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.     

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.     

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.     

The Dynamic Impact Experiments Under Active Confining Pressure and the Constitutive Equation of PP/PA Blends at Multi-Axial Compressive Stress State

Summary : A special active hydraulic confining pressure installation matched with Φ14.5 mm SHPB apparatus was developed. A series of active confining pressure impact experiments for PP/PA blends are performed in this special SHPB system under two kinds of axial strain rate: 8.0*10², 1.4*10³ s⁻¹ and the active confining pressure of 0 MPa, 4 MPa, 8 MPa, 12 MPa, 15 MPa, 20 MPa. The axial strain-time profile, the axial stress-time profile and the hoop strain-time profile of the specimen are recorded online respectively. According to the equilibrium equation, the complete state of principal stress and principal strain of PP/PA blends under multi-axial stress state is analyzed. The experimental results reveal that the axial stress-strain curves all are related to the confining pressure and the strain rate. It can also be seen that under a constant effective strain rate the effective stress- effective strain curves at different confining pressures are coincident basically. This manifests that under a certain effective strain rate there is only one unique effective stress- effective strain curve. The multi-axial constitutive equation for PP/PA blends is suggested finally as: where σ_conf is the confining pressure value. For 113 PP/PA blends, E = 2.98 GPa, α = −31.15 GPa, β = 93.31 GPa, θ₂ = 8.54 µS, E₂ = 0.82 GPa.     

关于一维波向有机玻璃透射声抗偶合的粘弹效应

用SHPB技术测定了20—60℃范围内,有机玻璃的应力-应变曲线。用标准线性固体粘弹模型讨论了有机玻璃log(t) 随时间近似线性衰减的特性,得到反映声抗偶合的参数κ与粘弹性常数b=Ε’/η’相一致的结论。由κ值对温度的依赖性讨论了有机玻璃声抗偶合的粘弹效应。     

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.     

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.     

数字图像相关技术在应力应变测量中的发展与最新应用

在现代材料的应力应变测量中,数字图像相关测量法具有非接触、全场性、对实验条件要求低、精度高等优点,是一种有效、实用的应力应变测量方法,被广泛的应用于实验力学及其他学科领域。本文综述了数字图像相关技术在材料应力应变方面的发展;将数字图像相关技术与其它测量技术进行了比较;介绍了数字图像相关技术的系统结构与相关算法、搜索方法;最后列举了数字图像相关技术在材料应力应变中的最新应用和新产品。     

High strain rate behavior of graphene-epoxy nanocomposites

This work consists of the synthesis of high purity graphene nanoflakes (GNF), the manufacturing of GNF-epoxy nanocomposites and the mechanical characterization of the nanocomposite at high and quasi static strain rates, (2750/s - 1.E−5/s). GNF were synthesized by using the electric arc discharge technique. Thermogravimetry/Differential Thermal Analysis (TG/DTA) of synthesized graphene reveals high purity and high crystallinity. Raman spectra and the broad Brunauer-Emmet-Teller (BET) specific surface area indicate that the synthesized graphene has several layers. Following the solution mixing manufacturing process of GNF-epoxy nanocomposites, the influences of strain rate on the mechanical behaviors are investigated under quasi static and dynamic loadings. High strain rate uniaxial compression tests (1270–2750/s) using Split Hopkinson Pressure Bar (SHPB) and quasi static compression tests (1.E−3 and 1.E−5/s) of GNF-epoxy with two graphene contents (0.1 and 0.5 wt %) are performed at room temperature. The maximum elasticity modulus achieved by the GNF-epoxy with 0.5 wt% at the strain rate of 2350/s corresponds to a 68% increase compared to the neat epoxy. The yield strength of the material is doubled under dynamic loading conditions compared to the quasi static loading.     

Dependency of dynamic interlaminar shear strength of composites on test technique used

Studies are presented on dependency of dynamic interlaminar shear (ILS) strength on the experimental technique used for a typical plain weave E-glass/epoxy composite. Dynamic ILS strength was determined based on two experimental techniques, namely torsional split Hopkinson bar (TSHB) apparatus using thin walled tubular specimens and compressive split Hopkinson pressure bar (SHPB) apparatus using single lap specimens. The results obtained from these techniques are compared. In general, it is observed that dynamic ILS strength for composites obtained by TSHB testing using thin walled tubular specimens is lower than the dynamic ILS strength obtained using single lap specimens in compressive SHPB. The issues involved in TSHB testing of thin walled tubular specimens made of composites are discussed and the reasons for reduced dynamic ILS strength using thin walled tubular specimens are highlighted. Finite element analysis (FEA) of thin walled tubular specimens made of composite and resin subjected to quasi-static torsional loading is presented. Using FEA results, the reasons for lower ILS strength of composite thin walled tubular specimens are substantiated.     

Delayed yielding of polycarbonate under constant load

The time-dependent yielding of glassy polycarbonate subjected to constant tensile loads has been studied. Application of a constant stress of a magnitude between the yield stress and the stress required for propagation of a neck in constant strain tests results in inhomogeneous yielding after a well-defined time lag. This delay time increases with decreasing stress and temperature. The critical stress for slowly cooled material is greater than that for quenched material in which the delay time is divided in two regions. The delay time is regarded as the time required for the initiation of inhomogeneous yielding at either edge of the specimen and growth over a certain distance across the specimen. Geometrical observations revealed that the inhomogeneous yielding is shear yielding which is initiated due to stress inhomogeneities caused by mechanical imperfections at the edge of the specimen. The Eyring treatment of delayed yielding can describe fairly well the stress and temperature dependence of the delay time.     

Experimental set-up for determination of the large-strain tensile behaviour of polymers at low temperatures

In this study, we present a method to determine the large-strain tensile behaviour of polymers at low temperatures using a purpose-built temperature chamber made of polycarbonate (PC). This chamber allows for several cameras during testing. In our case, two digital cameras were utilized to monitor the two perpendicular surfaces of the test sample. Subsequently, the pictures were analysed with digital image correlation (DIC) software to determine the strain field on the surface of the specimen. In addition, a thermal camera was used to monitor self-heating during loading. It is demonstrated that the PC chamber does not influence the stress-strain curve as determined by DIC. Applying this set-up, a semi-crystalline cross-linked low-density polyethylene (XLPE) under quasi-static tensile loading has been successfully analysed using DIC at four different temperatures (25 °C, 0 °C, −15 °C, −30 °C). At the lower temperatures, the conventional method of applying a spray-paint speckle failed due to embrittlement and cracking of the spray-paint speckle when the tensile specimen deformed. An alternative method was developed utilising white grease with a black powder added as contrast. The results show a strong increase in both the Young’s modulus and the flow stress for decreasing temperatures within the experimental range. We also observe that although the XLPE material is practically incompressible at room temperature, the volumetric strains reach a value of about 0.1 at the lower temperatures.     

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 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.     

Experimental approach for microscale mechanical characterization of polymeric structured materials obtained by additive manufacturing

In this paper, an original experimental method is developed for local strain characterization at the surface of additively manufactured polymeric materials. The process used herein is material extrusion. This experimental method is based on the use of microscopic speckle pattern deposited at the surface of micro single edge notched specimen (μ_SENT) made of acrylonitrile butadiene styrene (ABS). Two configurations of filament orientation were used for the specimen manufacturing. Images of the μ_SENT specimen surface were recorded during in-situ tensile test. The quantitative analysis of images was made by digital image correlation (DIC). The evolutions of the local strain heterogeneities and the crack tip are evidenced on the kinematic fields. It is shown that the crack propagates in the low resistance path which is the interface area between filaments. It is also evidenced that the intersection of perpendicular filaments in two adjacent layers blocks crack growth. The local strain evolutions at the surface of the specimen are compared to the macroscopic response of the material. The method developed herein allows the determination of the materials mechanical properties. The identification of the crack tip location using digital image correlation (DIC) and J-integral calculation lead to plot the J-R curve. The J-R curves comparison of the two specimen configurations shows that the fracture toughness is directly related to the material structure.     

Influence of water on damage and mechanical behaviour of single hemp yarn composites

This study aims to investigate the influence of water ageing on local deformation around a yarn in a hemp/epoxy composite. Specific single yarn composites were manufactured with the yarn oriented at 90° with respect to the tensile direction and with two types of epoxy resin, one being fully synthetic and the other one partially bio-based. First, a quantification of damage due to the water ageing is realised and photoelasticimetry analysis is used to study the evolution of the state of stress. Then, dumbbell samples with or without water ageing were tested under an optical microscope, and strain fields around the yarn were measured with the Digital Image Correlation (DIC) technique. The experimental results showed an increase in the measured strain after the water ageing. The local constitutive behaviour of the different constituents of the specimens could be approached by local analyses, and the evolution of the apparent stiffness values are discussed.     

The physical properties of pressure sensitive rubber composites

Nitrile butadiene rubber, NBR, structural foam of different apparent densities was obtained by using different concentrations of foaming agent, azodicarbonamide, ADC/K. The true stress-strain characteristics, in case of compression, of foamed samples after the application of cyclic stress-strain were measured. The effect of the cyclic stress-strain on strain energy density of ADC/K foaming agent-filled NBR rubber composites was studied. The mechanical parameters were found to depend on the foaming agent concentration and on the pre-cyclic fatigue number. Results also indicated that the strain energy decreased with filler concentration.The effects of the cyclic stress-strain on the conductivity of ADC/K foaming agent-filled NBR rubber composites were studied. The electrical properties were found to depend on the foaming agent concentration, the strain amplitude and the number of stress-strain cycles of pre-strain. This study was assisted by the current-voltage characteristics which were measured under the effect of different compression ratios: 0%, 5%, 10%, 15%, 20%, 25% and 30%. The free current carrier mobility and the equilibrium concentration of charge carriers in the conduction band were produced as functions of compressive strain. Results also indicate that there is a linear variation between pressure and conductivity for all samples, which means that these samples can be used as a pressure sensor.     

Uniaxial compression testing of polymeric materials

Two methods for uniaxial compression testing were investigated and compared using polypropylene as a model material. An overview of various parameters affecting compression test results is provided with particular emphasis on friction between the specimen and the compression plate. A procedure for the determination of the compressive modulus is introduced and results are shown. To enable the detection of instability associated with friction and barreling and to calculate true stress-true strain curves, the measurement of transverse expansion of the specimen at large strains is necessary. Nominal and true Poisson's ratio values in the pre- and post-yield regime are presented and the resulting true stress-true strain curves are compared and discussed. While in the post-yield regime nominal stress values misleadingly result in strain hardening, significant strain softening was observed using true stress values representing the intrinsic material behavior.