Irreversible Plastic Deformation Research Articles

Evaluation of the Energetic Release During Tensile tests in Notched Specimens by means of Experimental and Numerical Techniques

Static Thermographic Method is a rapid test procedure able to predict the fatigue limit of the material evaluating the end of the thermoelastic phase during a static tensile test. In order to investigate the energetic release of the material, experimental tests have been carried out on double edge steel notched specimens. During the test, the surface temperature was monitored with an infrared camera and the true stress-strain curve of the material was retrieved. Numerical simulations have been carried out to predict the temperature evolution of the material under monotonic load. The relationship between the elasto-plastic behavior and the energetic release has been investigated comparing numerical and experimental data. The rise of irreversible plastic deformation severally affects the temperature trend introducing an additional heat amount that leads to a deviation from the linearity of the thermoelastic effect. The analysis of the temperature evolution during a static tensile test may be adopted as a novel approach to relate the local stress state with a macroscopic stress value that introduce in the material the first micro plasticization.

Fatigue damage assessment in AM polymers evaluating their energy release

New Additive Manufacturing (AM) technologies require a deep knowledge of the process parameters to assure proper mechanical performances of structural components. In addition, fatigue loads may lead to premature failures of AM components compared to their traditional-manufactured counterparts. It is of fundamental importance to adopt a time and material saving strategy to investigate the fatigue behaviour of such components. Considering that fatigue is an energy-dissipative process, monitoring the specimen temperature provide useful information regarding the damage evolution within the material. Recently, the Static Thermographic Method (STM) has been proposed as a rapid test procedure to assess the initiation of damage in the material monitoring the temperature evolution during static tensile tests. In this work, the energy release of plain PA12 specimens, obtained by Selective Laser Sintering process, has been assessed allowing the estimation of a macroscopic limit stress that causes in the material the first irreversible plastic deformation.

Research on Real Surface Stability of Cone Bit Sliding Bearing Under Repeated Contact Condition

The cone bit is the most principal rock breaking tool in drilling engineering. The statistical analysis of field data shows that the early damage and failure of sliding bearing is one of the main reasons that affect the service life and performance of the cone bit in the real drilling process. Aiming at the complex working environment of cone bit sliding bearing with heavy load, high temperature and mixed lubrication, the paper establishes a multilevel line contact model of rough surface and carries out the finite element modeling and analysis from the perspective of contact mechanics. Then the Von Mises stress, Residual normal deformation and Inelastic dissipated energy are obtained under the different cycles of loading-unloading. The results show that with the increase of cycles of loading-unloading, the part of edge area of the line contact enters into elastic shakedown and the asperities in the central area occur irreversible plastic deformation, which has a significant influence on the stability of the sliding bearing of the cone bit.

Study on the deformation mechanism of abutment slope and its influence on the dam during the impoundment of high arch dam

The deformation of abutment slope during impoundment poses great challenges to the working behavior and long-term safety of the arch dam. According to the monitoring data of impoundment and slope, slope deformation, valley width change, and arch dam chord length change are systematically analyzed and studied. It is pointed out that the deformation of the dam body and foundation is an irreversible plastic deformation and is evidently correlated with impoundment. Based on the unsaturated effective stress principle of fractured rock mass and the action mode of the reservoir basin pressure, the deformation mechanism of abutment slope has been discussed. The large-scale 3D nonlinear finite element model is established based on numerical analysis, and the deformation mechanism during impoundment and the effect of the effective stress change on the displacement and stress of the dam body are analyzed. The results indicate that effective stress can elucidate the valley contraction to some extent and that the contribution of the reservoir basin pressure is small. Slope deformation is beneficial to the dam displacement to some extent. Moreover, it has a negligible influence on the dam surface stress. However, the junction of the dam and foundation needs to be given special attention.

Effect of stress induced damage on gas permeability of Beishan granite

Permeability is the key factor to control long-term stability of surrounding rock of high level radioactive waste (HLW) geological repository. In view of the low permeability of the intact granite in Beishan area, the preferred site for HLW repository in Northwest China, argon was used as the fluid medium to measure the gas permeability of Beishan granite under hydrostatic pressures and cyclic loading-unloading triaxial tests. It is found that the gas permeability decreases with the increase of confining pressure and their relationship approximately conforms to the power function. Due to irreversible plastic deformation, the permeability during unloading is less than that in loading process under the same confining pressure level. In the triaxial loading-unloading test, the acoustic emission technique applied well reflects the micro-crack propagation process and quantifies the cumulative damage in granite samples. It shows that the dilatancy stress is the threshold stress of the micro cracks in rock turn from stable growth to accelerated growth. This is the physical mechanism of significant increase of permeability after reaching the dilatancy stress.

Experimental Study on the Mechanical Behavior of Rock Bolts Subjected to Complex Static and Dynamic Loads

In underground mining practice, the rock bolt support system is the major support pattern to control the deformation and stability of openings. A rock bolt is generally subjected to complex loads including tension, torsion, bending and shear, which result from the deformation of excavations and exposure to dynamic loads that are generated by rockbursts. An understanding of the response of rock bolt under complex conditions is of great importance for rock bolt support design and practice. New sophisticated equipment has been developed for this purpose. This work involved a comprehensive experimental study on the mechanical behavior of rock bolts under complex loads. The results show that rock bolt pre-tensioning by torque application to the nut can result in decreases in tensile strength and elongation because the rock bolt is subjected to a combination of tension and distortion. When a pre-tensioned rock bolt is subjected to a shear load, the maximum shear force can reach up to 80% of the tensile capacity of the rock bolt. Higher impact energy results in a longer period of dynamic loading and a larger irreversible plastic deformation on the rock bolt, in contrast to a rock bolt that is subjected to low impact energy. The capacity and especially the deformation capacity of a rock bolt may decrease significantly after successive containment of the deformation of the surrounding rock mass from rockbursts.

Macro-Rheology Characterization of Gill Raker Mucus in the Silver Carp, Hypophthalmichthys molitrix.

The silver carp, Hypophthalmichthys molitrix, is an invasive planktivorous filter feeder fish that infested the natural waterways of the upper Mississippi River basin due to its highly efficient filter feeding mechanism. The characteristic organs called gill rakers (GRs), found in many such filter feeders, facilitate the efficient filtration of food particles such as phytoplankton that are of a few microns in size. The motivation to investigate the rheology of the GR mucus stems from our desire to understand its role in aiding the filter feeding process in the silver carp. The mucus-rich fluid, in a 'thick and sticky' state may facilitate the adhesion of food particulates. The permeation and transport through the GR membrane arefacilitated by the action of external shear forces that induce varying shear strain rates. Therefore, mucus rheology can provide a vital clue to the tremendous outcompeting nature of the silver carp within the pool of filter feeding fish. Based on this it was posited that GR mucus may provide an adhesive function to food particles and act as a transport vehicle to assist in the filter feeding process. The main objective of the protocol is to determine the yield stress of the mucus, attributed to the minimum shear stress required to initiate flow at which irreversible plastic deformation is first observed across a structured viscoelastic material. Accordingly, rheological properties of the GR mucus, i.e., viscosity, storage, and loss moduli, were investigated for its non-Newtonian, shear-thinning nature using a rotational rheometer. A protocol presented here is employed to analyze the rheological properties of mucus extracted from the gill rakers of a silver carp, fished at Hart Creek location of the Missouri River. The protocol aims to develop an effective strategy for rheological testing and material characterization of mucus assumed to be a structured viscoelastic material.

On the Prediction of High-Temperature Creep Processes in Metals

Some issues related to the development of methods for forecasting creep processes are considered. For this purpose, the temporal equivalence principle, the temperature-time analogy, the stress-time analogy, other analogies, and the modern relations between stresses and strains of the heriditary type in the form that allows describing irreversible plastic deformations are used. The expressions for multiple reduction coefficients are obtained using the time equivalence principle. A brief description is given to implementation of the method of analogies. An example of application of the concept of reduced time directly to the creep diagram is provided.

Development of Pressure-Responsive PolyPropylene and Biochar-Based Materials.

In this research paper, we reported the synthesis of biochar-based composites using biochar derived from exhausted tea leaves and polypropylene. The resulting materials were deeply characterized investigating mechanical (dynamic mechanical thermal analysis), thermal (thermogravimetrical analysis and differential scanning calorimetry), morphological (field emission scanning microscopy) and electrical properties vs. temperature. Furthermore, electrical conductivity was studied for a wide range of pressures showing an irreversible plastic deformation. An increment of one order of magnitude in the conductivity was observed in the case of 40 wt% biochar loading, reaching a value of 0.2 S/m. The material produced exhibited the properties of an irreversible pressure sensor.

Fatigue Deformation and Energy Change of Single-Joint Sandstone After Freeze-Thaw Cycles and Cyclic Loadings

In cold regions, saturated fractured rock is prone to crack initiation, extension, and branching along the original fracture end under the effect of freeze-thaw (F-T) cycles and cyclic fatigue loadings. The resulting strength deterioration is accompanied by large amounts of pore growth, which results in localized damage and fracturing along the crack end and even causes overall failure. To study the fatigue damage behaviors of fractured rocks under F-T cycles and cyclic loadings, single-joint quasi-sandstone specimens with joint angles of 45° and 90° were prepared. Subsequently, F-T damage tests with 0, 10, and 20 cycles and cyclic loading tests with different stress levels were performed. The F-T damage features are discussed based on the binarization image of localized damage along the joint ends and their wave velocity variations. It is found that the frost damage of single-joint quasi-sandstone tends toward a strip with localized fatigue characteristics. Moreover, the changes of strain compliance and dissipated energy are studied under the effect of loading-unloading fatigue. Some of the interesting phenomena observed are as follows: (i) during the early stage of cyclic loading, the joint can be compacted; nevertheless, it tends to expand along the fracture direction once it passes the elastic stage, and the irreversible plastic deformation is stable at this stage. (ii) The cracks caused by F-T damage in the 45° single-joint samples deflect along the fracture direction, in contrast to the 90° single-joint specimens. Moreover, the samples with a 45° single joint are more susceptible to original fracture at the early stage of the failure process, which results in a different failure mode from that of the 90° single-joint samples. (iii) The F-T cycles and cyclic loadings have a coupling effect on the single-joint sandstone. The strain compliance and hysteresis energy keep increasing uniformly after the F-T cycles and cyclic loadings.

Extended finite element method and anisotropic damage plasticity for modelling crack propagation in concrete

In this article, a new algorithm is developed for predicting crack initiation and growth direction in 3D solid concrete. The algorithm combines the anisotropic damage-plasticity model used for simulation of concrete material degradation (crack) and irreversible plastic deformation and the eXtended Finite Element Method (X-FEM). The Voyiadjis local anisotropic damage-plasticity model is expressed in a non-local form to prevent strain and damage localization in damaged points. Also, a new crack propagation direction criterion based on second order damage tensor of anisotropic damage, containing different material degradation components in different directions is proposed for 3D problems. Computational algorithms with an elastic predictor and plastic and damage corrector returning map are also developed. Finally, three numerical examples are solved and results of crack propagation paths predicted by the proposed algorithm are compared with available experimental data and those of other numerical simulations. It is shown that the results from the proposed method are in better agreement with experimental data compared to other reported numerical methods.

Modeling the transient behavior of the NiTi shape memory actuator using finite element analysis: Parametric study of the rate effects

Finite element simulation is presented to study the rate effects of the shape memory response of bent NiTi wire “C-shaped” exposure to load. A three-dimensional constitutive model for shape memory alloys proposed by Auricchio is implemented in the built-in library of Ansys Workbench 18.2 to capture the shape memory behavior of three different NiTi alloys that are subjected to different thermomechanical treatments. The quasi-static (slow) loading rate and the dynamic (fast) loading rate have been applied isothermally or at adiabatic conditions. Other models assumed the classical shape memory stress-strain behavior, but the realistic behavior of the NiTi SMA is different. The NiTi SMA is influenced by its microstructural changes, such as internal defects, grain size, chemical compositions, content of dislocations and density; all these effects should be added when modeling, which has been considered in this study. The results showed a significant effect of the dynamic loading of NiTi on decreasing the recoverable strain. As a result, it could be expected that the actuator lifetime could be reduced when a rapid, as opposed to a slow, loading rate is adopted. Inhomogeneity of stresses and deformations after loading can cause irreversible plastic deformation. The complete deformation recovery is noticed in the reorientation scenario, unlike in the SIM scenario, which exhibits unrecoverable deformation. Experimental validation is done by comparing the macroscopic displacement of the bent NiTi actuator with the model results, which shows good agreement.

Continuous laser illumination for in situ investigation of tungsten erosion under transient thermal loads

Two types of in situ optical diagnostics of erosion processes on tungsten surface caused by transient heat loads were developed and applied on the BETA (Beam of Electrons for materials Test Applications) facility at Budker Institute of Nuclear Physics. Tungsten plates were exposed to an electron beam with a duration of up to 350 μs and with an absorbed heat flux factor (HFF) of 20-50 MJ/m2s0.5, which is below the melting threshold. The distribution of thermal radiation on the sample front surface was imaged by a CCD camera. In addition, the illumination of the target surface with a continuous wave laser was used to record various types of material modification: roughening, cracking, bending, and local melting. The tile bending is associated with thermal expansion and irreversible plastic deformation of the heated thin layer. The lift of edges of the cracks during their formation is detected by recording the intensity of reflected and scattered radiation, and the resulting structure of the cracks is visualized with a CCD camera. The latter system can also identify the appearance of molten grains on the irradiated surface due to their low thermal bond to the material bulk. The described optical systems are able to detect first wall damaging during a plasma discharge in devices with magnetic confinement.

Adhesion force of W dust on tokamak W plasma-facing surfaces: The importance of the impact velocity

The adhesion of tungsten (W) dust particles on W tokamak plasma-facing surfaces is investigated. When spherical dust hits the tokamak walls, its shape is altered as a contact area is created by the adhesion work and irreversible plastic deformation. The van der Waals adhesion force between such a deformed grain and a flat surface is estimated, and the link between the force and the impact velocity is determined. We show that adhesion can be increased by several orders of magnitude when the impact velocity approaches the adhesion velocity. Finally, the effects of surface roughness are incorporated.

Experimental Study on Seismic Response of a Large‐Span and Column‐Free Subway Station in Composite Strata

In this paper, a shaking table test was conducted to investigate the seismic response of the large‐span and column‐free subway station in the upper‐soft and lower‐hard strata. The acceleration of the structure and the soil, the dynamic soil pressure, and the strain response of the subway station were obtained and analyzed. The results demonstrate the reasonable test design as the boundary effect was eliminated. The seismic response of the structure and soil became more severe as the acceleration amplitude of the input motion increased. It is indicated that possible shear damage of the soil and irreversible plastic deformation of the structure might have occurred as the test proceeded. The soft clay had a greater effect on the structure than that of the artificial rock. For the model structure, the tensile strain amplitude in the support region was larger than that in the midspan region. The support regions of the roof slab, lateral wall, and middle slab were the vulnerable components of the model structure during earthquakes.

Detection and Characterization of Damage in Quasi-Static Loaded Composite Structures using Passive Thermography.

Real-time nondestructive evaluation is critical during composites load testing. Of particular importance is the real time measurement of damage onset, growth, and ultimate failure. When newly formed damage is detected, the loading is stopped for further detailed characterization using ultrasound inspections or X-ray computed tomography. This detailed inspection data are used to document failure modes and ultimately validate damage prediction models. Passive thermography is used to monitor heating from damage formation in a hat-stiffened woven graphite epoxy composite panel during quasi-static seven-point load testing. Data processing techniques are presented that enable detection of the small transient thermographic signals resulting from damage formation in real time. It has been observed that the temperature rise due to damage formation at the surface is composed of two thermal responses. The first response is instantaneous and conforms to the shape of the damage. This heating is most likely due to irreversible thermoelastic, plastic deformation, and microstructural heating. The second response is a transient increase in temperature due to mechanical heating at the interface of failure. Two-dimensional multi-layered thermal simulations based on quadrupole method are used to investigate the thermal responses. In particular, the instantaneous response is used as the transient response start time to determine damage depth. The passive thermography measurement results are compared to ultrasonic measurements for validation.

Electron structure and thermodynamics of solid solutions in Ni–H system

The widespread concept of nickel hydride in the Ni–H system is discussed based on the first–principle atomic calculations and experimental X–ray diffraction data. The total cohesion energy in Ni–H solid solution has been determined using the density functional theory and program package Wien2k. Its dependence on hydrogen concentration is shown to be linear, which suggests the absence of any energy barrier for precipitation reaction. Moreover, the second derivative of the calculated solution enthalpy is negative within the hydrogen–to–nickel ratios, H/Ni, of 0.03 to 0.75, which is a sign of spinodal decomposition. These hydrogen concentrations are consistent with the measurements of X–ray diffraction, of which results are traditionally interpreted in terms of Ni hydride. The density of electron states has been calculated, and its non–monotonous concentration dependence correlates with that of solution enthalpy, which is also expected for spinodal decomposition. The obtained results are interpreted as miscibility gap in the Ni–H system with spinodal decomposition having the electron origin. In addition, using mechanical spectroscopy, the strain dependent internal friction has been observed in the hydrogen–charged nickel with H/Ni ratio of about 0.7. This effect is controlled by irreversible plastic deformation, which is typical for solid solutions, not for brittle chemical compounds. Finally, the “hydrides” in a number of metals are discussed in terms of two Gibb’s types of precipitation reactions. Keywords: nickel, hydrogen, ab initio calculations, spinodal decomposition, hydride

Characterizing the Stimulated Reservoir Volume During Hydraulic Fracturing-Connecting the Pressure Fall-Off Phase to the Geomechanics of Fracturing

Microseismic imaging of the hydraulic fracturing operation in the naturally fractured rocks confirms the existence of a stimulated volume (SV) of enhanced permeability. The simulation and characterization of the SV evolution is uniquely challenging given the uncertainty in the nature of the rock mass fabrics as well as the complex fracturing behavior of shear and tensile nature, irreversible plastic deformation and damage. In this paper, the simulation of the SV evolution is achieved using a nonlocal poromechanical plasticity model. Effects of the natural fracture network are incorporated via a nonlocal plasticity characteristic length, ℓ. A nonlocal Drucker–Prager failure model is implemented in the framework of Biot's theory using a new implicit C0 finite element method. First, the behavior of the SV for a two-dimensional (2D) geomechanical injection problem is simulated and the resulting SV is assessed. It is shown that breakdown pressure and stable fracturing pressure are the natural outcomes of the model and both depend upon ℓ. Next, the post-shut-in behavior of the SV is analyzed using the pressure and pressure derivative plots. A bilinear flow regime is observed and it is used to estimate the flow capacity of the SV. The results show that the flow capacity of the SV increases as ℓ decreases (i.e., as the SV behaves more like a single hydraulic fracture); however, for 0.1m≤ℓ≤1m, the calculated flow capacity indicates that the conductivity of the SV is finite. Finally, it is observed that as ℓ tends to zero, the flow capacity of the SV tends to infinity and the SV behaves like a single infinitely conducting fracture.

On the accumulation of irreversible plastic strain during compression loading of open-pore metallic foams

The accumulation of plastic strain as an essential element of the compression behavior of metal foams is investigated by analyzing effective stress-strain curves which were recorded during testing. By applying loading/unloading cycles within the low-strain region until reaching the stress plateau, it is studied how reversible elastic deformation is gradually transformed into irreversible plastic deformation and it is shown that both, elastic and plastic strains, contribute to the total strain ε. This behavior is found to be independent on the investigated mesostructural foam morphologies. Furthermore, a method is derived which can be used to determine a proof stress σϕPl=0.5 at which yielding dominates the deformation of a metal foam.

Crack path predictions in anisotropic structures considering residual stresses

AbstractDifferent mechanical processes induce a directional microstructure (texture) during the production of a material, e.g. during the hot rolling of aluminum plates or the injection molding process of natural fiber reinforced bio‐polymers. In such materials the anisotropic mechanical features are related to the texture and have a decisive impact on the fracture process. Rolling or shot peening processes, on the other hand, induce an irreversible plastic deformation and residual compressive stresses for the purpose of a surface toughening. Similarly, residual stresses are induced if a circular core with interference fit is inserted into a drilled hole. The measurement of crack paths in specimens with residual stresses due to a core reveals an influence of the core's interference with respect to the crack deflection and the resulting paths [1,2]. Numerical investigations of fatigue crack growth show that conventional crack deflection criteria are not valid in such specimens, as the deflection angle depends on the magnitude of the external loading, going along with a rotation of the principal stress axes and thus the Jk‐vector [2]. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)