Experimental and numerical study on dynamic mechanical behaviors of shale under true triaxial compression at high strain rate

High-energy gas fracturing of shale is a novel,high efficacy and eco-friendly mining technique,which is a typical dynamic perturbing behavior.To effectively extract shale gas,it is important to understand the dynamic mechanical properties of shale.Dynamic experiments on shale subjected to true triaxial compression at different strain rates are first conducted in this research.The dynamic stress-strain curves,peak strain,peak stress and failure modes of shale are investigated.The results of the study indicate that the intermediate principal stress and the minor principal stress have the significant influence on the dynamic mechanical behaviors,although this effect decreases as the strain rate increases.The characteristics of compression-shear failure primarily occur in shale subjected to triaxial compression at high strain rates,which distinguishes it from the fragmentation characteristics observed in shale under dynamic uniaxial compression.Additionally,a numerical three-dimensional Split Hopkinson Pressure Bar(3D-SHPB),which is established by coupling PFC3D and FLAC3D methods,is validated to replicate the laboratory characteristics of shale.The dynamic mechanical characteristics of shale subjected to different confining stresses are systematically investigated by the coupling PFC3D and FLAC3D method.The numerical results are in good agreement with the experimental data.

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-stressσy andσz,but negatively correlated with axial static prestressσx.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.

Dynamic mechanical characteristics of frozen subgrade soil subjected to freeze-thaw cycles

As a widely-applied engineering material in cold regions, the frozen subgrade soils are usually subjected to seismic loading, which are also dramatically influenced by the freeze-thaw(F-T)cycles due to the varying temperature. A series of dynamic cyclic triaxial experiments were conducted through a cryogenic triaxial apparatus for exploring the influences of F-T cycles on the dynamic mechanical properties of frozen subgrade clay.According to the experimental results of frozen clay at the temperature of-10℃, the dynamic responses and microstructure variation at different times of F-T cycles(0, 1, 5, and 20 cycles) were explored in detail.It is experimentally demonstrated that the dynamic stress-strain curves and dynamic volumetric strain curves of frozen clay are significantly sparse after 20F-T cycles. Meanwhile, the cyclic number at failure(Nf) of the frozen specimen reduces by 89% after 20freeze-thaw cycles at a low ratio of the dynamic stress amplitude. In addition, with the increasing F-T cycles,the axial accumulative strain, residual deformation,and the value of damage variable of frozen clay increase, while the dynamic resilient modulus and dynamic strength decrease. Finally, the influence of the F-T cycles on the failure mechanisms of frozen clay was discussed in terms of the microstructure variation. These studies contribute to a better understanding of the fundamental changes in the dynamic mechanical of frozen soils exposed to F-T cycles in cold and seismic regions.

Dynamic mechanical characteristics of NdFeB in electromagnetic brake

With the continuous development of artillery,the disadvantages of hydraulic recoil brakes gradually appear.At the same time,the appearance of high-performance Nd Fe B permanent magnet makes it possible to apply electromagnetic braking technology to recoil mechanism.In this paper,prototype tests of a certain artillery were carried out to verify the feasibility of the electromagnetic brake(EMB)and obtain the electromagnetic braking force.Due to the brittleness of Nd Fe B,in order to eliminate the worry about the safety of EMB,SHPB experiments of Nd Fe B were carried out.Then,based on the assumption of uniform crack distribution,the law of crack propagation and damage accumulation was described theoretically,and the damage constitutive model suitable for brittle materials was proposed by combining the Zhu-Wang-Tang(ZWT)equation.Finally,the numerical simulation model of the artillery prototype was established and through calculation,the dynamic mechanical characteristics of Nd Fe B in the prototype were analyzed.The calculation results show that the strength of Nd Fe B can meet the requirements of the use in the working process.From the perspective of damage factor,the damage value of the permanent magnet on the far right is larger,and the damage value of the inner ring gradually decreases to the outer ring.

Temperature Dependency of Dynamic Mechanical Properties of Cement Asphalt Paste by DMTA Method

We investigated the temperature dependency of the dynamic mechanical properties of cement asphalt paste by the dynamic mechanical thermal analysis（DMTA） method. The experimental results show that the dynamic mechanical properties of cement asphalt pastes are sensitive to temperature due to the inclusion of asphalt, and may go through different states within a temperature range of-40 ℃ to 60 ℃, which is different from that of pure cement and asphalt. As the temperature of the cement asphalt paste increases, a considerable change of dynamic mechanical properties, including storage modulus（E＇）, loss modulus（E＇＇） and loss factor（tand） is observed. Moreover, the influence of asphalt to cement（A/C） ratio on the temperature sensitivity of the dynamic mechanical properties of cement asphalt composites was investigated. The temperature dependency of cement asphalt composites is ascribed to the temperature dependency of the asphalt and its interaction with cement paste. A simple fractional model is proposed to describe the viscoelastic behavior of cement asphalt composites.

Investigating the dynamic mechanical behaviors of polyurea through experimentation and modeling

Polyurea is widely employed as a protective coating in many fields because of its superior ability to improve the anti-blast and anti-impact capability of structures.In this study,the mechanical properties of polyurea XS-350 were investigated via systematic experimentation over a wide range of strain rates(0.001-7000 s^-1)by using an MTS,Instron VHS,and split-Hopkinson bars.The stress-strain behavior of polyurea was obtained for various strain rates,and the effects of strain rate on the primary mechanical properties were analyzed.Additionally,a modified rate-dependent constitutive model is proposed based on the nine-parameter Mooney-Rivlin model.The results show that the stress-strain curves can be divided into three distinct regions:the linear-elastic stage,the highly elastic stage,and an approximate linear region terminating in fracture.The mechanical properties of the polyurea material were found to be highly dependent on the strain rate.Furthermore,a comparison between model predictions and the experimental stress-strain curves demonstrated that the proposed model can characterize the mechanical properties of polyurea over a wide range of strain rates.

Dynamic Mechanical Behavior of a Novel Polymeric Composite Damping Material

The dynamic mechanical behavior of a novel polymeric composite damping material has been investigated in this article. The composite consists of chlorinated polyethylene （CPE）, N,N-dicyclohexyl-2-benzothiazolylsufenamide （DZ）, 4,4＇-thio-bis（3-methyl-6-tert-buthylphenol） （BPSR） and vapor-grown carbon fiber （VGCF）. It is found that either the position or the intensity of damping peak can be controlled by changing the composition of CPE/DZ/BPSR composite. Within a certain composition region, damping peak maximum depends on CPE/DZ ratio, whereas damping peak position is controlled by BPSR content. Moreover, the improvement of storage modulus can be achieved by incorporation of VGCF. These results may imply that a damping material possessing both good damping properties and high strength can be designed and obtained.

Dynamic mechanical behavior of a Zr-based bulk metallic glass during glass transition and crystallization

The dynamic mechanical behaviors of the Zr41Ti14Cu12.5Ni8Be22.5Fe2 bulk metallic glass （BMG） during continuous heating at a constant rate were investigated. The glass transition and crystallization of the Zr-based BMG were thus characterized by the measurements of storage modulus E′ and internal friction Q^-1. It was found that the variations of these dynamic mechanical quantities with temperature were interre-lated and were well in agreement with the DSC trace obtained at the same heating rate. The origin of the first peak in the internal friction curve was closely related to the dynamic glass transition and subsequent primary crystallization. Moreover, it can be well described by a physical model, which can characterize atomic mobility and mechanical response of disordered condense materials. In comparison with the DSC trace, the relative position of the first internal friction peak of the BMG was found to be dependent on its thermal stability against crys-tallization.

The Effects of Stacking Sequence on Dynamic Mechanical Properties and Thermal Degradation of Kenaf/Jute Hybrid Composites

This research focused on the dynamic mechanical and thermal properties of woven mat jute/kenaf/jute(J/K/J)and kenaf/jute/kenaf(K/J/K)hybrid composites.Dynamic mechanical analysis(DMA)and Thermo-gravimetric Analysis(TGA)were used to study the effect of layering sequence on the thermal properties of kenaf/jute hybrid composites.The DMA results;it was found that the differences in the stacking sequence between the kenaf/jute composites do not affect their storage modulus,loss modulus and damping factor.From the TGA and DMA results,it has been shown that stacking sequence has given positive effect to the kenaf/jute hybrid composite compared to pure epoxy composite.This is because kenaf and jute fibre has increased the Tg values of the composites,thus affect the thermal degradation.Results showed that the storage modulus for kenaf/jute hybrid composites increased compared with pure epoxy composites with increasing temperature and the values of remained almost the same at glass transition temperature(Tg),the hybrid composite perhaps due to the improved fibre/matrix interface bonding.The preliminary analysis could provide a new direction for the creation of a novel hybrid composite which offers unique properties which cannot be accomplished in a single material system.

Dynamic Mechanical Characterizations and Road Performances of Flame Retardant Asphalt Mortars and Concretes

To research the dynamic mechanical properties and road performances of flame retardant asphalt mortars and mixtures, four different asphalt mortars/mixtures were prepared: a reference group and three asphalt mortars/mixtures containing composite flame retardant materials(M-FRs) of different proportions. Temperature sweep, frequency sweep, repeated creep test, force ductility test and bending beam rheological test were carried out to research the dynamic mechanical properties of asphalt mortars containing M-FRs; wheeltracking test, low-temperature bending test and freeze-thaw split test were used to study the road performances of asphalt mixtures containing M-FRs. The results show that high-temperature performances of the three flame retardant asphalt mortars improve greatly, while low-temperature cracking resistances decline. Both hightemperature performances and water stabilities of asphalt mixtures containing M-FRs are quite good and exceed the specification requirements. However, their low-temperature performances decline in different degrees. In summary, besides their good flame retardancy, the flame retardant asphalt mortars and mixtures also exhibit acceptable road performance.

Microstructure Evolution and Dynamic Mechanical Properties of Laser Additive Manufacturing Ti-6Al-4V Under High Strain Rate

The dynamic mechanical properties of the Ti-6Al-4V(TC4)alloy prepared by laser additive manufacturing(LAM-TC4)under the high strain rate(HSR)are proposed.The dynamic compression experiments of LAM-TC4 are conducted with the split Hopkinson pressure bar(SHPB)equipment.The results show that as the strain rate increases,the widths of the adiabatic shear band(ASB),the micro-hardness,the degree of grain refinement near the ASB,and the dislocation density of grains grow gradually.Moreover,the increase of dislocation density of grains is the root factor in enhancing the yield strength of LAM-TC4.Meanwhile,the heat produced from the distortion and dislocations of grains promotes the heat softening effect favorable for the recrystallization of grains,resulting in the grain refinement of ASB.Furthermore,the contrastive analysis between LAM-TC4 and TC4 prepared by forging(F-TC4)indicates that under the HSR,the yield strength of LAM-TC4 is higher than that of F-TC4.

Dynamic Mechanical Behavior and Prediction for PP/POE Blends

Polypropylene（PP）/ethylene-octene copolymer（POE） blends were prepared with a twin-screw extruder.Their dynamic mechanical behavior were systematically investigated.The results show that PP/POE blends are heterogeneities with a partial compatible two-phase structure,the glass transition temperature of PP phases in the blends tends to shift towards high temperature with increasing the POE content,and the glass transition temperature of POE phases shifts towards the low temperature with increasing the PP content.The Kerner＇s dispersed phase model and co-continuous phase model can reasonably predict the visco-elasticity of PP/POE blends with different compositions.Additionally,the morphological structure of the blends can be estimated via comparing the predicted DMA behavior with the experimental data.

Effects of laser power density on static and dynamic mechanical properties of dissimilar stainless steel welded joints

The mechanical properties of laser welded joints under impact loadings such as explosion and car crash etc. are critical for the engineering designs. The hardness, static and dynamic mechanical properties of AISI304 and AISI316L dissimilar stainless steel welded joints by CO2 laser were experimentally studied. The dynamic strainstress curves at the strain rate around 103 s-1 were obtained by the split Hopkinson tensile bar （SHTB）. The static mechanical properties of the welded joints have little changes with the laser power density and all fracture occurs at 316 L side. However, the strain rate sensitivity has a strong depen- dence on laser power density. The value of strain rate factor decreases with the increase of laser power density. The welded joint which may be applied for the impact loading can be obtained by reducing the laser power density in the case of welding quality assurance.

Dynamic Mechanical and Thermal Properties of Cross-linked Polystyrene/Glass Fiber Composites

Cross-linked polystyrene/glass fiber composites were fabricated using cross-linked polystyrene （CLPS） as matrix and E-glass fiber as the reinforcement. Surfaces of E-glass fibers were modified by vinyl triethoxysilane （VTES）, vinyl trimethoxysilane （VTMS） and γ-methacryloylpropyl trimethoxysilane （MPS）. The treated glass fibers were analyzed by fourier transform infrared spectroscopy （FTIR）. Dynamic mechanical thermal analysis （DMTA） and thermo-gravimetric analysis （TGA） were employed to investigate the effect of glass fibers surface modification on viscoelastic behavior and thermal properties. The morphology of fracture surfaces of various composites was observed by scanning electron microscopy （SEM）. The results revealed that these coupling agents were connected to the surfaces of the fibers by chemical bonding. Dynamic mechanical properties as well as thermal stability of the composites were improved considerablely, but to varying degrees depending on the fiber modification. The diversities of improvement of properties were attributed to the different interfacial adhesion between CLPS matrix and the glass fibers.

Effect of Thermal Crosslink Conditions on Dynamic Mechanical Behaviors of Flexible Epoxy

The dynamic mechanical behavior of a new kind of flexible epoxy FE-1,which was crosslinked under four different thermal crosslink conditions,was studied.Dynamic mechanical measurement was carried out from 10 ℃ to 120 ℃,and loss factor,tan δ and the storagemodulus as functions of temperature were presented under five different frequencies of 0.1 Hz, 1 Hz,5 Hz,50 Hz and 100 Hz. The experimental results show that temperature has dramatic effects on the dynamic mechanical behavior of flexible epoxy. Compared with other common available epoxy, the flexible epoxy has higher loss factor over broad frequency and common temperature range. Activation energy corresponding to glass transition process of FE-1 was calculated from the temperature corresponding to tan 8 rna~ values, obtained at different measurement frequencies. The maximum value of loss factor is 0.75 and the Tg varies from 6 ℃ to 50 ℃, indicating the flexible epoxy can be used as damping polymer materials at common temperature or frequency range.

The study of measuring technology on the dynamic mechanical properties of welded joint with high strainrate

In this paper, to meet the needs of studying work of dynamic mechanical properties of welded joint, the dynamic mechanical properties of welded joint were measured by means of SHPB(Split Hopkinson Pressure Bar).The dynamic mechanical property's curves of every part of welded joint were obtained. For studying the dynamic behavior of mechanical heterogeneity of welded joint, important data were offered. The method of test creates a new way of studying dynamic mechanical properties of welded joint.

A DYNAMIC MECHANICAL ANALYSIS STUDY OF THE TRANSITION BEHAVIOUR OF I-PP/EPDM BLENDS

The transition behaviour of the blends of isotactic polypropylene (i-PP) with ethylene-propylene terpolymer (EPDM) containing 42 wt% propylene was investigated by dynamic mechanical analysis technique (DMA). Owing to its high propylene content, EPDM is compatible with i-PP to some degree. The interaction between the two components was strengthened. As expected, for partially compatible system the glass transition temperature of i-PP in the blends shifted to lower temperature. It was found that there existed two transitions, αEPDM and βEPDM, for the EPDM used in this work. The former was considered to be the glass transition of the random chain segments of EPDM, while the latter the local motion of the long ethylene sequences in EPDM. The unusual transition behaviour of αEPDM in the blends was explained in terms of the greater thermal expansion of EPDM and the compatibility of the two components. On the other hand, the βEPDM changed with the composition of the blends in a regular manner.

EFFECTS OF MOLECULE AGGREGATION STATE OF HINDERED PHENOL ON DYNAMIC MECHANICAL PROPERTIES OF CHLORINATED POLYETHYLENE

Hindered phenol compound 3,9-bis{1,1-dimethyl-2[β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl}- 2,4,8,10-tetraoxaspiro[5,5]-undecane (AO-80) is a polymorphous material with different physical structures. The initial AO- 80 is highly crystalline, whereas AO-80 obtained by cooling from its molten state is an amorphous material. Annealing treatment below the melting point of AO-80 results in structural development. The mixture of chlorinated polyethylene (CPE) and vitrified AO-80 particles exhibits a dramatic change in the dynamic mechanical properties during heat treatment at 130'C. This change can be attributed to the decomposition of the vitrified AO-80 particles and the hybridization of two constituents. The vitrified AO-80 particles can crystallize again in a CPE matrix by annealing at 100'C, but this crystal is different from that of the initial AO-80 in its microstructure. In addition, the incorporation of CPE chains caused a dramatic increase in the modulus. As a result, the AO-80 crystal particles that contain some CPE chains act as multifunctional cross- links and the CPE/AO-80 hybrid was found to be a new type of elastomer.

STUDY ON DYNAMIC MECHANICAL BEHAVIOUR OF POLY BUTYLACRYLATE GRAFT VINYL CHLORIDE COPOLYMERS

Dynamic mechanical behaviours of PVC ho-mopolymer. poly butylacryiate (PBA ) graft vinylchloride copolymers and blend of PVC and ACR have been investigated. The results reveal that there is semicompatible nature in the two phases in these materials,that the compatibility of the two phases decrease, -with the increase of crosslinking density of PBA backbone in graft copolyers. For different copolymer types-PB A graft copolymers had better compatibility of the two phases compared ivith the graft copolymers 晈ith core/shell backbone poly-

Crystallization and Dynamic Mechanical Behavior of Coir Fiber Reinforced Poly(Butylene Succinate)Biocomposites

The crystallization behavior,crystal morphology and form,and viscoelastic behavior of poly(butylene succinate)(PBS)and coir fiber/PBS composites(CPB)were investigated by differential scanning calorimetry(DSC),polarized optical microscopy(POM),X-ray diffraction(XRD)and dynamic mechanical analysis(DMA).The results of DSC measurement show that the crystallization temperature increases with the filling of coir fibers.POM images reveal that the spherulitic size and crystallization behavior of PBS are influenced by the coir fibers in the composites.XRD curves show that the crystal form of pure PBS and CPB are remaining almost identical.In addition,the storage modulus of CPB significantly increases comparing with the pure PBS.This predicted the dimensional stability and improved load-deformation temperature.In conclusion,the addition of coir fibers has a significant effect on the thermal properties of the matrix.