| 1. |
- Jivkov, Andrey
(författare)
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Evolution of fatigue crack corrosion from surface irregularities
- 2003
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Ingår i: Theoretical and applied fracture mechanics (Print). - 0167-8442 .- 1872-7638. ; 40:1, s. 45-54
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Tidskriftsartikel (refereegranskat)abstract
- A moving boundary model is presented for crack nucleation and growth from surface flaws. It concerns with chemical attack that results in material dissolution. A controlling mechanism for evolution is the rupture of a brittle corrosion-protective film that is built up along the corroding surface. The evolution rate is a function of the degree of protective film damage caused by the surface straining. The problem is formulated for an elastic body containing a single and double pits. Low-frequency cyclic loading is considered. Numerical solution is proposed. The behaviours of a growing crack and of two competing cracks are described. Stages of incubation, blunting and steady-state growth characterise a single crack evolution. The steady-state growth rate is found independent of the initial geometry. Stages of independent growth, interactive growth and arrest of one crack characterise the evolution of two competing cracks. The lengths of the arrested cracks are presented as functions of the ratio between the pit depth for a series of different distances between the pits. It is emphasized that the solutions correspond to a homogeneous material. Further work is required to account for the material microstructure.
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| 2. |
- Azeez, Ahmed, et al.
(författare)
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Low cycle fatigue life modelling using finite element strain range partitioning for a steam turbine rotor steel
- 2020
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; 107
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Tidskriftsartikel (refereegranskat)abstract
- Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.
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| 3. |
- Azeez, Ahmed, 1991-, et al.
(författare)
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Numerical prediction of warm pre-stressing effects for a steam turbine steel
- 2023
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; 125
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Tidskriftsartikel (refereegranskat)abstract
- In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS’s beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.
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| 4. |
- Busse, Christian, et al.
(författare)
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Criteria evaluation for the transition of cracking modes in a single-crystal nickel-base superalloy
- 2020
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Ingår i: Theoretical and applied fracture mechanics (Print). - : ELSEVIER. - 0167-8442 .- 1872-7638. ; 106
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Tidskriftsartikel (refereegranskat)abstract
- Single-crystal nickel-base superalloys frequently experience two distinct fatigue crack growth modes. It has been observed that, under certain conditions, cracks transition from a path perpendicular to the loading direction to a crystallographic slip plane. As crystallographic cracking is associated with an increased fatigue crack growth rate, it is important to be able to predict when this transition occurs. In this work three different criteria for crystallographic cracking based on resolved anisotropic stress intensity factors are evaluated in a three-dimensional finite element context. The criteria were calibrated and evaluated using isothermal fatigue experiments on two different specimen geometries. It is suggested by the results, that a threshold value of a resolved shear stress intensity factor can act as a conservative criterion indicating cracking mode transition. Further, a trend hinting towards a loading frequency dependency could be observed.
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| 5. |
- Bödeker, Felix, et al.
(författare)
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An FFT-based homogenization scheme for cohesive zones with an application to adhesives and the core material of thin metal sandwich plates
- 2024
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; 129
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Tidskriftsartikel (refereegranskat)abstract
- Cohesive Zone Models with finite thickness are widely used for the fracture mechanical modeling of material layers, e.g., adhesive layers. Within this approach, the whole layer is modeled as a cohesive zone. Moreover, computational homogenization techniques are crucial for the development of advanced engineering materials, which are often heterogeneous. Compared to the commonly used Finite Element Method (FEM), solvers based on the Fast Fourier Transform (FFT) are expected to reduce the computational effort needed for the homogenization. Originated from an existing method for the computational homogenization of cohesive zones using FEM, a novel FFT-based homogenization scheme for cohesive zone models is presented. Our implementation of the FFT solver uses a displacement-based Barzilai–Borwein scheme and a non-local ductile damage model for the fracture behavior. Finally, the practical application of the method is discussed using an adhesive layer and the core material of HybrixTM metal sandwich plates as examples.
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| 6. |
- Chen, Shaohui, et al.
(författare)
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An experimental analysis of the defect sensitivity of solid foams
- 2018
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Ingår i: Theoretical and applied fracture mechanics (Print). - : ELSEVIER SCIENCE BV. - 0167-8442 .- 1872-7638. ; 96, s. 768-774
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Tidskriftsartikel (refereegranskat)abstract
- Single edge notched bending (SENB) and single edge notched tensile (SENT) fracture experiments were conducted to study the influence of the defect size on the global fracture behavior of three different brittle low density PVC solid foams. It was found that for sufficiently small defects (initial cracks), the continued fracture process was through breakage of cell edges located at random positions far away from the defect while the global fracture load was fairly constant and hence unaffected by the initial defects. At defects larger than about four cells, however, the continued cell edge fractures were localized to the near vicinity of the defect, resulting in a decreasing global fracture load in accordance with classical linear elastic fracture mechanics theory. Hence a size of about four cells is considered to be a critical microstructure transition length, meaning that the foam is unaffected for defects smaller than this transition length from a fracture point of view.
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| 7. |
- Di Stasio, Luca, 1988-, et al.
(författare)
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Energy release rate of the fiber/matrix interface crack in UD composites under transverse loading : Effect of the fiber volume fraction and of the distance to the free surface and to non-adjacent debonds
- 2019
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; 103
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Tidskriftsartikel (refereegranskat)abstract
- The effects of crack shielding, finite thickness of the composite and fiber content on fiber/matrix debond growth in thin unidirectional composites are investigated analyzing Representative Volume Elements (RVEs) of different ordered microstructures. Debond growth is characterized by estimation of the Energy Release Rates (ERRs) in Mode I and Mode II using the Virtual Crack Closure Technique (VCCT) and the J-integral. It is found that increasing fiber content, a larger distance between debonds in the loading direction and the presence of a free surface close to the debond have all a strong enhancing effect on the ERR. The presence of fully bonded fibers in the composite thickness direction has instead a constraining effect, and it is shown to be very localized. An explanation of these observations is proposed based on mechanical considerations.
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| 8. |
- He, Junjing, et al.
(författare)
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Basic modelling of creep rupture in austenitic stainless steels
- 2017
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier BV. - 0167-8442 .- 1872-7638. ; 89, s. 139-146
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Tidskriftsartikel (refereegranskat)abstract
- Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves.
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| 9. |
- Heshmati, Mohsen, 1987, et al.
(författare)
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On the strength prediction of adhesively bonded FRP-steel joints using cohesive zone modelling
- 2018
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier BV. - 0167-8442 .- 1872-7638. ; 93, s. 64-78
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Tidskriftsartikel (refereegranskat)abstract
- The variety of failure modes that are likely to occur in fibre-reinforced polymer (FRP)/steel joints used in the construction industry adds to the complexity associated with the design of these joints. This variation in possible failure modes is mainly attributed to the lack of a controlled application environment and to rather insufficient quality assurance protocols and procedures. The use of energy-based methods such as, cohesive zone modelling (CZM), can be a solution to circumvent such complexities. This paper investigates a number of issues related to CZM analyses of FRP/steel adhesive joints using various test configurations and a comprehensive numerical study. Parameters such as the effect of shape and type of cohesive law, crack path location, length of damage process zone, variations of adhesive and FRP properties, and different failure modes including cohesive, interfacial debonding and FRP failure on the strength of joints are investigated. The results show that the behaviour of the tested joints is accurately predicted provided that the variation of failure modes are taken into account. Moreover, it is shown that the damage process zone in adhesive layer is directly proportional to the shape of cohesive laws. This feature can be employed in the design phase to ensure sufficient overlap length and to account for important in-service parameters such as temperature and moisture.
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| 10. |
- Lin, Qibin, et al.
(författare)
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Strength and failure characteristics of jointed rock mass with double circular holes under uniaxial compression : Insights from discrete element method modelling
- 2020
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Ingår i: Theoretical and applied fracture mechanics (Print). - : Elsevier. - 0167-8442 .- 1872-7638. ; 109
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Tidskriftsartikel (refereegranskat)abstract
- Underground projects are typically performed in nonpersistent jointed rock masses. Excavation of tunnels or chambers causes crack initiation, propagation and coalescence, resulting in the instability and destruction of surrounding rocks. The failure behaviour of rocks is complicated owing to the interaction between holes and joints. Therefore, understanding the failure characteristics of holes in jointed rock masses is essential. In this paper, the mechanical characteristics of a jointed rock mass with double circular holes under uniaxial loading was investigated using the discrete element method. The effects of different joint parameters on the strength and failure behaviour of the jointed rock mass with double circular holes were studied. The results show that the existence of joints degrades the mechanical behaviour of the rock mass. Specifically, the peak strength and elastic modulus of the specimens show a “U” shape change with the joint dip angle and reach the minimum value when the angle is 30°. With an increase in joint spacing, the peak strength and elastic modulus increase gradually. An investigation of the crack coalescence process and displacement field of the specimens reveal the crack propagation mechanism based on the interaction between holes and joints under uniaxial loading. Five types of crack coalescence among the holes and adjacent joints were summarized. Four typical failure modes were observed: tensile failure across joints, block rotation failure around holes, shear failure through joint planes and tensile–shear mixed failure through holes.
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