Force-penetration Curves Research Articles

Energy transfer efficiency and rock damage characteristics of a hydraulic impact hammer with different tool shapes

The energy transfer efficiency and damage characteristics are the theoretical basis for the selection and optimization of hydraulic impact hammer tools. In this paper, quarter three-dimensional models of the piston-tool-rock system are established based on the continuum and discontinuum element method. The incident and reflected stress waves and rock damage characteristics obtained from the simulation model are in good agreement with experimental and theoretical results. The effects of piston impact velocity, tool shape, rock strength, confining pressure and repeated impacts on rock damage zone and energy transfer efficiency between tool and rock are analyzed. The force-penetration curves and penetration coefficients under different impact velocities, rock strength, and confining pressure conditions are obtained. The results illustrate that the energy transfer efficiency and penetration coefficient increase with the increase of rock strength, confining pressure and tip area of tool, but decrease with the increase of impact velocity. Based on the simulation results, a prediction formula for penetration force with consideration of the shape of the tool tip is proposed and the calculation accuracy is verified. Finally, the proportion of energy actually used for rock fracturing is analyzed.

ボタンチップの岩石への静的貫入特性に及ぼすチップ径の影響

Rock drills have been widely used for blast hole drilling in tunnel construction and ore extraction. For the advancement of performance and efficiency of rock drills, it is essential to understand the penetration behavior of a button bit and each of the button tips into rock. In this study, static penetration tests were conducted with three rocks using each of button tips with six diameters. The test results showed that the force-penetration curves are linear in the loading phase and convex downward in the unloading phase for sandstone and andesite and that the curves are convex downward in both the loading and unloading phases for granite, if rock chipping does not occur. The force-penetration curves in the loading phase and deformation mechanisms of the rocks could be explained with the contact mechanics; plastic deformation is dominant for sandstone and andesite, and in contrast elastic deformation is dominant for granite. In addition, the relations of the diameter of a button tip, the Young’s modulus and yield stress of rock to the force-penetration curves were clarified. These results indicate that force-penetration curves are estimated beforehand from mechanical properties of the target rock. Occurrence of rock chipping was found to be influenced by not only the diameter of a button tip but also the heterogeneity of rock texture.

Crack-front model for adhesion of soft elastic spheres with chemical heterogeneity

Adhesion hysteresis can be caused by elastic instabilities that are triggered by surface roughness or chemical heterogeneity. However, the role of these instabilities in adhesion hysteresis remains poorly understood because we lack theoretical and numerical models accounting for realistic roughness. Our work focuses on the adhesion of soft elastic spheres with low roughness or weak heterogeneity, where the indentation process can be described as a Griffith-like propagation of a nearly circular external crack. We discuss how to describe the contact of spheres with chemical heterogeneity that leads to fluctuations in the local work of adhesion. We introduce a variational first-order crack-perturbation model and validate our approach using boundary-element simulations. The crack-perturbation model faithfully predicts contact shapes and hysteretic force-penetration curves, provided that the contact perimeter remains close to a circle and the contact area is simply connected. Computationally, the crack-perturbation model is orders of magnitude more efficient than the corresponding boundary element formulation, allowing for realistic heterogeneity fields.

Temperature control for high-quality oil-free sweet potato CHIPS produced by microwave rotary drying under vacuum

Healthy and convenient foods have drawn consumers' attention and represent an opportunity for the food industry. This study is about the production of sliced sweet potato snacks by microwave vacuum drying (MWVD) in a rotary container with temperature control by power modulation. The average temperature of samples' surfaces was measured with an infrared sensor and controlled at 60 °C by a PID controller. The resulting product properties were compared to those from freeze- (FD), air- (AD), and conductive multiflash (KMFD) drying. Rotary container coupled to the temperature control guaranteed uniform heating and resulted in chips without burnt spots in MWVD.MWVD and KMFD produced sweet potato chips in less than 3 h, while AD and FD required 16 h. The drying method influenced chips' porosity: 58% (FD), 44% (MWVD), 39% (KMFD), and 18% (AD). AD chips had a compact structure, while FD chips were spongy with small pores. MWVD and KMFD expanded the material during fast water evaporation under vacuum, creating large and irregular pores. The puncture tests of these chips resulted in jagged force-penetration curves, typical of crispy products, with higher acceptance. Therefore, MWVD and KMFD have the potential for the industrial production of high-quality oil-free and crispy chips.

Response surface methodology calibration for DEM study of the impact of a spherical bit on a rock

The discrete element method (DEM) is frequently used for the numerical analysis of rock fractures. The DEM model requires the specification of microparameters that cannot be measured and are not directly related to the macroscopic properties of the material. Therefore, a calibration process for the microparameters is required to simulate the rock behavior. Since the calibration is usually performed iteratively using the trial and error method, several DEM simulations are run. Therefore, the calibration process is computationally expensive. In this work, we propose a calibration method based on the response surface methodology (RSM) that significantly reduces the numerical simulations to be performed during the calibration process, and thus, the associated computational cost. This methodology is novelty applied to impact problems and, it is validated by means of a benchmark, the good agreement obtained between the predicted and observed results verifies the applicability of the proposed method. A force-penetration curve is obtained through this approach, and the energy–time history is plotted. Moreover, several samples are tested to demonstrate the robustness of the solution. The effect of the stress wave under infinite boundary condition is studied. The method is capable of capturing the stress wave movement, and the velocity of the stress wave computed from the numerical results is in good agreement with experimental data.

A damage model for granite subjected to quasi‐static contact loading

AbstractAn anisotropic damage model is employed in order to simulate the fracture pattern of Bohus granite under quasi‐static (Q‐S) spherical indentation loading. The chosen damage description is added to the previously employed Drucker–Prager (DP) plasticity model with variable dilation angle. The resulting constitutive model is implemented to simulate the behavior of Bohus granite under indentation up to the load capacity of the material. The initial fragmentation, corresponding to the first small load‐drop in the force–penetration curve, is likely due to the high radial tensile stress state at or close to the contact boundary. Both predicted fracture pattern and force–penetration results from the numerical simulation are compared to experimental data and a good agreement is found. The variability in tensile strength of the material is included in the chosen damage model by taking advantage of Weibull statistics. In this work, it is suggested that indentation test results by themselves may be used to calibrate the statistical distribution of tensile strength at small size scales such as in indentation applications.

Development and application of an in-situ indentation testing system for the prediction of tunnel boring machine performance

Prediction of the tunnel boring machine (TBM) performance is critically important for project scheduling and cost estimation. Various models have been proposed to estimate the TBM performance. However, most of these models used the parameters of rock specimens acquired from the laboratory tests, which differed from the in-situ conditions. It is difficult to apply these models to the engineering practice. In the present study, to overcome these limitations, an in-situ indentation testing system for the force-penetration response was proposed and applied to a TBM-excavated tunnel. A database that integrates 33 groups of in-situ indentation testing (235 tests in total) and the corresponding TBM operating parameters was established, including 12 indentation indices extracted from the force-penetration curve and 6 operating parameters recorded by the TBM. A series of correlation analysis was carried out to investigate the relationships between these indices/parameters. The analysis indicates that the 12 indentation indices are highly or moderately correlated with each other. A similar phenomenon was found in the TBM operating parameters. Consequently, four indentation indices and one operating parameter (field penetration index, FPI) were selected to establish the predictive models. Predictive models of the TBM performance (FPI) with high correlation coefficient ( r = 0.960) were proposed using the multiple indices obtained from the in-situ indentation testing.

Mechanical and Acoustic Emission Responses of Rock Fragmentation under Disc Cutter Penetration

To better understand the rock fragmentation mechanism and optimize cutter design and selection for rock excavation by TBM, a set of three-dimensional indentation tests was conducted with different rock and cutter types. Acoustic emission (AE) monitoring technique was employed to capture the information of the rock damage evolution real-timely. It is found that the penetration by using the constant cross section (CCS) cutter tends to induce inconspicuous rock chips formation before the sudden occurrence of the macrocrack, but that by using the V-type cutter tends to induce gradual rock fragmentation accompanied by the multiple local rock chips formation and the sawtooth force-penetration curve. Meanwhile, rock fragmentation models for CCS and V-type cutters were compared, and the spatio-temporal evolution of AE events was quantitatively analyzed to reflect the rock damage zone development process. Results indicate that the V-type cutter has greater penetration ability and the CCS cutter can cause larger unit damage zone radius. Microscopic observation by using the scanning electron microscope (SEM) reveals that the fracture mechanism for the crushed zone of rock is mainly shear type and that for the major crack is tensile type. It does not matter with rock types or cutter types.

Experimental Investigation and Numerical Analyses for Red Sandstone Rock Fragmentation

The effects of initial stress conditions and rock breaking parameters on rock fragmentation were investigated through indentation tests and numerical analysis with a discrete element method (DEM). Different initial stress conditions and the thrusting velocity of the cutter were the focus in the design of indentation tests. Some indicators were used to reflect the instability of the thrusting force, rock breaking efficiency, failure patterns, and the fluctuation of force-penetration response. The indentation results show that rock failure patterns have a closed correlation with initial stress conditions and loading rate. All tests showed several distinct chipping phases, and obvious chiseled pit and crushed rock could be observed on the top surface of each rock specimen. Moreover, rock brittleness was evaluated by the fluctuation of force-penetration curves; this study also revealed the changing law of peak and mean thrusting force. In addition, the numerical simulation of indentation tests reproduced the development process of rock fragmentation and crack propagation. Increasing confining stress restrained the propagation of vertical and lateral cracks, whereas the axial stress had the effect of restraining the propagation of radial cracks. However, there is a critical confining stress that causes different results of fractured depth. Therefore, this investigation is beneficial for optimizing the cutting parameters to promote rock breaking efficiency.

Mixed-mode crack propagation during needle penetration for surgical interventions

An accurate description of the penetration mechanics of flexible needles into target soft tissues is a complex task, including friction at the needle-tissue interface, large strains, non-predetermined penetration trajectories, fracture under mixed-mode loading and so on. In the present work, a finite element algorithm is employed to simulate the two-dimensional deep penetration of a flexible needle in a soft elastic material. The fracture process of the target material during penetration is described by means of a cohesive zone model, with a suitable mixed-mode criterion for determining the propagation direction of the crack. To illustrate the potential of the numerical algorithm, we have performed some simulations of the insertion of a flexible needle with an asymmetric tip, and the results are presented in terms of force-penetration curves as well as of the obtained penetration paths in the target tissue.

Ship collision damage assessment and validation with experiments and numerical simulations

Closed-form expressions to estimate the energy absorption and damage extent for severe ship collision damages were initially developed in 1999 [1, 2], and further validated with experimental data in 2016 [3]. To gain further confidence for applications within design using the proposed analytical procedure, it is evident that more detailed and comprehensive comparisons and validations with experiments and numerical simulations are necessary. The purpose of the present paper is to use the analytical approach and finite element analyses to study in depth model-scale and full-scale collision tests so that to further quantify key calculation parameters and to verify the capability and accuracy of the proposed analytical method. In total 18 experimental tests and one full-scale collision accident are evaluated. The 18 experimental energy absorption-penetration and collision force-penetration curves, and the associated finite element simulations, are compared with results obtained from the analytical calculations. It can be concluded that the analytical method gives consistently good agreement with all experiments analysed here. Finally, an application of the analytical method is demonstrated by an example where speed restrictions are determined in a port to avoid LNG cargo leakage in an event of an LNG carrier being struck by another ship.

Development of a new rock drillability index for oil and gas reservoir rocks using punch penetration test

Punch penetration test has shown a good relationship with the rate of penetration (ROP) of tunnel boring machine (TBM) into hard rock. In addition, various indices have been determined from the analysis of the force-penetration curve of the penetration test to present rock drillability, boreability, and brittleness. Therefore, in the present paper, punch penetration test will be used to determine rock drillability index (RDI). In this research, 9 samples were taken from the same formation in two wells drilled at southwestern Iran. Considering the limitations in taking more samples, and also to enhance reliability of the results, each of the samples was divided into smaller specimens, so as to conduct multiple tests at each sampling point. Evaluation of the force-penetration curves of samples indicated that most samples of well 1 have little changes of forces early in the test, but they increase as the test proceeds. Because the samples from the Well 1 were taken from shallow depths, The change in the slope of the curve determined on the basis of the samples taken from Well 1 can be explained by the pores existing in these samples. Different indices were extracted from the analysis of the force-penetration curve of each specimen, including first peak slope index (BI1), maximum peak slope index (BI2), area under the curve (BI3), and RDI. Initial examination of relationships between each of indices and the measured ROP follow an either exponential or power-law model. Evaluation of these models between each of these indices and actual ROP indicated that, an exponential curve gives the best fit to the obtained values. A comparison between estimated ROPs from the fitted relationships against measured values showed that the estimated ROP using the proposed index is highly accurate and reliable and can present a good measure of the reservoir rock drillability.

Modeling of force-penetration curves for a button bit during impact penetration into rock

The discrete element method (DEM) is frequently used for the numerical analysis of rock fractures. The DEM model requires the specification of microparameters that cannot be measured and are not directly related to the macroscopic properties of the material. Therefore, a calibration process for the microparameters is required to simulate the rock behavior. Since the calibration is usually performed iteratively using the trial and error method, several DEM simulations are run. Therefore, the calibration process is computationally expensive. In this work, we propose a calibration method based on the response surface methodology (RSM) that significantly reduces the numerical simulations to be performed during the calibration process, and thus, the associated computational cost. This methodology is novelty applied to impact problems and, it is validated by means of a benchmark, the good agreement obtained between the predicted and observed results verifies the applicability of the proposed method. A force-penetration curve is obtained through this approach, and the energy–time history is plotted. Moreover, several samples are tested to demonstrate the robustness of the solution. The effect of the stress wave under infinite boundary condition is studied. The method is capable of capturing the stress wave movement, and the velocity of the stress wave computed from the numerical results is in good agreement with experimental data.

Quantitative Hardness Measurement by Instrumented AFM-indentation

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness measurements on an Au(111) surface with atomistic resolution of single plasticity events. A careful experimental procedure is described that includes the force sensor selection, its calibration, the calibration of the cantilever deflection detection system, and the minimization of instrumental drift for accurate and reproducible force-distance measurements. Also, a method for the data analysis is presented that allows the extraction of force-penetration curves from recorded force-distance curves. A typical curve displays a clear elastic deformation regime up to the first plasticity event, or pop-in, with a length in the range of one to two Burger's vectors. Later plasticity events exhibit the same magnitude. The work of plasticity is further extracted from the measurements. Finally, the hardness is determined in combination with the indentation curve using non-contact atomic force microscopy images of the remaining indents.

Experimental and numerical study of Kuru granite under confined compression and indentation

The paper presents an experimental and numerical investigation on the response of Kuru granite rock under confining compression and static indentation. Triaxial compression tests were executed at different levels of confining pressure. Indirect tensile tests were carried out with Brazilian disks, and quasi-static indentation tests with a spherical indenter were conducted. An elasto-plastic material model with piecewise yield function, plastic hardening/softening, coupled damage and non-associated plastic flow is presented and calibrated using the experimental test results. Finite element simulations of the triaxial compression tests and static indentation are performed. The influence of the material model features (e.g., the yield function and non-associativity) is discussed, and the effect of confining pressure during indentation is evaluated. It was found that, within the investigated pressure range, the dilatancy angle of the rock linearly decreased with pressure. With a saturating yield function, the simulated force-penetration curve is in good agreement with the experimental curve for the loading phase, but the simulated stiffness during unloading and the subsequent residual penetration are overestimated. The numerical simulation showed that the peak force of indentation increased approximately 11% with a confining pressure of 150MPa compared to the unconfined case. The shape of the equivalent stress field near the free surface of the rock is affected by the confining pressure as well.

Configurational phases in elastic foams under lengthscale-free punching

We carry out experiments with brick-like specimens of elastic open-cell (EOC) foams of three relative densities. Individual specimens may be “tall” (height = width = depth) or “short” (2height=width=depth). We place each specimen on a supporting plate and use a lengthscale-free (wedge-shaped or conical) punch to apply forces downward along the specimen's height. Regardless of the type of specimen, the force–penetration curves remain linear, for the wedge-shaped punch, or quadratic, for the conical punch, up to a sizable penetration commensurate with the smallest lengthscale of the specimen. After that there is an abrupt, all-but-discontinuous change in stiffness: if the specimen is tall, the stiffness drops; if the specimen is short, the stiffness shoots up. To analyze these curious experimental results, we posit that EOC foams can be found in either of two configurational phases, here termed the low-strain phase and the high-strain phase, which share a two-dimensional interface (a surface of strain discontinuity). The analysis may be outlined as follows. In the first part of an experiment, there obtains a “similarity regime” in which the penetration of the punch and the radius of the interface are the only prevailing lengthscales (because the punch is lengthscale free). In this case, it is possible to show that the force–penetration curve must be linear, or quadratic, depending on whether the punch be wedge-shaped or conical, respectively. This prediction of the analysis is consistent with the experiments. In time, the similarity regime breaks down when the interface reaches one of the specimen's boundaries distal to the tip of the punch. If the specimen is tall, the soft, stress-free lateral boundary is reached first, and the stiffness must drop; if the specimen is short, the hard boundary in contact with the supporting plate is reached first, and the stiffness must shoot up. These predictions too are consistent with the experiments. To provide direct empirical evidence of the interface, we use a digital-image correlation method. Lastly, we run computational simulations of all the experiments, using finite elements and the skeleton-and-bubble model of EOC foams. The computational results are in good accord with the experimental ones, and they allow us to carry out a detailed validation of the analysis. Our findings evince the cardinal role of configurational phases in the mechanics of EOC foams.

Force-penetration curves of a button bit generated during impact penetration into rock

To improve the performance and efficiency of percussion rock drills, it is essential to understand the impact penetration behavior of a bit drilling into rock. However, little knowledge to date has been published about the penetration behavior of the button bit, which is commonly used for hard rock. In this study, the impact penetration behavior of the button bit was investigated in laboratory tests. An impact penetration tester built from the same components used in an actual rock drill was developed and used in single-blow impact penetration tests. Unnatural fluctuations were observed in the force-penetration curves calculated using a two-point strain measurement method, which were probably due to not only the difference in rod stress as measured at two points on the rod, but also to the mismatch between the actual bit and the calculation model. A data correction method, in which the bit force calculated in a free end test is subtracted from that in the impact penetration test, was proposed using a numerical simulation. The method was applied to the measured rod stress, and force-penetration curves with a button bit were obtained from more than 40 impact penetration tests. Graph curves showed that peak force, initial slope and elastic penetration increased, while the maximum and final penetrations decreased with an increase in the secant slope. The crushing and elastic strain energies changed, but the sum of the energies was constant with an increase in the secant slope. The variations in the force-penetration curves are caused by the contact conditions between the bit and rock, the rock properties, and damage to the rock caused with each blow, not by the test conditions. The findings of this study will contribute to progress in the performance and efficiency of percussive rock drilling.

衝撃貫入試験における貫入抵抗曲線の算出方法の検討

衝撃貫入試験における貫入抵抗曲線の算出方法の検討

Mechanical Properties of the Honeycomb Nanoporous Membranes

Mechanical properties of the nanoporous film with the relative density of can be determined using the nanoindentation. Using the 3D finite element method, the force-penetration curves are discussed. The elastic and plastic properties of the nanoporous membranes are derived from the loading-unloading curves.

Special Issue on Multi-Scale Experimental Measurements in Mechanics

The development of micro/nano science and technology in recent years has created new challenges to the traditional macro-scale based experimental measurement. As such, multi-scale experimental measurements in mechanics have become a key area of research. Both the metrology and its applications have drawn great interest from researchers in related fields and have been progressing rapidly. It is our pleasure to present the nine papers in this special issue, entitled “Multi-Scale Experimental Measurements in Mechanics.” The special issue attempts to gather papers in the developments of multi-scale metrologies and related applications. It covers a range of topics to reflect the recent advances in optical measurement methods, non-destructive techniques, mechanical behavior characterization under different high resolution microscopes, fabrication techniques on micro-deformation sensor, etc. A brief introduction of these papers is as follows: In regard to multi-scale experiments including uniaxial tensile testing, in situRaman spectroscopy and scanning electron microscopy, Wei-Lin Deng et al. investigated the deformation in multi-scale and interfacial mechanical behavior of carbon nanotube fibers with multi-level structures. A twolevel interfacial mechanical model is presented to analyze interfacial bonding strengths of mesoscopic bundles and microscopic nanotubes. This study shows that properties of the multi-level interfaces are critical factors for determining fiber strength and toughness. With the aid of the atomic force microscopy (AFM) indentation system, Dongchuan Su et al. studied the near-surface elastic modulus via force-penetration curves acquired during indentation. An analysis algorithm is proposed based on the secant modulus method to extract the true penetration depths from force-displacement curves in AFM indentation testing. The results show that the AFM indentation method is capable of producing high spatial resolution maps of the elastic modulus with small forces and shallow indentations. MEMS sensors are typically fabricated out of materials that are mechanically sound at the microscale, but can be relatively poor electrical conductors. For this reason, areas ofMEMS are coated with various thin metal films to provide electrical pathways. These films, however, adversely alter resonant properties of a device. The paper by Pryputniewicz reviews a theoretical analysis of the effect that thermoelastic internal friction has on the Q-factor of microscale resonators and shows that the internal friction relating to TED is a fundamental damping mechanism in determination of quality of high-Q resonators over a range of operating conditions. As a method for measuring full-field out-of-plane displacement, projection moire provides high quality results with simple test setup. B. Gulker et al. implemented a projection moire system in low-velocity impact testing using an image processing program. Results from the projection moire experiments agree reasonably well with those obtained from the commonly used load cell method. The technique is further applied to composites with various microstructures. Y.R. Zhao et al. developed the electron grid fabricating technique by using a common scanning electron microscope (SEM). An error analysis for the multi-scanning grating was Y. Kang School of Mechanical Engineering, Tianjin University, Tianjin, China