Fatigue crack growth and crack closure under variable loadings on aluminum alloys. (Effect of load variation on characteristic form of crack growth rate curve in region II)

The fatigue crack growth behavior of one compact tension specimen of 16MnR steel under high-low sequence loading was investigated. The symmetric half finite element model under plane-stress state was used to calculate the elastic-plastic stress-strain responses, in which the Armstrong-Frederick type cyclic plasticity model was implemented as a user material subroutine UMAT of ABAQUS. A recently developed dynamic crack growth model was used to simulate the effects of high loading step on the successive low loading step. The detailed evolution process of the crack closure and cyclic plastic zone within the retardation region of fatigue crack growth was obtained. The extend of the crack closure, the size of cyclic plastic zone and the stress gradient have significant influence on the fatigue crack growth rate. The predicted fatigue crack growth rate is in good agreement with the experimental data.

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Plane Strain Crack Growth Models for Fatigue Crack Growth Life Predictions

Journal of Pressure Vessel Technology ◽

10.1115/1.2842167 ◽

1996 ◽

Vol 118 (1) ◽

pp. 78-85 ◽

Cited By ~ 1

Author(s):

J. M. Bloom ◽

S. R. Daniewicz ◽

J. L Hechmer

Keyword(s):

Growth Rate ◽

Fatigue Crack ◽

Crack Growth Rate ◽

Fatigue Crack Growth ◽

Crack Growth ◽

Crack Closure ◽

Crack Opening ◽

Constraint Factor ◽

R Ratio ◽

Point Bend

Experimental data and analytical models have shown that a growing fatigue crack produces a plastic wake. This, in turn, leads to residual compressive stresses acting over the crack faces during the unloading portion of the fatigue cycle. This crack closure effect results in an applied stress intensity factor during unloading which is greater than that associated with the Kmin, thus producing a crack-driving force which is less than ΔK = Kmax − Kmin. Life predictions which do not account for this crack closure effect give inaccurate life estimates, especially for fully reversed loadings. This paper discusses the development of a crack closure expression for the 4- point bend specimen using numerical results obtained from a modified strip-yield model. Data from tests of eight 4-point bend specimens were used to estimate the specimen constraint factor (stress triaxiality effect). The constraint factor was then used in the estimation of the crack opening stresses for each of the bend tests. The numerically estimated crack opening stresses were used to develop an effective stress intensity factor range, ΔKeff The resulting crack growth rate data when plotted versus ΔKeff resulted in a material fatigue crack growth rate property curve independent of test specimen type, stress level, and R-ratio. Fatigue crack growth rate data from center-cracked panels using Newman's crack closure model, from compact specimens using Eason 's R-ratio expression, and from bend specimens using the model discussed in this paper are all shown to fall along the same straight line (on log-log paper) when plotted versus ΔKeff, even though crack closure differs for each specimen type.

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