Crack Growth Morphology Research Articles

Research on the Evolution Law Physical Short Fatigue Crack and Tip Deformation Fields during Crack Closure Process of the Q&P Steel.

In this paper, a novel dual microscopic fatigue-crack and tip-deformation-fields measurement method based on a hybrid image-processing technique is proposed that was used to research the physical short fatigue crack (SFC) closure effect and the evolution law of the tip deformation fields of Quenching–Partitioning (Q&P) steel during the crack-closure process. The measurement problems are solved, such as the small SFC tip region, large deformation gradient, and strong material anisotropy. Microscopic crack and speckle images are acquired simultaneously on both sides of a compact tensile (CT) specimen of Q&P steel by dual microscopic cameras. A digital image processing (DIP) method is used to identify crack-growth morphology and measure crack length in Q&P steel, and the SFC growth rates are analyzed under different stress ratios. Microscopic digital image correlation (Micro-DIC) is used to analyze displacement fields at the crack tip of SFC and, combined with virtual extensometer technology, analyze the evolution law of crack closure and the evolution of crack-growing morphologies during the closure process under different lengths and stress ratios. Accordingly, the evolution of strain fields at the crack tip in one load cycle for different crack lengths and stress ratios during the SFC closure process is analyzed. The results show that the stress ratio affects the crack-closure behavior and crack growth rate of Q&P steel in the physical SFC crack-growing stage. The crack-closure effect has an obvious influence on the evolution process of displacement and strain fields at the crack tip. The evolution of short-fatigue-crack-tip morphology and strain field of Q&P steel conforms to the crack-closure law. The research results provide experimental and theoretical support for the further study of the SFC growth mechanism and fatigue life prediction of Q&P steel.

Realization of Tensile-Bending Mechanical-Thermal Coupling Fatigue Based on a Uniaxial Tensile-Fatigue Testing Device

Lack of advanced testing technology and instrument restricted development and application of material subjected to multiaxial mechanical-thermal coupling fatigue load. This paper developed the mechanical-thermal coupling tensile-bending combined fatigue testing instrument by modifying the existing uniaxial tensile testing device and the shape of specimens. The thermal loading unit, thermal insulation unit, and temperature monitoring unit were integrated into the tensile testing device to obtain fatigue properties of different temperatures. A single v-notch used to adjust the ratio of bending component to tensile component was prepared on the symmetric specimen to establish the tensile-bending coupling stress state of the minimum cross-section of specimens. The v-notch specimens and conventional symmetrical specimens were used on the modified instrument to test at room temperature to 600 °C. The performance of the modified instrument was verified through the stability of load response, the consistency of fatigue fracture process and displacement response, and the softening trend of material with the increase of temperature. The tensile-bending stress state caused by v-notch was proved through finite element analysis and the stress analysis based on the crack growth morphology. A deep neural network model was established connecting the testing parameters and the tensile-bending combined stress state, and the feasibility of the model was verified by small enough training error and testing error.

A comparison between two parameter and ductility exhaustion approaches for creep life assessment

To improve the accuracy of creep life assessment for cracked components, the two-parameter fracture mechanics approaches with considering the constraint effect have been investigated and developed over the past decade. To examine the accuracy and applicability of different approaches, in this work, the creep life (including both creep crack initiation and growth life) is comparatively assessed using the two-parameter C*-R*, two-parameter C*-Ac and ductility exhaustion based approaches for pressurized pipes with different initial axial crack sizes. The results show that the life assessment results of the C*-R* approach are sensitive to initial crack sizes. For the shallower initial cracks, the C*-R* approach can give reasonable life prediction results, while for the deeper and longer cracks, it produces conservative results due to the sensitivity of the in-plane constraint parameter R* to initial crack sizes. The ductility exhaustion based approach verifies that the two-parameter C*-Ac approach has better life prediction accuracy for a wide range of initial crack sizes due to that the parameter Ac can characterize both in-plane and out-of-plane constraints and it is not very sensitive to initial crack sizes. The C*-Ac approach also can accurately predict the creep crack growth morphology which is mainly dependent on the initial crack aspect ratio.

A more appropriate FE model to predict the creep crack initiation and growth behavior of brazed joint

In this paper, a more appropriate finite element (FE) model was proposed to predict the creep crack initiation (CCI) and growth (CCG) behavior of HastelloyC276-BNi2 brazed joint at high temperature. The effects of single-material, bi-material and three-material method on CCI and CCG behavior have been discussed fully. The relationships between CCG rate and parameter C∗ for different simulation methods were obtained. The results show that the simulation method has a great influence on CCI and CCG behavior of brazed joint. The brazed residual stress (RS) can accelerate CCI and enhance CCG rate, leading to the reduction of creep life. The CCI time and creep life predicted by single-material method is smaller than those by bi-material and three-material method. The creep life and crack growth morphology predicted by single-material method have a good consistent with the experimental results. The single-material method can accurately predict the CCG behavior of brazed joints. Bigger uniaxial creep strain can prolong the CCI time and reduce the CCG rate. The creep life of brazed joint can be enhanced by increasing uniaxial creep strain.

Crack propagation simulation of rock-like specimen using strain criterion

A digital image correlation (DIC) method was used to obtain the full-field strain and crack propagation path for a rock-like specimen with a single prefabricated crack under uniaxial compression. Additionally, scanning electron microscopy (SEM) was used to scan the fracture surface of the specimen after compression. The macro-mechanical response of the loading process was found to be in accordance with the micro-mechanism obtained by SEM, which demonstrates that the DIC system is accurate and that the full-field strain is accordant with crack propagation. Thereafter, a strain criterion was proposed and was applied to the extending discrete element method to simulate crack propagation. The strain criterion was found to be appropriate for simulating the crack initiation stress, peak strength and crack growth morphology of the specimen. Moreover, both the numerical simulation and experimental results show that the specimen failure process had four typical stages – initiation, unstable propagation, stable propagation and failure.

Effect of head wear and lateral forces on underhead radius crack propagation

This paper investigated the effect of rail head wear (HW) and lateral forces on underhead radius stress conditions and resulting stress intensity factors (SIFs) of a long transverse crack. The occurrence of tension spikes at the underhead radius of the rail as a result of localised vertical and lateral bending of the head-on-web was significantly exacerbated with increasing rail HW. The extended finite element method (X-FEM) modelling, which had previously been validated by comparison with in-track measurements to verify the prediction of tension spikes, was used to model a single rail on discrete elastic foundations. SIFs along the crack front were parametrically evaluated in terms of changes in the contact patch offset (CPO), the ( L/ V) ratio of lateral ( L) to vertical ( V) loads, the rail HW, and crack size and shape. The X-FEM results revealed that for a long transverse crack, extending the crack length to the underhead radius position results in higher SIFs across the underhead radius as a result of tensile bending stresses. These higher SIFs can contribute to a massively higher crack growth rate at this location, as was evidenced by the crack growth morphology at the underhead radius.

Effect of Environmental Corrosion on Fatigue Crack Growth Morphology: A Detailed Investigation of the Boundary between Fatigue and Corrosion Control Regimes

Legacy 7XXX series aluminum alloys were developed primarily for their high strength with less regard for their fatigue properties, corrosion resistance and fracture toughness. The constituent alloying elements in these materials (used to achieve high strengths) markedly increased their corrosion susceptibility. Consequently, aircraft structures made from these alloys have exhibited fatigue and corrosion damage. In the present work, we have investigated a crack finding in a fuselage skin of AA7XXX series alloy. This investigation revealed the crack propagated by a combination of fatigue and corrosion. Through the use of extensive metallography, mechanical analysis and laboratory experiments, we have separated the contributions to the damage growth due to corrosion and fatigue. We have also confirmed that in-service mixed-mode failures like this, observed in these alloys, can be reproduced reliably in the laboratory. Furthermore, it was observed that the presence of corrosion can actually change the propagation of a fatigue crack from mode I, the preferable orientation for fatigue crack propagation, to mode II, the preferable orientation for corrosion propagation. Even though the mechanical driving force is enough to grow the crack in mode I, the presence of corrosion can change it to mode II by electrochemical degradation of the material. Using electrochemical measurements, we relate the change in failure mode to the frequency of cyclic loading. At slow enough cyclic frequency the electrochemical energy released due to galvanic corrosion degrades the material such that the crack turns and propagates in the orientation which has only one third the mechanical driving force as compared to the original crack propagation path. This is the first time such phenomenon has been successfully replicated in the laboratory and modeled with finite element analysis.

超音波疲労試験機を用いたZr<sub><font size="-1">55</font></sub>Al<sub><font size="-1">10</font></sub>Ni<sub><font size="-1">5</font></sub>Cu<sub><font size="-1">30</font></sub>金属ガラスの超高サイクル疲労特性

In this work, the ultra-high cycle fatigue behavior of Zr55Al10Ni5Cu30 metallic glass was investigated by using ultrasonic fatigue testing method. Test alloy rods with a diameter of 8mm were prepared by tilt-casting with copper mold. Test specimens were machined to hourglass shape type (the minimum diameter ; 2.0mm), and after machining the surface of specimens was manually polished with polishing paste. The specimens were tested at a frequency of 20kHz under a stress ratio of -1. The fatigue limit (σw) (half a total stress amplitude) and fatigue ratio (σw/σB) of the alloy showed 893MPa and 0.53, respectively. Furthermore, we did not see any fish-eye like crack-growth morphology on fracture surfaces. The initiation region of fatigue crack was always observed at the side surface of the specimens. A viscous-flow fracture region with large dimples between the fatigue crack-growth region and the final unstable fracture region was observed. This peculiar fracture morphology may be due to adiabatic heating of the metallic glass specimens during the fracture, resulting in transition to supercooled liquid, under extremely high frequency of fatigue stress using ultrasonic loading.

Influence of hydrogen and frequency on fatigue crack growth behavior of Cr-Mo steel

Fatigue tests on Cr-Mo steel quenched at 860°C and tempered at 580°C were carried out under the frequencies of 0.2, 2, and 20 Hz with specimens containing a small artificial hole. An additional test in which the test frequency was alternately switched between 0.02 and 2 Hz was carried out. Hydrogen charging to the specimens was carried out by an immersion method. The fatigue life of the hydrogen-charged specimens remarkably decreased in comparison with that of the uncharged specimens. The fatigue crack growth rate da/dN increased with decreasing the test frequency f. The acceleration of da/dN saturated at \({\Delta K\,<\,17\,{\rm MPa}\sqrt{m}}\) for f ≤ 2 Hz. The presence of the upper bound for the fatigue crack growth acceleration was found with respect to the effects of hydrogen and test frequency in a hydrogen environment. The test switching the frequency between 0.02 and 2 Hz resulted the difference in fatigue crack growth morphology which is presumed to be caused by the difference in hydrogen concentration in the vicinity of crack tip. The particular crack morphology under the low test frequency with hydrogen was the localization of the slip around the crack tip and the linearization of the crack growth path. The hydrogen-enhanced striation formation model which was proposed to explain the effect of hydrogen on the fatigue crack growth for an austenitic stainless steel and low carbon steel can be applied also to the quenched and tempered Cr-Mo steel in this study.

The Prediction of Fatigue Crack Initiation Life in Spot Welds

Abstract: In this paper, strain‐based fatigue life prediction method has been used to estimate the fatigue crack initiation life of spot‐welded joints of Mild Steel JSC270D and Ultra‐High Strength Steel JSC980Y. To do so, the joints were simulated using three‐dimensional finite‐element (FE) models, and then nonlinear FE analysis was performed to obtain the local stress and strain ranges and finally, the Morrow equation was applied to estimate the crack initiation lives. The results have been compared with those obtained from experimental crack growth morphology. In addition, the difference between fatigue limits for smooth specimens and spot‐welded joints for mentioned materials has been briefly discussed. It has been shown that mean stress values in the Ultra‐High Strength Steel can significantly decrease the fatigue limit of spot‐welded joint because even at very low load level the stresses exceed the yield point at the root of nugget of spot‐welded joint, while the amount of mean stress in the Mild Steel for the same load level is much less than that of Ultra‐High Strength Steel. The comparison between numerical results of fatigue crack initiation lives and experimental data provided good agreement between numerical predictions and crack growth morphology observations. The results also shows that in some cases, depending on the joint type, the life spent in the nucleation phase can be an important part of the final failure lifetime.

2336 構造用鋼スポット溶接継手における疲労き裂進展形態に及ぼす強度レベルの影響(S07-4 疲労き裂進展,S07 構造材料の疲労強度とき裂進展問題)

2336 構造用鋼スポット溶接継手における疲労き裂進展形態に及ぼす強度レベルの影響(S07-4 疲労き裂進展,S07 構造材料の疲労強度とき裂進展問題)

Effect of split-sleeve cold-expansion on the fatigue resistance of hot-rolled 1020 steel

The split-sleeve cold-expansion process was applied to hot-rolled 1020 steel specimens containing a central notch and a central notch plus an EDM corner notch. The EDM notch was made to simulate a small corner crack in the notch. Constant amplitude axial fatigue tests were performed in the as-machined condition and the cold-expansion condition with the stress ratio, R, equal to 0.05 and −1. Fatigue life improvement occurred in the cold-expansion specimens at lower stress amplitudes in three out of four test series. EDM specimen crack growth monitoring indicated that the life improvement was due to small increases in crack initiation life (crack length equal to 0.25 mm) and to a larger extent fatigue crack growth retardation. Cyclic relaxation of residual compressive stresses due to higher stresses was the apparent reason for those cases where fatigue life improvement did not occur. SEM fractographic analysis of EDM specimens showed similar fatigue crack growth morphology for all test conditions.

Investigations on the integrity of fast breeder reactor piping and components

Abstract Results of investigations made with respect to the integrity of LMFBR (Liquid Metal Fast Breeder Reactor) piping and components are presented. The classification of sodium systems as Moderate Energy Fluid Systems is shown to be an important principal element to determine the failure mechanisms which are relevant. Based upon the selection of materials, design features and high-quality engineering standards the evaluation of the crack growth morphology of surface flaws contribute to the ensurance of the structural integrity. The crack shape development for bending stress distribution over the wall-thickness, which is a typical loading of FBR structures is discussed. It has been shown that even for such unfavourable loading conditions the through crack lengths are bounded. There is a considerable distance from critical crack configurations calculated by tearing modulus concept. Results from a large scale elbow test at operating temperature are reported. They contribute to the crack shape development under fatigue loading with bending type stress distribution over the wall thickness and are in good accordance with calculations. Acceptance criteria for flaws in structures are proposed showing that the structural integrity for coolant boundary and components of FBR can be assessed with a high degree of reliability.