of Electric Fields on Mechanical Properties and Microstructure of Ex Vivo Porcine Brain Tissues.

Electrotherapy plays a crucial role in regulating neuronal activity. Nevertheless, the relevant therapeutic mechanisms are still unclear; thus, the effects of electric fields on brain tissue's mechanical properties and microstructure need to be explored. In this study, focusing on the changes in mechanical properties and microstructure of ex vivo porcine brain tissues under different types of electric fields, directional and alternating electric fields (frequencies of 5, 20, 50, and 80 Hz, respectively) integrate with a custom-designed indentation device. The experimental results showed that for the ex vivo brain tissue, the directional electric field (DEF) can reduce the elastic properties of brain tissue. Simultaneously, the DEF can increase the cell spacing and reduce the proteoglycan content. The transmission electron microscope (TEM) analysis observed that the DEF can reduce the integrity of the plasma membrane, the endoplasmic reticulum's stress response, and the myelin lamella's separation. The alternating electric field (AEF) can accelerate the stress relaxation process of brain tissue and change the time-dependent mechanical properties of brain tissue. Meanwhile, with the increase in frequency, the cell spacing decreased, and the proteoglycan content gradually approached the control group without electric fields. TEM analysis observed that with the increase in frequency, the integrity of the plasma membrane increases, and the separation of the myelin lamella gradually disappears. Understanding the changes in the mechanical properties and microstructure of brain tissue under AEF and DEF enables a preliminary exploration of the therapeutic mechanism of electrotherapy. Simultaneously, the essential data was provided to support the development of embedded electrodes. In addition, the ex vivo experiments build a solid foundation for future in vivo experiments.

Instantaneous and long-term mechanical properties of Polyethylene Terephthalate Glycol (PETG) additively manufactured by pellet-based material extrusion

Polyethylene Terephthalate Glycol (PETG) is a highly popular feedstock for extrusion-based additive manufacturing. While data are available on the instantaneous properties of additively manufactured PETG, few research have been done on forecasting the creep behaviour of additively manufactured PETG while accounting for the material altering effects of ageing. This research article aims to enhance the understanding of both the instantaneous and time-dependent mechanical properties of additively manufactured PETG through a series of tensile, FEA simulations, Dynamic Mechanical Analysis (DMA), and two types of creep experiments. The details of experimental and mathematical calculations of the instantaneous and time-dependent properties of additively manufactured PETG are provided. Nine independent material parameters have been determined including three Young’s moduli, three shear moduli and three Poisson’s ratios, to fully quantify an orthotropic material model of additively manufactured PETG. The printed material exhibited a Young’s modulus that is 86.5% of the theoretically possible value in direction 1, a Young’s modulus in direction 2 is 66.0% of the theoretical optimum, and a Young’s modulus in direction 3 is within 1% of its theoretical maximum. In addition to reporting the creep behaviour of PETG, the novel application of the Time-Temperature Superposition Principle (TTSP) to additively manufactured PETG has been shown to produce an age-affected creep prediction for up to 3.88 years based on samples aged for 221 h and at 23 °C. The methodology and data models have been found to enable predictions for other ages and temperatures. It was concluded that the application of the TTSP creep methodology was limited by the creep test temperature, 60 °C, after which the material began to behave in a non rheologically-simple manner.

Unified transient creep constitutive model based on the crack evolution of micritic bioclastic limestone.

The surrounding rock at the exit of the No. 1 drainage tunnel of the Artashi Water Conservancy Project is micritic bioclastic limestone with 55% bioclastic material. This rock underwent unpredictable large and time-dependent deformation during excavation. To date, the mechanical behaviour of this kind of rock has rarely been studied. In this study, traditional triaxial compression tests and multilevel creep tests were conducted on micritic bioclastic limestone, and the results clarified the instantaneous and time-dependent mechanical properties of the rock. Considering that the essence of rock failure is crack growth, the crack strain evolution properties were revealed in rock triaxial compression tests and multilevel creep tests. Based on triaxial compression tests, the evolution of axial cracks with increasing deviatoric stress ratio Rd (ratio of deviatoric stress to peak deviatoric stress) was observed, and an axial crack closure element and new crack growth element were proposed. To simulate the creep behaviour of a rock specimen, the relationship of the rock creep crack strain rate with Rd was studied. A creep crack element was created, and the creep crack strain evolution equation was obtained, which closely fit the experimental data. Combining the 4 element types (elastic element, crack closure element, crack growth element, and creep crack element), a unified transient creep constitutive model (Mo’s model) was proposed, which represented both the transient and time-dependent mechanical properties of the micritic bioclastic limestone.

Dynamic viscoelastic properties of porcine gastric tissue: Effects of loading frequency, region and direction

The gastric biomechanics influences digestive function as well as a range of topics of medical and scientific interests such as interaction between the stomach and gastric devices. Hence, the mechanical properties are essential for understanding gastric tissue and function in health and disease, and for the development of diagnostic or therapeutic devices. A key characteristic to be characterized is the time dependent mechanical tissue properties. The aim of this study was to characterize viscoelastic properties of the stomach across a frequency range. Longitudinal and circumferential stomach samples from the porcine fundus, corpus and antrum were pre-stretched 10 % and sinusoidally loaded with 10 % dynamic strain. The viscoelastic properties were assessed from 0.01 − 15 Hz using dynamic mechanical analysis. The storage moduli, loss moduli and tan δ had a significant second-order polynomial trend with increasing frequency. For the loss moduli, significant differences were observed between 0.01 and 15 Hz and between 0.05 and 15 Hz (p = 0.023 to 0.041). Significant differences were not found for storage moduli. Tan δ was frequency-independent, indicating that the two moduli varied proportionally. Fundus had significantly smaller storage moduli for longitudinal samples compared to corpus (p = 0.034) and antrum (p = 0.014) but was not significantly different for circumferential samples. Analysis of direction-dependency showed significant differences between longitudinal and circumferential samples (p = 0.002 to 0.042). The presented work provides insight into tensile viscoelastic properties of gastric tissue, which is useful for developing biomaterials, devices and computational models for device development specification calibrations.

A new fractional-order model for time-dependent damage of rock under true triaxial stresses

To reasonably describe the time-dependent characteristics of rock under true triaxial stresses and analyze the long-term stability of deep underground engineering, based on the understanding of time-dependent mechanical properties of rock under true triaxial stresses, a new fractional-order damage element and the method in which the crack initiation stress is taken as the starting point of creep and the crack damage stress as the starting point of creep accelerating stage were established. Then, combined with the Mogi-Coulomb strength criterion, a fractional-order creep damage model of rock under true triaxial stresses was proposed. The proposed model was further verified based on uniaxial, conventional triaxial and true triaxial creep test results. The theoretical curves of the model are highly consistent with the test curves, indicating that the proposed model has wide applicability and rationality. Meanwhile the sensitivity of true triaxial stress states and model parameters to the creep characteristics was further investigated. Finally, based on the proposed creep model, the time-dependent characteristics of rock under untested true triaxial stress levels were predicted. The research provides a theoretical basis for the prediction of time-dependent disasters and the long-term stability analysis of surrounding rock in deep engineering.

Analysis of Surface Grinding of Thermoplastics Specimens with Inline Measurements

This paper analyzes the surface grinding of unfilled and glass-filled polyamides. The process is performed by varying the workpiece velocities to evaluate applied practical applications in the industry while being energy efficient. During the machining, the temperatures, normal forces, tangential forces, and spindle power were collected, and the surface quality was evaluated by a scanning electron microscope (SEM), helping to determine material removal mechanisms and study their behavior under grinding. One of the primary outcomes of the present research was that, different from most metallic and ceramic materials, polyamides benefited from the material removal rate increase. We had higher quality material removed efficiently. Also, the specific energy of both materials converged to previously demonstrated values, showing once again that it is highly dependent on the matrix. Moreover, the time-dependent mechanical properties of the material during processing were identified. The fast application of the force at high speed gave less time to respond to the mechanical strain, determining an improvement in the surface quality of the samples. Consequently, the surface quality of the final product improved with a speed increase, leading to low roughness values.