Prediction Of Fracture Toughness Research Articles

Fracture toughness prediction of reactor grade materials using pre-notched small punch test specimens

Pre-notched small punch test (p-SPT) samples (10 × 10 × 0.5 mm) have been fabricated to determine the crack initiation toughness. The machined notch profile is fabricated longitudinally from the centre of one side of the sample to the centre of the opposite side, producing a notch depth to thickness ratio a/t = 0.3.Plastic damage analysis of the p-SPT specimen has been performed by Gurson-Tvergaard-Needlemen (GTN) material damage model which shows the distribution sites of maximum equivalent plastic strain and maximum total void volume fraction (TVVF) is found to approximately similar pattern at different corresponding punch displacements. Numerical results show that the maximum equivalent plastic strain and maximum TVVF location is shifting from centre of the p-SPT specimen to the circumference side with the increment of punch displacement. Due to this numerical observation, the initiation of ductile tearing starts away from the centre of the p-SPT specimen.The crack initiation point has been predicted by numerical and experimental methods. The numerical method involves the GTN material damage model to identify crack initiation point. This is observed when first Gauss point failed completely. The GTN parameters which are used to invoke the crack initiation point in the p-SPT specimen were calibrated by experimental results of 3 mm diameter disk SPT specimens. However, the experimental method involves the analysis of the slope just before the maximum load to predict the crack initiation point. The predicted crack initiation points using both methods are utilized to estimate crack initiation toughness of structural steel. The crack initiation toughness values were obtained for two structural steels viz. T91 and SS304LN using the p-SPT test and compared with the standard test. The differences found were duly justified.

Influence of joint anisotropy on the fracturing behavior of a sedimentary rock

Fracture toughness (FT) and tensile strength (TS) indicate a rock's susceptibility to fracturing. Past studies showed that mechanical properties and FT of homogeneous rocks can be correlated well. In the present study, this capability has been extended to the jointed sedimentary rocks. Sandstone with a wide combination of analogue joints were tested in the laboratory, and the control of joint geometrical properties on the FT, TS, and development of fracture process zone (FPZ) were investigated. A FT prediction model was developed based on the TS of the jointed specimens. Multifractal scaling law (MFSL) was utilized to extend the laboratory results to the field scale. Further, the interaction of propagating crack with the already existing joints were investigated and post-peak behavior were studied. Results show that FT and TS decrease with decreasing joint spacing, but they are more sensitive at higher joint spacing. It is also possible to construct a linear prediction model between the FT and TS of rock. FPZ of jointed rock is less sensitive to joint orientation and more related to joint spacing. A negative correlation between the FPZ size and joint spacing is established in this study. Post-peak behavior of the jointed specimens indicate that crack-joint interaction increases the frictional resistance and dissipated fracture energy of the specimens. Further, comparison of mixed-mode fracture criteria with the experimental results show that Maximum Tangential Stress (MTS) criterion can successfully predict the mixed-mode fracture behavior of jointed sandstone.

Microstructure and mechanical properties of in-situ laminated Nb/Nb5Si3 composites

The in-situ laminated Nb/Nb5Si3 composite was prepared by spark plasma sintering. The microstructures and mechanical properties of the laminated Nb/Nb5Si3 composite were investigated in present work. It could be found that, alternant distribution microstructures of Nbss and Nb5Si3 layers were observed after sintering. The Nb/Nb5Si3 composite exhibited fracture toughness of 11.2MPa·m1/2, and the predicted fracture toughness was estimated by the Ashby model. Moreover, the deviation of the experimental value from the predicted value was discussed. In addition, the compressive strength of the in-situ laminated Nb/Nb5Si3 composite was measured at 1400°C and compared with that of the unidirectionally solidified Nb-17.5Si alloy.

Quantitative prediction of the mechanical properties of precipitation-hardened alloys with special application to Al–Mg–Si

Proper design of the heat treatments of metallic materials is crucial for creating a suitable microstructure resulting in the desired properties. In this paper, we present a multi-component simulation procedure for the quantitative prediction of yield strength, fracture strain, and fracture toughness of precipitation-hardened alloys, with special emphasis on Al–Mg–Si alloys. The procedure combines thermokinetic and micromechanical models, taking the microstructural information in terms of different populations of various second-phase particles into account. The predictions are compared to the results of experiments performed on AA6082 samples subjected to different artificial aging times, and good quantitative agreement is demonstrated.

Controlled relative humidity storage for high toughness and strength of binderless green pellets

AbstractAn equation was developed to predict fracture toughness of green powder compacts. The model combines crack tip toughness predicted by Kendall's model with crack tip shielding due to bridging of moisture meniscuses across the crack. The model predicts that crack tip shielding due to moisture should be dominant. Fracture tests on ceria green pellets verified that storing pellets at a high relative humidity (98% RH) for an extended period of time led to fracture strength more than double those stored at lower RH. However, at lower RH there is no significant increase in fracture strength with increased RH as predicted by the model. The lower strength at low RH is due to insufficient capillary and surface forces but may also be related to the lack of sufficient adsorbed moisture to form bridging meniscuses. The high green strengths achieved by storing pellets at a high RH suggest a method of strengthening green parts without adding binder.

Mode-I Fracture Toughness Prediction of Diamond at the Nanoscale

AbstractIn this paper, the fracture process of nanoscale diamond is analyzed using atomistic simulations and fracture toughness obtained using four different continuum fracture-mechanics theories. ...

Numerical Prediction of Fracture Behavior for Austenitic and Martensitic Stainless Steels

Two types of stainless steels are compared in this paper, austenitic X15CrNiSi25-20 and martensitic X20Cr13, based on their numerically predicted fracture behavior. There are engineering applications where both of the steels can be considered for use and where these materials can be exposed to crack occurrence and growth, so proper distinction between them is desirable. Comparison is made on the basis of [Formula: see text]-integral values that are numerically determined using finite element (FE) stress analysis results. FE analysis is performed on compact tensile (CT) and single-edge notched bend (SENB) type specimens that are usually used in standardized [Formula: see text]-integral experimental procedures. Calculated [Formula: see text]-integral values are plotted versus crack growth lengths for mentioned specimens. Results show somewhat higher values of [Formula: see text]-integral for steel X20Cr13 than X15CrNiSi25-20. Further, when comparing [Formula: see text]-integral values obtained through FE model of CT and SENB specimen, it is noticed that CT specimens give somewhat conservative results. Results obtained by this analysis can be used in predicting fracture toughness assessment during design process.

Some default values to estimate the critical distance and their effect on structural integrity assessments

When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. Thus, it is necessary to derive models that take into account the higher fracture resistance developed by structural materials when notches (and not cracks) are present. In this sense, the use of the Theory of Critical Distances (TCD) for the estimation of the apparent fracture toughness (KNc) observed in notched components has been validated for different types of materials, such as ceramics, polymers, composites and metals. The estimations, for U-shaped notches, arise from the combination of the TCD with the Creager-Paris stress distribution ahead of the notch tip, and apply a notch correction factor to the material fracture toughness observed in cracked conditions (Kc). Such correction only depends on the geometry (notch radius) and the material critical distance (L). The latter is the critical issue when applying the TCD, given that it generally requires calibration through experimental results and simple statistics (best fitting), or through a combination of experimental results with finite elements modelling. This paper provides some default safe values of the material inherent strength that may be used to derive safe estimates of the corresponding value of L, without any further calibration, to be finally used in the apparent fracture toughness predictions and subsequent structural integrity assessments.

Graphene and its elemental analogue: A molecular dynamics view of fracture phenomenon

Graphene and some graphene like two dimensional materials; hexagonal boron nitride (hBN) and silicene have unique mechanical properties which severely limit the suitability of conventional theories used for common brittle and ductile materials to predict the fracture response of these materials. This study revealed the fracture response of graphene, hBN and silicene nanosheets under different tiny crack lengths by molecular dynamics (MD) simulations using LAMMPS. The useful strength of these two dimensional materials are determined by their fracture toughness. Our study shows a comparative analysis of mechanical properties among the elemental analogues of graphene and suggested that hBN can be a good substitute for graphene in terms of mechanical properties. We have also found that the pre-cracked sheets fail in brittle manner and their failure is governed by the strength of the atomic bonds at the crack tip. The MD prediction of fracture toughness shows significant difference with the fracture toughness determined by Griffth's theory of brittle failure which restricts the applicability of Griffith's criterion for these materials in case of nano-cracks. Moreover, the strengths measured in armchair and zigzag directions of nanosheets of these materials implied that the bonds in armchair direction have the stronger capability to resist crack propagation compared to zigzag direction.

Predicting environmental degradation of adhesive joints using a cohesive zone finite element model based on accelerated fracture tests

A framework was developed to predict the fracture toughness of degraded adhesive joints by incorporating a cohesive zone finite element (FE) model with fracture data of accelerated aging tests. The developed framework addresses two major issues in the fracture toughness prediction of degraded joints by significant reduction of exposure time using open-faced technique and by the ability to incorporate the spatial variation of degradation with the aid of a 3D FE model. A cohesive zone model with triangular traction-separation law was adapted to model the adhesive layer. The degraded cohesive parameters were determined using the relationship between the fracture toughness, from open-faced DCB (ODCB) specimens, and an exposure index (EI), the time integration of the water concentration. Degraded fracture toughness predictions were done by calculating the EI values and thereby the degraded cohesive parameters across the width of the closed joints. The framework was validated by comparing the FE predictions against the fracture experiment results of degraded closed DCB (CDCB) joints. Good agreement was observed between the FE predictions and the experimental fracture toughness values, when both ODCB and CDBC were aged in the same temperature and humidity conditions. It was also shown that at a given temperature, predictions can be made with reasonable accuracy by extending the knowledge of degradation behavior from one humidity level to another.

Investigation of fatigue crack growth rate in CARALL, ARALL and GLARE

AbstractFibre metal laminates (FMLs) are being used to manufacture many structural components in aerospace industry because of their very high strength to weight ratios, yet the exact model for estimating fatigue crack propagation in FMLs cannot be developed because of many variable parameters affecting it. In this research, tensile strength, fatigue life and fracture toughness values of 2/1 configuration carbon reinforced aluminium laminate (CARALL), aramid reinforced aluminium laminate and glass laminate aluminium reinforced epoxy specimens have been investigated. Mechanical, chemical and electrochemical surface treatments were applied to AA 1050 face sheets to improve the adhesive properties of the laminates. The specimens were prepared using vacuum assisted resin transfer moulding technique and were cut to desired shapes. Fatigue tests were conducted on centre notched specimens according to ASTM Standard E399. Real time material data and properties of adhesive were used in definition of numerical simulation model to obtain the values of stress intensity factor at different crack lengths. It was observed that CARALL shows very superior tensile and fatigue strength because of stress distribution during failure. Numerical simulation model developed in this research accurately predicts fracture toughness of aramid reinforced aluminium laminate, CARALL and glass laminate aluminium reinforced epoxy with less than 2% error. An empirical analytical model using experimental data obtained during research was developed which accurately predicts the trend of FMLs fatigue life.

Applicability of Fracture Toughness Prediction by Coupled Damage Mechanics Model and Brittle Fracture Model for Ductile

Applicability of Fracture Toughness Prediction by Coupled Damage Mechanics Model and Brittle Fracture Model for Ductile

A probabilistic model including constraint and plastic strain effects for fracture toughness predictions in a pressure vessel steel

This work describes a probabilistic model based upon a local failure criterion incorporating the potential effects of plastic strain on cleavage fracture coupled with the statistics of microcracks. A central objective is to explore and further extend application of a multiscale methodology incorporating the influence of plastic strain on cleavage fracture phrased in terms of a modified Weibull stress (σ˜w) to correct fracture toughness for effects of geometry and constraint loss. Fracture toughness testing conducted on an ASTM A285 Gr C pressure vessel steel provides the cleavage fracture resistance data needed to assess specimen geometry effects on experimentally measured Jc-values. Non-linear finite element analyses for 3-D models of fracture specimens with varying geometries provide the relationship between σ˜w and J from which the variation of fracture toughness across different crack configurations is predicted. This study shows that the modified Weibull stress methodology effectively removes the geometry dependence of fracture toughness values.

Higher order Cauchy–Born rule based multiscale cohesive zone model and prediction of fracture toughness of silicon thin films

In this work, we extend the multiscale cohesive zone model (MCZM) (Zeng and Li in Comput Methods Appl Mech Eng 199:547–556, 2010), in which interatomic potential is embedded into constitutive relation to express cohesive law in fracture process zone, to include the hierarchical Cauchy–Born rule in the process zone and to simulate three dimensional fracture in silicon thin films. The model has been applied to simulate fracture stress and fracture toughness of single-crystal silicon thin film by using the Tersoff potential. In this study, a new approach has been developed to capture inhomogeneous deformation inside the cohesive zone. For this purpose, we introduce higher order Cauchy–Born rules to construct constitutive relations for corresponding higher order process zone elements, and we introduce a sigmoidal function supported bubble mode in finite element shape function of those higher order cohesive zone elements to capture the nonlinear inhomogeneous deformation inside the cohesive zone elements. Benchmark tests with simple 3D models have confirmed that the present method can predict the fracture toughness of silicon thin films. Interestingly, this is accomplished without increasing of computational cost, because the present model does not require quadratic elements to represent heterogeneous deformation, which is the inherent weakness of the previous MCZM model. Quantitative comparisons with experimental results are performed by computing crack propagation in non-notched and initially notched silicon thin films, and it is found that our model can reproduce essential material properties, such as Young’s modulus, fracture stress, and fracture toughness of single-crystal silicon thin films.

Enhanced fracture toughness of TiBw/Ti3Al composites with a layered reinforcement distribution

The prospect of combining laminated structure design and tailored reinforcement distribution to toughen brittle materials is investigated. TiBw/Ti3Al–Ti3Al laminated composites consisting of alternating TiBw/Ti3Al and Ti3Al layers were fabricated by reaction annealing of stacked, alternating foils of TiBw/Ti and Al. Enhancement in the fracture toughness is experimentally observed, which is attributed to stress redistribution and stress state change in laminated composites. Theoretical prediction of fracture toughness depending on a shear ligament toughening model agrees well with the experimental value. It is expected that the design strategy can be extended to other brittle intermetallic compounds, such as Ni–Al, Fe–Al, and Nb–Al systems.

Fracture toughness and tensile strength of 316L stainless steel cellular lattice structures manufactured using the selective laser melting technique

Selective Laser Melting (SLM) process is a metallic additive manufacturing technique that directly manufactures strong, lightweight and complex three dimensional parts in a layer-by-layer to scan and melt the metal powder for aerospace applications. However, there are still certain evaluation criteria such as fracture toughness and tensility of cellular structure made by SLM which were not reported before.This study presents new and novel methods in additive manufacturing and evaluates the local failure mechanism of 316L cellular lattice structures made by SLM under uniaxial tension and three point pending load. The effect of different build directions of the 316L lattice structure on the fracture toughness properties are compared to the Ashby and Gibson models. Also, the effect of different build directions on tensile properties of 316L cellular structures has been investigated.Microcomputer tomography (CT) reveals that the cellular structure parts with different build directions were manufactured free of defect by the SLM. The density of the lattice structure samples was found at 1.35g/cm3 for both vertical and horizontal building directions while the relative density of solid struts is 96.25%. The tensile and fracture toughness properties in vertical building direction samples are higher than those samples that were built in horizontal building direction. There was no big difference between the Ashby and Gibson micromechanical model to predict fracture toughness and Single Edge Notch Bend (SENB) test results from 0.2 to 0.5MPam0.5.

Interfacial delamination of thin-film PTFE (polytetrafluoroethylene) coatings

ABSTRACTInterfacial adhesion is a major concern with respect to successful performance of thin polymer films in developing new thin-film processes. Micro-indentation was used to induce interfacial delamination of polytetrafluoroethylene (PTFE) films deposited on glass substrates using hot filament chemical vapour deposition (HFCVD). Film thickness (1, 2, 3, 5, 10 µm) and indentation load (0.5, 0.75, 1, 2, 3 N) effects on the delamination diameter were investigated. A three-dimensional finite element model using shear material failure criterion and cohesive zone model (CZM) was developed to simulate the delamination. A normalized load–delamination radius relationship was obtained to evaluate the interfacial fracture toughness. The experimental observations showed that the delamination diameter depends on film thickness and indentation load. The numerical simulation indicates the delamination diameter depends on film thickness, material properties, and indentation force. The predictions of interfacial fracture toughness for 5- and 10-µm PTFE films are much smaller than those values using Rosenfeld et al.’s equation, which excludes the energy spent during the penetration.

On the correlation between minimum thickness and central deflection during small punch test

Present paper deals with a detailed study on the correlation between minimum thickness (t/t0) and central deflection (δ/t0). Such data are obtained during the deformation of a small punch test of miniaturized specimen. Finite element studies have been carried out to investigate the effect of various parameters which are expected to influence this correlation. The parameters under consideration are material hardening, material yield stress, coefficient of friction and initial thickness of the specimen. It is shown that the correlation remains unaffected with respect to change in material parameters. Similarly, the coefficient of friction beyond 0.2 also does not affect the correlation. However, change in thickness has significant effect on the correlation. A modification has been suggested in the existing correlation to consider the influence of thickness change. The modified correlation is then used to calculate fracture toughness using the experimental results quoted in the literature. It is shown that the modified correlation improves the fracture toughness prediction considerably.

A discussion of “An engineering methodology for constraint corrections of elastic–plastic fracture toughness – Part II: Effects of specimen geometry and plastic strain on cleavage fracture predictions” by C. Ruggieri, R.G. Savioli, R.H. Dodds [Eng. Fract. Mech. 146 (2015) 185–209

Ruggieri et al. (2015) proposed a modified Weibull stress model incorporating plastic strain effect and employed it in combination with the distribution of measured toughness values from one specimen geometry to predict fracture toughness distribution of another specimen geometry, and also estimated reference temperature for the Master Curve approach from small fracture specimens. This discussion raises the concern on the transferability of the characteristic fracture toughness between a two-parameter Weibull distribution of fracture toughness inherent to the modified Weibull stress model and a three-parameter Weibull distribution of measured toughness values or for the Master Curve approach. It also emphasizes the necessity for the modified Weibull stress model as a cumulative density function to conform to the probability normalization requirement.

Numerical and experiment fracture modeling for multiple cracks of a finite aluminum plate

In the present paper, an extended finite element method for prediction of stress intensity factor, fracture toughness, and failure load for multiple cracks configurations of finite Aluminum alloy plate are examined using ABAQUS software. The tensile experiment is performed to evaluate failure load, and fracture toughness for multiple cracks configurations. The extended finite element method predicted fracture parameters are compared with experimental results and conventional finite element results, shows an excellent agreement. Typical numerical results are presented to examine the effect of specimen thickness, characteristic length, crack inclination angle, and mode – I crack offset distance on the fracture parameters of multiple cracks configurations. Based on the numerical predictions, new mathematical models for estimation of stress intensity factor for multiple cracks configurations are introduced. The proposed mathematical model is useful while assessing the damage tolerance behavior of structural components/assemblies mainly for aircraft structure.