No description available.

No description available.

No description available.

No description available.

No description available.

La motivació de la tesis deriva en el interès de la indústria aeronàutica a explotar, per mitjà d'un disseny adaptat, la utilització del gir d'esquerda per protegir els reforços situats davant una esquerda que s'està propagant en la xapa d'una estructura integral. L'objectiu principal és l'avaluació i predicció del gir d'esquerda en situacions de càrrega pròximes a Mode I, proporcionant una eina de modelització i un criteri confident. L'entorn industrial sota el qual s'ha realitzat aquest treball requereix una predicció ràpida del comportament estructural proporcionant informació útil als constructors. Per aquest motiu la predicció del gir d'esquerda s'ha investigat utilitzant la teoria linear elàstica de la mecànica de la fractura (LEFM) i l'anàlisi amb elements finits (FEA).Durant aquest treball s'ha demostrat la importància i necessitat de caracteritzar el camp de tensions a la punta de l'esquerda amb el factor d'intensitat de tensió (SIF) conjuntament amb un segon paràmetre. La tensió uniforme, no singular, normal a la línea de l'esquerda i dependent en la geometria i càrrega de la proveta, es a dir la tensió T, ha estat seleccionada com a segon paràmetre per dur a terme les prediccions del gir d'esquerda.El criteri més desenvolupat per predir el gir d'esquerda en situacions pròximes a Mode I és el proposat per Buczek, Herakovich, Boone et al., anomenat WEFO en la tesis. Aquest combina el criteri de tensió principal màxima amb la tensió T i considera efectes d'anisotropia. LEFM s'ha utilitzat també en la predicció del gir d'esquerda sota càrregues quasi estàtiques controlant en tot moment la plastificació del lligament.En la investigació d'eines de modelització/simulació s'ha tingut en compte les capacitats d'aquestes en el camp de la mecànica de la fractura, de disseny, d'implementació, així com la complexitat d'ús. Tot i que hi ha un gran ventall de Softwares que compleixen els requeriments assenyalats, només aquells que es trobaven a l'abast de l'autor s'han analitzat. StressCheck ha estat escollit com a resultat de la investigació. L'avaluació de la propagació de l'esquerda en provetes compactes en tensió (CT) i en provetes amb dos elements reforçants (2SP) sota els règims de Paris i Forman ha estat satisfactòria.Un pas important ha estat la implementació de la capacitat d'extracció de la tensió T. La demostració de la fiabilitat en el seu càlcul s'ha demostrat mitjançant resultats en la literatura i càlculs analítics en provetes de doble biga en volada (DCB). Un aspecte a tenir en compte és la importància en realitzar anàlisis no linears geomètrics pel càlcul del SIF i la tensió T.Prediccions en la trajectòria de l'esquerda s'han realitzat en base amb els resultats obtinguts en l'estudi de modelització. La millor trajectòria s'ha predit per mitjà del criteri WEFO. No obstant, les diferents trajectòries obtingudes per una esquerda propagant-se en la direcció T-L o L-T no són comparables amb els resultats experimentals.Aquestes deficiències estan relacionades en la definició del punt d'inestabilitat de l'esquerda. Algunes referències posen de manifest que hi ha experiències on l'esquerda es comporta de forma estable tot i mostrar T > 0. Per un altre banda, els criteris WEF i WEFO defineixen la inestabilitat dependent d'una distancia específica del material, rc. Però la seva definició no és única i no existeix cap acord sobre el seu càlcul.L'autor proposa un criteri derivat dels criteris existents i basant-se en els assajos, simulacions i resultats obtinguts. Aquest deriva del treball de Pettit i la tensió T normalitzada, TR, proposada per Pook. La fiabilitat d'aquest criteri es demostra amb la proveta DCB. Les prediccions de la trajectòria de l'esquerda en la proveta cruciforme no són tant satisfactòries. Tot i així, s'ha d'accentuar que el criteri desenvolupat proporciona la predicció més acurada. / The motivation of this thesis started from the interest of aeronautical industry to exploit the utilization of crack turning to protect stiffeners in front of an approaching skin-crack in integral structures by a tailored design. The main objective was to assess and predict crack turning under nearly Mode I situations on structures that reproduce aeronautical conditions by providing a modelling tool and a reliable criterion. The industrial environmental in which this work has been carried out requires a fast prediction of the structural behaviour to provide useful inputs to aircraft designers. It is for this reason that the crack turning prediction was investigated by means of LEFM and FEA. During this work it has been shown the importance and necessity of a second parameter for the characterisation of the stress field at the crack tip besides the SIF. Among the different proposed second parameters, the uniform non-singular stress, normal to the crack line and dependent on the type of loading and specimen geometry, i.e. the T-stress, was selected for crack turning predictions due to both calculation simplicity and its independence of the crack tip distance. The most developed criterion for crack turning predictions near Mode I loading is the criterion proposed by Buczek, Herakovich and Boone et al., called the WEFO-criterion. This is the Maximal Principal Stress criterion implemented with the T-stress and taking into account anisotropic effects. A challenge of this thesis was to overcome the lack of prediction on crack turning provided by this last criterion.Although the validity of LEFM is restricted, it was applied for the prediction of crack turning for quasi-static loading while paying attention to possible plastification. A screening of existent commercial and non commercial tools was carried out in respect to their fracture mechanics capabilities, their design abilities, implementation as well as their complexity. Although, there are many software possibilities, only those within the reach of the author were evaluated. This resulted in the selection of the commercial tool StressCheck®. The assessment of crack propagation on compact tension and two stringer specimens governed by the Paris and Forman regimes was satisfactory compared with experimental results using the material data from simple standard specimens.An important step was the implementation of the T-stress extraction facility in the tool and the evidence of its reliability. The latter was proved by literature and analytical calculations on DCB specimens. An important finding was the importance to perform geometric non-linear analyses for computing SIF and T-stress to find values comparable with literature data and analytical calculations. Taking into account the results obtained on the modelling study, crack path predictions were performed. The best prediction by means of existing criteria was reached by the WEFO-criterion. Different crack paths were predicted for a crack propagating in T-L or L-T directions. However, these predictions were not satisfactorily reliable: the point in the crack path where crack turning should take place was not predicted adequately. Additionally, the crack paths were similar for T L and L-T directions. These deficiencies are related with the definition of the crack path instability. Some literature results have shown that in some experiences the crack behaved in a stable manner even if T > 0. Moreover, WEF and WEFO criteria define crack instability to be related with a material specific distance, rc, but, there is no agreement about its definition. Based on tests, simulation results and observations noted during this work, a compilation criterion was proposed. This is based on the work of Pettit and the normalised T-stress, TR, proposed by Pook. Its reliability was successfully proved on the DCB. The crack path predictions on the CFS were not as satisfactory. But even at its worst the developed criterion was the most accurate.

It is generally accepted that the fatigue crack growth depends on the stress intensity factor range (ΔK) and the maximum stress intensity factor (K_max). Numerous driving forces were introduced to analyze fatigue crack growth for a wide range of stress ratios. However, it appears that the effect of the crack tip stresses and strains need to be included into the fatigue crack growth analysis as well. Such an approach can be successful as long as the stress intensity factors are correlated with the actual elastic-plastic crack tip stress-strain field. Unfortunately, the correlation between the stress intensity factors and the crack tip stress-strain field is often altered by residual stresses induced by reversed plastic deformations. A two-parameter model (ΔK_tot, K_max,tot) based on the elastic-plastic crack tip stress-strain history has been proposed. The applied stress intensity factors (ΔK_appl, K_max,appl) were modified and converted into the total stress intensity factors (ΔK_tot, K_max,tot) in order to account for the effect of local crack tip stresses and strains on the fatigue crack growth. The fatigue crack growth was regarded as a process of successive crack re-initiations in the crack tip region and predicted by simulating the stress-strain response in the material volume adjacent to the crack tip and estimating the accumulated fatigue damage. The model was developed to predict the mean stress effect for steady-state fatigue crack growth and to determine the fatigue crack growth under simple variable amplitude loading histories. Moreover, the influence of the applied compressive stress on fatigue crack growth can be explained with the proposed two-parameter model. A two-parameter driving force in the form of: Δκ = K_max,tot^p ΔK_tot^(1-p) was derived based on the local stresses and strains at the crack tip using the Smith-Watson-Topper (SWT) fatigue damage parameter: D = σ_maxΔε/2. The parameter p is a function of material cyclic stress-strain properties and varies from 0 to 0.5 depending on the fatigue crack growth rate. The effects of the internal (residual) stress induced by the reversed cyclic plasticity manifested themselves in the change of the resultant (total) stress intensity factors driving the crack. Experimental fatigue crack growth data sets for two aluminum alloys (7075-T6 and 2024-T351), two steel alloys (4340 and 4140), and one titanium alloy (Ti-6Al-4V) were used for the verification of the model under constant amplitude loading. This model was also capable of predicting variable-amplitude fatigue crack growth. Experimental fatigue crack growth data sets after single overloads for the aluminum alloy 7075-T6, steel alloy 4140, and titanium alloy Ti-6Al-4V were also used for the verification of the model. The results indicate that the driving force Δκ can successfully predict the stress ratio R effect and also the load-interaction effect on fatigue crack growth.

It is generally accepted that the fatigue crack growth depends on the stress intensity factor range (ΔK) and the maximum stress intensity factor (K_max). Numerous driving forces were introduced to analyze fatigue crack growth for a wide range of stress ratios. However, it appears that the effect of the crack tip stresses and strains need to be included into the fatigue crack growth analysis as well. Such an approach can be successful as long as the stress intensity factors are correlated with the actual elastic-plastic crack tip stress-strain field. Unfortunately, the correlation between the stress intensity factors and the crack tip stress-strain field is often altered by residual stresses induced by reversed plastic deformations. A two-parameter model (ΔK_tot, K_max,tot) based on the elastic-plastic crack tip stress-strain history has been proposed. The applied stress intensity factors (ΔK_appl, K_max,appl) were modified and converted into the total stress intensity factors (ΔK_tot, K_max,tot) in order to account for the effect of local crack tip stresses and strains on the fatigue crack growth. The fatigue crack growth was regarded as a process of successive crack re-initiations in the crack tip region and predicted by simulating the stress-strain response in the material volume adjacent to the crack tip and estimating the accumulated fatigue damage. The model was developed to predict the mean stress effect for steady-state fatigue crack growth and to determine the fatigue crack growth under simple variable amplitude loading histories. Moreover, the influence of the applied compressive stress on fatigue crack growth can be explained with the proposed two-parameter model. A two-parameter driving force in the form of: Δκ = K_max,tot^p ΔK_tot^(1-p) was derived based on the local stresses and strains at the crack tip using the Smith-Watson-Topper (SWT) fatigue damage parameter: D = σ_maxΔε/2. The parameter p is a function of material cyclic stress-strain properties and varies from 0 to 0.5 depending on the fatigue crack growth rate. The effects of the internal (residual) stress induced by the reversed cyclic plasticity manifested themselves in the change of the resultant (total) stress intensity factors driving the crack. Experimental fatigue crack growth data sets for two aluminum alloys (7075-T6 and 2024-T351), two steel alloys (4340 and 4140), and one titanium alloy (Ti-6Al-4V) were used for the verification of the model under constant amplitude loading. This model was also capable of predicting variable-amplitude fatigue crack growth. Experimental fatigue crack growth data sets after single overloads for the aluminum alloy 7075-T6, steel alloy 4140, and titanium alloy Ti-6Al-4V were also used for the verification of the model. The results indicate that the driving force Δκ can successfully predict the stress ratio R effect and also the load-interaction effect on fatigue crack growth.

Thermally cycling a thin plate of nickel-based superalloy with an intense in-plane thermal gradient, or hot spot, produces thermally induced crack growth not represented by classic thermo-mechanical fatigue (TMF). With the max hot spot temperature at 1093 C (2000 F) of a 1.5 mm thick, 82.55 mm diameter circular plate of B-1900+Hf, annular buckling and bending stresses result during each thermal cycle which drive the crack initiation and propagation. A finite element analysis (FEA) model, using ANSYS 7.1, has been developed which models the buckling and as well as represents the stress intensity at simulated crack lengths upon cool down of each thermal cycle. The model approximates the out-of-plane response at heat-up within 5% error and a difference in the final displacement of 0.185 mm after twelve thermal cycles. Using published da/dN vs. Keff data, the number of cycles needed to grow the crack to the experimental arrest distance is modeled within 1 mm. The number of cycles to this point is within 5 out of 462 in comparison to the experimental test.

No description available.