Identification of damage mode in AZ31 magnesium alloy under tension using acoustic emission

In order to characterize different damage modes, real-time detection of the tensile cracking process for AZ31 magnesium alloy was performed using acoustic emission （AE） technique. Results showed that elastic deformation, plastic deformation, microcracking, stable and unstable propagation occurred during crack damage. Four damage modes were determined using AE multiparameter analysis. Dislocation motion signals with amplitudes 〈70 dB and twinning signals with 70-100 dB were found. Microcrack signal energy was concentrated from 2400 aJ to 4100 aJ, mainly at a rise time of less than 800 gs. A stable crack propagation signal had high peak to counts in the 20 to 50 range, whereas its ring count was in the 20 to 2000 range. The average frequency of unstable propagation signals was approximately 100 kHz, with duration from 2000 gs to 10s gs. The damage mechanisms and AE resources from different crack propagation steps were discussed. Various damage modes could be characterized by different AE signal parameters when they appeared simultaneously during crack propagation.

Indirect tension test of epoxy asphalt mixtureusing microstructural finite-element model

A finite-element model of the thermosetting epoxy asphalt mixture（EAM） microstructure is developed to simulate the indirect tension test（IDT）.Image techniques are used to capture the EAM microstructure which is divided into two phases：aggregates and mastic.A viscoelastic constitutive relationship,which is obtained from the results of a creep test,is used to represent the mastic phase at intermittent temperatures.Model simulation results of the stiffness modulus in IDT compare favorably with experimental data.Different loading directions and velocities are employed in order to account for their influence on the modulus and the localized stress of the microstructure model.It is pointed out that the modulus is not consistent when the loading direction changes since the heterogeneous distribution of the mixture internal structure,and the loading velocity affects the localized stress as a result of the viscoelasticity of the mastic.The study results can provide a theoretical basis for the finite-element method,which can be extended to the numerical simulations of asphalt mixture micromechanical behavior.

Influence of chloride corrosion on tension capacity of rebars

The reinforcement corrosion is the pitting corrosion of chloride corrosion.Hence,in this study,the variations of reinforcement tensile strength due to stress concentration of pitting corrosion are analyzed.The stress concentration consequence of corrosion on the reinforcement tensile capacity is studied utilizing tension tests and creating different ABAQUS software models.According to the modelling in various corrosion depths,strength reduction is less than 5%in corrosion with pit radius to reinforcement diameter ratio up to 0.3 and for corrosions higher than 0.4,the measure of capacity reduction is increased more to 30%.

Extended application of the tension test in thermal simulator Thermecmastor-Z

With the redesigned jigs for the Thermecmastor-Z thermal simulator,the feasibility of using 3 kinds of Gleeble specimens in the Thermecmastor-Z simulator was investigated. Results show that Gleeble specimens can be used in the Thermecmastor-Z simulator. The tension tests in the Gleeble and Thermecmastor-Z simulators produced results with the same trend,which proves that the high temperature ductility of Gleeble specimens can be reflected by the Thermecmastor- Z simulator. In addition,as the Thermecmastor-Z simulator offers a wider heating zone,better cross-section shrinkage and elongation of specimens can be achieved under the same test conditions.

Evaluation of Contact Pressure in Bending under Tension Test by a Pressure Sensitive Film

The contact pressure acting on the sheet/tools interface has been studied because of growing the concern about the wear of tools. Recent studies make use of numerical simulation software to evaluate and correlate this pressure with the friction and wear generated. Since there are many studies that determine the coefficient of friction in sheet metal forming by bending under tension (BUT) test, the contact pressure between the pin and the sheet was measured using a film that has the ability to record the applied pressure. The vertical force applied to pin was also measured. The results indicate that the vertical force is more accurate to set the contact pressure that using equations predetermined. It was also observed that the contact area between the sheet and the pin is always smaller than the area calculated geometrically. The friction coefficient was determined for the BUT test through several equations proposed by various authors in order to check if there is much variation between the results. It was observed that the friction coefficient showed little variation for each equation, and each one can be used. The material used was the commercially pure aluminum, alloy Al1100.

Tensile Stiffness Analysis on Ocean Dynamic Power Umbilical

Tensile stiffness of ocean dynamic power umbilical is an important design parameter for functional implementation and structural safety. A column with radial stiffness which is wound by helical steel wires is constructed to predict the tensile stiffness value of umbilicals in the paper. The relationship between the tension and axial deformation is expressed analytically so the radial contraction of the column is achieved in the relationship by use of a simple finite element method. With an agreement between the theoretical prediction and the tension test results, the method is proved to be simple and efficient for the estimation of tensile stiffness of the ocean dynamic power umbilical.

Influence of precipitation on the Portevin-Le Chatelier effect in Al-Mg alloys

In the alloy with solute content higher than the limiting solubility,the solute atoms that have failed to dissolve will precipitate from the solid solution and form precipitations.In this study, the Portevin-Le Chatelier(PLC) effects in annealed 5456 and 5052 aluminum alloys with different precipitation contents have been investigated under different applied strain rates.The results suggest that precipitations have significant effect on the PLC effect and the more the precipitations are, the greater the influence is.Furthermore,the solute diffusion is pipe diffusion in 5052 alloy with lower precipitation content.However,for 5456 alloy with higher precipitation content,the diffusion is no longer the case but more complex.

Modeling Methods and Test Verification of Root Insert Contact Interface for Wind Turbine Blade

Two modeling methods of the root insert for wind turbine blade are presented,i.e.,the local mesh optimization method(LMOM)and the global modeling method(GMM).Based on the optimized mesh of the local model for the metal contact interface,LMOM is proposed to analyze the load path and stress distribution characteristics,while GMM is used to calculate and analyze the stress distribution characteristics of the resin layer established between the bushing and composite layers of root insert.To validate the GMM,a tension test is carried out.The result successfully shows that the shear strain expresses a similar strain distribution tendency with the GMM′s results.

Experimental and Numerical Study on Mechanical Properties of Z-pins Reinforced Composites Adhesively Bonded Single-Lap Joints

The mechanical properties of Z-pins reinforced composites adhesively bonded single-lap joints(SLJs)under un-directional tension loading are investigated by experimental and numerical methods.Three kinds of joint configurations,including SLJs with three/two rows of Z-pins and“I”array of Z-pins,are investigated by tension test.The failure modes and mechanism of reinforced joints with different Z-pins numbers and alignment are analyzed,and the comparison is performed for the failure strengths of no Z-pins and Z-pins reinforced joints.According to experimental results,failure modes of three kinds of joints are all mixed failure.It turns out that the Z-pins are pulled out ultimately.The strength of joints of more Z-pins at the end of the overlap area is relatively bigger for the joint of the same Z-pins numbers.The strength of joints with Z-pins compared with non Z-pins joints is growing at 16%.Finally,the three-dimensional distribution of interfacial stress in the lap zone of three kinds of Z-pins reinforced joints is simulated,and the numerical results are in good agreement with the experimental results.It is effective that the numerical calculation of stress analysis is verified.

Heterogeneous Nature of Calcium Silicate Hydrate(C-S-H) Gel:A Molecular Dynamics Study

Structure and mechanical properties of Calcium silicate hydrate (C-S-H) at a molecular level act as "DNA" of cement-based construction materials.In order to understand loading resistance capability of C-S-H gel,research on molecular dynamics (MD) was carried out to simulate the uniaxial tension test on C-S-H model along x,y,and z directions.Due to the structure and dynamic differences of the layered structure,the C-S-H model demonstrates heterogeneous mechanical behavior.On an XY plane,the cohesive force can reach 4 GPa,which is mainly provided by the Ca-O and Si-O ionic-covalent bonds.The good plasticity of calcium silicate sheet is attributed to the silicate branch structure formation and the recovery role of interlayer calcium atoms.However,in z direction,C-S-H layers connected by the unstable H-bonds network,have the weakest tensile strength 2.2 GPa.This results in the brittle failure mode in z direction.The relatively low tensile strength and poor plasticity in z direction provides molecular insights into the tensile weakness of cement materials at macro-level.

Spot weldability comparison of high strength steel DP590 with different chemical composition

This paper introduces the cold-rolled DP590 high strength automotive steel sheets produced by Sougang Steel,which involve two different composition systems,one with high Al content and the other with C-Si-Mn.These two materials are pot-welded and the optimized welding current range and the best welding current are obtained.Both the two kinds of materials welding current's scope is 1 800 A,but the current of C-Si-Mn system DP590 is 1 400 A higher then the high Al content one' s;when these two kind of materials are welded with the optimal current,the nugget can be get by no defect.There have some difference in these two base materials.High Al DP590 has a bandy metallurgical structure compose with ferrite and martensite,the volume fraction of martensite is 8%,the grain size is 10.5.C-Si-Mn system DP590 compose with ferrite and martensite also while the volume fraction of martensite is 9%,the grain size is 9.5.Weld structure of high Al DP590 are bainite and lath martensite when C-Si-Mn system DP590's is lath martensite only.The variation of HV is same for these two materials nugget,the length of the are both 10 mm,there have no soft zone in the weld scope.The HV of the both materials are the same of 210 -220.The HV of high Al DP590 weld scope is 280,when C-Si-Mn system DP590 is much more then it with 425.After test these two welded sample get the same failure modes,the maximum shearing resistance and maximum positive tension of high Al DP590 are both less than the C-Si-Mn system DP590.

Mechanical Analysis of Sisal Fibers to Use as a Reinforced Material in Wood Beams

Natural fibers have recently raised attention for presenting adequate mechanical characteristics for reinforcement of structural elements. The use of both natural fibers, in especial Sisal fibers, in wood laminated beams and also wood from reforestation, is in accordance with the current economic interest and ecological appeal. Specifically, the strengthening of wood laminated beams with Sisal fibers is more effective for structures that require an increase in their structural capacity without a significant increase in height of the cross section. Furthermore, it is recommended that this type of reinforcement is used in wood structural elements where the elastic modulus is at least equal to the Sisal fibers. The composition of Sisal fibers is basically of cellulose, lignin and hemicelluloses. In particular, the amount of cellulose and the angle that the micro-fibers with the axis of the fiber characterize the failure strength and the modulus of elasticity. The average mechanical characteristics of the Sisal fiber are： tensile strength 347 to 378 （MPa） and elastic modulus 15.2 （GPa） whereas these properties are lower for strips of Sisal fibers. In this context, this paper deals with the analysis and the viability of the use of Sisal fibers in wood structures as a reinforced material.

Recent, development in low-constraint fracture toughness testing for structural integrity assessment of pipelines

Fracture toughness measurement is an integral part of structural integrity assessment of pipelines. Traditionally, a single-edge-notched bend （SE（B）） specimen with a deep crack is recommended in many existing pipeline structural integrity assessment procedures. Such a test provides high constraint and therefore conservative fracture toughness results. However, for girth welds in service, defects are usually subjected to primarily tensile loading where the constraint is usually much lower than in the three-point bend case. Moreover, there is increasing use of strain-based design of pipelines that allows applied strains above yield. Low-constraint toughness tests represent more realistic loading conditions for girth weld defects, and the corresponding increased toughness can minimize unnecessary conservatism in assessments. In this review, we present recent developments in low-constraint fracture toughness testing, specifically using single-edge- notched tension specimens, SENT or SE（T）. We focus our review on the test procedure development and automation, round-robin test results and some common concerns such as the effect of crack tip, crack size monitoring techniques, and testing at low temperatures. Examples are also given of the integration of fracture toughness data from SE（T） tests into structural integrity assessment.

Effects of twin orientation and twin boundary spacing on the plastic deformation behaviors in Ni nanowires

Spreading twins throughout nano metals has been proved to effectively mediate the mechanical behaviors in face-centered-cubic(fcc)metals.However,the experimental investigation concerning the roles of twin boundary(TB)during deformation is rarely reported.Here,with the joint efforts of in-situ nanomechani-cal testing and theoretical studies,we provide a systematic investigation regarding the effects of TB orien-tation(θ,the angle between tensile loading direction and the normal of TB)and spacing on deformation mechanisms in Ni nanowires(NWs).As compared with single-crystalline counterparts,it is found that nano-twinned(nt)NWs withθ∼0°exhibit limited ductility,whereas TB can serve as an effective block-age to the dislocation propagation.In contrast,in nt NWs withθ∼20°and 55°,TB migration/detwinning induced by TB-dislocation reaction or partial dislocation movement dominates the plasticity,which con-tributes to enhanced NW ductility.Regarding nt NWs withθ∼90°,dislocations are found to be able to transmit through the TBs,suggesting the limited effect of TB on the NW stretchability.Furthermore,de-creasing TB spacing(λ)can facilitate the detwinning process and thus greatly enhance the ductility of NW withθ∼55°.This study uncovers the distinct roles that TB can play during mechanical deforma-tions in fcc NWs and provides an atomistic view into the direct linkage between macroscopic mechanical properties and microscopic deformation modes.

Orientation-dependent ductility and deformation mechanisms in body-centered cubic molybdenum nanocrystals

The knowledge regarding anisotropic mechanical behaviors in nanoscale body-centered cubic (bcc) metals remains obscure. Herein, we report the orientation-dependent ductility in bcc Mo nanocrystals (NCs), which exhibit poor ductility along [110] direction but possess relatively better ductility along the [001] and [112] orientations. The origin of different deformability can be traced down to the distinct deformation mechanisms: the unexpected crack nucleation and propagation induce premature fractures in [110]-oriented NCs;in contrast, deformation twinning could contribute to the enhanced ductility in [001]-oriented NCs;interestingly, we find the activation of multiple dislocation slips in [112]-oriented NCs with the highest ductility. Further molecular dynamics simulations provide deeper insights into the defect dynamics that are closely interlinked with experimental observations. Our findings advance the basic understanding of orientation-dependent mechanical properties and help to guide endeavors to architecture the microstructures of bcc metals with enhanced ductility.

Concepts and implementation of strain-based criteria in design codes for steel structures

A uniaxial tension test is commonly used to determine the mechanical properties of steel,but it has no meaning for the response of the material in a structure.The test was developed as a consensus solution by producers,fabricators,designers and code writers,to have a standard by which similar materials could be compared to a common base.It does not represent the actual behavior of the steel in a structure,and was never intended to do so.To study the true behavior of the structure and how the material responds it would be better to determine the strains and deformations that will take place during actual service condition.Such characteristics reflect the real behavior,whether in the elastic or inelastic range.If stresses or forces are needed,these are easily determined by the value of the strain and the relevant material modulus,along with the type of cross section,whether elastic or inelastic.The paper addresses the properties of a range of structural steels,how these are incorporated into design standards and how the standards define deformation characteristics and demands for bolted and welded connections.

Nanofillers modification of Epocast 50-A1/946 epoxy for bonded joints

Epocast 50-A1/946 epoxy was primarily developed for joining and repairing of composite aircraft structural components. The objective of the present work is to modify the Epocast epoxy resin by different nanofillers infusion. The used nanofillers include multi-walled carbon nanotubes（MWCNTs）, SiC and Al2O3 nanoparticles. The nanofillers with different weight percentages are ultrasonically dispersed in the epoxy resin. The sonication time and amplitude for MWCNTs are reduced compared to Al2O3 and SiC nanoparticles to avoid the damage of MWCNTs during sonication processes. The fabricated neat epoxy and twelve nanocomposite panels were characterized via standard tension and in-plane shear tests. The experimental results show that the nanocomposites materials with 0.5wt% MWCNTs, 1.5wt% SiC and 1.5wt% Al2O3 nanoparticles have the highest improvement in the tensile properties compared to the other nanofiller loading percentages.The improvements in the shear properties of these nanocomposite materials were respectively equal to 5.5%, 4.9%, and 6.3% for shear strengths, and 10.3%, 16.0%, and 8.1% for shear moduli. The optimum nanofiller loading percentages will be used in the following papers concerning their effect on the bonded joints/repairs of carbon fiber reinforced composites.

Tensile Elastic Behavior of a Zr-Cu-Ag-Al Bulk Metallic Glass

Tensile elastic behavior of bulk Zr46（Cu4.5/5.5Ag1/5.5）46Al8 metallic glass was experimentally investigated. It exhibited linear and non-linear time-independent elastic deformation with a demarcative stress of approximately 500 MPa within the timescale in the present work, and repeated loading-unloading before yielding did not alter stress-strain relationship. The pure linear elastic strain limit is 0.6%, significantly lower than 2% as generally reported, but still much higher than 0.1% observed for typical crystalline alloys. Deviation from linear elastic behavior at stresses higher than 500 MPa is explained here as a macroscopic manifestation of local fluctuations in elastic strain, which becomes pronounced at stresses higher than the critical value. The occurrence of non-linear elasticity is possibly also related to the sinusoidal relationship between shear stress and atomic displacement.