scholarly journals In-situ SEM and optical microscopy testing for investigation of fatigue crack growth mechanism under overload

2018 ◽  
Vol 165 ◽  
pp. 13013
Author(s):  
Wei Zhang ◽  
Liang Cai
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Optical Microscopy ◽  
Crack Growth ◽  
Load Condition ◽  
Crack Tip ◽  
Initial Crack ◽  
Crack Growth Behavior ◽  
Single Overload

In this paper, the in-situ scanning electron microscope (SEM) and optical microscopy experiments are performed to investigate the crack growth behavior under the single tensile overload. The objectives are to (i) examine the overload-induced crack growth micromechanisms, including the initial crack growth acceleration and the subsequent retardation period; (ii) investigate the effective region of single overload on crack growth rate. The specimen is a small thin Al2024-T3 plate with an edge-crack, which is loaded and observed in the SEM chamber. The very high resolution images of the crack tip are taken under the simple variable amplitude loading. Imaging analysis is performed to quantify the crack tip deformation at any time instant. Moreover, an identical specimen subjected to the same load condition is observed under optical microscope. In this testing, fine speckling is performed to promote the accuracy of digital imaging correlation (DIC). The images around the crack tip are taken at the peak loads before, during and after the single overload. After that, the evolution of local strain distribution is obtained through DIC technique. The results show that the rapid connection between the main crack and microcracks accounts for the initial crack growth acceleration. The crack closure level can be responsible for the crack growth rate during the steady growth period. Besides that, the size of retardation area is larger than the classical solution.

An Empirical Model for Environmental Damage at the Crack Tip

1988 ◽  
Vol 125 ◽  
Author(s):  
K. -M. Chang
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Damage Zone ◽  
Crack Tip ◽  
Initial Crack ◽  
Environmental Damage ◽  
Crack Growth Behavior ◽  
Sustained Loading ◽  
Damaged Zone

ABSTRACTAn experimental method that is able to verify and to characterize the damage zone developed in front of crack tip under time-dependent fatigue crack propagation has been established. The crack growth rate was found to decrease exponentially in the damaged zone. Such a crack growth behavior can be described by a mathematical model which only requires three parameters: initial crack growth rate, (da/dN)q; normal crack growth rate (da/dN)o; and damaged zone size, D. An analyticat relationship has been developed to correlate the temperature and the time of sustained loading to the size of the induced damaged zone.

Effect of Constraint Induced by Specimen Geometries on Crack Growth Behavior Under Creep-Fatigue Interaction

2018 ◽  
Author(s):  
Lei Zhao ◽  
Lianyong Xu
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Growth Rates ◽  
Crack Tip ◽  
Crack Growth Behavior ◽  
Growth Behaviour ◽  
Crack Growth Behaviour ◽  
Creep Fatigue ◽  
Crack Growth Rates

Creep-fatigue interaction would accelerate the crack growth behaviour and change the crack growth mode, which is different from that presenting in pure creep or fatigue regimes. In addition, the constraint ahead of crack tip affects the relationship between crack growth rate and fracture mechanics and thus affects the accuracy of the life prediction for high-temperature components containing defects. In this study, to reveal the role of constraint caused by various specimen geometries in the creep-fatigue regime, five different types of cracked specimens (including C-ring in tension CST, compact tension CT, single notch tension SENT, single notch bend SENB, middle tension MT) were employed. The crack growth and damage evolution behaviours were simulated using finite element method based on a non-linear creep-fatigue interaction damage model considering creep damage, fatigue damage and interaction damage. The expression of (Ct)avg for different specimen geometries were given. Then, the variation of crack growth behaviour with various specimen geometries under creep-fatigue conditions were analysed. CT and CST showed the highest crack growth rates, which were ten times as the lowest crack growth rates in MT. This revealed that distinctions in specimen geometry influenced the in-plane constraint level ahead of crack tip. Furthermore, a load-independent constraint parameter Q* was introduced to correlate the crack growth rate. The sequence of crack growth rate at a given value of (Ct)avg was same to the reduction of Q*, which shown a linear relation in log-log curve.

Predicting the Crack Growth Behavior in a Filled Elastomer

2006 ◽  
Vol 3-4 ◽  
pp. 273-278
Author(s):  
C.T. Liu ◽  
M. Yen ◽  
H.K. Ching
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Length ◽  
Crack Growth ◽  
Growth Model ◽  
Initial Crack ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Initial Crack Length ◽  
Crack Growth Model

In this study, single-edge cracked uniaxial specimens with an initial crack length of 0.1 in. or 0.3 in. and wedge-shaped sheet specimens with an initial crack length of 0.3 in were tested at a constant displacement rate of 50 in/min under 1000 psi confining pressure. The specimens were made of a highly filled polymeric material, containing 86% by weight of hard particles embedded in a rubbery matrix, which was made of polybutadiene-acrylic acid-acrylonitrile rubber. The uniaxial crack growth data were used to develop a crack growth model, relating crack growth rate da/dt and Mode I stress intensity factor KI. The developed crack growth model was used to predict the crack growth behavior in the wedge-shaped specimen. The results of the analysis indicated that the predicted crack growth rate compared well with the experiment

Experimental Study of the Fatigue Crack Propagation and Overloading Retardation in TA2 Plate

2015 ◽  
Vol 1120-1121 ◽  
pp. 1008-1013
Author(s):  
Hui Fang Li ◽  
Yuan Hong ◽  
Cai Fu Qian
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Crack Tip ◽  
Stress Intensity Factor Range ◽  
Crack Growth Behavior ◽  
Mode I ◽  
Crack Growth Path ◽  
Inclined Angle

In this paper, I+II mixed mode notch-crack fatigue propagation in titanium alloy steel TA2 was tested with the emphasis on the crack mode transition and retardation under constant amplitude loading or overloading. Finite element method was employed to calculate the stress distribution at the crack tip in order to explain the crack growth behavior. It is found that after initiation from the crack tip, the new formed crack propagates in a mode I form, regardless of the magnitudes of the inclined angle of the crack and the overloading applied. The relationship between the fatigue crack growth rate and mode I stress intensity factor range was calculated. After overloading, crack initiation and propagation will be significantly slowed, or in other words, there exists overloading retardation. Calculation shows that after overloading, a plastic zone with residual compressive stress is formed which is responsible for the retardation of the crack growth rate. Micro-morphologies of the crack growth path and the crack fracture surface were also observed and analyzed.

Environmental Effect of Crack Growth Rate of Pipeline Steel in Near-Neutral pH Soil Environments

2004 ◽  
Author(s):  
Weixing Chen ◽  
Robert Sutherby
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Soil Chemistry ◽  
Pipeline Steel ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
General Corrosion ◽  
Neutral Ph

The laboratory work reported here was initiated to determine whether different soils can be shown to give rise to different growth rate for a given pipeline steel. Two soil synthetic environments with different near neutral pH value were designed based on various soil chemistries collected near the pipeline in the field where near-neutral pH SCC was found. The crack growth behavior in both the environments were determined using compact tension specimen. The crack growth rate was in situ monitored by the potential drop system. It was found that soil chemistry has a profound effect on crack growth rate. Although it is insensitive to the soil chemistry and cyclic frequency, the crack growth rate in the high ΔK regime has been significantly enhanced in comparison with that in air. In the low ΔK regime, the growth rate is shown to have minor dependence on ΔK value but strong dependence on the testing environments. The observed crack growth behavior in different ΔK regimes and environments was related to the crack tip sharpness and crack crevice wideness as a result of corrosion and room temperature creep deformation. Soil solutions with low general corrosion rate are associated with a blunt crack tip and wide crack crevice, which would result in lower stress intensity at the crack tip and weaker crack closure effect, respectively. Similarly, a loading wave allowing shorter creep time on a given volume of material at the crack tip at high loading stress tends to produce a sharper crack tip and narrow crack crevice. These two factors have opposite effect on crack growth rate, and the observed crack growth rate reflects the combined effect of these two opposite factors.

In-Situ SEM Testing for Transient Fatigue Crack Growth Behavior Investigation Subjected to a Single Tensile Overload

2014 ◽  
Author(s):  
Wei Zhang ◽  
Yongming Liu
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Closure ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Fatigue Crack Growth Behavior ◽  
Single Overload

In this paper, the in-situ scanning electron microscopy (SEM) experiments are performed in the edge-cracked specimen under the single overload in order to investigate transient fatigue crack growth behavior. The specimen is made of Al7075-T6 and under the plane stress condition. During the testing, several loading cycles of interest are selected and divided into a certain number of steps. At each step, high resolution images around the crack tip region are taken under the SEM. Imaging analysis is used to quantify the crack tip opening displacement (CTOD) at each corresponding time instant in a loading cycle. In the current experimental work, the crack closure phenomenon is not only directly observed under constant amplitude loadings, but also under the variable amplitude loading. The experimental results provide the evidence that the crack closure may disappear or become inconsequential right after the single overload. And some observations imply that the crack closure is not the only parameter which controls fatigue crack growth rate, other factors need to be considered. A detailed discussion is given based on the current investigation.

EFFECTS OF OVERLOAD AND SUPPLYING OIL ON CRACK GROWTH BEHAVIOR IN MG ALLOYS

2021 ◽  
Author(s):  
XUECHAO ZHANG ◽  
CHOBIN MAKABE ◽  
TATSUJIRO MIYAZAKI
Keyword(s):  
Growth Rate ◽  
Crack Growth ◽  
Growth Process ◽  
Stress Amplitude ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Fracture Surface Roughness ◽  
Magnesium Alloy Az31 ◽  
Single Overload

A single overload was applied during the crack growth process under constant stress amplitude, and retardation of crack growth was observed in the case of magnesium alloys as well as carbon steel, aluminum alloys, etc. The retardation of crack growth was related to crack closure, the fracture surface roughness, and crack tip deformation. In addition, the effects of supplying oil into the crack on crack growth behavior of an overloaded specimen were investigated in this study. The crack growth rate in the case of supplying oil became lower than in the case without supplying oil. In the case of the magnesium alloy AZ31, powder of oxide magnesium appeared from the crack after overloading. It is one of the typical behaviors of AZ31. In the case of AZ31 and AZX912, the crack growth behavior after overloading was slightly different due to the deformation of the crack tip.

Evaluation of Excessive Loading Effect on Fatigue Crack Growth Behavior Based on Crack Blunting and Stress Distribution in Front of the Crack Tip

2013 ◽  
Author(s):  
Yoshihito Yamaguchi ◽  
Jinya Katsuyama ◽  
Kunio Onizawa ◽  
Yinsheng Li
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Stress Distribution ◽  
Crack Growth ◽  
Compressive Load ◽  
Crack Tip ◽  
Crack Growth Behavior ◽  
Crack Blunting

It is very important to establish an evaluation method of the structural integrity of piping beyond the small scale yielding condition due to large earthquakes. One of the key issues is the effect of excessive loading on the fatigue crack growth behavior. We performed fatigue crack growth tests under constant amplitude cyclic loading with a single excessive tensile/compressive load. The stress distribution in front of crack tip and crack blunting were estimated by FEM analyses. After the crack tip was blunted by the excessive tensile loading, the effect of the excessive loading on crack growth rate varied depending on the magnitude of the subsequent compressive loading. When a compressive load is enough to close the crack, the crack growth rate became higher than that before the excessive tensile loading while increasing the tensile stress in front of crack tip. A crack growth prediction method has been proposed considering the effects of the excessive loading based on the variation of the stress distribution in front of crack tip and the crack blunting. The predicted crack growth rate by the proposed method was correlated with the experimental ones.

scholarly journals Study on the Influence of the Gurson–Tvergaard–Needleman Damage Model on the Fatigue Crack Growth Rate

Metals ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 1183
Author(s):  
Edmundo R. Sérgio ◽  
Fernando V. Antunes ◽  
Diogo M. Neto ◽  
Micael F. Borges
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Plastic Strain ◽  
Crack Growth ◽  
Damage Model ◽  
Crack Tip ◽  
Strength Parameter ◽  
Stress Intensity Factor Range

The fatigue crack growth (FCG) process is usually accessed through the stress intensity factor range, ΔK, which has some limitations. The cumulative plastic strain at the crack tip has provided results in good agreement with the experimental observations. Also, it allows understanding the crack tip phenomena leading to FCG. Plastic deformation inevitably leads to micro-porosity occurrence and damage accumulation, which can be evaluated with a damage model, such as Gurson–Tvergaard–Needleman (GTN). This study aims to access the influence of the GTN parameters, related to growth and nucleation of micro-voids, on the predicted crack growth rate. The results show the connection between the porosity values and the crack closure level. Although the effect of the porosity on the plastic strain, the predicted effect of the initial porosity on the predicted crack growth rate is small. The sensitivity analysis identified the nucleation amplitude and Tvergaard’s loss of strength parameter as the main factors, whose variation leads to larger changes in the crack growth rate.

Estimation of Crack Growth Life in Rail with a Squat Defect

2009 ◽  
Vol 417-418 ◽  
pp. 313-316 ◽  
Author(s):  
Hyun Kyu Jun ◽  
Won Hee You
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Rail Steel ◽  
Crack Tip ◽  
Cost Effective ◽  
Rolling Contact ◽  
Hertzian Contact ◽  
Maintenance Strategy ◽  
Whole Life Cycle

Rolling contact fatigue initiated defects such as surface corrugation, head check, squat, are one of growing problems in high speed railway line. A squat is generally developed below the rail surface and grows parallel to surface until it turns down into rail. Estimation of critical crack size and crack growth rate is an essential to prevent rail from failure and develop cost effective railway maintenance strategy. In this study, we predict crack growth rate of a rail with a squat defect. For this purpose, a rail model with a squat defect is developed. Timoshenko’s beam theory is applied to calculate the global bending stress at the crack tip and Hertzian contact model is applied to calculate the local contact stresses at the surface of rail by simulating rolling over a railway wheel on a rail. Stress intensity factors are calculated from the total stress at the crack tip. Fatigue crack growth curve of 60kg rail steel is applied to calculated crack growth rate. Software to predict crack growth life through whole life cycle is developed. We expect that we can make a more cost effective rail maintenance strategy by considering the crack growth analysis for a defective rail.

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