Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura
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A comparison between S-N Logistic and Kohout-Věchet formulations applied to the fatigue data of old metallic bridges materials
Get PDFA new formulation of a Logistic deterministic S-N curve is applied to fatigue data of metallic materials from ancient Portuguese riveted steel bridges. This formulation is based on a modified logistic relation that uses three parameters to fit the low-cycle- (LCF), finite-life- and high-cycle-fatigue (HCF) regions. This model is compared to the Kohout-Věchet fatigue model, which has a refined adjustment from very low-cycle fatigue (VLCF) to very high-cycle fatigue (VHCF). These models are also compared with other models, such as, Power law and fatigue-life curve from the ASTM E739 standard. The modelling performance of the S-N curves was made using the fatigue data considering the stress fatigue damage parameter for the materials from the Eiffel, Luiz I, Fão and Trezói riveted steel bridges. Using a qualitative methodology of graphical adjustment analysis and another quantitative using the mean square error, it was possible to evaluate the performance of the mean S-N curve formulation. The results showed that the formulation of the S-N curve using the Logistic equation applied to the metallic materials from the old bridges obtained superior performance to the analysed models, both in the estimation of fatigue behaviour in the low-cycle fatigue (LCF) region and in the lowest mean square error
Numerical model of cracking pattern in laminated bamboo specimens under tensile and shear loads
Get PDFThis work describes a two-dimensional numerical model that allows detecting the appearance of cracks and calculating their propagation in elements made of laminated bamboo, under tension and shear. This composite material has long parallel strong cellulose fibers embedded in a weak lignin matrix. The mechanical model that represents the failure and fracture process of laminated bamboo is still unknown. This numerical model simulates localized strains, showing the beginning and progression fracture in the material. The model is based on a two-dimensional scheme for plane stresses, using the finite element method. A one-dimensional plasticity constitutive model, based on Weibull probability distribution, is used to describe the mechanical response of the fibers, and a continuum damage constitutive model controls the behavior of the matrix. The homogenization process is done with the rule-of-mixtures, and vanishing fiber diameter simplification. Continuum strong discontinuities approach is taken as a technique to detect a jump in the displacement field, during the fracture process. This numerical model is used to simulate the failure on tensile and shear tests of laminated bamboo Guadua angustifolia, which were then compared to experimental findings. The results show that the numerical model detects the same crack patterns obtained in tests
Prediction and optimizing residual stress profile induced by cold expansion in aluminum alloys using experimental design
Get PDFCold expansion by hardening is a common process used in the aerospace industry to extend the fatigue lifetime of assembly holes, through a field of high compressive tangential residual stresses. The understanding and the control of the residual stresses are thus important, since it can be beneficial to improve lifetime of the structures. The main objective of this work is to establish and validate a predictive model of residual stresses generated by cold hardening. This technique can be an effective ways for industrials allowing an estimation of the fatigue lifetime of parts according to the process parameters or to determine the optimal parameters to maximize fatigue lifetime. An experimental setup was used to highlight the effect of expansion degree, thickness of the part and yield strength on the residual stresses profiles. Moreover, the proposed mathematical models were used to determine the optimal values of the various factors for the residual stress profiles prediction at any factor values, in order to achieve maximum service life after repair of a cracked structure or to delay crack initiation and growth in riveted or bolted structures. Besides, the modelling permits us to highlight the effect of interaction of these factors on the residual stresses profiles
Contribution of longitudinal stiffener rigidity and position to bridge girder integrity
Get PDFTo increase the elastic critical load of a plate, such as I-shaped cross-welded section of bridge girders, slenderness is usually reduced by dividing the web into subpanels, by means of transversal stiffeners and a longitudinal stiffener to increase flexural and torsional stiffness. The optimal solution is defined when the stiffener maximizes the buckling coefficient, with a minimal cross-section area. For this purpose, seven forms of open and closed sections of longitudinal stiffeners, with differing second moment of area, are examined in terms of buckling coefficient by theoretical solution and numerical calculation, to compare their contribution in terms of weight per linear meter of beam. The optimum value of a conventional flat stiffener position, respect panel height and an useful practical law is given to correlate the best position with respect to variations in stress gradient, from pure bending to pure compression. This practical law facilities calculation regarding where to put a stiffener with respect to compressed edge in a web panel subjected to flexural-compressive loads, in order to maximize the benefit of its action and increase the stability of bridge girders
Crack growth based life prediction approach under LCF-HCF interaction
Get PDFPrediction of cyclic life under low cycle fatigue - high cycle fatigue (LCF-HCF) interaction is of paramount importance in the context of structural integrity of components in the primary side of fast reactors where such damage under LCF-HCF interaction occurs. The present investigation deals with the crack growth behavior of a type 316LN austenitic stainless steel subjected to simultaneous application of LCF and HCF cycles (block-loading). Tests were performed over a wide range of temperatures from ambient to 923 K. Experimental results indicate that a critical crack-length (acr) exists, beyond which the LCF-HCF interaction becomes significant. An attempt was made to predict life under block cycling by estimating the acr using fatigue crack threshold (ΔKth) since the latter is known to be affected significantly by the loading history. A universal equation, based on the concept of an equivalent critical crack length (acr.,eq) which incorporates the damage contribution from DSA and ratcheting under combined LCF-HCF loading, was proposed for life estimation
Fatigue crack growth simulation of two non-coplanar embedded cracks using s-version finite element method
Get PDFIn this paper, the fatigue crack growth simulation of two non-coplanar embedded cracks using the s-version finite element method is presented, and the validity and reliability of the alignment rule for two non-coplanar cracks are evaluated. According to the previous numerical and experimental studies on two non-coplanar surface cracks, the simulated fatigue crack growth behavior is categorized into five patterns to discuss the criteria for the application of the alignment rule. The results suggest that the strength of interaction between the non-coplanar embedded cracks is similar to that between non-coplanar surface cracks. Finally, the interaction of the cracks is evaluated by the stress intensity factor, and the categorization of the fatigue crack growth behavior is discussed by the stress intensity factor. It can be found that the boundary corresponding to the criteria of the application of the alignment rule can be determined as the ratio of the stress intensity factor is 4%. Thus, instead of making a decision of the fatigue crack growth pattern based on the visual inspection, the ratio of the stress intensity factor can be used, and should give more quantitative evaluation of the interaction of two non-coplanar embedded cracks
Low cycle fatigue and creep-fatigue response of the 316Ti stainless steel
Get PDFSS 316Ti is widely used in bellows industry and is a good candidate material for high temperature bellows in sodium cooled fast reactor (SFR) systems. Typical operating temperature experienced by SFR systems is around 823K. Design of bellows for nuclear applications need to be in compliance with the standard design codes such as ASME section-III and RCC-MR. The fatigue data and cyclic stress strain curve of SS316Ti are not available in design codes such as RCC-MR or ASME section-III/NH. Hence, the material data required for high temperature design of bellows are generated experimentally. Initially, the basic tensile data such as yield strength, ultimate tensile strength and % elongation of the material were obtained from tensile testing at 823K. Low cycle fatigue tests were carried out in strain controlled mode on SS316Ti at 823K different strain ranges in air and variation of number of cycles with strain range was obtained. Creep-fatigue interaction (CFI) experiment was also conducted at 823K and strain amplitude of ± 0.4% with 1 minute hold time in peak tensile strain. The stress response (peak stress variation with number of cycles) of the material showed continuous hardening up to saturation followed by crack nucleation and final failure. The fatigue life was found to decrease with increase in strain range. The fatigue life decreased in presence of hold period in tension. The design fatigue curve for SS316Ti at 823K has been generated using the LCF data by incorporating factors of safety on strain and number of cycles. Cyclic stress strain curve was generated for the material at 823K. The tensile, LCF and CFI data generated will be useful in design of SS316Ti bellows for SFR systems
Isothermal and thermomechanical fatigue interaction in fatigue crack propagation behavior of a low-carbon nitrogen-controlled 316 stainless steel
Get PDFIn this work, the effect of superimposing of isothermal low cycle fatigue loading to the thermomechanical fatigue loading on the crack propagation behavior of the naturally initiated short crack in Low-carbon nitrogen-controlled 316 stainless steel was investigated. The experimental results indicated that the crack propagation path depends on the loading condition; the cracks appear to be initiated and propagated at grain boundary perpendicular to the loading axis which might be a relatively week region at elevated temperature under the in-phase type TMF loading and the LCF loading at high temperature, on the other hand, the cracks initiated and propagated by the transgranular mode under the out-of-phase type TMF loading and the LCF loading at middle temperature. The short crack growth rate has also affected the microstructure, i.e., the intergranular crack exhibits higher growth rate compared with the transgranular crack. In addition, the crack growth rate was accelerated by superimposing of the isothermal low cycle fatigue loading to the main thermo-mechanical fatigue loading. The short crack growth rate could be predicted according to summation law of crack growth behavior based on the J-integral approach considering with crack propagation path
Crack propagation in a brittle DCB specimen assessed by means of the Williams’ power expansion
Get PDFA double cantilever beam geometry has been chosen in order to investigate the importance of the higher-order terms of the Williams’ power expansion for the crack path estimation. The crack propagation has been tested experimentally on a brittle polymethylmethacrylate (PMMA) specimen and although the mode I loading conditions were applied, the crack kinked from its original plane immediately and propagated towards the bottom side of the specimen. It has been shown that this phenomenon is connected to the magnitude and sign of the T-stress and to the level of the constraint generally. In this work, the influence of the third and higher terms of the Williams’ series on the crack propagation is investigated. The generalized form of the well-known maximum tangential stress fracture criterion for determination of the crack propagation angle has been tested and discussed. The observed differences in the crack trajectory of different specimens have been found to be related to the magnitude of the higher-order terms of the stress tensor components at the crack tip
Cruciform specimens used for determination of the influence of T-stress on fatigue crack growth with overloads on aluminum alloy Al 6061 T651
Get PDFThe publication presents a cruciform specimen for the determination of cyclic crack growth data under biaxial loading. The design of the specimen with slotted loading arms allows good decoupling between the two loading directions. For different initial crack geometries, the solutions for the stress intensity factors KI and KII as well as the crack-parallel T-stress are calculated by linear elastic finite element analysis (FEA) with the program ABAQUS. For two specimens with the same geometry made of aluminum alloy 6061 T651, the crack growth behaviour is measured at different T-stresses at a stress ratio of R=0.7 and overloads. It is shown that the crack retardation after an overload with crack-parallel tensile stress is less than without it. The reason for this behaviour is considered to be the reduced plasticity at the crack tip due to the higher triaxiality of the stress state