Decrease In Stress Intensity Factor Research Articles

Effect of different hole shapes on the stress intensity factor of the finite plate with edge crack

Generally, flaws or defects are formed while manufacturing the components either internally or externally. Not all such flaws are unstable under service conditions. Cracks are often developed in the structural member due to high-stress concentration factors. Fracture mechanics help us in evaluating the life and safety of structures in the presence of cracks. This is accomplished by analyzing the intensity of the crack condition by estimating certain fracture parameters. This study aims at evaluating the mode I stress intensity factor (SIF) of a finite plate with edge crack in the presence of different hole shapes. The holes are placed along and perpendicular to the crack axis. The objective is to study the effect of the size, number, and shape of holes on the mode I SIF through finite element simulation. A 2D finite plate with an edge crack submitted to tensile loading was validated against analytical results. It was found from the simulation the existence of a range of distance from the crack tip within which when the holes are placed, results in the decrease of SIF. Beyond a certain distance that is far from the crack tip, the introduction of holes does not affect the SIF. When the elliptical holes (either single or multiple) was placed along the crack axis, it reduces the SIF effectively better than the circular and square hole. On the other hand, a single square hole when placed about the vertical axis, was found to diminish the SIF effectively relative to circular and elliptical holes. Similarly, the multiple circular holes reduce the SIF significantly more than the square and elliptical holes about the vertical axis.

Fatigue crack propagation behaviors in Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with STA and BASCA heat treatments

Fatigue crack propagation behaviors of a novel high strength Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with equiaxed and lamellar microstructures were systematically investigated. Fatigue crack propagation tests at both increasing and decreasing stress intensity factor range values were carried out with a frequency of 15 HZ and stress radio R of 0.1 by using compact tension samples. The sample with lamellar microstructure displayed the faster fatigue crack propagation rates in near-threshold regime compared to the sample with equiaxed microstructure. The fatigue threshold (ΔKth) was 2.7 MPa·m1/2 for lamellar microstructure, and 3.2 MPa·m1/2 for equiaxed microstructure. The steady state crack growth rate in lamellar microstructure was slightly slower than that in equiaxed microstructure in the Paris regime, while much faster in the rapid growth regime. The interactions between crack path and microstructures as well as the microscopic fracture surface morphology in both microstructures were detected and analyzed by scanning electron microscope (SEM), revealing the different crack propagation behaviors in three regimes. The results showed that the crack propagation behaviors were a result of two contributing factors: interface obstacles and crack front profile. The increased fatigue crack propagation resistance of equiaxed microstructure in near-threshold regime can be attributed to the interface obstacles which dominated over the rougher crack front profile. While the higher fatigue crack growth resistance for lamellar microstructure in Paris regime was attributed to the more positive effect of rougher crack front profile. In the rapid growth regime, the crack in lamellar microstructure was found to be extended along the grain boundary α, which was regarded as a low energy crack path and led to the faster crack propagation rate.

A Study of Fatigue Crack Growth Rate in Steels in Relation to Crack-Tip Plastic Deformation and Fracture. Part 1. Test Methods and Results for 10GN2MFA Steel

The methods for low-cycle fatigue testing and cyclic crack growth testing of 10GN2MFA steel at 20 and 270°C are described. Based on test results, fatigue crack growth rate curves are plotted and the characteristics of low-cycle plastic deformation and fracture are found. A difference in the kinetics of low-cycle plastic deformation is shown – the cyclic softening at 20°C and hardening at 270°C – which has an influence on fatigue life and fatigue crack growth resistance of this steel. The last-mentioned characteristic has been studied using two test methods: (i) with a decreasing stress intensity factor range and (ii) with a constant magnitude of load.

Influence of crack closure and local near-tip stress on crack growth life estimation

Currently, among other models for crack growth prediction, crack closure models that consider the decrease in stress intensity factor (SIF) range associated with cyclic loading asymmetry are more popular. One of drawbacks of these models is impossibility of considering the loading history sequence.The theory related threshold SIF range and ΔKth and local near-tip stresses, induced by overloads, and postulated that the overload effect in the near-threshold region of growth rates is caused by residual local stresses. The proposed model applies the local stress and strain approach to estimate stress σ* in the stress concentration region for fatigue analysis. This region is characterized by local near-tip stress σ*, whose amplitude is determined by cyclic inelastic reaction at crack tip. Further development of the model is due to varying nature of the threshold SIF range ΔKth.As a basis, the Forman-Mettu formula was adopted, which describes the fatigue crack growth curve in all three regions of fatigue crack growth rate. The range ΔK was determined by the peak load ΔP of the load history. The crack closure was considered by the Schijve equation, considering the asymmetry of the half-cycle U = f(R), and effective SIF was estimated by ΔKeff = ΔK*U. Given known SIF range ΔK, the value of the local stress σ* at distance from the crack tip r* was determined for each half cycle by Neuber and Ramberg-Osgood equations, and threshold SIF was estimated from the analytical formula of Кth=f(σ). Thus, known loading history made it possible to determine ΔKeff, Kmax, and Kth on each cycle for fatigue life estimation.Mathematical modeling of fatigue crack growth life, especially in near-threshold region of its growth, according to the Sunder’s scheme, showed that investigated aluminum alloy 2024-T3 exhibited crack growth sensitivity to various types of force action, including various types of random loading.

Phase field simulations on domain switching-induced toughening in ferromagnetic materials

Abstract A real-space phase field model is employed to investigate the domain switching-induced shielding or anti-shielding of a crack in ferromagnetic materials with single and multi-domain states. Phase field simulations demonstrate that magnetization switching takes place from the crack tip due to the highly concentrated stress in a ferromagnetic thin plate with a stationary crack. Based on the stress obtained from phase field simulations, the crack-tip stress intensity factors (SIFs) are calculated by linear extrapolation in the ferromagnetic thin plate subjected to different values of mechanical loads. The calculation results indicate that domain switching decreases the crack-tip stress intensity factors, resulting in domain-switching toughening in ferromagnetic materials. Furthermore, the domain switching in the multi-domain ferromagnetic plate induces a much larger decrease of stress intensity factor than that in the single-domain one, which suggests that engineering magnetic domains leads to the design of tougher ferromagnetic materials.

Origin of the Static Fatigue Limit in Oxide Glasses

Oxide glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. The log of crack velocity decreases linearly with decreasing stress intensity factor in Region I. For some glasses, at a lower stress intensity, Ko, log v asymptotically diminishes where there is no measurable crack growth. The same glasses exhibit static fatigue, or a decreasing strength for increasing static loading times, as cracks grow and stress intensity eventually reaches the fracture toughness. In this case, some glasses exhibit a low stress below which no fatigue/failure is observed. The absence of slow crack growth under a low stress intensity factor is called the fatigue limit. Currently, no satisfactory explanation exists for the origin of the fatigue limit. We show that the surface stress relaxation mechanism, which is promoted by molecular water diffusion near the glass surface, may be the origin of the fatigue limit. First, we hypothesize that the slowing down of slow crack growth takes place due to surface stress relaxation during slow crack growth near the static fatigue limit. The applied stress intensity becomes diminished by a shielding stress intensity due to relaxation of crack tip stresses, thus resulting in a reduced crack velocity. This diminishing stress intensity factor should result in a crack growth rate near the static fatigue limit that decreases in time. By performing Double Cantilever Beam crack growth measurements of a soda‐lime silicate glass, a decreasing crack growth rate was measured. These experimental observations indicate that surface stress relaxation is causing crack velocities to asymptotically become immeasurably small at the static fatigue limit. Since the surface stress relaxation was shown to take place for various oxide glasses, the mechanism for fatigue limit explained here should be applicable to various oxide glasses.

Determination of fracture parameters for a bi-material center cracked plate subjected to biaxial loading using FEOD method

Article history: Received 6 October, 2015 Accepted 27 February2016 Available online 27 February 2016 Fracture parameters of a bi-material plate containing a cener crack and subjected to biaxial tensile loading was calculated numerically. Based on the crack tip stress field obtained numerically in a bi-material joint and using the finite element over deterministic (FEOD) method, the stress intensity factors (KI and KII) and also non-singular T-stress terms, were determined for different material properties and biaxial loading cases. Due to asymmetry of loading and material properties in the investigated dissimilar plate, the center crack experinces mixed mode I/II fracture in general. By increasing the bi-material constant value, which shows the difference between the mechanical properties of two materials, the amplitude of stress intensity factor decreases. The obtained results from this method were in good agreement with the displacment field method prevousiuly reported by other researchers. © 2016 Growing Science Ltd. All rights reserved.

Effect of microstructure on the fatigue crack growth behaviour of a near-α Ti alloy

Fatigue crack growth behaviour of Ti–6Al–2Zr–1.5Mo–1.5V (VT-20 a near-α Ti alloy) was studied in lamellar, bimodal and acicular microstructural conditions. Fatigue crack growth tests at both increasing and decreasing stress intensity factor range values were performed at ambient temperature and a loading ratio of 0.3 using compact tension samples. Lamellar and acicular microstructures showed lower fatigue crack growth rates as compared to the bimodal microstructure due to the tortuous nature of cracks in the former and the cleavage of primary α in the latter. The threshold stress intensity factor range was highest for acicular microstructure.

Micromechanics of brittle creep in rocks

In the upper crust, the chemical influence of pore water promotes time dependent brittle deformation through sub‐critical crack growth. Sub‐critical crack growth allows rocks to deform and fail at stresses well below their short‐term failure strength, and even at constant applied stress (“brittle creep”). Here we provide a micromechanical model describing time dependent brittle creep of water‐saturated rocks under triaxial stress conditions. Macroscopic brittle creep is modeled on the basis of microcrack extension under compressive stresses due to sub‐critical crack growth. The incremental strains due to the growth of cracks in compression are derived from the sliding wing crack model ofAshby and Sammis(1990), and the crack length evolution is computed from Charles' law. The macroscopic strains and strain rates computed from the model are non linear, and compare well with experimental results obtained on granite, low porosity sandstone and basalt rock samples. Primary creep (decelerating strain) corresponds to decelerating crack growth, due to an initial decrease in stress intensity factor with increasing crack length in compression. Tertiary creep (accelerating strain as failure is approached) corresponds to an increase in crack growth rate due to crack interactions. Secondary creep with apparently constant strain rate arises as an inflexion between those two end‐member phases. The minimum strain rate at the inflexion point can be estimated analytically as a function of model parameters, effective confining pressure and temperature, which provides an approximate creep law for the process. The creep law is used to infer the long term strain rate as a function of depth in the upper crust due to the action of the applied stresses: in this way, sub‐critical cracking reduces the failure stress in a manner equivalent to a decrease in cohesion. We also investigate the competition with pressure solution in porous rocks, and show that the transition from sub‐critical cracking to pressure solution dominated creep occurs with increasing depth and decreasing strain rates.

Effect of Hole Location on the Load-Carrying Capacity of Laminated Veneer Lumber Beams

Predicting load-carrying capacity of timber beams with holes requires a model capable of accounting for the microscopic material behaviour that influences crack initiation and propagation. The complex stress distribution around the periphery of a hole causes additional tension perpendicular to grain stresses, which can change the failure mode of the beam. This situation can also be affected with a change of hole location within the beam depth because stress intensity factor will be increased by tensile stresses and decreased by compressive stresses. This is not an unlikely situation as services often have to pass through beams at different depths. This paper investigates the effect of changing the hole location through the depth of laminated veneer lumber (LVL) beams utilising an experimental and numerical investigation. Experimental tests to failure of LVL beams and numerical simulations using finite element methods show that for a hole eccentricity of less than 20% of the beam depth, the load-carrying capacity of the beam does not change significantly. For uniformly distributed loading, a linearly decreasing stress intensity factor from the support to mid-span is exhibited, showing an increase in load-carrying capacity as the opening approached mid-span.

Fractographic analysis of fatigue crack growth in lightweight integral stiffened panels

PurposeThe purpose of this paper is to complement available macroscopic fatigue crack growth measurements in flat stiffened panels with scanning electron microscopy (SEM) measurements of striation spacing.Design/methodology/approachThe paper's approach is fatigue testing of two‐stiffener flat panels manufactured using three different processes, with a central initial crack perpendicular to the stiffeners and load, in order to identify striation spacing during crack growth up to final fracture, using SEM.FindingsAn increase of striation spacing as cracks grow was quantified. Although when cracks approach the stiffeners the stress intensity factor decreases, there is no clear decrease of striation spacing in that region. Striation spacing is roughly similar to macroscopic crack‐propagation rate da/dN measured in the panels testing. This observation is no longer valid once the stiffeners are reached; this stage is characterized by fast acceleration of the cracking process until final complete rupture is verified, and macroscopic crack growth measurements are made difficult because of the “T” geometry in that region.Originality/valueA complete picture of the striation spacing during the fatigue crack growth up to final fracture of a two‐stiffener flat panel is provided for three different manufacturing processes: high‐speed machining, laser beam welding and friction stir welding.

Primary water stress corrosion cracking in alloy 600 and intergranular crack growth rate

The effects of the stress intensity factor and dissolved hydrogen on the crack growth rate in primary water reactor environments was investigated with alloy 600 with continuous carbide films at grain boundaries. A primary water stress corrosion cracking of intergranular fracture mode was observed. The intergranular crack growth rate decreased with a decreasing stress intensity factor, and the dissolved hydrogen content influenced the crack growth rate. Such results are consistent with those reported previously.

Crack arrest in high cycle thermal fatigue crazing

Thermal crazing in high cycle thermal fatigue due to thermal fluctuation in residual heat removal (RHR) system of some nuclear power plants is explained by crack arrest in the depth due to a decreasing stress intensity factor. This is related to high frequencies of thermal loading. An attempt has been made through a parametric study to acquire some knowledge about the loading, knowing the crack depth. For this purpose, analytical as well as finite element simulations of crack propagation in 2D- and 3D-semi-elliptical cracks have been performed. In periodic loading, bounds for the number of cycles to fatigue life are proposed. Moreover, it is shown that in the absence of mean stress, fatigue damage in RHR may be produced in the macroscopic elastic-plastic regime. Finally, it is shown by FE simulations that for a semi-elliptical crack, a small error on stress intensity factor may result in significant error on crack length at high number of cycles, due to error accumulation cycle by cycle. Moreover in this paper is given the reason as to why shielding effect has not been taken into account in the study of crack arrest in RHR.

Threshold stress intensity factor for delayed hydride cracking in Zr–2.5%Nb pressure tube alloy

Delayed hydride cracking (DHC) velocity was determined at 203, 227, 250 and 283 °C using 17 mm width curved compact toughness specimens machined from an unirradiated Zr–2.5 wt.% Nb pressure tube spool, gaseously charged with 60 ppm of hydrogen by weight. Single CT specimen was used to determine DHC velocity at a constant temperature for a range of stress intensity factor ( K I) obtained by load drop method. For a given temperature and K I > 15 MPa m 1/2, DHC velocity was found to be practically independent of K I. For 15 > K I > 10 MPa m 1/2, DHC velocity decreased significantly with decrease in stress intensity factor and extrapolation of the data suggested the threshold stress intensity factor to be about 9–11 MPa m 1/2 in the aforementioned temperature range. The activation energy associated with DHC was observed to be 35.1 kJ/mol.

Effect of the transitional gradient of material property on the mechanical behavior of a non-homogeneous interlayer

The main idea of this paper is to study the effects of the transitional gradient of material property at interface on the mechanical behavior of a bi-interface structure with a sandwiched non-homogeneous interlayer. Four different types of transitional functions of material property are put forward and their influences are analyzed by the finite element method. It is indicated that, to diminish the value of the gradient of Young’s modulus at interface is beneficial to decreasing the stress concentration at interface. Under the mode-I loading, among all the mode-I and mode-II stress intensity factors at both interfaces, the dominant one is always the mode-I stress intensity factor at the more rigid interface. With the gradient of Young’s modulus at interface decreasing, the SIFs corresponding to the two different interfaces vary inversely, however, the dominant stress intensity factor decreases. To reduce the gradient of material property at interfaces, i.e., to reduce the weak-discontinuity of interfaces, will be beneficial to enhancing the ability of bi-interface structure against fracture.

621 焼戻脆化を受けた 2.25Cr-1Mo 鋼の水素脆化感受性

A large 3.5 inch thick compact tension specimens (CT) were hydrogen pre-charged in an extra high capacity autoclave and crack growth tests were conducted on temper embrittled 2.25Cr-1Mo steels. The recent generation steel with high purity, less temper embrittlement susceptibility, showed sub-critical flaw growth over 10 days at room temperature. The crack growth rate observed was relatively constant rate at 5 to 8x(10)^<-5> mm/sec regardless of the slow increase or decrease in stress intensity factor at the crack tip. On the other hand, the temper embrittled 60's generation steel showed brittle, fast fracture at low stress intensity factor (K_1=45 MPa√<m>). Since the fracture toughness of this steel without hydrogen condition was higher than this, it is considered that the fast fracture was occured due to the presence of hydrogen. This fast fracture took place remarkably below 86℃.

湿度を制御した環境中における高炭素クロム軸受鋼の疲労挙動

The present paper describes the effect of humidity on the fatigue behavior of a high carbon chromium steel having very high strength. Cantilever-type rotary bending fatigue tests were performed in controlled relative humidity of 5, 45, 55 and 85%. It was found that step-wise S-N curves were seen in relatively lower humidity up to 55%. The transition stress at which the crack initiation site changed from the surface to the subsurface decreased with increasing humidity. In high humidity of 85%, however, cracks were always generated at the specimen surface regardless of stress level, thus step-wise S-N curve was not observed. The decrease of the transition stress and the disappearance of step-wise S-N curve in the high humidity were attributed to humidity-assisted surface crack initiation. At stress levels where surface or subsurface crack initiation occurred, fatigue lives were nearly the same regardless of humidity because of shorter time to fracture in the former and crack initiation from an inclusion inside the specimens in the latter. In subsurface fracture, a fish-eye was always seen with an inclusion at its center from which cracks initiated, and the morphology of fish-eyes was independent of humidity. Fatigue life increased with decreasing stress intensity factor range for inclusion, which was not influenced by humidity.

230 形状記憶TiNiファイバ強化エポキシ樹脂複合材料のき裂閉鎖作用(OS 多機能・複合機能化への材料協調設計)

In this research, the composite material with shape memory function is used as model material of photoelastic experiment. Photoelastic experimental hybrid method is developed in this research, it is assured that it is useful on the obtaining stress intensity factor and the separation of stress components from only isochromatic data. The measuring method of stress intensity factor of intelligent material by photoelastic experiment is introduced. As the result, the fracture resistance of the matrix (base material) as well as the TiNi fiber improved. This is due to decreasing stress intensity factor K_I caused by raising the ambient temperature in the iso-temperature furnace.

Analysis of Propagating Crack In Isotropic Material under Dynamic Mode I Constant Displacement

It has been reported that the dynamic stress intensity factor for a propagating crack is increasing or decreasing according to the increasement of the crack propagating velocity. It is confirmed in this study that the increasement or decreasement of stress intensity factor with crack growing velocity is accused by loading condition. When the crack propagates under a constant displacement along upper and lower boundary in finite plate, the dynamic stress intensity factor decreases according to the increasement of the propagating crack velocity. When the crack propagates under a constant stress along upper and lower boundary in finite plate, the dynamic stress intensity factor increases according to the increasement of the propagating crack velocity. The increasement or decreasement of stress intensity factor with crack growing velocity is greater in a fast crack propagation velocity than in a slow one.

Liquid metal embrittlement mechanism

Liquid metal embrittlement was studied in the following two aspects. First the first principle and Chen-Nanxian three-dimensional lattice reverse method were employed to obtain the effective potentials for Al-Ga and Ga-Ga. Then with the molecular dynamics simulation, the influence of liquid metal adsorption on dislocation emission was studied. The simulated result shows that after Ga atoms are adsorbed on the crack plane in Al crystal, the critical stress intensity factor decreases, which changes from 0.5 MPam1/2 (without adsorption) to 0.4 MPam1/2 (with adsorption). The reason for the reduction in the critical intensity stress factor is that Ga adsorption reduces the surface energy of the crack plane. Moreover, 7075 Al alloy adsorbing liquid metal (Hg+3atm%Ga) wasin-situ studied in TEM by using a special constant deflection device. The experimental result showed that liquid metal adsorption could facilitate emission, multiplication and motion of dislocations. When this process reached a critical condition, the brittle microcrack was initiated and propagated from the crack tip or in a dislocation free zone.