Initial Compression Stage Research Articles

Understanding the compaction behavior of uncured thermoset prepreg: Experimental investigation and theoretical analyses

AbstractThe deformation behavior of uncured thermoset prepreg during hot compaction process was investigated by dividing the whole compaction process into an initial compression stage and the subsequent creep stage. At the compression stage, there existed a strain‐softening phenomenon in the temperature range of 50–90°C, indicating different deformation behavior that is mainly determined by the viscosity of prepreg. Following an appraisal of advantages and limitations of existing compaction models, a combination model consisted of a modified power‐law model including the influence of temperature and the generalized Kelvin‐Voigt model was proposed to describe the deformation behavior of prepreg during compression and creep stage, respectively. Finally, a discussion on the compaction mechanism was conducted to offer some insights into the deformation process.Highlights Strain‐softening phenomenon occurred during the compaction of uncured prepreg. Proposed combination model captures the deformation of uncured prepreg well. The percolation mechanism dominates prepreg deformation during compaction. The squeeze flow mechanism causes limited thickness reduction of prepreg.

Mechanical properties of teguline clay pellet mixtures during continuous oedometric compression

Clay pellet mixtures are generally compressed to improve their engineering performance. Deepening the comprehension of the mechanical properties of these mixtures in the complete compression process facilitates the benefit to the engineering design and their utilization. In this study, the effects of soil grain size distribution, water content and dry density on the mechanical properties and microstructure of Teguline clay pellet mixtures during a continuous oedometric compression process are explored. Three types of soil pellet mixtures, including mixture A (grain size ≤5 mm), mixture B (≤ 0.4 mm) and mixture C (2–5 mm), were prepared with different water contents of 7%, 8% and 12% respectively. Subsequently, continuous oedometeric compression was undertaken to explore their mechanical behaviours of the soil pellet mixtures. After that, the microstropic structures of the compacted pellet mixtures were investigated using mercury intrusion porosimetry (MIP). The results indicated that mixture A has a minimal initial packing density of pellet mixtures, while mixture C has a maximum one at the initial compression stage. After completion of compression, the compression curves of the pellet mixtures tended to converge uniformity at a semilogarithm coordinate as the vertical stress increased. All of the compression curves presented a concave shape at the plastic compression stage, which is significantly influenced by grain size distribution and water content. In contrast, the elastic compression and rebound behaviours are little affected by the grain size distribution and water content. As far as the microstructure is concerned, compacted samples prepared by mixture A or C presented a unimodal pore structure, while those by mixture B showcased a bimodal pore structure. In comparison with the unimodal pore distribution of the undisturbed stiff clay, the compacted samples displayed a pseudo-unimodal pore distribution because the inter-aggregate pores still existed. A double tangent method was proposed to determine the delimiting pore diameter of the pseudo-unimodal pore distribution curves and found that the delimiting pore diameter decreased with the increase of dry density and water content. Moreover, the inflexion point for the pore diameter of compacted samples prepared by coarse soil was larger than that of fine soil. Combining this work with previous research, it was found that the high compression of coarse soil easily causes the pseudo-unimodal shape, which is also impacted by water content and particle properties. This work could help deepen the understanding of the mechanical characteristics and microstructure of the stiff clay pellet mixtures during continuous oedometric compression.

Model test study of bridge pile horizontal bearing behavior under landslide deformation

Pile is a common foundation on the slope, which poses a serious threat to the construction and operation if the slope deformation and causes landslide. In this study, a model device of pile foundation on landslide was independently developed by relative displacement loading between pile and soil to explore the influence of landslide deformation on pile and analysis the soil failure rule and the deformation characteristics of pile in different stages of landslide deformation, a few model tests were completed including the relative displacement between soil and pile from 1 to 17 cm, and the pile diameter and the modulus of slide bed were also considered. The results indicated that: the evolution process of landslide deformation with pile foundation on could be divided into four stages including soil compaction, cracks growth, yield stage, and failure stage; ratios of the maximum soil pressure and bending moment growth from the soil compaction stage to the cracks growth stage to the total growth in these four stages are both exceeding 60%; the soil pressure increases with the increase of pile diameter and sliding bed modulus. Therefore, it is best to effectively monitor and control the landslide in the initial soil compression stage that in soil compaction stage and methods such as increasing pile foundations or reinforcing the sliding bed can be used for protection.

Model for fracture conductivity considering particle size redistribution caused by proppant crushing

Clarifying the dynamic evolution and control mechanisms of fracture conductivity is crucial for optimizing hydraulic fracturing design. Previous studies on fracture conductivity mechanisms in deep shale reservoirs have been insufficient, particularly when considering proppant crushing, rendering existing models inapplicable. Therefore, in this study, the coupled effects of particle size redistribution caused by proppant crushing were considered. A particle crushing model based on fractal theory was introduced. It was combined with the Kozeny–Carman equation, and equations were derived for the evolution of the equivalent diameter of the proppant pile gradation curve and permeability, establishing a new model for predicting the fracture conductivity considering the particle size redistribution caused by proppant crushing. Furthermore, it provides an optimized chart of conductivity that matches the supply conductivity of the reservoir. The results of the study reveal that the fracture conductivity decreases owing to proppant rearrangement, compaction, and crushing, with a more significant decline in the initial compression stage, where the rate of conductivity reduction is 2.5 times that of the compaction stage. Proppant crushing shifts the gradation curve leftward, reducing the equivalent particle diameter and, consequently, the fracture conductivity. By adjusting the crushing ratio, apparent Young's modulus, looseness factor, and proportion of large-particle components of the proppant, the fracture conductivity can be controlled. This provides a theoretical basis for the development of proppant properties and fracturing optimization in deep shale reservoirs.

Failure patterns and mechanical behavior of the cement mortar column partially confined by the FRP jacket

Fiber-reinforced polymer (FRP) composite has been recently adopted to maintain the integrity of infrastructure made of cement mortar, attributed to its high strength-to-weight ratio and easy-to-construct. To obtain an in-depth understanding about the compressive behavior of cement mortar samples with variable FRP center wrapping heights, both the acoustic emission (AE) response and deformation characteristics of which were comprehensively investigated in the present research. The relationship between the confinement provided the exterior FRP jacket and the mechanical characteristic was also explored, followed by the economic analysis on this reinforced technique. Test results indicated that both the deformation ability and the load-bearing capacity gradually increased with the increased center wrapping height of the FRP jacket. When the larger the AE energy generated at the initial compression stage, the more dispersed AE localization. Meanwhile, these main cracks occurred on the surface of reinforced cement mortar generally transformed from the FRP wrapping strip towards other extremities. Correspondingly, the number of tensile cracks of FRP partially wrapped cement mortar decreased with the increased axial deformation. Note that the residual integrity of these reinforced cement mortar with larger FRP-center wrapping height is much superior than its counterparts. The force state of partially reinforced cement mortar with FRP confinement was also discussed in the present research and the results of which were compared with existing theoretical models. Compared to other theoretical models, Teng's model shows the most accuracy in predicting the mechanical behavior of FRP partially reinforced cement mortar.

Influences of random imperfection distribution on the compressive properties of interlocking block wall

Block imperfections exist inevitably owing to manufacturing and construction quality control. For dry-stacked interlocking block structures, imperfections result in small gaps randomly distributed between blocks, which affect the compressive strength of the wall. In this study, stochastic analysis is conducted to predict the compressive properties of interlocking block walls with spatially varying randomly distributed block imperfections. Monte-Carlo simulation is conducted in the analysis. The number of block imperfections is assumed to follow the Binominal distribution in massive block production process; the imperfection sizes are assumed to follow the truncated normal distribution. Based on these hypotheses, the damage development mechanism and load-path of interlocking block walls with different imperfection distributions are investigated. It is found that the compressive strength of walls containing blocks with mixed imperfection levels is lower; for walls with a higher number of imperfections, a larger coefficient of variation of imperfections leads to a significant decrease in compressive strength, while the seating effect at the initial stage of compression is reduced. The results provide a guidance to the quality control of mortar-less interlocking block structures.

Liquid-gas heat transfer characteristics of near isothermal compressed air energy storage based on Spray Injection

Isothermal compressed air energy storage (I-CAES) could achieve high roundtrip efficiency (RTE) with low carbon emissions. Heat transfer enhancement is the key to achieve I-CAES, thus the liquid-gas heat transfer characteristics of near I-CAES system based on spray injection was analyzed in this paper. The liquid-gas heat transfer model and thermodynamic model were established and the effects of design variables on heat transfer rate and energy efficiency were analyzed. The results showed that spray injection can produce considerable heat transfer capacity and effectively suppress air temperature change, but spray injection at initial compression stage will not bring much heat transfer enhancement. Increasing spray flow rate shifts the dominant heat transfer contribution from water bulk to spray heat transfer and increases total heat transfer rate and RTE, however the increase effect of spray injection is more obvious in small-scale CAES. Low water flow rate and large working cylinder volume are recommended to achieve slow compression and expansion processes with enough liquid-gas heat transfer duration, realizing near I-CAES with high RTE. High air storage pressure increases heat transfer rate, but deviates CAES from isothermal system, resulting in low RTE. This research will provide scientific basis for heat transfer enhancement, system design and performance optimization of I-CAES.

The novel strength criterion and the associated constitutive model based on the finite deformation behavior for the rock under the disturbance stress paths

To develop a novel 3D Hoek–Brown strength criterion and an associated constitutive model based on the finite deformation behavior of rock under the disturbance stress paths, a series of mechanical tests are initially conducted under designed disturbance stress paths (hydrostatic stress stage (HSS), initial loading–unloading stage (ILUS), and disturbance stage (DS)). Additionally, low disturbance (LD), medium disturbance (MD), and high disturbance (HD) modes are taken into consideration. Then, the finite deformation theory is employed to study the nonlinear behavior. This nonlinear behavior can be regarded as a nonlinear relationship between the stress and strain of rock under loading, especially the initial compression stage and plastic stage, as well as a complex deformation field, which is described by the finite deformation theory and featured by the mean rotation angle (Θ). Afterwards, the damage variable (DV) regarding Θ is proposed and used to establish a novel strength criterion (NSC) based on the current Hoek–Brown strength criterion. Finally, by considering the NSC, an associated elasto-plastic constitutive model is developed. The results show that the initial confining pressure (ICP) and disturbance mode have a significant effect on the elastic modulus (E). Meanwhile, the ICP has a higher effect on E than the disturbance mode. Then, the evolution of Θ is closely associated with the nonlinear deformation of rock masses because the σ1−Θ curves can be divided into three stages. Those stages can match well with the initial compression stage, elastic stage, and plastic stage, respectively. Regarding Θ as a parameter of the damage variable (DV), the NSC is established. Additionally, it can be observed that the failure envelopes of the NSC scale down gradually with increasing Θ on the deviatoric plane. Meanwhile, as the value of the coefficient of determination (DC) is higher than that of other strength criteria, the NSC may exhibit better agreement with the experimental data. Furthermore, a constitutive model based on the aforementioned strength criterion and the hardening function that takes Θ and the disturbance of rock masses (D) into account is proposed and validated as a solution to predict the nonlinear behavior of rock specimens under the disturbance stress paths.

Development of multi-field rock resistivity test system for THMC

Abstract. To study the relationship between rock mechanical properties and resistivity under deep-underground environmental conditions, a rock resistivity test system, which can realize the simultaneous control of temperature, pressure, seepage, and the chemical environment, was developed; further, a corresponding specimen-sealing method was explored. The system primarily comprises a triaxial system, chemical permeation system, temperature control system, and test control system. The reliability of the system was verified through tests and preliminary experiments. The resistivity of fractured granite specimens under coupling of seepage and temperature and the resistivity of unfrozen and freeze–thawed coals during triaxial compression were tested with this test system. The test results show that the temperature-induced resistivity change is greater at low seepage pressures for fractured granites, and the effect of seepage on rock resistivity is greater at lower temperatures. The resistivity change patterns of unfrozen and freeze–thawed coals during triaxial compression differ quite significantly. The resistivity of unfrozen coal specimens exhibits a decreasing trend in the initial compression stage and then gradually increases with rises in the deviatoric stress. After freezing and thawing, the electrical resistivity of coal decreases during the entire compression process.

Mechanism of Plastic Deformation of As-Extruded AZ31 Mg Alloy during Uniaxial Compression

The deformation mechanism and texture evolution of AZ31 Mg alloy compressed in three different directions at room temperature were studied, and the relationship between the two was compared through experiments and viscoplastic self-consistent (VPSC) modeling. Setting up only one specific deformation mode was the predominant mechanism by changing the CRSS ratio for the different deformation modes. The following conclusions were drawn: (1) It was demonstrated that basal slip causes a slow and continuous deflection of the grain toward the transverse direction (TD). When the sample is compressed in the extruded direction (ED), prismatic slip leads to grains being deflected toward the ED in the initial stages of compression, and when the sample is compressed 45° to the extrusion direction (45ED) and perpendicular to the extrusion direction (PED), prismatic <a> slip contributes little to the texture evolution. (2) When the sample is compressed along three different directions, pyramidal <c+a> slip leads to the grain being deflected toward the normal direction (ND), and the {10-12} extension twin deflects the grain at a large angle. (3) When only the {10-11} compression twin is activated, the grain will be deflected in the ND while the sample is compressed along the ED and 45ED, but when the sample is compressed in the PED, the grains are concentrated from both sides of the ED to the center.

A sinusoidal beam lattice structure with negative Poisson's ratio property

In this study, a three-dimensional (3D) beam lattice structure (BLS) with negative Poisson's ratio (NPR) effect is proposed by applying sinusoid-shaped beams. The finite element model of the BLS is established, and its sample is manufactured through selective laser sintering (SLS) technology. The axial compression experiment and simulation are performed to investigate the compression characteristics of BLS. BLS bends to one side in the later stages of compression. It can be concluded that the BLS possess strong NPR effect and excellent energy absorption performance. In addition, the effects of compression direction, amplitude and wavenumber on the compression characteristics of BLS are further analyzed. These BLSs exhibit significant anisotropic features, and they often show the deformation modes of S- and <-shaped. When the loading directions are z- and y-axes, these BLSs have more obvious NPR effect and better energy absorption characteristics compared with those of the x-axis. In addition, it is also found that the minimum values of PR (MPR) of these BLSs often appear in the initial stages of compression, and the maximum values of Poisson's ratio (PR) usually appear around the end of compression. In summary, the BLS has excellent mechanical properties and broad application prospects.

Modelling of Depth Prediction Algorithm for Intra Prediction Complexity Reduction

Video compression gained its relevance with the boon of the internet, mobile phones, variable resolution acquisition device etc. The redundant information is explored in initial stages of compression that’s is prediction. Inter prediction that is prediction within the frame generates high computational complexity when working with traditional signal processing procedures. The paper proposes the design of a deep convolutional neural network model to perform inter prediction by crossing out the flaws in the traditional method. It briefs the modeling of network, mathematics behind each stage and evaluation of the proposed model with sample dataset. The video frame’s coding tree unit (CTU) of 64x64 is the input, the model converts and store it as a 16-element vector with the goodness of CNN network. It gives an overview of deep depth decision algorithm. The evaluation process shows that the model performs better for compression with less computational complexity.

A Unified Nonlinear Elastic Model for Rock Material

Under conditions of low or medium stress, rocks that are in the compression state exhibit perceivable nonlinear elastic characteristics. After a comprehensive review of the existing nonlinear elastic models of rocks and joints, we proposed a new unified nonlinear elastic model. This new model contains two parameters with clear definitions, namely, the initial elastic modulus Eni and the modulus change rate m. This model covers a variety of existing models, including the simple exponential model, BB model and two-part Hooke’s model, etc. Based on the RMT experimental system, a large number of uniaxial compression tests for dolomite, granite, limestone and sandstone were performed, and their nonlinear deformation stress‒strain curves were obtained and fit with the unified nonlinear elastic model. The results show that the rocks have obvious nonlinear elastic characteristics in their initial compression stage, and that the unified nonlinear elastic model is able to describe these phenomena rather well. In addition, an empirical formula for predicting the uniaxial compressive strength of the rock was constructed, corresponding to the unified nonlinear elastic model.

Dynamic transformation of a near alpha high-temperature titanium alloy during hot deformation

The rapid drop of peak flow stress in the initial stage of hot compression experiment was found to be related to the occurrence of dynamic transformation from alpha phase (hcp) into beta phase (bcc) of a near α high-temperature titanium alloy. In order to predict the flow stress at all strain, the dynamic recovery (DRV) model and the Back-Error Propagation (BP) neural network architecture were established and comprehensively utilized to characterize the flow stress, which exhibited high accuracy in tracking the flow behavior at different deformation parameters. The variation of peak flow stress at initial stage of hot compression indicated that the rapid drop extent of peak value increased with the rise of deformation temperature, the decrease of strain rate and the increase of strain. It was worth noting that the dynamic transformation evolution in the microstructure exhibited the consistent variation of peak flow stress with different deformation parameters. The high-magnification microstructure analysis indicated that the dynamic transformation was accomplished by the immigration of α/β interface and the penetration of beta phase into alpha phase from edge to inside, all of which were related to the dislocation motion. The experimental result proved that the dynamic transformation was the dominant factor resulting in the rapid drop of peak flow stress at the initial stage of hot deformation.

Dynamic Response and Numerical Simulation of Closed-Cell Al Foams.

The drop hammer impact test was carried out to investigate the dynamic response of closed-cell Al foams. A relatively reasonable method was also developed to evaluate the velocity sensitivity of cellular material. The typical impact load-displacement curve exhibited two stages containing the initial compression stage and the progressive crushing stage. Three compressive damage behaviors and four failure modes of closed-cell Al foams were revealed, while the effect of velocity on the impact properties and the energy absorption capacity of different specimens were investigated. The results showed that the specific energy absorption of the specimens increased with the increasing density of the specimen and the impact velocity. However, the specimens with higher specific energy absorption seemed not to indicate better cushioning performance due to the shorter crushing displacement. In addition, the uniaxial impact simulation of two-dimensional (2D) Voronoi-based foam specimens was conducted at higher impact velocities. The simulation results of impact properties and deformation behavior agreed reasonably well with the experimental results, exhibiting similar velocity insensitivity of peak loads and deformation morphologies during uniaxial impact.

Dynamic Deformation Behavior and Fracture Characteristics of a near α TA31 Titanium Alloy at High Strain Rates.

The quasi-static and dynamic impact compression tests of the TA31 titanium alloy were conducted at the strain rates from 0.001 s-1 to 4000 s-1 and deformation temperatures from 293 K to 773 K, and the TA31 titanium alloy showed typical elastic-plastic characteristics. In the initial stage of compression (elastic deformation), the stress and strain are proportional, and the stress-strain curve is a straight line. In the plastic deformation stage, the flow stress decreases significantly with the increase of deformation temperature, while the strain rate has no significant effect on the flow stress during dynamic compression. A constitutive model has been established to predict the flow stress, and the relative error is 2.32%. It is shown by observing the microstructure that when the deformation temperature is 293 °C, and the strain rate reaches 1600 s-1, a shear band with an angle of about 45° to the axial direction of the specimen appears, and the severe shear deformation makes the α phase in the shear band fibrous and contains high-density dislocations. The formation process of the shear band and its influence on fracture are analyzed and discussed.

A novel concrete-filled auxetic tube composite structure: Design and compressive characteristic study

Concrete-filled stainless steel tube (CFSST) members take advantage of the high strength and the outstanding corrosion resistance to act as an important role in civil engineering structures. However, the steel tube could not provide the perfect confinement effect for the core concrete during the initial elastic compression stage because Poisson’s ratio of the concrete is smaller than that of the stainless steel tube (SST). In this paper, a novel concrete-filled auxetic stainless steel tube (CFASST) composite structure was designed and manufactured to actively restrain the concrete, making the best use of the desirable deformation characteristics of auxetic tubular structures. The axial compressive performance of these CFASST members and their control factors were investigated experimentally and numerically. Test results were discussed in detail which included failure modes, load versus displacement curves and strain analysis. Finally, parametric analyses were conducted to further study the effects of different parameters (Poisson’s ratio, thickness of the stainless tube) on the CFSST composite structure under axial compression. It was found that CFASST composite structures possess an unusual deformation mode and an improved confinement effect.

Development of a Model for Initial Compaction Stage in Rock Based on Energy Dissipation

An accurate model is essential for the numerical simulation of the rock failure occurring under different conditions. However, the expression of the initial compression stage is often overlooked in the widespread constitutive models. In this pursuit, several models considering the initial compression stage were gathered from the previous studies, and a model based on the energy dissipation principle was proposed in the present study. Subsequently, the experimental data of five different types of rock under uniaxial load was collected from the literature to verify the validity of the proposed model, and the performance was compared with the other models. It was found that all the models listed in this study exhibited good performance in expressing the stress-strain curve during the initial compression stage.

The role of β phase in the morphology evolution of α lamellae in a dual-phase titanium alloy during high temperature compression

The evolution behavior of α lamellae and evolution relationship between α lamellae and β phase in TC17 alloy during isothermal heating and subsequent compression were investigated. The results show that the α/β interface of TC17 alloy is made up of stable terrace plane and movable ledge plane. The ledge plane migrates along the length direction of α lamellae during isothermal heating, which results in rapid decrease of length and slow decrease of thickness of α lamellae. During compression, the continuous dynamic recrystallization (CDRX) are the main evolution mechanism of α and β phases. The slip systems of β phase are easily activated and the slip in β phase (i.e. (110)[1̅11]) will promote the slip in α phase (i.e. (0001)[112̅0]) at the initial stage of compression. Thus the CDRX of β phase is preferentially initiated by more activated slip and it will promotes the CDRX of α phase, then the HABs are formed in α lamellae. Meanwhile, the β stabilized elements Cr and Mo diffuse into the α/α boundaries in a short-time deformation. The α→β phase transformation is promoted by the enrichment of Cr and Mo elements and finally the α lamellae are separated with further compression.

Analogue sandbox modeling of orogenic wedge front faulting: Roles of inherited fault zones and topographic loading

Earthquakes reported in the frontal belts of active orogenic wedges such as the 2013 Mw6.6 Lushan and 1999 Mw7.6 Chi-Chi earthquakes, highlight the importance of loading exerted by mountainous topography and inherited fault zones. However, their effects on formation and activation of these wedge front fault systems remain poorly understood. This study presents three sets of sandbox experiments in which topographic loading and geometry of inherited faults were first separately investigated, then explored jointly. Experiments show that smaller topographic loading favors gentle-dipping wedge front thrusts, while larger topographic loading promotes the development of steep wedge front faults. Introduced as zones of frictional weakness, all inherited faults promote deformation localization, though the steep ones are characterized by buttressing nearby rather than direct slip on themselves. When the two factors were tested integrally, four major deformation modes arose, including (1) direct reactivation; (2) fault truncation in the initial stage of compression; (3) stacking of small thrusts against the inherited fault and possible truncation in a later evolutionary stage; (4) behavior as a fossil structure. These modes show differences not only in first-order deformation styles, but also in the distribution of secondary structures, defining a basic framework and providing new guides for the interpretation of complex wedge front fault systems and their activities, in areas such as the Longmen Shan (eastern Tibet), the Tian Shan and the western margin of the Taiwan orogen.