C.S. Shin

Fatigue crack growth under compressive loading

Abstract Fatigue cracks were grown in centre-notched sheet made from BS4360 50B structural steel. Despite the fact that loading was fully compressive, cracks initiated and grew in regions of residual tensile stress at the notch roots. It was observed that crack growth rates decreased with increasing crack length until arrest occurred. Near tip strain gauges were used to monitor crack closure; closure readings agreed well with those deduced from growth rates. The residual stress ahead of the slit tip and normal to the slit plane, σres, was estimated from the crack growth rate response. It was found that σres scaled with distance, x , from the slit tip as σ res αx −0.56 .

A practical method to estimate the stress concentration of notches

Abstract Knowledge of stress concentration factor and stress distribution of notches is important to engineers engaged in the design or fracture life assessment of engineering components. In a previous paper, it was pointed out that knowledge of the relevant solutions for stress analysis of cracks may be used to advantage to estimate the stress concentration factor and stress distribution ahead of a notch via the Creager-Paris stress field solution for notches. It was shown that this method accurately predicted the stress concentration factors and notch-tip stress distribution in a number of double- and single-edge-notched plates. Finite-element stress analysis results for plates with multiple notches are recently available. Stress concentration factors for a variety of notch configurations have also been collected from relevant handbooks. In the current work, a comparison was made between the finite element results and handbook solutions with estimates from fracture mechanics data, and reasonable agreement was found. A number of approximate expressions have been proposed to estimate notch-tip stress distributions. These estimates were also compared with finite element results, and a guideline is suggested for the choice of approximate expressions to ensure maximum accuracy.

Fatigue crack growth at stress concentrations—the role of notch plasticity and crack closure

Abstract Fatigue crack growth and closure behaviour in sharply notched specimens, made from AISI 316 stainless steel, BS 4360 grade 50B structural steel and a commercially pure aluminium, was monitored under constant amplitude cyclic loadings. The initial notch crack growth rates were in general high but decelerating. These observations are in contradiction with the predictions of linear elastic fracture mechanics. It is argued from experimental observations that plasticityinduced crack closure was mainly responsible for the initial deceleration of the notch crack. The effective stress intensity range, a parameter that has taken crack closure into account, was able to correlate most but not all of the notch crack growth data with the long crack data. When the applied loading is sufficiently high, a strain controlled condition exists just ahead of the notch tip. In this case, crack growth rates were better predicted by a shear decohesion model for high strain fatigue crack growth. Neither the nominal stress intensity range, a strain based intensity factor, a cyclic J integral nor a total shear parameter could bridge the gap between notch and long crack behaviour.

Fatigue crack growth from sharp notches

Abstract Notch-like stress raisers occur widely in engineering components. They are preferred sites for crack initiation when the components are subjected to cyclic loadings. Thus the growth of cracks initiated from notches is very relevant to design against fatigue failures. Schematic models proposed to explain the departure of notch crack growth from linear elastic fracture mechanics predictions are briefly reviewed. Different methods of measuring crack closure are compared. It is found that the commonly employed notch-mouth clip-gauge method is not sensitive enough to detect the closure of short cracks in regions of notch plasticity. Various mechanics parameters have been claimed to be able to bring the notch crack and long crack growth rate data to a single base. In the present work on double-edge notched AISI 316 stainless steel specimens, it is found that none of them is able to correlate satisfactory all the experimental data.

Hydrogen enhanced fatigue crack propagation of bainitic and tempered martensitic steels

Abstract The hydrogen-enhanced fatigue crack propagation (FCP) of bainitic and tempered martensitic structures in AISI 4130 steel has been evaluated. Three bainitic structures involved in the study were obtained by isothermal transformation at 330 °C, 370 °C and 470 °C. By selecting appropriate tempering temperatures, three tempered martensitic specimens with yield strengths corresponding to those of the bainitic structures, were prepared. For the bainitic specimens, their hydrogen-assisted FCP rates and fracture mode were found to be similar despite the fact that they have entirely different tensile strengths and hydrogen permeation behavior. The hydrogenassisted FCP rate for the tempered martensites increased significantly with decreasing tempering temperature. By comparing FCP rates in bainitic and tempered martensitic structures for AISI 4130 steel, it is clear that the effect of microstructure was the dominant factor in this steel-environment system.

Fatigue damage repair: a comparison of some possible methods

When a fatigue crack is discovered in an engineering component, some expeditious temporary repair may be needed before more thorough treatment is available. Conventional repair methods, such as grinding removal of the crack and stop drilling, are often employed. In this work, the method of drilling crack-flank holes near to but not at the crack tip has been studied. It was found that retardation still occurred. Moreover, if the holes were drilled a small distance ahead of the tip, the crack was likely to grow into the hole, achieving a considerable amount of life extension. The effects of inducing artificial crack closure by infiltrating epoxy resin, alumina powder and a mixture of both have also been investigated. These were compared with the effect of overload-induced retardation. In all the methods studied, different degrees of crack growth retardation have been achieved, and they have the potential to be developed into practical fatigue crack repair methods.

Residual properties of notched [0/90]4S AS4/PEEK composite laminates after fatigue and re-consolidation

Abstract Notch fatigue strengthening under different cyclic stress levels and elapsed number of cycles has been studied in [0/90] 4S AS4/PEEK laminates. Quick and extensive 0° fiber splitting and the corresponding 90° fiber shear off were found to be the underlying causes of stress concentration alleviation. This effectively raised the residual strength of the notched laminates and increased their fatigue lives to beyond one million cycles. On the other hand, re-consolidation of fatigued specimens removed most of the internal damages and in the meantime reverses the above strengthening. Detailed study of the residual strength changes and damage development history using re-consolidation lent support to the above deductions on the notch fatigue strengthening phenomenon in [0/90] 4S AS/PEEK laminate.

An Improved Cohesive Zone Model for Residual Notched Strength Prediction of Composite Laminates with Different Orthotropic Lay-ups

The traditional methods employing a characteristic length approach to predict residual strength of notched laminates did not recognize damage development prior to final failure. Due to this lack of physical basis, re-calibration is often needed when the specimen geometry and size are changed. More recent models such as the Damage Zone Model (DZM), Cohesive Zone Model (CZM) and the Effective Crack Growth Model (ECGM), have recognized the occurrence of progressive material softening ahead of the notch tip prior to catastrophic failure. However, since the anisotropic characteristic of the composite laminate has not been properly accounted for, these models may not be widely applicable for the laminates with different degrees of anisotropy. In this study, an Improved Cohesive Zone Model (ICZM) with proper consideration for anisotropy is proposed to predict the notched strength of composite laminate. Based on three fundamental parameters: namely the unnotched strength 0, the apparent fracture energy Gc and the effective longitudinal stiffness E0, this model successfully predicted the strength of notched composite laminates with various degrees of anisotropy. The current model also gives accurate prediction of the progressive damage zone size in quasi-isotropic and cross-ply laminates.

Some aspects of corner fatigue crack growth from holes

Abstract The early stages of fatigue failures often involved the initiation and growth corner cracks. To assess the residual strengths and to predict the remnant lives of components containing corner cracks, the associated stress intensity factors must be known. These stress intensity factors are sensitive to crack shape as well as crack size and vary along the crack front. The complex nature of the problem often precluded analytical treatments so that numerical methods are generally employed to obtain the related stress intensity factors. Different numerical solutions exist. These solutions usually assumed the corner cracks are exactly elliptical in shape. Each solution uses its own interpolation method to arrive at the stress intensity for different crack sizes and crack shapes. There exist few experimental data to check the usefulness of these solutions. In the current work, direct observations of corner fatigue crack growth were made. It was attempted to calibrate the stress intensity factor of some corner cracks using a backtracking method. Details about the experiment are reported. The empirical stress intensity factors are compared with two published numerical solutions. The agreement is found to be reasonable and the possible causes of discrepancy are discussed.

Impact Response of a Wind Turbine Blade Measured by Distributed FBG Sensors

An impact monitoring system using an array of fibre Bragg grating (FBG) sensors has been established on a wind turbine blade. The effect of sensor layout on detection capability has been investigated. The possibility to identify the location of an impact is also discussed. This preliminary work may shed some light on how to establish an optimal monitoring system with a minimum number of sensors while achieving an acceptable sensitivity and coverage. With such a system, the corresponding strain history and occurrence of local damage can be monitored, thus helping to narrow down the scope for detailed examination during an overhaul and enabling the integrity of a wind turbine blade to be more reliably assessed.