Application Of Fracture Mechanics Research Articles

On the coupled FE-SBFE method for fracture mechanics applications

On the coupled FE-SBFE method for fracture mechanics applications

Application of probabilistic fracture mechanics in evaluation of existing riveted bridges

Many existing riveted bridges in China are often required to carry an increasing volume of traffic, usually consisting of vehicles heavier than the original design. Recently Chinese government and engineers have paid more attention to the actual fatigue life and service safety of such infrastructures, and have decided to research evaluation methods using modern technologies. Therefore, based on investigation of fatigue – damage accumulation theory and probabilistic fracture mechanics, the evaluation methods have been developed to calculate the deterministic and probabilistic remaining fatigue life of existing riveted bridges. According to the fatigue failure mechanism of existing riveted bridges, a simple fracture evaluation model and a single-angle probabilistic fracture failure model (SAPFFM) were proposed, and based on Monte-Carlo method, a fatigue reliability analysis program was developed. Finally, as a case study, the fatigue evaluation models were used to predicate the fatigue safety of Zhejiang St...

Oxide films, pores and the fatigue lives of cast aluminum alloys

In the absence of gross defects such as cold shuts, the fatigue properties of castings are largely determined by the sizes of microstructural defects, particularly pores and oxide films. In contrast, the effects of grain size, second-phase particles, and nonmetallic inclusions are insignificant. The authors review the fatigue properties of castings made by gravity die casting, sand casting, lost-foam casting, squeeze casting, and semisolid casting, and compare A356/357 alloys with 319-type alloys. The application of fracture mechanics enables the properties to be rationalized in terms of the defects that are characteristic of each casting process, noting both the sizes and types of defect. The differences in the properties of castings are entirely attributed to their different defect populations. No single process is inherently superior. For defects of the same size (in terms of projected area normal to the loading direction), oxide films are less detrimental to fatigue life than pores. Areas of current controversy are highlighted and suggestions for further work are made.

Application of fracture mechanics to estimate fretting fatigue endurance curves

This paper presents approximations to fatigue curves in fretting conditions with spherical contact in the alloy Al 7075. The curves are defined for a specific contact geometry and loads applied in the tests (axial load on the specimen, the normal and tangential contact forces). In order to obtain a curve of this type it is necessary to fix all parameters except for one and analyse its influence on life. The method used to estimate life in fretting fatigue combines initiation with propagation. Different approaches to the growth of short cracks are employed and in some cases a fretting fatigue limit is predicted. Various groups of fretting tests have been analysed, evaluating the suitability of each approximation.

EDITORIAL

EDITORIAL

Numerical modelling and data assimilation of the Larsen B ice shelf, Antarctic Peninsula

In this study, the flow and rheology of pre-collapse Larsen B ice shelf are investigated by using a combination of flow modelling and data assimilation. Observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method. In particular, the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow. The application to the Larsen B ice shelf shows that a strong weakening of the ice in the shear zones, mostly along the margins, is necessary to fit the observed shelf flow. This pattern of bands with weak ice is a very robust feature of the inversion, whereas the ice rheology within the main shelf body is found to be not well constrained. This suggests that these weak zones play a major role in the control of the flow of the Larsen B ice shelf and may be the key to understanding the observed pre-collapse thinning and acceleration of Larsen B. Regarding the sensitivity of the stress field to rheology, the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments.

증기발생기 세관의 파괴저항 특성 측정에 관한 연구

The structural and leakage integrity of steam generator tubes should be sustained against all postulated loads even if a crack is present. During the past three decades, most of the efforts with respect to integrity evaluation of steam generator tubes have been focused on limit load solutions but, recently, the applicability of elastic-plastic fracture mechanics was examined cautiously due to its effectiveness. The purpose of this paper is to introduce a testing method to estimate fracture resistance characteristics of steam generator tubes with a through-wall crack. Due to limited thickness and diameter, inevitably, the steam generator tubes themselves were tested instead of standard specimen or alternative ones. Also, a series of three dimensional elastic-plastic finite element analyses were carried out to derive closed-form estimation equations with respect to J-integral and crack extension for direct current potential drop method. Since the effectiveness of JSUBIC/SUB as well as J-R curves was proven through comparison with those of standard specimens taken from pipes, it is believed that the proposed scheme can be utilized as an efficient tool for integrity evaluation of cracked steam generator tubes.

Fracture of cold jointed concrete interfaces

The behavior of concrete–concrete, transversely cold jointed interface beams is experimentally investigated to determine their fracture properties. Simply supported beams under three-point bending with different compressive strengths of concrete on either side of the interface are considered. It is found that the fracture energy decreases as the difference between the compressive strength of the material on either side of the interface increases. In addition, three different sizes of geometrically similar cold jointed interface specimens are tested to study the size effect using the Bažant’s size effect model of concrete. It is found that the interface specimens exhibit size effect similar to intact concrete specimens and the slope of the nominal strength versus size on a logarithmic plot is −1/2, indicating the applicability of linear elastic fracture mechanics. LEFM analyses are carried out on damaged concrete beams strengthened by external application of new concrete over the parent substrate, thus producing a cold jointed interface. It is found that the elastic properties of the repair material should be as close as possible to those of the parent one in order to get an efficient repair configuration.

Application of Probabilistic Fracture Mechanics to Prognosis of Aircraft Engine Components

It is generally accepted that traditional logistics functions including periodic nondestructive inspections and planned maintenance increase the reliability and readiness of turbine engines. Nevertheless, further significant enhancements in reliability and readiness are believed to be possible through the implementation of a prognosis system based on online monitoring and interpretation of critical engine operating parameters and conditions to diagnose potential problems and forecast readiness. An approach is presented for improving probabilistic life prediction estimates through the application of prognosis methods. Actual F-16/F100 usage data from flight data recorders were interfaced with a probabilistic life prediction code to quantify the influence of usage on the probability of fracture of an idealized titanium compressor disk. For the example cases considered, it is shown that usage variability leads to about 6 × × variability in life and from 10 × × to 100 × variability in the probability of fracture. The results suggest that variability in usage could provide a basis for selectively extending the life of aircraft engines.

Effect of constraint on fatigue crack propagation near threshold in medium carbon steel

The influence of constraint caused by different specimen geometry on fatigue crack growth in the threshold region is analyzed and a phenomenological model is proposed to correlate the fatigue crack propagation rate under different constraint levels. The approach is based on an application of two-parameter fracture mechanics and the corresponding calculations are performed according to the finite element method. The constraint level is quantified by the value of T-stress. The fatigue crack propagation rate is then expressed in terms of the stress intensity factor K and T-stress. The influence of K and T on the plastic zone size and crack opening near the fatigue crack tip is investigated. It is shown that assuming small scale yielding conditions, the plasticity induced crack opening and crack closure are not significantly influenced by the T-values, but the influence of T-stress on plastic zone size is significant and cannot be neglected. Plastic zone size is considered as a controlling variable for near threshold fatigue crack behavior and a simple procedure making it possible to estimate the changes of the fatigue crack propagation rate in the threshold region due to different constraint level quantitatively is formulated. The numerically obtained results are compared with experimental data. It is concluded that experimental values correspond well with those numerically predicted. The result presented makes it possible to relate experimentally measured data obtained from specimens with different geometries and thus contribute to more reliable estimates of the residual fatigue life of structures.

Application of linear fracture mechanics to notched timber elements

AbstractThe application of linear elastic fracture mechanics is ideally suited to estimating the strength of notched timber elements because wood is quite brittle with respect to tension across the grain. Elements with notches more than a few millimetres deep behave in a brittle manner. Some of the early research in developing this practical application of linear elastic fracture mechanics is presented.In mathematical terms the stress field in the vicinity of a notch contains at least one and sometimes two stress singularities. The magnitudes of these singularities are defined by stress intensity factors. Crack initiation, associated with a predictable size effect, occurs when these factors reach critical values.This paper gives equations for the stress fields around an orthotropic notch root. It also provides a simple method for computing stress intensity factors and size effects in notched structural elements. Some examples of stress intensity factors for timber elements containing zero and non‐zero angle notches are listed. Critical stress intensity factors for timber, related to density, are given for a series of notch types. Measured effects on strength arising from interactions between loading configurations are also presented. Copyright © 2006 John Wiley & Sons, Ltd.

Strain energy release rate calculation for a moving delamination front of arbitrary shape based on the virtual crack closure technique. Part I: Formulation and validation

This paper proposes a simple, efficient algorithm to trace a moving delamination front with an arbitrary and changing shape so that delamination growth can be analyzed by using stationary meshes. Based on the algorithm, a delamination front can be defined by two vectors that pass through any point on the front. The normal vector and the tangent vector for the local coordinate system can then be obtained based on the two delamination front vectors. An important feature of this approach is that it does not require the use of meshes that are orthogonal to the delaminations front. Therefore, the approach avoids adaptive re-meshing techniques that may create a large computational burden in delamination growth analysis. An interface element that can trace the instantaneous delamination front, determine the local coordinate system, approximate strain energy release rate components and apply fracture mechanics criteria has been developed and implemented into ABAQUS ® with its user-defined element (UEL) feature. In this Part I of a two-part paper, the approach and its implementation are described and validated by comparison to results from existing cases having analytical solutions or other established FEA predictions.

Application of Fracture Mechanics to Failure in Manatee Rib Bone

The Florida manatee (Trichechus manatus latirostris) is listed as endangered by the U.S. Department of the Interior. Manatee ribs have different microstructure from the compact bone of other mammals. Biomechanical properties of the manatee ribs need to be better understood. Fracture toughness (K(C)) has been shown to be a good index to assess the mechanical performance of bone. Quantitative fractography can be used in concert with fracture mechanics equations to identify fracture initiating defects/cracks and to calculate the fracture toughness of bone materials. Fractography is a standard technique for analyzing fracture behavior of brittle and quasi-brittle materials. Manatee ribs are highly mineralized and fracture in a manner similar to quasi-brittle materials. Therefore, quantitative fractography was applied to determine the fracture toughness of manatee ribs. Average fracture toughness values of small flexure specimens from six different sizes of manatees ranged from 1.3 to 2.6 MPa(m)(12). Scanning electron microscope (SEM) images show most of the fracture origins were at openings for blood vessels and interlayer spaces. Quantitative fractography and fracture mechanics can be combined to estimate the fracture toughness of the material in manatee rib bone. Fracture toughness of subadult and calf manatees appears to increase as the size of the manatee increases. Average fracture toughness of the manatee rib bone materials is less than the transverse fracture toughness of human and bovine tibia and femur.

Effects of Current and Voltage on Welding Strength of Lap Joint with CO<sub>2</sub> Welding Process

The application of fracture mechanics is being increased gradually to assess the safety of welding crack-containing structures. The joint parts of vehicle frame are mostly made up of welding joint. Due to the impact and fatigue load, cracks initiate and propagate in the welding joint part, as a result, the frame comes to a final failure. Therefore it is necessary to improve the welding performance and fracture resistance of welding part in structure. This study presents a most suitable way of welding process for lap joints by CO2 arc welding which is widely used in the the vehicle structure. Also this paper makes an evaluation to tensile strength and fatigue strength under various welding condition

Computational aspects of inverse analyses for determining softening curves of concrete

Inverse analyses of laboratory experiments for determining strain softening curves of cementitious materials are an important tool of applied fracture mechanics and provide the basis for computational simulations of failure processes in civil structures. In these analyses, experimental results are approximated by repeated numerical simulations of the tests. A new optimisation method for this fitting process is proposed. It is based on an evolutionary algorithm including local neighbourhood attraction for convergence improvement. Several aspects of applicability and performance of the method as well as the objectivity of the results are discussed and applications are presented.

A novel approach to the determination of the threshold for stress corrosion cracking (K ISCC) using round tensile specimens

This article discusses determination of threshold stress intensity for propagation of stress corrosion cracking (KISCC), using circumferential notch tensile (CNT) specimens. Use of round tensile specimens is a novel and cost-advantageous approach to determination of KISCC. However, compliance of this specimen geometry to the constraints for application of linear elastic fracture mechanics (LEFM) has traditionally been argued, and hence this aspect is addressed in detail. The LEFM suits best the materials that undergo brittle cracking, and hence a highly brittle material, cast iron, has been selected as the test material. However, susceptibility of this material to caustic embrittlement has been established employing another technique, viz. slow strain rate testing and fractography of the specimens. Using CNT specimens, KISCC has been determined for the cast iron in hot caustic solutions, and the features of intergranular caustic cracking and secondary cracking have been established using scanning electron microscopy.

Tire Design Methodology Based on Safety Factor to Satisfy Tire Life (Simulation Approach to Truck and Bus Tire Design)

Abstract A tire is not only simply made of rubber, but also twisted cord, which we can call FRR (Fiber Reinforced Rubber). The rubber in FRR is made of many materials, including rubber compound, carbon black, silica, and other materials. FRR is a double composite material, which means a particle reinforced material in a microscopic view and also a fiber reinforced one in a macroscopic view. Therefore, it is very difficult to apply fracture mechanics to the evaluation of tire durability on a practical tire design level. This paper gives a proposal for a new design methodology considering the actual tire condition which gives tire profile growth and rubber aging due to heat build-up under operation. These two issues are especially important for truck and bus tire design. Tire profile growth is a very important one because the change of tire profile induces different strains in the rubber at the same location in the tire during its life. We apply the FEM (Finite Element Modeling) based Double Inflation Pressure (DIP) method to simulate the experimental fact of the change of tire profile. In order to use the relationship between the change of the tire profile and change of rubber properties, like breaking strain in the tire as service time passes, the concept of safety factor is introduced. The low modulus rubber parts in the tire dominate the strain field and allow the large deformations of the tire. Safety factors derived from the strain field are called “Margin of Safety.” On the other hand, the durability of FRR, like the belt layer and carcass layer with the cord part, is evaluated by stress because the FRR part reacts to the stress of the internal pressure and load of the tire. But, the Margin of Safety of the rubber part is more important because the cord part has enough safety factor in comparison with the rubber part. Tire life is predicted with the Margin of Safety described above.

Applied Fracture Mechanics in Metallurgical Failure Investigations

Applied Fracture Mechanics in Metallurgical Failure Investigations

Could It Happen Again?

Most readers of “Twenty-Five–Year Experience With the Bjork-Shiley Convexoconcave Heart Valve: A Continuing Clinical Concern” in this issue of Circulation have not heard of the valve.1 Many elements of its story parallel those of cerivastatin2,3⇓ and rofecoxib,4 but with a twist: Recalling a heart valve may entail greater risks than leaving a possible “time bomb” in place. The Bjork-Shiley story raises many questions about balancing the benefits of drugs and devices with their risks to public safety. For example: See p 2850 My personal involvement began when a lawyer for Shiley, Inc, with a biostatistical degree read our account of the Braunwald-Cutter heart valve and wanted to understand why the University of Alabama at Birmingham and the Mayo Clinic decided to prophylactically remove every one.5 Subsequently, I served on scientific panels, spearheaded a multi-institutional effort to quantify the risks of prosthesis explant,6 and suggested ways to use this information for the tough decision making ahead.7 The Bjork-Shiley heart valve, developed by Shiley, Inc, in collaboration with Dr Viking Bjork of Sweden, was …

A Model for Mixed-Mode Fatigue

Abstract The initial use of fracture mechanics to characterize crack growth by fatigue was by Paris. Since then, the application of fracture mechanics to fatigue problems has become routine. In the case of fatigue under mode I loading, the method of analysis, under conditions of constant amplitude and small-scale yielding, is well established. However, apart from the conditions of variable amplitude loading, large scale yielding, and short existing cracks, the procedures for analyzing fatigue under mode II and mixed-mode loading still have not been resolved. A general model, applicable to pure and mixed-mode fatigue crack growth, has been developed based on the unified model. The formulation of the general model is in terms of a power law relationship that is an extension of Paris. The experimental procedures for the measurement of mode II and mixed-mode fatigue crack growth, and verification of the proposed fatigue model, are dealt with herein. The verification has been confined to conditions of constant amplitude and small-scale yielding.