scholarly journals Investigating slow shock in low-impedance materials using a direct impact Hopkinson bar setup

2021 ◽  
Vol 250 ◽  
pp. 06009
Author(s):  
Puneeth Jakkula ◽  
Georg Ganzenmüller ◽  
Samuel Beisel ◽  
Stefan Hiermaier
Keyword(s):  
Dynamic Compression ◽  
Hopkinson Bar ◽  
Polyurethane Foams ◽  
Image Correlation ◽  
Dynamic Force ◽  
Strain Rates ◽  
Direct Impact ◽  
Compression Tests ◽  
Low Impedance Materials ◽  
Force Equilibrium

This work implements a direct impact Hopkinson bar, suitable for investigating the evolution of dynamic force equilibrium in low-impedance materials. Polycarbonate as the bar material favours for a long pulse duration of 2.6 ms for an overall length of only 5 m, allowing to compress large specimens to high strains. This setup is applied to polyurethane foams with different densities ranging from 80 - 240 kg/m3. Dynamic compression tests are performed at strain rates of 0.0017, 0.5 and 500 /s on the foams at room temperature. Depending on density, they show a saturation in increase of yield strength at strain rates of 500 /s, or even show a negative strain rate sensitivity for the lowest density. This behaviour is explained by comparing the dynamic force equilibrium to a phenomenon similar to shock in solid materials: For low densities and high rates of strain, homogeneous compression is replaced by a localized collapse front with a jump in stress across the front. Digital image correlation is performed to analyse elastic and plastic compaction waves by means of Lagrange diagrams.

scholarly journals Compression, Tension and Shear Testing of Fibrous Composite with the Split Hopkinson Bar Technique

2018 ◽  
Vol 183 ◽  
pp. 02006 ◽  
Author(s):  
Amos Gilat ◽  
Jeremy D. Seidt
Keyword(s):  
Strain Rate ◽  
High Speed ◽  
Fibrous Composite ◽  
Hopkinson Bar ◽  
Image Correlation ◽  
Compression Tests ◽  
Strain Rate Effects ◽  
Full Field ◽  
Split Hopkinson Bar ◽  
Split Hopkinson

The Split Hopkinson Bar (SHB) technique is used for high strain rate testing of T800/F3900 composite in compression, tension and shear. Digital Image Correlation (DIC) is used for measuring the full-field deformation on the surface of the specimen by using Shimadzu HPV-X2 high-speed video camera. Compression tests have been done on specimens machined from a unidirectional laminate in the 0°and 90° directions. Tensile tests were done in the 90° direction. Shear tests were done by using a notched specimen in a compression SHB apparatus. To study the effect of strain rate, quasi-static testing was also done using DIC and specimens with the same geometry as in the SHB tests. The results show that the DIC technique provides accurate strain measurements even at strains that are smaller than 1%. No strain rate effect is observed in compression in the 0° direction and significant strain rate effects are observed in compression and tension in the 90° direction, and in shear.

Characterization of Mechanical Behavior of Polyurethane Foams Using Digital Image Correlation

2005 ◽  
Author(s):  
Helena (Huiqing) Jin ◽  
Wei-Yang Lu ◽  
Simon Scheffel ◽  
Michael K. Neilsen ◽  
Terry D. Hinnerichs
Keyword(s):  
Digital Image Correlation ◽  
Digital Image ◽  
Polyurethane Foams ◽  
Image Correlation ◽  
Compression Tests ◽  
Steel Cylinder ◽  
Full Field ◽  
Strain Concentration ◽  
The Impact ◽  
Dic Technique

Polyurethane foams have good energy absorption properties and are effective in protecting sensitive components from damages due to impact. The foam absorbs impact energy by crushing cells and undergoing large deformation. The complex deformation of the foam needs to be modeled accurately to simulate the impact events. In this paper, the Digital Image Correlation (DIC) technique was implemented to obtain the deformation field of foam specimens under compression tests. Images of foam specimen were continuously acquired using high-speed cameras. The full field displacement and strain at each incremental step of loading were calculated from these images. The closed-cell polyurethane foam used in this investigation was nominal 0.32 kg/m^3 (20 pcf). In the first experiment, cubic specimens were compressed uniaxially up to 60%. The full-field displacements and strains obtained using the DIC technique provide detailed information about the inhomogeneous deformation over the area of interest during loading. In the second experiment, compression tests were conducted for a simple foam structure - cubic foam specimens with a steel cylinder inclusion. The strain concentration at the interface between steel cylinder and foam was studied to simulate the deformation of foam in a typical application. In the third experiment, the foam was loaded from the steel cylinder during the compression. The strain concentration at the interface and the displacement distribution over the surface were compared for cases with and without a confinement fixture to study the effects of confinement. These experimental results demonstrate that the DIC technique can be applied to polyurethane foams to study the heterogeneous deformation. The experimental data is briefly compared with the results from modeling and simulation using a viscoplastic model for the foam.

Compression Tests of High Strength Steels

2008 ◽  
Vol 59 ◽  
pp. 293-298
Author(s):  
Vaclav Mentl ◽  
Josef Bystricky
Keyword(s):  
Mechanical Properties ◽  
Mechanical Testing ◽  
High Speed ◽  
Dynamic Compression ◽  
Testing Machine ◽  
High Strength Steels ◽  
High Strength ◽  
Strain Rates ◽  
Compression Tests ◽  
Impact Compression

Mathematical modelling and virtual testing of components and structures represent a useful and economic tool for design and safety assessment. The basic mechanical properties which can be found in material standards are not relevant in cases where the real service conditions differ from those applied during standardised testing. Thus e.g. mechanical behaviour at higher strain rates can be interesting for the car components when the simulation of crash situations is used during structure development. The dynamic compression tests are usually performed by means of drop towers, by means of high speed hydraulic testing machines or Hopkinson bar method. At the Mechanical Testing Laboratory of the SKODA Research Inst. in Pilsen, Czech Republic, an instrumentation of Charpy pendulum testing machine was realised in order that it was possible to perfom dynamic compression tests, [1], and the compatibility of obtained results in comparison with traditional impact compression tests was verified within the round–robin carried out by TC5 ESIS Sub-Committee on “Mechanical Testing at Intermediate Strain Rates“, [2]. A new striking tup and load measurement system were designed and callibrated. At the same time, a new software was developed which makes it possible to evaluate the test force-deformation record. The goal of this study was 1. to check the possibility of compression testing of high strength materilas by mens of Charpy pendulum, and 2. to study the strain rate influence on basic mechanical properties.

Rate-Dependent Constitutive Modeling of Polymer Subjected to Dynamic Loading

2012 ◽  
Vol 185 ◽  
pp. 119-121
Author(s):  
Jian Ming Yuan ◽  
Jan Ma ◽  
Geoffrey E.B. Tan ◽  
Jian Fei Liu
Keyword(s):  
Test Data ◽  
Constitutive Modeling ◽  
Dynamic Compression ◽  
Fem Simulation ◽  
Material Model ◽  
Strain Rates ◽  
Compression Tests ◽  
Material Models ◽  
Rate Dependent ◽  
Tension Tests

This paper proposes an effective and systematical method to obtain reliable rate-dependent material models used in FEM simulation for polymers. Compressive stress-strain curves of two types of polymer are obtained at different strain rates. Rate-dependent elastic-plastic models are applied to describe the observed rate-dependent behaviors, whereby the input data of material model are determined from the test data obtained. Verification of the material models is proposed via comparing FEM simulation with test data of quasi-static tension tests and dynamic compression tests of different strain rates.

The Tensile Properties of PVB at Intermediate Strain Rate Using the In Situ Hopkinson Bar

2015 ◽  
Vol 782 ◽  
pp. 204-209
Author(s):  
K. Li ◽  
Rong Chen ◽  
H. Zhang ◽  
X. Wen
Keyword(s):  
Tensile Properties ◽  
High Speed ◽  
Hopkinson Bar ◽  
Polyvinyl Butyral ◽  
Local Deformation ◽  
Image Correlation ◽  
Strain Rates ◽  
Pulse Shaper ◽  
Full Field

Polyvinyl butyral (or PVB) is commonly used as interlayer in architectural laminated glass and windshield in automobiles for its strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility. A modified in-situ Hopkinson bar system is used to measure the tensile properties of the PVB with the strain rates of 30~100 s-1. In this system, a high impedance striker tube with the rubber pulse shaper is use to generate a long loading pulse of 50ms. Two X-cut quartz piezoelectric force transducers are sandwiched between the specimen and two bars respectively to directly measure the dynamic loading forces, and the strain field of the specimen is calculated by the Digital Image Correlation (DIC) method via photos obtained by the high speed camera. The local deformation of the full-field specimen was clearly displayed and the fracture strain of the specimen was evaluated. The results show that the tensile strengths of the PVB increase with increasing loading strain rates.

scholarly journals The Symmpact: A Direct-Impact Hopkinson Bar Setup Suitable for Investigating Dynamic Equilibrium in Low-Impedance Materials

2021 ◽  
Author(s):  
P. Jakkula ◽  
G. C. Ganzenmüller ◽  
S. Beisel ◽  
P. Rüthnick ◽  
S. Hiermaier
Keyword(s):  
Strain Rate ◽  
Strain Rate Sensitivity ◽  
Rate Sensitivity ◽  
Hopkinson Bar ◽  
Dynamic Compaction ◽  
Strain Gauges ◽  
Negative Strain Rate Sensitivity ◽  
Compaction Waves ◽  
Low Impedance Materials ◽  
Force Equilibrium

Abstract Background Measuring the dynamic behavior of low-impedance materials such as foams is challenging. Their low acoustic impedance means that sensitive force measurement is required. The porous structure of foams also gives rise to dynamic compaction waves, which can result in unusual behavior, in particular if the foam material is so thick, that dynamic force equilibrium is not reached. Objective This work investigates comparatively large polyurethane foam specimens with densities in the range of 80 – 240 kg/m3 to deliberately achieve a state away from force equilibrium during high-rate compaction. The aim is to understand how an increase in strain rate leads to a reduction in strength for such materials. Methods A specialized direct-impact Hopkinson bar is employed. It uses polycarbonate bars to achieve the required long pulse duration of 2.6 ms to compress the large specimens into the densification regime. In contrast to existing setups, both striker and output bar are instrumented with strain gauges to monitor force equilibrium. The absence of an input bar allows monitoring force equilibrium more accurately. Special attention is paid to the calibration of strain gauges, taking non-linear effects, wave dispersion and attenuation into account. Digital Image Correlation is employed to analyze elastic and plastic compaction waves by means of Lagrange diagrams. Results Depending on density, the specimens show saturation of dynamic strength increase at high rates of strain $$\approx$$ ≈  500 /s, or even negative strain rate sensitivity in case of the lowest density. The occurrence of apparent negative strain rate sensitivity is accompanied by a localized structural collapse front, moving at a low velocity of $$\approx$$ ≈ 10 m/s through the material. This apparent strain rate sensitivity is a structural effect which is related to the thickness of the specimen. Conclusions The primary aim of material characterization using Hopkinson bars is to achieve a state of force equilibrium. For this reason, very thin specimens are usually employed. However, data gathered in this way is not representative for thick foam layers. Here, an increase of strain rate can lead to a decrease of strength if homogeneous deformation is replaced by a dynamic compaction wave. This behavior can occur at strain rates encountered under conditions such as automotive crash.

The effect of loading rate on the elasto-plastic flexure of steel beams

1966 ◽  
Vol 290 (1421) ◽  
pp. 266-285 ◽  
Keyword(s):  
Loading Rate ◽  
Dynamic Compression ◽  
Bending Moment ◽  
Steel Beams ◽  
Beam Specimen ◽  
Strain Rates ◽  
Structural Elements ◽  
Compression Tests ◽  
Pure Bending ◽  
Yield Behaviour

The plastic behaviour of structural elements subjected to dynamic bending is discussed in relation to the strain-rate dependence of the flow stress. Theoretical and experimental work based on the concept of a dynamic fully plastic moment is summarized, with special reference to the yield behaviour of mild steel. A new dynamic loading machine is described which permits the deformation of a beam specimen in pure bending at speeds giving maximum strain rates up to 20 s -1 . The apparatus incorporates transducers which permit the recording of bending moment and deformation rate throughout the test. The paper gives the results of dynamic flexure tests on a 0.10 % carbon steel and also the results of dynamic compression tests on the same material. The relation between the two sets of results is discussed, and it is concluded that the behaviour in bending exhibits features which are not predictable from the compression test data.

scholarly journals Experimental Study on the Mechanical Behavior of EN08 Steel at Different Temperatures and Strain Rates

Metals ◽  
2018 ◽  
Vol 8 (9) ◽  
pp. 736 ◽  
Author(s):  
Farid Abed ◽  
Akrum Abdul-Latif ◽  
Ayatollah Yehia
Keyword(s):  
Strain Rate ◽  
Mechanical Response ◽  
Dynamic Compression ◽  
Damage Model ◽  
Tensile Tests ◽  
Dynamic Strain ◽  
Uniaxial Tensile ◽  
Strain Rates ◽  
Compression Tests ◽  
Sem Images

The objective of this paper is to investigate the mechanical response of EN08 steel at quasi-static and dynamic strain rates. Uniaxial tensile tests under quasi-static regime (from 0.0015 s−1 to 0.15 s−1) are conducted on EN08 steel at a range of temperatures between 298 K and 923 K. Dynamic compression tests are also performed by using a drop hammer and by considering different masses and heights to study the material response at strain rates up to 800 s−1. Through the stress-strain responses of EN08 steel, a strong dependency of the yield stress as well as the ultimate strength on the strain rate and temperature is recognized. Furthermore, the strain hardening is highly affected by the increase of temperature at all levels of strain rate. The microstructure of the steel is also examined at a fracture by using SEM images to quantify the density of microdefects and define the damage evolution by using an energy-based damage model.

Compression and Hysteresis Curves of Nonlinear Polyurethane Foams Under Different Densities, Strain Rates and Different Environmental Conditions

2011 ◽  
Author(s):  
M. F. Alzoubi ◽  
E. Y. Tanbour ◽  
R. Al-Waked
Keyword(s):  
Polyurethane Foams ◽  
Strain Rates ◽  
Compression Tests ◽  
Dissipation Energy ◽  
Slip Rates ◽  
Static Compression ◽  
Hysteresis Curves ◽  
Pu Foam ◽  
Different Temperatures ◽  
Loading And Unloading

Polyurethane (PU) closed cell foam samples with different densities were tested under loading and unloading compression tests at different temperatures and strain rates. Quasi-static compression tests were performed using the Lloyd LR5K Plus instrument at strain rates ranges from 0.033–0.267 s−1. Tests were conducted in a pre´cised enclosure to control the dependency of PU foam cells on temperature and humidity. In order to have an accurate comparison in compression and hysteresis curves for all tests; all PU foam samples were selected intentionally from the same foam block but with different location densities. Furthermore, all foam samples were tested in the direction of foam rise (thickness). First, PU foam samples were compressed with a circular platen up to 70% strain at different strain rates and different temperatures. Then, the platen was raised completely from the foam samples. During the experiment; stress-strain responses were measured and plotted for loading and unloading curves to determine stored energy, dissipation energy and peak stresses were calculated at 70% strain. Results have shown that PU foam sample responses under compression testing gets softer at higher temperatures when conducted at a constant strain rates. At constant temperatures, PU foam samples get harder at higher slip rates. Finally, both stored and dissipation energies were found to be dependent heavily on foam density, ambient temperature and strain rate.

scholarly journals Experimental Study on Dynamic Compression Mechanical Properties of Aluminum Honeycomb Structures

2020 ◽  
Vol 10 (3) ◽  
pp. 1188 ◽  
Author(s):  
Sheng Zhang ◽  
Wei Chen ◽  
Deping Gao ◽  
Liping Xiao ◽  
Longbao Han
Keyword(s):  
Mechanical Properties ◽  
Strain Rate ◽  
High Strain ◽  
Dynamic Compression ◽  
Honeycomb Structure ◽  
Strain Rates ◽  
Compression Tests ◽  
High Strain Rates ◽  
Honeycomb Structures ◽  
Aluminum Honeycomb

In this paper, dynamic compression tests are developed to investigate the dynamic compression mechanical properties of the aluminum honeycomb structures at different strain rates, especially at the high strain rates. The difficulties at the high strain rates exist due to the large deformation, the low wave resistance and the size effect of the honeycomb structures. The Split Hopkinson Pressure Bar (SPHB) test method is carried out and special measures such as the adoption of waveform shaper, the size optimization of the impact bar and the specimen, and employment of the semiconductor strain gauge, etc. are taken to overcome the difficulties. It is discovered that the dynamic compression mechanical properties possess a stress hardening effect at a high strain rate from 1.3 × 103 s−1 to 2.0 × 103 s−1, but then a stress softening effect at a high strain rate of 4.6 × 103 s−1. It is also discovered that the yield strength and the average plateau stress at the strain rate of 2.0 × 103 s−1 is higher than that at the strain rate of 1.3 × 103 s−1. However, the yield strength and the average plateau stress at the strain rate of 4.6 × 103 s−1 is lower than that at the strain rate of 2.0 × 103 s−1 and 1.3 × 103 s−1, but higher than that at a quasi-static state. This indicates that the aluminum honeycomb structure is sensitive to the strain rate. Additionally, the damage mode of the aluminum honeycomb structure is plastic buckling, collapse and folding of the cell wall, which is carried out using dynamic compression tests. The folding length of the cell wall at a higher strain rate is found to be longer than that at a lower strain rate. The test results can also be used as the stress–strain curves of the honeycomb constitutive model at the high strain rates to carry out the numerical simulation of high-speed impact.

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