An optical correlation technique for characterizing the crack velocity in concrete

In the present work, an experimental method named “the rocking spalling test” is proposed to investigate the crack-propagation velocity in concrete and rock-like materials under dynamic tensile loading. This method is based on the use of double-notched specimens loaded in spalling tests. A compressive pulse is transmitted to a rectangular specimen by means of a Hopkinson bar. It is reflected as a tensile wave on the opposite free surface of the sample. A large notch provides a rocking effect of the rear part of the specimen whereas a short notch is used to trigger a single unstable crack. This experimental configuration has been optimized through a series of numerical simulations. Finally, a series of tests have been conducted on dry and wet concrete specimens. Crack gauges and ultra-high speed camera coupled to Digital Image Correlation have been used to characterize the crack speed in dry and wet concrete samples.     

A comparison of DIC and grid measurements for processing spalling tests with the VFM and an 80-kpixel ultra-high speed camera

During the last decades, the spalling technique has been more and more used to characterize the tensile strength of geomaterials at high-strain-rates. In 2012, a new processing technique was proposed by Pierron and Forquin [1] to measure the stress level and apparent Young’s modulus in a concrete sample by means of an ultra-high speed camera, a grid bonded onto the sample and the Virtual Fields Method. However the possible benefit to use the DIC (Digital Image Correlation) technique instead of the grid method has not been investigated. In the present work, spalling experiments were performed on two aluminum alloy samples with HPV1 (Shimadzu) ultra-high speed camera providing 1?Mfps maximum recording frequency and about 80?kpixel spatial resolution. A grid with 1?mm pitch was bonded onto the first sample whereas a speckle pattern was covering the second sample for DIC measurements. Both methods were evaluated in terms of displacement and acceleration measurements by comparing the experimental data to laser interferometer measurements. In addition, the stress and strain levels in a given cross-section were compared to the experimental data provided by a strain gage glued on each sample. The measurements allow discussing the benefit of each (grid and DIC) technique to obtain the stress-strain relationship in the case of using an 80-kpixel ultra-high speed camera.     

Spalling uniaxial strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> at high strain rates

In this article research into the uniaxial tensile strength of Al₂O₃ monolithic ceramic is presented. The experimental procedure of the spalling of long bars is investigated from different approaches. This method is used to obtain the tensile strength at high strain rates under uniaxial conditions. Different methodologies proposed by several authors are used to obtain the tensile strength. The hypotheses needed for the experimental set-up are also checked, and the requirements of the set-up and the variables are also studied by means of numerical simulations. The research shows that the shape of the projectile is crucial to achieve successfully tests results. An experimental campaign has been carried out including high speed video and a digital image correlation system to obtain the tensile strength of alumina. Finally, a comparison of the test results provided by three different methods proposed by different authors is presented. The tensile strength obtained from the three such methods on the same specimens provides contrasting results. Mean values vary from one method to another but the trends are similar for two of the methods. The third method gives less scatter, though the mean values obtained are lower and do not follow the same trend as the other methods for the different specimens.     

Shockless spalling damage of alumina ceramic

Ceramic materials are commonly used to build multi-layer armour. However reliable test data is needed to identify correctly models and to be able to perform accurate numerical simulation of the dynamic response of armour systems. In this work, isentropic loading waves have been applied to alumina samples to induce spalling damage. The technique employed allows assessing carefully the strain-rate at failure and the dynamic strength. Moreover, specimens have been recovered and analysed using SEM. In a damaged but unbroken specimen, interactions between cracks has been highlighted illustrating the fragmentation process.     

Failure-delay effect in destruction of steel samples under spalling conditions

Dynamic spalling tests have been run on two batches of 30KhN4M steel samples. Experimental data have been processed with the classical technique based on solution of the elastic wave equation. Three samples have been revealed that demonstrated the failure-delay effect under testing. The incubation-time criterion has been used to show the conditions of emergence of failure delay with the example of triangular loading pulses. A rate strength curve has been constructed for the other samples. It has been shown that the limiting strengths under dynamic loads considerably differ for samples from different batches despite the same chemical composition and static strength.     

Statistical analysis of the strength of ultra-oriented ultra-high-molecular-weight polyethylene film filaments in the framework of the Weibull model

A statistical analysis of the distribution of the tensile strength σ of ultra-oriented ultra-high-molecular-weight polyethylene (UHMWPE) film filaments has been performed in the framework of the Weibull model using the results obtained from a large number (50) of measurements. The UHMWPE film filaments have been produced by means of high-temperature multistage zone drawing of xerogels prepared from 1.5% UHMWPE solutions in decalin. The Weibull modulus has been determined for this type of materials. It has been shown that, for the ultimate draw ratio λ = 120, the average tensile strength is equal to 4.7 GPa, which is significantly higher than the tensile strength σ = 3.5 GPa for commercial gel-spun UHMWPE fibers manufactured by the DSM Company (The Netherlands) and the Honeywell International Incorporation (United States). It has been demonstrated that, for 20% of the specimens thus prepared, the tensile strength reaches record-high values σ = 5.2–5.9 GPa.     

On some principal features of data processing of spall fracture tests

A method for processing the results of dynamic spall fracture tests, based on the exact solution of the wave equation, and its commonly used simplified version based on the assumed unique relation between the free surface velocity drop and the ultimate medium fracture stress, are analyzed. Using the considered exact solutions of the wave technique, tensile stress pulses during spalling are determined. The obtained stress levels at the fracture point are compared with the spall strength calculated by the velocity drop technique. The cases of agreement and disagreement of the results obtained using both techniques are shown. By the example of differently shaped loading pulses, possible scenarios of sample fracture are presented, in particular, the probability of the fracture delay effect is shown, which can be lost in the simplified processing method.     

Design of an experimental configuration for studying the dynamic fragmentation of ceramics under impact

To characterize the penetration resistance of a ceramic target during impact, it is necessary to take into consideration the mechanical behaviour of the fragmented ceramic. In the present work, a series of numerical simulations have been performed to design an experimental configuration named the “normal impact test”. The aim of this configuration is to generate a strong fragmentation of the ceramic tile and to prevent any ejection of debris in order to analyse the fragmentation properties of the impacted ceramic. An anisotropic damage model has been used to perform these computations. The Weibull parameters that constitute input data of this model have been identified from four-point flexural tests performed on the so-called Hexoloy® silicon carbide. Finally, the numerical predictions are compared to the failure pattern of the fragmented ceramic.     

Resonant elastic X-ray scattering: Where from? where to?

A series of quasi-static and dynamic tensile tests at varying temperatures were carried out to determine the mechanical behaviour of Ti-45Al-2Nb-2Mn+0.8vol.% TiB₂ XD as-HIPed alloy. The temperature for the tests ranged from room temperature to 850  ∘C. The effect of the temperature on the ultimate tensile strength, as expected, was almost negligible within the selected temperature range. Nevertheless, the plastic flow suffered some softening because of the temperature. This alloy presents a relatively low ductility; thus, a low tensile strain to failure. The dynamic tests were performed in a Split Hopkinson Tension Bar, showing an increase of the ultimate tensile strength due to the strain rate hardening effect. Johnson-Cook constitutive relation was used to model the plastic flow. A post-testing microstructural of the specimens revealed an inhomogeneous structure, consisting of lamellar α₂ + γ structure and γ phase equiaxed grains in the centre, and a fully lamellar structure on the rest. The assessment of the duplex-fully lamellar area ratio showed a clear relationship between the microstructure and the fracture behaviour.     

Measurement of the biaxial properties of nineteenth century canvas primings using electronic speckle pattern interferometry

This paper reports on the use of electronic speckle pattern interferometry (ESPI) for the measurement of the biaxial tensile properties of English 19th century canvas primings and their constituent materials. Typically, such primings are comprised of a complex structure of layers, each with different mechanical properties. ESPI has been shown to be an effective technique for investigating complex composite structures and it is especially useful for understanding the behaviour of heterogeneous materials in which non-uniform strains can occur. The flexibility of canvas primings presents a more difficult application for both biaxial tensile testing and ESPI strain measurements. A series of experiments have been carried out to measure the Poisson's ratio of the three main constituents of a 19th Century priming as composites and of an original 19th century primed canvas. The samples have been uniaxially tensioned on a biaxial tensile tester designed specifically to investigate the mechanical properties of paintings on canvas. Measurements of deformation have made using a two-dimensional in-plane ESPI configuration. The results have shown that Poisson's ratio decreases as the constituents of a painting are built up. Preliminary tests on thermally aged and original primings suggest that for a painting without cracks it is the embrittled paint which determines the mechanical response of the painting at an relative humidity of 35–40%.     

A refined method for estimating fiber and interfacial shear strength by using a single-fiber composite

In estimating interfacial shear strength from the fragmentation process of fibers in single-fiber composites, a problem arises as to the value of the fiber strength if the fiber strengths distribute widely and strongly depend on the fiber length. To overcome this problem, a refined analysis method for simultaneously estimating the fiber and the interfacial shear strength from the fragmentation process has been shown. Agreements between the values estimated with the proposed method and the results of the single-fiber tensile and the direct shear tests have been obtained. It has been shown that the estimation of the interfacial shear strength using the proposed method is insensitive to the matrix properties if the interfacial shear strength is unaltered by the matrix properties, and that the variations of the distribution parameters of the fiber strength is significantly smaller for the proposed method as compared with the single-fiber tensile tests. The results obtained by applying the proposed method to various carbon fibers have been shown.     

Numerical Modeling of the Effect of Inflow Water Mineralization in the Dynamics of the Autumnal Thermal Bar in Kamloops Lake

The results of numerical modeling of hydrodynamic processes that occur during the period of existence of an autumnal thermal bar in Kamloops Lake, Canada are presented. These results were obtained using a nonhydrostatic 2.5D model in the Boussinesq approximation with allowance for the diurnal variability of radiative and turbulent heat fluxes at the surface of the lake. A series of numerical experiments with various values of water mineralization in the Thompson River have been performed. The calculations show that the mineralization of inflow river waters has a significant effect on the dynamics of horizontal movements of the thermal front and on the pattern of circulation flows induced by the thermal bar during the period of lake cooling.     

Influence of Particle Morphology and Flow Conditions on the Dispersion Behavior of Fumed Silica in Silicone Polymers

The dispersion behavior of agglomerates of several grades of fumed silica in poly(dimethyl siloxane) liquids has been studied as a function of particle morphology and applied flow conditions. The effects of primary particle size and aggregate density and structure on cohesivity were probed through tensile and shear strength tests on particle compacts. These cohesivity tests indicated that the shear strength of particle compacts was two orders of magnitude higher than the tensile strength at the same overall packing density. Experiments carried out in both steady and time‐varying simple‐shear flows indicate that dispersion occurs through tensile failure. In the steady‐shear experiments,enhanced dispersion was obtained at higher levels of applied stress and, at comparable levels of applied stress, dispersion was found to proceed faster at higher shear rates. Experiments conducted in time‐varying flows further corroborated the results obtained in tensile cohesivity tests. Experiments in which the mean and maximum stresses in the time‐varying flows were matched to the stresses produced in steady shear flows highlight the influence of flow dynamics on dispersion behavior.     

Optimization of laser beam welding process parameters to attain maximum tensile strength in AZ31B magnesium alloy

An empirical relationship is developed to predict tensile strength of the laser beam welded AZ31B magnesium alloy by incorporating process parameters such as laser power, welding speed and focal position. The experiments were conducted based on a three factor, three level, central composite face centered design matrix with full replications technique. The empirical relationship can be used to predict the tensile strength of laser beam welded AZ31B magnesium alloy joints at 95% confidence level. The results indicate that the welding speed has the greatest influence on tensile strength, followed by laser power and focal position.     

Influence of mesoscale three-phase parameters to the tensile behavior of concrete

In the mesoscopic level, concrete is regarded as three-phase composite material with cement matrix, aggregate, and the interfacial transition zone (ITZ) between them. The mechanical properties of ITZ are regarded weaker than those of the cement matrix and aggregate. In this study, a mesoscale mechanical model based on the interface specimen with a single aggregate is established to study the influence of three-phase parameters on the interface specimen under quasi-static and dynamic direct tensile loading. Besides, the loading rate effect is also considered in this study to further analyze the dynamic performance of ITZ and the whole interface specimen. According to the numerical results, it is indicated that the ITZ properties (elastic modulus and strength) play significant roles in the performance of the interface specimen under quasi-static direct tensile loading. However, the cement matrix is dominant to the mechanical properties of interface specimen under dynamic tensile loading. Moreover, the properties of ITZ (elastic modulus, strength, and DIF values) and the ITZ thickness have some influence on the dynamic performance of ITZ and the whole interface specimen under dynamic tensile loading. In contrast, the Poisson’s ratio and density of ITZ have little influence on the dynamic behavior of the whole interface specimen. Additionally, the aggregate diameter is influential to the time reaching peak stress of ITZ and the whole interface specimen, and the loading rate only influences the time to reach the peak stress of ITZ under dynamic tensile loading.     

Rebuilding of metal components with laser cladding forming

Laser cladding forming (LCF) is a novel powerful tool for the repairing of metal components. Rebuilding of V-grooves on medium carbon steel substrates has been carried out with laser cladding forming technique using stainless steel powder as the cladding material. Microstructure of the deposited layers has been characterized using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDAX), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Mechanical properties of the rebuilt V-groove samples have been evaluated by tensile and impacting tests and microhardness measurement. Experimental results show that good fusion bonding between the rebuilt layers and the substrate has been formed, and the microstructure of the cladding layers is mainly composed of fine, dense and defect-free epitaxial columnar dendrites. Due to the effect of grain size refinement, the tensile strength, impacting toughness, elongation and microhardness of the rebuilt samples have been greatly enhanced compared to those of the substrate. Microhardness is also very uniform throughout the rebuilt regions. With the growth of the deposited layers, the microhardness increases gradually. The good ductility of the deposited regions is verified by the SEM fracture analysis.     

Quantum-chemical study on the mechanical behaviour of polydimethylsiloxane under deformation: interaction with fumed silica surfaces

Silicone elastomers filled by fumed silica reveal peculiar technical behaviour. Chemical modification of the silica surface is commonly used to control the rheological and mechanical properties of uncured and cross-linked silicone elastomers. To study the microscopic aspect of the problem, quantum-chemical (QCh) modelling in the framework of originally elaborated semiempirical techniques has been performed. The response of a silicone polymer fragment, interacting with a silica model cluster, to external stress is considered in terms of a mechanochemical reaction. The impact of silicone-water and silicone-modified filler interactions on stress-strain characteristics, i.e. Young's modulus, energy and force of deformation and tensile strength have been studied on a molecular level.     

Dynamic photoelastic study of the transient stress field in solids during shock wave lithotripsy

Photoelastic and shadowgraph imaging techniques were used to visualize the propagation and evolution of stress waves, and the resultant transient stress fields in solids during shock wave lithotripsy. In parallel, theoretical analysis of the wavefront evolution inside the solids was performed using a ray-tracing method. Excellent agreement between the theoretical prediction and experimental results was observed. Both the sample size and geometry were found to have a significant influence on the wave evolution and associated stress field produced inside the solid. In particular, characteristic patterns of spalling damage (i.e., transverse and longitudinal crack formation) were observed using plaster-of-Paris cylindrical phantoms of rectangular and circular cross sections. It was found that the leading tensile pulse of the reflected longitudinal wave is responsible for the initiation of microcracks in regions inside the phantom where high tensile stresses are produced. In addition, the transmitted shear wave was found to play a critical role in facilitating the extension and propagation of the microcrack.     

Microwave bonding of poly(methylmethacrylate) microfluidic devices using a conductive polymer

Component binding within microfluidic devices is a problem that has long been seeking a solution. In this investigation, the use of microwave radiation to seal PMMA components has been investigated using polyaniline as an absorber that is capable of inducting interfacial bonding. Straight microchannels were machined into PMMA using a Datron CAT3DM6 CNC machine with widths and depths across a range of 100-1000 μm. Prototype fluidic devices were prepared with channel patterns utilizing varying feature sizes, bends and flow profiling to demonstrate the application of the technique to real microfluidic devices. Experimental data illustrated the successful bonding of channels in the range stated previously and bonding (tensile) strength was assessed via pull tests on bonded PMMA using an Engstrom Zwick 100 tensile testing system (Engstrom Ltd, US). Coherent, defect free seals were attained with breakage tests requiring an excess of 1 kN force.     

Adhesive strength, superhardness, and the phase and elemental compositions of nanostructured coatings based on Ti-Hf-Si-N

New superhard coatings based on Ti-Hf-Si-N with good physical and mechanical properties have been fabricated. A comparative analysis of the physical, mechanical, and tribomechanical characteristics of the coatings has been performed. The values of hardness, modulus of elasticity, elastic recovery, adhesive strength, friction coefficient, and wear rate of the coatings have been determined and calculated. The specific features of deformation and fracture of the coatings deposited on a steel substrate during the adhesion tests have been described. It has been shown that the parameters measured during scratching make it possible to distinguish the threshold values of the critical load, which lead to different (cohesive and adhesive) types of failure of the coatings during tribological tests. The stoichiometry for different series of samples with Ti-Hf-Si-N coatings has been determined using Rutherford backscattering, secondary ion mass spectrometry, and energy dispersive microanalysis.