Low Crack Growth Research Articles

Study on micro-crack propagation mechanism in different positions of single crystal titanium at nanoscale

In order to study the nanoscale mechanism of micro-crack propagation at different positions in single-crystal titanium, a molecular dynamics model of single-crystal titanium was established to analyze the micro-crack propagation behavior at different locations and to explore the influence of crack position on the expansion. The nanoscale mechanism of micro-crack propagation in single-crystal titanium is revealed. The simulation results show that the edge crack instability and expansion gradually form a gap under the tensile load in [12–30] crystal direction. However, the gap and absorption energy could be higher, making the morphology change at the gap not obvious. When the tensile load of the [0001] crystal orientation reaches the yield strength of the material, the material will plastically flow, making the model's overall shape symmetric and uniform. The tensile load has no excitation effect on the upright micro-crack. The upper crack is dominant when the single-crystal titanium model is preloaded with parallel double cracks. It expands to form holes, inhibiting the lower crack's growth.

Research on slow crack growth resistance of polyethylene pipe based on strain hardening modulus

AbstractSlow crack growth (SCG) is the principal factor for determining the operating lifetime of polyethylene (PE) pipes. Exploring the relationship between the molecular structure and the strain hardening (SH) modulus is helpful to evaluate the SCG resistance of PE pipe materials accurately. This paper uses an efficient SH test method to study the SH modulus for four kinds of PE pipes. Meanwhile, the correlation between SH modulus and sample thickness, material crystallinity (Xc), comonomer type and content, weight‐average molecular weight, and molecular weight distribution has been established and investigated respectively. The results represent that the comonomer type and the Xc of PE materials played significant roles in the SH modulus. The SH modulus of ethylene‐hexene copolymer is 5–7 MPa higher than that of ethylene‐butene copolymer. The amount in the range of 1 × 103 to 1 × 105 of the molecular weight distribution of PE materials has a significant contribution to the establishment of the tie molecular network. In addition, the higher Xc and the thinner thickness were beneficial to obtain the higher measured SH modulus.

Initiation and Propagation Characteristics of Multi-Source Fatigue Crack in Superalloy Disk

The multi-source fatigue crack growth characteristics of a turbine disk were studied via high-speed rotating low cycle fatigue test, fracture analysis and crack growth simulation analysis. The results show that the fatigue initiation life N0 of Superalloy disk and its simulation parts is less dispersive and similar, and the S-N0 curve of the material is not conservative to predict the N0 of the disk. The fracture of eccentric hole of disk is multi-source fatigue fracture, and the surface characteristic of the crack L/D decreases rapidly from 7.5 to 2, which is obviously different from the single source fatigue crack growth. Because of the long and thin stress distribution at the eccentric hole and the small dispersivity of GH698 crack initiation life, it's relatively easy to crack on the plane A of the eccentric hole, which is prone to multi-source fatigue. At the stage of a ≥ 1.23 mm, the reverse life of crack propagation fracture of 2# and 5# eccentric holes is consistent, and the error between the simulation value of single source fatigue crack propagation life and the reverse life is below 5%. In the stage of 0.61 mm ≤ a < 1.23 mm, the simulation crack growth rate of 5# eccentric hole of multi-source is close to that of fracture inversion. Accurate simulation analysis of multi-source crack growth needs to define the initial crack accurately, at the same time, consider the influence of existing fatigue damage on crack growth.

SEM Study of the Influence of Microstructure on Low Cycle Fatigue Crack Growth in Martensitic Steel I

Distribution of fatigue cracks in chromium martensitic steel after low cycle fatigue (LCF) tests at room temperature has been studied using SEM, and the experimental evidences of localized plastic flow (LPF) are presented. The influence of the location of LPF and the microstructure elements on the trajectory and growth of microcracks is also considered. The dimensions of plastic zones ahead of macrocrack tip as well as at its edges were measured in the process of crack propagation inside of the sample. The processes occurring in plastic zone, particularly ahead of macrocrack tip, were analyzed. Distribution, orientation and the reasons of slip bands’ formation as well as the microstructure elements at which they were nucleated have been studied. The impact of the slip bands’ orientation on the process of macrocrack growth was also analyzed. In addition the interactions of a crack with the boundaries of former austenite grains, martensitic packets, martensitic laths, slip bands and precipitates have been discussed.

Modeling of creep-fatigue interaction effects on crack growth at elevated temperatures

The well-known load frequency effect on creep-fatigue crack growth is explained by the interactions between fatigue and creep loading and is quantified using the concept of plasticity-induced crack closure. It is shown that the hold time during creep loading affects crack growth rates during subsequent fatigue cycles. Longer hold times lead to lower crack-tip opening stresses and faster crack growth rates during fatigue loading. To model the impact of hold time on crack opening stresses during fatigue loading, a strip-yield model was developed for creep-fatigue crack growth. The strip-yield model computes crack-tip opening stresses, which determine the effective stress intensity factor range and crack growth rate during the fatigue portion of each loading cycle. Maximum stress intensity factor is used to compute the crack growth rate during the creep portion of each cycle. The proposed strip-yield model is used to compute creep-fatigue crack growth rates for several structural materials, i.e., an Astroloy, aluminium alloy 2650 and 316 stainless steel. The model predictions of crack growth rates compare well with published experimental data for these alloys. This model achieves reliable predictions of crack growth rates and life prediction on components subjected to creep-fatigue loading at elevated temperatures by considering loading interaction effects.

Characteristics of Microscopic Inclusions and their Influence on Fatigue Crack Propagation in FGH96 Superalloy

FGH96 superalloy has been used in aircraft engine turbine discs due to the superior damage tolerance. This work investigated the characteristics of microscopic inclusions and their influence on fatigue crack propagation in hot isostatic pressed and heat treated commercial FGH96 superalloy. Low cycle fatigue crack growth tests were conducted. Several kinds of microscopic inclusions were identified by optical microscope (OM), scanning electric microscope (SEM) and energy dispersive spectrometer (EDS). Microscopic inclusions and secondary cracks were analyzed near the main cracks. Combined with the finite element method (FEM) results, it can be proposed that the microscopic inclusions have no critical influence on fatigue crack propagation in the specimens.

Failure analysis for power car wheels based on contact positions and tread slope

Abstract During service operation, the thermal cracks and shelling at power car wheel are initiated before a regular profiling of railway wheel. The damaged railway wheel is reprofiled prior to regular inspection and the maintenance cost for the railway wheel is sharply increased. Therefore, it is necessary to analyze and understand the damage mechanism of power car wheel in order to reduce maintenance cost. In the present paper, to analyze the damage cause and reduction for the power car wheel with tread brakes, the observations of by replication test was performed and fracture characteristics depend on contact zones are carried out. Contact stress is analyzed with respect to the wheel tread slope and contact positions. The result shows the zone between tread center and wheel rim demonstrated white layer by higher hardness and lower fracture toughness and faster crack growth rate in view point of fracture mechanic characteristics. The 1/20 of wheel tread slope can effectively mitigates surface damage of power car wheel.

ひずみ拡大係数に基づくステンレス鋼の低サイクル疲労き裂進展評価

ひずみ拡大係数に基づくステンレス鋼の低サイクル疲労き裂進展評価

OS0606-477 316ステンレス鋼の低サイクル疲労き裂発生・成長における環境効果

OS0606-477 316ステンレス鋼の低サイクル疲労き裂発生・成長における環境効果

Cyclic Thermal Stress-Induced Degradation of Cu Metallization on Si3N4 Substrate at −40°C to 300°C

The high-temperature reliability of active metal brazed copper (AMC) on Si3N4 ceramic substrates used for fabricating SiC high-temperature power modules was investigated under harsh environments. The AMC substrate underwent isothermal storage at 300°C for up to 3000 h and a thermal cycling test at −40°C to 300°C for up to 3000 cycles. During isothermal storage at 300°C, the AMC substrate exhibited high reliability, characterized by very little deformation of the copper (Cu) layer, low crack growth, and low oxidation rate of the Cu layer. Under thermal cycling conditions at −40°C to 300°C, no detachment of the Cu layer was observed even after the maximum 3000 cycles of the experiment. However, serious deformation of the Cu layer occurred and progressed as the number of thermal cycles increased, thus significantly roughening the surface of the Cu metallized layer. The cyclic thermal stress led to a significant increase in the crack growth and oxidation of the Cu layer. The maximum depth of the copper oxides reached up to 5/6 of the Cu thickness. The deformation of the Cu layer was the main cause of the decrease of the bond strength under thermal cycling conditions. The shear strength of the SiC chips bonded on the AMC substrate with a Au-12 wt.%Ge solder decreased from the original 83 MPa to 14 MPa after 3000 cycles. Therefore, the cyclic thermal stress destroyed the Cu oxides and enhanced the oxidation of the Cu layer.

Thermal fatigue life prediction of particle reinforced composites

This article proposes a thermal fatigue life prediction model for particle reinforced composites. First, a three-phase model was used to determine the effective eigen strain of a two-phase element formed by an ellipsoidal matrix shell surrounding an ellipsoidal particle, and then the disturbance strain in the two-phase element caused by thermal inconsistency was obtained. If the average stress in the effective medium was considered as vanished, the thermal stress field in the matrix and particles was obtained by considering random orientation of two-phase elements. Second, based on the interface cracks propagating around the ellipsoidal particles, the strain energy release rate was examined. When the strain energy release rate reached the critical value of the matrix, the debonding stress was determined. The subtending angle and initiation crack length were available. Finally, according to the low cycle fatigue crack growth rate formation, the thermal fatigue life was computed. The results show that the bigger the thermal stress is, the smaller the thermal fatigue life is, indicating that thermal fatigue life is an exponential function of crack initiation length and critical length.

Effect of Intermetallics on the Low Cyclic Fatigue Crack Growth Behavior of a Heavily Alloyed Al-Si Piston Alloy

This paper focuses on a typical automotive piston material to characterize its fractographic appearance after low cyclic fatigue test. The fatigue fracture of this heavily alloyed Al-Si alloy takes place in a brittle manner. The crack nucleated from a large intermetallic colony close to the specimen surface. When the crack encountered the intermetallics, it might progress along the interface among the intermetallics or the interface between the intermetallics and the eutectic. The debonded Al-Si eutectic region and intermetallics provided a weak material path for the crack propagation. A fatigue fracture mechanism in the heavily alloyed Al-Si alloy is elucidated.

The Effect of Microstructure on Mechanical Properties and Fatigue Crack Growth Behavior of TC4-DT Alloy

This paper examines the influence of different microstructure on the mechanical properties and fatigue crack growth behavior of TC4-DT alloy at room temperature. Different heat treatments were carried out in order to produce the microstructure of bimodal and lamellar conditions. Results show that, the tensile properties and fracture toughness of bimodal and lamellar microstructure was varied by different heat-treatment. In contrast on bimodal microstructure, the lamellar microstructure presents high fracture toughness and low fatigue crack growth rate, but the yield stress and ductility declines. The analysis results indicted that the lamellar microstructure obtained by air cooling from the  phase field showed the integrated condition of tensile properties and fracture toughness duo to the fine secondary  lamella, and the better fatigue crack growth behavior.

부식을 고려한 항공기재료의 부식피로수명예측 연구

In this study, test results for fatigue crack propagation of Al-2024-T3511 are presented considering corrosion effects, and fatigue lifes are compared for the cases of corrosion and non-corrosion. Higher corrosion environments show lower fatigue life and faster crack growth. To predict the corrosion fatigue life, a corrosion factor concept is newly introduced and applied in Paris` law. The predicted results show good agreement with experimental data and this corrosion fatigue model can be successfully used for the prediction of fatigue life of aluminum structure with corrosion effects. ᜎࠀ㘲㐮㄰㔻Ԁ䭃䑎䷕੿6〰㬅K䍄乍쀗ᘍ ܀㘲㐮ㄷ㬅K䍄乍ꠗᘍ頀܀㘲ㄮ㌸㬅K䍄乍

2331 CT試験片における応力三軸度を考慮した低サイクル疲労き裂進展特性に関する研究(S07-4 疲労き裂進展,S07 構造材料の疲労強度とき裂進展問題)

2331 CT試験片における応力三軸度を考慮した低サイクル疲労き裂進展特性に関する研究(S07-4 疲労き裂進展,S07 構造材料の疲労強度とき裂進展問題)

Evaluation of the Effect of Crack Tip Constraint on Fatigue Crack Growth Rate in Inconel 718

Abstract The objective of this work has been to characterize the fatigue crack growth rate of Inconel 718 in the elastic and elastic-plastic regimes. The major new contribution here is to develop fatigue crack growth rate data on this alloy using shallow crack specimens subjected to cyclic loadings that involve material plasticity exceeding what is allowed by the standard Linear Elastic Fracture Mechanics (LEFM) procedures of ASTM E 647. Compact (C(T)) and three point bend (SE(B)) specimen geometries were used in this investigation. The SE(B) specimens were used to obtain shallow crack data using crack to depth ratios (a/W) as small as 0.08 in standard bend specimens with W = 50.8 mm. Compliance methods were used to estimate the crack length during the cyclic testing. The C(T) specimens were used to investigate the effect of fully reversed loading, i.e., R = −1.0. These specimens were tested only in deep crack configurations with a/W > 0.3. Both C(T) and SE(B) specimens were used to obtain high cycle fatigue crack growth data as well as some component of the low cycle fatigue crack growth rate data. The cyclic elastic stress intensity range was used to characterize the crack growth driving “force” in the high cycle regime as defined in ASTM E 647. A cyclic J integral range, as originally utilized by Dowling and Begley [1], was used in the elastic-plastic regime. High cycle and low cycle fatigue crack growth data were successfully obtained from tests on shallow crack SE(B) specimens. High cycle fatigue crack growth was not affected by specimen geometry or by crack length ratio, even for a/W ratios as low as 0.08. The low cycle fatigue crack growth rate was similar, but not identical, to what one would get by extrapolating the high cycle fatigue crack growth rate, as proposed by Dowling and Begley [1]. Under conditions of increasing J range, the crack growth rate under elastic-plastic conditions was accelerated, while under decreasing J range conditions the crack growth rate was decelerated in comparison to the extrapolated high cycle fatigue crack growth rate measurements. The R ratio did not affect the crack growth rate for the two cases tested here, namely R = 0.1 and R = −1.0. More rapidly increasing J range conditions resulted in greater crack growth rate acceleration. In the elastic-plastic fatigue regime, shallow crack specimens demonstrated slower crack growth rates than deep crack specimens.

Fatigue crack growth and wear behaviors in rheocast Al−Si−Mg alloys

Rheocast aluminum alloys, which consist of globular α-Al cells, refined grains and eutectic Si particle, were used to investigate fatigue crack growth and wear characteristics. The Si particles were systematically varied from coarse and acicular shapes to small and globular ones. At low ΔK fatigue crack growth rates decreased in samples consisting of acicular Si particles of large grain size, which induced a large amount of crack closure. Large and acicular Si particles were easily cracked and separated the particle/matrix interface, which promoted to fracture at smaller ΔK. On the other hand, small Si particles made fatigue crack grow, even at a high ΔK region, and increased the fracture toughness of the alloy. However, in the wear test, small eutectic Si particles were pulled out by friction force during sliding wear and the wear loss amount increased with increase in sliding distance.

334 表面き裂を有する配管の低サイクル疲労き裂進展解析(OS1-6 熱影響部と配管の健全性評価)(OS1 高強度材・表面改質材の疲労と破壊)

334 表面き裂を有する配管の低サイクル疲労き裂進展解析(OS1-6 熱影響部と配管の健全性評価)(OS1 高強度材・表面改質材の疲労と破壊)

Influence of Density and Sintering Temperature on Fatigue Crack Growth and Fracture Toughness of FL4405 High Strength PM Steel

Density and sintering temperature effects were investigated with FL4405 low alloy PM high strength steel. Room temperature, plane strain fracture toughness KIc, and constant amplitude fatigue crack growth with (min/max) load ratios of 0·05 and 0·5 and macroand microfractography were evaluated. Two density levels, 7·0 and 7·4 g cm−3, and two sintering temperatures, 1120 and 1315°C, were investigated. Increasing the density and sintering temperature had no influence on stress intensity threshold and near threshold fatigue crack growth resistance where transcrystalline/transparticle cleavage existed. At higher fatigue crack growth rates, where ductile dimples existed, both higher density and higher sintering temperature enhanced fatigue crack growth resistance. The KIc and Kmax values at fatigue crack growth fracture increased at the higher density and increased further at the higher sintering temperature. All final fracture regions were brittle at the macroscale while containing ductile dimples at the micro scale. No crack closure was found in fatigue crack growth tests and no load ratio effects were present at low fatigue crack growth rates. Significant load ratio effects were present at high fatigue crack growth rates. PM/0770

A sensitive mechanical test for slow crack growth in polyethylene

AbstractSlow crack growth (SCG) in a wide variety of polyethylenes has been investigated by a constant tensile load test (the PENT test) for a single edge notched specimen. The PENT test is very sensitive to the changes in molecular structure and morphology of polyethylene. The resistance to SCG depends on the density of the tie molecules and the strength of the crystals.