Investigation of Critical Stress Intensity Factors for AISI 4340 and ASTM A533 Alloy Steels at Different Murakami Area Parameters

Micro-crack initiation at tip of a rigid line inhomogeneity in piezoelectric materials

Mixed-mode dynamic fracture parameters for soda-lime glass

Micro-crack initiation at the tip of a semi-infinite rigid line inhomogeneity in piezoelectric solids

The second-order statistics of critical stress intensity factor (SIF) of single edge notched fiber reinforced composite plates with random system properties and subjected to uniaxial tensile loadings is investigated. This paper is an extension of reference (Lal and Kapania, 2013) by the present authors by considering more number of input random system parameters for higher accuracy. A C0 finite element method based on a higher-order shear deformation plate theory using displacement correlation method via isoparametric quarter point element is proposed for basic formulation. A stochastic finite element method using first-order perturbation technique and Monte Carlo simulation (MCS) is employed to examine the mean, coefficient of variance, and probability density faction of critical first mode SIF. The effect of different fiber orientations, crack length, plate thickness, a number of layers, and the lamination schemes with random system properties on the statistics of SIF of single edge crack laminated composite plate is evaluated. The tensile failure load is predicted using Hashin’s failure criteria. The present approach is validated with results available in literature and by employing independent MCS.

The cracked chevron notched semi-circular bending (CCNSCB) method for measuring the mode I fracture toughness of rocks combines the merits (e.g., avoidance of tedious pre-cracking of notch tips, ease of sample preparation and loading accommodation) of both methods suggested by the International Society for Rock Mechanics, which are the cracked chevron notched Brazilian disc (CCNBD) method and the notched semi-circular bend (NSCB) method. However, the limited availability of the critical dimensionless stress intensity factor (SIF) values severely hinders the widespread usage of the CCNSCB method. In this study, the critical SIFs are determined for a wide range of CCNSCB specimen geometries via three-dimensional finite element analysis. A relatively large support span in the three point bending configuration was considered because the fracture of the CCNSCB specimen in that situation is finely restricted in the notch ligament, which has been commonly assumed for mode I fracture toughness measurements using chevron notched rock specimens. Both CCNSCB and NSCB tests were conducted to measure the fracture toughness of two different rock types; for each rock type, the two methods produce similar toughness values. Given the reported experimental results, the CCNSCB method can be reliable for characterizing the mode I fracture toughness of rocks.

The interaction between nanotwin and dislocation pileup at twin boundary and interface cracks are of vital importance in studying the toughening mechanism of nanocrystalline materials. We have developed a theoretical model to discuss the effects of nanotwin and dislocation pileup on dislocation emission from an interfacial collinear crack tip in nanocrystalline bimaterials. The nanotwin as a stress source can be descripted by two disclination dipoles, and some dislocations produced by the grain boundary pile up at twin boundary. Using the complex potential method, the complex form of dislocation force and critical stress intensity factors (SIFs) corresponding to dislocation emission under remote plane loadings are deduced. Through numerical analysis, it is discussed that the effects of twin strength, twin orientation, twin size, twin position, dislocation pileup, crack length, and material constant on the critical SIF corresponding to dislocation emission in detail. The results show that the nanoscale twin and the dislocation pileup at the twin boundary will hinder dislocation emitted from the collinear crack tip and reduce toughness caused by dislocation emission. There is a critical nanotwin size making the critical SIF equal to zero and a best twin position minimizing the value of the critical SIF making dislocations emission easiest. The shear modulus of the two half plane have great influence on the critical SIF.

The initiation of a longitudinal shear crack (mode III fracture) in homogeneous structures and weld joints of elastoplastic materials with the limiting strain is considered. To obtain critical fracture parameters, a coupled (sufficient) fracture criterion in an elastoplastic material is used. Simple relations for critical shear stresses, lengths of pre-fracture zones and critical stress intensity factor (SIF) for mode III fracture were obtained. There is a limiting transition from a sufficient fracture criterion to the necessary criterion, when the length of the prefracture zone tends to zero. Critical stresses obtained according to necessary and sufficient criteria differ significantly. In the framework of the proposed model, the critical SIF obtained by a sufficient criterion is a variable, it depends on the grain diameter, the parameters of the material strain diagram and the crack length. The rule of selection of the parameters of the proposed fracture model is indicated by two laboratory experiments: by approximating the material strain diagram with a simple shear and critical SIF. The obtained critical parameters are conservative estimates of critical stresses. These estimates should be used in the calculation of steel structures operating at temperatures below the cold brittleness.

The chirality-dependent mixed-mode I-II fracture toughness and crack growth angles of single-layer graphene sheets are determined using molecular dynamics (MD) simulations and the finite element (FE) method based on the boundary layer model, respectively. The carbon–carbon bond in the FE method is equivalent to a nonlinear Timoshenko beam based on the Tersoff–Brenner potential. All the results of the present FE method agree well with those of our MD simulations performed using the REBO potential. The chiral crack angles of α = 0° (zigzag), 15°, 30° (or 90°, armchair), and 45° at different loading angles from 0° ≤ φ ≤ 90° (φ = 90° for mode I and φ = 0° for mode II) are studied. The present results show that both critical stress intensity factors (SIFs) and crack growth angles strongly depend on the chiral angle α, the dimensions [in two-dimensional (2D) or three-dimensional (3D) states], as well as the temperature, for a given loading angle φ. The critical equivalent SIFs change from 2.52 to 4.07 nN Å−3/2 in the 2D state and from 2.46 to 5.06 nN Å−3/2 in the 3D state at different loading angles. The SIFs are around one order of magnitude smaller than those of ordinary steel, which indicates that chiral graphene is remarkably brittle in contrast to its ultrahigh strength. These findings should be of great help in understanding the chirality-dependent fracture properties of graphene sheets and designing graphene-based nanodevices.

Investigation of role of alloy microstructure in hydrogen-assisted fracture of AISI 4340 steel using circumferentially notched cylindrical specimens

The continuous enhancement of reliability and durability requirements for many machinery components is significantly pushing the experimental research on the Very-High-Cycle Fatigue (VHCF) response of metallic materials. In order to significantly reduce testing time, ultrasonic testing machines are widely adopted when carrying out VHCF tests. Several fundamental material properties can be estimated from the fracture surfaces of specimens failed during ultrasonic VHCF tests. In the VHCF literature the critical Stress Intensity Factor (SIF) is generally estimated by applying analytical SIF formulations to the typical semi-circular surface crack geometry revealed by fracture surfaces at final failure. However, when subjected to ultrasonic VHCF tests, analytical SIF formulations valid for static loading conditions could eventually lead to significant estimation errors. The present paper aims at comparing the critical SIF at failure estimated from conventional analytical formulations and from Finite Element Models (FEMs). The fracture surfaces of two specimens with different shapes (Hourglass and Gaussian) are taken into account for modeling crack geometry at final failure. Through an implicit solving procedure, the critical SIF in resonance condition at 20 kHz is computed from the 3D geometry of the cracked specimens and compared with the corresponding analytical prediction.

A localized stress field approach for calculating the critical stress intensity factor for an isotropic solid at atomistic scale

Subcritical growth of a crack interacting with the stress field of mobile solutes in an elastic solid in the presence of image effects: A kinetic Monte Carlo study

A semi-quantitative model of pop-in behavior

This paper explores the fracture toughness of mild steel through experimental and finite element mixed-mode loading modeling. The experiment set the plate of size rectangular of a through-edge inclined crack to find out the critical stress. The experimental results were then applied as input for modeling the specimen in ANSYS, where both the Mode I and Mode II stress intensity factors were computed. The hoop stress approach obtained the maximum hoop stress theory by use of which the critical stress intensity factor is calculated, which shows the fracture toughness of the material. These showed that the mild steel fracture toughness was between 53 and 78 MPa/m1/2. An experimental parametric study of crack length as well as crack inclination on stress intensity factors was carried out, giving insightful conclusions regarding material behavior in fracture in mixed-mode conditions.

A review is presented of the experimental data on subcritical crack growth in geological materials. The main parameters describing subcritical crack growth are the critical stress intensity factor Kc, the subcritical crack growth limit Ko, and the stress intensity factor‐crack velocity (K‐v) relationship between Koand Kc. The K‐v data are presented in terms of an equation in which the crack velocity depends on stress intensity factor raised to a power n because this is common practice in experimental studies. These data are presented as tables and in synoptic diagrams. For silicates the value of n increases as the environment becomes depleted in hyroxyl species and with increase in the microstructural complexity of the solid. Values of n as low as 9.5 have been found for tensile cracking of quartz in basic environments and as high as 170 for tensile cracking of basalt in moist air. Insufficient experimental data are available to predict subcritical crack growth behavior at depth in the earth's crust without major extrapolations of the data base. Schematic outlines are presented, therefore, of the probable influence on subcritical crack growth of some key parameters in the crustal environment. These include stress intensity factor, temperature, pressure, activity of corrosive environmental agent, microstructure, and residual strains. In addition, a discussion is presented of the likely magnitude of the subcritical crack growth limit. For stress corrosion tensile crack growth of quartz a limit of approximately 0.2 of the critical stress intensity factor is inferred from theoretical calculations. Further problems discussed with regard to the extrapolation of experimental data to crustal conditions include the choice of a suitable equation to describe crack growth and the magnitude of parameters in these equations. A brief discussion of the double torsion testing method is presented in order to aid the interpretation of experimental results because it is almost the sole method used to study subcritical cracking in rocks.