Direct Observation and Mechanism of Fatigue Crack Propagation

Kikukawa, Makoto; Jono, Masahiro; Adachi, M.

doi:10.1520/stp35892s

Cited by 31 publications

(16 citation statements)

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“…The mechanism of formation of fatigue striations has been discussed by several researchers ( Ref 7,9,10). Following the explanations presented by Furukawa et al (Ref 7), there are minor differences in the hypotheses assumed by these researchers because the slip-off mechanism is the fundamental aspect commonly considered in these hypotheses.…”

Section: Resultsmentioning

confidence: 98%

Load Ratio Estimation Through Striation Height and Spacing Analysis of an Aerospace Al Alloy 7475-T7351

Ruckert

Filho

et al. 2010

J. of Materi Eng and Perform

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It is well known that striation spacing may be related to the crack growth rate, da/dN, through Paris equation, as well as the maximum and minimum loads under service loading conditions. These loads define the load ratio, R, and are considered impossible to be evaluated from the inter-spacing striations analysis. In this way, this study discusses the methodology proposed by Furukawa to evaluate the maximum and minimum loads based on the experimental fact that the relative height of a striation, H, and the striation spacing, s, are strongly influenced by the load ratio, R. Fatigue tests in C(T) specimens were conducted on SAE 7475-T7351 Al alloy plates at room temperature and the results showed a straightforward correlation between the parameters H, s, and R. Measurements of striation height, H, were performed using scanning electron microscopy and field emission gun (FEG) after sectioning the specimen at a large inclined angle to amplify the height of the striations. The results showed that for increasing R the values of H/s tend to increase. Striation height, striation spacing, and load ratio correlations were obtained, which allows one to estimate service loadings from fatigue fracture surface survey.

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Section: Resultsmentioning

confidence: 98%

Load Ratio Estimation Through Striation Height and Spacing Analysis of an Aerospace Al Alloy 7475-T7351

Ruckert

Filho

et al. 2010

J. of Materi Eng and Perform

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“…The larger the COD, the higher is the crack growth rate. This phenomenon was first observed by Laird and Smith [51] and has been well established in long-crack growth [52,53]. The exponent n ¢ has been found to have a value near unity when the COD is replaced by crack-tip-opening displacement (CTOD) (42.13) where the CTOD, denoted by f t , is measured at the location of crack extension for the previous cycle.…”

Section: Micromechanics Of Small-crack Growthmentioning

confidence: 86%

Using Probabilistic Microstructural Methods to Predict the Fatigue Response of a Simple Laboratory Specimen

Tryon¹

2004

Engineering Design Reliability Handbook

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For the purpose of this chapter, fatigue is defined as the entire range of damage-accumulation sequences, from crack nucleation of the initially unflawed bar to final fast fracture. Background Various Stages of FatigueCurrent fatigue-life prediction methods in metallic components consider three stages: crack initiation, long-crack propagation, and final fracture. Long-crack propagation and final fracture are the stages of damage accumulation that are well characterized using linear elastic or elastic-plastic fracture mechanics. Crack initiation is the early stage of damage accumulation where small cracks (cracks with depths less than several grain diameters) have been observed to deviate significantly from predicted long-crack fracture mechanics behavior [1]. The deviation is attributed to the heterogeneous media in which small cracks evolve [2].The crack-initiation stage can be broken down into two phases: crack nucleation and small-crack growth. Crack nucleation is the locally complex process of crack formation on the microstructural scale. Crack nucleation is characterized by smooth fracture surfaces at angles inclined to the loading direction. This type of failure is indicative of shear stress Mode II (sliding mode) fracture. Although loading has been shown to affect the nucleation size [3,4], experimental evidence suggest that the nucleation size is on the order of the grain size [5][6][7].Small-crack growth is characterized by fracture-surface striations perpendicular to the loading direction. This type of failure is indicative of tensile stress Mode I (opening mode) fracture. The behavior of small cracks tends to transition to linear or elastic-plastic fracture mechanics behavior when the crack depth reaches about ten mean grain diameters [8]. Crack nucleation and small-crack growth must be modeled separately because different mechanisms control each phase. The relative importance of the crack-nucleation stage on overall fatigue life depends on several factors. Materials that exhibit a strong preference for planar slip show a strong correlation between the crack causing final fracture and the earliest nucleated cracks [9]. Materials that prefer cross slip showed almost no correlation between the crack causing final fracture and the earliest nucleated cracks [9]. The relative importance of the crack nucleation may also depend on the loading condition. If the loading is relatively low (high-cycle fatigue), the majority of life will be spent in the nucleation of a crack. If the loading is high (low-cycle fatigue), cracks may nucleate early and spend the remainder of the fatigue life in the crack-growth stages. However, high-strength materials have been shown to spend the majority of fatigue life in the crack-nucleation stage, even during low-cycle fatigue [10]. Scatter in Fatigue LifeThe coefficient of variation (COV) of fatigue-life tests range widely, depending on the material alloy and load level. Even for well-controlled laboratory tests of annealed smooth specimens at room temperature, the COV varies fro...

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“…9 is developed. The direct metal-metal contacts can occur during the unloading process, causing cold-welding (re-bonding) at the contact surfaces [15][16][17]. During the process, microstructure refinement can also occur near the crack surfaces.…”

Section: Discussionmentioning

confidence: 99%

Fatigue crack propagation properties of Ti–6Al–4V in vacuum environments

Oguma

Nakamura

2013

International Journal of Fatigue

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Abstract:To determine the effects of vacuum environment on fatigue crack propagations in a Ti-6Al-4V alloy, K-decreasing tests were conducted in air and vacuum. The fatigue crack propagation rate became slower and threshold stress intensity factor range became larger with decreasing vacuum pressure.The tendency cannot be fully explained by the crack closure. Based on fracture surface observations, granular region of a few micrometer size asperities was observed on the fracture surface only in high vacuum and ultra high vacuum. The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of adsorbed gas on fracture surfaces relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between vacuum and air environments. A new mechanism for the formation of the granular region was proposed, and that is one of the phenomena which can explain the reduction of crack propagation rate in vacuum.

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Direct Observation and Mechanism of Fatigue Crack Propagation

Cited by 31 publications

References 0 publications

Load Ratio Estimation Through Striation Height and Spacing Analysis of an Aerospace Al Alloy 7475-T7351

Load Ratio Estimation Through Striation Height and Spacing Analysis of an Aerospace Al Alloy 7475-T7351

Using Probabilistic Microstructural Methods to Predict the Fatigue Response of a Simple Laboratory Specimen

Fatigue crack propagation properties of Ti–6Al–4V in vacuum environments

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