Advances in Fracture and Damage Mechanics VI

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Authors: Shan Wang, Xing Hua Shi, Jian He
Abstract: Based on the characteristics of loading condition of ship structure in service and the degeneration of the strength under fatigue and corrosion, the random time-variant model of ship hull section modulus was set up using the theory of random process. Then the time-variant reliability was analyzed with the method of up-crossing analysis and parallel system reliability. This method could not only present successive decrease of reliability of the ship structure under fatigue and corrosion, but also avoid the complicated numerical integration. It could be seen that this method was easy to achieve. Furthermore, the time-variant reliability was compared with the annual instantaneous reliability through an example, which indicated that this method was more precise, and could provide some references to the decision-making of the reliability, maintenance and safeguard.
Authors: Yu Zhu, Shi Lang Xu
Abstract: For micro-fiber reinforced strain-hardening cementitious materials, in addition to the basic characteristics and mechanical properties of fiber and interfacial properties between fiber and matrix, mechanical properties of matrix such as strength and crack resistance are essential parameters for material design, too. Therefore, the fracture properties of cement paste and mortar which are two most basal cementitious materials were studied, using three-point bending beams of which strength and depth are varied. Complete load versus crack mouth opening displacement (P-CMOD) curve directly obtained, and double-K fracture parameters ini Ic K and un Ic K were subsequently determined. The initial cracking load Pini was determined using resistant strain gauges. The results show that an apparent stable crack propagation before unstable failure was observed both in cement paste and in mortar. For cement paste, due to the influence of shrinkage crack, the divergence of the unstable fracture toughness un Ic K is more evident than initial fracture toughness ini Ic K .
Authors: Friedrich G. Buchholz, J. Wiebesiek, M. Fulland, Hans A. Richard
Abstract: In this paper the rather complex 3D fatigue crack growth behaviour in a shaft with a quarter-circular crack under torsion is investigated by the aid of the programme ADAPCRACK3D and by application of a recently developed 3D fracture criterion. It will be shown that the computationally simulated results of fatigue crack growth in the FE-model of the shaft are in good agreement with experimental findings for the development of two anti-symmetric cracks, which originate from the two crack front corner points, that is where the crack front intersects the free surface of the cylindrical laboratory test-specimens. Consequently, also for this case with a rather complex 3D crack growth of two anti-symmetric cracks, the functionality of the ADAPCRACK3Dprogramme and the validity of the proposed 3D fracture criterion can be stated.
Authors: Jeffrey Vogwell, Jose Maria Minguez
Abstract: Anchor chocks are used in the sport of rock climbing for providing secure attachment to a rock face. They are used at regular intervals and must be light weight (since many are carried) and also sufficiently strong to withstand an impact force should a climber fall from a height. In chock design, steel wire cable is widely used for connecting the nut component, which is wedged into a rock crevice, to the free end which attaches, via a karabiner link, to the safety rope. However, the wire cable is vulnerable to failure as it can fray with use at exposed ends - especially when folded into a loop using tight bends. Also, the ferrule end connections are considered a potential design weakness. In a research programme tests have been carried out on new and also some well used anchor chocks and has revealed very different, and some unpredicted, failure modes – depending on the state of the wire rope and whether the applied load at failure was static or impact. This paper presents the results of test failures for a range of chocks and discusses the benefits of using single lengths of wire cable with suitably swaged end ferrules.
Authors: Shi Lang Xu, Yu Zhu
Abstract: As cement-based composite, concrete can be properly represented by three phases in microstructure: cement paste, aggregate as well as interfacial transition zone between them. As the matrix compositions of concrete, fracture properties of cement paste and mortar have great influence on fracture performance of concrete, and fracture energy is an important parameter for concrete non-linear fracture mechanics research. Therefore, three-point bending beams of cement paste and mortar with different sizes and strengths were tested. A complete load-deflection (P-δ) curve was directly obtained, and a fit was made for the tail of the P-δ curve using power and exponential function, respectively. And then the fitting results of the two function were compared. It was found that with the increase of specimen size the influence of tail curve on fracture energy is decrease,and considering the influence of tail curve, fracture energy of cement paste and mortar are size-independent. The discussion on the obtained results was made.
Authors: Orlando León-García, Roumen H. Petrov, Leo Kestens
Abstract: The microstructure evolution in the deformation zone around second phase particles of IF steel sheets subjected to tensile deformation has been investigated in order to correlate it to the damage at microstructural scale. The experimental set-up consisted of a series of interrupted tensile tests which were carried out at different tensile deformations up to fracture. The microstructure of the deformed samples was investigated by EBSD analysis in which the EBSD scans were focused on the areas containing Ti (C, N) particles of cubical shape. It was found that at tensile strains below 25%, the ferrite matrix exhibited the evolution of slip bands inside specific grains depending on their crystallographic orientation although no special strain localization around the particles was observed. After 35% of tensile strain, the strain concentrates around the particles and particle-matrix decohesion was observed. The lattice rotations of the matrix surrounding the particles as well as the selective deformation of the grains are analyzed and discussed.
Authors: Guang Lan Liao, Tie Lin Shi, Zi Rong Tang
Abstract: Machine fault diagnosis is essentially an issue of pattern recognition, which heavily depends on suitable unsupervised learning method. The Self-Organizing Map (SOM), a popular unsupervised neural network, has been used for failure detection but with two limitations: needing predefined static architecture and lacking ability for the representation of hierarchical relations in the data. This paper presents a novel study on failure detection of gearbox using the Growing Hierarchical Self-Organizing Map (GHSOM), an artificial neural network model with hierarchical architecture composed of independent growing SOMs. The GHSOM can adapt its architecture during unsupervised training process and provide a global orientation in the individual layers of the hierarchy; hence the original data structure can be described correctly for machine faults diagnosis. Gearbox vibration signals measured under different operating conditions are analyzed using the proposed technique. The results prove that the hierarchical relations in the gearbox failure data can be intuitively represented, and inherent structure can be unfolded. Then gearbox operating conditions including normal, tooth cracked and tooth broken are classified and recognized clearly. The study confirms that GHSOM is very useful and effective for pattern recognition in mechanical fault diagnosis, and provides a good potential for application in practice.
Authors: Hong Yu Qi, Xiao Guang Yang, Rui Li
Abstract: Thermal barrier coating (TBC) is an essential requirement of a modern gas turbine engine. The TBC failure is the delamination and spallation. The oxidation damage under high temperature results in the reduction of interfacial adhesion. The interfacial fracture toughness is an important property to analyze the TBC failure. The interfacial fracture toughness of ceramic coating - bond coating has been researched in the past. However, the facture toughness of the bond coating - substrate due to the Al depletion was very few studied. In this study, a NiCrAlY bond coating by air plasma spray (APS) was deposited. The substrate was directionally solidified superalloy (DZ40M). Isothermal oxidation was performed at 10500 for 100h. Using the HXZ-1000 micro-hardness equipment, the five different times was chosen to test the hardness and the crack length, and then the fracture toughness was obtained. While the oxidation exposure time increased at 10500, the hardness of the substrate close to the bond coating decreased with the increase of the bond coating’ hardness. Meanwhile, the interfacial fracture toughness of the bond coating - substrate decreased because of the Al depletion.
Authors: Martin Riedler, Martin Stockinger, Michael Stoschka, Wilfried Eichlseder
Abstract: With the objective of creating a simulation model for the lifetime calculation of forged aerospace components it is necessary to clarify the damage mechanisms in the materials used. This has been researched for the Ni-base alloy Inconel 718 by varying the forging parameter effective plastic strain rate, which is realised by using three types of equipment: hydraulic press, screw press and hammer. Specimens processed at the screw press show the highest lifetime by keeping all other forging parameters unvaried. Micro structural investigations show that the amount and morphology of dominant as-large-as grains play a important role. This methodology is currently investigated for Ti-6Al-4V. Lifetime tests show that besides effective strain and anisotropy the influence of morphology is important. As soon as the model status allows lifetime analyses the thermo-mechanical process (forging and heat treatment) can be developed depending on the desired lifetime specifications in order to realise an interdisciplinary lifetime optimisation of forgings. A further aim is the use of basic coherences of safe-life and fail-safe approaches in the low and high cycle fatigue region in order to reasonable handle with flaws and defects at the edge layer.
Authors: Le Wen Zhang, Shu Cai Li, Ying Wang
Abstract: Based on the hypothesis of equivalent strain energy and the theories of fracture mechanics and damage mechanics,the constitutive model and fracture damage mechanism of bolted discontinuous jointed rockmass are systematically studied under the state of complex stresses. Initially , considering the equivalent strain energy , the constitutive relation of anchored discontinuous jointed rockmass is derived under the state of compression-shear stresses. The constitutive relation under the state of tension-shear stresses is also developed according to the theory of self-consistence. Next,the damage evolution equations of discontinuous multi-crack rockmass under compression-shear and tension-shear are put forward according to the wing crack-initiating criterion. Finally,based on the above constitutive models and the damage evolution equations three-dimensional finite element procedures have been developed to evaluate the stability and deformability of the surrounding rock mass during excavation and supporting. The calculated results indicate that above-mentioned constitutive relation and the damage evolution equations are available.

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