Effect of fracture surface interference on shear crack growth

Abstract In this paper dynamic fracture behavior of unidirectional graphite-epoxy composite plates is investigated experimentally and numerically. Crack propagation experiments are conducted on thick unidirectional graphite-epoxy composite plates subjected to in-plane, symmetric and asymmetric, impact loading. The coherent gradient sensing technique (CGS) is used in conjunction with high-speed photography to visualize the crack growth events. Cracks are found to propagate at subsonic speeds in the Mode-I case, whereas in both mixed mode and Mode-II the crack tip speed clearly exceeds the shear wave speed of the laminate. In the case of symmetric loading (Mode-I), the crack tip speeds approach the Rayleigh wave speed of the composite (1500 m/s), however it never exceeds it as predicted by asymptotic analysis. The situation is found to be entirely different for growing shear (Mode-II) cracks. A shock wave emanating from the crack tip is observed in the optical patterns. This provides direct evidence that the crack propagates faster than the shear wave speed of the composite. The crack tip speed is then observed to jump to a level close to the axial longitudinal wave speed along the fibers (7500 m/s) and then to stabilize to a lower level of approximately 6500 m/s. This speed corresponds to the speed at which the energy release rate required for shear crack growth is non-zero as determined from asymptotic analysis. The CGS interferograms also reveal the existence of large-scale frictional contact of the crack faces behind the moving shear cracks. In addition high speed thermographic measurements are conducted that show concentrated hot spots behind the crack tip indicating crack face frictional contact. Finally, these experiments are modeled by a detailed dynamic finite element calculation involving cohesive elements, newly developed adaptive remeshing using subdivision and edge collapse, composites element, and penalty contact. The numerical calculations are calibrated on the basis of fundamental material properties measured in the laboratory. The numerical methodology is subsequently validated by direct comparison to optical experimental measurements (crack speed record and near tip deformation field structure). For shear crack growth the numerics also reveal the experimentally observed shock wave structure and confirm the optical observation of large-scale crack face contact.

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Estimation of Applied Stress at Fatigue Cracks of a Single Crystal Superalloy

Volume 10B: Structures and Dynamics ◽

10.1115/gt2020-14840 ◽

2020 ◽

Author(s):

Daisuke Kobayashi ◽

Katsuhiro Takama ◽

Tomihiko Ikeda

Keyword(s):

Growth Rate ◽

Fatigue Crack ◽

Crack Growth Rate ◽

Fatigue Crack Growth ◽

Fracture Surface ◽

Single Crystal ◽

Crack Growth ◽

Applied Stress ◽

Fatigue Crack Growth Rate ◽

Single Crystal Superalloy

Abstract Needless to say, it is important to estimate the stress applied to a material when conducting failure analysis. In recent years, a material assessment method using electron back-scatter diffraction (EBSD) has been developed. It has been reported that a characteristic misorientation distribution corresponding to the fracture mode is seen in cross-sectional EBSD observation near the fracture surface of a Ni-based superalloy. Furthermore, the authors discovered EBSD striations on the crack cross-section, which is formed with each fatigue crack growth during a turbine shut-down process. This was discovered in misorientation analysis on a single-crystal superalloy blade used in a commercial land-based gas turbine. Since Ni-based superalloys have high deformation resistance, they do not undergo enough ductile deformation to form striations at the crack tip on the fracture surface during fatigue crack growth, and, as a result, striations corresponding to cyclic loadings are rarely observed in fractography. Even in such a Ni-based superalloy with brittle crack growth, the fatigue crack growth rate and the applied stress can be estimated by measuring EBSD striation spacing in misorientation analysis. However, a practical problem in assessment is that the resolution limit fixed with field emission scanning electron microscopes (FE-SEM) determine the range in which crack growth rate can be assessed. Hence, it is difficult to clearly discriminate the EBSD striations when the fatigue crack growth rate is too low, such as in the low stress intensity factor range (ΔK) region. The applied stress can be calculated from ΔK. Therefore, in this paper, in order to estimate the applied stress during fatigue crack growth, we focused on estimating ΔK by measuring the plastic zone size along the crack growth.

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Tensile fracture and fractographic analysis of 1045 spheroidized steel under hydrostatic pressure

Journal of Materials Research ◽

10.1557/jmr.1990.0089 ◽

1990 ◽

Vol 5 (1) ◽

pp. 83-91 ◽

Cited By ~ 42

Author(s):

A. S. Kao ◽

H. A. Kuhn ◽

O. Richmond ◽

W. A. Spitzig

Keyword(s):

High Pressure ◽

Hydrostatic Pressure ◽

Fracture Surface ◽

Atmospheric Pressure ◽

Fracture Mechanism ◽

Shear Crack ◽

Applied Pressure ◽

Void Formation ◽

Tensile Fracture ◽

Central Crack

Void formation in tensile test under hydrostatic pressure is characterized through quantitative metallography, and the fracture mechanism under pressure is analyzed by fractography. Transition of the fracture surface from the cup-and-cone under atmospheric pressure to a slant structure under high pressure is explained on the basis of the void development leading to fracture and the concomitant change in fracture mechanism. The concept of “shear blocks” is introduced to illustrate the features observed on the fracture surface of specimens tested under high pressure. It is postulated that shear blocks evolve to connect the central crack regions with the shear crack initiated on neck surface due to the severe necking deformation under applied pressure.

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Analysis on Low Cycle Fatigue Life and Fatigue Fracture Surface Morphology of TC25 Titanium Alloy

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.697.652 ◽

2016 ◽

Vol 697 ◽

pp. 652-657

Author(s):

Rong Guo Zhao ◽

Yi Yan ◽

Yong Zhou Jiang ◽

Xi Yan Luo ◽

Qi Bang Li ◽

...

Keyword(s):

Titanium Alloy ◽

Fatigue Crack ◽

Fatigue Life ◽

Fracture Surface ◽

Crack Growth ◽

Fatigue Fracture ◽

Low Cycle Fatigue ◽

Observation System ◽

Cycle Fatigue ◽

Stress Strain

At room temperature, the low cycle fatigue tests for smooth specimens of TC25 titanium alloy under various stress ranges are operated at a CSS280I-20w Electro Hydraulic Servo Universal Testing Machine with a microscopic observation system, and the low cycle fatigue lifetimes are measured. Based upon the analysis of stress-strain hysteresis loop of low cycle fatigue of TC25 titanium alloy, a simplified Manson-Coffin formula is derived according to both the experimental characteristics and the stress-strain constitutive model, the fatigue lifetimes are plotted against stress ranges, and a stress-fatigue life curve for TC25 titanium alloy is obtained by the linear regression analysis method. Finally, the fracture surface morphologies of TC25 specimens are investigated using a JSM-6360 Scanning Electron Microscopy, and the fatigue fracture mechanisms of low cycle fatigue are studied. It shows that the plastic deformations are mainly formed at the accelerated fracture stage, and various shear lips can be observed on the fracture surfaces, which demonstrates that the shear stress results in the final rupture of TC25 titanium alloy. During the fracture of low cycle fatigue, the cleavage nucleation leads to the formation of fatigue crack initiation region, the fatigue crack growth exhibits a mixed transgranular and intergranular crack growth mode, and in the final rupture region, the fracture surface of low cycle fatigue of TC25 titanium alloy appears as a typical semi-brittle fracture mode.

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Effective Stress Intensity Factor in Mode III Crack Growth in Round Shafts

Recent Advances in Mechanics of Solids and Structures ◽

10.1115/imece2003-43478 ◽

2003 ◽

Cited By ~ 1

Author(s):

A. Vaziri ◽

H. Nayeb-Hashemi

Keyword(s):

Stress Intensity Factor ◽

Stress Intensity ◽

Intensity Factor ◽

Fracture Surface ◽

Crack Growth ◽

Effective Stress ◽

Mode Iii ◽

Mode Iii Crack ◽

Fracture Surfaces ◽

Effective Stress Intensity Factor

Turbine-generator shafts are often subjected to a complex transient torsional loading. Such transient torques may initiate and propagate a circumferential crack in the shafts. Mode III crack growth in turbo-generator shafts often results in a fracture surface morphology resembling a factory roof. The interactions of the mutual fracture surfaces result in a pressure, and a frictional stress field between fracture surfaces when the shaft is subjected to torsion. This interaction reduces the effective Mode III stress intensity factor. The effective stress intensity factor in circumferentially cracked round shafts is evaluated for a wide range of applied torsional loadings by considering a pressure distribution in the mating fracture surfaces. The pressure between fracture surfaces results from climbing the rought surfaces respect to each other. The pressure profile not only depends on the fracture surface roughness (height and width (wavelength) of the peak and valleys), but also depends on the magnitude of the applied Mode III stress intensity factor. The results show that the asperity interactions significantly reduce the effective Mode III stress intensity factor. However, the crack surfaces interaction diminishes beyond a critical applied Mode III stress intensity factor. The critical stress intensity factor depends on the asperities height and wavelength. The results of these analyses are used to find the effective stress intensity factor in various Mode III fatigue crack growth experiments. The results show that Mode III crack growth rate is related to the effective stress intensity factor in a form of the Paris law.

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