Variation Of Fracture Toughness Research Articles

Experimental and numerical assessment of fracture toughness variations in steam boiler components after a long period of operation

This study presents an experimental and numerical assessment of the fracture toughness of steam boiler components, which have been in operation for more than 100,000 h. The materials tested are components of two different parts of a steam turbine unit: the pipeline section that connects the headers with turbines, which is made of 13HMF steel, and the valve chamber section, which is made of L17HMF steel grades. Full-size arc-shaped tension specimens (AT) are employed for the experimental evaluation. AT and single-edge notch tension (SENT) specimens are used for numerical assessment. The finite element method (FEM) formulations are developed and examined in Abaqus, which is set up as a shell model with the specimen thickness designated as the mid-surface for the analysis. The notch is modeled using a contour integral definition based on the maximum energy release rate criterion. The fracture toughness is quantified using the stress intensity factor (KI), and the results are compared. The valve chamber is found to have a higher fracture toughness than the pipeline section. The effect of specimen geometry is also noticeable, and the fracture toughness of SENT and AT specimens are compared. The results are useful in assessing the component's durability after long operation.

Effect of high-temperature thermal fatigue on mode I fracture toughness of gabbro

Investigating variations of rock fracture toughness after high-temperature thermal fatigue is greatly significant for developing deep geothermal resources. This study investigated the mode I fracture toughness of gabbro after high-temperature fatigue treatment by heating a cracked straight-through Brazilian disc (CSTBD) gabbro specimen to 200, 400, 500, and 600 °C, followed by 100 thermal cycles. Additionally, acoustic emission (AE) and digital image correlation (DIC) were used to monitor the failure process of gabbro. The results showed that the peak load, fracture toughness, and fracture energy of gabbro decreased with the increased temperature and number of cycles, which was most significant at 500 and 600 °C. Under mode I loading, cracks developed along the loading direction and connected with the prefabricated crack. Additionally, the regions with higher von Mises equivalent strain (Evm) values were concentrated at the loading points and prefabricated crack of the rock sample. The degree of bending of the primary cracks increased, the number of secondary cracks increased, and the Evm values changed significantly as the temperature increased. The phase transformation of plagioclase feldspar at 450 °C and the oxidation of iron- and magnesium-containing pyroxene at 600 °C changed the internal structure of the rock. Cyclic treatment further exacerbated the thermal damage, developing microcracks within the rock and reducing fracture toughness significantly.

A comparative study on characterizing the fracture toughness variations in thick-wall hydrogenation reactor welded joint by spherical indentation tests

This study provides an investigation on the existing models in predicting the fracture toughness distributions in thick-wall hydrogenation reactor welded joint from SITs. Three criterions, namely the critical stress, the critical strain, and the critical damage variable, were used in determining the critical indentation depth in the IEF model, together with the ERR model. Experimental results proved that none of the existing four criterions (or models) works for all zones in the welded joint. The critical stress criterion well reproduces the decrease of KIC in the weld (compared with that of the base metal) and the KIC distributions along the thickness, but may provide highly non-conservative predictions (more than 100% over-estimation). Highly stable KIC predictions were obtained from the critical strain criterion and the ERR model, but the critical strain criterion failed in characterizing weakest parts of the welded joint (i.e., the HAZ and weld) as assuming all zones with the same plasticity, and the ERR model failed in characterizing one of the weakest parts (i.e., weld) due to the existence of initial damage. Compared with the critical strain criterion and the ERR model, highly instable prediction results were observed in the CDV criterion, with the maximum error exceeds 50%. Source of errors in the existing fracture toughness prediction models were extensively explored through the physical basis of each criterion (or model) and from the solidarity of the phenomenologically summarized criterions.

Novel framework for predicting constraint effects on fracture toughness using an elastic–plastic phase field model and modified boundary layer formulations

Fracture toughness is dependent on geometry according to a phenomenon known as the constraint effect on fracture toughness. In this paper, we propose a framework to predict constraint effects on fracture toughness using an elastic–plastic phase field model and modified boundary layer formulations. An elastic–plastic material was adopted because constraint effects are typically related to plastic deformation at the crack tip. Modified boundary layer formulations were used as loading methods to control phase field evolution at the crack tip. Numerical results indicate that fracture toughness increases with negative T-stress. The effects of yield stress and the linear hardening coefficient on constraint effects were also investigated. The results revealed that the variation in fracture toughness was greater in materials with low-strain hardening and yield stress compared to that in materials with high-strain hardening and yield stress. Finally, the variation in cleavage fracture toughness as predicted by our framework with a limited set of available experimental data was evaluated. The trend of the predicted results is consistent with that of the experimental results.

Impact of metastable graphene-diamond coatings on the fracture toughness of silicon carbide.

Silicon carbide has excellent mechanical properties such as high hardness and strength, but its applications for body armor and protective coating solutions are limited by its poor toughness. It has been demonstrated that epitaxial graphene-coated SiC can enhance SiC mechanical properties due to the pressure-activated phase transition into a sp3 diamond structure. Here, we show that atomically thin graphene coatings increase the hardness of SiC even for indentation depths of ∼10 μm. Very importantly, the graphene coating also causes an increase of the fracture toughness by 11% compared to bare SiC, which is in contradiction with the general indirect variation of hardness and fracture toughness. This is explained in terms of the presence of a diamond phase under the indenter while the rest of the coating remains in the ultra-tough graphene phase. This study opens new venues for understanding hardness and toughness in metastable systems and for the applications of graphene-coatings.

Fracture research of adhesive-bonded joints for GFRP laminates under mixed-mode loading condition

Abstract The mixed-mode fracture characterization of adhesively bonded glass fiber-reinforced polymer (GFRP) plate joints is studied based on theoretical and experimental techniques. An improved beam model is used to estimate the compliance and the mixed-mode energy release rate (ERR) for GFRP four-point mixed-mode bending specimens. In this model, the deformation of the upper and lower GFRP plates is mutually independent, and the deformable adhesive is considered to satisfy the displacement compatibility conditions on the bonding interface. The explicit theoretical solutions of the compliance and ERR using the compliance method are reduced. The present theoretical solutions fit well with the finite element solutions by comparison with the rigid joint model. From the critical loads in experiment, the variation in fracture toughness on mixed-mode I/II has been determined by modifying the stiffness of the composite GFRP/GFRP substrate.

The influence of recycled asphalt pavement materials on low and intermediate fracture behavior of asphalt concrete (AC) after exposing to different thawing and freezing periods

The effects of compaction process, mixture type and environmental conditions on mode I cracking resistance of asphalt concrete (AC) materials was investigated experimentally. To do that, a large number of edge notched disc bend specimens containing 0, 25 and 50% recycled asphalt mixture (RAP) material and exposed to different numbers of thawing and freezing cycles (0 to 10 cycles) were fractured. The critical values of stress intensity factor and fracture energy indexes were obtained for the AC mixtures at three test temperatures of −25, 0 and + 25 °C. Based on the results, the fracture resistance capacity of the AC mixture was decreased by increasing the RAP content, number of thawing/freezing cycles and decreasing the test temperature. The effect of thawing/freezing cycles on decreasing the cracking resistance was more pronounced for the first cycles and the variations of fracture toughness and fracture energy values with the numbers of thawing/freezing cycles showed a plateau form for higher numbers of cycles. The fracture curves of the tested samples manufactured using the Gyratory compaction method was upper than the related curves of samples made by Marshall compaction method. The sensitivity of fracture results (i.e., gradient of reduction in the fracture resistance versus freeze/thaw cycles) became low by increasing the test temperature.

Investigation of the compression-shear fracture characteristics of flaw-filled sandstone under confining pressure

In the field of rock engineering, the coupled effects of confining pressure and crack filling material on rock mechanical properties have not received widespread attention. In this study, the central cracked Brazilian disk specimen was used to investigate the fracture properties of sandstone in compression-shear loading mode in a confining pressure and crack-filling environments. Variations in peak load and fracture toughness were quantitatively evaluated for different confining pressures and fillings, accompanied by the establishment of quantitative relationships related to fracture toughness. The trend of linear increase in effective fracture toughness versus confining pressure arises primarily from the contribution of confining pressure. The filling material weakens the fracture toughness of the rock to a certain extent, which is attributed to the accumulation of energy during the failure of the sandstone and the reduction of the stress concentration at the crack tip. The crack propagation trajectory mainly exhibits the characteristics of a stretched wing crack, but shows a transition trend towards a shear crack to a certain extent.

Statistical Distribution Model of Charpy Absorbed Energy in Transition Temperature Range for Reactor Pressure Vessel Steel

Abstract A statistical model to estimate the distribution characteristics of Charpy absorbed energy in the transition temperature range was proposed based on the variation in fracture toughness addressed by the Master Curve and the relationship between fracture toughness and Charpy absorbed energy associated with the Charpy Master Curve. Charpy absorbed energy in the transition temperature range can be well approximated by both the Weibull and normal distributions, and the parameters to determine the shape of the distributions (scale and shape parameters of the Weibull distribution, mean and standard deviation of the normal distribution) can be deductively defined as the functions of two independent variables, median of the fracture toughness and the difference between reference temperature and Charpy reference temperature. A series of Charpy impact tests were conducted under the same conditions to obtain the characteristics of the variation in Charpy absorbed energy for a reactor pressure vessel steel SQV2A base metal, and the experimental variation was reasonably predicted by the proposed model.

Evaluation of XD 10 Polyamide Electrospun Nanofibers to Improve Mode I Fracture Toughness for Epoxy Adhesive Film Bonded Joints

The demand for ever-lighter structures raises the interest in bonding as a joining method, especially for materials that are difficult to join with traditional welding and bolting techniques. Structural adhesives, however, are susceptible to defects, but can be toughened in several ways: by changing their chemical composition or by adding fillers, even of nanometric size. Nanomaterials have a high surface area and limited structural defects, which can enhance the mechanical properties of adhesives depending on their nature, quantity, size, and interfacial adhesion. This work analyzes the Mode I fracture toughness of joints bonded with METLBOND® 1515-4M epoxy film and XantuLayr electrospun XD 10 polyamide nanofibers. Two joint configurations were studied, which differed according to the position of the nanomat within the adhesive layer: one had the nanofibers at the substrate/adhesive interfaces, and the other had the nanofibers in the center of the adhesive layer. Double cantilever beam joints were manufactured to evaluate the Mode I fracture toughness of the bonding with and without nano-reinforcement. The nanofibers applied at the substrate/adhesive interface improved the Mode-I fracture toughness by 32%, reaching the value of 0.55 N/mm. SEM images confirm the positive contribution of the nanofibers, which appear stretched and pulled out from the matrix. No fracture toughness variation was detected in the joints with the nanofibers placed in the middle of the adhesive layer.

Dynamically propagating cracks in anisotropic plates subjected to hyperbolic thermal shock

ABSTRACT In this paper, dynamically propagating cracks in anisotropic plates exposed to a generalised thermal shock are investigated. Here, the governing equations are considered according to the Lord-Shulman (LS) model as a fully coupled generalised thermoelasticity theory. The crack is modelled in the context of the eXtended Finite Element Method (XFEM). To evaluate Stress Intensity Factors (SIFs), a technique based on the J integral and crack tip opening and sliding displacements, previously applied for stationary cracks, is developed for a dynamically propagating crack in orthotropic solids. Besides, a variety of functions for enriching the displacement field are derived relying on the behaviour near the tip of a dynamically propagating crack in anisotropic materials. The maximum hoop stress criterion is also modified by using the asymptotic stress field for a dynamically propagating crack. In addition, the effect of the angular variation of fracture toughness in orthotropic materials is accounted in computing the crack propagation speed. In some examples, dynamic crack growth in complex cracked orthotropic structures subjected to LS thermal shock is studied in detail. The impacts of fibre orientations and LS relaxation time are also investigated in these examples. In conclusion, applying non-Fourier thermal shock for modelling dynamically propagating cracks in orthotropic structures is recommended to obtain more realistic results. In addition, by increasing the LS relaxation time, the crack grows less oscillatory and is more aligned along the material fibres.

Modeling fracture of multidirectional thin-ply laminates using an anisotropic phase field formulation at the macro-scale

An equivalent single layer approach to model fracture events of multidirectional balanced thin-ply laminates via the use of the Phase Field method is explored. The inherent anisotropic nature of a multidirectional laminate is taken into account through the use of a structural tensor, defined from scaled directional vectors, which can account for the variation in fracture toughness of the laminates in varying directions. The scaling constants are defined using the lay-up of the laminate and the intra-laminar fracture toughness of the lamina, minimizing the number of input parameters required while also alleviating the structural tensor of a pure numerical and geometric meaning. They have a significant effect in the solution, and are here related to materials properties, not only providing a new perspective on their definition but also allowing the reduction of the number of numerical parameters used to calibrate the anisotropic PF model. The numerical implementation of the proposed formulation is performed using a simple and robust thermal analogy in Abaqus by exploiting the use of an anisotropic conductivity matrix that plays the role of the structural tensor in the anisotropic phase field formulation, which reduces the complexity of the simulations. Experimental results, based on open-hole tension and double edge-notched tension, are reproduced via simulation validating the model for size effects and for the response to off-axis loading. Successful prediction of notch size effects in multidirectional composite laminates is achieved by means of an equivalent single layer approach, incl. the off-axis open-hole tension strengths of a directional thin-ply laminate. All numerical strength predictions were well within acceptable errors of the respective experimental values.

Crystallized fraction and crystal size effects on the strength and toughness of lithium disilicate glass-ceramics

This work investigated the variation in fracture strength and toughness of stoichiometric lithium disilicate (LS2) glass-ceramics as a function of crystal size (d) and crystallized volume fraction (f), with three average crystal sizes (8, 13 and 34 µm) and a wide range of crystallized fractions (0–100 %). The fracture strength and toughness increased with increasing the crystallized volume fraction. For constant crystallized fraction, KIC increased with crystal size, indicating an R-curve behavior. The mean free path between the crystals limits the maximum size of the critical defect and is the crucial feature controlling fracture strength. Finally, we verified that the contribution to the toughness of R-curve mechanisms in this glass-ceramic is proportional to (f.d)1/2, which agrees with R-curve models for ceramics.

Structure dependence of fracture toughness and ionic conductivity in lithium borophosphate glassy electrolytes for all-solid-state batteries

Glass materials are potential candidates as solid electrolytes for batteries, but the atomistic origins of the variations in their properties and functionalities with composition are not well understood. Here, based on combined experimental and simulation techniques, we investigate the structural origin of the variation in fracture toughness and ionic conductivity of lithium borophosphate glass electrolytes with varying compositions. We focus on these properties since they are critically important for mechanical stability and electrochemical performances of glassy electrolytes. To this end, we have performed molecular dynamics simulations combined with X-ray total scattering experiments to provide the atomic picture of the disordered structure of borophosphate glass. The mechanical properties have been characterized through single-edge precracked beam measurements and axial tensile simulations. We find that the deformation and fracture behaviors of the electrolytes are governed by bond switching events of boron, which dissipate the strain energy during fracture. The migration of lithium ions in the electrolyte network is facilitated by hopping between superstructural rings, which reflects the important role of medium-range order structure in determining the lithium-ion diffusion. These findings have important implications for the design of future glassy electrolytes.

Alteration in the mechanical properties of the Bakken during exposure to supercritical CO2

Carbon neutrality, a balance between emitting and removing carbon from the atmosphere, has become a global aspiration that has resulted in significant scientific advancements. The effective long term geological storage of CO2 as one of the efficient ways to achieve such goal requires a deep understanding of interactions between CO2 and geologic formations. In this study, a sample from the Bakken Formation in North Dakota, which is a target layer for both enhanced oil recovery (EOR) and storage of CO2, was incubated for 3, 8, 16, 30 and 60 days. Then, mineral assemblages and mechanical properties including fracture toughness and contact creep modulus were assessed following each reaction time using XRD analysis and the nanoindentation technique, respectively. Results showed that fracture toughness variation of all three mechanical phases that were recognized based on the force-displacement curves, exhibited an N-shape: an increase (from 1.48 to 2.43 MPa m0.5 after 8 days) proceeding by a decrease (1.26 MPa m0.5after 16 days) and then an increase (2.17 MPa m0.5 after 60 days). Similarly, contact creep modulus of these three different mechanical phases showed the similar N-shape variation pattern. Furthermore, the p-value of the t-Test of fracture toughness and the contact creep modulus values was found less than 0.05, verifying that these two mechanical parameters were truly affected as a result of exposure to ScCO2. These alterations in mechanical properties were attributed to mineral evolution, microstrain, and microstructural alterations which were observed in electron micrographs from the sample after 60 days. Collectively, findings from this study can enable us to predict physico-chemical response of the shale formations that simultaneously produce hydrocarbons, should undergo hydraulic fracturing, EOR and will become future CO2 storage sites.

Variation of Fracture Toughness with Biaxial Load and T-Stress under Mode I Condition

The effect of a biaxial load on brittle fractures of solids under predominantly elastic deformation and mode I loading conditions is studied in this article. A cross-shaped specimen is used in this work, and the variation of T-stress is achieved by changing the load applied to the arm parallel to the crack. In the tests, under a series of different loads parallel to the crack, a series of load values perpendicular to the crack are obtained, and the stress intensity factor is calculated by FEM (Finite Element Analysis). The test data demonstrate that the apparent fracture toughness of the material varies with the load parallel to the crack and the T-stress, since the T-stress is directly related to the load parallel to the crack. If only the first term of the Williams series solution is used to describe the stress field near the crack tip, the variation of KC is unexplained; therefore, more than one term in the Williams series solutions is used to develop the theory of biaxial effect on solid fracture behavior. The fracture criterion based on this consideration is also used to predict crack instability or crack curving under mode I load conditions. Experimental data are presented and compared with the theory.

Experimental Study on the Fracture Toughness of Granite Affected by Coupled Mechanical-Thermo

Abstract Hot dry rock geothermal energy is deep geological energy. Its ability to resist fracture is an important basis for effective reconstruction and scientific evaluation of the stability of geothermal reservoirs. Hot dry rock is typically buried deeply, and the reservoir is often in a high-temperature and high stress environment. There have been limited studies conducted on the effect of different three-dimensional stress and temperature on granite fracture toughness. Thereby, herein an experimental study is conducted on the heat treatment of granite under different external loads and temperatures. The variation in fracture toughness of granite with temperature and pressure is studied using a three-point bending fracture mechanics experiment, scanning electron microscope (SEM) observation, and acoustic wave velocity measurement. The results show that under the joint influence of 25 MPa deviator stress and 200 °C temperature, the Mode I, Mixed mode (I + II), and Mode II fracture toughness of granite show a nonlinear change trend of decreasing and increasing. Among the three modes, the change range of Mode I fracture toughness is not more than 10% which is not significant. Contrarily, the degradation effect of rock mechanical properties caused by the joint action of stress and temperature in Mode II and Mixed mode (I + II) is predominant. The maximum range of Mode II fracture toughness is reduced by 22%, whereas the maximum range of Mixed mode (I + II) fracture toughness is reduced by 18%. However, the compression action of three-dimensional stress causes a slight enhancement in granite mechanical properties, wherein the maximum range of Mixed mode (I + II) fracture toughness is increased by 12%. Furthermore, the change of granite’s ability to resist tensile, shear, and composite (tensile + shear) fracture is not coordinated under the joint action of different temperatures and external loads. This may be due to the small deviator stress effect, which is similar to the early loading stage of uniaxial compression. External loads and thermal stress damage occur in the rock along with the compaction of pore cracks. These mechanisms have different dominant positions under varied temperature and three-dimensional stress coupling conditions, resulting in either the enhancement or weakening of the mechanical properties of granite. The results of this experimental study are conducive to gaining an in-depth understanding of the change law of deep rock mechanical properties and the exploration of hot dry rock reservoir reconstruction.

Assessing the fracture and fatigue resistance of nanostructured thin films

Fatigue failure through sustained loading of ductile materials manifests in irreversible motion of dislocations, followed by crack initiation and growth. This contrasts with the mechanisms associated with brittle ceramics, such as nanostructured physical vapor deposited thin films, where inhibited dislocation mobility typically leads to interface-controlled damage. Hence, understanding the fatigue response of thin films from a fundamental viewpoint – including altered atomic bonds, crystal structures, and deformation mechanisms – holds the key to improved durability of coated engineering components. Here, a novel method utilizing quasi-static and cyclic-bending of pre-notched, unstrained microcantilever beams coupled with in situ synchrotron X-ray diffraction is presented to study the fracture toughness and fatigue properties of thin films under various loading conditions. Investigating a model system of sputter-deposited Cr and Cr-based ceramic compounds (CrN, CrB2, and Cr2O3) demonstrates that the fatigue resistance of such thin films is limited by the inherent fracture toughness. In fact, cantilever cycling close to the critical stress intensity is sustained up to 107 load cycles on all materials, without inducing noticeable material damage, structural or stress-state changes. The observed variation in fracture toughness is put into context with linear-elastic fracture theory and complementary micro-pillar compression, thereby elucidating the wide range of values from as low as 1.6±0.2 MPa√m for Cr1.79O3 up to 4.3±0.3 MPa√m for Cr1.03B2, respectively. Moreover, possible mechanisms governing the elastic-plastic deformation response of all coatings, both in quasi-static and cyclic-loading conditions, are discussed. Our findings contribute key-insights into the underlying mechanisms dictating the damage tolerance of PVD coated components by relating fatigue strength limits to fundamental material properties.

Microstructural and residual stress effects on toughening of stoichiometric BaO.2SiO2 glass-ceramics

Barium disilicate (BaO.2SiO2=BS2) glass is one of the few stoichiometric glasses that nucleates internally, homogeneously via thermal treatment. This system has been scarcely assessed in microstructure-property studies. Here we address fracture strength and toughness (KIC) variation as a function of crystallized volume fraction and crystal size, as well as the possible effect of residual stresses (RS) in BS2 glass-ceramics (GCs) by independently varying these two microstructural parameters. KIC increased with spherulite size and crystallized volume fraction. KIC variation with crystallized volume fraction was similar for GCs with different crystal sizes. Combination of the current findings on BaO.2SiO2 (crystals under tensile RS) with previous studies of Li2O.2SiO2 GCs (compressive RS) indicates that crystallization of a tougher phase – not type of residual stress in the crystals – is the crucial parameter controlling fracture toughness and strength. These findings are quite useful to design novel strong and tough GCs.

Characterization of fracture toughness for surface-modified layer of 18CrNiMo7-6 alloy steel after carburizing heat treatment by indentation method

Advanced processing of a workpiece, such as by carburization, will modify its surface. Accurate measurement of fracture toughness of such a surface-modified layer is important for fatigue resistance manufacturing, but is still a challenge. A method to characterize fracture toughness of the surface-modified layer using a layering treatment and continuous indentation test is proposed. The load–depth curve obtained from continuous indentation testing with a Vickers indenter was used to characterize the fracture toughness of the surface-modified layer produced by carburizing heat treatment of 18CrNiMo7-6 alloy steel. Variation of fracture toughness with depth of the surface-modified layer was determined.