Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura
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Coupled identification of the hardening behavior laws and Gurson–Tvergaard–Needleman damage parameters - Validation on tear test of 12NiCr6 CT specimen
Get PDFThis work is devoted to the application of the micromechanical Gurson-Tvergaard-Needleman (GTN) model to study the ductile tearing of 12NiCr6 steel. GTN model is widely used to describe the three stages of ductile tearing: nucleation, growth and the coalescence of micro-voids. A new approach consists of a coupled identification of the GTN damage model with and without hardening laws using an inverse analysis is proposed. After identification, the obtained results show a good agreement between the experimental curve tensile test of an axisymetric notched bar (AN2) and those numerically obtained for GTN model coupled with the hardening laws. In order to validate the identified GTN parameters, a simulation of tear test is conducted on 12 NiCr6 steel CT specimen. The numerical results are compared with experimental results found in the literature and a good agreement is obtained. This identification procedure is more accurate than when the damage parameters are identified independently of the hardening laws
Comparison of the constraint parameters in elastic-plastic fracture mechanics
Get PDFTo describe the effect of crack-tip constraint on the stress field for finite cracked bodies, the following most widely used constraint parameters can be employed in elastic-plastic fracture mechanics, namely, local triaxiality parameter, constraint parameter Q, the second fracture mechanics parameter A using a three-term elastic-plastic asymptotic expansion. To establish the relationship between different constraint parameters, the crack-tip stress fields are employed. Relationship between the crack-tip constraint parameters A, A2, Q and of elastic-plastic fracture mechanics is investigated
Environmental effects on the reliability of an AISI 304 structure
Get PDFThis work is based on the application of probabilistic fracture mechanics (MRP) models to predict the reliability of pressure pipelines in nuclear power reactors. The simulation of cracking of stainless steel piping under the conditions of intergranular stress corrosion cracking (IGSCC) is based on the improved modified piping reliability analysis including seismic events (PRAISE) code. The IGSCC is characterized by a unique damage parameter that depends on residual stresses and environmental conditions. Case studies of IGSCC failures in nuclear power plants are presented and the contribution of environmental effects on crack initiation and leakage is discussed. For small damages we observed that the change in temperature or oxygen concentration does not affect the initiation process but their decrease contribute favourably to the decrease in the leakage probabilities
Computational multi-scale analysis of simultaneous processes of delamination and damage accumulation in laminated composites
Get PDFWhen studying mechanical behavior of structures made of laminated polymer composite materials, issues related to their relatively high susceptibility to damage, such as delamination, fiber breakage and matrix failure, play significant role. Key aspects are also related to accurate description of the internal structures and heterogeneity of the material. This work is aimed at a numerical study of the processes of damage accumulation in plies as well as development of delamination in laminated polymer composite L-shaped specimen taking into account microstructural parameters. The influence of contact conditions between the plies on the model’s behavior has been studied. It was established that the delamination processes inside the specimen can initiate at the early stages of deformation, which is reflected in the difference of the results obtained using models with various numbers of separated plies. Comparison of the results of modelled elastic behavior of the specimen and models with the developed delamination leads to conclusions about influence of a number of model parameters on the results obtained before and after propagation of delamination. In particular, such factors as the geometry and mechanical properties of models, the number and conditions for specifying contact pairs in the model were analyzed
Numerical analysis of a caprock integrity during oil production by steam-assisted gravity drainage method
Get PDFThe work is devoted to the investigation of a caprock integrity during oil production by steam-assisted gravity drainage method. An originally proposed thermo-hydro-mechanical model was used for the evaluation of mechanical loading acting on the over-burden. The model includes mass conservation laws, the energy conservation law and the linear momentum balance. Filtration of each phase of the three-phase flow (steam, oil and water) is described by Darcy’s law. Inelastic deformations are described by the phenomenological viscoplastic model based on Drucker-Prager yield criterion. The effect of the porosity evolution induced by the propagation of the steam chamber within the reservoir on the oil production rate and the caprock integrity is studied. It has been shown that the oil production rate is strongly depend on the prevailing physical mechanism of the porosity evolution. Reservoirs characterized by the volumetric strain mechanism of the porosity evolution produce slightly higher values of the mechanical loadings acting on the over-burden
The approach to fracture diagnosis by means of experimental measurements of the stored energy
Get PDFThe energy dissipation in metals in the process of irreversible deformation leads to intensive heat generation in the areas in the strain localization zones. This work is devoted to the measuring the power of heat sources by means of the data of temperature generated using IR thermography. Verification of calculated values of power of heat sources is carried out by the use of heat flux sensor that based on the Seebeck effect. The quasi-static experimental studies on the tensile were conducted on the flat titanium alloy Grade 2 specimens. A good agreement between the thermography and the measured directly using heat flux sensors was shown. The measurements showed the relationship between the moment of fracture of metal specimens and the change in the nature of localized heat generation caused by irreversible deformation
Averaged strain energy density estimated rapidly from the nodal stresses by FEM for cracks under mixed mode loadings including the T-stress contribution
Get PDFThe present contribution reviews a recently proposed method to rapidly estimate the averaged SED at the tip of short as well as long cracks under in-plane I+II and long cracks under out-of-plane I+III mixed mode loadings. Short cracks are distinguished from long cracks by considering that the stress fields within the control volume of short cracks are no longer governed solely by the stress intensity factors (SIFs), but the contribution of higher order terms, and primarily the T-stress, becomes significant to estimate the averaged SED. According to the proposed method, the averaged SED is calculated using the linear elastic nodal stresses evaluated by FEM either at the crack tip, to account for the SIFs contribution, and at selected FE nodes of the crack free edges, to include the T-stress contribution. The advantage of the proposed approach is two-fold: coarse FE meshes can be adopted; moreover, geometrical modelling the control volume is no longer necessary. To validate the approach, cracked plates subjected to in-plane I+II mixed mode loading as well as bars weakened by circumferential outer cracks subjected to out-of-plane mixed mode I+III loading have been analysed. A comparison between approximate values of the averaged SED according to the nodal stress approach and those derived directly from the FE strain energy adopting very refined FE meshes has been successfully performed
The ab-initio aided strain gradient elasticity theory: a new concept for fracture nanomechanics
Get PDFWhen the width of cracked nanocomponents made of brittle or quasi-brittle materials is less than approximately , the size of the - dominance zone becomes smaller than and comparable to the fracture process zone ( ). The fracture process starts to be dominated by far-stress field terms and the critical stress intensity factor can no more represent the total fracture driving force. This means a breakdown of a classical linear elastic fracture mechanics suffering from the undesirable crack-tip stress singularity. The contribution presents a new concept expected to properly predict the critical crack driving force for nano-components: The ab-initio aided strain gradient elasticity theory (AI-SGET). In contrast to the Barenblatt cohesive model, the strain gradient elasticity theory does not require to prescribe a suitable field of cohesive tractions along the crack faces in order to eliminate the stress singularity and to exhibit cusp-like profiles of crack flanks close to the crack front in accordance with atomistic models. The only unknown and necessary quantity is the material length scale parameter which can be, e.g., determined by best strain gradient elasticity fits of ab-initio computed phonon-dispersions and near-dislocation displacement fields. Atomistic approaches can also be employed to determine fracture mechanical parameters (crack driving force, crack tip opening displacement) related to the moment of crack instability in a given material. Such AI-SGET codes can then be utilized to a successful prediction of fracture of cracked nanocomponents made of brittle or quasi-brittle materials.  
Experimental fracture resistance study for cracked bovine femur bone samples
Get PDFCrack growth and brittle fracture is one of the major failure modes in bone materials and therefore understanding the fracture behavior and affecting parameters on the crack growth resistance of bone is necessary for biomechanics researchers. In this paper, mode I fracture toughness value for the left and right femur bones of same bovine were measured experimentally using several single edge notch bend beam specimens (SENB) subjected to three-point bend loading in dry condition. The SENB specimens were cut along the longitudinal axis of bone but from different hoop directions. Fracture toughness results of sample prepared from the frontal part of bone were higher than the back or side sections. Depending on the location of sample, the fracture toughness of femur bone was varied from 5 to 10 MPa . Furthermore, the results obtained for similar location of both left and right femur bones were nearly identical. The fracture energy (Gf) of the tested specimens was also measured and it was found that a linear relation can be fitted to the (KIc)2 versus Gf results of the tested bovine bone
Using a Hashin Criteria to predict the Damage of composite notched plate under traction and torsion behavior
Get PDFThe volume fraction effect of carbon fiber in an epoxy matrix in the vicinity of the notch on the traction and torsion behavior were studied. This volume fraction which increases with the reduction of the thickness is a parameter which proved effective on the capacity of resistance. The calculations are done with the numerical code (ABAQUS 2009) using the model Hashin with shell elements of the structure. The objective of this study is to simulate the damage of stratified composite materials, the effect of several parameters such as the stacking sequence and the increase of the thickness of the folds having the same orientation of the structure, were evaluated and presented by load-displacement curves. The results obtained from the study illustrate the variation of damage as a function of these effects which act simultaneously and which also show that these modifications have a higher absorption capacity than that without modification