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Abstract: Macroscopic, freestanding graphene-based paper-like materials are of interest for use as mechanically strong, stiff, and flexible and electrically conductive materials. Chemically reduced graphene oxide paper shows promise for such applications. In this work, we studied the mechanical and electrical properties of a set of paper materials prepared by filtration of homogeneous colloidal suspensions of hydrazine-reduced graphene oxide with different concentrations. Young’s modulus, fracture strength, and fracture strain of each type of sample was determined by tensile tests. The paper sample prepared from the colloidal suspension with the lowest concentration of reduced graphene oxide platelets had the highest modulus and fracture strength and showed the smoothest surface morphology. The electrical conductivity measured by the four-probe measurement method increased as the concentration was increased. [Copyright &y& Elsevier]

The dynamics of a “peeling front” or an elastic line is studied under creep (constant load) conditions. Our experiments show in most cases an exponential dependence of the creep velocity on the inverse force (mass) applied. In particular, the dynamical correlations of the avalanche activity are discussed here. We compare various avalanche statistics to those of a line with non-local elasticity, and study various measures of the experimental avalanche-avalanche and temporal correlations such as the autocorrelation function of the released energy and aftershock activity. From all these we conclude, that internal avalanche dynamics seems to follow “line depinning”-like behavior, in rough agreement with the depinning model. Meanwhile, the correlations reveal subtle complications not implied by depinning theory. Moreover, we also show how these results can be understood from a geophysical point of view. [ABSTRACT FROM AUTHOR]

Ice fractures when subject to stress that exceeds the material failure strength. Previous studies have found that a von Mises failure criterion, which places a bound on the second invariant of the deviatoric stress tensor, is consistent with empirical data. Other studies have suggested that a scaling effect exists, such that larger sample specimens have a substantially lower failure strength, implying that estimating material strength from laboratory-scale experiments may be insufficient for glacier-scale modeling. In this paper, we analyze the stress conditions in crevasse onset regions to better understand the failure criterion and strength relevant for large-scale modeling. The local deviatoric stress is inferred using surface velocities and reanalysis temperatures, and crevasse onset regions are extracted from a remotely sensed crevasse density map. We project the stress state onto the failure plane spanned by Haigh–Westergaard coordinates, showing how failure depends on mode of stress. We find that existing crevasse data are consistent with a Schmidt–Ishlinsky failure criterion that places a bound on the absolute value of the maximal principal deviatoric stress, estimated to be 158±44 kPa. Although the traditional von Mises failure criterion also provides an adequate fit to the data with a von Mises strength of 265±73 kPa, it depends only on stress magnitude and is indifferent to the specific stress state, unlike Schmidt–Ishlinsky failure which has a larger shear failure strength compared to tensile strength. Implications for large-scale ice flow and fracture modeling are discussed. [ABSTRACT FROM AUTHOR]

Ice fractures when subject to stress that exceeds the material failure strength. Previous studies have found that a von Mises failure criterion, which places a bound on the second invariant of the deviatoric stress tensor, is consistent with empirical data. Other studies have suggested that a scaling effect exists, such that larger sample specimens have a substantially lower failure strength, implying that estimating material strength from laboratory-scale experiments may be insufficient for glacier-scale modelling. In this paper, we analyze the stress conditions in crevasse onset regions to better understand the failure criterion and strength relevant for large-scale modelling. The local deviatoric stress is inferred using surface velocities and reanalysis temperatures, and crevasse onset regions are extracted from a remotely sensed crevasse density map. We project the stress state onto the failure plane spanned by Haigh–Westergaard coordinates, showing how failure depends on mode of stress. We find that existing crevasse data is consistent with a Schmidt–Ishlinsky failure criterion that places a bound on the absolute value of the maximal principal deviatoric stress, estimated to be (158 ± 44) kPa. Although the traditional von Mises failure criterion also provides an adequate fit to the data with a von Mises strength of (265 ± 73) kPa, it depends only on stress magnitude and is indifferent to the specific stress state, unlike Schmidt–Ishlinsky failure which has a larger shear failure strength compared to tensile strength. Implications for large-scale ice-flow and fracture modelling are discussed. [ABSTRACT FROM AUTHOR]

The relevance of problems related to the fracturing of engineering materials and structures will not decrease over time. Fracture mechanics methods continue to be developed, which, combined with numerical methods of computer modeling, are implemented in software packages. However, this is only one facet of the complex of actual problems related to modeling and analyzing the behavior of brittle materials. No less important are the problems of developing not only numerical, but also new analytical models. In this paper, analytical models of only one class are considered, the distinguishing feature of which is that they describe the full load–strain curve using only one equation. However, the determination of model parameters requires tests for which the destruction of the test object is necessary, which may be unacceptable if controlled destruction is technically impossible or economically unreasonable. At the same time, in practice, it is possible to obtain values of stresses and strains caused by loads smaller than the peak load. Pre-peak loads can be used to predict strength using numerical methods, but it is desirable to have a suitable analytical model to extend the capabilities and to reduce the cost of applied research. Such a model was not found in the known literature, which motivated this work, which aims to modify the analytical model to predict strength and the full load–displacement (or stress–strain) curve using only pre-peak loading. This study is based on the analysis of known data and synthesis using mathematical modeling and fracture mechanics. The input data for the model do not include the particle size distribution and other physical and mechanical properties of the components of the material under study. These properties may remain unknown, but their influence is taken into account indirectly according to the "black box" methodology. Restrictions of the scope of the model are defined. The simulation results are consistent with experiments known from the literature. [ABSTRACT FROM AUTHOR]

Glued-in Rods (GiR) are among the high-performance joining technologies used in timber engineering. Engineering design procedures for GiR almost exclusively regulate softwood and softwood engineered wood products (EWP) under quasi-static loads. Since the use of hardwood is expected to significantly increase due to climate change, and potentially establish itself as predominant in Europe's forests, GiR in combination with hardwood and corresponding EWP need to be investigated. Timber constructions subjected to cyclic loads are increasingly being used, as for example in timber bridges, high rise buildings, and wind turbine towers. No normative regulation is yet available for fatigue of GiR, which is for most part due to incomplete or completely missing experimental data. This paper is the first of a two part series; it presents fatigue investigations on GiR in combination with hardwood EWP with different types of rods, wood species and adhesives. Further, the embedment length and stress ratio were varried. In total, more than 70 cyclic tests were performed resulting in a high number of SN-curves showing the fatigue characteristics of GiR. The results show that two different damage mechanisms exist: in low cycle fatigue range (LCF), timber and adhesive fracture become dominant, while rod failure is the limiting factor in high cycle fatigue range (HCF). Based upon the documented experimental findings, existing standards for cyclic load on softwood connection technologies were discussed. The compagnion paper will present a design methodology based on a wood damage accumulation, giving a complete picture combined with existing metal fatigue models. [ABSTRACT FROM AUTHOR]

Copyright of Maintenance & Reliability / Eksploatacja i Niezawodność is the property of Polish Scientific & Technical Society Consumables, Polish Maintenance Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

BACKGROUND McClintock 1,2 was one of the first to model continuous-cycling fatigue crack growth by assuming a succession of miniature low-cycle-fatigue (LCF) specimens at the crack tip. Elements ahead of the crack amass damage until the arrival of the tip itself. Such models had been summarized by Majumdar and Morrow, 3 but the author was unaware of these papers at the time. The ideas were pursued further by Chakrabortty 4 whose paper, again unknown to the author, was published in the same issue (1979, Vol. 2) of FEMS. In the original paper 5 crack propagation is represented by successive regeneration at the crack tip, the process becoming progressively easier as the crack grows owing to an increase in strain concentration. These ‘initiation’ cycles were related to the ‘Coffin’ expression for crack initiation, thereby introducing two empirical constants k and α. The paper is of the class ‘ρ/ N’ where ρ is the assumed size of a crack-tip process zone and N is the cycles required to traverse that zone. Expressions had been previously derived linking LCF with linear-elastic fracture-mechanics (LEFM) crack growth, using the parameter Δ J. 6 (It was later shown that this was identical to using an equivalent stress intensity parameter. 7 ) It has been shown that the approach does not apply for crack depths < 180 μm. 6 At elevated temperature the Δ J approach was successful for describing crack growth in the range 0.2–1.2 mm for the cobalt-based alloy MAR-M509 at 600 °C. 8 Other studies have in fact shown 9 that crack growth rates are approximately constant below a depth of 200 μm. Even to this day short crack growth relations in LCF and their practical use are not as familiar as those in LEFM. It may justifiably be argued that for every paper published on LCF short crack growth, several hundred have appeared dealing with LEFM crack growth in terms of the ‘Paris’ law. One of the original referees' comments was that the paper 5 should have been presented in terms of LEFM. The author nevertheless defended his approach because for small cracks it was thought useful to retain the crack depth explicitly. It will be shown that LCF crack growth data have a definite part to play in the assessment of components and structures. [ABSTRACT FROM AUTHOR]

Since the pioneering paper by Mandelbrot (Nature, 308:721–722, 1984) on the fractal character of the fracture surfaces in metals, the fractal aspects in the deformation and failure of materials have been investigated by several Researchers (see the reviews by Bouchaud (J Phys Condens Matter, 9:4319–4344) and Carpinteri et al. (Appl Mech Rev, 59:283–305, 2006)) and the attempts to apply fractals to fracture have grown exponentially. Aim of this paper is 2-fold: on one hand, it summarizes in a detailed yet concise fashion the major results of the fractal approach to the scaling of mechanical properties in solid mechanics; on the other hand, it reports some recent results concerning the size effect in the failure of reinforced concrete (RC) beams. These recent findings clearly show that the picture of the size-scale effects is much more complex when interaction among different collapse mechanisms occurs. The consequences on the size-scale effects are discussed in detail. [ABSTRACT FROM AUTHOR]

This paper presents the recent progress and achievement in the application of fiber-reinforced polymers (FRP) on strengthening reinforced/prestressed concrete beams subjected to fatigue loading. Although the performance of FRP-strengthened structures under monotonic loading has been intensively investigated, fatigue behavior is relatively less known to date. This paper summarizes most of the currently available literature, including the codes and design manuals, on reinforced/prestressed concrete beams externally strengthened with FRP. The review focuses specifically on the fatigue life as a function of the applied load range, bond behavior of externally bonded FRP, damage accumulation, crack propagation, size effects, residual strength, and failure modes. Research needs including considerations for design guidelines are presented. [ABSTRACT FROM AUTHOR]

This paper is aimed at modeling the propagation of multiple cohesive cracks by the extended Voronoi cell finite element model or X-VCFEM. In addition to polynomial terms, the stress functions in X-VCFEM include branch functions in conjunction with level set methods and multi-resolution wavelet functions in the vicinity of crack tips. The wavelet basis functions are adaptively enriched to accurately capture crack-tip stress concentrations. Cracks are modeled by an extrinsic cohesive zone model in this paper. The incremental crack propagation direction and length are adaptively determined by a cohesive fracture energy based criterion. Numerical examples are solved and compared with existing solutions in the literature to validate the effectiveness of X-VCFEM. The effect of cohesive zone parameters on crack propagation is studied. Additionally, the effects of morphological distributions such as length, orientation and dispersion on crack propagation are studied. [ABSTRACT FROM AUTHOR]

Details of an extensive collaborative project to study tension stiffening have been recently published in a number of papers. Among other factors, the project investigated the loss of tension stiffening with time. It has been concluded that the reduction of tension stiffening to a constant long-term value is fairly rapid, being complete in 10-30 days. This paper explores the mechanisms that may operate to cause the reduction of tension stiffening with time. It is concluded that the loss is mainly attributed to cumulative damage resulting from either the formation of additional surface cracks or the formation or extension of internal cracks. It would appear that the final tension stiffening is only minimally dependent on concrete strength. [ABSTRACT FROM AUTHOR]

In a previous paper, it was shown that an estimation approach to crack tip properties under cyclic creep loading conditions was described, using a methodology based on the linear matching method (LMM). The calculations revealed the possibility of obtaining the crack tip parameters, C* ( n), for cracked structures subjected to both mechanical loads and temperatures, using the HRR field criterion as the crack tip condition. In this paper, those calculated values of C* ( n) are re-interpreted and presented in terms of the reference stresses, insensitive to the constitutive models and creep exponent. These reference stress values are compared with those currently used for life assessment of high-temperature plant, showing that current practice is significantly conservative for thermal loading. [ABSTRACT FROM AUTHOR]

The existence of numerous microcracks causes changes in the stiffness or fracture toughness of materials. In this paper, the manifestations of mechanical properties in the damaged materials caused by the microcracks are evaluated by the present homogenization method based on the superposition method together with the VNA solution. Moreover, it is known that the stress concentration at the macrocrack tip decreases due to the stress relaxation effect caused by the existence of the microcracks. In order to evaluate the manifestations of mechanical behavior, the mechanical effects of the existence of the microcracks on the macrocrack, the component separation method for mixed-mode stress intensity factors of the macrocrack in the damaged materials is newly developed in this paper. Various numerical analyses are successfully conducted for the two topics, the mechanical properties of the damaged materials and the mechanical behavior of the macrocrack in the damaged materials. [ABSTRACT FROM AUTHOR]

This paper deals with a basic methodology for the application of the finite element (FE) simulation and linear elastic fracture mechanics (LEFM) to predict the onset of delamination growth. The first step of the proposed methodology is the experimental characterisation of material interlaminar fracture in pure mode I (DCB test), pure mode II (ENF test) and mixed mode I+II (MMB test). In the second step, the results of the interlaminar fracture tests are translated from load/displacement data to strain energy release rates by means of test FE simulations; thus, the critical strain energy release rate (Gc), or fracture toughness are obtained as a function of mode ratio. The third step of the methodology consists of applying FE simulations to determine the strain energy release rate (G) and mode ratio for the delamination and then comparing the result to the values of the Gc curve. The main innovative aspect of the paper is that FE simulation is used as a part of the material characterisation process; in fact, data reduction of interlaminar fracture tests is carried out by means of FE simulations. In addition, regarding the practical details of FE application to the proposed methodology, some innovative aspects are introduced, such as the analysis is carried out as a non-linear geometric problem (taken into account large displacements) and considering inside each simulation different elastic properties for the material depending on whether it is under tension or compression. These aspects and the rest of the proposed methodology are validated by the experimental results presented in the paper. [ABSTRACT FROM AUTHOR]

This paper deals with the study of thermal stresses due to a plane crack lying on an interface in a microperiodic two-layered composite under a uniform perpendicular heat flow. An approximate analysis is carried out within the framework of linear stationary thermoelasticity with microlocal parameters. Taking this approach, a general method of solving the resulting boundary-value problem is presented. Effective results have been achieved through the use of suitable potential solutions and reducing the thermal crack problem to its skew-symmetric isothermal mechanical counterpart. An illustrative example is given in the paper by considering an insulated stress-free penny-shaped crack. For this problem the solution is obtained in terms of elementary functions and discussed from the point of view of fracture theory. [ABSTRACT FROM AUTHOR]

The Callovo Oxfordian clay-rock (COx) is studied in France for the disposal of radioactive waste, because of its extremely low permeability. This host rock is governed by a hydromechanical coupling of high complexity. This paper presents an experimental study into the mechanisms of water uptake in small, unconfined, prismatic specimens of COx, motivated by the comprehension of cracking observed during concrete/COx interface sample preparation. Water uptake is monitored using both X-ray tomography and neutron radiography, the combination of these imaging techniques allowing material deformation and water arrival to be quantified, respectively. Given the speed of water entry and crack propagation, relatively fast imaging is required: 5-min X-ray tomographies and 10-s neutron radiographs are used. In this study, pairs of similar COx samples from the same core are tested separately with each imaging technique. Two different orientations with respect to the core are also investigated. Analysis of the resulting images yields with micro- and macro-scale insights into hydromechanical mechanisms to be obtained. This allows the cracking to be interpreted as a rapid breakdown in capillary suction (supposed large both to drying and rebound from in situ stress state) due to water arrival, which in turn causes a loss of effective stress, allowing cracks to propagate and deliver water further into the material. [ABSTRACT FROM AUTHOR]

The delamination fracture in four-point bending beams made of adhesively bonded lengthwise vertical layers is studied assuming that each layer exhibits smooth material inhomogeneity along the width and length of the layer. The study aims at determining the strain energy release rate with applying the Ramberg-Osgood equation for modeling the non-linear mechanical behavior of the material in each layer. Cosine laws are used to describe the continuous variation of the modulus of elasticity in width and length directions of layers. Beams made of an arbitrary number of vertical layers which have individual widths and material properties are considered. Besides, the delamination crack is located arbitrary between layers, i.e. the two crack arms have different widths. The J-integral is applied for verification of the non-linear solution to the strain energy release rate derived in the present paper. The solution is used to investigate the influence of material inhomogeneity in width and length directions of layers, the crack location along the beam width, the non-linear mechanical behavior of the material and the crack length on the delamination fracture behavior. The approach developed is expected to be useful in structural design of multilayered inhomogeneous beams with considering the delamination fracture behavior. [ABSTRACT FROM AUTHOR]

Recently, the advent of optical systems has made possible the fast and accurate measurement of roughness discontinuity in three dimensions. In this study, a number of 3D roughness parameters such as fractal parameter (D), amplitude parameter (A), roughness parameter Rs and Grasselli's roughness parameter (2A0θ*max/(C + 1)) have been presented to quantify the geometry of the fracture surfaces. However, all these roughness parameters are scale dependent and their values change with scale. In this paper, the variations in theses parameters have been investigated with regard to scale. For this purpose, initially, three natural fracture surfaces with the dimensions 500 × 500 mm2 were selected and then, the four mentioned roughness parameters were calculated for these three surfaces in the range from 50 × 50 mm2 to 500 × 500 mm2. Finally, to evaluate the accuracy of roughness estimation by these roughness parameters, several joint replicas from the surfaces were constructed and the direct shear tests were carried out on them. Based on the experimental results, the Grasselli's roughness parameter gives more accurate estimation of the variations in surface roughness in regard to scale. Also the effect of small-scale evenness on surface roughness has been investigated. [ABSTRACT FROM AUTHOR]

The present paper aims to assess the effect of freeze/thaw cycles on fracture behaviour of a natural stone: the red Verona marble. A wide variety of specimen types and methods to determine Mode I fracture toughness of natural stones are available in the literature and, in this context, the model originally proposed for plain concrete, i.e. the Two-Parameter Model (TPM), is adopted. Such a method is able to take into account the slow nonlinear crack growth occurring before the peak load, typical of quasi-brittle materials, with the advantage of easy specimen preparation and simple test configuration. In the present paper, the atmospheric ageing is simulated by means of thermal pre-treatments consisting of freeze/thaw cycles. Experimental tests are carried out using three-point bending Single-Edge Notched (SEN) specimens, according to the TPM procedure. The effects of thermal treatment on both mechanical and fracture parameters are examined in terms of elastic modulus and fracture toughness, respectively. [ABSTRACT FROM AUTHOR]

The unconventional fracture model can simulate complex fracture network propagation in a formation with pre-existing natural fractures. The interaction between hydraulic fracture branches, or stress shadowing effect, could be modeled by 2D or 3D displacement discontinuity method (DDM). In this paper, we concentrate on a hydraulic fracture model that integrates 3D DDM for computing the induced 3D stress field around the propagating hydraulic fractures and incorporates the changes in induced stress into the fracture height calculations and propagation criterion. Examples show that for parallel fractures, the height growth may be promoted or suppressed depending on the relative fracture height. For fractures initiated from different formation layers, the fracture growth into the layer occupied by the other fractures is reduced due to the vertical stress shadowing effect. [ABSTRACT FROM AUTHOR]

Complete closed-type carbon fiber-reinforced polymer stirrups with a rectangular section (CFRPRS) were developed and tested in the research reported in this paper. The use of a CFRPRS is intended to relieve stress concentrations and reduce the number of kinked fibers at the bent portion. A total of 16 B.5 specimens were tested with consideration of the bend strength of the CFRPRS as well as carbon fiber-reinforced polymer (CFRP) stirrups with circular sections. Observations were made regarding the bend strength and failure modes. Three different types of CFRPRS were tested with different width-thickness ratios (34.4 for a sectional area of and 4.5 or 8.0 for ) and three different ratios of the bend radius to the area-equivalent bar diameter (1.5, 3.0, and 4.5). The test results indicated that CFRPRSs exhibited improved bend strength as compared to its counterpart, the conventional CFRP stirrups with circular sections. CFRPRSs possessing a larger stirrup width-thickness ratio provided more strength for a given sectional area. The best correlation between the test results and predictions of the CFRPRS bend strength was observed when sections of the CFRPRS were modeled as a collection of transformed individual circular sections. [ABSTRACT FROM AUTHOR]

Studying the thermal damage constitutive model of rock using statistical theory can better reflect the damage evolution process and the stress–strain relationship of rock under temperature and loading, which is one of the key problems especially in deep rock mechanics. The thermal-mechanical coupling damage constitutive model of rock is established using the Hoek–Brown strength criterion, based on the Weibull distribution and the continuous damage theory. The rationality of the model is also verified by experiments. The main conclusions are as follows. The stress–strain curves of rock can be divided into four stages according to the damage evolution characteristics, including the non-damage of loading, damage stability expansion, damage intensification expansion, and damage stability expansion to saturation, and the method of determining the demarcation points of each stage is given clearly. The initial damage point of the rock is about 25% of the peak stress, the damage value is about 0.3 when the rock reaches the peak stress and about 0.6 when reaches the residual stress. Both the damage value and the strain energy release rate of the rock corresponding to the peak stress show exponential growth with the increase in confining pressure. The maximum damage evolution rate of the rock shows exponential decay as the confining pressure rises, indicating that the confining pressure can delay the development of cumulative damage. The modified damage constitutive model considering compaction coefficient is in good agreement with the test curves in the stage of compaction, linear elasticity, yield, and pre-peak strength. It is hoped that through the research of this paper, it can provide references for studying the macroscopic mechanical response from the damage propagation characteristics of the rock in the future. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate the characteristics of the rate of fatigue fracture propagation in members under maximum force on a steel arch bridge and to identify the fatigue life of those members under excessive loads. The method for bridge fatigue prediction used was numerical S-N curve controlled by cumulative damage rule, fracture critical member, and fatigue fracture determination based on SNI 1729-2015, S-N category. The distribution of stress, strain and life cycle was obtained. The result revealed 1 critical built-up weld beam on the steel arch bridge, specifically double welded wide flange beam TB7. Excessive loads from heavy vehicles, namely trailer types 7C1, 7C2 and 7C3, were simulated. Initial fracturing would occur after 53 years and 5 months of service with an initial crack size of 1.717 mm, leading to a propagation cycle of 45 years, which would cause critical fractures of 10.55 mm after 99 years. Thus, the maximum number of vehicles permitted to enter the bridge, assuming a 5% increase per year, was 13,450 per year for trailer type 7C3 at a planned capacity of 6,984,000 vehicles. This paper illustrates how fatigue life prediction can be a useful guide for the sustainability of bridges and provide a theoretical basis for developing optimized fatigue life of steel arch bridges. [ABSTRACT FROM AUTHOR]

In the lifetime prediction and extension of a nuclear power plant, a reactor pressure vessel (RPV) has to demonstrate the exclusion of brittle fracture. This paper aims to apply fracture mechanics to analyse the non-uniform cooling effect in case of a loss-of-coolant accident on the RPV integrity. A comprehensive framework coupling reactor system, fluid dynamics, fracture mechanics, and probabilistic analyses for the RPVs integrity analysis is proposed. The safety margin of the allowed RTNDT is increased by more than 16°C if a probabilistic method is applied. Considering the non-uniform plume cooling effect increases KI more than 30%, increases the failure frequency by more than 1 order of magnitude, and increases the crack tip constraint due to the resulting higher stress. Thus, in order to be more realistic and not to be nonconservative, 3D computational fluid dynamics may be required to provide input for the fracture mechanics analysis of the RPV. [ABSTRACT FROM AUTHOR]

The paper presents results from experimental studies on the influence of loading-unloading processes on the mechanical, linear, and nonlinear properties of the strain-hardening polycrystalline aluminum alloy AMg6 (Rus). The stress-strain curve is measured for AMg6 samples under high-cycle loading-unloading up to fracture of a sample. The microhardness of the sample is measured before and after its fracture. It has been found that the loading-unloading process leads to strain hardening of the AMg6 alloy. The influence of strain hardening of AMg6 on its linear and nonlinear elastic properties is studied by an ultrasonic method. To study the nonlinear elastic properties for different domains of the loading curve, we used the Thurston-Brugger method and spectral method by studying the efficiency of second acoustic harmonic generation. The experimental results are discussed. [ABSTRACT FROM AUTHOR]

The hardening behaviour of metals is generally described in terms of a stress-strain curve derived from experiments. In this paper, a linear method to identify the stress-strain curve starting from full-field measurement data is presented. This method can be applied to any stress state using a generic yield function, the only requirement is that the full-fieldmeasurement is extended up to the border of the specimen. The method is presented and validated using a finite element model of a notched specimen. Moreover, experiments were performed on specimens cut from a BH340 steel sheet to illustrate the viability to actual cases. Two geometries were considered, a standard uniaxial test, where the method was used to evaluate the post-necking behaviour, and a notched specimen with a heterogeneous strain field. The proposed method, named linear stress-strain curve identification (LSSCI), can be a useful tool in combination with inverse methods to identify the constitutive behaviour of metals in large strain plasticity. [ABSTRACT FROM AUTHOR]

The paper investigated the creep-fatigue behavior for directionally solidified nickel-based superalloy DZ445 at 900 °C. It is found that the fatigue life shows an abnormal increase when the dwell time exceeds a critical value during creep-fatigue deformation. The area of hysteresis loop and fractograph explain the phenomenon quite well. The shortest life corresponds to the maximal area of hysteresis loop, i. e. the maximum energy to be consumed during the creep-fatigue cycle. The fractographic observation of failed samples further supports the abnormal behavior of fatigue life. [ABSTRACT FROM AUTHOR]

Fatigue failure occurs for many metal materials, and it affects the product reliability. Therefore, the fatigue life predication is important for metal materials under conditions of heavy and high-speed loading. Presently, many researches have focused on the fatigue life predication and many fatigue life predication methods have been used industrially, such as Manson-Coffin relation and damage strain model. However, few reports can be found on the predication accuracy. Based on Manson-Coffin relation and damage strain model, this paper analyzes the effect of the material performance ɛf on the predication accuracy. Furthermore, it is pointed out that single material performance will cause the low calculation accuracy. Then, an improved formula is derived to predicate the strain fatigue life. This formula is related to two material performances ɛf and Δɛc. Moreover, the predication accuracy analysis is conducted through calculating error. Finally, experiments prove that this formula can greatly improve the calculation accuracy. Therefore, this proposed formula is reliable and reasonable. [ABSTRACT FROM PUBLISHER]

In this paper, the stresses near the tip of interface corners are expressed in matrix power function form. To make this matrix power function form valid for all possible cases of interface corners, all kinds of singularities including oscillatory and logarithmic singularity are discussed in this paper. The coefficient vector of this matrix power function form solution is then defined as a vector of stress intensity factors along the radial direction. Since the stress intensity factors are functions of radial direction, the maximum stress intensity factor of a certain radial direction may be useful for the fracture prediction of interface corners. This new definition of stress intensity factors is applicable for all possible singular orders-repeated or distinct, real or complex, and keeps the same unit for all possible interface corners. Therefore, it will be helpful for bridging the problems of cracks, corners, interface cracks and interface corners. Finally, the matrix form solutions are reduced to the scalar form solutions for two special cases. One is a special case of corner angles-cracks, and the other is a special case of anisotropic materials-isotropic materials. Through this reduction, new scalar form analytical solutions are obtained and specialized further to compare with the existing analytical solutions. [ABSTRACT FROM AUTHOR]

The direct strength method (DSM) recently included in the North American Specification and Australian/New Zealand Standard AS/NZS 4600:2005 gives design rules for compression and bending. No rules are presented in this standard for shear or for combined bending and shear. Two series of tests on C-section can be used to develop and calibrate rules for design in shear and for combined bending and shear. These are the University of Missouri Rolla tests of the 1970s and recent tests on high-strength C-sections at the University of Sydney. Both series of tests use a similar test rig, although different levels of tension field action have been observed. Two features researched are the effect of full-section shear buckling (as opposed to web-only shear buckling), and tension field action. Full-section buckling is a feature of the DSM but requires software that can evaluate full sections for shear. The paper proposes DSM design rules for C-sections in shear and for combined bending and shear both with and without tension field action. The test results are compared with the proposed design rules. [ABSTRACT FROM AUTHOR]

The failure anisotropy of geomaterial is inherently related to the microstructure such as grain arrangement, bedding, and crack pattern. This paper presents a formulation of an anisotropic failure criterion in three-dimensional (3D) stress space on the basis of the combination of the SMP criterion and a material microstructure tensor proposed in previous publications. The criterion is specialized for cross-anisotropic granular-material based on the loading direction and the direction of material bedding determined by two angles in 3D stress space. Consequently, it can describe the variation of strength resulting from the changing of the direction of material bedding and the changing of the stress state. Furthermore, the proposed criterion can capture the cross-anisotropic strength behavior of granular materials, which is concluded from the comparisons between its predicted and relevant experimental results. [ABSTRACT FROM AUTHOR]

Owing to its importance in evaluating the fracture behavior of concrete, the crack extension resistance curve of concrete has been widely studied, both experimentally and theoretically. In this paper, a numerical approach is developed for the crack extension resistance curve of concrete by considering the variation of the fracture process zone (FPZ) length during the whole fracture process. In this approach, the FPZ length is determined by using the linear asymptotic superposition assumption. Dividing the whole fracture process into three different stages of the cohesive stress distribution within the FPZ, the crack extension resistance curve is formulated by superposition of the intrinsic fracture toughness of concrete and the fracture toughness caused by the cohesive stress within the FPZ. The developed numerical approach is applied to the tested and simulated standard three-point bending notched concrete beams. The effect of the variation of the FPZ length on the crack extension resistance curve is evaluated on the basis of the numerical results. The crack extension resistance first increases with an increase in ratio of the effective crack length to the beam depth and then reaches a plateau value when the FPZ is fully developed. When the effective crack length is normalized to the beam depth, the crack extension resistance is basically independent of the beam depth within the beam size range studied. [ABSTRACT FROM AUTHOR]

New knowledge has raised a concern about the cost-ineffective design methods and the true performance of railroad prestressed concrete ties. Because of previous knowledge deficiencies, railway civil and track engineers have been aware of the conservative design methods for structural components in any railway track that rely on allowable stresses and material strength reductions. In particular, railway sleeper (or railroad tie) is an important component of railway tracks and is commonly made of prestressed concrete. The existing code for designing such components makes use of the permissible stress design concept, whereas the fiber stresses over cross sections at initial and final stages are limited by some empirical values. It is believed that the concrete ties complying with the permissible stress concept possess unduly untapped fracture toughness, based on a number of proven experiments and field data. Collaborative research run by the Australian Cooperative Research Centre for Railway Engineering and Technologies (Rail CRC) was initiated to ascertain the reserved capacity of Australian railway prestressed concrete ties that were designed using the existing design code. The findings have led to the development of a new limit-states design concept. This paper highlights the conventional and the new limit-states design philosophies and their implication to both the railway community and the public. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a new mechanistic-empirical (ME) reflective cracking model developed for hot-mix asphalt (HMA) overlay thickness design and analysis. After reviewing existing models, the reflective cracking model based on Paris’ law of fracture mechanics was considered as the state of the practice and was selected as the basis for the ME model development in this study. The model consists of stress intensity factor and fracture properties (A and n) as the fundamental input parameters for modeling reflective crack propagation caused by both traffic loadings (bending and shearing) and thermal effects (temperature variations). For practical application, 32 SIF regression equations were developed for HMA overlays with three levels of load transfer efficiencies (10, 50, and 90%) at joints/cracks under various traffic loading spectrums (bending and shearing) based on more than 1.6 million finite element simulations and computations. For the thermal induced reflective cracking, a “hybrid” approach, similar to the Strategic Highway Research Program low temperature cracking model, was proposed. In this hybrid approach, the viscoelastic properties of the HMA mixes were considered through the thermal stress at the “far field” (σVE-far), which then ties with the stress intensity factor (Kthermal) determined through regression equations. Also, the required fracture properties (A and n) can easily be determined in the laboratory using the overlay tester that has unique features with respect to specimen size, specimen preparation, and relatively short testing time (about 15 min). Additionally, the proposed reflective cracking model was preliminarily calibrated using three HMA overlay field-case studies, and then verified using California’s heavy vehicle simulator test results. Currently, more calibration is underway, but more field performance data are definitely needed for further model calibration and verification. [ABSTRACT FROM AUTHOR]

In this paper the contact fatigue resistance of gearwheel teeth, subjected to shot-peening treatment, was investigated experimentally and analytically. The main objective was the evaluation and prediction of fatigue crack initiation, propagation, direction, and rate. A specially designed experimental rig was used to test a number of spur gears with the following characteristics: (a) unhardened, thermally untreated unpeened surfaces, (b) thermally treated unpeened surfaces, (c) unhardened peened surfaces, and (d) thermally treated peened surfaces. The theoretical model assumed initiation and propagation of surface cracks of gears operating in the elastohydrodynamic lubrication regime while loading was due to simultaneous rolling and sliding. Finite element modeling was used for the calculation of the stress field at the gear teeth. Comparison of the experimental and analytical results showed considerable improvement in the contact fatigue strength of thermally treated gear teeth and especially those that underwent shot peening, which increased surface durability. The residual stresses induced by shot peening are mainly effective in stopping microcrack propagation. When shot peening is applied on thermally untreated gear teeth surface, it increases the contact fatigue life of the material by 17% at 7 × 105 loading cycles. If shot peening is applied on carburized gear teeth surfaces, it increases the surface fatigue life by approximately 8% at 106 cycles. Contact fatigue and eventual pitting are treated as a normal consequence of the operation of machine elements. To study this failure process different types of testing machines have been designed. The purpose of this paper is the presentation and evaluation of a new design experimental rig for studying contact fatigue damage of gear teeth subjected to different load patterns. [ABSTRACT FROM AUTHOR]

We describe the application of the dual boundary element method for the determination of stress intensity factors in plate bending problems. The loadings considered include internal pressure, and also combined bending and tension. Mixed mode stress intensity factors are evaluated by a crack surface displacement extrapolation technique and the J-integral technique. The boundary element results for the case studies considered in the paper have been compared with either analytical or finite element results and in all cases good agreement has been achieved. [ABSTRACT FROM AUTHOR]

The hybrid simulation test method is a versatile technique for evaluating the seismic performance of structures by seamlessly integrating both physical and numerical simulations of substructures into a single test model. Hybrid simulation results have been shown to be reliable by comparison to shake-table simulations when propagation of experimental errors is successfully mitigated. Techniques have been proposed to assess the severity of experimental errors prior to or after tests have been completed. In this paper, an error monitoring method is presented that predicts the reliability of the test results in real time using data that is readily available. Two error monitors are derived based on estimates of energy added to the hybrid simulation as a result of errors in the experimental setup. The proposed method can provide valuable decision-making information by generating early warnings of unacceptable levels of experimental errors. The warning signals can serve as an indicator to pause the simulation, examine the source of error, and correct the problem, preferably before the specimen is irreparably damaged. Use of the proposed hybrid simulation error monitors to provide a measure of confidence of the quality of the test results is verified in two hybrid simulations. [ABSTRACT FROM AUTHOR]

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In order to avoid introduction of an error when a local crack growth criterion is used in an incremental crack growth formulation, each straight crack extension would have to be infinitesimal or have its direction corrected. In this paper a new procedure to correct the crack extension direction is proposed in connection with crack growth analyzed by the Dual Boundary Element Method (DBEM). The proposed correction procedure and a reference correction procedure already described in the literature are evaluated by solving two different computational crack growth examples. In the two examples it is found that analyses of the crack paths performed with the proposed crack correction procedure using big increments of crack extension are in excellent agreement with analyses of the crack paths performed by using very small increments of crack extension. Furthermore, it is shown that the reference correction procedure has a tendency to overcorrect the crack growth direction if the stop criterion for the iterative correction procedure is not calibrated in each new crack growth analysis. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a methodology for the assessment of the near-crack tip fields in structures operating at elevated temperatures in the creep range and subjected to cyclic mechanical and thermal loads. The method involves generating simplified extreme solutions, which corresponds to very short (rapid cycle) and long (slow cycle) cycle times, bounding the behaviour for intermediate cycle times. In such situations, by examining the increments of strain over a cycle of loading at the crack tip, the solutions may be related to the HRR fields and, hence, related to an equivalent constant load. Here, these solutions are generated using a nonlinear programming method, the linear matching method (LMM), on two bounding constitutive relationships with identical uniaxial data, Norton's law and the Bailey–Orowan model. These solutions are generated for the classical cracked axisymmetric Bree problem, presented as a contour of an equivalent path-independent integral in creep, C*, by using a sampling point within an identified best-matched HRR field. [ABSTRACT FROM AUTHOR]

The shear tests performed by Bazant and Kazemi in 1991 have been cited as strong evidence that the theory of fracture mechanics applies to the shear strength of beams without stirrups. It has also been noted in the past, however, that these results are not totally consistent with other size effect experimental series. This paper provides a detailed summary of duplicate experiments carried out to determine if the original 1991 results were reproducible. It was found that the repeat experiments were 31 to 70% stronger than the original results. All experimental results were stronger than the predictions of the ACT and CSA codes. Cracks patterns were observed to sortie with the beam size. A comparison of historical data indicated that the higher strengths were likely trot a result of random variation, but due to a systematic effect in the original testing, making them inconsistent with other tests. Potential reasons for the differences from the 1991 results are presented and discussed. [ABSTRACT FROM AUTHOR]

The present paper provides a statistical model to the size effect on grained materials tensile strength and fracture energy. It has been already demonstrated by using extreme value theory that the scaling law obtained for the tensile strength introducing a doubly truncated distribution of flaws, under the hypothesis of Weibull's weakest link, resembles the Multi-Fractal Scaling Law (MFSL), already proposed by the first Author through fractal concepts. Since the weakest link in grained materials is usually represented by the interface between the matrix and the grains, in the present paper we assume that the flaw distribution can be represented by the grain size distribution, rather than by an arbitrary flaw distribution. Furthermore, introducing a micromechanical model for the critical displacement wc, we draw a link between the fracture energy and the largest aggregate grain on crack surfaces. In this way we are able to compute the tensile strength and the fracture energy as a function of the specimen size. The obtained scaling laws are substantially in agreement with the MFSL for the fracture energy proposed by the first Author. A further result provided by the proposed approach is the description of the scatter increase of both tensile strength and fracture energy values when testing small specimens. This trend is confirmed by experimental data available in the literature. Eventually, the influence of the aggregate grading upon the size effect for strength and toughness is analyzed. [ABSTRACT FROM AUTHOR]

The objective of this paper is to show the advantages of the cohesive crack model for predicting fracture of V-notched components. Critical values of the generalized stress intensity factor can be obtained from the knowledge of the material softening function and the elastic parameters, avoiding a cumbersome experimental work. The results were checked successfully against experimental ones, from other authors, in different materials: steel, aluminium, PMMA and PVC. A non dimensional formulation of the fracture criterion for sharp V-notched components was obtained and a simple approximate expression derived for easy appication. [ABSTRACT FROM AUTHOR]

A criterion to predict crack onset at a sharp notch in homogeneous brittle materials has been presented in a previous paper of one of the authors. It is reviewed and improved herein. It fulfils both the energy and the strength criteria and takes an Irwin-like form involving the generalized intensity factor of the singularity governing the elastic behaviour in the vicinity of a notch tip. The prediction agrees fairly well with the experiments although it slightly underestimates the experimental measures. A cause of this discrepancy can be that a small notch tip radius blunts the sharp corner. It is analysed in this paper by means of matched asymptotics involving 2 small parameters: a micro-crack increment length and the notch tip radius. A correction is brought to the initial prediction and a better agreement is obtained with experiments on PMMA notched specimens. Experiments performed on a stiffer material (Alumina/Zirconia) show that it is less sensitive to small notch tip radii. A remaining small discrepancy between experiments and predictions can be due to some non linear behaviour of the materials near the notch tip. In addition, without new developments, the method allows to determine the stress intensity factor at the tip of a short crack emanating from a sharp or a rounded v-notch. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the fatigue behaviour of shot-peened open-hole plates with Q345 steel. The beneficial effects induced by shot peening on the fatigue life improvement are highlighted. The characteristic fatigue crack initiation and propagation modes of open-hole details under fatigue loading are revealed. The surface hardening effect brought by the shot peening is analyzed from the aspects of in-depth micro-hardness and compressive residual stress. The fatigue life results are evaluated and related design suggestions are made as a comparison with codified detail categories. In particular, a fracture mechanics theory-based method is proposed and demonstrated its validity in predicting the fatigue life of studied shot-peened open-hole details. [ABSTRACT FROM AUTHOR]

The subject of this paper is to describe some of the aspects manifesting in the use of the elastoplastic material model library multiPlas, which was developed to support non-linear computations in the ANSYS system. The text focuses on the analysis of numerical simulations of a virtual tension test in several case studies, thereby the text endeavours to describe the problems connected with modelling non-linear behaviour of concrete in a tensile area. [ABSTRACT FROM AUTHOR]

Structural components made of fibre-reinforced materials are frequently used in engineering applications. Fibre-reinforced composites are multiphase materials, and complex mechanical phenomena take place at limit conditions but also during normal service situations, especially under fatigue loading, causing a progressive deterioration and damage. Under repeated loading, the degradation mainly occurs in the matrix material and at the fibre-matrix interface, and such a degradation has to be quantified for design structural assessment purposes. To this end, damage mechanics and fracture mechanics theories can be suitably applied to examine such a problem. Damage concepts can be applied to the matrix mechanical characteristics and, by adopting a 3-D mixed mode fracture description of the fibre-matrix detachment, fatigue fracture mechanics concepts can be used to determine the progressive fibre debonding responsible for the loss of load bearing capacity of the reinforcing phase. In the present paper, a micromechanical model is used to evaluate the unixial or multiaxial fatigue behaviour of structures with equi-oriented or randomly distributed fibres. The spatial fibre arrangement is taken into account through a statistical description of their orientation angles for which a Gaussian-like distribution is assumed, whereas the mechanical effect of the fibres on the composite is accounted for by a homogenization approach aimed at obtaining the macroscopic elastic constants of the material. The composite material behaves as an isotropic one for randomly distributed fibres, while it is transversally isotropic for unidirectional fibres. The fibre arrangement in the structural component influences the fatigue life with respect to the biaxiality ratio for multiaxial constant amplitude fatigue loading. One representative parametric example is discussed. [ABSTRACT FROM AUTHOR]

Considerations are made for the fracture morphology and its evolution in relation to the brittle fracture along butt-weld, where crack path stability may play important roles in various aspects. Analyses of a kinked and curved crack are first reviewed in the present paper. Having learned the solutions of a kinked and curved crack, several crack path criteria are compared for the crack path prediction. Then, some aspects of stability of a crack path are examined as to whether it may propagate, keeping its original direction or not in a homogeneous material, and whether crack kinking in a material having inhomogeneous fracture toughness may be predicted by seeking the state which gives rise to an energetically stable one. Having reviewed the morphology of brittle crack propagation along butt-weld, brittle fracture of extremely thick plates of high tensile steel is discussed for the fracture control of recently emerging large container ships. [ABSTRACT FROM AUTHOR]

Corrosion will compromise safety operation of oil and gas pipelines, accurate determination of failure pressure finds importance in residual strength assessment and corrosion allowance design of onshore and offshore pipelines. This paper investigates failure pressure of high strength pipeline with single and multiple corrosions using nonlinear finite element analysis. On the basis of developed regression equations for failure pressure prediction of high strength pipeline with single corrosion, the paper proposes an assessment procedure for predicting failure pressure of high strength pipeline with multiple corrosions. Furthermore, failure pressures predicted by proposed solutions are compared with experimental results and various assessment methods available in literature, where accuracy and versatility are demonstrated. [ABSTRACT FROM AUTHOR]