Stress Intensity Factors For Specimens Research Articles

Investigation of the size of the fracture process zone and the cleavage stress in cracked steel parts

AbstractThe study is devoted to the development of methods for predicting the brittle fracture of a steel part with a crack. To describe the limit state of the fracture process zone, a mathematical model of the fracture process zone in an elastic–plastic stress state and the generalized brittle fracture theory have been used. The cleavage stress and the size of the fracture process zone are used as parameters for the fracture toughness of the material. A finite element analysis approach was developed to determine these parameters by considering the elastic–plastic stress state of cracked specimens. An analytical procedure for calculating the above parameters for low‐carbon and low‐alloy steels is given. The proposed models allow the analytical calculation of critical values of the stress intensity factor for specimens with cracks at negative temperatures. The development of this study is linked to the improvement of the technology for determining the physical strength criteria of materials. The application of the proposed models makes it possible to create a methodology for predicting crack resistance of welded structures of arctic design considering their geometry and structural and technological characteristics.

Crack Front Geometry and Stress Intensity Factor of Semi-circular Bend Specimens with Straight Through and Chevron Notches

The semi-circular bending (SCB) test is one of the most useful testing methods for determining the mode-I fracture toughness of rocks. An SCB specimen with an artificial notch is loaded at three points including two lower points and a single upper point during the test. In general, there are two types of geometries for artificial notches: straight through and chevron notches. The straight through notch is commonly adopted for the SCB (STNSCB) test as the suggested method for estimating mode-I fracture toughness of rocks in ISRM, while the cracked chevron notch SCB (CCNSCB) test using a specimen with a chevron notch has been performed by some researches. In this paper, by means of the commercial finite-element software ABAQUS, cracking behavior from the tip of an artificial notch during STNSCB and CCNSCB tests is analyzed with Extended Finite-Element Method (XFEM) to clarify crack front geometry in the process of cracking. The relationship between the crack length and the stress intensity factor can be obtained by analyzing stress intensity factors of the specimen with FEM, based on crack front geometries calculated with XFEM during the cracking process. Using this relationship, the minimum stress intensity factor at a critical crack length is determined for estimating mode-I fracture toughness of the rock for the CCNSCB test. Furthermore, by performing the SCB tests using Kimachi sandstone specimens with three different artificial notch geometries, fracture toughness of the sandstone is determined from the SCB test. Consequently, it is concluded that the values of fracture toughness from the test with three notch geometries are almost the same.

Numerical Analysis of Stress Intensity Factor in Specimens with Different Fillet Geometry Subjected to Bending

Abstract The article presents the maps of xx stress component and compares values of analytical and numerical calculations for the stress intensity factor range of welded specimens with fillet welds which subjected to cyclic bending. The tests were performed under constant value of moment amplitude Ma = 9.20 N·m and stress ratio R = σmin/ σmax = −1. The specimens were made of drag steel rod S355. The specimens were solid and welded. The numerical models were simulated with ABAQUS suite and numerical calculations performed with FRANC3D software.

Elastic and plastic stress intensity factor in specimen of aluminum alloys under tension and bending in the temperature range

The elastic and plastic stress intensity factors (SIF) are studied through computations for two aluminum alloys in the temperature range. Subject for studies is central notched specimens with external semi-elliptical surface crack. Wide range of relative crack depth and aspect ratio is studied. The calculations are carried out under uniaxial tension and a three-point bending for aluminum alloys D16 and B95 at -60°С, 20°С and 250°С. The elastic and plastic stress intensity factor distributions along the curvilinear semi-elliptic crack front are obtained as a function of the relative sizes of defects, the loading type and temperature. Fundamental differences in the behavior of the elastic and plastic SIFs for surface cracks are established at high, room and low temperatures. The distributions of elastic SIF are the same at different temperatures for both materials. Contrary to that, the plastic SIF is sensitive to the plastic properties of the materials. The plastic SIF gradually increases by increasing the temperature. The plastic SIF, which is sensitive to the constraint effects and elastic-plastic material properties, is attractive as the self-dependent unified parameter for characterization of the material fracture resistance properties.

Stress Intensity Factors of Semi-Circular Bend Specimens with Straight-Through and Chevron Notches

Semi-circular bend specimen is one of the useful test specimens for determining fracture toughness of rock and geo-materials. Generally, in rock test specimens, initial cracks are produced in two shapes: straight-edge cracks and chevron notches. In this study, the minimum dimensionless stress intensity factors of semi-circular bend specimen (SCB) with straight-through and chevron notches are calculated. First, using finite element analysis, a suitable relation for the dimensionless stress intensity factor of SCB with straight-through crack is presented based on the normalized crack length and half-distance between supports. For evaluating the validity and accuracy of this relation, the obtained results are then compared with numerical and experimental results reported in the literature. Subsequently, by performing some experiments and also finite element analysis of the SCB specimen with chevron notch, the minimum dimensionless stress intensity factor of this specimen is obtained. Using the new equation for the dimensionless stress intensity factor of SCB with straight-through crack and an analytical method, i.e., Bluhm’s slice synthesis method, the minimum (critical) dimensionless stress intensity factor of chevron notched semi-circular bend specimens is calculated. Good agreement is observed between the results of two mentioned methods.

Impact compressive and bending behaviour of rocks accompanied by electromagnetic phenomena.

It is well known that electromagnetic phenomena are often observed preceding earthquakes. However, the mechanism by which these electromagnetic waves are generated during the fracture and deformation of rocks has not been fully identified. Therefore, in order to examine the relationship between the electromagnetic phenomena and the mechanical properties of rocks, uniaxial compression and three-point bending tests for two kinds of rocks with different quartz content, granite and gabbro, have been carried out at quasi-static and dynamic rates. Especially, in the bending tests, pre-cracked specimens of granite were also tested. Using a split Hopkinson pressure bar and a ferrite-core antenna in close proximity to the specimens, both the stress-strain (load-displacement) curve and simultaneous electromagnetic wave magnitude were measured. It was found that the dynamic compressive and bending strengths and the stress increase slope of both rocks were higher than those observed in static tests; therefore, there is a strain-rate dependence in their strength and stress increase rate. It was found from the tests using the pre-cracked bending specimens that the intensity of electromagnetic waves measured during crack extension increased almost proportionally to the increase of the maximum stress intensity factor of specimens. This tendency was observed in both the dynamic and quasi-static three-point bending tests for granite.

EXPERIMENTAL INVESTIGATION ON FRACTURE TOUGHNESS OF INTERFACE CRACK FOR ROCK/CONCRETE

Fracture toughness is a critical input parameter for fracture-mechanics based fitness-for-service assessments, and it is preferable to determine this by experiment. In the present paper, fracture toughness of rock/concrete bimaterial interface was obtained by the tests which were performed on the universal material tester. A beam specimen with single-edge crack is used to form the different fracture mode mixity. The stress intensity factors of specimen per unit load with different combinations of K1 and K2 were calculated by the mixed hybrid finite element method on the principals of linear elastic interface fracture mechanics. By regressing the critical stress intensity factors of 7 specimen groups, two experiential fracture criterions of mixed crack interface were derived, and the fracture toughness([Formula: see text], [Formula: see text]) of rock/concrete were obtained further.

Fracture Toughness and Subcritical Crack Growth in Polycrystalline Silicon

The fracture behavior of polycrystalline silicon in the presence of atomically sharp cracks is important in the determination of the mechanical reliability of microelectromechanical system (MEMS) components. The mode-I critical stress intensity factor and crack tip displacements in the vicinity of atomically sharp edge cracks in polycrystalline silicon MEMS scale specimens were measured via an in situ atomic force microscopy/digital image correlation method. The effective (macroscopic) mode-I critical stress intensity factor for specimens from different fabrication runs was 1.00±0.1MPa√m, where 0.1MPa√m is the standard deviation that was attributed to local cleavage anisotropy and grain boundary effects. The experimental near crack tip displacements were in good agreement with the linearly elastic fracture mechanics solution, which supports K dominance in polysilicon at the scale of a few microns. The mechanical characterization method implemented in this work allowed for direct experimental evidence of incremental (subcritical) crack growth in polycrystalline silicon that occurred with crack increments of 1-2μm. The variation in experimental effective critical stress intensity factors and the incremental crack growth in brittle polysilicon were attributed to local cleavage anisotropy in individual silicon grains where the crack tip resided and whose fracture characteristics controlled the overall fracture process resulting in different local and macroscopic stress intensity factors.

Influences of Isotropic and Anisotropic Material Properties on Stress Intensity Factors of the Crack Tip under Biaxial Stress

In order to clarify the effect of anisotropy on local deformation and the stress intensity factors in the vicinity of the crack tip under complicated stress states, biaxial loading tests were carried out on isotropic (Photoelastic sheet: PSM-1, Measurements Group, Inc) and anisotropic (LEXAN 9030, General Electric Company) polycarbonate materials. The biaxial dynamic loading device, which was developed by the authors, was fully utilized in the experiments. We carried out the experiments using isotropic and anisotropic specimens with a pre-processed 0°, 15°, 30°, 45°, 60°, 75° or 90° crack angle. We applied the equal biaxial stress, that is, the loads for both the Y- and X-axis as 0N : 0N-3920N : 3920N (1 : 1). We determined the stress intensity factors of specimens using both photoelasticity and caustics method. The results of the experiments were compared. As a result, when the materials with anisotropic property are used as machine or structural members, it is important to consider the extrusion direction of the material in their design.

Effect of crack configuration and pre-crack length on stress intensity factors

Abstract In this paper, the results for the stress intensity factor of a mode III (tearing mode) have been investigated using the method of finite element analysis. A compact tension specimen containing a pre-crack and subjected to out-of-plane tearing loads has been used. A three-dimensional finite element analysis (FEA) model using the ANSYS program has been built for specimens made of different bio-medical materials such as stainless steel, titanium, alumina, high density polyethylene(HDPE) and polymethyl methacrylate(PMMA). Such materials are used for hip and knee replacement and for dental implants. The effects of notch angle, notch tip radius and pre-crack length on the stress intensity factor for the mode III have been studied. The model was extended to study the effect of crack and bonding line separation distance on stress intensity factors for specimens made of different homogeneous materials such as stainless steel bonding with epoxy as a filler material. The number of elements along the crack front and crack tip has been varied to determine its effects on the stress intensity factors. It is concluded that the pre-crack must be greater than or equal to 33% of the total crack lengt and the notch must be a blunt notch in order to obtain the stress intensity factor, KII I, independently, of the crack notch angle. However, the stress intensity factor is independent of the crack and bonding line separation distance when it is less than or equal to 15% of the specimen width. The results for the stress intensity factors, KII I, are obtained using a linear elastic fracture mechanics (LEFM) approach.

Effect of notch configuration and pre-crack length on stress intensity factors

Abstract The determination of stress intensity factors for specimens with pre-cracks is important in fracture analysis. In this paper, the evaluation of stress intensity factors of modes I and II for compact tension specimens is presented. A 3-D finite element analysis (FEA) model using the ANSYS program is constructed for specimens made of different bio-medical materials such as stainless steel, titanium, alumina, high density polyethylene (HDPE) and polymethyl methacrylate (PMMA). Such materials are used for hip and knee replacement and for dental implants. The effects of notch angle, notch tip radius and pre-crack length on the stress intensity factor are studied. The number of elements along the crack front and crack tip is considered and has been varied to determine its effect on the value of the stress intensity factor. It is concluded that, in order to evaluate the reasonable values for the stress intensity factor, the pre-crack lengt should be greater than 10% and 33% of the total crack lengt for modes I and II, respectively. The results for the stress intensity factors KI and KII are obtained using a linear elastic fracture mechanics (LEFM) approach.

Fatigue strength of gas turbine compressor blades

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed into the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied considering the effect of the medium (sea water) and stress ratio in a cycle, as well as fatigue strength of newly-manufactured blades and those after being in operation. Specific features of fatigue crack propagation in blades have been considered and a method for predicting the life of blades with cracks has been justified.

Fatigue strength and life of compressor blades for marine gas turbine engines

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed on the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on the fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied taking into account the effect of the medium (sea water), stress ratio in a cycle, and programmed mode of loading. The authors also consider fatigue strength of newly-manufactured blades and those in operation, as well as specific features of fatigue crack propagation in blades. They have substantiated a method for predicting the life of blades with cracks.

Dynamic stress intensity factors for compact tension and bend specimens.

A computational procedure is presented for evaluating the dynamic stress intensity factors for compact tension (CT) and bend specimens under impact loading conditions. The dynamic stress intensity factors for the specimens subjected to arbitrary impact loads can be computed by super-imposing the step-impact response functions. The response functions for CT and bend specimens are obtained by the finite element method. The recent dynamic fracture tests indicate that a loss of contact between the bend specimen and the anvils occurs during the impact loading. The present procedure is effective for the determination of the dynamic stress intensity factors for the specimens, even if the loss of contact occurs.

Critical stress intensity factor of specimens with corrosion fatigue cracks

Critical stress intensity factor of specimens with corrosion fatigue cracks