Showing papers on "Dynamic load testing published in 2010"

Studies of dynamic crack propagation and crack branching with peridynamics

Abstract: In this paper we discuss the peridynamic analysis of dynamic crack branching in brittle materials and show results of convergence studies under uniform grid refinement (m-convergence) and under decreasing the peridynamic horizon (δ-convergence) Comparisons with experimentally obtained values are made for the crack-tip propagation speed with three different peridynamic horizons We also analyze the influence of the particular shape of the micro-modulus function and of different materials (Duran 50 glass and soda-lime glass) on the crack propagation behavior We show that the peridynamic solution for this problem captures all the main features, observed experimentally, of dynamic crack propagation and branching, as well as it obtains crack propagation speeds that compare well, qualitatively and quantitatively, with experimental results published in the literature The branching patterns also correlate remarkably well with tests published in the literature that show several branching levels at higher stress levels reached when the initial notch starts propagating We notice the strong influence reflecting stress waves from the boundaries have on the shape and structure of the crack paths in dynamic fracture All these computational solutions are obtained by using the minimum amount of input information: density, elastic stiffness, and constant fracture energy No special criteria for crack propagation, crack curving, or crack branching are used: dynamic crack propagation is obtained here as part of the solution We conclude that peridynamics is a reliable formulation for modeling dynamic crack propagation

Optimization of Variable-Stiffness Panels for Maximum Buckling Load Using Lamination Parameters

Abstract: With the large-scale adoption of advanced fiber placement technology in industry, it has become possible to fully exploit the anisotropy of composite materials through the use of fiber steering. By steering the composite fibers in curvilinear paths, spatial variation of stiffness can be induced resulting in beneficial load and stiffness distribution patterns. One especially relevant area in which fiber steering has proved its effectiveness is in improving buckling loads of composite panels. Previous research used predefined forms of fiber angle variations and the coefficients of these analytic expressions were used as design variables. Alternatively, the local ply angles were used as design variables directly. In this paper, a framework is developed to treat the most general approach that considers the largest possible design space. The use of lamination parameters efficiently defines stiffness variation over a structural domain with the minimum number of variables. A conservative reciprocal approximation scheme is introduced. The inverse buckling factor is expanded linearly in terms of the in-plane stiffness and in terms of the inverse bending stiffness. The new approximation scheme is convex in lamination parameter space. Numerical results demonstrate improvements in excess of 100% in buckling loads of variable-stiffness panels compared to the optimum constant stiffness designs. Buckling load improvements are attributed primarily to in-plane load redistribution, which is confirmed both by the prebuckling stress distribution as well as by comparing the performance of designs optimized with variation of both in-plane and bending stiffness to those optimized with only bending stiffness variation. A tradeoff study between in-plane stiffness and buckling performance is also presented and shows the benefits of variable-stiffness design in enlarging the design possibilities of composite panels.

An Experimental Method to Determine the Tensile Strength of Concrete at High Rates of Strain

Abstract: In the present work, dynamic tensile strength of concrete is experimentally investigated by means of spalling tests. Based on extensive numerical simulations, the paper presents several advances to improve the processing of spalling tests. The striker is designed to get a more uniform tensile stress field in the specimen. Three methods proposed in the literature to deduce the dynamic strength of the specimen are discussed as well as the use of strain gauges and a laser extensometer. The experimental method is applied to process data of several tests performed on wet micro-concrete at strain rates varying from 30 to 150/s. A significant increase of the dynamic tensile strength with strain-rate is observed and compared with data of the literature. In addition, post-mortem studies of specimens are carried to improve the analysis of damage during spalling tests.

A Semi-Circular Bend Technique for Determining Dynamic Fracture Toughness

Abstract: We propose and validate a fracture testing method using a notched core-based semi-circular bend (SCB) specimen loaded dynamically with a modified split Hopkinson pressure bar (SHPB) apparatus. An isotropic fine-grained granitic rock, Laurentian granite (LG) is tested to validate this dynamic SCB method. Strain gauges are mounted near the crack tip of the specimen to detect the fracture onset and a laser gap gauge (LGG) is employed to monitor the crack surface opening distance. We demonstrate that with dynamic force balance achieved by pulse shaping, the peak of the far-field load synchronizes with the specimen fracture time. Furthermore, the evolution of dynamic stress intensity factor (SIF) obtained from the dynamic finite element analysis agrees with that from quasi-static analysis. These results prove that with dynamic force balance in SHPB, the inertial effect is minimized even for samples with complex geometries like notched SCB disc. The dynamic force balance thus enables the regression of dynamic fracture toughness using quasi-static analysis. This dynamic SCB method provides an easy and cost-effective way to measure dynamic fracture toughness of rocks and other brittle materials.

Dynamic compression of metallic sandwich structures during planar impulsive loading in water

Abstract: The compressive response of rigidly supported stainless steel sandwich panels subject to a planar impulsive load in water is investigated. Five core topologies that spanned a wide range of crush strengths and strain-dependencies were investigated. They included a (i) square-honeycomb, (ii) triangular honeycomb, (iii) multi-layer pyramidal truss, (iv) triangular corrugation and (v) diamond corrugation, all with a core relative density of approximately 5%. Quasi-statically, the honeycombs had the highest peak strength, but exhibited strong softening beyond the peak strength. The truss and corrugated cores had significantly lower strength, but a post yield plateau that extended to beyond a plastic strain of 60% similar to metal foams. Dynamically, the transmitted pressures scale with the quasi-static strength. The final transmitted momentum increased slowly with core strength (provided the cores were not fully crushed). It is shown that the essential aspects of the dynamic response, such as the transmitted momentum and the degree of core compression, are captured with surprising fidelity by modeling the cores as equivalent metal foams having plateau strengths represented by the quasi-static peak strength. The implication is that, despite considerable differences in core topology and dynamic deformation modes, a simple foam-like model replicates the dynamic response of rigidly supported sandwich panels subject to planar impulsive loads. It remains to ascertain whether such foam-like models capture more nuanced aspects of sandwich panel behavior when locally loaded in edge clamped configurations.

Dynamic crack propagation analysis of orthotropic media by the extended finite element method

Abstract: Dynamic crack propagation of composites is investigated in this paper based on the recent advances and development of orthotropic enrichment functions within the framework of partition of unity and the extended finite element method (XFEM). The method allows for analysis of the whole crack propagation pattern on an unaltered finite element mesh, defined independent of the existence of any predefined crack or its propagation path. A relatively simple, though efficient formulation is implemented, which consists of using a dynamic crack initiation toughness, a crack orientation along the maximum circumferential stress, and a simple equation to presume the crack speed. Dynamic stress intensity factors (DSIFs) are evaluated by means of the domain separation integral method. The governing elastodynamics equation is first transformed into a standard weak formulation and is then discretized into an XFEM system of time dependent equations, to be solved by the unconditionally stable Newmark time integration scheme. A number of benchmark and test problems are simulated and the results are compared with available reference results.

Dynamic fragmentation process in concrete under impact and spalling tests

Abstract: Intense damages as scabbing on front face, spalling on rear face, radial cracks are observed in concrete structures when subjected to the impact of a kinetic striker. To characterize the dynamic strength and damage of concretes under such loadings one may perform spalling tests and EOI (edge-on impact) tests. Both tests have been conducted with dry and wet specimens of a micro-concrete named MB50. The tests revealed a remarked effect of strain-rate and free water on the dynamic response of the concrete. In parallel, bending tests and direct tensile tests have been performed with dry and saturated concrete samples considering large and small effective volumes and the results have been compared with five sets of data given in the literature with the same material. A scale effect is observed in agreement with prediction of Weibull theory. Moreover, an anisotropic damage model that describes the initiation of cracks and the obscuration of critical defects under high strain-rate tensile loading is presented. It allows accounting for the influence of loading-rate and free-water on the dynamic strength and damage of concrete observed in EOI and spalling tests.

High speed tensile behavior of sisal fiber cement composites

Abstract: The experimental behavior of sisal fiber reinforced cement composites subjected to high speed tension load was studied. High strain rates were achieved by using a high rate servo-hydraulic testing machine. A state-of-the-art high speed Phantom camera was also used to take images from the specimen during the test. The images were used in a digital image correlation model to determine the displacement fields and to calculate crack spacing. The effect of strain rate was investigated by comparing static and dynamic tensile tests which were performed at strain rates ranging from 5.5 × 10−6 to 24.6 s−1, respectively. A numerical tension stiffening model based on nonlinear finite difference method was used to simulate tensile cracking behavior of sisal fiber cementitious composites. The composite presented strain rate sensitivity for ultimate tensile strength and strain capacity with a dynamic amplification factor of 1.26.

Communication across an inductive link with a dynamic load

Abstract: The present invention provides a load used for communication in a remote device having a dynamic communication load configuration. In one embodiment, the dynamic communication load configuration varies as a function of a characteristic of power in the remote device. The remote device toggles between load configurations to communicate with the inductive power supply. A sensor in the remote device detects a characteristic of power in the remote device and configures the communication load based on the sensor output. In another embodiment, the remote device adjusts the dynamic communication load configuration in the remote device in response to a failure to receive a response from the inductive power supply.

In situ determination of the static inductance and resistance of a plasma focus capacitor bank.

Abstract: The static (unloaded) electrical parameters of a capacitor bank are of utmost importance for the purpose of modeling the system as a whole when the capacitor bank is discharged into its dynamic electromagnetic load. Using a physical short circuit across the electromagnetic load is usually technically difficult and is unnecessary. The discharge can be operated at the highest pressure permissible in order to minimize current sheet motion, thus simulating zero dynamic load, to enable bank parameters, static inductance L0, and resistance r0 to be obtained using lightly damped sinusoid equations given the bank capacitance C0. However, for a plasma focus, even at the highest permissible pressure it is found that there is significant residual motion, so that the assumption of a zero dynamic load introduces unacceptable errors into the determination of the circuit parameters. To overcome this problem, the Lee model code is used to fit the computed current trace to the measured current waveform. Hence the dynamics...

Component-based aggregate load models for combined power flow and harmonic analysis

Abstract: Aggregate load models traditionally used for steady state analysis of power systems are based on standard constant impedance/current/power representation of static load component, with induction motor representation of dynamic load component These traditional load models cannot accurately represent characteristics of non-linear power electronic loads, the impact of local distributed generation (DG) on the aggregate load demand, or the effects of demand-manageable loads Furthermore, the assessment of the contribution of non-linear loads and inverter-interfaced DG to the flow and propagation of harmonic currents is usually not possible using the traditional aggregate load models This paper uses a component-based approach to build improved aggregate load models that are capable of preserving full information on electrical characteristics of aggregated load, enabling to use the same load models for both the analysis of power flows and voltage profiles, and analysis of harmonics Furthermore, by modelling aggregate system load as a composition of general load categories, it is possible to identify the exact portion of available demand-manageable load in the aggregate demand, and then investigate the actual effects of the controlled changes in demand on system performance The developed load models are implemented in two "sister papers" that accompany this one, where they are combined with the models of small/large DG to perform steady state analysis of network performance (power flows, voltage profiles, overloading of system components, harmonic analysis and a simple case of demand-side management) (10 pages)

Erratum to: Rock Dynamic Fracture Toughness Tested with Holed-Cracked Flattened Brazilian Discs Diametrically Impacted by SHPB and its Size Effect

Abstract: Dynamic fracture initiation toughness of marble was tested using two types of the holed-cracked flattened Brazilian disc (HCFBD) specimens, which were diametrically impacted at the flat end of the disc by the split Hopkinson pressure bar (SHPB) of 100 mm diameter. One type of the discs is geometrically similar with different outside diameter of 42 mm, 80 mm, 122 mm and 155 mm respectively, and with crack length being half the diameter; another type of the discs has identical 80 mm diameter and different crack length. Issues associated with determination of the stress wave loading by the SHPB system and the crack initiation time in the disc specimen were resolved using strain gage technique. The stress waves recorded on the bars and the disc failure patterns are shown and explained. The tested dynamic fracture toughness increases obviously with increasing diameter for the geometrically similar HCFBD specimens. It changes moderately for the one-size specimens of identical diameter and different crack length. The size effect of rock dynamic fracture toughness is mainly caused by the fracture process zone length l and fracture incubation time τ, the latter being an additional influencing factor for the dynamic loading as compared with the counterpart static situation. Hence a method is proposed to determine a unique value for the dynamic fracture initiation toughness, the approach takes average of the local distribution and time history for dynamic stress intensity factor in the spatial-temporal domain, which is defined by l and τ jointly. In this way the dynamic size effect is minimized.

Influence of dynamic load on friction behavior of human articular cartilage, stainless steel and polyvinyl alcohol hydrogel as artificial cartilage.

Abstract: Many biomaterials are being developed to be used for cartilage substitution and hemiarthroplasty implants. The lubrication property is a key feature of the artificial cartilage. The frictional behavior of human articular cartilage, stainless steel and polyvinyl alcohol (PVA) hydrogel were investigated under cartilage-on-PVA hydrogel contact, cartilage-on-cartilage contact and cartilage-on-stainless steel contact using pin-on-plate method. Tests under static load, cyclic load and 1 min load change were used to evaluate friction variations in reciprocating motion. The results showed that the lubrication property of cartilage-on-PVA hydrogel contact and cartilage-on-stainless steel contact were restored in both 1 min load change and cyclic load tests. The friction coefficient of PVA hydrogel decreased from 0.178 to 0.076 in 60 min, which was almost one-third of the value under static load in continuous sliding tests. In each test, the friction coefficient of cartilage-on-cartilage contact maintained far lower value than other contacts. It is indicated that a key feature of artificial cartilage is the biphasic lubrication properties.

Elasto-plastic instability of single-layer reticulated shells under dynamic actions

Abstract: This paper addresses the issue on dynamic collapse mechanism of single-layer reticulated shells subjected to harmonic load, sudden load and seismic load. The method for failure mechanism has reviewed the relationship between the response of the reticulated shell and the peak acceleration of dynamic action. Besides, the steel damage accumulation is considered in the method by compiling a user subroutine based on the computing program ABAQUS. An example is introduced to describe dynamic instability collapse resulting from geometric nonlinearity of the shell and the other example is presented to describe strength failure resulting from excessive development of plastic deformation, for it is discovered that the structure is not only to be prone to instability collapse in dynamic action. According to the responses of the single-layer reticulated shell under dynamic loads, this study discusses the relationship between the failure model and the corresponding dynamic load parameters. Then, the method for distinguishing failure modes is proposed based on the fuzzy synthetic evaluation theory and the structural responses of sufficient samples at the failure state. The technique feasible to be used to distinguish different failure modes of single-layer reticulated shells under different dynamic loads is validated.

Reliability-based calibration of resistance factors for static bearing capacity of driven steel pipe piles

Abstract: As part of a study to develop load and resistance factor design (LRFD) codes for foundation structures in South Korea, resistance factors for the static bearing capacity of driven steel pipe piles were calibrated in the framework of the reliability theory. A database of 52 static load test results was compiled, and the data from these load test piles were sorted into two cases: a standard penetration test (SPT) N-value at pile tip (i) less than 50 and (ii) equal to or more than 50. Reliability analyses and resistance factor calibration for the two static bearing capacity analysis methods adopted in the Korean Design standards for foundation structures were performed using the first-order reliability method (FORM) and the Monte Carlo simulation (MCS). Reliability indices and resistance factors computed by the MCS are statistically identical to those computed by FORM. Target reliability indices were selected as 2.0 and 2.33 for the group pile case and 2.5 for the single pile case. The resistance factors rec...

Numerical Study on Dynamic Swing of Suspension Insulator String in Overhead Transmission Line under Wind Load

Abstract: Numerical modeling of an overhead transmission line section and the digital simulation of stochastic wind field, to which the transmission line section is exposed, are presented. Time-domain analyses of typical transmission line sections in stochastic wind fields are carried out by ABAQUS software. It is discovered that the numerically determined dynamic swing angles of the suspension insulator strings in the transmission line sections are larger than those calculated with the formulas proposed in the technical code for designing overhead transmission lines. A dynamic wind load factor is suggested to be introduced into the formulas proposed in the design code.