Development of the split-Hopkinson pressure bar technique for viscous fluid characterization

The split Hopkinson pressure bar (SHPB) technique has been employed to evaluate the dynamic squeeze flow behavior of viscous Newtonian fluids. In this paper, the conditions under which classic Hopkinson bar data analysis is applicable for fluid specimens are discussed in detail. Requirements include the development of a parabolic flow profile and associated pressure distribution across the specimen. The times required for these processes to occur are calculated and compared with the experimental timescale in order to establish a specimen design criterion for valid SHPB testing. To evaluate this design criterion, an isothermal squeeze flow model describing the behavior of a cylindrical fluid specimen which includes inertial forces is used to predict the experimental results for a model Newtonian fluid. Good agreement between the theory and the experiment is obtained for thin specimens (1.0 mm) across a wide range of shear strain rates (over 10⁵ s⁻¹). As a result of this study, the conditions under which valid SHPB experimental results may be obtained for a Newtonian fluid specimen are identified.     

Influence of specimen size and inner defects on high strain rates compressive behaviors of plain woven composites

This paper reports the influence of specimen size and inner defects on high strain rates compressive behaviors of plain woven composites. The compressive behaviors of plain woven composites along out-of-plane direction were investigated from experimental and numerical approaches. In experimental, the compressive stiffness and strength decreased as the size of plain woven composite specimens increased. In finite element analysis (FEA) model, a new microstructure model with random defect distribution was established to find the influence of inner defects and specimen size effect on the compressive behaviors under high strain rates. From the numerical results, the compressive strength, modulus and fracture strain decreased obviously with the increase of volume fraction and size of defects. We found that the good agreement existed between the testing and the FEA results. The defects size and distribution were the main factors to weaken the compressive stiffness and strength.     

High strain rate out-of-plane compression properties of aramid fabric reinforced polyamide composite

The composite-structure protective systems in head-on collision with objects are largely subjected to dynamic compression load along the thickness of composite structure. A typical plain weave aramid fabric reinforced polyamide (PA) composite, which is defined as one of single polymer composites (SPCs), is addressed in this paper. Firstly, in the process of sample preparation, processing characteristics of the single polymer composites are skillfully achieved and discussed using differential scanning calorimetry (DSC) and capillary rheometer. Secondly, the out-of-plane compression properties of the composite are studied on Split Hopkinson Pressure Bar (SHPB) apparatus in the strain rate range of 400–1200s⁻¹. Effects of fiber content and strain rate on dynamic off-plane compression properties are investigated and quasi-static properties are obtained on a universal testing machine as a comparison. Results provide a basis for selecting composite composition and lay-up for designing armor with improved impact resistance. Additionally, penetration of the resin through the fabric is observed by the digital microscope and the internal damage of the laminates is qualitatively predicted by the microstructure of the internal fabric yarns.     

Synthesis and characterization of the physical, chemical and mechanical properties of isocyanate-crosslinked vanadia aerogels

A strong lightweight material (X-VOx) was formulated by nanocasting a conformal 4 nm thin layer of an isocyanate-derived polymer on the entangled worm-like skeletal framework of typical vanadia aerogels. The mechanical properties were characterized under both quasi-static loading conditions (dynamic mechanical analysis, compression and flexural bending testing) as well as high strain rate loading conditions using a split Hopkinson pressure bar (SHPB). The effects of mass density, moisture concentration and low temperature on the mechanical properties were determined and evaluated. Digital image correlation was used to measure the surface strains through analysis of images acquired by ultra-high speed photography, indicating nearly uniform compression at all stages of deformation during compression. The energy absorption of X-VOx was plotted as a function of the density, strain rate and temperature, and compared with that of plastic foams. X-VOx remains ductile even at ?180 °C, a characteristic not found in most materials. This unusual ductility is derived from interlocking and sintering-like fusion of nanoworms during compression. X-VOx emerges as an ideal material for force protection under impact.     

Mechanical behavior of basalt and glass textile composites at high strain rates: A comparison

The aim of this paper is to study and compare the mechanical behavior of woven basalt and woven glass epoxy composites at high strain rates, in order to assess the possibility of replacing glass fiber composites with basalt fiber composites for aircraft secondary structures, such as radomes, fairings, wing tips, etc. Both composites were produced using the same epoxy matrix, the same manufacturing technique, and with comparable densities, fiber volume fractions, and static stiffnesses. Dynamic tensile and shear experiments were performed using a split Hopkinson tension bar, in addition to reference quasi-static experiments to compare both material behaviors over a wide range of strain rates. Normalized results with respect to the material density and fiber volume fraction showed that basalt epoxy composite had higher elastic stiffness, ultimate tensile strength, ultimate tensile strain, and absorbed energy in tension compared to glass epoxy composite. This suggests a promising potential in replacing glass fibers composites with basalt fiber composites in aircraft secondary structures and, more generally, components prone to impact. However, for the basalt epoxy composite, improvements in the fiber-matrix adhesion and in the manufacturing technique are still required to enhance their shear properties compared to glass fiber composites, and fully exploit the potential of basalt epoxy composites in aeronautical applications.     

High strain rate in-plane shear behavior of composites

Studies are presented on in-plane shear properties of a typical plain weave E-glass/epoxy composite under high strain rate loading. In-plane shear properties were determined with ±45 degree off-axis compression and tension tests using a split Hopkinson pressure bar apparatus. In-plane shear properties are presented as a function of axial and shear strain rates. The range of axial strain rates for off-axis compression tests was 819–2003 per sec, and for off-axis tension tests was 91–180 per sec, whereas the range of shear strain rates for off-axis compression tests was 1388–3442 per sec and for off-axis tension tests was 153–303 per sec. In general, it was observed that in-plane shear strength was enhanced at high strain rate loading compared to that at quasi-static loading. Also, it was observed that in-plane shear strength increased with increasing strain rate within the range of strain rates considered.     

Strain rate sensitive compressive response of gelatine: Experimental and constitutive analysis

Gelatine is widely used as a soft tissue simulant in physical surrogates for the human body. Historically, gelatine has been used to evaluate penetrating impacts and, more recently, to evaluate blunt impact and blast loading effects on soft tissue. There is a need for material characterisation data across a wide range of strain rates, and appropriate constitutive relationships that can be used in models, particularly finite element models, to accurately predict the response of gelatine under various loading conditions. In this study, dynamic experiments were conducted using a split Hopkinson pressure bar and quasi-static tests performed on gelatine to investigate its compressive stress-strain response at both quasi-static and dynamic rates of deformation. The experimental results show that gelatine exhibits rate-sensitive and nonlinear behaviour. The Zhu-Wang-Tang constitutive model can adequately describe the rate-sensitive compressive behaviour of gelatine as good agreement was found between experimental results and model prediction.     

Effect of thermal cycling and open-hole size on mechanical properties of polymer matrix composites

This paper investigates the effects of thermal cycling on mechanical degradation of polymer matrix composites (PMCs). Un-notched and open-hole specimens are tested using developed thermal cycling apparatus and tensile test machine. In addition, the hole-size effect of open-hole tension glass/epoxy composite laminates is investigated. The tensile strength, mass loss and surface degradation of the specimens were obtained during 250 cycles. Experimental results showed that the holes diameter is the main parameter to control the thermal cycling effects on open hole structure. Also, it is found that laminates with smaller holes have higher tensile strength variation than those with larger holes. The results showed that increment of the hole diameter and number of cycles decreases the tensile strength.     

Influence of strain rate on the compressive yield stress of CMDB propellant at low,intermediate and high strain rates

The mechanical properties of composite modified double base (CMDB) propellant significantly depend on the strain rate. In particular, the yield stress increases dramatically at higher strain rates. To study this behaviour, low, intermediate and high strain rate compression testing (1.7 × 10⁻⁴ to 4 × 10³ s⁻¹) of CMDB propellant at room temperature was conducted by using a universal testing machine, a hydraulic testing machine and a split Hopkinson pressure bar (SHPB) system, respectively. The yield stress was observed to increase bilinearly with the logarithm of strain rate, with a sharp increase in slope at a strain rate of 5 × 10¹ s⁻¹, which was supported by dynamic mechanical analysis (DMA) testing. The Ree-Eyring model, involving two rate-activated processes, was employed to predict the yield behaviour of CMDB propellant over a wide range of strain rates. The predictions are in excellent agreement with the experimental data.     

Characterization of 3D angle-interlock thermoplastic composites under high strain rate compression loadings

In the present work, dynamic compression response of polypropylene (PP) based composites reinforced with Kevlar/Basalt fabrics was investigated. Two homogeneous fabrics with Kevlar (K3D) and Basalt (B3D) yarns and one hybrid (H3D) fabric with a combination of Kevlar/Basalt yarns were produced. The architecture of the fabrics was three-dimensional angle-interlock (3D-A). Three different composite laminates were manufactured using vacuum-assisted compression molding technique. The high strain rate compression loading was applied using a Split-Hopkinson Pressure Bar (SHPB) set-up at a strain rate regime of 3633–5235/s. The results indicated that the dynamic compression properties of thermoplastic 3D-A composites are strain rate sensitive. In all the composites, the peak stress, toughness and modulus were increased with strain rate. However, the strain at peak stress of Basalt reinforced composites (B3D, H3D) decreased approximately by 25%, while for K3D specimens it increased approximately by 15%. The K3D composites had a higher strain rate as compared to the B3D and H3D composites. In the case of K3D composite, except strain at peak stress, remaining dynamic properties were lower than the B3D composite, however, hybridization increased these properties. The failure mechanisms of 3D-A composites were characterized through macroscopic and scanning electron microscopy (SEM).     

Strain and damage sensing in additively manufactured CB/ABS polymer composites

In this study, an experimental investigation is performed to observe the electromechanical response of CB (carbon black)/Acrylonitrile butadiene styrene (ABS) additive manufactured composite under quasi-static (tensile, shear, and mode-I fracture) and dynamic (mode-I fracture) loading conditions for the potential damage sensing applications. Dog bone tensile, double v-notch shear, and single edge notch bending (SENB) specimen printed with three different configurations (0°/90°, +45°/-45°, and 0°) are considered for the quasi-static condition. A modified split Hopkinson pressure bar along with high-speed video camera is used for dynamic fracture experiments. Four-point probe technique coupled with a high-resolution data acquisition system is employed to obtain the real-time electrical response. In the case of tensile loading, +45°/-45° printed specimens show a nonlinear change of electrical resistance due to unique failure mode. Under the shear loading, electrical resistance remains unchanged during the elastic deformation. After the damage evolution, +45°/-45° printed specimens exhibit a higher rate of change in electrical resistance due to alignment of the filaments along the maximum principle shear stress direction. For both static and dynamic fracture loading, a minimal change of electrical resistance is observed before crack initiation. However, after the crack initiation, a sharp change of electrical resistance for 0°/90° printed specimens indicates a faster crack propagation as compared to the +45°/-45° printed specimens.     

Investigation of sub‐micron size cenosphere fillers and filler loading on the mechanical and tribological peculiarity of polyester composites

This paper investigates the effect of sub‐micron size cenosphere filler and filler loading on mechanical and dry sliding wear property of polyester composites. Composites are fabricated by filling with 10 and 20 wt% of 800 and 200‐nm size of cenosphere filler particles. Neat polyester composite is also prepared for comparison analysis. Dry sliding wear test is conducted for these composites over a range of sliding distance with different sliding velocities and applied loads on a pin‐on‐disc wear test machine. Taguchi methodology and analysis of variance (ANOVA) is used to analyze the friction and wear characteristics of the composites. The artificial neural network (ANN) approach is implemented to the friction and wear data for corroboration. In this work, mechanical properties of composites such as hardness, tensile strength, tensile modulus, flexural strength, and compressive strength revealed that mechanical properties and wear resistance of the composites increase with a decrease in the particle size. The measured Young's moduli are comparable to standard theoretical prediction models. The morphology of worn composite specimens has been examined by scanning electron microscopy to understand the dominant wear mechanisms. Finally, optimal factor settings are determined using a genetic algorithm (GA). Copyright © 2017 John Wiley & Sons, Ltd.     

Tension testing of polycarbonate at high strain rates

Dynamic tensile tests were performed on polycarbonate using a split Hopkinson tension bar (SHTB) system. A prefixed short metal bar was used to generate the incident stress pulse. The shape of the incident pulse was controlled to meet the requirement of the one-dimensional experimental principle of SHTB. The dynamic tensile stress–strain responses of polycarbonate at high strain rates up to a rate of 1750 s⁻¹ were obtained. Experimental results indicate that the tensile behavior of polycarbonate is dependent on the strain rate. Its yield stress and unstable strain all increase with the increased strain rate. The yield behavior was modeled for a wide range of strain rates based on the thermally activated theory. The correlation between the experimental data and the model is good.     

Comparative mechanical property characterization of three indirect composite resin materials compared with two direct composites

Various new indirect composite materials have been developed with required advantages. In this study three indirect composite material (Artglass, Belleglass HP, Targis) were tested for flexural strength, fracture toughness, wear resistance and hardness against Filtek P60 and Z‐100. Five specimens of each material were fabricated according to the manufacturer's directions. The flexural strength and fracture toughness was measured using the bending test. The wear test was performed to accelerated wear in a toothbrushing apparatus. Vickers hardness was measured for each of the tested materials. The statistical tests used for flexural strength, fracture toughness, wear and hardness were One‐way ANOVA and Kruskal–Wallis test. The level of statistical significance chosen was p = 0.05. Results of the study showed that Filtek P60 was superior to the other composites in all tests. Significant differences were found among the materials. The differences in flexural strength, fracture toughnes, wear and hardness may have been due to differences in chemistry or method of polymerization of the composites. Copyright © 2003 John Wiley & Sons, Ltd.     

Compression yielding of polypropylenes above glass transition temperature

Yielding behaviour under compressive loading of two materials based on polypropylene, an isotactic homopolymer and an ethylene-propylene block copolymer, is studied at different strain rates and temperatures. Quasi-static tests, performed in electromechanical machines, and dynamic tests, carried out in a Hopkinson bar, were compared and simultaneously analyzed to generate a master curve representative of the material yielding, assuming the strain rate-temperature superposition principle. Experimental data were fitted to equations based on the cooperative model for semi-crystalline polymers.     

Preparation,characterization and properties of fiber reinforced composites using silicon‐containing hybrid polymers

A novel glass fiber reinforced composite was prepared by using silicon‐containing hybrid polymers, poly(methylhydrogen‐diethynylsilyene) (PMES) and poly(phenylethynyl‐silyloxide‐phenylborane) (APABS), as matrix resins. The curing behavior and rheological properties of the matrix resins were investigated by differential scanning calorimetry (DSC) and rotational rheometer. The dynamic viscoelastic properties, mechanical properties, and microstructures of the composites were studied by dynamic mechanical analysis (DMA), universal testing machine (UTM), and scanning electron microscopy (SEM), respectively. The results show that the composite can be well cured between 200 and 300 °C through reactive groups like Si‐H, N‐H, and C≡C units, the possible thermosetting mechanism is also proposed. The composites exhibit excellent mechanical properties with bending strength reach up to 261 and 178 MPa before and after heat‐treating, respectively. SEM analysis clearly indicates that crack in the matrix, matrix/fiber interface debonding, and fiber pull out are predominate failure mechanism for the composites which are heat‐treated in different temperatures. All these obtained results can give theoretical guiding reference for their further applications. Copyright © 2016 John Wiley & Sons, Ltd.     

The role of organoclay on the properties of Polymethylsilsesquioxane: A systematic study

In this study, an attempt is made to improve the properties of PMSQ, an organosilicone polymer which possesses distinguished properties, through an easy and facile route by the inclusion of organically modified montmorillonite clay. PMSQ-clay composites were prepared by solution blending of the components initially and then heat curing under load. The effect of clay content, varied at 5–40 wt.%, on mechanical, thermal and dynamic mechanical properties was evaluated and the optimum was obtained for 20%. Morphology investigation as well as microstructure analysis revealed intercalated to exfoliated morphology of PMSQ-clay composite. An appreciable improvement in mechanical properties of PMSQ, compressive strength and impact strength in particular, was achieved by clay inclusion up to 20%. The properties declined at ≥ 30% clay loading. The composites showed increased thermal stability compared to unmodified PMSQ up to 400 °C. Also, increase in clay content accelerated conversion to ceramic SiOC. PMSQ-clay composites exhibited good visco-elastic characteristics with higher T_g probably due to enhanced polymer-clay interactions. Thus, a simple and viable method to enhance the mechanical and thermal characteristics of PMSQ by way of preparing its composite with the reinforcing filler organoclay is demonstrated here.     

Experimental assessment of stiffness and energy dissipation properties of disk-shaped polymer-based composite specimens by in-plane torsion testing

Torsion testing machines are widely used either to measure the strength, stiffness and stress-strain properties of materials or to replicate real-life service conditions. In this paper, a novel experimental method is presented, based on the development of a dedicated steel structure to be used in conjunction with a universal testing machine. This equipment allows applying cyclic in-plane torsion loads on disk-shaped components. The proposed approach aims to enable the assessment of stiffness and damping properties on specimens enabling the application of higher loads in comparison with the traditional machines.Specifically, dynamic trials were performed by attaching the composite specimens and the steel structure to the testing machine, such that the uniaxial controlled displacements can be used to exert the desired cyclic loads onto the specimen. Both torsional stiffness and energy loss were measured from the steady-state load-displacement hysteresis cycles. Amplitudes of sine signals, from 0.05 to 0.2 mm, and a frequency ranging between 1 and 20 Hz, were used in the experiments. The results are presented comparing the behaviour of two polymer matrix composites, with the same number of identical laminae, but characterized by different stacking sequences, namely unidirectional and quasi-isotropic configurations.     

An experimental investigation of wear of glass fibre-epoxy resin and glass fibre-polyester resin composite materials

In this paper, the effects of resin content on the wear of woven roving glass fibre-epoxy resin and glass fibre-polyester resin composite materials have been examined. Furthermore, composite materials are experimentally investigated under different loads and speeds by using a block-on-shaft wear tester. The influences of two thermosetting resins epoxy and polyester on the wear of glass-woven roving reinforced composites under has been investigated dry conditions. The glass fibre-epoxy resin and the glass fibre-polyester resin composite materials specimens have been tested under different experiment conditions. Tests were conducted for 0.39 and 0.557 m/s speeds, at two different loads of 5 and 10 N. The weight losses were measured after measuring different sliding distances. Wear in the experiments was determined as weight loss. For each experiment, one specimen was used. The amount of wear was measured before the experiment and after the experiment with the apparatus of balance scales with the accuracy of 10⁻³ g. Glass fibre-epoxy resin composites generally showed higher strength and minimum wear when compared with glass fibre-polyester resin composites materials. In addition, Scanning electron microscopy (SEM) is used to study the worn surface to verify the results.     

Impact testing of polymeric foam using Hopkinson bars and digital image analysis

The present investigation aims at testing polymeric foam under impact loading using large diameter nylon Hopkinson bars and optical field measurements. Accurate average stress-strain relations can be obtained when soft large diameter polymeric pressure bars and the appropriate data processing are used. However, as there are generally no homogeneous strain and stress fields for polymeric foams, an optical field observation is needed. In contrast to quasi-static tests where the digital image correlation (DIC) measurement is commonly used, technical difficulties still remain for the reliable use of DIC under impact conditions. In this paper, an accurate synchronization method based on the displacement measurement of the end of pressure bars (calculated by a robust DIC algorithm) is preferred to conventional MCDL box time synchronization. Also, the bar end displacement measurement offers a complementary calibration method for the tension/strain conversion coefficient. Strain fields are obtained for tests on foam sample at impact velocities up to 20 m/s. The localized strain fields permit better understanding of the observed stress plateau from SHPB results. The relevance of the present method for establishing mechanical response of polymeric foam is then demonstrated.