X-ray microtomographic characterization and quantification of the strain rate dependent failure mechanism in cenosphere epoxy syntactic foams

This work investigates the failure mechanism in cenosphere epoxy syntactic foams at the quasi-static and dynamic strain rates. Split-Hopkinson pressure bar experiments are controlled to stop dynamic deformation of the foams at various strain stages. The internal microstructure at each strain is characterized in the x-ray microtomography and compared to the microstructure in the foams deformed quasi-statically. The microscopic observations reveal that the failure process in syntactic foams at the low and high rates is dominated by the crushing of cenospheres and the cracking of the epoxy matrix. However, the mechanism of failure in the foam is significantly affected by the strain rate. Compared to quasi-static compression, macro-cracks form earlier in the matrix at dynamic rates and can propagate to split cenospheres. The volume of the damage as defined by the failure of both cenospheres and the matrix is calculated from the x-ray microtomographic images. It is found that the damage can be quantitatively related to the strain and the strain rate using an empirical equation

The high strain-rate behaviour of three molecular weights of polyethylene examined with a magnesium alloy split-Hopkinson pressure bar

A traditional split-Hopkinson pressure bar system has been modified by the addition of ZK60A magnesium alloy pressure bars in order to increase the resolution of data when examining specimens of low-density, high-density and ultra-high molecular weight polyethylene. It was found that the low density of the ZK60A allowed a decent increase in transmitted pulse amplitude, whilst its relatively high yield strength afforded long-term reliability of the system. The accuracy of data obtained from the fitted strain gauges was verified with the use of a high-speed video camera, and was found to be an excellent match.Comment: Will be shortly submitted to 'Polymer Testing

On the high strain rate behavior of 63-37 Sn-Pb eutectic solders with temperature effects

This study presents experimental results performed on samples of Eutectic solder material (63 wt. % Sn 37 wt. % Pb) loaded at high strain rates and elevated temperatures. The tests were performed at high strain rates using Split Hopkinson Pressure Bar (SHPB). The strain rates were in the range of 400 s-1to 1300 s-1. Heating unit was added to conventional SHPB to vary sample' s initial temperature conditions. Tests were conducted at three initial temperatures, i.e. room temperature, 60 °C and 120 °C for compressive mode. The effects of temperature on the behavior of material were compared. Transient temperature changes during dynamic loading conditions are calculated by an analytical approach using measured stress-strain data for plastic work. Test results were fitted into the Johnson-Cook model (JC model). In addition, dynamic tests were performed in tension mode using Split Hopkinson Tensile Bar (SHTB) at room temperature

Temporal characterization of rock dynamic destruction

Dynamic strength tests published in literature have been analysed by structuraltemporal damage criteria. Parameter τ - incubation time - describing material stability behaviour under high-rate influences have been estimated for Kimachi sandstone, Inada granite and Tage tuff. Two types of dynamic tensile experiments have been used: split Hopkinson pressure bar and spalling. Purely dynamic effect of fracture delay have been observed in the case of Kimachi sandstone and discussed

Wave propagation through alumina-porous alumina laminates

A Brazilian disk geometry of an alumina layered composite with alternating dense and porous layers was dynamically loaded using a Split-Hopkinson Pressure Bar (SHPB)apparatus under compression. High-speed imaging and transmitted force measurements were used to gain an insight into stress wave propagation and mitigation through such a layered system. Uniformly distributed porosities of 20 and 50 vol % were introduced into the interlayers by the addition of fine graphite particles which volatilized during heat treatment. Brazilian disk samples were cut from the cylinders which were drilled out of the sintered laminated sample. The disks were subjected to dynamic impact loading in perpendicular and parallel orientations to the layers in order to investigate the influence of the direction of impact. The dynamic failure process of the layered ceramic consisted of the initiation and propagation of the cracks mainly along the interphases of the layers. Upon impact, the impact energy was dissipated through fracture in parallel orientation (0 deg) but transmitted in perpendicular (90 deg) orientations. The high degree of correlation between the transmitted force, microstructure and orientation in which the layered systems were impacted is discussed.Comment: 39 pages, 14 figures, 2 tables. Accepted for publication in the Journal of the European Ceramic Societ

A framework for analyzing hyper-viscoelastic polymers

Hyper-viscoelastic polymers have multiple areas of application including aerospace, biomedicine, and automotive. Their mechanical responses are therefore extremely important to understand, particularly because they exhibit strong rate and temperature dependence, including a low temperature brittle transition. Relationships between the response at various strain rates and temperatures are investigated and a framework developed to predict large strain response at rates of c. 1000 s$^{-1}$ and above where experiments are unfeasible. A master curve of the storage modulus's rate dependence at a reference temperature is constructed using a DMA test of the polymer. A frequency sweep spanning two decades and a temperature range from pre-glass transition to pre-melt is used. A fractional derivative model is fitted to the experimental data, and this model's parameters are used to derive stress-strain relationships at a desired strain rate.Comment: 6 pages, 11 figures, conference paper from ECCMR X, 2017, p529-53

Progressive Damage and Failure Analysis of Bonded Composite Joints at High Energy Dynamic Impacts

Both wing and fuselage structures utilize bonded composite joints for structural efficiency in modern commercial and military aircraft. To ensure compliance with certification requirements mechanical fasteners are typically used as a failsafe mechanism for appropriate strength in the event of complete stiffener disbond. However, the use of fasteners decreases the structural efficiency of the structure by adding weight. This establishes the requirement to better exploit the efficiency of bonded structures and fully understand the failure behavior of adhesively bonded composite structures, particularly when subjected to elevated loading rates due to high energy dynamic impacts (HEDI). For this reason, the NASA Advanced Composite Consortium (ACC) HEDI team developed an experimentation and numerical modeling program for high rate loading of composite joints. In the present work, the response of adhesively bonded composite joints subjected to elevated loading rates is studied numerically and validated against experimental results. Due to dynamic considerations of experiments, the idea of wedge insert was extended to use with Split Hopkinson Pressure Bar (SHPB) testing techniques. Mode-I and Mode-II test configurations were simulated to evaluate the capability of two continuum damage material (CDM) models in LS-DYNA, namely MAT162 and MAT261. Three different levels of fidelity were considered to investigate the level of detail required to numerically predict the failure behavior and the results from high fidelity analysis are presented

Characterisation of dynamic behaviour of alumina ceramics: evaluation of stress uniformity

Accurate characterisation of dynamic behaviour of ceramics requires the reliable split-Hopkinson pressure bar (SHPB) technique and the condition of uniaxial homogeneous specimen deformation. In this study, an experimentally validated 3D finite element model of the full scale SHPB experiment was developed to quantitatively evaluate the wave propagation in the bars and the stress distribution/evolution in the alumina specimen. Wave signals in both the SHPB experiments and the finite element model were analysed to characterise the dynamic behaviour of alumina. It was found that the equilibrium of both stresses within the specimen and forces at the specimen ends can be established in the intermediate stage of deformation. The validity of stress uniformity in the alumina specimen supports the assumption of uniaxial homogeneous specimen deformation in the SHPB and validates the characterisation of dynamic behaviour of alumina ceramics

Intermediate strain rate behavior of cancellous bone: From the lower to the higher strain rate

Not applicabl

Dynamic behavior and microstructural properties of cancellous bone

The aim of the presented study is to identify some properties of the dynamic behavior of the cancellous bone and to identify the link between this mechanical behavior and the microstructural properties. 7 cylinders of bovine cancellous bone (diameter 41 mm, thickness 14 mm) were tested in quasi static loading (0.001 s-1), 8 in dynamic loading (1000 s-1) and 10 in dynamic loading (1500 s-1) with a confinement system. All the specimens were submitted to imaging before the tests (pQCT) in order to indentify two microstructural properties: Bone Volume / Total Volume ? BV/TV ? and Trabeculae Thickness ? Tb.Th. The behavior of bovine cancellous bone under compression exhibits a foam-type behavior over the whole range of strain rates explored in this study. The results show that for the quasi-static tests only the stresses are correlated with BV/TV. For the unconfined dynamic tests, the yield stress is correlated to BV/TV and the plateau stress to BV/TV and Tb.Th. For the confined tests, only the plateau stress is correlated to BV/TV and Tb.Th. The effect of strain rate is an increase of the yield stress and the plateau stress. The confinement has an effect on the measured values of compression stresses that confirms the importance of marrow flow in the overall behavior