Existence Of Microcracks Research Articles

Evaluation of Bond (Adhesive) Strength Between Conventional Concrete and Steel Fiber Reinforced Concrete Interface

Good bond strength between overlay and substrate is a key factor in performance of concrete repairs. This thesis was aimed at studying the evaluation of bond strength between conventional concrete substrate and steel fiber reinforced concrete. Many factors such as surface roughness, existence of micro cracks, compaction, curing etc. influence `the bond strength. The quality assurance of the bond strength requires test methods that can quantify the bond strength as well as identify the failure mode. There have been numerous investigations led to development of different test methods. The forces which are applied in each test and the failure mode are important in order to choose the proper test. An interpretive study on test methods is presented. While this study can provide individually useful information on bond strength and bond characterization.

High Sensitivity, Wide Range Pressure Sensor Based on Layer-by-Layer Self-Assembled MXene/Carbon Black@Polyurethane Sponge for Human Motion Monitoring and Intelligent Vehicle Control

Flexible pressure sensors with high sensitivity, low cost, lightweight, and convenience are of significance in the field of human movement monitoring and sensing. This article demonstrated a piezoresistive pressure sensor prepared by layer-by-layer (LbL) self-assembly of MXene and carbon black (CB) on a polyurethane (PU) sponge. The electronegativity of MXene and the electropositive of CB enable them to combine more stably. The assembled sensor achieves reliable repeatability (over 3700 cycle testing), high sensitivity (−5.26 kPa <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula> ), wide sensing range (0–240 kPa), acceptable response time (300/300 ms), and good reproducibility under various pressures. MXene and CB greatly increase the electrical conductivity and measurement range of the piezoresistive sensors. The existence of microcracks allows the sensor to sensitively capture micropressure signals, such as human radial artery pulse and respiration. In addition, the large-area conductive framework permits the sensor to maintain high responsiveness over a huge strain range and to distinguish various degrees of human movement. Furthermore, the sensor has shown excellent performance in controlling smart devices through external pressure as a human–machine interface. The pressure sensor based on LbL self-assembled MXene/CB on a PU sponge has potential applications in motion signal detection, artificial intelligence, and other fields.

Study on Mechanical Properties of crack Healing of low carbon Steel based on cyclic Phase Transformation Heat treatment

With the rapid development of materials, metal materials are used less and less, but at this stage, metal materials are still widely used, and iron and steel materials are the most widely used. Cracks often appear in the process of metal material processing and use, and these cracks will have a certain impact on the use of metal materials. The existence of microcracks will affect the mechanical properties of materials to some extent, but in most cases, the mechanical properties of materials will be greatly reduced, and in serious cases, metal materials will break directly in the process of use or processing. The crack healing process needed after the emergence of cracks can effectively change this situation, but so far, the research on metal crack healing is still not perfect. In this paper, taking the internal crack of low carbon steel as the object, the recovery of mechanical properties of low carbon steel by cyclic phase transformation heat treatment was studied. The results show that with the increase of the healing area, the microhardness of the area after crack healing also increases, and the tensile strength of the specimen also increases after the healing. When the healing area is similar, increasing the healing time and temperature will result in grain coarsening, resulting in the decrease of microhardness and tensile strength in the crack healing zone.

Effect of pulse frequency on the nanosecond pulsed laser welded Al/steel lapped joint

Nanosecond pulse laser welding of 6063 aluminum alloy and 316L stainless steel sheets with a thickness of 0.2 mm was performed. The effect of pulse frequency on the joining characteristics was investigated. The results showed that porosities and cracking defects with larger dimension and numbers were produced in joints at the frequency of 125 kHz, while fewer defects were obtained in the joints with the pulse frequency larger than 175 kHz. As the pulse thermal effect played a major role at the frequency of 125 kHz, diffusion and enrichment of aluminum elements occurred in almost the entire aluminum side of the weld, generating various intermetallic compounds such as FeAl, Fe2Al5 and FeAl3. As the frequency increased, the banding structures in the weld became more congregate, which were mainly the mixed phases of FeAl and Fe2Al5. The shear force test results indicated that the highest shear force of 135.4 N was obtained when the pulse frequency was 225 kHz. The nailing effect of the wave-shaped weld structures and the absence of porosities led to desirable mechanical properties. Besides, three fracture modes, named by interfacial fracture (IF), substrate tearing failure (STF) and nugget pullout (NP), could be classified. The large interfacial porosities and dispersive distribution of brittle phases led to the IF mode. The distribution of the Fe-Al brittle phases at the weld edge resulted in the STF mode. The existence of microcracks at the bottom of the weld contributed to the NP mode.

Assessment the Influence of Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Micro and Nanoparticles on Properties of Concrete

This work focuses on investigating the behavior of micro and nanoFe2O3 in concrete through the study of microscopy, compressive strength and flexural strength. Seven mixes of concrete are prepared. Three of which include micro Fe2O3 comprising 1%, 3% and 5% as partial substitutions of cement weight and the other three mixes containing 1%, 3% and 5% nanoFe2O3 as a partial substitutions of cement weight, as well as the reference mix. SEM study exhibits that the microstructure of sample produced with micro Fe2O3 show more uniformity and dense compared with reference sample. However, existence of micro cracks in the microstructure in mixes with micro Fe2O3 is clear especially with increasing micro Fe2O3 content to 3% and 5%. In mixes with nanoFe2O3, the new shapes of needle like crystals and flower like crystals can be distinguished. The presence of these types of crystals in thrives amount approve that nanoFe2O3 producing new phases of gel. Hence, the microstructure of samples is condensed, and with the increasing of nanoFe2O3 content the microstructure is completely compacted. The results of compressive strength and flexural strength showed that the mechanical properties are related to microstructure properties. When micro Fe2O3 is used, the mechanical properties drop obviously. While, the mechanical properties are improved in mixes containing nanoparticles in comparison with the control mix.

Investigation on microstructure and hardness of nickel-alumina functionally graded material

In this research study, six-layered nickel-alumina (Ni-Al2O3) functionally graded material (FGM) was prepared using powder metallurgy (PM) method. The objectives of this study were to investigate the microstructure and hardness of the graded composite layer by layer. Using a cylindrical steel die, the six-layered nickel-alumina graded structure was fabricated considering 0%, 20%, 40%, 60%, 80% and 100% weight percentage of ceramic concentration for different layers. A hydraulic press was used for fabrication of the FGM layered structure and 30 ton compaction load was applied. Considering two-step sintering cycle, sintering was carried out at sintering temperature 1200 °C and sintering time 4 h. The sintered specimens were characterized using optical microscopy (OM), scanning electron microscopy (SEM) and hardness testing. It was observed that uniform particle distribution within the graded layers and smooth microstructural transition occurred between adjacent layers. It was also observed that the interface lines are obvious, less wavy, straight and parallel which confirms proper layer stacking process. On the other hand, from the SEM micrographs, the existence of microcracks and voids are identified in the alumina-rich layer and mostly around alumina particles.

Characteristics and controlling factors of tight sandstone gas reservoirs in the Upper Paleozoic strata of Linxing area in the Ordos Basin, China

Due to the large resource potential of tight sandstone gas, the Upper Paleozoic strata of the Linxing area has become an important exploration target in the Ordos Basin. In this study, by analyzing the distribution characteristics of tight sandstone gas (TSG), the relations of source-reservoir assemblages were divided into three types containing interbedded with source rocks (Type I), adjacent to source rocks (Type II) and far from source rocks (Type III). Five factors were identified to control their genesis, migration, and accumulation. Firstly, the gas generation intensity of source rocks is larger than 10 × 108 m3/km2, which indicated that source rocks could provide gas to the reservoirs. The gas generation centers of source rocks control the distribution of TSG reservoirs, while the areas among the hydrocarbon generation centers were the enrichment zones for TSG. Secondly, under the condition of generally tight reservoirs, the sandstone reservoir which has good quality (high porosity, high permeability) provided the favorable accumulation condition for TSG. Thirdly, the study area experienced one stage of gas charging mainly from the late early Jurassic to late early Cretaceous, which was 178 to 100Ma. Continuous gas generation and expulsion led to the sustained accumulation of TSG. Fourthly, with the general development of overpressure in the Upper Paleozoic strata, gas expansion force as the main power drive natural gas to migrate into the reservoirs of type I and type II during the accumulation period. Finally, the development of faults and fractures provided a migration path for natural gas charging in the reservoir of type III vertically. The existence of microcracks can improve the petrophysical properties of tight reservoirs. Under the joint control of these factors, the reservoir of Type I is the most favorable exploration target.

Failure mechanism of rigid polyurethane foam under high temperature vibration condition by experimental and finite element method

ABSTRACTThe failure mechanism of rigid polyurethane foam (RPUF) under room temperature (RT) and high temperature vibration conditions was investigated by experiment and finite element stimulation. Damaged RPUF specimens were prepared at different vibration amplitudes ranging from 0 to 19.879 mm at RT and 150 °C for different vibration times. The tensile test was utilized to evaluate the vibration damage degree of RPUFs, and the results exhibited that tensile strength decreased gradually with the increase of vibration amplitude and time at both RT and 150 °C. Thermogravimetric analysis and Fourier transform infrared spectroscopy illustrated that thermal degradation of RPUF is attributed to the decomposition of carbonyl urethane groups at 150 °C. The scanning electron microscopy analysis of the tensile fracture surfaces revealed that the vibration failure of RPUF mainly resulted from the existence of microcracks in cell structure. A finite element simulation was established by ABAQUS to study stress distribution of RPUF under different vibration loads, which then demonstrated that the microcracks are most likely to exist on the junction of two microcell units, which is due to convergence of stress in the process of vibration. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48343.

Steel Material Degradation Assessment Via Vibro-Acoustic Modulation Technique

The increasing probability of collapse in defective structures owing to aging is one of the major issues in transportation. Therefore, different methodologies that are capable of monitoring structural components have been used to identify defects and predict failure. Among these methods, the non-linear vibro-acoustic modulation (VAM) technique has been implemented for many years in a variety of industries, such as aerospace. This method utilizes the effect of nonlinear interactions between a high frequency ultrasonic wave (carrier signal) and a much lower frequency structural vibration (modulating signal). This interaction takes place at nonlinear interfaces (cracks, bolted connections, delaminations, etc.) manifesting itself in the spectrum as side-band components around the carrier frequency. In this study, the VAM method was investigated as a non-destructive evaluation (NDE) method for fracture critical members (FCMs) in steel bridges. The results of the experimental studies revealed that using the VAM technique on test specimens during the tension only fatigue tests would provide some useful information on the existence of micro-cracks and on failure prediction. The use of the VAM technique for center-notched rectangular test specimens of structural steel under low-amplitude fatigue loading at a frequency of 10 Hz is capable of predicting the failure at 70–80% of the fatigue lifetime of the specimen. Moreover, in this investigation utilizing fatigue cycling as a modulating signal was successfully substituted for the conventional utilization of resonance structural bending vibrations.

Nucleation location and propagation direction of radial and median cracks for brittle material in scratching

At present, diamond wire sawing technology is a mainstream method for wafering brittle materials such as silicon crystal, SiC crystal, sapphire, ceramics, optical glass and so on. Microcracks will occur on the wafers surface inevitably during sawing owing to the high brittleness and hardness of brittle material. The existence of microcracks and their size, orientation and distribution not only affect the surface quality and fracture strength of wafers, but also have an important impact on the subsequent processing effect of wafers after sawing. Therefore, the further investigation on the formation and propagation direction of microcracks is needed during diamond wire sawing. In this paper, an analytical model of the elastic stress field for brittle material is established during single abrasive scratching. The stress fields in front of and behind the indenter are analyzed respectively. Two different material removal modes, ductile removal and brittle removal, are considered. To evaluate the two different material removal modes, influence factor μ is proposed. The nucleation location and propagation direction of radial crack and median crack of several materials are predicted by analytical model. The result show that the radial crack nucleates behind the indenter. The deflection angle between propagation direction and scratching direction is about 35–60°, and its size depend on the material removal modes. The nucleation location of median crack is no longer at the bottom of the plastic zone, but will deflect toward the scratching direction during scratching. The size of deflection angle depends on the tip half angle of indenter. When the tip half angle is 55–80°, the deflection angle decreases with the increase of the indenter half angle. All predicted results are in good agreement with the experimental data of relevant scholars. At the end, the half-penny crack (radial/median crack system) presented in indentation process is extended to scratching process.

A new method to model the non-linear crack closure behavior of rocks under uniaxial compression

The existence of micro cracks inside rocks influences the rock strength and deformation significantly and controls the presence of crack closure stage under compression. However typical discrete element method (DEM) fails to model the gradual crack closure behavior of rock due to insufficient consideration for pre-existing micro-cracks. The stress-strain curves simulated in previous DEM are usually linear in initial stage until numerous cracks are induced under stress. Thus a new method implemented in Discrete Element Method is proposed to model the crack closure behavior of rocks under uniaxial compression. The pre-existing micro-cracks inside the rock sample are created and opened by introducing two notional contact surfaces and giving a negative reference gap. These opened cracks are gradually closed under stress as the adjacent discs sustain sufficient relative movement (re-contact). Thus the gradual crack closure behavior of rock under compression is reproduced. The crack distribution respect to the crack orientation is described with the crack tensor analysis to consider the effect of crack density and anisotropy. Three factors including crack intensity, crack orientation and crack gap are discussed with parametric study to study their influence on the crack closure behavior. The effects of pre-existing cracks on the properties of rocks (such as rock strength, Young's modulus, crack closure stress levels) are also analyzed, and are in well agreement with observations by experiment. A little modification is made to previous calibration process to consider the crack closure stage of rock. Finally two types of rocks including limestone and granite are calibrated and compared to the experiment results. The simulated stress-strain curves match the curves of experiment, especially the initial crack closure segment perfectly, which validates the proposed method.

Coating properties of 1H,1H-perfluorooctylamine-terminated polyimides based on hexafluoroisopropylidene diphthalic anhydride and 1,4,5,8 naphthalenetetracarboxylic dianhydride

This study attempted to prepare polyimide coatings (PIs) containing 6FDA (hexafluoroisopropylidene diphthalic anhydride) and/or NTDA (1,4,5,8 naphthalenetetracarboxylic dianhydride) in various ratios. 1% of these polyimide chains were terminated with 1H,1H-perfluorooctylamine (PFOA) and PFOA, and terminated polyimide coatings (PIFs) were prepared in the same ratios as PIs. Coatings were produced on aluminum substrates by a 75-μm wire-wound applicator and thermally imidized. Finally, thermal, wear, and physical properties of PIFs and PIs were compared to each other. PIFs exhibited higher abrasion resistance than PIs due to the presence of PFOA. The increase in 6FDA content also resulted in elevated abrasion resistance. Both NTDA and 6FDA compounds improved thermal stability. The increase in NTDA content raised the Tg values of the polyimides. PIFs exhibited higher static water contact angle values than PIs, but the existence of microcracks decreased their hydrophobicity. The gloss values of PIFs were excellent. Improved values were obtained for pencil hardness, crosscut adhesion, and MEK (methyl ethyl ketone) rubbing properties for all coatings. In addition to the pencil hardness test, a pendulum hardness test was also carried out. The presence of PFOA affected pendulum hardness values positively, and the increase in the amount of 6FDA on polymer backbone improved the pendulum hardness values despite the rigidity of NTDA.

Micromechanical characterisation of Indian rail steel

Railway transport dynamics cause rails in general to experience a time varying rolling contact load to add shear strain to that from traction and normal strain from the axle loads. These loading conditions together with the frictional heat at the contact interface alters the microstructure in the near-surface region of the rails and wheels. This paper presents the hardness and microstructural study of used Indian rail steel to expose the influence of load parameter active at the interface. Samples are prepared from the rail section taken from Dhanbad railway division. The hardness variation along the rail head cross-section and metallurgical variations of the test sample are presented. Microstructural examinations reveal existence of micro cracks near to the rail surface favouring formation of wear debris. Also, the presence of the pro-eutectoid ferrite and cementite phase at the grain boundaries of the pearlitic rail material is observed. The fatigue cracks are found to initiate across the grain boundaries containing the pro-eutectoid ferrite phase. This study will help to develop a better understanding of rolling contact fatigue (RCF) crack initiation and wear debris formation mechanisms on the rails. Thus, these results can be used to improve rail maintenance planning and risk assessments.

Analysis of a main fatigue crack interaction with multiple micro-cracks/voids in a compact tension specimen repaired by stop-hole technique

Fatigue is a process in engineering materials in which damage accumulates due to the fluctuating loading. One solution for a component under the fatigue process is to arrest the crack propagation before the final failure using different available retardation methods, such as drilling/stop-hole technique. In addition, structural components may also suffer from the existence of micro-cracks or voids due to their forming process or service lives. These micro-cracks/voids are very critical to study, since they can effectively play an important role in the behavior of the existing main crack in a component. This article aims to investigate the effect of the stop-hole retardation technique and multiple micro-cracks/voids with different characteristic lengths and geometries on the fatigue crack propagation in a compact tension specimen. A modified Forman equation, the so-called NASGRO equation is used to define the transition between crack initiation and crack growth period. Also, the extended finite element method is adapted in the crack propagation phase in order to model crack path in the geometry eliminating the need for remeshing procedure. The whole analyses are conducted in a commercial package through a user-written code that handles all fatigue crack growth analysis. The reference solutions from the literature are used to compare and to validate results obtained from current work.

THE CRUSTAL ANISOTROPY PATTERN IN THE EPICENTRAL AREA OF THE 2008 MW 6.4 EARTHQUAKE IN NORTHWEST PELOPONNESE, GREECE

An anisotropic upper crust has been revealed in the W. Gulf of Corinth with potentially changing properties. During 2013, a unique opportunity to conduct a shear-wave analysis was presented, as a combination of the significantly increased seismicity in the area, including a seismic swarm between May and August, and the existence of local seismological networks. The Hellenic Unified Seismological Network (HUSN) and the Corinth Rift Laboratory Network (CRLN) provided invaluable data during the unrest period. While shear-waves travel through an anisotropic medium, the splitting phenomenon takes place and, as a result, their propagation is characterized by two discernible components: the fast (Sfast) and the slow (Sslow) one, which arrives to the station in a subsequent temporal point. Modern advances in seismology and geophysics have rendered shear-wave splitting a valuable tool in determining properties of the anisotropic propagation media. One of the predominant causes of this phenomenon is the existence of microcracks throughout the upper crust. The current study presents results for 8 stations from 535 analyzed events that are in agreement with the anisotropy models of EDA and APE.

PRELIMINARY SHEAR-WAVE SPLITTING RESULTS IN THE BROADER AIGION AREA (GREECE) DURING 2013

An anisotropic upper crust has been revealed in the W. Gulf of Corinth with potentially changing properties. During 2013, a unique opportunity to conduct a shear-wave analysis was presented, as a combination of the significantly increased seismicity in the area, including a seismic swarm between May and August, and the existence of local seismological networks. The Hellenic Unified Seismological Network (HUSN) and the Corinth Rift Laboratory Network (CRLN) provided invaluable data during the unrest period. While shear-waves travel through an anisotropic medium, the splitting phenomenon takes place and, as a result, their propagation is characterized by two discernible components: the fast (Sfast) and the slow (Sslow) one, which arrives to the station in a subsequent temporal point. Modern advances in seismology and geophysics have rendered shear-wave splitting a valuable tool in determining properties of the anisotropic propagation media. One of the predominant causes of this phenomenon is the existence of microcracks throughout the upper crust. The current study presents results for 8 stations from 535 analyzed events that are in agreement with the anisotropy models of EDA and APE.

Study on snail trail formation in PV module through modeling and accelerated aging tests

In recent years, a defect on the surface of photovoltaic (PV) modules called ‘snail trails’ has become a widespread reliability issue, affecting many modules installed in the field. In this work, silver acetate is identified as one component of snail trails through non-destructive Raman analysis. The chemical reaction between the silver grid line, oxygen and acetic acid on top of the micro-crack is proposed as the mechanism. The generation and/or diffusion of acetic acid released from ethylene vinyl acetate (EVA) encapsulant, oxygen and moisture are modeled using finite element method to predict the formation of silver acetate. The simulation results indicate that the existence of micro-crack plus cell gap are necessary for snail trail formation and act as the pathway for transport. As the source of acetic acid, encapsulant plays an important role in snail trail formation. Oxygen transmission rate of backsheet also has significant influence on snail trail formation. Water vapor transmission rate is shown to have no effect on snail trials over a wide rate of transmission rates. An accelerated aging snail trail test has been developed that can simulate snail trails within days, and is compared to modeling and field results.

Microstructure Contributions to Vibrational Damping and Identification of Damage Mechanisms in Arundo Donax L: Reed Cane for Woodwind Instruments

ABSTRACTNatural cane reeds (Latin name Arundo Donax L and here termed ADL) have been used on woodwind instruments for centuries with little change. The reed acts as a mechanical valve controlling the energy input into the musical instrument and it is the musician’s first option for altering the instrument’s sound and response characteristics. Despite this, their consistency, variable performance, durability and sensitivity to ambient conditions make it difficult for the musician to find and maintain a reed that responds to their liking. Thus it is desirable to examine the material, microstructural and anatomical properties of the reed and their contributions to vibrational performance with input from mechanical engineers, materials scientists and musicians.The current study is part of an on-going research project, and this paper presents preliminary results. In the present work raw samples of ADL obtained from a manufacturer in pre-cut form are sectioned into longitudinal and transverse specimens for mechanical characterization. Prior to testing, samples are conditioned using an incubation system to 37 degrees Celsius and 90% relative humidity, mimicking in-use conditions of the reed. Initial microstructure analysis of each specimen is completed using optical microscopy to quantify fiber spatial arrangement, size and the existence of micro-cracks along the fiber-matrix interface. X-ray diffraction is also used to quantify the fraction of crystalline cellulose present in each sample. Specimens are then excited over a specific frequency range similar to that of in-use reeds using pressure waves in a non-contact setup. Values of internal friction are obtained as logarithmic decrement values for frequency-dependent decay. One set of specimens is then subjected to cyclic mechanical loading at low frequency (&lt; 1Hz) and stresses up to 15MPa. The other set is maintained at the given environmental conditions using the incubator and aged through temperature and humidity cycling. Comparisons of post-testing microstructure damage and internal friction measurements are then completed to delineate specific degradation mechanisms due to mechanical/fatigue deterioration and moisture cycling. Internal friction is found to be dependent on both frequency, moisture and cyclic loading. Furthermore, the existence of microstructural cracks contributes to increasing decrement values at high frequencies in both fatigued and moisture cycled samples. Statistically significant correlations are discovered between logarithmic decrement and vascular bundle orientation at 700 Hz and logarithmic decrement and parenchyma cell diameter at 1000 Hz. Reductions in internal friction below 400 Hz indicate a decreasing loss modulus (E’’) with increased moisture cycles, although this trend will be tested against a larger sample set in further work.

鉛フリーはんだにおけるクリープ・疲労き裂進展の微視的観察

Creep-fatigue crack propagation tests of lead-free solder were conducted under various loading waveforms. The waveforms adapted in displacement-controlled tests were four triangular waveforms: pp waveform having fast loading-unloading, cc waveform having slow loading-unloading, cp waveform with slow loading followed by fast unloading, and pc waveform with fast loading followed by slow unloading, and three waveforms with hold time: cc-h waveform having a hold time under tension and compression, cp-h waveform having a hold time under tension, and pc-h waveform having a hold time under compression. In load-controlled conditions, three waveforms are adapted: pp, cc-h and cp-h. Microscopic observation using SEM and EBSD was conducted near the crack tip region and on fracture surfaces. Plenty of microcracks were observed near the main crack tip on the surfaces of specimens, while almost all of them disappeared after removing the surface layer of about 0.4mm. On the removed surfaces, microcracks were observed near the main crack tip for unsymmetrical waveforms, cp, pc, cp-h, while no microcracks for symmetrical waveforms, pp, cc, cc-h. The existence of microcracks is responsible for crack acceleration under unsymmetrical waveforms. EBSD observation showed the formation of subgrains within original grains of Sn and eutectic phases near the crack tip, and the grain size decreased with increasing crack propagation rates. The grain average of GROD also decreased with increasing crack propagation rate, while that of KAM was nearly constant without respect to the crack propagation rate. Striations, fragmentation, and intergranular fracture facets are three main features of creep-fatigue fracture surfaces. The features of striations were clear for the cases of pp, cc, pc, while vague for the other cases. The spacing of striations was nearly equal to the crack propagation rate, supporting the linear summation rule of creep and fatigue crack propagation rates. Plenty of intergranular facets were observed for cp, cp-h. Fragmentation was abundant for waveforms including creep contribution.

On Continuum Damage Modeling of Fiber Reinforced Viscoelastic Composites with Microcracks in terms of Invariants

A continuum damage model is developed for the linear viscoelastic behavior of composites with microcracks consisting of an isotropic matrix reinforced by two arbitrarily independent and inextensible fiber families. Despite the fact that the matrix material is isotropic, the model in consideration bears the characteristic of directed media included in the transverse isotropy symmetry group solely due to its fibers distributions and the existence of microcracks. Using the basic laws of continuum damage mechanics and equations belonging to kinematics and deformation geometries of fibers, the constitutive functions have been obtained. It has been detected as a result of the thermodynamic constraints that the stress potential function is dependent on two symmetric tensors and two vectors, whereas the dissipative stress function is dependent on four symmetric tensors and two vectors. To determine arguments of the constitutive functionals, findings relating to the theory of invariants have been used as a method because of the fact that isotropy constraint is imposed on the material. As a result the linear constitutive equations of elastic stress, dissipative stress, and strain energy density release rate have been written in terms of material coordinate description. Using these expressions, total stress has been found.