Elastic Deformation Region Research Articles

Prediction of Surface Wear of Involute Gears Based on a Modified Fractal Method

A new wear prediction method of tooth surfaces of involute gears based on a real tooth surface model and a modified fractal method is developed. The real tooth surface model of an involute gear pair is introduced, and microgeometry feature detection of tooth surfaces is achieved by monitoring variations of normal vectors of each discrete data point of the real tooth surface model. To predict wear progression of tooth surfaces of a gear pair, an abrasive wear analysis model and the modified fractal method are used to analyze contact performance and its changes with accumulation of surface wear. The abrasive wear analysis model can analyze wear depths of gear tooth surfaces with sliding distances, local contact pressure, and directions of wear progression based on Archard's model. The modified fractal method is proposed to calculate instantaneous contact stiffness and estimate elastic and plastic deformation regions based on an asperity contact model. Microgeometry features of tooth surface asperities can be described as the basis of an asperity contact model and allow tooth contact analysis of real tooth surface models with their local microgeometry feature changes due to plastic deformations. Feasibility and effectiveness of this wear prediction method were verified by comparing predicted results of gear surface wear progression with gear wear test results.

Size Effects in the Impact of Carbon Fiber Reinforced Composite Structures

In this study, target size effects in the low energy impact response of plain CFRP plateswere investigated. It was found that increase the target size leads to a reduction in the maximumimpact force recorded during the test. This is due to the reduction on flexural rigidity of the largerpanels. The experimental results indicated that at energies above the first failure threshold, themaximum impact force does not coincidence with the predicting value. Two mathematical modelswere used to predict the maximum impact force including single degree of freedom (SDOF)spring-mass model and Energy-Balance (E-B) model. The predicting results were then comparedwith the experimental results, and both of the two models show good agreement with theexperimental results in elastic deformation region. In addition, the level of agreement between thepredictions and the experimental results indicate that both models are capable of modelling theimpact response of these CFRP panels at elastic regime.

Application of the Virtual Fields Method to determine dynamic properties at intermediate strain rates

Crash analysis simulation is now very important in automotive industry to assess automotive crashworthiness and safety. In order to acquire reliable crash simulation results, precise material behaviors at intermediate strain rates should be used as input data. To determine the stress-strain curves at various strain rates, the number of expensive and complicated experiments is large. The present study aims at determining the stress-strain curves of sheet metals at various strain rates from a single dynamic experiment. A new type of high speed tensile tester for sheet metal specimens was built and high speed tensile tests were carried out. Full-field heterogeneous strain fields were measured by a digital image correlation technique using a high-speed camera. The load data was acquired from strain gauges attached to the elastic deformation region on the specimen. Then, an inverse identification scheme with a rate dependent hardening law was applied to retrieve dynamic parameters. The stress-strain curves of advanced high strength steel at intermediate strain rates (100/s - 300/s) were successfully obtained from a single experiment.

Mechanism for amorphization of boron carbide under complex stress conditions

As an excellent material, the application of boron carbide (B4C) is limited by pressure-induced amorphization. To understand the mechanism for amorphization in B4C, first-principles methods based on density functional theory were employed to investigate the mechanical behaviors and the deformation process in B4C under complex stress conditions with six different biaxial perpendicular compression directions. The angle (θ) between one of the loading directions and the [0 0 0 1] c-axis ranged from 0° to 75° with every 15° interval. We found that the maximum stress at θ = 30° is 124.5 GPa, which is the lowest among six biaxial compressions. Simulation results show that the mechanism for amorphization in B4C under complex stress conditions is complicated. We take the θ = 30° biaxial compression as an example to explain the complicated deformation process. In the elastic deformation region, sudden bending of three-atom chains occurs and results in a stress fluctuation. Then the formation of new B–B bonds between the three-atom chains and the icosahedra leads to the first stress drop. After that, the B–C bonds in the chains are broken, resulting in the second stress drop. In this process, the icosahedra are partially destroyed. The stress increases continuously and then drops at the critical failure strain. Finally, the fully destruction of icosahedra leads to amorphization in B4C. However, under other five biaxial compressions, the B–C bonds in three-atom chains are not fractured before structural failure. Understanding the deformation mechanism for amorphization of B4C in real applications is prime important for proposing how to resist amorphization and enhance the toughness of B4C.

Improved fracture properties of Ni-Ti superelastic alloy under sustained tensile load in physiological saline solution containing hydrogen peroxide by hydrogen charging

We have attempted to improve the fracture properties of Ni-Ti superelastic alloy under a sustained tensile load in physiological saline solution containing hydrogen peroxide by charging with a small amount of hydrogen, which causes negligible hydrogen embrittlement. For hydrogen charged in the parent phase, no pitting potential is observed in anodic polarization curves for the present test solution; the time to fracture increases under an applied stress even in the elastic deformation region of the martensite phase. Upon aging after hydrogen charging, the pitting potential varies widely and the increase in the time to fracture slightly decreases. Hydrogen charging increases the time until a rapid shift of the corrosion potential in the less noble direction, corresponding to the breakdown of surface oxide films. The final fracture is mainly caused by overloading due to localized corrosion irrespective of hydrogen charging. For hydrogen charged in the martensite phase, i.e., the specimen not subjected to interactions between hydrogen and the stress-induced phase transformations, the time to fracture increases by more than that of the specimen subjected to the interactions. The present study indicates that a small amount of hydrogen charging is effective for inhibiting the localized corrosion of Ni-Ti superelastic alloy under applied stress in physiological saline solution containing hydrogen peroxide, thereby improving the fracture properties. In addition, changes in the hydrogen states and/or defects induced by the interactions between hydrogen and phase transformations probably play an important role in improving the fracture properties.

Analytical and Experimental Investigation of Ballistic Impact on Thin Laminated Composite Plate

Penetration of flat-ended cylindrical projectiles into thin laminated composite plates was investigated analytically and experimentally. An analytical modeling was carried out for thin laminated composite plates by developing a new function for deflection by computing Von Karman nonlinear strains and by using the principle of energy balance. During the perforation process, different regions were considered for the plate, such as fracture region, elastic deformation region, delamination region, and undeformed region. The energy absorbed by each region was measured in small time intervals. To validate this model, the ballistic experiment is performed on the thin laminated composite plate near and beyond ballistic limit velocity. The samples were made from plain woven glass/epoxy using a hand lay-up method. In addition to the initial velocity, the residual velocity of the projectile was also measured using two parallel laser curtains. A comparison drawn between analytical and experimental results demonstrated a good consistency in the residual velocity of the projectile. Finally, the distribution of strains along the plate thickness direction over time, the different amounts of absorbed energy of the failure modes, delamination radius, and energy are assessed at near and beyond ballistic limit velocity.

Disentanglement induced by uniaxial pre-stretching as a key factor for toughening poly(l-lactic acid) sheets

Poly(l-lactic acid) (PLLA) sheets with dramatic improvement of mechanical properties, especially toughness, were obtained by uniaxial pre-stretching around Tg. It was found that a network structure consisting of cohesional entanglements was formed during the usual compression molding process, leading to the brittleness of PLLA, and the destruction of the network structure due to disentanglement after pre-stretching, leading to the toughness of PLLA. The network structure could not be destroyed when pre-stretching ratio (PSR) was in the elastic deformation region I and strain softening region II (PSR < 0.2). However, when PSR got into the strain hardening region III (PSR = 0.2), the network structure was destroyed resulting from the disentanglement, leading to the brittle to ductile transition. With PSR increasing from 0.2 to 0.4, the orientation was formed in pre-stretched PLLA, leading to further increase of the elongation at break. And then the degree of orientation increased along with the occurrence of the mesophase (PSR ≥ 0.5), therefore, the modulus and strength increased, while the elongation at break decreased. However, the elongation at break was still larger than that of undrawn PLLA.

Design and development of a new magnetic sensor for stress monitoring and annihilation/control in steels

This paper describes a new method and apparatus for monitoring the distribution of the hydraulic stresses (type stresses I) and the residual stresses (type stresses II and III) in ferromagnetic steels within the elastic region and plastic deformation region, determining the deforming step them and use them for elimination and / or control of the stresses in the steel.

THE COMPARATIVE ANALYSIS OF THEORETICAL STUDIES RESULTS OF THE BOUNDARY STRESS STATE OF ANISOTROPIC MATERIALS

The purpose of the study is to identify mechanical theories of strength and mathematical models for determining the limiting stress state of anisotropic materials suitable for the adequate description of the elastic range of deformation of softwood and hardwood in biaxial, planar and bulk mechanical loads. The relevance of the study is due to the fact that today there is no unified methodology for approximating the results of experimental studies of the short-term strength of composite materials with a complex stressed state. In the mathematical formulation of the problem, one and the same short-term strength surface can be satisfactorily described by several criteria. To achieve this goal, a classification and a comparative analysis of the known mechanical theories of the short-term strength of anisotropic materials and the main provisions of the general theory of quadrics was made. It has been established that, to date, there is no single generally accepted theory, which methods would adequately assess the limits of elastic, viscoelastic and viscoelastoplastic deformation regions of biological origin anisotropic composite materials under conditions of complex mechanical and temperature-humidity loads. In particular, the strength criteria of Ashkenazi, von Mises, Marin-Hu, Prager, Norris-McKeenen, Hill, Tsai-Hill, Tsai-Wu, Hoffman, Norris, Fisher, Zakharov, Malmeister, and Goldenblat-Kopnov were analysed. As a result, it has been established that the strength conditions for materials with a weak asymmetry of strength limits in the direction of structural symmetry are unsuitable for describing the strength surfaces of materials with a strong asymmetry of ultimate strength. We have revealed that the biaxial and flat stress-strain state in the tangential-radial plane of structural symmetry of hardwood is satisfactorily described by the Ashkenazi criterion and softwood by the Goldenblatt-Kopnov criterion. A promising direction of further research is the identification of a universal strength criterion, suitable, in contrast to the standard tensor-polynomial criteria, to adequately predict the plasticity surface and the region of viscoelastic deformation of anisotropic materials.

Coating of inner surface of cylindrical pipe for hydrogen entry prevention using plasma process

Aluminum alloy A6061, which is highly resistant to hydrogen entry, was prepared on a carbide steel cylindrical pipe using a magnetron sputtering deposition method. In this method, plasma was generated between the cylindrical pipe substrate and the cylindrical rod targets, and it moved toward the axial direction with the generation of a modulated magnetic field by a low-frequency alternating coil current. Uniform A6061 thin films were deposited inside the cylindrical pipe using the magnetron sputtering deposition method. The surface morphologies of the films were smooth, and the uniformity of the films was increased by the modulated magnetic field. Moreover, hydrogen content measurements revealed that the A6061 plasma coating is highly resistant to hydrogen entry in corrosive environments, suggesting that the coating was applicable to the elastic deformation region of the base material.

First interactions between hydrogen and stress-induced reverse transformation of Ni–Ti superelastic alloy

The first dynamic interactions between hydrogen and the stress-induced reverse transformation have been investigated by performing an unloading test on a Ni–Ti superelastic alloy subjected to hydrogen charging under a constant applied strain in the elastic deformation region of the martensite phase. Upon unloading the specimen, charged with a small amount of hydrogen, no change in the behaviour of the stress-induced reverse transformation is observed in the stress-strain curve, although the behaviour of the stress-induced martensite transformation changes. With increasing amount of hydrogen charging, the critical stress for the reverse transformation markedly decreases. Eventually, for a larger amount of hydrogen charging, the reverse transformation does not occur, i.e. there is no recovery of the superelastic strain. The residual martensite phase on the side surface of the unloaded specimen is confirmed by X-ray diffraction. Upon training before the unloading test, the properties of the reverse transformation slightly recover after ageing in air at room temperature. The present study indicates that to change the behaviour of the reverse transformation a larger amount of hydrogen than that for the martensite transformation is necessary. In addition, it is likely that a substantial amount of hydrogen in solid solution more strongly suppresses the reverse transformation than hydrogen trapped at defects, thereby stabilising the martensite phase.

After-effects induced by interactions between hydrogen and the martensite transformation in Ni–Ti superelastic alloy

The effects of dynamic interactions between hydrogen and a stress-induced martensite transformation on the recovery of deteriorated tensile properties by ageing in air at room temperature have been investigated for a Ni–Ti superelastic alloy. A specimen is subjected to single stress-induced martensite and reverse transformations immediately after hydrogen charging. Upon tensile testing, brittle fracture occurs in the latter half of the elastic deformation region of the martensite phase after the stress-induced martensite transformation. Upon ageing before the tensile test, fracture occurs during the stress-induced martensite transformation. In addition, the nano- and micro-morphologies of the brittle outer part of the fracture surface of the specimen are changed by ageing. Thus, the tensile properties markedly deteriorate, rather than recover, by ageing. The present results clearly indicate that dynamic interactions between hydrogen and the stress-induced martensite transformation have serious after-effects on hydrogen embrittlement of Ni–Ti superelastic alloy.

Acoustic emissions and pressure stimulated currents experimental techniques used to verify Kaiser effect during compression tests of Dionysos marble

The damage development due to externally applied mechanical stress is a hot topic of interest involving several applications of everyday life, like civil engineering, monument restoration, construction evaluation and health monitoring. Repetitive loadings of brittle materials cause internal damages that gradually extend, leading to inevitable failures. Such processes are studied under the concept of the materials’ mechanical memory effect that is widely known as Kaiser effect. The Kaiser effect states that a structure will only suffer further internal damaging when exposed to applied stresses higher than previously encountered. Certain conditions lead to a violation of the Kaiser effect, known as the Felicity effect, quantitatively measured using the Felicity Ratio. This work presents the experimental results when repetitive mechanical load loops are applied on marble specimens, while concurrent Acoustic Emission (AE) and Pressure Stimulated Currents (PSC) measurements are conducted. The collected AE and PSC data are studied in combination with the mechanical data, like mechanical stress and strain, under the frame of the Kaiser effect. It is clearly seen that the Felicity ratio strongly depends on the stress range the material is subjected to, with regard to the elastic or plastic deformation region.

Direct Visualization of Stress Distribution Related to Adhesive through Mechanoluminescence

Elasticoluminescent material is a novel ML functional ceramic powder (controllable size: 10 nm―100 μm, representative and the most efficient ML material: SrAl2O4:Eu2+) and it can emit intensive light repeatedly accompanied by mechanical stress such as deformation, friction, impact, even in elastic deformation region. The ML intensity is proportional to strain energy of the material. Thus, when dispersedly coated onto a structure, each particle acts as a sensitive mechanical sensor, while the two-dimensional (2D) emission pattern of the whole assembly reflects the dynamical strain/stress distribution inside the structure and the mechanical information around the defect and crack or the in-visible tip. This ML visualization technique provides a novel way of diagnosing the structural health, especially in early detection for safe & security measure. Actually, the ML sensor has been applied to real social infra-structure such as bridge, building, welding point of pipeline and high pressure vessel, and successfully demonstrated the ability to detect active crack including likelihood for growth, real crack propagation and mechanically weak points etc. Meanwhile, recently NEW advanced material such as CFRP, high tensile strengthen steel and non-steel metal for light weight vehicle are strongly required from the viewpoint of energy saving and environment. Accordingly, joint & adhesive technologies especially for different kinds of materials and the non-destructive evaluation have been investigating for application of these materials and the reliability in use. Especially, as to adhesion, kissing bond and weak bond, in which strengthen of the bonding is nothing or weak respectively without void and crack, have been remaining as challenge for the application and has been vigorously investigated in the field of non-destruct evaluation so far. In the presentation, we would like to report on suitability of the ML sensing for diagnosing (1) a stress distribution in adhesive, (2) visualization of mechanical distribution related to adhesive condition. In fact, as the result of (1), we succeeded to directly visualize stress distribution in adhesive for the first time through mechanoluminescence. We will discuss the results with the result of conventional FEM analysis, simulation of mechanical stress distribution. In addition, as the result of (2), we succeeded to visualize strass/strain distribution of adherend, clearly find out the stress/strain distribution are related to adhesion condition and also find out the area of pseud kissing bond through mechanoluminescence for the first time. [Acknowledgement] The research was partially supported by Innovative Structural Materials Association (ISMA), program for the promotion of scientific research with a view to creating intellectual resources through New Energy and Industrial Technology Development Organization (NEDO), AIST Strategic budget 2015 and KAKENHI, Grant-in-Aid for Scientific Research (B).

Visualization of Relative Strain Distribution for Carbon Fiber Reinforced Plastic Plate By Mechanoluminescent Technique

In the past few decades, there has been much attention paid towards carbon fiber reinforced plastics (CFRPs) as a structural material for various applications, such as aerospace, transport, infrastructure, wind energy and so on, because of superior mechanical, electrical, thermal, and chemical characteristics compared to steel or metal. However, they are practically used as a complicated architecture including laminates of different ply orientations and woven fabrics, due to the improvement in an anisotropic mechanical strength. Thus, when the CFRP laminates was used in real products with a complicated architecture, it is difficult to investigate and predict the stress/strain distributions and rupture models. The development of in-situ evaluation techniques capable of providing a stress/strain distribution has been strongly demanded. Recently, we have reported that the mechanoluminescence (ML) techniques using composite films consisting of an ML material and organic compounds can be useful technique for visualizing the dynamic equivalent strain distributions in complicated architectures. The ML is the luminescence phenomenon induced by mechanical actions, such as compression, tension, friction, or torsion. Especially, the ML intensity is proportional to the strain energy in the elastic region. In the present study, therefore, we examined the feasibility of a non-destructive testing (NDT) for carbon fiber reinforced plastic (CFRP) laminates by using a mechanoluminescence (ML) technique. Specifically, we investigated to visualize the relative strain distribution of the CFRP laminates with bidirectional fiber bundles (twill woven) by using the fabricated ML sensor consisting of a mixture of SrAl2O4:Eu (SAOE) ML material, which shows the highest ML intensity among various materials reported, and epoxy resin attached to the surface of CFRP laminates. For comparison, the CFRP with unidirectional fiber bundles was also investigated. An ML sensor was fabricated by using a composite of SAOE powder and epoxy resin. The SAOE powder was prepared by using a solid-state reaction method. SrCO3, α-Al2O3, Eu2O3, and small amount of Ho2O3 powders were mixed in an agate mortar with ethanol solution. The obtained mixture powder was calcined at 800ºC for 1 h in air, and then sintered at 1350ºC for 4 h in reducing atmosphere (H2+Ar). The sensing characteristics of the ML sensors painted on the surface of the CFRP laminates were evaluated under a tensile testing in a dark room at room temperature. The commercial strain gage was also applied on the opposite side of CFRP laminates with a commercial adhesive. Tensile tests were performed with a material testing machine. The ML images of the ML sensor and longitudinal strains during the tensile tests were recorded by using a CCD camera and a uniaxial strain gage connected a data logger, respectively. Both devices were connected to a personal computer to simultaneously obtain the data. For quantitative and reproducible measurement, the ML sensor was once irradiated by blue light-emitting diode (LED) for 1 min and kept under dark condition for 5 min. From recording the ML images in the tensile tests, the ML sensor attached to the CFRP laminates with unidirectional fiber bundles gave uniform ML intensity distribution over the whole of ML sensor, and the ML intensity increased with increasing the tensile load, indicating that the whole of CFRP laminate deforms uniformly by the applying tensile loads. On the other hand, the CFRP laminates with bidirectional fiber bundles exhibited a twill weave ML intensity distribution similar to the original twill weave pattern. Specifically, the ML intensities on the areas of a longitudinal fiber bundle (tensile direction) were found to be higher than those on transverse fiber bundle areas. This indicates that a longitudinal fiber bundle deform much larger than the transverse ones. Therefore, it is concluded that ML sensor could directly visualize the load share between fiber bundles with different directions in CFRP laminate with high spatial resolution. Based on the obtained results in this study, we concluded that the ML sensor could visualize a relative strain distribution of CFRP laminates with both unidirectional and bidirectional fiber bundles. Additionally, the ML sensor could detect a tiny strain difference at the interface between longitudinal fiber bundles even in the elastic deformation region. Thus, it is considered that the ML sensor could be a possible candidate for a dynamic strain distribution technique of the CFRP products.

Stress and strain concentration in weld-joint flaws

Finite-element modeling is used to investigate the stress–strain state at the tip of a blunt weld-seam flaw. A multifactorial experiment yields formulas for the concentration coefficients and the stress and strain gradients in elastic and plastic deformation regions of the local zone.

Measurement and correlation of acoustic emissions and pressure stimulated voltages in rock using an electric potential sensor

Previous laboratory-based studies have demonstrated pressure stimulated currents and electromagnetic emissions in rock and cement mortar can be used to assess damage. There is some debate whether the current and electromagnetic emission measurement technologies used thus far are viable for field application. The results presented here provide evidence that electric potential sensors are a viable new technology for field monitoring of electrical emissions. Cylindrical specimens of rock were loaded at constant stress or strain rates until failure occurred; strain gauges, piezoelectric transducers and electric potential sensors were used to monitor the strain, acoustic emissions and pressure stimulated voltages. Pressure stimulated voltages were observed in the linear elastic and inelastic deformation regions of loading, suggesting that pressure stimulated voltages are associated with microcracking and macrocracking events. Electric potential sensors are potentially a cost effective and more advanced alternative to piezo transducers and geophones for structural health monitoring of rocks.

Effects of temperature, phase, elastic deformation and transformation on inhibition of localized corrosion of hydrogen-charged Ni–Ti superelastic alloy in NaCl solution

The effects of the solution temperature, phase, elastic deformation and the phase transformation on the inhibition of localized corrosion of a small amount of hydrogen-charged Ni–Ti superelastic alloy in 0.9% NaCl solution have been investigated by electrochemical measurements. No pitting potentials are observed in the anodic polarization curves obtained above room temperature and under static applied stress in the presence of parent or stress-induced martensite phases. In a dynamic cyclic tensile test in the elastic deformation region of the parent phase under various constant applied anodic potentials, the critical applied potential for an increase in the current density, which corresponds to the occurrence of localized corrosion, considerably shifts in the noble direction. At the stress plateau, i.e., in the superelastic region, caused by stress-induced martensite and reverse transformations, the critical applied potential for a marked increase in the current density slightly shifts in the noble direction. In the elastic deformation region of the stress-induced martensite phase, the critical applied potential for a marked increase in the current density shifts in the noble direction. The present results indicate that hydrogen charging is effective for inhibiting the localized corrosion of Ni–Ti superelastic alloy in NaCl solution at various temperatures, irrespective of the phase itself, under dynamic elastic deformation and the phase transformation.

Mechanical characteristics of CNT-reinforced metallic glass nanocomposites by molecular dynamics simulations

Carbon nanotubes (CNTs) have been widely used to strengthen different types of materials including polymers, ceramics, and metals due to their extraordinarily high strength. In this work, we investigated the influence of CNTs on the mechanical characteristics of amorphous metallic glasses by using molecular dynamics simulations. We constructed long (continuous) and short (discontinuous) CNT-reinforced metallic glass nanocomposites, determined their stress–strain responses and elastic modulus, and compared their tensile and compressive behaviors with monolithic metallic glasses. It was found that long CNTs dramatically increased the stiffness and yield strength of the metallic glass, and improved their elastic region of deformation and ability to absorb energy. In contrast, metallic glasses reinforced with short CNTs showed almost no improvement of mechanical properties.

Effect of Hydrogen on the Stress Relaxation of Aged NiTi Shape Memory Alloys

The susceptibility of the NiTi shape memory alloy to relaxation after the hydrogen charging in an aqueous solution has been investigated with respect to ageing during one to six days in air at room temperature. The orthodontic wires have been prepared by immersing in a 0.9% NaCl solution for 3 h, under applied current density of 10 A/m and then relaxed with an imposed deformation in a fully austenite state of structure and in a state with 1/3 and 2/3 of the martensite volume fraction. Through the stress relaxation, the hydrogen-charged specimen has shown a significant decrease of the stress, compared to the non-immersed alloy, when the imposed deformation was located in the plateau of the austenite-martensite transformation. It was also found that a longer ageing period is important and the properties of the wires with longer stress relaxation are similar to the those of non-charged wires. Nevertheless, no difference has been detected between the as-received and the as-charged specimens when the imposed deformation was located in the elastic deformation region of the fully austenitic structure. This behavior is attributed to the effect of the gradient of absorbed hydrogen, existing between the surface and the center axis of the studied wires, which facilitates the mobility of the martensite bands during the stress relaxation.