This study presents the results of an experimental investigation of 18 short concrete columns con... more This study presents the results of an experimental investigation of 18 short concrete columns confined by carbon fiber-reinforced polymer (CFRP) and transverse spiral reinforcement (TSR) under uniaxial compression. Longitudinal rebars are not installed in the specimens in order to eliminate their confinement effect to concrete which affects the analysis of 3-D compression of concrete. The paper only consider for FRP and spiral reinforcement confinement in transverse direction. Two key experimental parameters were investigated: the thickness of the CFRP tube (0.167, 0.334, and 0.501 mm) and the spacing of the TSR (25 and 50 mm). The failure mode, axial and transverse stress-strain relationship, confinement effectiveness, Poisson's ratio and dilatation performance of the specimens were discussed. Test results show that the ultimate strength of concrete has a linear proportional enhancement with an increase in the FRP layer in each TSR category and a decrease in the TSR spacing in each FRP layer category. The ultimate load carrying capacity of the confined concrete depends on the confinement pressure during failure in terms of ultimate strength and axial strain.
Materials such as bone, teeth, and seashells possess remarkable combinations of properties despit... more Materials such as bone, teeth, and seashells possess remarkable combinations of properties despite the poor structural quality of their ingredients (brittle minerals and soft proteins). Nacre from mollusk shells is 3,000 times tougher than the brittle mineral it is made of, a level of toughness amplification currently unmatched by any engineering material. For this reason, nacre has become the model for bioinspiration for novel structural materials. The structure of nacre is organized over several length scales, but the microscopic brick-and-mortar arrangement of the mineral tablets is prominent. This staggered structure provides a universal approach to arranging hard building blocks in nature and is also found in bone and teeth. Recent models have demonstrated how an attractive combination of stiffness, strength, and toughness can be achieved through the staggered structure. The fabrication of engineering materials that duplicate the structure, mechanics, and properties of natural nacre still present formidable challenges to this day. FIGURE 3.2 Tensile stress-strain behavior of nacre. Adapted from Ref. 14.
This paper presents a material model suitable for simulating the behavior of dry fabrics subjecte... more This paper presents a material model suitable for simulating the behavior of dry fabrics subjected to ballistic impact. The developed material model is implemented in a commercial explicit finite element (FE) software LS-DYNA through a user defined material subroutine (UMAT). The constitutive model is developed using data from uniaxial quasi-static and high strain rate tension tests, picture frame tests and friction tests. Different finite element modeling schemes using shell finite elements are used to study efficiency and accuracy issues. First, single FE layer (SL) and multiple FE layers (ML) were used to simulate the ballistic tests conducted at NASA Glenn Research Center (NASA-GRC). Second, in the multiple layer configuration, a new modeling approach called Spiral Modeling Scheme (SMS) was tried and compared to the existing Concentric Modeling Scheme (CMS). Regression analyses were used to fill missing experimental data – the shear properties of the fabric, damping coefficient and the parameters used in Cowper-Symonds (CS) model which account for strain rate effect on material properties, in order to achieve close match between FE simulations and experimental data. The difference in absorbed energy by the fabric after impact, displacement of fabric near point of impact, and extent of damage were used as metrics for evaluating the material model. In addition, the ballistic limits of the multi-layer fabrics for various configurations were also determined.
Teleost Fish skin Fish scale Scalation pattern Puncture resistance Biomimetics Body armor a b s t... more Teleost Fish skin Fish scale Scalation pattern Puncture resistance Biomimetics Body armor a b s t r a c t
The objective of this research was to study the behavior of 3D AR glass fabric cement-based compo... more The objective of this research was to study the behavior of 3D AR glass fabric cement-based composites under impact loading. It was found that 3D fabrics significantly improve the toughness and energy absorption of cement-based composites under impact loading, compared to short AR glass fibers reinforcement. The 3D fabric improves the toughness in as high as 200 folds compared to short fiber composites. The energy absorption was highly affected by the thickness of the element and the location of the 3D fabric faces.
Woven fabrics are used in many applications, including ballistic armors, propulsion engine contai... more Woven fabrics are used in many applications, including ballistic armors, propulsion engine containment systems and fabric reinforced composites. In order to facilitate the design and improvement of such applications, this paper investigates the stress–strain response in warp and fill directions, the apparent Poisson’s ratio, and the in-plane shear response of Kevlar 49 fabric including the possible effects of specimen size and pre-loading on the mechanical responses of the fabric. Full-field image analysis of the fabric under shear deformation is used to better understand the mechanisms related to in-plane shear. The experimental results show that the fabric exhibits non-linear and orthogonal behavior in tension, and can deform up to 20% before complete failure. It has identical Young’s modulus (pre-peak elastic stiffness) in warp and fill directions, but has different crimp strain, tensile strength and ultimate strain. The apparent Poisson’s ratio is a nonlinear function of strain and dependent upon the levels of pre-loading. It increases with strain quickly at the beginning and decreases gradually until the fabric fails. The shear response is highly nonlinear and has four distinct regions: linear elastic rotation region, dissipative rotation region, yarn compression region and shear locking region, and it is not dependent upon specimen size after normalization.
In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both ... more In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10−4 s−1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s−1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.
The toughness of fiber-reinforced composites largely relies on crack bridging. More specifically,... more The toughness of fiber-reinforced composites largely relies on crack bridging. More specifically, intact fibers left behind the tip of a propagating crack are progressively pulled out of the matrix, dissipating energy which translates into toughness. While short fibers are traditionally straight, recent work has showed that they can be shaped to increase the pullout strength, but not necessarily the energy to pullout. In this work we have modeled, fabricated and tested short fibers with tapered ends inspired from a high-performance natural material: nacre from mollusc shells. The main idea was to duplicate a key mechanism where a slight waviness of the inclusion can generate strain hardening and energy dissipation when the inclusion is pulled out. We have incorporated a similar feature to short fibers, in the form of tapered ends with well defined opening angles. We performed pullout tests on tapered steel fibers in epoxy matrices, which showed that the pullout of tapered fiber dissipates up to 27 times more energy than straight fibers. The experimental results also indicated the existence of an optimum taper angle to maximize work of pullout while preventing the brittle fracture of the matrix. An analytical model was developed to capture the pullout mechanism and the interaction between fiber and matrix. The analytical model can guide the design of tapered fibers by providing predictions on the influence of different parameters.
Nacre from mollusk shell is a high-performance natural composite composed of microscopic mineral ... more Nacre from mollusk shell is a high-performance natural composite composed of microscopic mineral tablets bonded by a tough biopolymer. Under tensile stress, the tablets slide on one another in a highly controlled fashion, which makes nacre 3000 times tougher than the mineral it is made of. Significant efforts have led to nacre-like materials, but none can yet match this amount of toughness amplification. This article presents the first synthetic material that successfully duplicates the mechanism of tablet sliding observed in nacre. Made of millimeter-size wavy poly-methylmethacrylate tablets held by fasteners, this "model material" undergoes massive tablet sliding under tensile loading, accompanied by strain hardening. Analytical and finite element models successfully captured the salient deformation mechanisms in this material, enabling further design refinements and optimization. In addition, two new mechanisms were identified: the effect of free surfaces and "unzipping." Both mechanisms may be relevant to natural materials such as nacre or bone.
This paper presents the development of a test procedure and application of non-contacting strain ... more This paper presents the development of a test procedure and application of non-contacting strain measurement for cement-based composites under moderately high strain rate tensile tests. The strain time histories of test specimens measured by a laser extensometer in high speed mode were derived by a phase-shift technique based on zero-crossing method. The accuracy of the linear variable differential transformer (LVDT) of the actuator in a servo-hydraulic high rate testing machine was verified by image analysis using sisal fiber reinforced cement composite at a strain rate of 25 s−1. The same procedure was then applied to Alkaline Resistant (AR) glass fabric reinforced cement composite tested at an average strain rate of 17 s−1. Comparison between the strain values measured by the laser extensometer and the LVDT shows a good agreement between these two measurement techniques. The test results show that the Young’s modulus, tensile strength, maximum strain, and toughness of the AR-glass fabric–cement composite increase with increasing strain rate. However, under both static and dynamic loadings the composite has similar behavior: multi-crack development and one dominant crack leading to final failure. In order to ensure the accuracy of dynamic tensile test procedures, non-contacting devices and techniques should be used as an independent means of verification of test results. The accuracy required in quantifying relative improvements in mechanical properties necessitates the various methods of measuring the displacement and strain rate properties.
Dynamic tensile tests were conducted using a high speed servo-hydraulic testing machine on three ... more Dynamic tensile tests were conducted using a high speed servo-hydraulic testing machine on three types of fabric reinforced cement composites. The high speed testing procedure and data processing method are presented. Quasi-static tests were also conducted on the composites. Effects of strain rate on the mechanical properties of fabric–cement composites are noted. A good correlation was found between the properties of the fabrics and the composites, with the carbon fabric exhibiting the highest strength and ductility performance in high speed tensile tests. The differences in tensile behavior of the various composites were correlated with the differences in the role of the fabric materials. Composites tested under high speed loading exhibited different responses as compared to similar composites tested under quasi-static condition.
Fabric–cement composites developed using the pultrusion production process have demonstrated impr... more Fabric–cement composites developed using the pultrusion production process have demonstrated impressive tensile and flexural properties. For instance fabric reinforced composites with bonded Alkali Resistant (AR) glass fabrics exhibit strain-hardening behavior, tensile strength in the range of 20–25 MPa, and strain capacity of the order of 2–5% under static conditions. Properties of these composite systems were investigated under three point bending conditions using an instrumented drop weight impact system. Samples were studied from the viewpoint of the variations of impact load, deflection response, acceleration and absorbed energy. Development of the testing system in terms of components and acceleration response are discussed in detail. Methods of the impact load measurement using three different ways of acceleration response, piezoelectric load washer and conventional strain gage based load cell are discussed. Cement composites with two different fabric contents and four different drop heights of hammer (dropping mass) were tested. Experimental results indicate that for the same drop height, the stiffer beam type specimens have a lower ultimate deflection but a higher load carrying capacity than the plate type specimens. The maximum flexural stress and absorbed energy of composites increase with drop height. In beam specimens, complete fracture does not take place as cracks form and close due to rebound and significant microcracking in the form of radial fan cracking is observed, whereas interlaminar shear is the dominant failure mode in the plate specimens.
In this research project the behaviour of strain-hardening cement-based composites (SHCC) subject... more In this research project the behaviour of strain-hardening cement-based composites (SHCC) subjected to low and high strain rates was studied. Uniaxial tension tests on dumbbell-shaped SHCC specimens were performed at rates ranging from 10 -5 s -1 to 50s -1 . For the tests performed at strain rates of 10 -2 s -1 and below, SHCC yielded a moderate increase in tensile strength and simultaneous decrease in strain capacity with increasing strain rate. When tested for higher strain rates from 10 to 50s -1 a considerable increase in tensile strain and strain capacity was measured. Microscopic investigation of the fracture surfaces showed that almost no fibre failure and an average pullout length of 2.5mm were found in the high strain rate test. This observation is in contrast to that of rapid quasi-static testing, where the average fibre pullout length of 300μm was much shorter. Furthermore, the fibres on the fracture surfaces produced in the high rate tests exhibited pronounced plastic deformations. Finally, quasi-static and high-speed tension tests on individual fibres and single fibre pullout tests were performed. While the increase in the tensile strength of the fibre was only moderate in the range of strain rates investigated, a considerable increase in bond strength between fibre and matrix was determined.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Jan 1, 2010
The experimental behavior of sisal fiber reinforced cement composites subjected to high speed ten... more The experimental behavior of sisal fiber reinforced cement composites subjected to high speed tension load was studied. High strain rates were achieved by using a high rate servo-hydraulic testing machine. A state-of-the-art high speed Phantom camera was also used to take images from the specimen during the test. The images were used in a digital image correlation model to determine the displacement fields and to calculate crack spacing. The effect of strain rate was investigated by comparing static and dynamic tensile tests which were performed at strain rates ranging from 5.5 × 10−6 to 24.6 s−1, respectively. A numerical tension stiffening model based on nonlinear finite difference method was used to simulate tensile cracking behavior of sisal fiber cementitious composites. The composite presented strain rate sensitivity for ultimate tensile strength and strain capacity with a dynamic amplification factor of 1.26.
The impact response of unidirectional continuous sisal fiberreinforced cement composites was inve... more The impact response of unidirectional continuous sisal fiberreinforced cement composites was investigated. An impact test setup based on a free-fall drop of an instrumented hammer on a three-point bending configuration test was used. The effect of impact energy on the composite response was investigated by using three different drop heights. Mechanical properties obtained under impact loading were compared with static three-point bending tests. A comparison with the impact behavior of glass fabricreinforced composites was addressed. Furthermore, damage mechanisms were investigated by characterizing the cracking patterns during the impact event using a high-speed digital camera. It was observed that while the maximum flexural stress values were in the same range for static and impact tests, the absorbed energy increased up to a maximum level of 0.0090 kJ/in. 2 (14.0 kJ/m 2 ) for the impact energy of 20 J. The damage process was initiated by tensile and shear cracks. Delamination of the plies within the continuous fiber composites was observed only when the impact energy was 27 J.
This paper presents the results of an experimental investigation into the strength, deformation, ... more This paper presents the results of an experimental investigation into the strength, deformation, and fracture behaviour of textile-reinforced concrete (TRC) subjected both to low and high-rate tensile loading ranging from 0.0001 to 50 s−1. High strain rates were achieved using a high-rate servo-hydraulic testing machine. The effect of the addition of short fibres on the static and dynamic response of TRC has been investigated, and the microstructure of both composite and fibre was observed after the tests using an ESEM. An increase in tensile strength, strain capacity, and work-to-fracture was observed for strain rates up to 0.1 s−1 with increasing strain rate. The addition of short glass fibres increased the tensile strength and first crack strength of the TRC. For high-speed tests (rates above 5 s−1) an increase in the tensile strength, first crack strength and work-to-fracture was also observed, but at the same time there was a decrease in the strain capacity. The tests at high loading rates showed a pronounced effect of the specimen length on the measured mechanical properties: with increasing gauge length the tensile strength and strain capacity decreased, while the work-to-fracture increased.▶ Tensile strength, strain capacity, and work-to-fracture increased when ε˙≤0.1 s−1. ▶ When ε˙≤5 s−1 a decrease in the strain capacity was observed. ▶ Different fibre-fracture morphologies were observed for high and low strain rates. ▶ The addition of short fibres increased the tensile strength and the crack stress.
Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydrau... more Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydraulic machine at intermediate strain rates to validate the testing technique and to investigate the strain-rate effect on the material's stress-strain behavior and failure mode. We present the experimental procedures and results discussing the constitutive response of the alloy at strain rates up to approximately 200 s À1 . The predominant frequencies of the high-speed testing machine were characterized by modal analysis, and we analyzed the effect from vibration of the system and loading rate on flow stress by using a single degree-of-freedom (SDOF) spring-mass-damper model. We tested two different specimen sizes at a wide range of actuator velocities to achieve the desired strain rates. Results show that the yield strength, ultimate strength, and failure strain were dependent on strain rate. We fitted the data to the Johnson-Cook (JC) constitutive model, and the resulting parameters are comparable to published results in the literature. The fracture surface of specimens tested at different strain rates obtained by scanning electron microscopy (SEM) showed that the ductile failure mode was dominant for the alloy, and strain rates within the range examined affected the fracture morphology.
This study presents the results of an experimental investigation of 18 short concrete columns con... more This study presents the results of an experimental investigation of 18 short concrete columns confined by carbon fiber-reinforced polymer (CFRP) and transverse spiral reinforcement (TSR) under uniaxial compression. Longitudinal rebars are not installed in the specimens in order to eliminate their confinement effect to concrete which affects the analysis of 3-D compression of concrete. The paper only consider for FRP and spiral reinforcement confinement in transverse direction. Two key experimental parameters were investigated: the thickness of the CFRP tube (0.167, 0.334, and 0.501 mm) and the spacing of the TSR (25 and 50 mm). The failure mode, axial and transverse stress-strain relationship, confinement effectiveness, Poisson's ratio and dilatation performance of the specimens were discussed. Test results show that the ultimate strength of concrete has a linear proportional enhancement with an increase in the FRP layer in each TSR category and a decrease in the TSR spacing in each FRP layer category. The ultimate load carrying capacity of the confined concrete depends on the confinement pressure during failure in terms of ultimate strength and axial strain.
Materials such as bone, teeth, and seashells possess remarkable combinations of properties despit... more Materials such as bone, teeth, and seashells possess remarkable combinations of properties despite the poor structural quality of their ingredients (brittle minerals and soft proteins). Nacre from mollusk shells is 3,000 times tougher than the brittle mineral it is made of, a level of toughness amplification currently unmatched by any engineering material. For this reason, nacre has become the model for bioinspiration for novel structural materials. The structure of nacre is organized over several length scales, but the microscopic brick-and-mortar arrangement of the mineral tablets is prominent. This staggered structure provides a universal approach to arranging hard building blocks in nature and is also found in bone and teeth. Recent models have demonstrated how an attractive combination of stiffness, strength, and toughness can be achieved through the staggered structure. The fabrication of engineering materials that duplicate the structure, mechanics, and properties of natural nacre still present formidable challenges to this day. FIGURE 3.2 Tensile stress-strain behavior of nacre. Adapted from Ref. 14.
This paper presents a material model suitable for simulating the behavior of dry fabrics subjecte... more This paper presents a material model suitable for simulating the behavior of dry fabrics subjected to ballistic impact. The developed material model is implemented in a commercial explicit finite element (FE) software LS-DYNA through a user defined material subroutine (UMAT). The constitutive model is developed using data from uniaxial quasi-static and high strain rate tension tests, picture frame tests and friction tests. Different finite element modeling schemes using shell finite elements are used to study efficiency and accuracy issues. First, single FE layer (SL) and multiple FE layers (ML) were used to simulate the ballistic tests conducted at NASA Glenn Research Center (NASA-GRC). Second, in the multiple layer configuration, a new modeling approach called Spiral Modeling Scheme (SMS) was tried and compared to the existing Concentric Modeling Scheme (CMS). Regression analyses were used to fill missing experimental data – the shear properties of the fabric, damping coefficient and the parameters used in Cowper-Symonds (CS) model which account for strain rate effect on material properties, in order to achieve close match between FE simulations and experimental data. The difference in absorbed energy by the fabric after impact, displacement of fabric near point of impact, and extent of damage were used as metrics for evaluating the material model. In addition, the ballistic limits of the multi-layer fabrics for various configurations were also determined.
Teleost Fish skin Fish scale Scalation pattern Puncture resistance Biomimetics Body armor a b s t... more Teleost Fish skin Fish scale Scalation pattern Puncture resistance Biomimetics Body armor a b s t r a c t
The objective of this research was to study the behavior of 3D AR glass fabric cement-based compo... more The objective of this research was to study the behavior of 3D AR glass fabric cement-based composites under impact loading. It was found that 3D fabrics significantly improve the toughness and energy absorption of cement-based composites under impact loading, compared to short AR glass fibers reinforcement. The 3D fabric improves the toughness in as high as 200 folds compared to short fiber composites. The energy absorption was highly affected by the thickness of the element and the location of the 3D fabric faces.
Woven fabrics are used in many applications, including ballistic armors, propulsion engine contai... more Woven fabrics are used in many applications, including ballistic armors, propulsion engine containment systems and fabric reinforced composites. In order to facilitate the design and improvement of such applications, this paper investigates the stress–strain response in warp and fill directions, the apparent Poisson’s ratio, and the in-plane shear response of Kevlar 49 fabric including the possible effects of specimen size and pre-loading on the mechanical responses of the fabric. Full-field image analysis of the fabric under shear deformation is used to better understand the mechanisms related to in-plane shear. The experimental results show that the fabric exhibits non-linear and orthogonal behavior in tension, and can deform up to 20% before complete failure. It has identical Young’s modulus (pre-peak elastic stiffness) in warp and fill directions, but has different crimp strain, tensile strength and ultimate strain. The apparent Poisson’s ratio is a nonlinear function of strain and dependent upon the levels of pre-loading. It increases with strain quickly at the beginning and decreases gradually until the fabric fails. The shear response is highly nonlinear and has four distinct regions: linear elastic rotation region, dissipative rotation region, yarn compression region and shear locking region, and it is not dependent upon specimen size after normalization.
In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both ... more In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10−4 s−1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s−1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.
The toughness of fiber-reinforced composites largely relies on crack bridging. More specifically,... more The toughness of fiber-reinforced composites largely relies on crack bridging. More specifically, intact fibers left behind the tip of a propagating crack are progressively pulled out of the matrix, dissipating energy which translates into toughness. While short fibers are traditionally straight, recent work has showed that they can be shaped to increase the pullout strength, but not necessarily the energy to pullout. In this work we have modeled, fabricated and tested short fibers with tapered ends inspired from a high-performance natural material: nacre from mollusc shells. The main idea was to duplicate a key mechanism where a slight waviness of the inclusion can generate strain hardening and energy dissipation when the inclusion is pulled out. We have incorporated a similar feature to short fibers, in the form of tapered ends with well defined opening angles. We performed pullout tests on tapered steel fibers in epoxy matrices, which showed that the pullout of tapered fiber dissipates up to 27 times more energy than straight fibers. The experimental results also indicated the existence of an optimum taper angle to maximize work of pullout while preventing the brittle fracture of the matrix. An analytical model was developed to capture the pullout mechanism and the interaction between fiber and matrix. The analytical model can guide the design of tapered fibers by providing predictions on the influence of different parameters.
Nacre from mollusk shell is a high-performance natural composite composed of microscopic mineral ... more Nacre from mollusk shell is a high-performance natural composite composed of microscopic mineral tablets bonded by a tough biopolymer. Under tensile stress, the tablets slide on one another in a highly controlled fashion, which makes nacre 3000 times tougher than the mineral it is made of. Significant efforts have led to nacre-like materials, but none can yet match this amount of toughness amplification. This article presents the first synthetic material that successfully duplicates the mechanism of tablet sliding observed in nacre. Made of millimeter-size wavy poly-methylmethacrylate tablets held by fasteners, this "model material" undergoes massive tablet sliding under tensile loading, accompanied by strain hardening. Analytical and finite element models successfully captured the salient deformation mechanisms in this material, enabling further design refinements and optimization. In addition, two new mechanisms were identified: the effect of free surfaces and "unzipping." Both mechanisms may be relevant to natural materials such as nacre or bone.
This paper presents the development of a test procedure and application of non-contacting strain ... more This paper presents the development of a test procedure and application of non-contacting strain measurement for cement-based composites under moderately high strain rate tensile tests. The strain time histories of test specimens measured by a laser extensometer in high speed mode were derived by a phase-shift technique based on zero-crossing method. The accuracy of the linear variable differential transformer (LVDT) of the actuator in a servo-hydraulic high rate testing machine was verified by image analysis using sisal fiber reinforced cement composite at a strain rate of 25 s−1. The same procedure was then applied to Alkaline Resistant (AR) glass fabric reinforced cement composite tested at an average strain rate of 17 s−1. Comparison between the strain values measured by the laser extensometer and the LVDT shows a good agreement between these two measurement techniques. The test results show that the Young’s modulus, tensile strength, maximum strain, and toughness of the AR-glass fabric–cement composite increase with increasing strain rate. However, under both static and dynamic loadings the composite has similar behavior: multi-crack development and one dominant crack leading to final failure. In order to ensure the accuracy of dynamic tensile test procedures, non-contacting devices and techniques should be used as an independent means of verification of test results. The accuracy required in quantifying relative improvements in mechanical properties necessitates the various methods of measuring the displacement and strain rate properties.
Dynamic tensile tests were conducted using a high speed servo-hydraulic testing machine on three ... more Dynamic tensile tests were conducted using a high speed servo-hydraulic testing machine on three types of fabric reinforced cement composites. The high speed testing procedure and data processing method are presented. Quasi-static tests were also conducted on the composites. Effects of strain rate on the mechanical properties of fabric–cement composites are noted. A good correlation was found between the properties of the fabrics and the composites, with the carbon fabric exhibiting the highest strength and ductility performance in high speed tensile tests. The differences in tensile behavior of the various composites were correlated with the differences in the role of the fabric materials. Composites tested under high speed loading exhibited different responses as compared to similar composites tested under quasi-static condition.
Fabric–cement composites developed using the pultrusion production process have demonstrated impr... more Fabric–cement composites developed using the pultrusion production process have demonstrated impressive tensile and flexural properties. For instance fabric reinforced composites with bonded Alkali Resistant (AR) glass fabrics exhibit strain-hardening behavior, tensile strength in the range of 20–25 MPa, and strain capacity of the order of 2–5% under static conditions. Properties of these composite systems were investigated under three point bending conditions using an instrumented drop weight impact system. Samples were studied from the viewpoint of the variations of impact load, deflection response, acceleration and absorbed energy. Development of the testing system in terms of components and acceleration response are discussed in detail. Methods of the impact load measurement using three different ways of acceleration response, piezoelectric load washer and conventional strain gage based load cell are discussed. Cement composites with two different fabric contents and four different drop heights of hammer (dropping mass) were tested. Experimental results indicate that for the same drop height, the stiffer beam type specimens have a lower ultimate deflection but a higher load carrying capacity than the plate type specimens. The maximum flexural stress and absorbed energy of composites increase with drop height. In beam specimens, complete fracture does not take place as cracks form and close due to rebound and significant microcracking in the form of radial fan cracking is observed, whereas interlaminar shear is the dominant failure mode in the plate specimens.
In this research project the behaviour of strain-hardening cement-based composites (SHCC) subject... more In this research project the behaviour of strain-hardening cement-based composites (SHCC) subjected to low and high strain rates was studied. Uniaxial tension tests on dumbbell-shaped SHCC specimens were performed at rates ranging from 10 -5 s -1 to 50s -1 . For the tests performed at strain rates of 10 -2 s -1 and below, SHCC yielded a moderate increase in tensile strength and simultaneous decrease in strain capacity with increasing strain rate. When tested for higher strain rates from 10 to 50s -1 a considerable increase in tensile strain and strain capacity was measured. Microscopic investigation of the fracture surfaces showed that almost no fibre failure and an average pullout length of 2.5mm were found in the high strain rate test. This observation is in contrast to that of rapid quasi-static testing, where the average fibre pullout length of 300μm was much shorter. Furthermore, the fibres on the fracture surfaces produced in the high rate tests exhibited pronounced plastic deformations. Finally, quasi-static and high-speed tension tests on individual fibres and single fibre pullout tests were performed. While the increase in the tensile strength of the fibre was only moderate in the range of strain rates investigated, a considerable increase in bond strength between fibre and matrix was determined.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Jan 1, 2010
The experimental behavior of sisal fiber reinforced cement composites subjected to high speed ten... more The experimental behavior of sisal fiber reinforced cement composites subjected to high speed tension load was studied. High strain rates were achieved by using a high rate servo-hydraulic testing machine. A state-of-the-art high speed Phantom camera was also used to take images from the specimen during the test. The images were used in a digital image correlation model to determine the displacement fields and to calculate crack spacing. The effect of strain rate was investigated by comparing static and dynamic tensile tests which were performed at strain rates ranging from 5.5 × 10−6 to 24.6 s−1, respectively. A numerical tension stiffening model based on nonlinear finite difference method was used to simulate tensile cracking behavior of sisal fiber cementitious composites. The composite presented strain rate sensitivity for ultimate tensile strength and strain capacity with a dynamic amplification factor of 1.26.
The impact response of unidirectional continuous sisal fiberreinforced cement composites was inve... more The impact response of unidirectional continuous sisal fiberreinforced cement composites was investigated. An impact test setup based on a free-fall drop of an instrumented hammer on a three-point bending configuration test was used. The effect of impact energy on the composite response was investigated by using three different drop heights. Mechanical properties obtained under impact loading were compared with static three-point bending tests. A comparison with the impact behavior of glass fabricreinforced composites was addressed. Furthermore, damage mechanisms were investigated by characterizing the cracking patterns during the impact event using a high-speed digital camera. It was observed that while the maximum flexural stress values were in the same range for static and impact tests, the absorbed energy increased up to a maximum level of 0.0090 kJ/in. 2 (14.0 kJ/m 2 ) for the impact energy of 20 J. The damage process was initiated by tensile and shear cracks. Delamination of the plies within the continuous fiber composites was observed only when the impact energy was 27 J.
This paper presents the results of an experimental investigation into the strength, deformation, ... more This paper presents the results of an experimental investigation into the strength, deformation, and fracture behaviour of textile-reinforced concrete (TRC) subjected both to low and high-rate tensile loading ranging from 0.0001 to 50 s−1. High strain rates were achieved using a high-rate servo-hydraulic testing machine. The effect of the addition of short fibres on the static and dynamic response of TRC has been investigated, and the microstructure of both composite and fibre was observed after the tests using an ESEM. An increase in tensile strength, strain capacity, and work-to-fracture was observed for strain rates up to 0.1 s−1 with increasing strain rate. The addition of short glass fibres increased the tensile strength and first crack strength of the TRC. For high-speed tests (rates above 5 s−1) an increase in the tensile strength, first crack strength and work-to-fracture was also observed, but at the same time there was a decrease in the strain capacity. The tests at high loading rates showed a pronounced effect of the specimen length on the measured mechanical properties: with increasing gauge length the tensile strength and strain capacity decreased, while the work-to-fracture increased.▶ Tensile strength, strain capacity, and work-to-fracture increased when ε˙≤0.1 s−1. ▶ When ε˙≤5 s−1 a decrease in the strain capacity was observed. ▶ Different fibre-fracture morphologies were observed for high and low strain rates. ▶ The addition of short fibres increased the tensile strength and the crack stress.
Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydrau... more Dynamic tensile tests are conducted on aluminum alloy (AA) 6061-T6 using a high-speed servohydraulic machine at intermediate strain rates to validate the testing technique and to investigate the strain-rate effect on the material's stress-strain behavior and failure mode. We present the experimental procedures and results discussing the constitutive response of the alloy at strain rates up to approximately 200 s À1 . The predominant frequencies of the high-speed testing machine were characterized by modal analysis, and we analyzed the effect from vibration of the system and loading rate on flow stress by using a single degree-of-freedom (SDOF) spring-mass-damper model. We tested two different specimen sizes at a wide range of actuator velocities to achieve the desired strain rates. Results show that the yield strength, ultimate strength, and failure strain were dependent on strain rate. We fitted the data to the Johnson-Cook (JC) constitutive model, and the resulting parameters are comparable to published results in the literature. The fracture surface of specimens tested at different strain rates obtained by scanning electron microscopy (SEM) showed that the ductile failure mode was dominant for the alloy, and strain rates within the range examined affected the fracture morphology.
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Papers by Deju Zhu