Comparison of Fracture Assessments of Corrosion Pits Using Sharp and Blunt Notched Crack Procedures

Historically, when determining failure criteria for a cracked structure, the structure has been assessed with the crack tip assumed to be infinitely sharp. For scenarios where it can be shown that the crack tip is blunt, such as for corrosion pits, this may lead to overly pessimistic assessments which can have implications for remnant life assessments of structures. Recent research has been carried out to evaluate structures with the crack tip as a blunt notch with a radius, and to develop a notch acuity parameter. That research led to formulae for adapting either the failure assessment curve or the failure assessment points utilising the notch acuity parameter for ferritic materials. This paper analyses various configurations of crack and specimen geometry, crack tip notch acuity parameter and crack spacing. It takes a finite element model of a cracked structure with an infinitely sharp crack and compares the failure parameters to those of equivalent notched cracks. The comparison is completed using the formulae derived in recent literature and those determined using finite element models with the blunt notched radii at the crack tip incorporated into the model.

Research on the Heat Insulation Performance of Advanced Thermal Barrier Coatings Based on Dense Vertical Cracked Structure

The Thermal barrier coatings (TBCs) applied to gas turbine can effectively protect the metallic components from overheating. It makes contribution to raising turbine entry temperatures, which can improve the cycle thermal efficiency of turbine and prolong its service life. To understand the failure mechanism of TBCs and extend its lifetime, it is vital to prepare TBCs with excellent heat insulating performance. TBCs with dense vertical cracked structure is an essential kind of advanced thermal barrier coatings and has received great attention recently. However, most studies are based on laboratory-scale, since the complex coating preparation process makes it challenging to obtain the controllable microstructure distribution of the coating. Moreover, the thin slices of TBCs are difficult to get during the reconstruction process of the coating microstructure. Therefore, in this work, an optimized numerical reconstruction method of TBCs was applied to obtaining the TBCs with dense vertical cracked structure. The influences of the microstructural TBCs with different numbers of vertical cracks as well as different length and width of vertical cracks were herein discussed, together with the analysis by the developed numerical calculation program based on the lattice Boltzmann method (LBM). The results showed that the performance of heat insulation in dense vertical cracked coatings was improved as the characteristic network structure. Meanwhile, it indicated that the optimal heat insulation performance can get when the length and width of vertical cracks were around 125μm, 15μm respectively. The results can play a guiding role in choosing and designing the turbine blades for further development of gas turbines.

A hybridised CSAGA method for damage detection in structural elements

In recent years, significant developments have been noticed in nondestructive techniques for damage detection in cracked structures. Some of the proposed methods can be used to find out the existence of the crack; other methods locate and simultaneously find out the damage severity. In the current investigation, a novel hybridised method is proposed for damage detection in structural elements. The proposed method can be used to investigate both location and nature of damage in structures within a reasonable time limit. The problem in the current analysis requires a set of dynamic parameters that depend on the dynamics of the cracked structure due to the presence of the crack. In the present study, the first three natural frequencies of a structure are considered as the inputs to find out the damage location. A finite element method is used to generate the first three natural frequencies of a cracked cantilever beam with multiple cracks. A method hybridizing the nature-inspired artificial intelligence techniques has been implemented for crack detection. Here, clonal selection algorithm and genetic algorithm have been integrated to design the framework of the hybrid technique. The changes in the natural frequencies are given as inputs to the hybrid technique and the output from the technique is the locations of damage.

Dynamic Instability Analysis of a Cantilever Beam With Breathing Crack

In this paper, dynamic characteristics of a beam with breathing crack is considered. Breathing crack is modeled as a bilinear oscillator. The stiffness of the cracked beam is estimated by using influence coefficients based on castigliano’s theorem and strain energy release rate (SERR). The equation of motion of breathing cracked beam is formulated using finite element method using Hamilton’s principle. The equation of motion of breathing cracked beam is converted into Mathieu-Hill type equation to obtain the regions of dynamic instability of beam using Harmonic balance method and it is further solved for Eigen frequency of the cracked beam. The increase in breathing crack depth increases the instability region and it is found that the effect of the crack location near to the fixed end is more for the cantilever beam, and this also increases the instability region. It is found that increase in dynamic instability index increases the instability regions of the cracked structure. In addition to that, the effect of static and dynamic loads are also investigated and discussed. The study has been conducted for the first two instability boundaries of the cracked structure only. It is hence seen that assuming the crack to remain open underestimates the stability boundaries of the system. Permitting the crack to open and close (breath) yields a stability boundaries in between the open and uncracked beam.

Peri-Ultrasound Modeling of Dynamic Response of an Interface Crack Showing Wave Scattering and Crack Propagation

A cracked structure made of two different elastic materials having a Griffith crack at the interface is analyzed when it is subjected to pure shear loading and ultrasonic loading. The waves generated by the applied load and the crack propagation resulted from the shear loading are investigated. Peri-ultrasound modeling tool is used for this analysis. A comparison between experimental results and numerical predictions shows a very good matching between the two. Furthermore, the increase in nonlinear ultrasonic response in presence of the interface crack could also be modeled by this technique. The computed results show that when the interface crack propagates, then it breaks the interface at one end of the crack and breaks the material with lower elastic modulus at the other end. The unique feature of this peridynamics-based modeling tool is that it gives a complete picture of the structural response when it is loaded—it shows how elastic waves propagate in the structure and are scattered by the crack, how the crack surfaces open up, and then how crack starts to propagate. Different modeling tools are not needed to model these various phenomena.