Fracture Toughness Measurements Research Articles

Investigation of nanostructured hard metal composites obtained from powder mixtures (WC-Co)-Al2O3

A new stereological model of nanostructured hard metal composites is suggested which describes the packing density, structural parameters and fracture toughness depending on the sizes, volume of the carbides and addition of nanoparticles Al2O3. The results of complex theoretical and experimental investigations of hard metal composites based on powder mixtures (WC-Co)-Al2O3(nano) are presented, which confirm the possibility of improvement of the structure parameters (with the average size of the binder interlayer being up to 0.12 μm and carbide grain up to 1μm), increasing the hardness 15-18 HV, GPa, fracture toughness, K1c, 16-19 MPa m1/2, transverse rupture toughness, σи - 2700 MPa. The results of the modeling experiments are in satisfactory agreement with the measurements of structure parameters and fracture toughness.

Water as Veneering Modeling Liquid Affects Microhardness of Glassy Matrix Veneering Ceramic.

To evaluate the effect of different veneering liquids used for modeling on microhardness, fracture toughness and biaxial flexural strength of a glass-veneering ceramic. The manufacturer recommended modeling liquid (ML), distilled water (DW), isopropyl alcohol (IA), 0.5% (P05), 1% (P1), and 2% (P2) polyethylene glycol solutions were mixed with feldspathic ceramic powder to form disc-shaped samples (n=20, 15 mm × 1.2 mm). After sintering, samples were mirror-polished and subjected to Vickers indentation (n=5) for measurement of microhardness and fracture toughness. The remaining 15 samples from each group were subjected to biaxial flexural strength. Data were subjected to one-way ANOVA and Weibull analysis. The microhardness was affected by veneering liquid (p=0.002): DW promoted higher microhardness values than ML and IA. Fracture toughness (p=0.301) and flexural strength (p=0.930) were not affected by the veneering liquid but Weibull parameters were affected. All groups presented surface pores under high magnification. Even though the use of DW led to higher values of surface microhardness than the ML, all obtained values are inside the range of enamel microhardness values reported in the literature. Such parameters may affect antagonist wear and should be reported in clinical trials.

Temperature effect on elastic and fracture behaviour of lead-free piezoceramic BaTiO3

Ferroelectric ceramics, especially piezoceramics, are widely used in the field of sensors, micromanipulators, and capacitors. Barium titanate and its derivates are the first choices between lead-free materials. The main aim of the paper is to clarify fundamental processes taking place in the vicinity of Curie temperature with a focus on the fracture behaviour of pure polycrystalline barium titanate. The explanation of observed changes in the mechanical behaviour of this material is based on the experimental approach supported by numerical simulations utilising features of the real microstructure on the grain level. Several model materials with various grain microstructures were manufactured from the submicron barium titanate powder sintered at various temperatures. Two resulting materials with a suitable distribution of grains were selected for further investigation. The grain size influenced not only the exact position of the temperature of the Curie point but also the kinetics of the lattice transformation, elastic, and fracture properties. The significant drop observed in the fracture resistance was attributed to the development of localised internal thermal stresses, which was supported by the results of the performed numerical simulations. The coincidence of the volume change of neighbouring grains due to lattice transformation together with a significant variation in elastic properties can lead up to a 20% decrease in the measured fracture toughness. Understanding this behaviour is essential for the processing and correct application of lead-free barium titanate materials.

Atomistic measurement and modeling of intrinsic fracture toughness of two-dimensional materials

Quantifying the intrinsic mechanical properties of two-dimensional (2D) materials is essential to predict the long-term reliability of materials and systems in emerging applications ranging from energy to health to next-generation sensors and electronics. Currently, measurements of fracture toughness and identification of associated atomistic mechanisms remain challenging. Herein, we report an integrated experimental-computational framework in which in-situ high-resolution transmission electron microscopy (HRTEM) measurements of the intrinsic fracture energy of monolayer MoS2 and MoSe2 are in good agreement with atomistic model predictions based on an accurately parameterized interatomic potential. Changes in crystalline structures at the crack tip and crack edges, as observed in in-situ HRTEM crack extension tests, are properly predicted. Such a good agreement is the result of including large deformation pathways and phase transitions in the parameterization of the inter-atomic potential. The established framework emerges as a robust approach to determine the predictive capabilities of molecular dynamics models employed in the screening of 2D materials, in the spirit of the materials genome initiative. Moreover, it enables device-level predictions with superior accuracy (e.g., fatigue lifetime predictions of electro- and opto-electronic nanodevices).

Prediction of Mixed-mode I and II effective fracture toughness of several types of concrete using the extreme gradient boosting method and metaheuristic optimization algorithms

Complex sample preparation procedures, the necessity for highly accurate and sensitive instruments, early sample failure, and brittle samples all contribute to the difficulty of measurement of concrete fracture toughness (CFT) in the laboratory. With such limitations on the measuring CFT, it is necessary to develop new and more effective tools. Machine learning (ML) methods are promising tools for non-linear time series prediction. In this study, six extreme gradient boosting (XGBoost)-based meta-heuristic algorithms, including standard XGBoost, XGBoost-particle swarm optimization (PSO), XGBoost-imperialist competitive algorithm (ICA), XGBoost-grey wolf optimization (GWO), XGBoost-shuffled frog leaping algorithm (SFLA), and XGBoost-genetic algorithm (GA) were developed to predict the effective fracture toughness (K_eff) of concrete, utilizing 560 datasets obtained from the central straight notched Brazilian disc (CSNBD) test. Additives of micro silica and powdered stone, which are the most widely used building materials were used in the concrete samples to investigate their effect on the physical and mechanical properties. Therefore, 10 different materials made from a combination of concrete and different percentages of additives, were used in the experiment. The datasets included six input parameters effective on the K_eff, including concrete type (ST), diameter (D), thickness (t), length (L), force (F), and crack angle (α). The XGBoost-based models' performances were compared with those of each other and with four other ML methods of Gaussian process regression (GPR), decision trees (DT), support vector regression (SVR), and K-nearest neighbors (KNN). The ML models’ performance in the prediction of K_eff for both holdout and 5-fold methods from high to low is XGBoost-PSO, XGBoost-ICA, XGBoost-GWO, XGBoost-SFLA, XGBoost-GA, XGBoost, GPR, SVR, DT, and KNN. Considering the datasets utilized in this research, the feature selection method's sensitivity analysis revealed that, except for the parameter D, all other parameters considered in this study, significantly affect the K_eff. By developing the meta-heuristic methods, the ability of the XGBoost-based methods was significantly improved. Finally, this article recommends using the XGBoost-PSO hybrid model to predict the K_eff. This work’s significance is that it allows geotechnical engineers to accurately estimate the K_eff of different types of concrete. In this way, the high time and cost required for the CSNBD test can be eliminated.

Effect of heat treatment on the microstructure and mechanical properties of 2.4 GPa grade maraging steel fabricated by laser powder bed fusion

The effects of three different heat treatments (direct aging, austenitizing–aging, and solutionizing–austenitizing–aging) on the meso-/micro-structures and the mechanical properties of the 18 wt% Ni-350 maraging steel (M350), specifically designed for ultra-high strength, additively manufactured using the laser powder bed fusion (LPBF) are studied. In both the as printed and directly aged conditions, the dendritic growth induced cellular structures constitute the unit microstructural features of the martensite; the martensitic transformation in these conditions occurs within the cell matrix and the cell boundaries act as the preferred sites for the austenite retention/reversion. The cell structures partially dissolve upon the austenitizing–aging heat treatment, while solutionizing fully erases them; both these heat treatments can facilitate the formation of the conventional packet–block–lath hierarchy, which is absent in the as printed and directed aged samples. Uniaxial tensile and mode I fracture toughness measurements reveal that the presence of the austenite in the directly aged M350 maraging steel lowers its strength but increases the fracture toughness. The absence of austenite in the other two heat treatment conditions results in high strength, but lower toughness. These findings shed light on understanding fundamental relationship between the unique features of the LPBF process and the martensitic transformation in maraging steels.

Improving the Accuracy of Fracture Toughness Measurement in Burst Experiments

Improving the Accuracy of Fracture Toughness Measurement in Burst Experiments

Integrated simulation, machine learning, and experimental approach to characterizing fracture instability in indentation pillar-splitting of materials

Measuring fracture toughness of materials at small scales remains challenging due to limited experimental testing configurations. A recently developed indentation pillar-splitting method has shown promise of improved flexibility in fracture toughness measurements at the microscale, partly due to the occurrence of an unusual fracture instability, i.e., a transition from stable to unstable crack propagation. In spite of growing interest in this method, the underlying mechanism of this phenomenon is yet to be elucidated. Here, we provide a comprehensive description of fracture instability in indentation pillar-splitting by combining in situ experiments with high-fidelity simulations based on cohesive zone and J-integral methods. In addition, a machine-learning-based solution for predicting the critical indentation load of fracture instability is established through Gaussian processes regression for broad use of this method by the community.

Remarkably high fracture toughness of HfNbTaTiZr refractory high-entropy alloy

• The fracture behavior of the Hfnbtatizr refractory HEA was investigated. • The HEA exhibits a remarkably high fracture toughness of K JIC = 211 MPa•m 1/2 . • Fully-ductile fracture mode with the formation of kink bands was observed. • The excellent damage-tolerance plus high strengths in a wide temperature range endow this HEA a promising structural material. High-entropy alloys (HEAs) with face-centered cubic (FCC) phase such as CoCrFeMnNi or CoCrNi generally exhibit ultra-high fracture toughness but relatively low strength. In contrast, body-centered cubic (BCC) HEAs often display higher strengths, but the few reports on fracture toughness measurement demonstrate low toughness values. Here we show that the BCC HfNbTaTiZr refractory HEA, which possesses a combination of high strength, good tensile ductility and excellent high-temperature properties, also exhibits a remarkably high fracture toughness. By using the "single specimen" compliance method for J -integral measurement according to the ASTM E1820–17 standard, its fracture toughness K JIC was experimentally determined to be 210 MPa m 1/2 , which renders this HEA among the toughest metallic materials. The excellent damage tolerance makes the HEA promising for applications as high-temperature structural materials such as in aerospace field.

Investigation on the Feasibility of Different Semicircular Bend Methods for Asphalt Concrete

Cracking of asphalt pavement is mostly caused by the mixed fracture of asphalt concrete. Determining a simple, repeatable, and accurate method is necessary for evaluating the fracture resistance of asphalt concrete. To explore the mixed fracture performance of asphalt concrete and determine the feasibility of different semicircular bend methods, the mixed fracture performances of asphalt concrete at medium and low temperatures were measured. The mixed fracture modes were realized through changing the position of the support and notch or changing the notch angle. After that, the crack propagating characteristics, crack initiation angle, and fracture toughness were analyzed. Results show that temperature has a significant impact on the fracture path, and crack initiation angle at low temperature follows the generalized maximum tangential stress (GMTS) theory. The measured fracture toughness ratios are lower than the theoretical value of GMTS criterion, but the established empirical model has higher accuracy. In the semicircular bend method, changing the positions of the support and notch is stable and repeatable. Therefore, this method may be preferred to evaluate the mixed fracture performance of asphalt concrete in the future.

Evaluation of the fracture toughness of butt-welded joints using the boundary effect model

Yielding loads (Py) from small three-point-bending (3-p-b) specimens with notch tips in the heat-affected zone, fusion zone, weld metal, and base metal of Q235 (common carbon steel in China) were used to estimate the corresponding fracture toughnesses. To model the elastic and plastic fracture around the Q235 welded joint, the boundary effect model initially developed for quasi-brittle fracture of heterogeneous solids with a large crack-tip fracture process zone was adopted in this study, bypassing the stringent ASTM standard requirements on the specimen size, initial crack length, and un-notched ligament. It was found that the weld metal had the highest fracture toughness value around 130 MPa√m, followed by the heat-affected zone of 109 MPa√m, the base metal of 83 MPa√m, and the fusion zone of 77 MPa√m. Microstructures and notch-tip plastic zones in each section of the welded joint were measured and used to explain the fracture toughness measurements.

Mechanical, fracture toughness, and Dynamic Mechanical properties of twill weaved bamboo fiber‐reinforced Artocarpus heterophyllus seed husk biochar epoxy composite

AbstractThis research aimed to find the effect of Artocarpus heterophyllus fruit seed husk biochar (ASB) and twill weaved bamboo fiber (TBF) in mechanical, DMA, and fracture toughness behavior of epoxy‐based composite. The surface of both weaved bamboo fiber and biochar was treated with amino silane before composite making. In this investigation, the laminates were fabricated by hand layup process and analyzed by appropriate ASTM standards. Results of this study revealed that the incorporation of 40 vol.% twill weaved bamboo fiber (EB) into the resin improved the mechanical properties. The load distribution was uniform with twilled weaving. For composite designation EBJ3, the maximum measured tensile strength was 168 MPa, the tensile modulus is 6.24 GPa, the flexural strength is 195 MPa, and the flexural modulus is 6.85 GPa, the izod impact is 6.24 J, and the hardness is 90 shore‐D. Similarly, the maximum storage modulus and lowest loss factor for the composite designation EBJ3 were measured to be 3.8 GPa and 0.4, respectively. Moreover, the maximum measured fracture toughness and energy release rate is 39.4 MPa.√m and 0.88 MJ/m2 for composite designation EBJ3 with 5.0% biochar and 40% bamboo fibers in volume. The SEM fractography revealed that the matrix molecules are adhered to the fiber, indicating enhanced bonding. These biomass waste converted fruit seed husk biochar and industrial crop bamboo fiber epoxy composites could be used as workable material in structural, defense, automotive, and domestic product‐making applications.

A critical evaluation of cortical bone fracture toughness testing methods

Cortical bone fracture mechanics which quantifies the tissue's resistance to fracture is widely regarded as important to finding key determinants of bone fragility and fracture. Currently, the most widely used fracture mechanics approach is the J-integral resistance (J-R) curve as defined in ASTM E1820 standard. This standard employs an unloading compliance (UC) method to estimate crack extension, necessary for fracture toughness and resistance curve (R-curve) quantification. Further, this UC method requires a series of unload-reload cycles to be conducted during the fracture test. However, cortical bone violates some assumptions on which the UC method is based, which are: no energy loss during the unload-reload cycles and any change in unloading compliance is only due to crack extension. Consequently, the aim of this study was to examine the impact of the UC method on the accuracy of fracture toughness measurement for bovine cortical bone. Ten pairs of single edged notched bend specimens were prepared from the posterior diaphysis of bovine tibiae and underwent three-point bending fracture tests. The paired specimens were divided into two groups: a cyclic loaded group and a monotonic loaded group. Further, crack extension was determined by the UC method for the cyclic group and by an optical method for both the cyclic and monotonic groups. From these, three different approaches were used to generate J-R curves from which three fracture toughness parameters were computed and compared between the three approaches. This comparison allowed the impact of crack extension estimation by the UC method as well as the unload-reload cycles on the accuracy of the fracture toughness measures to be assessed. Results show that the UC method underestimates crack extension by an average error of 73%. In addition, the combined effects from crack extension estimation using the UC method and the unload-reload cycles lead to a significant overestimation of the specimen's fracture toughness measures. This highlights the need for more studies to establish a standardized approach to cortical bone fracture testing.

Additive effect of micro-damage zones from nano-scale crack tip and nano-grains for fracture toughness measurements of 3Y-TZP zirconia ceramics

A nano-scale crack tip around 500 nm wide introduced by femtosecond laser still affects the accuracy of fracture toughness KIC measurements of 3Y-TZP zirconia ceramics with average grain size G from 200 to 500 nm. A simple formula was proposed to estimate the additive effect of crack-tip damage zones from an infinitely sharp crack to a nano-scale blunt notch. The error in fracture toughness measurements is less than 8 % if the nano-scale crack-tip width < 0.5·G. The intrinsic KIC can be deduced from the simple formula if the nano-scale crack tip > 0.5·G. This study shows the same KIC was deduced from two different sets of 3Y-TZP measurements with nano- and micro-scale notches of 500 nm and 18 µm wide. Furthermore, the simple formula specifies the relation between the fracture toughness KIC and intrinsic strength ft via grain size G, which means KIC can also be estimated from ft and G without testing pre-cracked specimens. KIC values of 3Y-TZP from specimens with and without pre-cracks were compared.

Evaluations on Ceramic Fracture Toughness Measurement by Edge Chipping

Advanced ceramics, such as alumina and zirconia, are widely used in modern industries, due to their superior properties, such as high hardness and strength. Fracture toughness is a significant property that describes the capability of materials to resist crack instability and expansion to failure, and also helps when calculating the allowable working stress, and crack size, of the component. This paper comprehensively lists the current toughness-testing methods. With comparative investigations of various methods, edge chipping is found to be the simplest way of measuring the ceramic fracture-toughness, in terms of the dominant median crack, chip geometrical similarity, and force-distance relations, giving consideration to its potential application in industry. Moreover, to avoid the data variance from crack-length measurement in edge chipping, it is further proposed that the energy analyses can be used to improve the way in which the toughness expression is formulated.

Influence of Block Copolymer Concentration and Resin Crosslink Density on the Properties of UV‐Curable Methacrylate Resin Systems

AbstractAdditive manufacturing is on the verge of replacing established processes in dentistry, as it offers the possibility of manufacturing individual parts simply and cost‐effectively. Due to its suitability for a wide variety of materials and, above all, its high precision, the focus is currently on stereolithographic processes. Intrinsic brittleness of the used multifunctional acrylic monomers remains however one of the major challenges. One promising concept is the use of block copolymers (BCPs) guaranteeing minor effects on 3D‐printing processing and UV‐curing due to initially at least partial solubility, and hence low viscosity impact. A polycaprolactone‐polysiloxane (PCL‐PDMS‐PCL) triblock copolymer is synthesized via ring‐opening polymerization of caprolactone and used in radical UV‐cured methacrylic resin systems. Small angle X‐ray scattering measurements reveal the self‐assembly of the BCPs to objects of around 20 nm prior to curing. Subsequently, thermo‐mechanical characterization is carried out by dynamic mechanical analysis, flexural testing, and fracture toughness measurements (KIC). Transmission electron microscopy and scanning electron microscopy micrographs show a homogenous distribution of the BCPs and effective toughening via cavitation and shear yielding. The influence of the crosslink density on the toughness and the high effectiveness of block copolymers for improving fracture toughness is clearly shown.

Novel XFEM variable strain damage model for predicting fracture in small-scale SENT and full-scale pipe tests

The tensile strain capacity (TSC) of a welded pipeline under bending and/or axial loads is critical to pipeline integrity assessment, and the fracture toughness expressed as a J-R curve plays a key role in predicting the TSC. The single edge notched tension (SENT) tests have been recommended for fracture toughness measurement of pipelines due to the similar low constraint geometries with the full-scale pipelines. The extended finite element method (XFEM) has been increasingly implemented in predicting the fracture of pipelines from small-scale to full-scale tests. However, current damage criteria based on a fixed critical stress or strain value have not been well calibrated, and limitations have been found. This paper develops a novel XFEM variable strain-based damage model by adopting a critical varying strain profile that accounts for the crack-tip constraint derived from a modified Mohr-Coulomb (MMC) fracture criterion. The novel damage criterion is calibrated by reproducing results like those measured from small-scale SENT tests of X100 pipe steel and full-scale pressurized tests of X52 circumferentially surface-cracked pipeline. Two sets of optimal damage parameters are calibrated specifically for X100 and X52. The initial crack in each model is simulated by cutting a notch in the XFEM domain resembling the original crack pre-machined on the specimen. Previously, researchers inserted a XFEM planar crack into the finite element mesh. The comparisons between results from notched models and planar cracked models reveal a distinct difference in the crack tip constraint and fracture resistance. Predictions from the novel variable damage criterion demonstrate advantages over those from the simple fixed damage criterion.

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.

Experiments on the mode II fracture toughness in ENF tests of CFRP curved beams

The purpose of this study is to establish end-notched flexure (ENF) tests of curved beam specimens cut from tanks or cylinders. To provide data reduction methods for experiments, a compliance-based beam method (CBBM) for curved beams in ENF tests is proposed based on the previously established Euler–Bernoulli curved beam theory formulations of ENF tests. ENF tests were conducted using curved beam specimens with three different curvature radii, and the results were in good agreement with the curved beam theory. In addition, the fracture toughness values obtained from the proposed CBBM for curved beams were validated using three-dimensional finite element (FE) analysis. The results revealed that the FE analysis with the measured fracture toughness agreed well with the experimental load–displacement curves, verifying the validity of the obtained fracture toughness values for curved beams.

The rising crack resistance curve behavior and mechanism of La2O3 doped zirconia toughened alumina composites prepared via vat photopolymerization based 3D printing

In this study, we combined liquid precursor infiltration of high introduction amounts of bi-additives (20 wt%) and vat photopolymerization-based 3D printing to fabricate zirconia toughened alumina, and the infiltration systems consist of the four following systems Zr4+/La3+, Zr4+/Ce4+, Zr4+/Gd3+, and Zr4+/Er3+. The sample immersed with Zr4+/La3+ shows intense peaks of m-ZrO2 phase compared to the other samples while a new phase of rod-like LaAl11O18 grain occurs in the Zr4+/La3+ immersed sample, the existence of which could be confirmed by XRD and EDS. The fracture toughness of the Zr4+/Ce4+, Zr4+/Gd3+, and Zr4+/Er3+samples remained basically unchanged versus the crack size, with the fracture toughness between 4 MPa m1/2 and 5 MPa m1/2; while the measured fracture toughness values for the Zr4+/La3+ system show a rising crack resistance curve behavior, with the toughness enhanced from 6.37 MPa m1/2 to 8 MPa m1/2. The obvious enhancement of the toughness could be attributed to thermal expansion misfit and rod-like LaAl11O18 in the Zr4+/La3+ system. The toughening mechanism and the effect of residual stresses on the fracture mode are discussed on the basis of theoretical calculation and analysis. The rising crack resistance curve behavior could provide a new toughening strategy in ceramic 3D printing.