Fracture Mechanics Analysis on Structural Failure of Ship

This study presents the effects of the geometry of the notches on the fatigue life, crack initiation, and growth in mixed mode and first mode of the fracture. For this purpose, some samples were tested with U-shaped notches with different radii and depths and various orientations with respect to the loading directions under fatigue loads. Using the fatigue crack growth rates, the Paris law coefficients were obtained for the used material in different conditions. It was shown that these coefficients are independent of the geometry of the samples. Fatigue crack growth behaviors in the mixed mode tests were also in good agreement with the result of the numerical simulations. Predictions of the maximum tangential stress criterion and numerical simulation for the position of the crack initiation on the notch root were compared with the tested observations, which showed good agreement. The fatigue life of the test samples was compared with the analytical results provided by the Manson-Coffin law. It was shown that a 25% decrease in notch depth increases the fatigue life by 4 to 6 times, also, an increase of 0.5 mm in the notch radius increases the fatigue life by 40%. Finally, the fracture surface of the samples was checked using an optical microscope. This study showed that the fracture surfaces have one or two lateral shear lips and plane stress conditions were established.

Recent developments in polymer nanocomposites have led to improvements in conventional short‐term, but the long‐term mechanical properties have received little attention. The objective of the present study was to characterize the effect of nanoparticles on the fatigue crack initiation and propagation mechanisms and on the fatigue properties of polyamide‐6 (PA6) nanocomposite (PA6NC) prepared by in situ polymerization with montmorillonite clay. Two approaches were employed: fatigue life measurements and crack growth monitoring. Compared with non‐filled PA6 at the same stress amplitude, the number of cycles to fracture was higher for the nanocomposite, which suggests an increase in the intrinsic resistance of the material to crack initiation. However, the crack growth rate results indicated that nanoparticles decreased the resistance to crack propagation. Post‐fatigue fractographic observations indicated a change in the fatigue crack propagation mechanism resulting from the addition of nanoparticles, primarily attributed to the increase in yield stress, which favors the development of a fibrillated deformation zone. The fibrillation process in the relatively high crack propagation rate regime appeared to be preceded by plastic deformation at approximately constant volume. Most of the effect of nanoparticles on the fatigue behavior and properties results probably from the mechanical reinforcement on the microstructure and its effect on the yield stress and Young's modulus rather than from the effect of the inorganic filler to act as a stress concentrator. Polym. Compos. 25:433–441, 2004. © 2004 Society of Plastics Engineers.

Effect of hydride precipitation on the fatigue cracking behavior in a zirconium alloy cladding tube

ABSTRACTIn modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.

Local unified probabilistic model for fatigue crack initiation and propagation: Application to a notched geometry

The reliability or lifetime of micro-joints on printed circuit boards (PCBs) is significantly affected by fatigue processes, including fatigue crack initiation and propagation to failure. Accordingly, the industries producing electronic devices and components strongly desire a new nondestructive inspection technology, which detects micro-cracks appearing as thermal fatigue fractures in the joints. In this investigation, we applied a synchrotron radiation X-ray micro-tomography system called the SP-μCT to three-dimensionally and nondestructively evaluate the fatigue crack propagation process in complex-shaped solder joints. The observed specimens have a typical joint structure in which chip type resistors 1.0 mm in length and 0.5 mm in width are mounted on an FR-4 substrate by joining with Sn-3.0Ag-0.5Cu solder. A thermal cycle test was carried out, and specimens were collected at fixed cycle numbers. The same solder joints were observed successively using the SP-μCT at beamline BL20XU at SPring-8, the largest synchrotron radiation facility in Japan. An X-ray energy of 29.0 keV was selected to obtain computed tomography (CT) images with high contrast among some components, and a refraction-contrast imaging technique was also applied to the visualization of fatigue cracks in the solder joints. The following results were obtained. At the early stage in the fatigue process of normal joints, the main fatigue cracks were clearly observed to initiate from the region around the solder joint tip and the vicinity of the chip corner. Additionally, many micro-cracks roughly 5 to 10 μm in length also formed in the thin solder layer between the chip and substrate. The important observed fact is that these micro-cracks deform, grow, and connect to each other due to the thermal cyclic loading, prior to main crack propagation. On the other hand, in case of solder joints which included relatively larger initial voids, the voids deformed, and the fatigue cracks initiated and propagated from the surface of the voids. Furthermore, by employing the three-dimensional crack images, the crack dimensions were quantified straightforwardly by measuring the surface area of the fatigue crack, and the fatigue crack propagation process was also accurately evaluated via the average crack propagation rate. Consequently, the obtained CT images clearly illustrate the process of crack propagation due to the thermal cyclic loading of a solder joint. In contrast, such information has not been obtained in any form by industrially employed X-ray CT systems or finite element analyses.

The effect of the embrittlement on the fatigue limit and crack propagation in a duplex stainless steel during high cycle fatigue

In this paper, fatigue crack initiation and propagation properties of three structural steels with different static strength and cyclic softening behavior were evaluated and compared with the estimation results based on the local strain response. Steel C had the highest cyclic softening rate, which was 10 times that of steel A and twice that of steel B. The relationship between strain range and fatigue life in the fatigue life region shorter than 105 cycles was almost the same regardless of the test steels. The fatigue crack initiation life from the notch bottom of the SENT specimen was almost the same independent of static strength and cyclic softening rate. The crack initiation life estimated from the Strain range versus fatigue life equation using the local strain response measured by DIC was roughly in agreement with the experimental results. Steel C had the highest crack opening load and the slowest fatigue crack propagation rate compared to the other two steels. The local strain range at the fatigue crack tip showed a good correlation with the fatigue crack propagation rate irrespective of the steel grade. In addition, the estimates of fatigue crack propagation rate based on the local strain response were in almost agreement with the experimental results.

The initiation and propagation of fatigue cracks from notches in 10 mm thick sections of two engineering steels O80M15 and O80A47 has been studied in three point bending fatigue. A range of notch root radii varying from 0.13 mm to 3.17 mm provided different absolute distances within which the fatigue crack propagation is influenced by the notch stress field. The fatigue crack propagation data from the regions affected by the notch stress field were compared with that not influenced by the notch stress field, to obtain the modified stress intensity factor coefficient for cracks emanating from notches. It was found that the modified stress intensity factor coefficient does not fall to zero at the tip of a blunt notch. The effective stress intensity factor coefficient for blunt notches could be described as a function of notch geometry and microstructural features of steels, viz. grain size. The initiation and propagation of fatigue cracks from notches in 10 mm thick sections of gas carburised, gas carbonitrided and nitrocarburised O80M15 and induction hardened 080A47 were tested in three point bending fatigue. The number of cycles to produce a detectable fatigue crack, N., correlated well with the effective stress intensity factor range. The microstructural feature in case hardened steel was the prior austenitic grain size. The fracture surface examination did not indicate subsurface crack initiation generally associated with the case hardened steels subjected to fatigue. The results of fatigue crack initiation in carburised and carbonitrided notched geometries indicated that fatigue crack initiation is not affected by the case depth. The fatigue crack propagation data for cracks emanating from case hardened notches in O80M15 could be correlated to the effective stress intensity factor range. The results provided an adequate engineering method for design against fatigue failure from a range of stress concentration for the surface hardened steels examined.

Many factors, such as the amplitude of the cyclic load, the relative rigidity of components and the frictional condition, play important roles in the fretting fatigue process. This study has focused on the local singular stress field at the fatigue crack tip or at the contact edge where the initial crack occurs, based on the numerical analysis results with the use of finite element method (FEM) program ABAQUS. It is proposed that all the effect of test conditions can be contained by the local stress field, which would dominate the fatigue crack initiation and propagation. By comparing with the experimental results, it was found that the initial fatigue crack should occur at the point of maximum shear stress range near the contact edge, and would propagate in the direction of maximum tangential stress range. An analytical simulation of fretting fatigue crack propagation has been carried out. It was found that a fatigue crack was always under the mixed mode conditions, the fracture path could be estimated by the maximum tangential stress theory (Δσθmax), and the propagation curve could be characterized by the stress intensity factor range in this direction ( ΔKσθmax ).

Cyclic deformation behavior of steels with a nanolamellar microstructure and tensile strength of 1500 MPa

Numerical analysis of pitting corrosion fatigue in floating offshore wind turbine foundations

Fatigue crack initiation and propagation behavior in Al – 7075 alloy under in-phase bending-torsion loading

3 - Initiation and propagation of fatigue cracks

Fatigue crack initiation and propagation in turbine discs can lead to catastrophic failures in aeroengines, compromising structural integrity and highlighting the critical need for effective crack detection and propagation prediction. This study proposes an adaptive parameter adjusting and online forecasting DLinear model, a data-driven model aimed at predicting surface fatigue crack propagation at turbine disc bolt holes utilizing limited experimental datasets. The model’s performance is evaluated through prediction error analysis and compared with existing machine learning and long-term time-series forecasting methods. Results demonstrate high predictive accuracy, with mean absolute percentage errors of 1.75%, 1.64%, 1.71%, 1.04%, 1.40%, and 1.41% for six bolt holes. The proposed model significantly outperforms ARIMA, RNN, and Informer, achieving average MAPE reductions of 5.36%, 17.02%, and 30.51%, respectively. This model enables reliable crack propagation prediction with small datasets, offering a practical solution for real-time monitoring and forecasting during component-level engineering tests.