Conference proceedings of the Society for Experimental Mechanics, Sep 8, 2016
This paper describes a dynamic experimental technique for characterizing the uniaxial stress-stra... more This paper describes a dynamic experimental technique for characterizing the uniaxial stress-strain relationship of rubbers under medium strain rate deformation. This method combines the Virtual Fields Method (VFM) and high-speed imaging with digital image correlation. The VFM can be expressed so that force measurement during dynamic loading is no longer required but acceleration data on the specimen surface, which occurs as a result of wave propagation in the specimen, are measured and used as a 'virtual load cell'. In a previous paper, the authors have utilized this technique for characterizing material parameters for the dynamic behaviour of rubbers using a drop-weight apparatus [Int. J. Solids Struct. 69-70:553-568, 2015]. One limitation of this technique is that the stability of the parameter estimation depends on the length of a specimen. When the loading stress wave reaches the fixed end of the rubber specimen, a static equilibrium state is instantaneously achieved. At this instant, the acceleration fields are no longer able to provide information, and the identification is unstable. In order to overcome this limitation, the present paper proposes a VFM able to produce stable identification even at the equilibrium instant. This procedure utilizes both inertial and material forces, and a new experiment apparatus has been developed for simultaneously measuring these two sets of data. This new procedure is described using results from simulations; then, the experimental system and its results will be presented.
Polymers are frequently used in applications in which they may be exposed to high rate or impact ... more Polymers are frequently used in applications in which they may be exposed to high rate or impact loading and there is growing industrial importance in understanding their mechanical behavior at different strain rates. This paper describes research that has been performed to better understand and predict the dependence on strain rate of mechanical properties such as yield and flow stress, by investigating the relationship between rate dependence and temperature dependence. In particular, the mechanical behavior of poly(vinyl chloride) with 20 wt% plasticizer has been studied at strain rates from 0.001 s À1 to 4700 s À1 and temperatures from À60 to þ80 C. In addition to the use of timeetemperature superposition, this paper presents a novel experimental method in which the yield stress and the effects of adiabatic heating at high strain rates are simulated in the low rate loading.
This research utilises a novel heat stage combined with a Zeiss scanning electron microscope to i... more This research utilises a novel heat stage combined with a Zeiss scanning electron microscope to investigate phase changes in iron alloys at temperatures up to 800 ℃ using SE and EBSD imaging. Carbon steel samples with starting structures of ferrite/pearlite were transformed into austenite using the commercial heat treatment process whilst imaging within the SEM. This process facilitates capturing both grain and phase transformation in real time allowing better insight into the microstructural evolution and overall phase change kinetics of this heat treatment. The technique for imaging uses a combination of localised EBSD high temperature imaging combined with the development of high temperature thermal-etching SE imaging technique. The SE thermal etching technique, as verified by EBSD images, enables tracking of a statistically significant number of grains (>100) and identification of individual phases. As well as being applied to carbon steel as shown here, the technique is part...
This dissertation examines the high strain rate mechanical properties of polymers, polymer bonded... more This dissertation examines the high strain rate mechanical properties of polymers, polymer bonded explosives and solders. In addition, it examines experimental developments that will allow more detailed investigation of these properties in the future. The split Hopkinson pressure bar is a standard apparatus used to measure the mechanical strength of materials at strain rates between 500 and 10,000 s- 1 � Measurements are performed by dynamically loading a small cylindrical specimen between two rods that are instrumented with strain gauges. The data from these strain gauges are used to calculate stress and strain in the specimen as functions of time. Chapter two contains an account of the Hopkinson bar system and its application. The advantages and limitations of the system are presented and discussed. Once of these limitations, the time taken for the specimen to reach mechanical equilibrium, is examined in detail in chapter three. In chapter four the variation of strength with strain rate of two polymers, polycarbonate and polyvinyladine difluoride, is investigated. It is shown that at high stain rates their mechanical strength is increased by freezing out of low order molecular relaxations. Chapter five presents data from measurements made on polymer-bonded explosives. The effect of crystal size on the mechanical properties of the mixture is examined, and it is shown that the strength of the material varies as the inverse square root of crystal spacing. In chapter six measurements of the high strain rate properties of different solder materials are presented. These measurements were made to provide high quality experimental results for the development of material models for solder at high strain rates. Chapter seven presents results from the application of speckle metrology, a wellestablished technique for making optical measurements of displacements, to the Hopkinson bar system. Along with a line laser device for measuring specimen radius this allowed accurate measurements of the Poisson's ratio of Hopkinson ba [...]
Crystallographic texture in polycrystalline HCP and cubic materials, often developed during therm... more Crystallographic texture in polycrystalline HCP and cubic materials, often developed during thermomechanical deformations, has profound effects on properties at the macroscopic or component level; given the respective natures of current detection techniques, a non-destructive, three-dimensional bulk texture detection method for these materials is yet to be developed. This thesis tries to achieve this goal through systematic studies on the relationship between ultrasonic wave velocity and texture. The feasibility of such development is firstly reviewed via the combination of computational and experimental studies on exemplary HCP materials. Numerical results obtained via a representative volume element (RVE) methodology reveal that the wave speed varies progressively and significantly with changing texture, and experimental ultrasound studies combined with EBSD characterisation demonstrate distinguished velocity profiles for samples with different textures. Thus the possibility of the development is proved by the combined results. A novel convolution theorem is then presented, which couples the single crystal wave speed (the kernel function) with polycrystal orientation distribution function to give the resultant polycrystal wave speed function. Firstly developed on HCP and then successfully extended to general anisotropic materials, the theorem expresses the three functions as harmonic expansions thus enabling the calculation of any one of them when the other two are known. Hence, the forward problem of determination of polycrystal wave speed is solved for all crystal systems with verifications on varying textures show near-perfect representation of the sensitivity of wave speed to texture as well as quantitative predictions of polycrystal wave speed. More importantly, the theorem also presents a solution to the long-standing inverse problem for HCP and cubic materials, with proof of principle established where groups of HCP and cubic textures are recovered solely from polycrystal wave velocities through the theorem and the results show good agreements with the original textures. Therefore the theorem opens up the possibility of developing a powerful package for bulk texture measurement and wave propagation studies in HCP, cubic materials and beyond.
The Photon Doppler Velocimetry (PDV) technique is used to capture simultaneously propagating elas... more The Photon Doppler Velocimetry (PDV) technique is used to capture simultaneously propagating elastic waves of longitudinal and shear nature in a Tension-Torsion Hopkinson Bar (TTHB) apparatus. The system uses a pair of probes per velocity measurement, which were taken on the opposite sides of the TTHB bar with a laser irradiated spot size of ~35 µm. The collected data were compared to the measurements obtained from the conventional strain gauge technique, and were in good agreement. The PDV method was effective in separating longitudinal and rotation signals even when they were superimposed on each other at the gauge location. This approach is also shown to be effective in detecting and accounting for the presence of bending waves in the TTHB bars.
This paper presents a novel apparatus, combined with application of the Virtual Fields Method (VF... more This paper presents a novel apparatus, combined with application of the Virtual Fields Method (VFM) for the high frequency characterisation of the mechanical response of ultra-soft materials. The viscoelastic response is characterised under harmonic deviatoric loading at a range of frequencies using the dynamic VFM in the frequency domain. Obtaining useful high rate data on soft materials is challenging using typical test methodologies, as the low speed of sound in the material makes stress equilibrium difficult to obtain; additionally, the low stiffness results in small stresses and weak measurement signals. This novel apparatus and analysis method have been shown capable of obtaining important material characterisation data that would be impossible to access using currently existing test techniques.
Background Understanding the mechanical response of elastomers to applied deformation at differen... more Background Understanding the mechanical response of elastomers to applied deformation at different strain rates and temperatures is crucial in industrial design and manufacture; however, this response is often difficult to measure, especially at high strain rates (e.g. > 100 s− 1), and more predictive methods to obtain constitutive relationships are required. Objective The objective of the research described in this paper is to develop such methods. Method The paper outlines a novel approach combining quasi-static monotonic tests in tension and compression, quasi-static cyclic tests in tension, and high strain rate tests in compression, with dynamic mechanical analysis and time-temperature superposition. A generalized viscoelastic model incorporating continuum damage is calibrated. Results The results show that a model calibrated using data from quasi-static compression and dynamic mechanical analysis can be used to adequately predict the compressive high strain rate response: he...
A novel instrument for investigating the dynamic microstructure evolution of high temperature ser... more A novel instrument for investigating the dynamic microstructure evolution of high temperature service materials up to 1150 °C in scanning electron microscope Review of Scientific Instruments 91, 043704 (2020);
Polymers are frequently used in fields as diverse as aerospace, biomedicine, automotive and in-du... more Polymers are frequently used in fields as diverse as aerospace, biomedicine, automotive and in-dustrial vibration damping, where they are often subjected to high strain rate or impact loading. Poly(vinyl chloride) (PVC), and its plasticised variants (PPVC), are just two examples of this broad category of materi-als. Since many polymers exhibit strong rate and temperature dependence, including a low temperature brittle transition, it is extremely important to understand their mechanical responses over a wide range of loading con-ditions.PVC with 60 wt% plasticiser is used in this study, as its highly rubbery nature lends itself well to being used in various load mitigation and energy absorption applications. It is challenging to obtain high strain rate data on rubbery materials using conventional techniques such as the split-Hopkinson (Kolsky) bar. Therefore, alternative approaches are required. Based on previous work developing a framework to predict high rate re-sponseusing a fract...
Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, ... more Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, and automotive. Understanding their mechanical responses is therefore extremely important, particularly because they often exhibit strong rate and temperature dependence, including a low temperature brittle transition. Relationships between the response at various strain rates and temperatures are investigated and a framework developed to predict response at rates where experiments are unfeasible. A master curve showing the rate dependences of the storage and loss moduli at a reference temperature is constructed using the results of a dynamic mechanical analysis (DMA) experiment. A frequency sweep spanning two decades and a temperature range from pre-glass transition to pre-melt is used. A fractional derivative model is fitted to the experimental data, and this models parameters are used to show how the stress-strain relationships at a desired strain rate could be derived.
We report the synthesis and characterization of a kinetically controlled, thermoreversible supram... more We report the synthesis and characterization of a kinetically controlled, thermoreversible supramolecular polyurethane whose mechanical properties depend unusually strongly on the processing history. Materials were prepared by solution casting, quenching and annealing of quenched material, allowing pronounced micro-structural evolution, which leads to rapid increases in modulus as determined by rheological analysis. Tensile tests showed that the quenched material is soft, weak and ductile (shear modulus ~ 5 MPa, elongation ~ 250 %), but after annealing, at 70 °C for one hour, it becomes stiffer, stronger and more brittle (~ 20 MPa, ~ 20 %). FTIR and NMR spectroscopic analysis, coupled with MDSC and SAXS, were performed to investigate the network's dynamic structural changes. SAXS results suggest the presence of a lamellar structure in the sample when solution cast at high temperature, or annealed. This ordering is unique when compared to structurally-related supramolecular bisurethane and bisurea polymers, and may be the cause of the observed path dependence. These mechanical properties, which can be switched repeatedly by simple thermal treatments, coupled with its adhesion properties as determined from peel and tack tests, make it an excellent candidate as a recyclable material for adhesives and coatings.
This work investigates the single rod impact test technique, from which wave propagation characte... more This work investigates the single rod impact test technique, from which wave propagation characteristics, i.e. the propagation coefficient and wave speed, can be obtained and used to identify the material modulus as a function of frequency. To accomplish this, an elastic Titanium rod and a viscoelastic Polymethyl Methacrylate (PMMA) rod have been tested. With a known modulus, the Titanium rod modulus was successfully recovered from the wave propagation coefficient in a frequency range of 5 kHz ∼ 30 kHz, which verifies the validity of this technique. The PMMA modulus identified by this technique agrees reasonably with the modulus derived from DMA tests in the range 2 kHz ∼ 20 kHz. The good agreement between the theoretical and simulation-based modulus further confirms the effectiveness of the single rod impact test in this use. It is also demonstrated that the single rod impact technique is capable of detecting the minor mechanical changes induced by temperature variations as small as 2 • C.
Anyone having need of a copy of this report should first contact the Defense Technical Informatio... more Anyone having need of a copy of this report should first contact the Defense Technical Information Center (DTIC) at the address shown below. If you are a registered DTIC User and qualify as a recipient of this document, DTIC can provide you with a copy. If you are a registered DTIC User and do not qualify as a recipient, DTIC can submit a request for release, on your behalf, to the controlling DoD agency for their review and determination.
This paper reviews the literature on the response of polymers to high strain rate deformation. Th... more This paper reviews the literature on the response of polymers to high strain rate deformation. The main focus is on the experimental techniques used to characterize this response. The paper includes a small number of examples as well as references to experimental data over a wide range of rates, which illustrate the key features of rate dependence in these materials; however this is by no means an exhaustive list. The aim of the paper is to give the reader unfamiliar with the subject an overview of the techniques available with sufficient references from which further information can be obtained. In addition to the 'well established' techniques of the Hopkinson bar, Taylor Impact and Transverse impact, a discussion of the use of time-temperature superposition in interpreting and experimentally replicating high rate response is given, as is a description of new techniques in which mechanical parameters are derived by directly measuring wave propagation in specimens; these are particularly appropriate for polymers with low wave speeds. The vast topic of constitutive modelling is deliberately excluded from this review.
The present study focuses on the identification of the evolution of the dynamic elasto-plastic pr... more The present study focuses on the identification of the evolution of the dynamic elasto-plastic properties of Al 5456 FSW welds. An innovative method is proposed to make best use of the data collected with full-field measurements during dynamic experiments, and achieve identification of the mechanical properties of heterogeneous materials without requiring measurement of the load. Compressive specimens have been submitted to high strain-rate loading through a split Hopkinson pressure bar device while displacement fields were obtained using fullfield measurement techniques. Two sets of experiments have been performed using two different methods: the grid method and digital image correlation. Afterwards, the identification of the elastic and plastic properties of the material was carried out using the Virtual Fields Method. Finally, identification of the evolution of the yield stress throughout the weld has been achieved for strain-rates of the order of 10 3 s-1. Keywords Friction stir welding Á Virtual fields method Á Dynamic deformation Á Digital image correlation Á Grid method Á Split Hopkinson pressure bar & F. Pierron
Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried... more Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130°C and 1250°C spanning strain rates from 0.01 s À 1 to 4000 s À 1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall-Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener-Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000 s À 1 where mechanical testing's results were absent.
We report a non-cytotoxic supramolecular polyurethane network whose mechanical properties can be ... more We report a non-cytotoxic supramolecular polyurethane network whose mechanical properties can be recovered efficiently (>99%) at body temperature.
Conference proceedings of the Society for Experimental Mechanics, Sep 8, 2016
This paper describes a dynamic experimental technique for characterizing the uniaxial stress-stra... more This paper describes a dynamic experimental technique for characterizing the uniaxial stress-strain relationship of rubbers under medium strain rate deformation. This method combines the Virtual Fields Method (VFM) and high-speed imaging with digital image correlation. The VFM can be expressed so that force measurement during dynamic loading is no longer required but acceleration data on the specimen surface, which occurs as a result of wave propagation in the specimen, are measured and used as a 'virtual load cell'. In a previous paper, the authors have utilized this technique for characterizing material parameters for the dynamic behaviour of rubbers using a drop-weight apparatus [Int. J. Solids Struct. 69-70:553-568, 2015]. One limitation of this technique is that the stability of the parameter estimation depends on the length of a specimen. When the loading stress wave reaches the fixed end of the rubber specimen, a static equilibrium state is instantaneously achieved. At this instant, the acceleration fields are no longer able to provide information, and the identification is unstable. In order to overcome this limitation, the present paper proposes a VFM able to produce stable identification even at the equilibrium instant. This procedure utilizes both inertial and material forces, and a new experiment apparatus has been developed for simultaneously measuring these two sets of data. This new procedure is described using results from simulations; then, the experimental system and its results will be presented.
Polymers are frequently used in applications in which they may be exposed to high rate or impact ... more Polymers are frequently used in applications in which they may be exposed to high rate or impact loading and there is growing industrial importance in understanding their mechanical behavior at different strain rates. This paper describes research that has been performed to better understand and predict the dependence on strain rate of mechanical properties such as yield and flow stress, by investigating the relationship between rate dependence and temperature dependence. In particular, the mechanical behavior of poly(vinyl chloride) with 20 wt% plasticizer has been studied at strain rates from 0.001 s À1 to 4700 s À1 and temperatures from À60 to þ80 C. In addition to the use of timeetemperature superposition, this paper presents a novel experimental method in which the yield stress and the effects of adiabatic heating at high strain rates are simulated in the low rate loading.
This research utilises a novel heat stage combined with a Zeiss scanning electron microscope to i... more This research utilises a novel heat stage combined with a Zeiss scanning electron microscope to investigate phase changes in iron alloys at temperatures up to 800 ℃ using SE and EBSD imaging. Carbon steel samples with starting structures of ferrite/pearlite were transformed into austenite using the commercial heat treatment process whilst imaging within the SEM. This process facilitates capturing both grain and phase transformation in real time allowing better insight into the microstructural evolution and overall phase change kinetics of this heat treatment. The technique for imaging uses a combination of localised EBSD high temperature imaging combined with the development of high temperature thermal-etching SE imaging technique. The SE thermal etching technique, as verified by EBSD images, enables tracking of a statistically significant number of grains (>100) and identification of individual phases. As well as being applied to carbon steel as shown here, the technique is part...
This dissertation examines the high strain rate mechanical properties of polymers, polymer bonded... more This dissertation examines the high strain rate mechanical properties of polymers, polymer bonded explosives and solders. In addition, it examines experimental developments that will allow more detailed investigation of these properties in the future. The split Hopkinson pressure bar is a standard apparatus used to measure the mechanical strength of materials at strain rates between 500 and 10,000 s- 1 � Measurements are performed by dynamically loading a small cylindrical specimen between two rods that are instrumented with strain gauges. The data from these strain gauges are used to calculate stress and strain in the specimen as functions of time. Chapter two contains an account of the Hopkinson bar system and its application. The advantages and limitations of the system are presented and discussed. Once of these limitations, the time taken for the specimen to reach mechanical equilibrium, is examined in detail in chapter three. In chapter four the variation of strength with strain rate of two polymers, polycarbonate and polyvinyladine difluoride, is investigated. It is shown that at high stain rates their mechanical strength is increased by freezing out of low order molecular relaxations. Chapter five presents data from measurements made on polymer-bonded explosives. The effect of crystal size on the mechanical properties of the mixture is examined, and it is shown that the strength of the material varies as the inverse square root of crystal spacing. In chapter six measurements of the high strain rate properties of different solder materials are presented. These measurements were made to provide high quality experimental results for the development of material models for solder at high strain rates. Chapter seven presents results from the application of speckle metrology, a wellestablished technique for making optical measurements of displacements, to the Hopkinson bar system. Along with a line laser device for measuring specimen radius this allowed accurate measurements of the Poisson's ratio of Hopkinson ba [...]
Crystallographic texture in polycrystalline HCP and cubic materials, often developed during therm... more Crystallographic texture in polycrystalline HCP and cubic materials, often developed during thermomechanical deformations, has profound effects on properties at the macroscopic or component level; given the respective natures of current detection techniques, a non-destructive, three-dimensional bulk texture detection method for these materials is yet to be developed. This thesis tries to achieve this goal through systematic studies on the relationship between ultrasonic wave velocity and texture. The feasibility of such development is firstly reviewed via the combination of computational and experimental studies on exemplary HCP materials. Numerical results obtained via a representative volume element (RVE) methodology reveal that the wave speed varies progressively and significantly with changing texture, and experimental ultrasound studies combined with EBSD characterisation demonstrate distinguished velocity profiles for samples with different textures. Thus the possibility of the development is proved by the combined results. A novel convolution theorem is then presented, which couples the single crystal wave speed (the kernel function) with polycrystal orientation distribution function to give the resultant polycrystal wave speed function. Firstly developed on HCP and then successfully extended to general anisotropic materials, the theorem expresses the three functions as harmonic expansions thus enabling the calculation of any one of them when the other two are known. Hence, the forward problem of determination of polycrystal wave speed is solved for all crystal systems with verifications on varying textures show near-perfect representation of the sensitivity of wave speed to texture as well as quantitative predictions of polycrystal wave speed. More importantly, the theorem also presents a solution to the long-standing inverse problem for HCP and cubic materials, with proof of principle established where groups of HCP and cubic textures are recovered solely from polycrystal wave velocities through the theorem and the results show good agreements with the original textures. Therefore the theorem opens up the possibility of developing a powerful package for bulk texture measurement and wave propagation studies in HCP, cubic materials and beyond.
The Photon Doppler Velocimetry (PDV) technique is used to capture simultaneously propagating elas... more The Photon Doppler Velocimetry (PDV) technique is used to capture simultaneously propagating elastic waves of longitudinal and shear nature in a Tension-Torsion Hopkinson Bar (TTHB) apparatus. The system uses a pair of probes per velocity measurement, which were taken on the opposite sides of the TTHB bar with a laser irradiated spot size of ~35 µm. The collected data were compared to the measurements obtained from the conventional strain gauge technique, and were in good agreement. The PDV method was effective in separating longitudinal and rotation signals even when they were superimposed on each other at the gauge location. This approach is also shown to be effective in detecting and accounting for the presence of bending waves in the TTHB bars.
This paper presents a novel apparatus, combined with application of the Virtual Fields Method (VF... more This paper presents a novel apparatus, combined with application of the Virtual Fields Method (VFM) for the high frequency characterisation of the mechanical response of ultra-soft materials. The viscoelastic response is characterised under harmonic deviatoric loading at a range of frequencies using the dynamic VFM in the frequency domain. Obtaining useful high rate data on soft materials is challenging using typical test methodologies, as the low speed of sound in the material makes stress equilibrium difficult to obtain; additionally, the low stiffness results in small stresses and weak measurement signals. This novel apparatus and analysis method have been shown capable of obtaining important material characterisation data that would be impossible to access using currently existing test techniques.
Background Understanding the mechanical response of elastomers to applied deformation at differen... more Background Understanding the mechanical response of elastomers to applied deformation at different strain rates and temperatures is crucial in industrial design and manufacture; however, this response is often difficult to measure, especially at high strain rates (e.g. > 100 s− 1), and more predictive methods to obtain constitutive relationships are required. Objective The objective of the research described in this paper is to develop such methods. Method The paper outlines a novel approach combining quasi-static monotonic tests in tension and compression, quasi-static cyclic tests in tension, and high strain rate tests in compression, with dynamic mechanical analysis and time-temperature superposition. A generalized viscoelastic model incorporating continuum damage is calibrated. Results The results show that a model calibrated using data from quasi-static compression and dynamic mechanical analysis can be used to adequately predict the compressive high strain rate response: he...
A novel instrument for investigating the dynamic microstructure evolution of high temperature ser... more A novel instrument for investigating the dynamic microstructure evolution of high temperature service materials up to 1150 °C in scanning electron microscope Review of Scientific Instruments 91, 043704 (2020);
Polymers are frequently used in fields as diverse as aerospace, biomedicine, automotive and in-du... more Polymers are frequently used in fields as diverse as aerospace, biomedicine, automotive and in-dustrial vibration damping, where they are often subjected to high strain rate or impact loading. Poly(vinyl chloride) (PVC), and its plasticised variants (PPVC), are just two examples of this broad category of materi-als. Since many polymers exhibit strong rate and temperature dependence, including a low temperature brittle transition, it is extremely important to understand their mechanical responses over a wide range of loading con-ditions.PVC with 60 wt% plasticiser is used in this study, as its highly rubbery nature lends itself well to being used in various load mitigation and energy absorption applications. It is challenging to obtain high strain rate data on rubbery materials using conventional techniques such as the split-Hopkinson (Kolsky) bar. Therefore, alternative approaches are required. Based on previous work developing a framework to predict high rate re-sponseusing a fract...
Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, ... more Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, and automotive. Understanding their mechanical responses is therefore extremely important, particularly because they often exhibit strong rate and temperature dependence, including a low temperature brittle transition. Relationships between the response at various strain rates and temperatures are investigated and a framework developed to predict response at rates where experiments are unfeasible. A master curve showing the rate dependences of the storage and loss moduli at a reference temperature is constructed using the results of a dynamic mechanical analysis (DMA) experiment. A frequency sweep spanning two decades and a temperature range from pre-glass transition to pre-melt is used. A fractional derivative model is fitted to the experimental data, and this models parameters are used to show how the stress-strain relationships at a desired strain rate could be derived.
We report the synthesis and characterization of a kinetically controlled, thermoreversible supram... more We report the synthesis and characterization of a kinetically controlled, thermoreversible supramolecular polyurethane whose mechanical properties depend unusually strongly on the processing history. Materials were prepared by solution casting, quenching and annealing of quenched material, allowing pronounced micro-structural evolution, which leads to rapid increases in modulus as determined by rheological analysis. Tensile tests showed that the quenched material is soft, weak and ductile (shear modulus ~ 5 MPa, elongation ~ 250 %), but after annealing, at 70 °C for one hour, it becomes stiffer, stronger and more brittle (~ 20 MPa, ~ 20 %). FTIR and NMR spectroscopic analysis, coupled with MDSC and SAXS, were performed to investigate the network's dynamic structural changes. SAXS results suggest the presence of a lamellar structure in the sample when solution cast at high temperature, or annealed. This ordering is unique when compared to structurally-related supramolecular bisurethane and bisurea polymers, and may be the cause of the observed path dependence. These mechanical properties, which can be switched repeatedly by simple thermal treatments, coupled with its adhesion properties as determined from peel and tack tests, make it an excellent candidate as a recyclable material for adhesives and coatings.
This work investigates the single rod impact test technique, from which wave propagation characte... more This work investigates the single rod impact test technique, from which wave propagation characteristics, i.e. the propagation coefficient and wave speed, can be obtained and used to identify the material modulus as a function of frequency. To accomplish this, an elastic Titanium rod and a viscoelastic Polymethyl Methacrylate (PMMA) rod have been tested. With a known modulus, the Titanium rod modulus was successfully recovered from the wave propagation coefficient in a frequency range of 5 kHz ∼ 30 kHz, which verifies the validity of this technique. The PMMA modulus identified by this technique agrees reasonably with the modulus derived from DMA tests in the range 2 kHz ∼ 20 kHz. The good agreement between the theoretical and simulation-based modulus further confirms the effectiveness of the single rod impact test in this use. It is also demonstrated that the single rod impact technique is capable of detecting the minor mechanical changes induced by temperature variations as small as 2 • C.
Anyone having need of a copy of this report should first contact the Defense Technical Informatio... more Anyone having need of a copy of this report should first contact the Defense Technical Information Center (DTIC) at the address shown below. If you are a registered DTIC User and qualify as a recipient of this document, DTIC can provide you with a copy. If you are a registered DTIC User and do not qualify as a recipient, DTIC can submit a request for release, on your behalf, to the controlling DoD agency for their review and determination.
This paper reviews the literature on the response of polymers to high strain rate deformation. Th... more This paper reviews the literature on the response of polymers to high strain rate deformation. The main focus is on the experimental techniques used to characterize this response. The paper includes a small number of examples as well as references to experimental data over a wide range of rates, which illustrate the key features of rate dependence in these materials; however this is by no means an exhaustive list. The aim of the paper is to give the reader unfamiliar with the subject an overview of the techniques available with sufficient references from which further information can be obtained. In addition to the 'well established' techniques of the Hopkinson bar, Taylor Impact and Transverse impact, a discussion of the use of time-temperature superposition in interpreting and experimentally replicating high rate response is given, as is a description of new techniques in which mechanical parameters are derived by directly measuring wave propagation in specimens; these are particularly appropriate for polymers with low wave speeds. The vast topic of constitutive modelling is deliberately excluded from this review.
The present study focuses on the identification of the evolution of the dynamic elasto-plastic pr... more The present study focuses on the identification of the evolution of the dynamic elasto-plastic properties of Al 5456 FSW welds. An innovative method is proposed to make best use of the data collected with full-field measurements during dynamic experiments, and achieve identification of the mechanical properties of heterogeneous materials without requiring measurement of the load. Compressive specimens have been submitted to high strain-rate loading through a split Hopkinson pressure bar device while displacement fields were obtained using fullfield measurement techniques. Two sets of experiments have been performed using two different methods: the grid method and digital image correlation. Afterwards, the identification of the elastic and plastic properties of the material was carried out using the Virtual Fields Method. Finally, identification of the evolution of the yield stress throughout the weld has been achieved for strain-rates of the order of 10 3 s-1. Keywords Friction stir welding Á Virtual fields method Á Dynamic deformation Á Digital image correlation Á Grid method Á Split Hopkinson pressure bar & F. Pierron
Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried... more Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130°C and 1250°C spanning strain rates from 0.01 s À 1 to 4000 s À 1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall-Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener-Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000 s À 1 where mechanical testing's results were absent.
We report a non-cytotoxic supramolecular polyurethane network whose mechanical properties can be ... more We report a non-cytotoxic supramolecular polyurethane network whose mechanical properties can be recovered efficiently (>99%) at body temperature.
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