Experimental Determination of Fatigue Crack Growth Behavior and Surface Strain Distribution of ‘Faint-Waist Pure Shear’ Specimens with Different Crack Tip Radii

2006 ◽  
Vol 79 (4) ◽  
pp. 712-733 ◽  
Author(s):  
Christian Feichter ◽  
Zoltan Major ◽  
Reinhold W. Lang
Keyword(s):  
Crack Growth ◽  
Strain Gradient ◽  
Strain Distribution ◽  
Strain Analysis ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Analysis Method ◽  
Growth Initiation ◽  
Tip Radius

Abstract The influence of the initial crack-tip radius on the fatigue behavior and the strain distribution in the vicinity of the blunted crack tip was determined experimentally using a servo-hydraulic testing machine and an optical full-field strain analysis method. Two different elastomer grades (SBR, EPDM) were selected for the experimental work. The strain analysis method used, based on the image correlation technique, was found to be an effective tool to determine strains, strain distributions and gradients near the crack tip for elastomeric materials. Different material behavior was observed in the two rubber types investigated. While the crack tip was regularly blunted (half circle shape) for EPDM and the strain gradient was low (less steep), the crack tip was sharp (less blunted) with a higher strain gradient for SBR. Furthermore the crack tip radius was found to be an important influencing factor on the initiation of crack growth, but not on the crack growth behavior after initiation. Based on the experimental results a phenomenological, first-order, crack-growth initiation law was proposed, dependent on the crack tip radius, the tearing energy at crack growth initiation for a sharp crack and a parameter representing the influence of the crack tip radius.

Effects of Loading Spectra on High pH Crack Growth Behavior of X65 Pipeline Steel

CORROSION ◽  
10.5006/3472 ◽  
2020 ◽  
Vol 76 (6) ◽  
pp. 601-615 ◽  
Author(s):  
Hamid Niazi ◽  
Karina Chevil ◽  
Erwin Gamboa ◽  
Lyndon Lamborn ◽  
Weixing Chen ◽  
...  
Keyword(s):  
Free Surface ◽  
Crack Initiation ◽  
Crack Propagation ◽  
Strain Rate ◽  
Crack Growth ◽  
Pipeline Steel ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Secondary Crack

The effects of mechanical factors on crack growth behavior during the second stage of high pH stress corrosion cracking in pipeline steel were investigated by applying several loading scenarios on compact tension (CT) specimens. The main mechanism for stage 2 of intergranular crack propagation is anodic dissolution ahead of the crack tip which is highly dependent on crack-tip strain rate. The maximum and minimum crack growth rates were 3 × 10−7 mm/s and 1 × 10−7 mm/s, respectively. It was observed that several factors such as mean stress intensity factor, amplitude, and frequency of loading cycles determine the crack-tip strain rate. Low R-ratio cycles, particularly high-frequency ones, enhance secondary crack initiation, and crack coalescence on the free surface. This mechanism accelerates crack advance on the free surface which is accompanied with an increase in mechanical driving force for crack propagation in the thickness direction. These findings have implications for pipeline operators and could be used to increase the lifespan of the cracked pipelines at stage 2. For those pipelines, any loading condition that increases the strain rate ahead of the crack tip enhances anodic dissolution and is detrimental. Additionally, secondary crack initiation and coalescence could be minimized by avoiding internal pressure fluctuation, particularly rapid large pressure fluctuations.

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.

scholarly journals FATIGUE CRACK GROWTH BEHAVOR OF CARBON BLACK–REINFORCED NATURAL RUBBER

2021 ◽  
Author(s):  
Lewis B. Tunnicliffe
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Carbon Black ◽  
Natural Rubber ◽  
Crack Growth ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Fatigue Crack Growth Behavior ◽  
Amplification Factors

ABSTRACT Fatigue crack growth behavior of carbon black–reinforced natural rubber is investigated. Rubber compounds of Shore A = 70 are prepared by varying the formulation loadings of a wide range of carbon black types based on their structure and surface area properties. The resulting fatigue crack growth behavior shows significant variation in β exponent values, depending on the properties of the carbon black. These variations are rationalized by considering the strain amplification of natural rubber by carbon black aggregates in the region of compound directly ahead of the crack tip. An assumption is made that little networking of the carbon black aggregates exists in this region of very high strain and that hydrodynamic calculations that consider occluded rubber can therefore provide realistic values for strain amplification. A reasonable scaling of power law crack growth parameters to calculated strain amplification factors is found, with the exponent, β, decreasing with increasing strain amplification. The implication here is that enhanced strain amplification promotes the formation of strain-induced crystallites in the crack tip region. Performance tradeoffs resulting from the crossover of crack growth data sets dependent on the carbon black type are discussed. Of practical significance is the fact that the strain amplification factors can be calculated directly from knowledge of carbon black type and loading in rubber formulations.

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.

scholarly journals Modeling of environmentally assisted fatigue crack growth behavior

2015 ◽  
Vol 33 (6) ◽  
pp. 351-359
Author(s):  
Daniel Kujawski ◽  
Phani C.R. Sree
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Extension ◽  
Crack Tip ◽  
Load Cycle ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Loading Frequency ◽  
Crack Tip Opening

AbstractIt is well recognized that environment has a significant role on the failure of mechanically loaded structures. In most cases of cyclic loading, fatigue crack growth (FCG) behavior exhibits lower threshold and faster growth rate in air than in vacuum. It is well documented that the effect of loading frequency on FCG behavior can be more pronounced in aggressive environment/material systems. This is seen in the Kmax term of the FCG. On the other hand, a weak dependence of FCG behavior with R ratio in inert environment indicates that a crack extension is governed mainly by ΔK. Existing experimental data indicate that the actual crack extension per cycle is associated with the rising part of the load cycle than the unloading part. In this paper, the synergetic role of environment and mechanical loading on crack growth behavior is considered to see their roles on FCG. In this article, we attempt to model how crack extension interplays between a crack-tip opening and crack-tip blunting angle associated with the applied load and environment, respectively. To support such a model for discussion, we have selected limited FCG data taken from literature corresponding to different environments ranging from vacuum to air and NaCl solution for a number of alloys and with different specimens geometries. We are also not discussing innate mechanisms for each alloy, due to space concerns.

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