Internal Damage Mechanisms Research Articles

The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

AbstractSuperaustenitic steel Sanicro 25 has been subjected to out‐of‐phase thermomechanical fatigue cycles with 10‐min dwell at peak temperature in the temperature range from 250°C to 700°C. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated. The internal structure and nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X‐ray spectroscopy in scanning transmission electron microscope. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for fatigue crack nucleation and propagation. Effect of dwells in in‐phase and in out‐of‐phase cycling on the fatigue behaviour, on the modification of internal structure and damage mechanisms, were analysed and discussed.

Effects of freezing conditions on the frost resistance and microstructure of concrete

Repeated freezing and thawing (F-T) cycles are the major reason for the frost damage and premature failure in concrete structures. Frost damage can be evaluated by its effects on formation of internal cracks and scaling at the surface of concrete. Both internal frost damage and surface scaling mechanisms depend on several environmental factors such as the rate of freezing, minimum temperature and the duration at the minimum temperature.The objective of this study is to investigate the effects of freezing conditions on the type and intensity of damage, and on microstructure of concrete. Investigations, within the scope of this research, indicated that freezing conditions significantly affect the F-T resistance assessment of the concrete mixtures. Microstructural examinations also revealed the severity of the scaling under slower freezing rate and longer exposure time at the minimum temperature. Therefore, examination of the local freezing and thawing regime is crucial for designing durable and sustainable concrete structures.

Modeling Tensile Damage and Fracture Behavior of Fiber-Reinforced Ceramic-Matrix Minicomposites.

Evolution of damage and fracture behavior of fiber-reinforced mini ceramic-matrix composites (mini-CMCs) under tensile load are related to internal multiple damage mechanisms, i.e., fragmentation of the brittle matrix, crack defection, and fibers fracture and pullout. In this paper, considering multiple micro internal damage mechanisms and related models, a micromechanical constitutive stress–strain relationship model is developed to predict the nonlinear mechanical behavior of mini-CMCs under tensile load corresponding to different damage domains. Relationships between multiple micro internal damage mechanisms mentioned above and tensile micromechanical multiple damage parameters are established. Experimental tensile nonlinear behavior, internal damage evolution, and micromechanical tensile damage parameters corresponding to different damage domains of two different types of mini-CMCs are predicted. The effects of constitutive properties and damage-related parameters on nonlinear behavior of mini-CMCs are discussed.

Modeling Temperature-Dependent Vibration Damping in C/SiC Fiber-Reinforced Ceramic-Matrix Composites.

In this paper, the temperature-dependent vibration damping in C/SiC fiber-reinforced ceramic-matrix composites (CMCs) with different fiber preforms under different vibration frequencies is investigated. A micromechanical temperature-dependent vibration damping model is developed to establish the relationship between composite damping, material properties, internal damage mechanisms, and temperature. The effects of fiber volume, matrix crack spacing, and interface properties on temperature-dependent composite vibration damping of CMCs and interface damage are analyzed. The experimental temperature-dependent composite damping of 2D and 3D C/SiC composites is predicted for different loading frequencies. The damping of the C/SiC composite increases with temperature to the peak value and then decreases with temperature. When the vibration frequency increases from f = 1 to 10 Hz, the peak value of composite damping and corresponding temperature both decrease due to the decrease of interface debonding and slip range, and the damping of 2D C/SiC is much higher than that of 3D C/SiC at temperature range from room temperature to 400 °C. When the fiber volume and interface debonding energy increase, the peak value of composite damping and the corresponding temperature decreases, mainly attributed to the decrease of interface debonding and slip range.

Evaluation of Hugoniot parameters for unreacted Al/PTFE reactive materials by modified SHPB test

Numerical simulation is one of the most effective ways to investigate the internal damage mechanism for reactive materials (RMS), and an adequate equation of state (EOS) is of great importance. Explosives and lasers are widely employed to drive a flying plate to obtain the EOS parameters. However, the velocity achieved by these ways is too high to obtain the corresponding EOS parameters for unreacted RMS because they tend to be deflagrated under the intense dynamic loads. In order to evaluate the shock EOS parameters for unreacted Al/PTFE RMS, this work introduces a direct method based on the split Hopkinson pressure bar (SHPB) test. The traditional SHPB test was developed by adding two polyvinylidene fluoride piezoelectric sensors pasted at the bottom surfaces of cylindrical RM specimens, which could directly obtain the real-time stress wave speed, and the particle speed is tested by the strain gauge in the transmit bar. Therefore, the two key parameters (the elastic wave speed C0 and slope S) in the shock EOS are achieved by the method of directly linear fitting. This work could provide favorable reference for relative simulation researchers of RMS.

Role of Internal Damage Mechanisms in Controlling Bond-Slip Behavior in Pullout Tests in Concrete

AbstractThe bond-slip relationship is determined from the measured load and the measured relative displacement between the rebar and the concrete in a pullout test. It is considered a characteristi...

Comparison between different non-destructive techniques methods to detect and characterize impact damage on composite laminates

This paper aims to investigate the ability of ultrasonic and electronic speckle pattern interferometry to analyse the low-velocity impact internal damage mechanisms on basalt composite laminates and to provide information on the shape and the extent of the delamination in non-destructive way. Basalt/epoxy composites with different thicknesses have been realised and characterised by mechanical tests to investigate both fibre-dominated (tensile and flexural behaviour) and matrix-dominated properties (interlaminar shear strength). Specimens were impacted at penetration and at increasing energy values, to explore the damage onset and propagation. The results showed that the damage was concentrated under the impactor–material contact point and that the composite with intermediate thickness had the best balance between the different kinds of impact damages: delamination and indentation. Further, a good agreement was found between the overall data obtained by the two non-destructive techniques, confirming the capability of both techniques to examine the composite impact damage.

On the microstructure evolution during isothermal low cycle fatigue of β-annealed Ti-6242S titanium alloy: Internal damage mechanism, substructure development and early globularization

The present work deals with the room and high temperature microstructure evolutions of a near alpha titanium alloy (Ti-6242S) during isothermal low cycle fatigue. The initial fully lamellar microstructure has been purposefully annealed to form α/β interface phase layers. The shearing and displacement of the β layers is identified as the main governing internal damage mechanism. This is phenomenal owning to the very low imposed strain; and is justified considering: (i) the large colony size and thin thickness of the β layers within the initial lamellar microstructure, (ii) the presence of the interface phase, and (iii) the intensified substructure development within the α and β phases. The shearing and displacement of the β layers lead to the segmentation and this is followed by initial stage of dynamic globularization even at room temperature condition. The globularization kinetic is intensified at higher temperature cyclic loading. In this regard, the sub-boundary induced boundary splitting/grooving is characterized as an involved mechanism. Interestingly, the early dynamic globularization of the α lamellae is also traced through the high temperature fatigued microstructures. The β phase penetration along the α phase sub-boundary is realized as the main globularization mechanisms. The interface phase layer stimulates the possibility of globularization via providing the high-energy non-coherent interfaces in structure. In addition, the deformation twinning within the interface phase, so called as “twinning induced micro-plasticity effect”, appears to have a significant effect on the fatigue damage behavior via compensation of incompatibility between α and β phases and deformation twin formation.

An experimental study on deformation and failure mechanical behavior of granite containing a single fissure under different confining pressures

Fissures in natural rocks play an important role in determining the strength, deformability and failure behavior of rock mass. However in the past, triaxial compression experiments have rarely been conducted for rock materials containing three-dimensional (3-D) fissures and the failure mechanical behavior of fissured rocks is not well known due to the difficulty of conducting triaxial experiments on fissured rocks. Therefore in this research, conventional triaxial compression experiments were performed to study the strength, deformability and failure behavior of granite specimens with one preexisting open fissure. Thirty-one specimens were prepared to perform conventional triaxial compression tests for intact and fissured granite. First, based on the experimental results, the effects of the confining pressure and the fissure angle on the elastic modulus and the peak axial strain of granite specimens are analyzed. Second, the influence of the confining pressure on the crack damage threshold and the peak strength of granite with respect to various fissure angles are evaluated. For the same fissure angle, the crack damage threshold and the peak strength of granite both increase with the confining pressure, which is in good agreement with the linear Mohr–Coulomb criterion. With increasing fissure angle, the cohesion of granite first increases and later decreases, but the internal friction angle is not obviously dependent on the fissure angle. Third, nine crack types are identified to analyze the failure characteristics of granite specimens containing a single fissure under conventional triaxial compression. Finally, a series of X-ray microcomputed tomography (CT) observations were conducted to analyze the internal damage mechanism of granite specimens with respect to various fissure angles. Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of granite specimens. The study helps to elucidate the fundamental nature of rock failure under conventional triaxial compression.

Strength, Deformability and X-ray Micro-CT Observations of Deeply Buried Marble Under Different Confining Pressures

In this research, a series of triaxial compression experiments and X-ray observations were conducted to explore the strength, deformability and internal damage mechanism of deeply buried marble. The results show that an increase in confining pressure results in obvious brittle–ductile transition characteristics of deeply buried marble. The Young’s modulus of the marble increased nonlinearly with increasing confining pressure. The peak and residual strength of the marble exhibit a clear linear relationship with the confining pressure, which can be described by the linear Mohr–Coulomb criterion. The sensitivity of the residual strength on the confining pressure was clearly higher than that of the peak strength. After uniaxial and triaxial compression failure, marble specimens were analyzed using a three-dimensional X-ray micro-CT scanning system. Based on horizontal and vertical cross-sections, the marble specimen is mainly dominated by axial splitting tensile cracks under uniaxial compression, but under confining pressure, the marble specimen is mainly dominated by a single shear crack. To quantitatively evaluate the internal damage of the marble material, the crack area and aperture extent for each horizontal cross-section were calculated by analyzing the binarized pictures. The system of crack planes under uniaxial compression is more complicated than that under triaxial compression, which is also supported by the evolution of the crack area and aperture extent. Finally, the brittle–ductile transition mechanism of the marble is discussed and interpreted according to the proposed conceptual models.

Analysis of the damage threshold of the GaAs pseudomorphic high electron mobility transistor induced by the electromagnetic pulse**Project supported by the National Basic Research Program of China (Grant No. 2014CB339900) and the Open Fund of Key Laboratory of Complex Electromagnetic Environment Science and Technology, China Academy of Engineering Physics (CAEP) (Grant No. 2015-0214.XY.K).

An electromagnetic pulse (EMP)-induced damage model based on the internal damage mechanism of the GaAs pseudomorphic high electron mobility transistor (PHEMT) is established in this paper. With this model, the relationships among the damage power, damage energy, pulse width and signal amplitude are investigated. Simulation results show that the pulse width index from the damage power formula obtained here is higher than that from the empirical formula due to the hotspot transferring in the damage process of the device. It is observed that the damage energy is not a constant, which decreases with the signal amplitude increasing, and then changes little when the signal amplitude reaches up to a certain level.

Experimental Study of Mechanical Behavior and X-Ray Micro CT Observations of Sandstone Under Conventional Triaxial Compression

This paper reports a series of triaxial compression experiments and X-ray observations carried out to explore the mechanical behavior and internal damage mechanism of sandstone material. The results show that the Young’s modulus of sandstone increased nonlinearly with increasing confining pressure, but the Poisson’s ratio remained unaffected. The nonlinear Hoek–Brown criterion can better reflect the peak strength properties than the linear Mohr–Coulomb criterion. However, the residual strength of sandstone exhibits a clear linear relationship with the confining pressure, which can be best described by the linear Mohr–Coulomb criterion. The sensitivity of the crack damage threshold on the confining pressure was clearly lower than that for the peak strength. After unaxial and triaxial compression failure, the sandstone specimens were analyzed using a 3D X-ray micro CT scanning system. Based on the horizontal and vertical cross-sections of sandstone specimens, we found that under uniaxial compression and lower confining pressure, the sandstone specimen is dominated mainly by axial splitting tensile cracks; however, under higher confining pressure, the sandstone specimen is mainly dominated by a single shear crack. To quantitatively evaluate the internal damage of sandstone material, crack area and aperture extent for each horizontal cross-section were calculated by analyzing the binarized pictures. The system of crack planes under uniaxial compression is much more complicated than that under triaxial compression, which is also testified by the evolution behavior of crack area and aperture extent. Finally, the fracture mechanism of sandstone during uniaxial and triaxial compression is discussed in detail and simplified models are proposed.

The damage effect and mechanism of the bipolar transistor induced by different types of high power microwaves

By analyzing the variations of the internal distributions of the temperature with time and the current density and the burnout time with the signal amplitude, we study the internal damage process and mechanism of the typical silicon-based n+-p-n-n+ structure bipolar transistor induced by three kinds of high power microwaves such as triangular wave, sinusoidal wave and square wave. The results show that the base-emitter junction is the damage position and the device is more susceptible to damage under the injection of the square waves. The displacement current and the burnout time increase but the proportion of the displacement current in the total current decreases with signal amplitude increasing. The injected power plays a determinative role in the damage process compared with the displacement current. Adopting the data analysis software, the relation equation between the burnout time t and the signal frequency f is obtained. It is demonstrated that the burnout time increases with signal frequency increasing, and the equations of the three kinds of high power microwaves all agree with the formula t= afb.

Study on the monofilament fracture damage pattern and stress field in fracture experiment

The fracture experiments of the monofilament composite material system is applied to evaluate the interface shear strength of the composite materials for a long time. This paper is based on the polarizing microscope, establishing the online observation fracture experimental platform and analyzing the formation process of the fracture according to fracture injury patterns of the carbon monofiber resin matrix composite system in the fracture experiment. Secondly, this paper also describes the forming process of the fracture injury patterns, diamond damage fracture, V damage fracture and double V damage fracture. The experiment shows under the tensile load. Matrix of different mechanical properties is the main factor to affect the carbon fiber fracture morphology and the stress distribution representation. Finally, by employing the finite element software ABAQUS as computing platform, and with the help of the user-defined subroutine, the numerical analysis method of the fracture process in the monofilament composite system is put forward. From The Micro Perspective, this new method explains the internal damage mechanism of composite material, which is the result of different fiber directional properties of interface between fiber and matrix due to the different chemical reaction extent of the interface phase.

Analysis of nonlinear deformations and damage in CFRP textile laminates

Carbon fibre-reinforced polymer (CFRP) textile composites are widely used in aerospace, automotive and construction components and structures thanks to their relatively low production costs, higher delamination and impact strength. They can also be used in various products in sports industry. These products are usually exposed to different in-service conditions such as large bending deformation and multiple impacts. Composite materials usually demonstrate multiple modes of damage and fracture due to their heterogeneity and microstructure, in contrast to more traditional homogeneous structural materials like metals and alloys. Damage evolution affects both their in-service properties and performance that can deteriorate with time.These damage modes need adequate means of analysis and investigation, the major approaches being experimental characterisation, numerical simulations and microtomography analysis. This research deals with a deformation behaviour and damage in composite laminates linked to their quasi-static bending. Experimental tests are carried out to characterise the behaviour of woven CFRP material under large-deflection bending. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behaviour and damage in woven CFRP laminates. Multiple layers of bilinear cohesive-zone elements are employed to model the onset and progression of inter-ply delamination process. X-ray Micro-Computed Tomography (MicroCT) analysis is carried out to investigate internal damage mechanisms such as cracking and delaminations. The obtained results of simulations are in agreement with experimental data and MicroCT scans.

Void Nucleation Model in a Steam Turbine Material under Creep Condition

In order to establish a reliable remaining life assessment method for high temperature components in thermal power plants, it is necessary to understand the internal damage mechanism in heat resistant alloys and establish the damage prediction method. In the case of commercial heat resistant alloy used for thermal plants, nucleation, growth and coalescence of creep voids are dominant damage mechanism. So far, the void growth mechanism is well investigated and various growth models are suggested. However, rational modeling of void nucleation was remained to be difficult subject because basic mechanism has been unclear. Therefore, in this study, void nucleation behavior of a turbine rotor material under creep condition was investigated by a scanning electron microscope and development of void nucleation models was examined. Void nucleation model was developed based on the idea that creep voids nucleate by vacancy condensation at the interface between precipitated carbides and matrix on the grain boundary. In addition, consideration of creep property was included in the model via a relationship between strain and vacancy density obtained from the molecular mechanics simulation. In addition, model was expanded to predict the void number density, by a statistical treatment of the different physical property related to void nucleation in the every grain boundary. It was confirmed that the present void nucleation model can reproduce evolution of void number density.

The damage effect and mechanism of the bipolar transistor induced by the intense electromagnetic pulse

A study of the internal damage process and mechanism of the typical n+-p-n-n+ structure bipolar transistor induced by the intense electromagnetic pulse (EMP) is carried out in this paper from the variation analysis of the distribution of the electric field,the current density and the temperature. Research shows that the damage position of the bipolar transistor is different with the different magnitude of the injecting voltage,when the magnitude of the injecting voltage is low the damage will appear firstly near the collector region under the center of the emitter region,and when the magnitude of the injecting voltage is sufficiently high the damage will appear firstly at the edge of the base near the emitter due to the breakdown of the PIN structure composed of the base-epitaxial layer-collector. Adopting the data analysis software,the relation equation between the device damage power P and the pulse width T under different injecting voltage is obtained. Owing to the variety of the device damage energy,it is demonstrated that the empirical formulas of the intense electromagnetic pulse P=AT-1 (A is a constant) is modified to P=AT-1.4 for the bipolar transistor.

The effect of impactor shape on the impact response of composite laminates

The effect of impactor shape on the impact response of thin woven carbon/epoxy laminates is investigated herein. Using a drop weight test rig, specimens were impacted at initial impact energies of 4 J and 6 J using steel hemispherical, ogival and conical impactors, all 12 mm in diameter. The specimens impacted by the conical impactor absorbed most energy as a result of local penetration. The hemispherical impactor produced the highest peak force and lowest contact duration as expected. Only the hemispherical impactor at an initial impact energy of 4 J produced barely visible impact damage (BVID) whereas the other impactor shapes produced permanent indentation and penetration. Destructive testing techniques are required to determine the internal damage mechanisms since non-destructive inspection (NDI) techniques provided similar damage areas for the three impactor shapes.

Modeling Stiffness Degradation in Filamentary Composite Materials

The effect of service‐load‐induced internal damage in advanced filamentary composite materials has for some time been a topic of extensive research. It is well known that the stiffness of the material system is degraded as load is applied and damage accumulates. A computational simulation of the processes by which the internal damage propagates was developed for studying the effect of the complex interactions among identified internal damage mechanisms. The approach considers the random distribution of initial matrix flaws (microcracks and micropores) and weak fiber‐matrix interfacial bonding, the random strength properties of the fibers, and load transfer in the vicinity of broken fibers through matrix shear, and it employs an iterative, time‐stepping technique to simulate damage propagation under increasing load and over periods of sustained constant load. The developed model is employed to simulate the response of strands of Kevlar 49/epoxy to specified loading conditions. The statistical nature of the effect of preloading on the stiffness and strength degradation of continuously loaded strands is demonstrated and discussed.