Higher Stress Intensity Factor Research Articles

Simulation on the Decrease in Threshold Stress Intensity Factor (SIF) Range due to High Maximum SIF

Abstract In this paper, a method is proposed to quantitatively estimate the decrease in threshold stress intensity factor (SIF) range due to high maximum SIF, that is observed for some materials in constant maximum SIF fatigue crack growth tests. The parameters for the simulation were inversely determined from the experimental data of carbon steel, Al and Ti alloys with the aid of genetic algorithm. The set of candidate parameters named as a “generation” were repeatedly generated and evaluated until the experimental data were reproduced by the simulation. A very interesting fact was that though the test conditions for these three materials were different, the obtained simulation parameters seemed to be independent of material. Finally, the proposed method was validated by comparing predictions with experimentally determined values of the decrease in threshold on an embrittled carbon steel.

P70 最大応力拡大係数(K値)増による下限界K値範囲漸減量の定量予測(GS1)

In this paper, we propose a method to quantitatively estimate the decrease in threshold stress intensity factor (SIF) range due to high SIF, that is observed for some materials in constant maximum SIF fatigue crack growth tests. The parameters cased for the simulation were inversely determined with the aid of Genetic Algorithm. The validity of our method was shown by comparing the simulation data with experimental data for embrittled S 55 C.

Assessment for Decrease in Threshold Stress Intensity Factor (SIF) Range Due to High Maximum SIF

Abstract In this paper, we consider the decrease in threshold stress intensity factor (SIF) range ΔKth due to high maximum SIF K, which is observed for several materials in the tests with the Kmax constant method when closure-free conditions are realized. We proposed an assessment criterion to predict this phenomenon from the past data for Al and Ti and showed that the criterion is valid also for steels. Finally, we proposed a simplified model to explain the phenomenon, considering the fact that marks of static fracture are observed on the fracture surface.

Evaluation of Crack Growth Characteristics in Float Glass and Thermally Tempered Glass Based on the Crack Length Measurement Using Au Ion Sputtered Film

Thermal tempered glass is made by rapidly cooling after heating float glass to near the softening point. Compressive residual stresses at the surface layer of thermally tempered glass strengthen the material, and tensile residual stresses are also generated at the same time in the interior of the material to balance with the surface compressive residual stresses. It is considered that the fatigue crack growth characteristics of thermally tempered glass are remarkably different from that of float glass, due to the existence of those residual stresses. In this study, Au film made by ion sputtering method and an optical microscope are used to measure the crack length of the thermally tempered glass and float glass, and these fatigue crack growth characteristics are investigated. As a result, it is confirmed that the crack length measurement with an Au film has high accuracy of measurement. Furthermore, it is found that the higher stress intensity factor is needed to cause the crack growth in thermally tempered glas than in flat glass.

Quantitative Estimation of Decrease in Threshold Stress Intensity Factor (SIF) Range due to High Maximum SIF

In this paper, we propose a method to quantitatively estimate the decrease in threshold stress intensity factor (SIF) range due to high SIF, that is observed for some materials in constant maximum SIF fatigue crack growth tests. The parameters cased for the simulation were inversely determined with the aid of Genetic Algorithm. The validity of our method was shown by comparing the simulation data with experimental data for embrittled S 55 C.

応力拡大係数（K値）増による下限界K値範囲漸減現象

応力拡大係数（K値）増による下限界K値範囲漸減現象

218 遺伝的アルゴリズムを用いた最大応力拡大係数(K値)増による下限界K値範囲漸減量の定量評価(OS1-3 損傷・欠陥の解析,オーガナイズドセッション1 計算固体力学の最新応用)

218 遺伝的アルゴリズムを用いた最大応力拡大係数(K値)増による下限界K値範囲漸減量の定量評価(OS1-3 損傷・欠陥の解析,オーガナイズドセッション1 計算固体力学の最新応用)

Fatigue crack propagation of 444 stainless steel welded joints in air and in 3%NaCl aqueous solution

Fatigue crack propagation (FCP) was studied on ferritic stainless steel welded joints in air and in 3%NaCl aqueous solution. In air, when the crack propagated normal to the weld line, FCP rates decreased temporarily as it reached the heat affected zone (HAZ) and then increased monotonously. When the crack propagated within HAZ and the weld metal, FCP rates were lower than those of the base metal. After allowing for crack closure, the FCP data of all the welded specimens became similar and coincided with those of the base metal. In 3%NaCl aqueous solution, all the welded specimens still showed enhanced FCP rates at high effective stress intensity factor range region, where extensive intergranular and quasi-cleavage fractures were seen.

Analysis of Toughening Mechanisms of ZrO2/Nano-SiC Ceramic Composites

Three effects of nano-particles on the toughness of nano-composite ceramics, namely, nano-particle clustering, crack pinning, and transgranular fracture, are identified from both the experimental and analytical studies. It is found that crack pinning toughens the nano-composite ceramics because a higher stress intensity factor is needed to cause crack propagation around or pull-out of the nano-particle. The nano-particle along the grain boundary steers the crack into the matrix grain due to the strong cohesion between the nano-particle and the matrix. Transgranular fracture increases with the increase of the volume fraction of nanoparticles. Since the fracture resistance of the grain boundary is lower than that of the grain lattice, the higher the probability of transgranular fracture induced by nanoparticles, the tougher is the nano-composite. Nano-particle clustering, which increases with increasing volume fraction of nano-particles, leads to the reduction of both the strength and toughness of the nano-composite ceramics. The larger the size of the clustered particle, the more defects it contains and, thus, the easier it is for the crack to pass through the clustered particle. That is, the nano-particle clustering can reduce toughening induced by crack pinning. The theoretical prediction, based on the combination of the three effects of nano-particles, is in agreement with the experimental data.

Assessment on Decrease in Threshold Stress Intensity Factor (SIF) Range due to Maximum SIF

In this paper, we consider the decrease in threshold stress intensity factor (SIF) range ΔKth due to high maximum SIF Kmax, that is observed for several materials in the tests by Kmax constant method when i) closure-free and ii) non-corrosive atmosphere conditions are realized. We proposed an assessment criterion to predict this phenomenon from the past data for Al and Ti and showed that the criterion is valid also for steels. Finally we proposed a simplified model to explain the phenomenon, considering the fact that marks of static fracture are observed on the fracture surface.

Corrosion fatigue characteristics in the weld of multi-pass welded A106 Gr B steel pipe

In order to investigate the corrosion fatigue characteristics in the weld of multi-pass welded A106 Gr B steel pipe, corrosion fatigue tests were performed under the various stress ratios and 3.5 wt% NaCl solution at room temperature. The corrosion fatigue characteristic curves were represented using crack closure concept. The obtained results are as follows ; when the load frequency is 1.0 Hz, the crack opening point is transited in the region of Kmax=20-32 MPa-msu1/2. In the low stress intensity factor range, the crack opening point is higher than that in air. However, in the high stress intensity factor range, it is lower than that in air. In the cases of 0.1 Hz and 0.01 Hz, the crack opening point gradually decreases to K(min) with Kmax increase.

Comparison of finite element techniques for 2D and 3D crack analysis under impact loading

For several technical applications the dynamic aspect in fracture mechanics cannot be neglected. When the reliability of components with macroscopic cracks has to be assessed, the consideration of dynamic effects may lead to much higher stress intensity factors than under static conditions. In this paper three different methods to calculate the dynamic stress intensity factor for the mode-I loading of stationary cracks are compared. Based on two- and three-dimensional finite element simulations, the dynamic stress intensity factor is computed with the dynamic J-integral, the modified crack closure integral and the displacement interpretation method. The theoretical fundamentals of all three methods are summarized in the paper and the numerical implementation is explained briefly. Results for different models are shown and compared to findings in the literature.

Sub-critical crack growth in sodium germanate glasses

Sub-critical crack growth in binary sodium germanate glass was investigated over a wide range of the crack velocities, 10 −7–10 −2 m s −1, by using small-size specimens with double cleavage drilled compression configuration. For evaluating the intrinsic sub-critical crack growth, crack initiation and subsequent propagation of the crack were performed in heptane. With increasing Na 2O content in sodium germanate glass, sub-critical crack growth curve shifted toward higher stress intensity factors first up to 10 mol% Na 2O, but more addition of Na 2O caused the curve to shift to lower stress intensity factor regions. In other words, fracture toughness shows a maximum at the composition of 10Na 2O·90GeO 2, whose value is 1.07 MPa m 1/2. This compositional dependence of fracture toughness originates from the so-called germanate anomaly. On the other hand, the slope of sub-critical crack growth curve for the glass containing >10 mol% Na 2O was much shallower than that for soda-lime glass. These glasses are very fatigable even in inert condition. It is considered that this fatigue behavior can be caused by the microscopic structural variation, which is the presence of GeO 6 units in GeO 2 glass network, and that these units can be the fatigue crack path.

On the effect of nano-particle clustering on toughening of nano-composite ceramics

In this paper, two and three-dimensional clustering models are developed to characterize the effect of nano-particle clustering on toughening of nanocomposite ceramics. It is found that crack pinning toughens the nano-composite ceramics because a higher stress intensity factor is needed for crack to propagate around or to pull-out the nano-particle. The nano-particle along the grain boundary steers the crack into the matrix grain due to the strong cohesion between the nanoparticle and the matrix. Since the fracture resistance of the grain boundary is lower than that of the grain lattice, the higher the probability of transgranular fracture induced by nano-particles, the tougher is the nano-composite. However, both crack pinning and transgranular fracture are affected by nano-particle clustering. Nanoparticle clustering, which increases with increasing volume fraction of nano-particles, leads to reduction of both the strength and toughness of the nano-composite ceramics. The larger the size of the clustered particle, and the more defects it contains, the easier it is for the crack to pass through the clustered particle, which means that the nano-particle clustering can reduce toughening induced by crack pinning and transgranular fracture. The theoretical prediction, based on the combination of the three mechanisms of nano-particles, is in agreement with the experimental data.

Fatigue Fracture Features of Pressure Vessel Steel in Simulated Light Water Reactor Environment

To evaluate fatigue crack growth of reactor pressure vessel steel at reactor operating conditions, tests were performed in air saturated hot water. The main test parameter was a loading frequency. Crack growth rate was increased with decreasing frequency up to a critical value. It was assumed through fractographic study and surface analysis that the enhancement of crack growth rate was environmentally assisted due to hydrogen embrittlement. In the low stress intensity factor range, cracks grew through crystallographic facets and tear ridges. Moreover, cleavage occurred at MnS inclusions. Larger cleavage occurred near MnS inclusions in the high stress intensity factor range. Brittle facets combined with voids and stair or step-like cracks with arrest marks appeared at sites some distance from the MnS inclusions. It was suggested that the environmentally assisted cracks were related to interactions of hydrogen with oxide film and also to dislocation motion at the crack tip.

Propagation and Arrest of Fatigue Cracks from a Precrack under Cyclic Torsional Loading in Medium-Carbon Steel.

Fatigue tests of crack propagation from a pre crack in thin walled tubular specimens made of a medium carbon steel were performed under cyclic torsion with and without superposed static tension. The condition of the transition from shear to tensile mode and the propagation behavior during this transition were investigated from the viewpoint of fracture mechanics. The direction of fatigue crack propagation follows the direction of the maximum of the total range of the tangential stress, Δσθ max near the crack tip and then gradually changes to the direction perpendicular to the maximum of the total range of the principal nominal stress. The propagation rate is faster than the uniaxial data when compared at the onset of crack extension and at high stress intensity factors. The former acceleration is caused by crack closure and the latter acceleration is due to the non singular T stress. The negative nonsingular stress induces excessive plasticity ahead of the fatigue crack, and then accelerates the fatigue crack.

Using PA11 and 12 as curing agents for epoxy networks: Influence of reactivity on miscibility and properties

The Rilsan PA11 prepolymer was evaluated as a curing agent of a diepoxy prepolymer (DGEBA). The miscibility, the glass transition temperature, and the melting of the blend were studied as a function of time at 200°C. A gelation phenomenon was evidenced by dynamic mechanical analysis and the gel time was determined at 200°C. The participation of amide groups to the reaction process at this temperature was confirmed by the study of the PA12 Orgasol®/DGEBA system and a reaction mechanism was elucidated by the study of a model system composed of ethylacetamide/phenyl glycidyl ether. The mechanical properties of DGEBA/Rilsan networks cured 7 h at 200°C were evaluated and indicate very high Young's modulus and critical stress intensity factor. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 857–865, 2000

Linear fracture mechanics analysis on growth of interfacial delamination in LSI plastic packages under temperature cyclic loading-Part II: Material properties and package geometry factors

For Part I see ibid., vol.21, pp.422-27 (Nov. 1998). A comprehensive study is made to evaluate the effects of material properties and package geometry factors on the delamination growth along the interfaces of a die-bonding layer and the bottom surface of the die pad in large scale integration (LSI) packages subjected to temperature cyclic loading. The effects of the coefficient of thermal expansion and Young's modulus of both the encapsulant resin and die-bonding material, as well as such package geometry factors as thickness, width, and chip size, are investigated. Numerical fracture mechanics analyses are conducted for three models of delamination geometries, for which relatively high stress intensity factors are induced. The effects of these design parameters on delamination growth are determined, and guidelines for designing packages with leadframes made of Cu alloy or Fe-42%Ni alloy (alloy 42) are established.

A study on tensile and fatigue properties of aged NAS 254N stainless steel at elevated temperatures

The effects of aging on tensile properties and fatigue crack growth behaviors of NAS 254N stainless steel was studied. Yield strength and ultimate tensile strength of the aged specimens were almost the same as the as-received (as-rec.). The fracture strain, however, was decreased significantly by the aging, and the fracture surface of the aged at room temperature (RT) test was intergranular. As test temperature increased, yield strength, ultimate tensile strength and elongation decreased. And a type of serration was observed at 550-650°C As strain rate decreased, yield strength and ultimate tensile strength decreased, but elongation increased. It was observed that tensile strength and strain had a sudden change at one point. And this critical temperatureT cr was 550°C. The effect of aging time on the tensile strength and strain was also investigated. Tensile strength and strain decreased significantly beyond 100hrs. Fatigue crack growth rate at RT was enhanced by the aging at high stress intensity factor range. This is due to the occurrence of the intergranular fracture in the aged specimen. At 650°C, the fatigue crack growth behavior was almost the same without intergranular fracture.

Investigation of the effects of shed-rate, initial Kmax, and geometric constraint on Δ Kth in Ti–6Al–4V at room temperature

A correct definition of threshold stress intensity is critical to understanding the growth of small cracks in advanced gas turbine engine applications. A study of the effect of specimen geometry, constraint, and load shedding rate on the measured threshold stress intensity of Ti–6Al–4V (AMS 4928) was conducted to compare conventional through cracks (compact tension specimen) to surface flaw defects ( K b specimen). At room temperature, stress ratios of 0.1 and 0.8 were examined. The effect of shed rate was assessed and it was found that shed rates as low as −20 in −1 (−0.8 mm −1) give accurate threshold measurements in both geometries at both stress ratios. These results indicate that the more gradual shed rate of −2 in −1 (−0.08 mm −1) recommended in ASTM E647 (Standard test method for measurement of fatigue crack growth rates, Annual Book of ASTM Standards, E647-95a, 1997:557–93) may be conservative for high strength titanium alloys. A study of the effect of starting stress intensity factor on the resultant threshold value was also completed. It was found that starting at high initial stress intensity factors gave false values for threshold.