|
|
Theoretical background for determining the fracture-mechanics parameters using the Boundary Element Method
Gröger, Roman
Boundary Element Method is a powerful tool for certain class of Solid Mechanics problems, where the fundamental solution of the governing differential equation is known. This article briefly outlines the theoretical background of BEM for plane problems as well as integral identities for calculation of internal stresses and displacements. Chapter 2 introduces the most useful contour integrals for calculation of fracture parameters in two-parameter linear elastic fracture mechanics. These coefficients are solved using the stress and strain tensor field calculated using BEM. All the proposed and discussed approaches are compared and the differences between FEM and BEM are discussed.
|
|
|
The Influence of T-stress on Mixed Mode Fracture Criterion
Seitl, Stanislav
In this paper, two-parameter constraint based linear elastic fracture mechanics is used to modify brittle fracture criterions under combination of normal and shear loading (Mode I and II). The value of constraint is characterized by T-stress. The results contribute to better description of influence of the geometry of a body on its fracture behaviour.
|
|
|
The effect of V-notch on the propagation rate of fatigue crack
Hutař, Pavel ; Knésl, Zdeněk
The rate of a fatigue crack propagating in a vicinity of a V-notch is studied. To assess the influence of the constraint on the rate of the propagation, two parameter constraint based linear elastic fracture mechanics is used. The corresponding values of the T-stress are calculated and values of the effective stress intensity factor Keff controlling the rate of a crack propagation are estimated as a function of the V-notch opening angle. The results obtained are compared with those for a crack emanating from nonsingular U- notch concentrator.
|
|
|
The influence of T-stress on the characteristics of a fatigue crack
Hutař, Pavel
The influence of the T-stress values on the propagation of fatigue cracks is analyzed. The parameters of the plastic zone are studied at two levels of applied stress, i.e. loading and unloading. The CTOD parameter and rp (plastic zone size) in the crack grow direction are discussed as a driving parameter of a fatigue crack propagation rate. The three ways of determination of propagation rate of a fatigue cracks are compared. Recommendations for applications of these methods are discussed.
|
|
|
Calculation of fracture parameters for the cracked bodies strengthened by circular patches
Gröger, Roman
The present article yields a quantitative characterization of the influence of geometry and material properties on the values of fracture parameters. Both the cracked sheets with single and double circular patches are used for the analysis. The desired value of elastic T stress is obtained using the conservation M integral resulting from the Eshelby integral concept. For the SIF calculation, the indirect method of J integral is used and the general concept for the SIF separation in case of a mixed mode of loading is discussed here. The reduction effect is specified by the ratios of SIF and T stress. The method of stiffening the region surrounding the crack tip can be used as an alternative method of the lifetime prolongation, but cannot be used in all cases. Sometimes the usual method of the crack tip singularity cancellation is the only one to repair the structure.
|
|
|
Analýza algoritmů výpočtu T napětí s ohledem na vliv vyšších členů Williamsova rozvoje
Gröger, Roman
A comparison between a couple of difference methods for quantification of T stress is presented. The stress distribution around a crack tip is expressed using the first four terms of Williams eigenfunction expansion for specimens corresponding to low constraint (SECT) as well as three-point bending specimens with high constraint. The dominant regions are quantified by accounting the different number of stress terms. An alternative method for calculation of T stress using the conservation integrals is used to compare the obtained results with the products of more accurate integral method. The appropriate ratios of r/a, on which the difference methods give the reasonable results, are outlined in conclusion.
|
|
|
Two-parameter characterisation of a crack growing in the vicinity of the material interface
Gröger, Roman ; Knésl, Zdeněk
A crack propagating in the vicinity of the material interface is analyzed. The problem is solved in the range of two-parameter linear elastic fracture mechanics. The aim of the contribution is to assess the influence of T stress on the direction of a crack growing to the interface. The interface is modelled for different material properties characterised by the Dundur's composite parameters. First the effect of the interface on T stress value is studied and values of KI, KII and T stress are estimated. Consequently the two-parameter modification of MTS criterion and Sih's SED criterion is used for prediction of a crack propagation direction. Both these approaches are compared. It is concluded that the negative values of T stress stabilize the crack propagation perpendicularly to the interface.
|
|
|
Calculation of the fracture parameters for a crack growing from the tip of a V-notch
Gröger, Roman ; Hutař, Pavel
This article yields a comparison of two principles for caculation of the fracture parameters - the direct finite element method and hybrid element method. The stress intensity factor and the T stress for a crack growing from the tip of a nonsingular stress concentrator for a large variety of notch angles is obtained. It is discussed the sensitivity of each parameter on the ratio of crack length to the width of the specimen. The obtained results can contribute to more precise characterisation of the crack instability state in a case of the combination V-notch + crack.
|
|
|
Two-Parameter Approach for Short Cracks at Notches
Seitl, Stanislav ; Hutař, Pavel ; Knésl, Zdeněk
A two parameter constraint-based fracture mechanics method is applied to the behavior of microstructurally and mechanically short fatigue cracks. A short crack in the vicinity of a grain boundary and a crack which is engulfed by the plastic strain field of a notch are studied as examples. The different levels of stress constraint are quantified by using the T-stress and the Q parameter. The corresponding calculations are performed by using a finite element method (ANSYS system). The influence of the T values on the propagation of short fatigue cracks is analyzed. The size of the plastic zone and the crack closure are significantly affected by the level of constraint. The influence of constraint on small fatigue crack growth kinetic is studied under the assumption that the controlling variable for the rate of propagation of the fatigue crack is the size of the plastic zone.
|
|
|
Quantitative Assessment of the T-stress in Short Fatigue Crack Propagation
Seitl, Stanislav ; Knésl, Zdeněk
Two parameter constrained based fracture mechanics is applied to quantitative assessment of microstructurally short fatigue cracks propagation. The different levels of a stress constraint are quantified by T-stress. Corresponding calculations are performed by finite element method (ANSYS system). The values of the T-stress are numerically estimated as a function of material properties, location and orientation of the short crack with respect to the grain boundary. The influence of constraint on small fatigue crack growth kinetic is discussed. It is shown that the level of constraint of the near-tip fields for microstructurally short fatigue cracks may influence the fatigue crack rate substantially.
|
guest ::
