Mechanical Fracture Toughness Research Articles

Computational Investigation of the Mechanical Response of a Bioinspired Nacre-like Nanocomposite under Three-Point Bending

Natural biological nanocomposites, like nacre, demonstrate extraordinary fracture toughness, surpassing their base materials, attributed to their intricate staggered hierarchical architectures integrating hard and soft phases. The enhancement of toughness in these composites is often linked to the crack-deflection mechanism. Leveraging the core design principles that enhance durability, resilience, and robustness in organic materials, this paper describes the use of computational modeling and simulation to perform a three-point bending test on a 3D staggered nanocomposite intentionally crafted to mimic the detailed microstructure of nacre. We adopted a previously proposed interfacial zone model that conceptualizes the “relatively soft” layer as an interface between the “hard” mineral tablets and the microstructure’s interlayer spaces to examine how the microstructure and interface characteristics affect the mechanical responses and failure mechanisms. By comparing the model’s predictions with experimental data on natural nacre, the simulations unveil the mechanisms of tablet separation through adjacent layer sliding and crack deflection across interfacial zones. This study offers a robust numerical method for investigating the fracture toughening mechanisms and damage evolution and contributes to a deeper understanding of the complex interplays within biomimetic materials.

Peridynamic analysis of thermomechanical effects in fracture toughening mechanisms of stop holes

In this study, a bond-based fully coupled thermomechanical peridynamic framework is implemented to study the fracture toughening of stop-holes in brittle materials subjected to thermomechanical loads. The developed modelling approach aims to provide comprehensive guidance about thermal effects on the fracture toughening mechanism of inclusions under realistic engineering conditions. First, the model is verified using simulation of several benchmark case studies and predicts the mechanical, thermal and damage parameters with high accuracy. Then, a fully coupled thermomechanical analysis of a 2D structure is performed to study the effect of the location of a single stop-hole on fracture toughening. A thorough investigation of the arresting and accelerating effect of the stop-hole is conducted. Additionally, the accuracy of various degrees of thermomechanical coupling including one-way weak coupling and no coupling is compared with full coupling. The study of crack propagation dynamics reveals that ignoring thermal effects underpredicts the toughness enhancement of stop-holes. Furthermore, although the weak thermomechanical coupling offers a reasonable prediction of crack propagation dynamics in most cases of our study, it is not accurate in certain cases. In the final section, some benchmark cases of complex arrangements of multiple stop-holes in literature are revisited. Our results indicate a different toughness trend for some cases when considering the full thermomechanical coupling. This suggests the proposed framework provides a more accurate tool for designing toughened structures using inclusions.

XFEM for Composites, Biological, and Bioinspired Materials: A Review.

The eXtended finite element method (XFEM) is a powerful tool for structural mechanics, assisting engineers and designers in understanding how a material architecture responds to stresses and consequently assisting the creation of mechanically improved structures. The XFEM method has unraveled the extraordinary relationships between material topology and fracture behavior in biological and engineered materials, enhancing peculiar fracture toughening mechanisms, such as crack deflection and arrest. Despite its extensive use, a detailed revision of case studies involving XFEM with a focus on the applications rather than the method of numerical modeling is in great need. In this review, XFEM is introduced and briefly compared to other computational fracture models such as the contour integral method, virtual crack closing technique, cohesive zone model, and phase-field model, highlighting the pros and cons of the methods (e.g., numerical convergence, commercial software implementation, pre-set of crack parameters, and calculation speed). The use of XFEM in material design is demonstrated and discussed, focusing on presenting the current research on composites and biological and bioinspired materials, but also briefly introducing its application to other fields. This review concludes with a discussion of the XFEM drawbacks and provides an overview of the future perspectives of this method in applied material science research, such as the merging of XFEM and artificial intelligence techniques.

Gradient changes in fiber bundle content and mechanical properties lead to asymmetric bending of bamboo

Natural biomaterial bamboo has excellent strength and toughness. The content of fiber bundles and the gradually increasing tensile strength of fiber bundles from the inside to the outside in the radial direction of the bamboo wall are the two main factors leading to significant differences in the fracture behavior and mechanical properties of bamboo under loading from different sides of bamboo strip. In this study, the flexural strength, toughness, and bending fracture energy of the bamboo loaded from the inside of the sample (Mode B) were found as 28%, 191%, and 174%, respectively, higher than those of the bamboo loaded from the outside of the sample (Mode A). The mechanical properties of samples loaded radially (Mode C) were higher than Mode A and lower than Mode B. The fracture process of the sample and the corresponding fracture morphology showed parenchyma fracture (matrix failure), fiber fracture, and interfacial debonding (fiber–fiber, fiber-parenchyma, parenchyma-parenchyma) occurring in all samples. Tearing of the cell wall, fiber debonding, fiber bridging, fiber pull-out, and crack deflection are the major fracture-toughening mechanisms of bamboo. This study reveals the relationship between the gradient changes in the mechanical properties and content of bamboo fiber bundles and the bending and fracturing behavior of bamboo, which provides theoretical basis and ideas for the development of new bamboo engineering materials, the bionic design of bamboo structures, and the structural optimization of fiber-reinforced composite materials.

Adhesion Strengthening Mechanism of Carbon Nanotube-Embedded Epoxy Composites: A Fracture-Based Approach.

Interfacial bonding integrity between different materials is critical to maintain the functionality of the entire physical system in any scale, ranging from building structures down to semiconductor transistors. For example, micro-patterned polymers embedded with conductive nanoparticles [e.g., carbon nanotubes (CNTs)] bonded with integrated circuits have been applied as many emerging chemical/biological microelectronic sensors. Nonetheless, it is challenging to measure and ensure the interfacial bonding integrity between materials for consistent and sustainable operations. Herein, we apply multiple interface characterization methods based on micro-engineering and microscopy as an integrative approach to reveal the mechanism of interfacial reinforcement by adding CNTs in a matrix material. An epoxy/CNT micro-beam is fabricated onto a silicon substrate, sandwiching a gold layer as an interfacial precrack. Superlayers of chromium are then repeatedly deposited onto the microstructure, inducing stepwise increasing stress over the materials and the corresponding micro-beam bending after detachment from the bonded interface. Accordingly, we can quantify key interfacial fracture parameters such as crack length, steady-state energy release rate, and fracture toughness. By further examining the formation and distribution of the micro-/nanostructures along the debonded interface using bright-field microscopy, 3D fluorescence imaging, and scanning electron microscopy, we can identify the underlying dominant interfacial strengthening and fracture toughening mechanisms. We further compare experimental results and theoretical predictions to quantify the interfacial bonding properties between epoxy/CNT and silicon and unveil the underlying reinforcement mechanisms. The results provide insights to develop polymer/nanoparticle composites with reinforced interfacial bonding integrity for more sustainable and reliable applications including microelectronics, surface coatings, and adhesive materials.

Experimental Proof of Reverse Bending Technique for Modifying Weld Residual Stress in Weld CTOD Specimen and Comparison of Effect with Other Techniques

Abstract In fracture toughness testing of welds, weld residual stress often makes it difficult to obtain a straight precrack front. Therefore, BS 7448-2, Fracture Mechanics Toughness Tests. Method for Determination of KIc, Critical CTOD and Critical J Values of Welds in Metallic Materials, and ISO 15653, Metallic Materials – Method of Test for the Determination of Quasistatic Fracture Toughness of Welds, recommend techniques for modifying weld residual stress to achieve a straight precrack front. BS 7448-2 prescribes three techniques: local compression, reverse bending, and a stepwise high stress ratio (R-ratio) technique. However, in ISO 15653, which was newly published in 2010, the reverse bending technique was rejected because detailed conditions for obtaining a reliable effect were not provided. In the revision of ISO 15653 in 2018, the reverse bending technique was revived with a proposal of appropriate conditions. The present paper reviews the conditions of reverse bending proposed in the revision of ISO 15653, together with the technical background. The effectiveness of the proposed reverse bending conditions is demonstrated experimentally by precracking in weld specimens with a thickness of 127 mm (5 inches). The effects of the two other techniques, i.e., the local compression and stepwise high R-ratio techniques, are also examined, and the effects of each technique on crack front straightness and the required fatigue cycles to finalize precracking are discussed.

Mechanical properties and fracture toughness of fumed silica epoxy composites containing glycidyl terminated polysiloxanes

Nanocomposites have received much attention as construction materials, adhesives, and sealants because of the strong interfacial interactions between nano-sized fillers and matrix. However, dispersing the nano-sized fillers in the polymer matrix is challenging because of their tendency to agglomerate. In this study, a glycidyl terminated polysiloxane (g-PDMS) was synthesized and used to disperse fumed silica (nano-sized silica aggregates) in a BPADG/D230 system. The kinetic parameters of the g-PDMS/BPADG/D230 curing were obtained through non-isothermal DSC analysis. The apparent activation energy and reaction order of the epoxy resin/D230 system were calculated to be 51.82 kJ/mol and 0.91, respectively. Next, the effect of fumed silica on the properties of the nanocomposites was investigated. Compared to neat epoxy, the maximum strength, Young's modulus, fracture toughness and fracture energy of the nanocomposite with 5 phr (parts per hundred parts of epoxy resin) fumed silica were increased by 27%, 61%, 76% and 79%, respectively. Lastly, the fracture toughening mechanisms of the nanocomposites were examined using SEM images of the fractured surfaces of single-edge notched specimens after a three-point bending test.

Residual stress as a fracture toughening mechanism: A Phase-Field study on a brittle material

Recent engineering design practice for materials and structures relies more and more on damage-tolerant criteria. Such a design approach is attained mainly by employing materials showing a certain level of fracture toughness.This work aims to explore a way to generate fracture toughness in materials that intrinsically shows no toughness at all, i.e. brittle materials. The key idea lies in the introduction of inelastically deformed sub-regions (e.g. circular inclusions) in the base material, which inevitably generate a residual stress field.To accomplish this purpose, the advanced Phase-Field method coupled with the eigenstrain theory is employed, respectively to simulate the crack propagation behavior and to introduce a residual stress field in a pre-notched sample. Information about crack propagation and displacement externally imposed is used to obtain the resistance curve (R-curve) for several configurations.One of the main findings of this research regards the possibility of originating fracture toughness in intrinsically brittle materials upon appropriate positioning of one inclusion - containing a certain amount of inelastic deformation – with respect to a notch tip. This result demonstrates that accurate design of residual stress is crucial to attaining unprecedented material or structure performance, and the method shown here represents a valid tool to exploit this advanced design capability.

Single and hybrid organoclay‐filled PLA nanocomposites: Mechanical properties, viscoelastic behavior and fracture toughening mechanism

AbstractPoly (lactic acid) (PLA) nanocomposites based on single and hybrid organic‐modified montmorillonites were previously studied in terms of their morphological, thermal and fire performance. As surfactants of the organoclays influenced the compatibility between the nanofillers and PLA with different degrees of clay platelets dispersion, the present work investigated the effect of these features in the mechanical properties of PLA nanocomposites. PLA nanocomposites specimens were analyzed by dynamic‐mechanical‐thermal analysis, which pointed out changes in the viscoelastic behavior of the materials by the incorporation of the organoclays, namely the increase of the storage modulus due to polymer chains movements restriction and reinforcement effects associated with the dispersion of the nanofillers. Flexural and impact testing showed that hybrid organomontmorillonites containing ester ammonium and ethoxylated amine improved PLA's ductility, toughness and impact resistance. This behavior was explained by the high level of compatibility and interaction between the surfactants and PLA chains due to the polar groups in their structures. These organoclays caused a transition on PLA's fracture from brittle to ductile in a way that the toughening mechanism was explained by crazing and multi‐shear banding induced by the plasticized interfacial region around these organoclays.

Study of the castable selection for blast furnace blowpipe

The contradictory properties required of castable refractories makes selecting castable refractories for industrial applications challenging. This paper seeks to describe the material selection for a blast furnace blowpipe application that is subjected to sudden temperature changes and must prevent heat loss. Three commercial high alumina castables containing andalusite or mullite from different manufacturers were characterized. Thermal shock damage resistance was evaluated using thermal shock damage resistance theory and experiments. The castables’ coefficient of thermal expansion was estimated using quantitative X-ray diffraction. Crack propagation resistance was measured using the work-of-fracture technique. Thermal shock damage was experimentally evaluated by measuring the modulus of elasticity and rupture prior to and after thermal cycles. Ultimately, the microstructure of the castables was related to the thermal shock damage behavior by estimating the aggregate size and the fracture toughening mechanisms using light optical and scanning electron microscopes. Heat loss was evaluated by calculating the blowpipe shell temperature using a one-dimensional steady-state heat conduction model. The best commercial castable refractory for blowpipe showed high thermal shock damage resistance and low thermal conductivity. The results in this study agreed with thermal shock damage resistance parameters and showed a correlation between coarse microstructure with large aggregate and higher thermal shock damage resistance.

Investigation of Fracture-Toughening Mechanism of Dentine-Enamel Junction in Human Teeth by Means of <i>J</i>-Integral Calculation

Investigation of Fracture-Toughening Mechanism of Dentine-Enamel Junction in Human Teeth by Means of <i>J</i>-Integral Calculation

Nanoscaled eutectic NiAl-(Cr,Mo) composites with exceptional mechanical properties processed by electron beam melting

Eutectic NiAl-(Cr,Mo) composites are promising high temperature materials due to their high melting point, excellent oxidation behavior and low density. To enhance the strength, hardness and fracture toughness, high cooling rates are beneficial to obtain a fine cellular-lamellar microstructure. This can be provided by the additive process of selective electron beam melting. The very high temperature gradient achieved in this process leads to the formation of the finest microstructure that has ever been reported for NiAl-(Cr,Mo) in-situ composites. A very high hardness and fracture toughening mechanisms were observed. This represents a feasibility study towards additive manufacturing of eutectic NiAl-(Cr,Mo) in-situ composites by selective electron beam melting.

Numerical study of crack initiation and growth in human cortical bone: Effect of micro-morphology

In this study, crack initiation and growth in four different groups of human cortical bones, i.e., young, aged, diseased (osteoporosis) and treated are investigated numerically with a zero-thickness Cohesive Element Method, employing statistical realisations of randomly distributed microstructural constituents. The obtained simulation results demonstrated distinct crack paths in bones with varying microstructures, based on analysis of initiation, propagation and branching of multiple cracks, with supporting fracture toughening mechanisms. It is shown that superior mechanical properties and fracture resistance in the young and treated groups originated from both the qualitative and quantitative features of microstructural constituents.

High-speed X-ray visualization of dynamic crack initiation and propagation in bone

The initiation and propagation of physiological cracks in porcine cortical and cancellous bone under high rate loading were visualized using high-speed synchrotron X-ray phase-contrast imaging (PCI) to characterize their fracture behaviors under dynamic loading conditions. A modified Kolsky compression bar was used to apply dynamic three-point flexural loadings on notched specimens and images of the fracture processes were recorded using a synchronized high-speed synchrotron X-ray imaging set-up. Three-dimensional synchrotron X-ray tomography was conducted to examine the initial microstructure of the bone before high-rate experiments. The experimental results showed that the locations of fracture initiations were not significantly different between the two types of bone. However, the crack velocities in cortical bone were higher than in cancellous bone. Crack deflections at osteonal cement lines, a prime toughening mechanism in bone at low rates, were observed in the cortical bone under dynamic loading in this study. Fracture toughening mechanisms, such as uncracked ligament bridging and bridging in crack wake were also observed for the two types of bone. The results also revealed that the fracture toughness of cortical bone was higher than cancellous bone. The crack was deflected to some extent at osteon cement line in cortical bone instead of comparatively penetrating straight through the microstructures in cancellous bone. Statement of SignificanceFracture toughness is with great importance to study for crack risk prediction in bone. For those cracks in bone, most of them are associated with impact events, such as sport accidents. Consequently, we visualized, in real-time, the entire processes of dynamic fractures in notched cortical bone and cancellous bone specimens using synchrotron X-ray phase contrast imaging. The onset location of crack initiation was found independent on the bone type. We also found that, although the extent was diminished, crack deflections at osteon cement lines, a major toughening mechanism in transversely orientated cortical bone at quasi-static rate, were still played a role in resisting cracking in dynamically loaded specimen. These finding help researchers to understand the dynamic fracture behaviors in bone.

Fracture analysis of cortical bone under the condition of cement line debonding and osteon pullout

Cortical bone consists of osteons embedded in interstitial bone tissue and there is a thin amorphous interface, named cement line, between osteon and interstitial bone. Due to fatigue and cyclic loading, the pullout or debonding phenomenon often occurs in osteonal and interstitial tissue bone. The study aims to construct a fiber-reinforced composite material debonding model for cortical bone, in which the bonding condition along the osteon, cement line and interstitial tissue bone are assumed to be imperfect. In the study, we used the complex variable method to obtain series representations for stress fields in the osteon, cement line and the interstitial tissue bone with a radial crack. The effects of material properties of osteon and cement line, crack position, and varying degrees of debonding on the fracture behavior were investigated by computing the stress intensity factor (SIF) in the vicinity of the microcrack tips. The investigation results indicated that the cement line was important for controlling the fracture toughening mechanisms and that the level of imperfect bonding among osteon, cement line and interstitial tissue bone had a pronounced effect on the crack behavior and should not be ignored.

Validation of a low blow specimen technique for R-curve determination using a drop tower

Abstract: Fracture mechanics based component design requires appropriate fracture mechanics toughness data with respect to both, loading rate as well as test temperature. Taking high-rate loading into account such as with accidental scenarios, different standards such as ASTM E 1820 or BS 7448-3 provide some information on dynamic fracture mechanics testing. Nevertheless, the designations differ so that a validation of the own material specific test method used for dynamic R-curve determination is mandatory. In order to address this for ductile cast iron materials an experimental method for the reliable determination of dynamic J-integral crack resistance curves at -40 °C following the multiple specimen approach has been established and validated. The experimental concept offers some additional valuable features. Single values of dynamic crack initiation toughness can be determined using a single specimen technique based on crack sensors. Furthermore, an experimentally independent method is provided according to which CTOD δ 5 R-curves can be established. The focus of the present paper is on the validation of the experimental low blow technique using a drop tower test system. A drop tower test system was developed and set up to perform low blow tests at temperatures down to -40 °C. The system allows for a variation of the impact mass and height and was optimized for testing of ductile cast iron at stress intensity rates from approximately 5∙10 4 to 3∙10 5 MPa√ms -1 . This range of loading rate is characteristic for instance with crash scenarios of heavy sectioned DCI casks for radioactive materials. In order to address characteristic challenges of impact tests (test duration of microseconds up to milliseconds, inertial effects, signal oscillations), an appropriate full bridge strain gage method for the measurement of force directly on the specimen as well as a non-contact measurement of load line displacement using an optical extensometer have been developed and validated. The low blow test requires either to prevent bouncing strikes of the hammer by using the stop block technique or to catch the hammer after its first strike. Both options are not part of the experimental concept and setup which have been realized here. The paper describes investigations which have been performed in order to make sure that bouncing strikes of the hammer do not cause additional crack extension in the specimen. This is necessary to ensure a unique relation between the work done and the achieved crack extension. The investigations covered the analysis of limit loads of the specimen with respect to the measured force. The measured stiffness of the specimen was assessed and signals of crack sensors were analyzed. Furthermore, an analysis of the mechanical behavior of the loading system and the specimen by optical observation was performed. Corresponding results are discussed in the paper. It had finally been proven that additional crack extension in the specimen due to bouncing strikes of the hammer is not to be expected under the given conditions of test setup, material and loading. It can be seen as a major experimental advantage that the striker does not have to be catched after the low blow test.

Finite element analysis of boron nitride nanotubes' shielding effect on the stress intensity factor of semielliptical surface crack in a wide range of matrixes using RVE model

Abstract In terms of fracture toughening mechanisms, this study carried out a finite element analysis to determine the shielding effect of boron nitride nanotubes (BNNTs), as a reinforcement element, on a preexisting semielliptical surface crack in both soft (polymeric) and stiff (ceramic and metal) materials. The BNNTs were distributed randomly using the random sequential adsorption algorithm in the hypothetical matrixes. The prepared representative volume elements (RVEs) contained 2%, 5%, and 8% volume fractions of BNNTs. The elastic modulus ratio of BNNTs to the RVEs matrixes' elastic modulus was set for 11 levels as 1, 5, 10, 20, 40, 100, 200, 500, 1000, 5000 and 10000. Then, semielliptical cracks at the crack depth to crack width ratio of 0.2, 0.4, 0.6 and 1 were embedded on the RVEs' surface to evaluate the mode I stress intensity factor, or K I value. The results showed that a higher mismatch difference between the matrix and BNNTs' elastic modulus caused further reduction in K I value. Moreover, in softer materials, the BNNTs' volume fraction changes were more effective and decreased K I was rather when compared to stiff materials. The further shielding effect of BNNTs for softer material can be explained by the higher participation of BNNTs in the load-bearing function in softer matrixes.

Characterisation of Fracture and HAC Resistance of an Individual Microstructural Constituent with Micro-Cantilever Testing

This paper focuses on the application of miniaturized fracture tests to evaluate the fracture and hydrogen assisted cracking (HAC) resistance of a selected microstructural constituent (acicular ferrite, AF) which only occurs in microscopic material volumes. Site-specific Focused Ion Beam (FIB) micro-machining was used to fabricate sharply notched micro-cantilevers into a region fully constituting of AF. The micro-cantilevers were subsequently tested under uncharged and hydrogen charged conditions with a nanoindenter. The load displacement curves were recorded and analysed with a simplified plastic hinge model for the uncharged specimen, as AF demonstrated an essentially ductile behaviour. The simplified model assisted with FE simulations provided values of the critical plastic crack tip opening displacement (CTOD). A value of the conditional fracture toughness was thereby determined as 12.1 MPa m1/2. With LEFM, a threshold stress intensity factor, Kth, to initiate hydrogen crack propagation in AF was found to range between 1.56 MPa m1/2 and 4.36 MPa m1/2. All these values were significantly below the corresponding values reported for various ferrous alloys in standard macro-tests. This finding indicates that the fracture and HAC resistance at the micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.

In situ characterization of fracture toughness and dynamics of nanocrystalline titanium nitride films

Abstract

Investigation of hydrogen assisted cracking in acicular ferrite using site-specific micro-fracture tests

Hydrogen assisted cracking (HAC) is a common type of failure mechanism that can affect a wide range of metals and alloys. Experimental studies of HAC are cumbersome due to various intrinsic and extrinsic parameters and factors (associated with stress, hydrogen and the materials microstructure) contributing to the hydrogen crack kinetics. The microstructure of many materials consists of diverse constituents with characteristic features and mechanical properties which only occur in very small material volumes. The only way to differentiate the effect of these individual constituents on the hydrogen crack kinetics is to miniaturise the testing procedures. In this paper we present a new experimental approach to investigate hydrogen assisted crack growth in a microstructural constituent, i.e. acicular ferrite. For this purpose, sharply notched micro-cantilevers were fabricated with a Focus Ion Beam within this selected microscopic region. Acicular ferrite can be found in many ferrous alloys including ferritic weld metal and has specific features that control its intrinsic susceptibility to HAC. These features were characterised via Electron Backscatter Diffraction and the specimens were subsequently loaded under uncharged and hydrogen charged conditions with a nano-indenter. The outcomes of the testing, demonstrated that the threshold stress intensity factor, Kth, to initiate crack propagation in acicular ferrite ranges between 1.56MPam1/2 and 4.36MPam1/2. This range is significantly below the values of Kth reported for various ferrous alloys in standard macro-tests. This finding indicates that the mechanisms and resistance to HAC at micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.