Optimum utilization of the loading capability of engineering materials is an important and active contribution to protect nature's limited resources,and it is the key for economic design methods.In order to make u...Optimum utilization of the loading capability of engineering materials is an important and active contribution to protect nature's limited resources,and it is the key for economic design methods.In order to make use of the materials' resources,those must be known very well;but conventional test methods will offer only limited informational value.The range of questions raised is as wide as the application of engineering materials,and partially they are very specific.The development of huge computer powers enables numeric modelling to simulate structural behaviour in rather complex loading environments-so the real material behaviour is known under the given loading conditions.Here the art of material testing design starts.To study the material behaviour under very distinct and specific loading conditions makes it necessary to simulate different temperature ranges,loading speeds, environments etc.and mostly there doesn't exist any commonly agreed test standard.In this contribution two popular,non-standard test procedures and test systems will be discussed on the base of their application background,special design features as well as test results and typically gained information:The demand for highspeed tests up to 1000 s^(-1) of strain rate is very specific and originates primarily in the automotive industry and the answers enable CAE analysis of crashworthiness of vehicle structures under crash conditions.The information on the material behaviour under multiaxial loading conditions is a more general one.Multiaxial stress states can be reduced to an equivalent stress,which allows the evaluation of the material's constraint and criticality of stress state.Both discussed examples shall show that the open dialogue between the user and the producer of testing machines allows custom-tailored test solutions.展开更多
The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenit...The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800-1100~C and strain rates of 0.001-1 s-1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-HoUomon parameter （Z） increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6. x 1015 （lnZ---35.5）, a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction （EBSD） confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.展开更多
Sheet metal undergoes different strain conditions during different forming processes.The investigation of mechanical properties under these conditions is very important in the forming techniques.Sheet metal is particu...Sheet metal undergoes different strain conditions during different forming processes.The investigation of mechanical properties under these conditions is very important in the forming techniques.Sheet metal is particularly liable to failure under plain strain state.Measure and investigate the necking strain under plane strain condition is a particularly important study for sheet formability forecasting.In this study,material behavior of DP780 high strength steel sheet under plain strain condition was studied.Conventional plane strain tensile tests were carried out on the MTS testing machine with a special designed specimen.A digital image correlation system was employed to measure the full-field strain distribution during plain strain tensile test.The strain evolution during deformation was obtained and investigated.The capability of the specimen for plane strain test was validated from the strain distributions.The necking strain and fracture strain of DP780 high strength steel sheet were determined from the strain field and strain history results.展开更多
An indirect tensile testing method is proposed for characterizing low strength graphite platelet reinforced vinyl ester nanocomposites at high-strain rate. In this technique, the traditional Brazilian disk (diametrica...An indirect tensile testing method is proposed for characterizing low strength graphite platelet reinforced vinyl ester nanocomposites at high-strain rate. In this technique, the traditional Brazilian disk (diametrical compression) test method for brittle materials is utilized along with conventional split-Hopkinson pressure bars (SHPB) for evaluating cylindrical disk specimens. The cylindrical disk specimen is held snugly in between two concave end fixtures attached to the incident and transmission bars. To eliminate the complexities of conventional strain gage application, a non-contact Laser Occluding Expansion Gage (LOEG) has been adapted for measuring the diametrical transverse expansion of the specimen under high-strain rate diametrical compressive loading. Failure diagnosis using high-speed digital photography validates the viability of utilizing this indirect test method for characterizing the tensile properties of xGnP (exfoliated graphite nanoplatelets) reinforced and additional CTBN (Carboxyl Terminated Butadiene Nitrile) toughened vinyl ester based nanocomposites. Also, quasi-static indirect tensile response agrees with previous investigations conducted using the traditional dog-bone specimen in direct tensile tests. Investigation of both quasi-static and dynamic indirect tensile test responses shows the strain rate effect on the tensile strength and energy absorbing capacity of the candidate materials. The contribution of reinforcement to the tensile properties of the candidate materials is presented.展开更多
This paper focuses on the dynamic tensile response of glass-graphite/epoxy composites illustrating improvement in energy absorption through hybridization. The dynamic response and energy absorption characteristics of ...This paper focuses on the dynamic tensile response of glass-graphite/epoxy composites illustrating improvement in energy absorption through hybridization. The dynamic response and energy absorption characteristics of pultruded hybrid combinations of glass and graphite fibers in an epoxy matrix subjected to induced transverse tension at high strain-rate in a modified Split Hopkinson Pressure Bar (SHPB) apparatus, are presented. Transverse tensile strength was determined by diametral compression of disc samples (Brazilian indirect tensile test method). Diametral crack initiation and strain to failure were monitored with a Shimadzu HPV-2 high-speed video camera at a recording speed of 500,000 fps and Digital Image Correlation (DIC). Adequate measures were taken to ensure that initiation of specimen failure occurred at the exact center of the disc specimen, and propagated through the diameter along the compressive loading axis, for the induced transverse tension tests to be valid. A study of the strength and specific energy absorption demonstrates the benefits of hybridization. Under induced transverse tensile loading condition, the pure glass/epoxy (GL60) exhibited higher strength than pure graphite/epoxy (GR60). Pure graphite/epoxy (GR60) has higher specific energy absorption capacity than pure glass/epoxy (GL60) in transverse tension. Among all hybrids, GR30 has the highest specific energy absorption under transverse tension. Overall, hybrid GL48, with 48% low-cost glass fibers in the inner core and 12% high-cost graphite fibers in outer shell, was found to exhibit better performance under induced transverse tension at high strain-rates, showing the benefits of hybridization.展开更多
A constitutive model is critical for the prediction accuracy of a metal cutting simulation. The highest strain rate involved in the cutting process can be in the range of 104-106 s 1. Flow stresses at high strain rate...A constitutive model is critical for the prediction accuracy of a metal cutting simulation. The highest strain rate involved in the cutting process can be in the range of 104-106 s 1. Flow stresses at high strain rates are close to that of cutting are difficult to test via experiments. Split Hopkinson compression bar （SHPB） technology is used to study the deformation behavior of Ti-6Al-4V alloy at strain rates of 10 -4-10 4s- 1. The Johnson Cook （JC） model was applied to characterize the flow stresses of the SHPB tests at various conditions. The parameters of the JC model are optimized by using a genetic algorithm technology. The JC plastic model and the energy density-based ductile failure criteria are adopted in the proposed SHPB finite element simulation model. The simulated flow stresses and the failure characteristics, such as the cracks along the adiabatic shear bands agree well with the experimental results. Afterwards, the SHPB simulation is used to simulate higher strain rate（approximately 3 × 10 4 s -1） conditions by minimizing the size of the specimen. The JC model parameters covering higher strain rate conditions which are close to the deformation condition in cutting were calculated based on the flow stresses obtained by using the SHPB tests （10 -4 - 10 4 s- 1） and simulation （up to 3 × 10 4 s - 1）. The cutting simulation using the constitutive parameters is validated by the measured forces and chip morphology. The constitutive model and parameters for high strain rate conditions that are identical to those of cutting were obtained based on the SHPB tests and simulation.展开更多
For the research on steel structure in fire,it is very important to determine the properties of structural steel at elevated temperature.Up to now,the high-temperature properties of material is believed to be related ...For the research on steel structure in fire,it is very important to determine the properties of structural steel at elevated temperature.Up to now,the high-temperature properties of material is believed to be related to only temperature state,which is not precise enough to simulate the behavior of steel structures under different combinations of heating,cooling,loading,and unloading.To analyze the influence of the temperature-load history on the steel properties,a series of tests were carried out under different temperature-load paths about steel Q235,which is widely used in steel structures in China.In this paper,the method to set the temperature-load paths was introduced;the variety regulation of steel properties changing with temperature was analyzed under different paths;according to experimental results,the formulas of elastic modulus and yield strength at elevated temperature were fitted,and the stress-strain-temperature 3D relationships of structural steel under different paths were presented.展开更多
文摘Optimum utilization of the loading capability of engineering materials is an important and active contribution to protect nature's limited resources,and it is the key for economic design methods.In order to make use of the materials' resources,those must be known very well;but conventional test methods will offer only limited informational value.The range of questions raised is as wide as the application of engineering materials,and partially they are very specific.The development of huge computer powers enables numeric modelling to simulate structural behaviour in rather complex loading environments-so the real material behaviour is known under the given loading conditions.Here the art of material testing design starts.To study the material behaviour under very distinct and specific loading conditions makes it necessary to simulate different temperature ranges,loading speeds, environments etc.and mostly there doesn't exist any commonly agreed test standard.In this contribution two popular,non-standard test procedures and test systems will be discussed on the base of their application background,special design features as well as test results and typically gained information:The demand for highspeed tests up to 1000 s^(-1) of strain rate is very specific and originates primarily in the automotive industry and the answers enable CAE analysis of crashworthiness of vehicle structures under crash conditions.The information on the material behaviour under multiaxial loading conditions is a more general one.Multiaxial stress states can be reduced to an equivalent stress,which allows the evaluation of the material's constraint and criticality of stress state.Both discussed examples shall show that the open dialogue between the user and the producer of testing machines allows custom-tailored test solutions.
文摘The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800-1100~C and strain rates of 0.001-1 s-1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-HoUomon parameter （Z） increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6. x 1015 （lnZ---35.5）, a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction （EBSD） confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.
文摘Sheet metal undergoes different strain conditions during different forming processes.The investigation of mechanical properties under these conditions is very important in the forming techniques.Sheet metal is particularly liable to failure under plain strain state.Measure and investigate the necking strain under plane strain condition is a particularly important study for sheet formability forecasting.In this study,material behavior of DP780 high strength steel sheet under plain strain condition was studied.Conventional plane strain tensile tests were carried out on the MTS testing machine with a special designed specimen.A digital image correlation system was employed to measure the full-field strain distribution during plain strain tensile test.The strain evolution during deformation was obtained and investigated.The capability of the specimen for plane strain test was validated from the strain distributions.The necking strain and fracture strain of DP780 high strength steel sheet were determined from the strain field and strain history results.
文摘An indirect tensile testing method is proposed for characterizing low strength graphite platelet reinforced vinyl ester nanocomposites at high-strain rate. In this technique, the traditional Brazilian disk (diametrical compression) test method for brittle materials is utilized along with conventional split-Hopkinson pressure bars (SHPB) for evaluating cylindrical disk specimens. The cylindrical disk specimen is held snugly in between two concave end fixtures attached to the incident and transmission bars. To eliminate the complexities of conventional strain gage application, a non-contact Laser Occluding Expansion Gage (LOEG) has been adapted for measuring the diametrical transverse expansion of the specimen under high-strain rate diametrical compressive loading. Failure diagnosis using high-speed digital photography validates the viability of utilizing this indirect test method for characterizing the tensile properties of xGnP (exfoliated graphite nanoplatelets) reinforced and additional CTBN (Carboxyl Terminated Butadiene Nitrile) toughened vinyl ester based nanocomposites. Also, quasi-static indirect tensile response agrees with previous investigations conducted using the traditional dog-bone specimen in direct tensile tests. Investigation of both quasi-static and dynamic indirect tensile test responses shows the strain rate effect on the tensile strength and energy absorbing capacity of the candidate materials. The contribution of reinforcement to the tensile properties of the candidate materials is presented.
文摘This paper focuses on the dynamic tensile response of glass-graphite/epoxy composites illustrating improvement in energy absorption through hybridization. The dynamic response and energy absorption characteristics of pultruded hybrid combinations of glass and graphite fibers in an epoxy matrix subjected to induced transverse tension at high strain-rate in a modified Split Hopkinson Pressure Bar (SHPB) apparatus, are presented. Transverse tensile strength was determined by diametral compression of disc samples (Brazilian indirect tensile test method). Diametral crack initiation and strain to failure were monitored with a Shimadzu HPV-2 high-speed video camera at a recording speed of 500,000 fps and Digital Image Correlation (DIC). Adequate measures were taken to ensure that initiation of specimen failure occurred at the exact center of the disc specimen, and propagated through the diameter along the compressive loading axis, for the induced transverse tension tests to be valid. A study of the strength and specific energy absorption demonstrates the benefits of hybridization. Under induced transverse tensile loading condition, the pure glass/epoxy (GL60) exhibited higher strength than pure graphite/epoxy (GR60). Pure graphite/epoxy (GR60) has higher specific energy absorption capacity than pure glass/epoxy (GL60) in transverse tension. Among all hybrids, GR30 has the highest specific energy absorption under transverse tension. Overall, hybrid GL48, with 48% low-cost glass fibers in the inner core and 12% high-cost graphite fibers in outer shell, was found to exhibit better performance under induced transverse tension at high strain-rates, showing the benefits of hybridization.
基金Supported by National Natural Science Foundation of China(Grant Nos.51205284,51575384)
文摘A constitutive model is critical for the prediction accuracy of a metal cutting simulation. The highest strain rate involved in the cutting process can be in the range of 104-106 s 1. Flow stresses at high strain rates are close to that of cutting are difficult to test via experiments. Split Hopkinson compression bar （SHPB） technology is used to study the deformation behavior of Ti-6Al-4V alloy at strain rates of 10 -4-10 4s- 1. The Johnson Cook （JC） model was applied to characterize the flow stresses of the SHPB tests at various conditions. The parameters of the JC model are optimized by using a genetic algorithm technology. The JC plastic model and the energy density-based ductile failure criteria are adopted in the proposed SHPB finite element simulation model. The simulated flow stresses and the failure characteristics, such as the cracks along the adiabatic shear bands agree well with the experimental results. Afterwards, the SHPB simulation is used to simulate higher strain rate（approximately 3 × 10 4 s -1） conditions by minimizing the size of the specimen. The JC model parameters covering higher strain rate conditions which are close to the deformation condition in cutting were calculated based on the flow stresses obtained by using the SHPB tests （10 -4 - 10 4 s- 1） and simulation （up to 3 × 10 4 s - 1）. The cutting simulation using the constitutive parameters is validated by the measured forces and chip morphology. The constitutive model and parameters for high strain rate conditions that are identical to those of cutting were obtained based on the SHPB tests and simulation.
文摘For the research on steel structure in fire,it is very important to determine the properties of structural steel at elevated temperature.Up to now,the high-temperature properties of material is believed to be related to only temperature state,which is not precise enough to simulate the behavior of steel structures under different combinations of heating,cooling,loading,and unloading.To analyze the influence of the temperature-load history on the steel properties,a series of tests were carried out under different temperature-load paths about steel Q235,which is widely used in steel structures in China.In this paper,the method to set the temperature-load paths was introduced;the variety regulation of steel properties changing with temperature was analyzed under different paths;according to experimental results,the formulas of elastic modulus and yield strength at elevated temperature were fitted,and the stress-strain-temperature 3D relationships of structural steel under different paths were presented.
