Machine Stiffness Research Articles

Synergizing Structural Stiffness Regulation with Compliance Contact Stiffness: Bioinspired Soft Stimuli‐Responsive Materials Design for Soft Machines

Stiffness regulation strategies endow soft machines with stronger functionality to cope with diverse application requirements, for example manipulating heavy items by improving structural stiffness. However, most programmable stiffness strategies usually struggle to preserve the inherent compliant interaction capabilities following an enhancement in structural stiffness. In this study, inspired by the musculocutaneous system, we propose a soft stimuli‐responsive material (SRM) by combining shape memory alloy into compliant materials. By characterizing the mechanical performance, the flexural modulus increases from 6.6 to 142.4 MPa under the action of active stimuli, crossing two orders of magnitude, while Young's modulus stays at 2.2 MPa during programming structural stiffness. This comparative result indicates that our SRMs can keep a lower contact stiffness for compliant interaction although structural stiffness increases. Then, we develop three diverse soft machines to show the application potential of this smart material, such as robotic grippers, wearable devices, and deployable mechanisms. By applying our materials, these machines possess stronger load‐bearing capabilities. Meanwhile, these demonstrations also illustrate the efficacy of this paradigm in regulating the structural stiffness of soft machines while maintaining their compliant interaction capabilities.

Influence of Machine Stiffness on the Portevin–Le Chatelier Effect of Ti-12Mo Alloy Based on a Modified McCormick Constitutive Model

Influence of Machine Stiffness on the Portevin–Le Chatelier Effect of Ti-12Mo Alloy Based on a Modified McCormick Constitutive Model

Study on the effect of rolling mill dynamic stiffness on coupled vibration of hydraulic machine

The productivity and capacity release of hot-strip rolling is severely limited by pendular vibration. Previous studies on mill vibration have investigated the influence of rolling parameters on vertical vibration; however, the influence of dynamic stiffness compensation factors on vertical vibration has not yet been considered owing to the limitations of modelling methods. Herein, we develop a simulation model of mill liquid–machine coupling with dynamic stiffness compensation using AMESim software. The established simulation model is used to investigate the effect of this dynamic stiffness compensation on mill vertical vibration under conditions of downfeed thickness difference excitation frequency, thickness difference excitation amplitude, thickness difference excitation phase, and dynamic stiffness compensation signal generation hysteresis. Evidently, the impact of the mill stiffness compensation on the mill vertical vibration differs with the thickness of the excitation frequency, and the impact should be discussed according to the excitation frequency segment. The amplitudes of the roll system vibration and incoming thickness difference excitation are essentially linear, whereas the excitation phase does not affect the frequency or amplitude of the mill vibration. The rolling machine stiffness compensation signal hysteresis also has a significant impact on the mill vibration amplitude. Our findings will improve the mechanism of the hot rolling process and suppress droop vibrations.

Structural and Modal Analysis of the Unloading Robot on the Broaching Machine

In order to meet the actual production of a machining company in the low production efficiency, labor intensity, process flow complexity and other real problems, the broaching machine processing upgraded and designed a broaching machine for the production of automatic loading and unloading device. This paper analyzes the layout scheme and workflow of automatic loading and unloading, designs the overall structure of the loading and unloading device and key components, and establishes a three-dimensional model through Solidworks software, and carries out a static analysis of the limit position in the operation process by using the finite element analysis software ANSYS Workbench, and obtains the maximum deformation under the limiting conditions of 0.1129mm, and the maximum stress value of 24.236MPa. The maximum deformation is 0.1129mm and the maximum stress is 24.236MPa. Subsequently, modal analysis is carried out to extract the first 6 orders of intrinsic frequency and corresponding vibration pattern, and harmonic response analysis is carried out on the weak components on the basis of modal analysis, which verifies that the structure complies with the design requirements in terms of strength, stiffness and resonance of the whole machine, and it provides the theoretical basis for the subsequent structural improvement and optimization design.

Calibration and data-analysis routines for nanoindentation with spherical tips

Instrumented spherical nanoindentation with a continuous stiffness measurement has gained increased popularity in microphysical investigations of grain boundaries, twins, dislocation densities, ion-induced damage, and more. These studies rely on different methodologies for instrument and tip calibration. Here, we test, integrate, and re-adapt published strategies for tip and machine-stiffness calibration for spherical tips. We propose a routine for independently calibrating the effective tip radius and the machine stiffness using standard reference materials, which requires the parametrization of the effective radius as a function of load. We validate our proposed workflow against key benchmarks and apply the resulting calibrations to data collected in materials with varying ductility to extract indentation stress–strain curves. We also test the impact of the machine stiffness on recently proposed methods for identification of yield stress. Finally, we synthesize these analyses in a single workflow for use in future studies aiming to extract and process data from spherical nanoindentation.Graphic abstract

Optimization of Square-shaped Bolted Joints Based on Improved Particle Swarm Optimization Algorithm

The bolted joint is widely used in heavy-duty CNC machine tools, which has huge influence on working precision and overall stiffness of CNC machine. The process parameters of group bolt assembly directly affect the stiffness of the connected parts. The dynamic model of bolted joints is established based on the fractal theory, and the overall stiffness of joint surface is calculated. In order to improve the total stiffness of bolted assembly, an improved particle swarm optimization algorithm with combination of time-varying weights and contraction factor is proposed. The input parameters are preloading of bolts, fractal dimension, roughness, and object thickness. The main goal is to maximize the global rigidity. The optimization results show that improved algorithm has better convergence, faster calculation speed, preferable results, and higher optimization performance than standard particle swarm optimization algorithm. Moreover, the global rigidity optimization is achieved.

Seismic Hazard Assessment in Machine Foundation Design: A Review Study

This paper presents a comprehensive review of seismic hazard assessment in machine foundation design and highlights key research directions for future studies. The review includes a detailed analysis of existing literature, that covers a wide range of subjects associated with seismic analysis and design of machine foundations. It begins with an overview of the seismic hazard assessment evaluation procedure and the various factors that influence it, such as soil properties, machine stiffness, and earthquake excitation. Then it discusses the key challenges associated with the design and seismic analysis of machine foundations, including the impact of dynamic loads and soil-structure interaction. Following the analysis of the current research, the paper identifies several important research directions for future studies. These include the development of more accurate seismic hazard assessment methodologies, using sophisticated numerical modeling methods. The paper also emphasizes the need for more comprehensive experimental studies to validate the numerical models and better understand the behavior of machine foundations under seismic loads. In brief, the paper provides a comprehensive review of the current state of knowledge in seismic hazard assessment in machine foundation design and highlights the importance of future research to proceed in this field.

Experimental and numerical analysis of the Portevin–Le Chatelier effect in a nickel-base superalloy for turbine disks application

High-pressure turbine disks are subjected to extreme mechanical and thermal loadings throughout their lifetime. This is the reason why during the design of turboshaft engines, regulation rules impose on manufacturers to demonstrate the integrity of rotating parts, especially turbine disks, with over-speed experiments. To improve the prediction of rotation speed at burst, it is essential to study the viscoplastic behavior and the fracture of the material. The present work focuses on the approach used to calibrate accurately a viscoplastic behavior model at high temperature (500 °C) of nickel-based superalloy Inconel718. The experimental testing campaign includes standard tensile tests on axisymmetric and flat specimens with and without notches. The behavior is described using the McCormick model for dynamic strain aging. This model accounts for the viscoplastic behavior including the range of negative strain rate sensitivity and the Portevin–Le Chatelier (PLC) effect. In addition to global measurements, digital image correlation at high temperature using a speckle directly engraved on the specimen is used to detect and measure the strain carried by the PLC bands. This information is used in the parameter calibration. The consideration of the machine stiffness is shown to be essential in the analysis for a correct description of serrations and of the critical strain for PLC effects. 3D simulations on full specimen geometries samples are performed to validate the finite element model. Finally, a full 3D representative disk geometry simulation is performed to detect instabilities and their propagation due to geometrical singularities.

Effect of Mounting Orientation on Testing Equipment on Elastic Compliance of Clamped SE(T) Specimens: an exploration

The accurate evaluation of fracture mechanics properties is key to the safety of operation of high responsibility structures such as nuclear reactor components. This paper evaluates the impact of mounting orientation on the test machine of clamped SE(T) (SE(T)c) specimens on its compliance. The elastic unloading compliance is a commonly used technique to measure the crack depth of specimens within fracture mechanics tests. Inaccurate measurements affect reliability of resulting measured properties. Two mounting orientations are evaluated in this paper together with two SE(T)c specimens with different width-to-thickness ratios (W/B=2 and W/B=4). Conclusions show that machine stiffness is significantly different in both orientations, and such value has the potential to affect crack depth prediction. On the other hand, the analysis scope of this work was unable to detect significant differences between both orientations, probably because loading was limited to maintain specimen integrity. Further investigation is necessary to precisely adress the impact of this effect.

Unstable Pillar Failure under Soft Loading Condition

Abstract Pillar rockburst is an unstable pillar failure and one of the most hazardous problems in the underground mining engineering of deep hard-rock mines. In order to study the mechanism of unstable pillar failure, laboratory tests, numerical simulation, and theoretical analysis are adopted. The disc spring group was used to realize the soft loading function of testing machine, where loading stiffness of testing machine can be adjusted by changing the number and combination mode of disc springs. The results show that the loading stiffness of testing machine has major effect on the post-peak failure behavior of rock specimen, which means that the elastic rebound of disc spring group determines the unstable failure characteristics of rock specimen. The sudden jump Δd of rock specimen deformation and the elastic energy release ΔW of disc spring group all increase with the decreased loading stiffness of testing machine, resulting in more severe rock unstable failure (pillar rockburst). The soft loading condition has buffering and delaying effects on rock failure, but it increases the unstable failure intensity of rock specimen. The numerical simulation reproduced the rock unstable failure and the elastic rebound behavior of disc spring group, which also illustrated the damage evolution process of rock unstable failure. The necessary condition of rock unstable failure and the analytical solution of sudden jump Δd and elastic energy release ΔW were derived based on catastrophe theory, which further verified the experimental results. This study reveals the physical essence of unstable pillar failure, which may help to under the mechanism of pillar rockburst and provide references for underground mining.

Effects of neutron irradiation and post-irradiation annealing on pop-in crack propagation instabilities in 6061 aluminium alloy

This study analyses the increase of crack propagations instabilities (pop-in) with irradiation during fracture toughness testing of aluminium alloy 6061 alloy after neutron irradiation. The aim is to identify if the crack propagation instabilities are related to microstructural heterogeneities, i.e. weak zones, or if they are linked to a competition between testing machine stiffness and tearing modulus.An in-depth mechanical instability analysis was performed comparing the testing machine stiffness to the tearing moduli of the irradiated materials. For this purpose an experimental program was carried out in the hot cells facility at CEA Saclay to conduct mechanical and microstructural studies, after irradiation in a Material Testing Reactor (mean conventional thermal fluence: 6.8 × 1021 nth.conv.cm−2).The material hardening was quantified in this study using tensile tests after annealing heat treatments. The effect of this hardening on crack propagation instabilities was measured by fracture toughness tests on irradiated and precracked CT12.5 samples. Damage mechanisms were studied by scanning electron microscope fractography and classical ductile fracture micromechanisms were found for all conditions without notable differences between non irradiated and irradiated materials. The evolution of toughness with irradiation and subsequent heat treatments was therefore attributed to the matrix hardening/softening and its effect on nucleation of voids on brittle second phase particles.To investigate experimentally if the crack propagation instabilities were related to the tearing moduli, annealing heat treatments of the irradiated material were applied that softened the material. It was shown that with softening, the number of crack propagation instabilities decreased and eventually the pop-ins disappeared. The instability criteria using J-Δa R-curves of the toughness tests were applied and it was concluded that the increase in pop-ins with irradiation resulted from the decrease of the material tearing modulus and its interaction with the machine stiffness.The toughness test results of the irradiated material of the present study were compared with those of the literature and show very good agreement.

The Role of Fault Rock Fabric in the Dynamics of Laboratory Faults

AbstractFault stability is inherently linked to the frictional and healing properties of fault rocks and associated fabrics. Their complex interaction controls how the stored elastic energy is dissipated, that is, through creep or seismic motion. In this work, we focus on the relevance of fault fabrics in controlling the reactivation and slip behavior of dolomite‐anhydrite analog faults. We designed a set of laboratory experiments where we first develop fault rocks characterized by different grain size reduction and localization at normal stresses of σN = 15, 35, 60, and 100 MPa and second, we reload and reactivate these fault rocks at the frictional stability transition, achieved at σN = 35 MPa by reducing the machine stiffness. If normal stress is lowered this way, reactivation occurs with relatively large stress drops and large peak‐slip velocities. Subsequent unstable behavior produces slow stick‐slip events with low stress drop and with either asymmetric or Gaussian slip velocity function depending on the inherited fault fabric. If normal stress is raised, deformation is accommodated within angular cataclasites promoting stable slip. The integration of microstructural data (showing brittle reworking of preexisting textures) with mechanical data (documenting restrengthening and dilation upon reactivation) suggests that frictional and chemically assisted healing, which is common in natural faults during the interseismic phase, can be a relevant process in developing large instabilities. We also conclude that fault rock heterogeneity (fault fabric) modulates the slip velocity function and thus the dynamics of repeating stick‐slip cycles.

Enhancement of constant normal stiffness direct shear testing protocols for determining geomechanical properties of fractures

Discontinuity behaviour can have a large impact on geotechnical engineering design; therefore, it is essential to determine their geomechanical properties to predict rockmass behaviour and mitigate any potential failure that may affect personnel safety or damage property. Geotechnical numerical software programmes that discretely simulate discontinuities rely on direct shear laboratory tests to provide the properties needed as inputs to these tools. This study presents valuable direct shear laboratory test results for the mechanical properties and behaviours of fresh, unweathered, rough fractures in the Pointe du Bois granite. The testing programme considers three separate boundary conditions: constant normal stress (CNL*) and two variations of constant normal stiffness (CNS). A new machine stiffness model is proposed to estimate the machine stiffness for tunnelling applications. Measurements of shear strength and a proposed secant dilation angle are assessed and critically evaluated. It is found in this study that there is no significant difference between shear strength parameters determined from direct shear data from CNL* and CNS boundary conditions when comparing maximum strength and residual strength. In addition, it is found that the proposed secant dilation angle, critical shear displacement, and total dilation potential all have a negative relationship with increasing normal stress.

Importance of machine compliance to quantify electro-plastic effect in electric pulse aided testing: An experimental and numerical study

Electric pulse aided deformation (EPAD) of metals reduces the flow stress due to both thermal (expansion as well as softening) and electro-plastic (EP) effects. To quantify the electro-plastic effect during pulse application, close prediction of stress-drop due to thermal effects is very important. Testing machine relaxes due to sudden drop in stress at the instant of pulse application and this aspect must be considered in deformation analysis of the predictive methodology to accurately estimate the stress-drop. Coupled electro-thermo-mechanical finite element analysis by incorporating the testing machine stiffness is carried out during the present work to estimate the stress-drop due to thermal effects and validated. Results indicate that the testing machine stiffness significantly influences the stress-drop. In addition, experimental and numerical studies carried out during the present work explain the role of relative rate of deformation and thermal expansion during pulse application to quantify the EP effect. In the present work, cold-rolled and annealed automotive (CMn 440) steel is used and there is a reasonable EP effect.

Experimental validation of a micro-CT finite element model of a human cadaveric mandible rehabilitated with short-implant-supported partial dentures

PurposeThis study aimed to address the predictive value of a micro-computed tomography (μCT)-based finite element (μFE) model of a human cadaveric edentulous posterior mandible, rehabilitated by short dental implants. Hereby, three different prosthetic/implant configurations of fixed partial dentures (“Sp”-3 splinted crowns on 3 implants, “Br” – Bridge: 3 splinted crowns on 2 implants, and “Si”- 3 single crowns) were analysed by comparing the computational predictions of the global stiffness with experimental data. MethodsExperimental displacement of the bone/implant/prosthesis system was measured under axial and oblique loads of 100 N using an optical deformation system (GOM Aramis) and the overall movement of the testing machine (Zwick Z030). Together with the measured machine force, an “Aramis” (optical markers) and “Zwick” (test machine) stiffness were calculated. FE models were created based on μCT-scans of the cadaveric mandible sample (n = 1) before and after implantation and using stl-files of the crowns. The same load tests and boundary conditions were simulated on the models and the μFE-results were compared to experimental data using linear regression analysis. ResultsThe regression line through a plot of pooled stiffness values (N/mm) for the optical displacement recording (true local displacement) and the test machine (machine compliance included) had a slope of 0.57 and a correlation coefficient R2 of 0.82. The average pooled correlation of global stiffness between the experiment and FE-analysis (FEA) showed a R2 of 0.80, but the FEA-stiffness was 7.2 times higher. The factor was highly dependent on the test configuration. Sp-configuration showed the largest stiffness followed by Br-configuration (17% difference in experiment and 21% in FEA). ConclusionsThe current study showed good qualitative agreement between the experimental and predicted global stiffness of different short implant configurations. It could be deduced that 1:1 splinting of the short implants by the crowns is most favorable for the stiffness of the implant/prosthesis system. However, in the clinical context, the absolute in silico readings must be interpreted cautiously, as the FEA showed a considerable overestimation of the values.

Experimental Study on Instability and Load Transfer Mechanism within Multi-Pillar System

In order to study the instability and load transfer mechanism among multiple parallel pillars during underground mining, compressive tests on single- and treble-pillar specimens were conducted under soft loading condition with adjustable stiffness. Disc spring group was used to realize the adjustable stiffness of the test machine. Experimental results showed that the instability of rock specimen was determined by the stiffness of test machine, where the sudden jump displacement Δd exponentially decreases with the increase of loading stiffness. The load transfer behavior of treble-pillar specimen could only be reproduced under this soft loading condition. Under the soft loading condition, the failure of single pillar in multi-pillar system induces the sudden increase of cumulative AE counts not only in itself, but also in adjacent pillars, which indicates the accelerated damage in adjacent pillars accelerated by load transfer effect. Finally, it was concluded that the soft loading condition was the necessary condition for the load transfer of multi-pillar system. Results of this study may contribute to the better understanding of load transfer mechanism and to optimizing the design of room-and-pillar stopes during underground mining.

A Novel Tunable Stiffness Mechanism Using Filament Jamming

Abstract The jamming mechanism is a crucial method to tune the stiffness of soft-bodied machines to adapt to their surroundings. However, it is difficult for the present jamming structures to integrate them into systems with complicated shapes such as twist, cylinder, and spiral. This paper introduces a novel jamming mechanism termed a filament jamming technique, which varies stiffness using jamming of a cluster of tiny and compliant filaments. The jamming structure demonstrated various characteristics such as softness, shape compatibility, lightweight, and high stiffness. These feats can meet a variety of application scenarios that the traditional jamming one cannot afford. The experimental test was used to explore the jamming structure's stiffness behavior and dynamic performance. The influence of the filament structure dimensions, material properties, and the vacuum pressure on the stiffness was revealed. With the negative pressure increasing, both the natural frequency and damping ratio increase due to the rigidity variation. It indicates that the filament jamming structure has excellent response rapidity and shock resistance. Our work demonstrated some versatile features of the filament jamming technology, like shape adaptation, shape-preserving, stiffness stability, and compliance. To demonstrate the advantage of the jamming technique, we constructed a soft gripper and a torsional actuator to illustrate how the mechanics of filament jamming can enhance real-world robotics systems’ performance. Therefore, the filament jamming mechanism provides various machines and structures with additional properties to increase forces transmitted to the environment and tune response and damping. This study aims to foster a new generation of mechanically versatile machines and structures with softness and stiffness.

Theoretical and experimental investigations of mechanical vibrations of hot hammer forging

The characterization of mechanical vibrations in a hammer forging process is a tremendously important parameter for machine design and production engineering. The dynamic response of a forging hammer to the reaction forces is affected by material behaviour, time, spring-damper system and foundation. In this research firstly, the effect of mass ratio and coefficient of restitution on the forging efficiency were theoretical characterized. The interesting influence of anvil initial velocity on the forging efficiency is also analytically presented. The mechanical vibrations of a LASCO HO U-315 hammer were experimentally investigated. Two steel grades a S355 and a 42CrMo4 were used to forge trial parts. The velocity of the ram and acceleration of the anvil during a hot die forging process were measured using a laser velocity meter type LSV-2000-45. The influences of forging time, coefficient of restitution, energy loss and time interval (delay) between blows on the efficiency of the forging process were examined. The energy loss before die contact was determined to be approximately 10%. The investigations also showed that a variation of the time interval between blows within the usual range has no effect on the intensity of the vibrations of the anvil nor on the energy loss of the hammer. The dependence of the free damped vibrations of the anvil on machine stiffness, damper coefficient and mass of machine has been confirmed. Additionally, the loss of energy due to hammer movement as well as the free damped mechanical vibrations of the anvil were theoretically analysed in order to verify the experimental findings. Theoretical analysis showed an anvil initial velocity of approximately 0.2 m/s results in a 4% increase of forging efficiency. A good agreement between the experimental and theoretical results was observed.

Kinematics analysis and performance testing of 6-RR-RP-RR parallel platform with offset RR-hinges based on Denavit-Hartenberg parameter method

A six degrees-of-freedom parallel platform in a 6-RR-RP-RR configuration with high accuracy, high stiffness and a large working stroke is studied for application to the sub-mirror adjustment system of a large-aperture telescope. To meet the performance requirements, the parallel platform adopts a self-centering and well-designed offset universal hinge. The two hinge axes of the offset hinge do not intersect but have a specific offset in space, which makes the kinematics more complex than that with a common universal hinge. Therefore, to solve this complex kinematics problem, this paper innovatively introduces the Denavit–Hartenberg (D-H) parameter method that is used for series mechanisms. The method has a simple modeling process, strong applicability and continuity, providing a new tool for the analysis and application of the parallel mechanisms. A kinematics model of the parallel platform can be constructed and solved using a numerical iteration method. The accuracy of the numerical kinematics solution is verified using a co-simulation method. This paper analyses the passive derivative motion and the leg length error is compensated. Finally, test studies of the motion resolution, the repetitive positioning accuracy, the motion stroke, the static stiffness of the legs, and the static stiffness and dynamic stiffness of the entire machine were also carried out to verify the platform’s performance.

Macroscopic and microscopic characterizations of Portevin-LeChatelier effect in austenitic stainless steel using high-temperature digital image correlation analysis

Using digital image correlation (DIC) analysis in an Fe-19Cr-13Ni-0.2C austenitic stainless steel (mass%), the tensile deformation behavior accompanied with the Portevin-LeChatelier (PLC) effect was investigated at high temperatures of 723-823 K under various applied strain rates of 10−4 s−1–5 × 10−3 s−1. With the help of the DIC analysis, the global strain in the gauge part of a tensile specimen and the local strain in a PLC band were precisely measured regardless of the lowering machine stiffness at a high temperature. The high-temperature DIC analysis could identify the critical strain and strain rate conditions where stress-strain curve indicated serrated flow. The analytical results revealed that dynamic strain aging in the austenitic stainless steel was controlled by the pipe diffusion of Cr atoms. Further, it was also confirmed that the local strain gently increased in a gauge part through a PLC band propagation, which promoted a repeatable A-type PLC band propagation in the same direction. Furthermore, the local strain was discontinuously developed by the repeatable formation of PLC bands; consequently, the local strain rate became remarkably high within a PLC band. In addition, the local strain rate in the PLC band monotonically increased with the increase in the number of PLC bands. From this viewpoint, it was discovered that necking deformation occurs suddenly at the PLC band, when the local strain rate exceeds the critical value, where dynamic strain aging is ineffective.