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EN
The article proposes the concept of experimental dynamic tests of aggregate behavior - sand used as a material for covering temporary protective shelters for the population and civil defense. These shelters are part of the construction shielding infrastructure useful in crisis situations. Laboratory tests of loose soil, including sand, with impact loads are based on the Split Hopkinson Pressure Bar using a clamp. The work describes both this experimental method and the original laboratory stand for impact tests based on a pneumatic launcher with a projectile - bar. This position was built at the Institute of Civil Engineering of the Military University of Technology taking into account the requirements of generally applicable legal provisions. Examples of preliminary experimental research on the behavior of selected aggregate - dry sand under dynamic load - are presented.
EN
Under certain extreme conditions in rock engineering works, fast change in temperature in the load-bearing rocks can happen. Known as thermal shock (TS), such process involves rapid temperature rise or drop, which causes fracturing in the rock material and thus can pose as a threat to the stability of the rock structures. To investigate the influence of thermal shock caused by fast cooling on the mechanical property of rock, laboratory tests are performed on heated granite which are cooled with different methods, with the highest cooling rate reaching 167.4 °C/min. The dynamic loading tests are performed on the heated granite specimens utilizing the split Hopkinson pressure bar (SHPB) system. The test results show that the dynamic compressive strength drops with the increase in heating level or cooling rate. This pattern is explained by the nuclear magnetic resonance (NMR) test data that the pores inside the heated granite increase both in size and quantity as heating level or cooling rate rises. Damage patterns of the tested granite specimen fragments are analyzed based on the observation with scanning electron microscope (SEM), and the mechanisms of thermal shock in granite are also discussed.
EN
The paper presents the results of the compression tests for carbon-epoxy composites in order to assess the amount of energy absorbed depending on the process velocity and content of axial fibres. Two types of prepreg (UD 200 g/m2 and woven 160 g/m2) were used to prepare the specimens with a diameter of 20 mm and a height of 34 mm. The specimens were subjected to compression under various speed conditions (static, dynamic and SHPB tests). The calculated specific energy absorption values showed a 50–60% decrease with increasing process velocity and depending on the type of specimens architecture. The highest energy values were absorbed by the specimens with the highest share of axial fibres in the sample.
EN
As the dynamic behavior of the concrete is different from that under static load, this research focuses on the study of dynamic responses of concrete by simulating the split Hopkinson pressure bar (SHPB) test. Finite element code LS-DYNA is used for modeling the dynamic behaviors of concrete. Three continuous models are reviewed and the Holmquist-Johnson-Cook model (HJC) is introduced in detail. The HJC model which has been implemented in LS-DYNA is used to represent the concrete properties. The SHPB test model is established and a few stress waves are applied to the incident bar to simulate the dynamic concrete behaviors. The stress-strain curves are obtained. The stress distributions are analyzed. The crack initiation and propagation process are described. It is concluded that: the HJC model can modeling the entire process of the fracture initiation and fragmentation; the compressive of the concrete is significantly influenced by the strain rates.
EN
Results of numerical analysis of the influence of pulse shaper geometry on wave signals in the split Hopkinson pressure bar experiment are presented. Five pulse shapers, i.e. square, ring, cross, star and disk ones have been analysed. It has been assumed that the disc pulse shaper is the reference geometry to assess the remaining types of pulse shapers. The results of numerical analyses have shown that pulse shapers with shapes different than disk are highly capable of minimizing high-frequency Pochhammer-Chree oscillations and, thus, reduce dispersion of waves propagating in the bar. The greatest damping ability has been observed while using the ring pulse shaper at both low and high impact velocities of the striker.
EN
The high strength concrete containing coarse aggregate (CA) exhibits excellent resistance performance to dynamic impact and penetration. In this paper, four different maximum sizes (0 mm, 10 mm, 15 mm and 20 mm) and three types (basalt, granitic and iron ore) of CAs were used to fabricate high performance cementitious composite (HPCC). Then, the closed-loop servo-controlled material testing machine and 74-mm-diameter split Hopkinson pressure bar (SHPB) were employed to conduct static and dynamic impact tests, respectively. On the basis of stress–strain curves, the mechanism of CA on static and dynamic behavior of HPCC was discussed. HPCC with different CAs present obvious strain rate effect. The CA with the maximum size of 10 mm shows the most notable effect on dynamic properties of HPCC. However, as the increasing of the maximum size of CA, the dynamic peak stress and toughness were decreased. The fracture pattern of HPCC with different CA is brittle failure, but the significant difference was not found.
7
Content available remote Strain rate effects on selected mechanical properties of glass-polyester laminates
EN
The study presents the results of experimental testing of a layered cross-ply [0/90]n E-glass/polyester composite in the range of the selected compression properties at high strain rates = 2300÷4600 s−1 and a quasi-static strain rate = 0.0067 s−1. The composite was manufactured using contact technology using Owens Corning CD-600 E-glass stitched fabric and Polimal 104 polyester resin. The circular cross-section specimens of three sizes 2.5, 5 and 7.5 mm in height and 15 mm in diameter were tested in the above described experiments. To determine the static properties, quasi static experimental tests were conducted using an Instron 8802 machine in the displacement control mode at a constant crosshead speed of 1, 2 and 3 mm/min respectively for the 2.5, 5.0 and 7.5 mm specimen types. The compression loading was monitored with a load cell Instron ±250 kN, whereas the axial strain with an Instron 2620-604 extensometer using additional fixing discs. The measuring base of the extensometer was equal to the specimen height. Identification included the stress-strain curve, strength, Young’s modulus and failure strain. For the high strain rates testing, a modified Split Hopkinson Pressure Bar test system was used, containing an LTT 500 Amplifier made by Tasler, Germany and an NI USB-6366 data acquisition device made by National Instruments, USA. The failure modes observed under high strain rate loading were similar to those observed under quasi static loading. The samples predominantly failed by shear fracture. Reduction of the specimen height implies an increase in the nonlinear effects in the initial part of the stress-strain diagrams (increasing strain at same stress), probably caused by the boundary effect. The main parts of the stress-strain plots are approximately linear, thus the linear elastic-brittle material model can be assumed. It was generally observed that the compressive strength and Young’s modulus along the thickness direction are higher at high strain rate loading compared to the results at quasi-static loading.
PL
W pracy przedstawiono wyniki badań eksperymentalnych laminatu krzyżowego [0/90]n szkło-E/poliester w zakresie wyznaczenia właściwości mechanicznych podczas ściskania z prędkością = 2300÷4600 s-1 w badaniach dynamicznych oraz = 0,0067 s-1 w badaniach quasi-statycznych. Badany kompozyt wytworzono metodą kontaktową z użyciem tkaniny zszywanej Owens Corning CD-600 oraz żywicy poliestrowej Polimal 104. Do badań przyjęto trzy typy próbek walcowych o wysokości 2,5; 5 i 7,5 mm oraz średnicy 15 mm. W celu wyznaczenia właściwości mechanicznych statycznych przeprowadzono badania eksperymentalne z wykorzystaniem maszyny wytrzymałościowej Instron 8802 w trybie sterowania przemieszczeniem, z prędkością przemieszczenia 1, 2 and 3 mm/min, odpowiednio dla próbek typu 2,5, 5,0 i 7,5 mm. Obciążenie rejestrowano przy użyciu głowicy pomiarowej Instron ±250 kN. Pomiar odkształcenia realizowano z użyciem ekstensometru Instron 2620-604 z dodatkowym oprzyrządowaniem w postaci dysków. Baza pomiarowa ekstensometru była równa wysokości próbki. Badania identyfikacyjne obejmowały wyznaczenie zależności naprężenie-odkształcenie, wytrzymałości, modułu sprężystości oraz odkształceń niszczących. Badania przy dużej prędkości odkształceń przeprowadzono na stanowisku Zmodyfikowany Pręt Hopkinsona (SHPB - Split Hopkinson Pressure Bar), stosując wzmacniacz LTT 500 firmy Tasler, Niemcy oraz kartę pomiarową NI USB-6366 firmy National Instruments, USA. Zaobserwowano podobny mechanizm niszczenia próbek w badaniach quasi-statycznych oraz dynamicznych. Próbki niszczyły się przez ścinanie. Zmniejszenie wysokości próbki powoduje ujawnienie się efektów nieliniowych w początkowej części wykresów (zwiększenie odkształcenia przy tym samym naprężeniu). Jest to związane z efektem brzegowym. Środkowa część zależności naprężenie-odkształcenie jest w przybliżeniu liniowa, co odpowiada materiałowi liniowo-sprężysto-kruchemu. W wyniku badań zaobserwowano wzrost wytrzymałości na ściskanie oraz modułu sprężystości w kierunku prostopadłym do powierzchni warstwy laminatu wraz ze wzrostem prędkości odkształceń.
8
EN
Split Hopkinson pressure bar (SHPB) is one of the most important and recognisable apparatus used for characterizing the dynamic behaviour of various materials. Incident pulse generated one the incident bar usually have a rectangular shape, which is proper for some materials but for others is not. Therefore, several methods of shaping the incident pulse are used for obtaining constant strain rate conditions during tests. Very often pulse shapers made of copper or similar material are implemented due to its softness properties. In this paper such material was investigated using the FE model of SHPB. Its mechanical behaviour was characterised with and without copper disc between the striker and incident bar. Numerical simulations were carried out using explicit LS-DYNA code. Two different methods were used for modelling the copper sample: typical finite Lagrangian elements and meshless Smoothed Particle Hydrodynamics (SPH) method. As a result of two techniques used axial stress-strain characteristics were compared for three different striker’s velocity with an influence of the copper pulse shaper taking into account. Finally, FE and SPH method was compared with taking into consideration: the efficiency, computer memory and power requirements, complexity of methods and time of simulation.
9
Content available Numerical simulation of dynamic weld compression
EN
The article presents some numerical results and experimental validation of Split Hopkinson pressure bar (SHPB) tests for welded S40NL steel. The goal of this research is to define material constants for modelling it in FEM. Steel was tested with Charpy impact test to determine properties of material. Next, the joint for welding was prepared. It was welded with electric arc welding method (MAG) with flux-cored wire. Hopkinson bar test is well-known experiment method used to determine material properties at high strain rates. The tests were performed in Institute of Fundamental Technological Research. Material properties for Johnson-Cook material model were obtained. Comparison between experimental results taken in quasi-static conditions and dynamic conditions proves that the behaviour of materials in those two states is quite different. Results from one type of loading condition cannot be used to create a realistic model of material when it is loaded dynamically. Numerical simulation of Hopkinson bars was performed on cylindrical model with known length and accelerated to high speed in direction of incident bar. For the purpose of the simulation, a Finite Element Code LS-DYNA was used. It allows simulation of dynamic response of SHPB system. The results show quite good agreement. The model can be used to simulate weld performance under high strain rate.
EN
The soft rock materials have special dynamic properties in deep mining. Their dynamic compression tests are performed with experimental system of Split Hopkinson Pressure Bar (SHPB). The results of experiment indicate that the dynamic stress-strain curves of rock materials are of distinct plastic yield characteristics. Based on modified excess stress model, according to the function relationship between strain, strain rate and, to simplify constitutive equation of modified excess stress model with dimensionless analysis, the simplified constitutive equation of modified excess stress model is obtained; And then, continuum damage theory and statistical damage theory are incorporated into the simplified constitutive equation of modified excess stress model account for the influence of damage to dynamic strength, the simplified constitutive equation of damage excess stress model is built up. The simplified constitutive functions are fitted out respectively on the basis of the experimental data. There is a good coherence between the fitting stress-strain curves and the experimental stress-strain curves.
PL
Miękkie materiały skalne w kopalniach głębokich posiadają specyficzne właściwości dynamiczne. Wykonano testy kompresji dynamicznej przy zastosowaniu urządzenie kompresyjnego Hopkinsona (Split Hopkinson Pressure Bar SHPB). Wyniki przeprowadzonych badań wykazały, krzywe wykresu rozciągania skał mają charakterystykę typową dla materiałów upłynnionych. W oparciu o model rozkładu naprężeń, zgodnie z funkcją zależności pomiędzy typem odkształcenia a jego szybkością, celem uproszczenia równania odkształcenia zmodyfikowanego przy pomocy analizy bezwymiarowej, otrzymano równanie opisujące model naprężeń zmodyfikowanych. Następnie wykorzystano teorię ciągłego kontinuum oraz teorię statystyki odkształceń w równaniu odkształcenia zmodyfikowanego celem ustalenia zależności z badanym modelem. Uproszczone w ten sposób funkcje skorelowano na podstawie otrzymanych danych doświadczalnych. Ustalono wysoką spójność pomiędzy krzywymi teoretycznymi i doświadczalnymi.
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