Crack Tip Closure Research Articles

Fatigue crack opening stress based on the strip-yield model

The modified strip-yield model based on the Dugdale model and two-dimensional approximate weight function method were utilized to evaluate the effect of in-plane constraint, transverse stress, on the fatigue crack closure. The plastic zone sizes and the crack opening stresses considering transverse stress were calculated for four specimens: single edge-notched tension (SENT) specimen, single edge-notched bend (SENB) specimen, center-cracked tension (CCT) specimen, double edge-notched tension (DENT) specimen under uniaxial loading. And the crack opening behavior of the center-cracked specimen under biaxial loading was also evaluated. Normalized crack opening stresses σ op/ σ max for four specimens were successfully described by the normalized plastic zone parameter Δω ′ rev/ ω ′ considering transverse stress, where Δω ′ rev and ω ′ are the size of the reversed plastic zone at the moment of first crack tip closure and the size of the forward plastic zone for maximum stress, respectively. The normalized plastic zone parameter with transverse stress also was satisfactorily correlated with the behavior of crack closure for CCT specimen under biaxial loading.

The equation of the second portion of the fatigue fracture kinetic diagram for various additional load ratios

The second portion of the fatigue fracture kinetic diagram is described by a reliable equation with additional load ratios R in the case of the crack-tip closure effect and by the Forman equation for the values of load ratio R* in the case without the crack closure effect. We have obtained the analytical dependence of the effective range of the stress intensity factor ΔKeff on the nominal range ΔKR for the first part of the second portion of the fatigue fracture kinetic diagram (ΔK1-2,R≤ΔKR≤ΔKfcl,R) with various load ratios.

An Investigation of The Effects of Temperature on Fatigue Crack Growth Behavior of a Cast Nearly Lamellar Ti-47A1–2Cr-2Mn + 0.8 Vol. %TiB2 Gamma Titanium Alloy

ABSTRACTThis paper presents the results of a study of the effects of temperature on fatigue crack growth in Ti-47A1–2Cr-2Mn + 0.8 Vol. %TiB2 gamma titanium aluminide intermetallics. Fatigue crack growth rate data are presented for the cast lamellar microstructure at 25,450 and 750°C. The trends in the fatigue crack growth rate data are explained by considering the combined effects of crack-tip deformation mechanisms and oxide-induced crack closure. Faster fatigue crack growth rates at 450°C are attributed to the high incidence of irreversible deformation-induced twinning, while slower crack growth rates at 700°C are due to increased deformation by slip and the effects of crack-tip shielding provided by oxide-induced wedging, which is analyzed using a modified Dugdale-Barenblatt model.

Fatigue crack growth of SM-1240/TIMETAL-21S metal matrix composites at elevated temperatures

A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process.

Monitoring of surface fatigue crack growth and crack tip closure behavior by the ultrasonic isoscanning technique.

The ultrasonic isoscanning technique using shear waves is employed to monitor the surface fatigue crack growth under a constant stress intensity factor range. It is shown that this technique has the potential to become an effective tool for the evaluation of quantitative surface fatigue crack growth characteristics. Not only can the three-dimensional crack profile be continuously measured with fairly high accuracy during fatigue crack growth testing, but this study indicates that the crack tip closure can be detected at the maximum depth point and near the surface. The surface regions under plane-stress conditions are closed for a greater portion of the fatigue load cycle than the maximum depth point under a predominantly plane-strain condition. The plane-strain crack tip opening load is smaller than the plane stress crack tip opening load.

Monitoring of surface fatigue crack growth and crack tip closure behavior by the ultrasonic isoscan technique.

The ultrasonic isoscan technique using shear waves is employed to monitor the surface fatigue crack growth under a constant stress istensity range. It is shown that this technique has the potential to become an effective tool for quantitative surface fatigue crack growth characteristics. Not only can the three dimensional crack profile be continuously measured with fairly high accuracy during fatigue testing, but this study indicates that the crack tip closure can be detected at the maximum depth point and near the surface. The surface regions under plane-stress conditions are closed for a greater portion of the fatigue load cycle than the maximum depth point under a predominantly plane-strain condition. The plane strain crack tip opening load is smaller than the plane stress crack tip opening load.

Fatigue crack growth behaviour of A533B steel in simulated pwr water

Fatigue crack growth data are reported for A533B material in a simulated primary PWR loop environment. Marked environmentally enhanced crack growth, characterised by the formation of a plateau in the growth rate versus ΔK curve, was observed in sinusoidal tests but was not evident in triangular waveform tests. Detailed fractography showed that inclusions can play an important role in fatigue crack growth behaviour. Of the two basic mechanisms suggested for environmentally enhanced crack growth, namely, dissolution-controlled and hydrogen induced cracking, the latter can more readily explain the results of the present investigation. At low R ratio, changing the waveform from triangular to sinusoidal induced environmentally enhanced crack growth and a possible explanation is given in terms of crack tip closure effects.

Surface Acoustic Wave Measurements of Surface Cracks in Ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.

Surface acoustic wave measurements of surface cracks in ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.

The noncontinuum crack tip deformation behavior of surface microcracks

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.

ASSESSMENT OF CRACK TIP CLOSURE IN AN ALUMINUM ALLOY BY ELECTRONFRACTOGRAPHY

Abstract— Crack tip closure measurements were performed in a 2124 T3 51 aluminum alloy by electronfractography. The technique makes use of high resolution fractography to correlate a closure in striation spacing with a sudden change of the stress intensity factor range following steady state crack growth. The crack growth tests under programmed loads were performed with a servohydraulic machine interfaced to a digital computer. An average value of Kop/Kmax equal to 0·15 was found in the range of 10 MPa√m < ΔK < 35 MPa√m. The higher closure developing after overloads was found a suitable explanation for crack growth retardation.

A theoretical explanation of the delaying effects of overloads on fatigue crack propagation

A simple theoretical model valid for generalized plane stress conditions is described which is able to predict the effects of overloads on crack tip deformation and fatigue crack propagation rates. The model is based upon a method of superimposing crack problems in an attempt to represent the effect of residual stresses in the plastic zones. An expression for the crack opening displacement is derived which is valid whilst the crack tip is propagating through the overload plastic zone. Crack tip closure is predicted during stress cycling and, following tradition, this is regarded as reducing the effect of the stress range intensity factor on crack propagation rate thus causing retardation. The model also predicts that, whenever a simple condition involving applied stress ratios is satisfied, crack arrest will take place. In keeping with experimental observations the model predicts that during its propagation through the overload plastic zone the crack growth rate first declines to a minimum and then increases to the appropriate value which would result if the overload had not been applied. In certain circumstances the model also predicts some acceleration effects and delayed retardation.

Crack tip closure and environmental crack propagation

Crack propagation rate, da/dN, and crack tip closure stress, σ cc, in part-through crack fatigue specimens of aluminum alloys are drastically affected by gaseous environments. The present studies indicate that the crack closure reflects the influence of the environment on the plastic deformation at the crack tip, and, therefore, on the crack propagation rates. Postulating that da/dN is mainly determined by ΔK eff ∝ (σ max−σ cc) (instead of ΔK ∝ (σ max−σ min) , as is done traditionally) leads to the relationship da/dN = A(ΔK eff) n in which A and n are virtually independent of the gaseous environment. The exponents are n ≈ 3.3 for Al 7075 T651 and n ≈ 3.1 for Al 2024 T351, respectively.