Stress-strain Curves Of Specimens Research Articles

Mechanical response to dynamic compressive load applied to shale after thermal treatment

In deep shale gas extraction, high temperature environment and dynamic loads are two important factors in affecting reservoir reconstruction and shale breaking. By employing the test techniques of Split Hopkinson pressure bar, dynamic compressive mechanical response of shale specimens with five different bedding dips after thermal treatment of 25–200 °C was investigated under the same impact energy. Results reveal that the increasing thermal treatment temperature weakens the post-peak behavior in stress-strain curves of specimens with bedding dips of 0°, 30° and 90°, and strengthens that of specimens with bedding dips of 45° and 60°. The dynamic compression strength variation of specimens versus increasing thermal treatment temperature shows two opposite trends bounded by the bedding dip of 30°. With increasing thermal treatment temperature, failure mode of specimens with bedding dips of 45° and 60° is affected evidently, but that of specimens with bedding dips of 0°, 30° and 90° has no fundamental change. The increasing thermal treatment temperature impacts the amount of absorbed energy in one of two ways depending on the bedding dip, and increases the dynamic fragmentation degree of all specimens, but that of specimens with bedding dips of 45° and 60° is much larger. Furthermore, the dynamic anisotropy of specimens after thermal treatment and its possible application in deep shale gas extraction are discussed. Results can be beneficial to the perforation design in hydraulic fracturing and the dynamic fracturing technology development in deep shale reservoirs.

Comparative studies on the effect of fabric structure on mechanical properties of carbon fiber/epoxy composites

The application of carbon fiber/epoxy composites places more requirements on the selection of fabrics to meet the needs of structural components. Due to the large design space for reinforced fabric patterns, the relationship between the reinforcements and properties is essential to further understand. Four typical fabrics were manufactured in this research, named non-interlaced bidirectional fabrics, plain weave laminated fabrics, angle-interlock fabrics and bidirectional angle-interlock fabrics. The structural features of fabrics were analyzed by using representative geometric unit, and the symmetry properties were discussed based on group theory. Vacuum assistant resin transfer molding was adopted to obtain the corresponding resin matrix composite specimens. Quasi-static tensile and bending tests were conducted on these specimens. The stress-strain curves of specimens were illustrated, and the failure characterizations were also analyzed in mesoscopic scale. The results showed that the high crimp of yarns reduced the stability of composites. Both the tensile and flexural properties were affected by the curvature of yarns. The research results provided a theoretical basis for the selection of fabric structure and the application of carbon fiber/epoxy composites.

Numerical simulation and experimental study of off-axis tensile performance of triaxial woven fabric reinforced rubber composites

In this paper, the off-axis tensile behaviors of triaxial woven fabric reinforced rubber composites (TWFR) was studied experimentally and numerically. The tensile failure morphologies and stress-strain curves of specimens cut at seven different off-axis angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°) were obtained by finite element analysis and experiment. The finite element analysis results were found to be in good agreement with the experimental results. Stress-strain curves indicated the TWFR exhibited nonlinear tensile behavior under off-axial tension loading, and could be divided into the coupling working stage of yarns and rubber matrix in TWFR, and the stressed stage of yarns. Similar tensile properties were observed for specimens cut along the direction of angle bisector of any two sets of adjacent yarns. The tensile failure morphologies indicated that the failure mechanism of TWFR was tensile-shear mixed failure.

Study on failure mechanism of porous concrete based on acoustic emission and discrete element method

Porous concrete is an environmentally friendly material. To evaluate the damage and discuss the failure mechanism of porous concrete, a uniaxial compression test was carried out in this study. Based on acoustic emission (AE) technology, the changes in AE ringing counts and energy were recorded, and the b-value was analysed. In this research, the evolution of cracks and stress-strain curves of specimens under uniaxial compression were also analysed by the discrete element method (DEM). The results showed that the failure process of porous concrete could be divided into an initial stage, crack propagation stage and failure stage. The mechanical properties and failure characteristics of porous concrete are different from those of conventional concrete. The main failure mode of porous concrete under compression is the failure of the bonding layer between the aggregates, whereas the aggregate failure of conventional concrete rarely occurs. There are many connected or unconnected pores in porous concrete. During the failure process of porous concrete, cracks propagate into the pores until penetrating cracks occur. This paper provides a basis for further study on the microscopic mechanism of porous concrete failure.

Tensile and fatigue behavior of carbon fiber reinforced magnesium composite fabricated by liquid-solid extrusion following vacuum pressure infiltration

Liquid-solid extrusion following vacuum pressure infiltration (LSEVI) technique is a new technology, which is suitable for the fabrication of carbon fiber reinforced magnesium composite (Cf/Mg composite). This work details an experimental investigation on understanding the fatigue performance of Cf/Mg composite fabricated by LSEVI technique. The S-N curve of the composite was determined and the fatigue behavior considering different loading levels and cycles was studied. Moreover, the fracture surfaces and microstructures of test samples were examined to understand the fatigue damage mechanism. The investigation finds that the residual tensile strength of specimens that survived during fatigue test was enhanced with the increase in fatigue cycles and applied stress. In addition, fatigue loading could change the tensile behavior of the composite, and a yielding point was observed at stress-strain curves of specimens which survived during fatigue test. Observations of the fracture surfaces revealed that more fibers were pulled out with the increase in fatigue cycles and applied stress.

Fatigue assessment of full-scale welded crane runway girders*

Abstract This publication is focused on a critical plane approach to assess the influence of the multiaxial stress state on the lifetime of welded crane runway girders cyclically stressed by travelling wheel loads. Cyclic stress-strain curves of specimens both from the weld region and the base material of the girder are evaluated in strain-controlled reversible fatigue tests as a basis for the assessment of the lifetime. Finite element analyses are performed to estimate the multiaxial and non-proportional local stress condition acting on sharp notches of the girder, i. e. the weld toes and the weld root. The MPA AIM-life concept is used to evaluate the lifetime of crane runways on the basis of advanced fatigue parameters and the cyclic material properties. Furthermore, the notch stress approach according to IIW guideline [1] is applied to investigate the fatigue behavior. The estimated lifetimes are compared to experimental results of component fatigue tests conducted with full-scale welded crane runway girders.

OS0704 実験とシミュレーションによるPLLA/PCLポリマーブレンド材の動的圧縮特性の決定

Coefficients of Cowper-Symonds constitutive equation for PLLA/PCL = 80/20 at 36℃ were determined using the results of compressive tests. The simulation of split Hopkinson pressure bar using the coefficients was carried out under the same condition as the experiments. The stress-strain curves of specimens, the thickness and the diameter in the simulation at high strain rate were compared with those in the experiments.

Mechanical Properties of Forged Ti-6Al-4V Structure by Electron Beam Welding

The blisk (bladed disk) is a new structural component of the modern aero-engine and plays an important role in improving its performance. Ti-6Al-4V alloy joints welded by electron beam have been widely used for compressor blisk in advanced aero engine. It is necessary to analyze microstructure and mechanical properties of Ti-6Al-4V welded structure by electron beam welding (EBW) for failure analysis and structure design of blisk. Microstructure of Ti-6Al-4V welded structure by EBW was investigated by microscopic observation and micro indentation testing. Experiment results show grain coarsening in fusion zone (FZ) and heat affected zone (HAZ) appears large gradient organization structure, which presents significant local heterogeneity. On the centerline perpendicular to the welding direction, Vickers microhardness was measured in increments of 1mm, 0.5mm, 0.25mm and 0.1mm. Due to the presence of martensite, microhardness of the fusion zone is about 20% higher than that of the base material. The size of joints in different regions was acquired, 2.5 to 3.0-mm-wide in FZ and about 0.7-mm-wide in HAZ respectively. Three different types of EBW samples were designed for tensile test, including welded structure, welded joint and base material. Three different stress-strain curves of specimens were acquired, including welded joint. The experiment data indicates that the tensile strength of welded joints is 8% more than that of the base metal.

Mechanical Properties of Forged Ti-6Al-4V Structure by Electron Beam Welding

The blisk (bladed disk) is a new structural component of the modern aero-engine and plays an important role in improving its performance. Ti-6Al-4V alloy joints welded by electron beam have been widely used for compressor blisk in advanced aero engine. It is necessary to analyze microstructure and mechanical properties of Ti-6Al-4V welded structure by electron beam welding (EBW) for failure analysis and structure design of blisk. Microstructure of Ti-6Al-4V welded structure by EBW was investigated by microscopic observation and micro indentation testing. Experiment results show grain coarsening in fusion zone (FZ) and heat affected zone (HAZ) appears large gradient organization structure, which presents significant local heterogeneity. On the centerline perpendicular to the welding direction, Vickers microhardness was measured in increments of 1mm, 0.5mm, 0.25mm and 0.1mm. Due to the presence of martensite, microhardness of the fusion zone is about 20% higher than that of the base material. The size of joints in different regions was acquired, 2.5 to 3.0-mm-wide in FZ and about 0.7-mm-wide in HAZ respectively. Three different types of EBW samples were designed for tensile test, including welded structure, welded joint and base material. Three different stress-strain curves of specimens were acquired, including welded joint. The experiment data indicates that the tensile strength of welded joints is 8% more than that of the base metal.

Experimental Method of Axial Compressive Existing Concrete Columns Strengthened with Externally Bonded FRP

To study the experimental method of axial compressive test of concrete columns in service strengthened with bonding FRP(fiber reinforced plastic), the post-tensioned pre-stressing method was employed to exert initial stress to the specimens before bonding FRP, Then FRP was bonded. Axial loading method of the specimens was that one main jack exerted axial force and two jacks provided stiffness for the specimens. In this way the complete stress-strain curves of specimens was obtained. The results indicated the technical target of axial compressive test of in service concrete columns strengthened with FRP was greatly achieved by using this experimental method.

斜交対称CFRP積層板の低サイクル疲労挙動に関する実験的考察

Cyclic zero-tension tests for symmetric angle-ply CFRP laminated specimens were carried out to investigate the low cycle fatigue behavior of graphite/epoxy and graphite/PEEK composites. Two types of stacking sequences were tested: [+θ/-θ]4s (Distributed ply) and [+θ4/-θ4]s (Blocked ply), where θ was 30º or 45º. Stress-strain curves of specimens under cyclic loadings were obtained by means of an extensometer. A mechanical ratcheting, which means progressive increase in plastic strain at each cycle, was observed on the cyclic stress-strain curves. Comparing the stress-strain curve of static tensile test with one of the cyclic zero-tension test, we made the assumption that the fatigue failure occurred when the ratcheting strain by cyclic loadings reached the static failure strain. Results of an ultrasonic scanning test revealed that the distributed ply specimens differed from the blocked ply specimens in an internal fatigue damage progress.

低炭素鋼の強度と降伏点現象に及ぼす結晶粒径と熱処理の影響

We examined the effect of grain size on flow stress and yield-point phenomena in variously heat-treated low-carbon steel specimens. We found that the experimental results are easily understood if the flow stress consists of two internal stresses, Yint and Yir. Yint is a common internal stress that is work hardened; Yir is the cause of yield-point phenomena, and its value is supposed to be rapidly decreased (work-softened) to zero with strain. Both Yint and Yir were affected by grain size. Furthermore, the effect of heat treatment was strong on Yint but weak on Yir. The Hall-Petch coefficient k was appreciably larger at lower yield points than in the large strain region. This comparison supports the hypothesis of the existence of two internal stresses because the grain size dependence of flow stress at lower yield points has two origins, Yint and Yir, whereas the latter case has only one origin, Yint. In the strain region exceeding yield-point elongation, the stress-strain curves of specimens with various grain sizes were perfectly superposed on each other by parallel translation, which means that if Yint is expressed as Yint(ε)=f(ε) for a specimen with a certain grain size, it is expressed as Yint(ε)=f(ε+Δε)−Δσ for arbitrary grain size specimens, where Δε and Δσ are the amounts of translation in strain and stress, respectively.

Restoration phenomena of TiNi shape memory alloys in a neutron irradiation environment

Transformation properties and deformation behavior of TiNi shape memory alloys, which were neutron-irradiated at 323 and 520 K with a maximumfluence of 1.4 × 10 25 m −2 and subsequently post-annealed at 473, 523 and 573 K, were measured by electrical resistance measurements and tensile tests. At an irradiation temperature of 323 K, abrupt changes in M s temperatures and stress-strain curves of specimens were observed at a dose of over 10 −2 dpa. Irradiation alterations were annihilated by post-irradiation annealing at temperatures above 523 K. On the other hand, 520 K irradiations brought about quite a few changes in those properties regardless of the magnitudes of displacement. The irradiated state of this alloy can be represented by two conflicting processes; ordering and disordering, which depends on temperature, displacement and displacement rate. The key temperature of that state is 520 K, at which the ordering becomes predominant over disordering and brings about the occurrence of a restoration phenomena.

The effect of strain rate on the properties of an aluminium-lithium alloy

The flow stress of metals is generally observed to increase with strain rate• The literature on the dependence of properties on strain rate is extensive, but for new alloys, and high-strength materials in particular, data are far more difficult to obtain. Commercial AI -L i alloys show combinations of strength and toughness that are comparable with, and in some cases superior to, traditional highstrength A1 alloys [1]. Investigations of properties at high strain rate have shown an increase in strength with increasing strain rate in the range of strain rates 10-5-103 s -1, with the effect being far more pronounced in tension than in compression [2]. Chiem et al. [2] showed that fracture strain increases with strain rate at high strain rates. However , their values reported at the high strain rates are well below the quasi-static ductilities• Kobayashi et al. [3], on the other hand, showed that the elongation at high strain rates is superior to that measured at quasi-static strain rates for A1-Li alloys• This letter contributes to the database on AI -L i alloys by characterizing the properties of an alloy in compression over the strain rate range 10 .3 to 6 x 103 S 1 . The AI L i alloy used in this study was Alcan 8090-T82551 in the form of rectangular extruded bar of dimensions 100 m m x 25 mm. The alloy has nominal values of 0.2% proof stress, ultimate tensile stress and elongation of 475 MPa, 525 MPa and 5.0%, respectively, which concur with values given in [1]. At the lowest strain rate (10 .3 s -1) compression tests were performed with the cylinder axis in the extrusion, short transverse (thickness) and long transverse (width) directions, and using two different lubrication conditions, viz. a Teflon tape and a Teflon grease (Dupont Kryotox 240 AC). At the higher strain rates all tests were with the specimen axis in the short transverse direction and using only the Teflon tape. The range of strain rates was achieved using a screw-type hydraulic drop weight with a piezoelectric load cell, and direct impact Hopkinsons bar techniques, which were described in detail in [4-6]• Fig. 1 illustrates four typical stress-strain curves for specimens with the cylinder axis in the short transverse direction• The two curves at the strain rate of 1.3 x 10 .3 s -I are typical results for the two types of lubrication. Up to the maximum load the curves were identical within observed specimen-tospecimen scatter• Beyond a maximum load the Teflon tape-lubricated specimens most commonly failed in shear at a strain of the order of 0.2, whereas 600

Effects of Surface Integrity on the Fatigue Life of Thin Flexing Membranes

It has been known for some time that surface integrity has an effect on the fatigue life of metals and “brittle” polymers. In cardiovascular applications of polymeric materials, emphasis is placed on elastomers having extended flexure lifetimes (i.e., fatigue life). The effect of surface integrity on the performance properties of Biomer® (Ethicon, Inc, Somerville, NJ) a segmented polyurethane used in many blood contacting devices, is being investigated using uniaxial tensile tests in air at room temperature, and biaxial fatigue tests in deionized water at body temperature. Tensile tests were done using ASTM-D-882: Standard Test Methods for Tensile Properties of Thin Plastic Sheeting. No significant differences were noted in the stress-strain curves for specimens with various surface finishes. Fatigue tests were performed using an apparatus developed to allow for the exposure of thin-sheet polymer specimens to fluid at body temperature, while being biaxially strained. Because no standard test method was available, a test protocol was developed with reference to ASTM-D-671–78: Standard Test Methods for the Flexural Fatigue of Plastics by Constant Amplitude of Force. Stress versus life cycle data for specimens with differing surface finishes are being collected. Results to date suggest fatigue life of thin flexing membranes will decrease with increasing order of surface roughness, and fatigue properties are more sensitive to effects of changes in surface integrity than tensile properties measured by monotonic loading.

Effects of surface integrity on the fatigue life of thin flexing membranes.

It has been known for some time that surface integrity has an effect on the fatigue life of metals and "brittle" polymers. In cardiovascular applications of polymeric materials, emphasis is placed on elastomers having extended flexure lifetimes (i.e., fatigue life). The effect of surface integrity on the performance properties of Biomer (Ethicon, Inc, Somerville, NJ) a segmented polyurethane used in many blood contacting devices, is being investigated using uniaxial tensile tests in air at room temperature, and biaxial fatigue tests in deionized water at body temperature. Tensile tests were done using ASTM-D-882: Standard Test Methods for Tensile Properties of Thin Plastic Sheeting. No significant differences were noted in the stress-strain curves for specimens with various surface finishes. Fatigue tests were performed using an apparatus developed to allow for the exposure of thin-sheet polymer specimens to fluid at body temperature, while being biaxially strained. Because no standard test method was available, a test protocol was developed with reference to ASTM-D-671-78: Standard Test Methods for the Flexural Fatigue of Plastics by Constant Amplitude of Force. Stress versus life cycle data for specimens with differing surface finishes are being collected. Results to date suggest fatigue life of thin flexing membranes will decrease with increasing order of surface roughness, and fatigue properties are more sensitive to effects of changes in surface integrity than tensile properties measured by monotonic loading.