•Room and elevated temperature FCP behavior of L-PBF IN718 was examined.•L-PBF IN718 showed higher FCP rates than CM IN718 in low and intermediate ΔK regime.•Effect of orientation on FCP behavior of ...L-PBF IN718 was negligible.•FCP rates of L-PBF IN718 at 650 °C were lower than those at 25 °C in low ΔK regime.
The fatigue crack propagation (FCP) behavior of Inconel 718 alloy manufactured by laser powder bed fusion (L-PBF) was studied at 25 and 650 °C, and compared to those of conventionally processed counterpart. The FCP rates of L-PBF Inconel 718 alloy were found to be significantly higher than those of conventionally processed counterpart at 25 °C in low and intermediate ΔK regimes. The FCP rates decreased in near-threshold ΔK regime at 650 °C, while the reversal occurred above ΔK of 7 MPa√m, as compared to those at 25 °C. The FCP behavior of L-PBF Inconel 718 alloy was discussed based on micrographic and fractographic analyses.
As one of the most common failure modes for helical gear system, the gear crack in helical gear pair is studied, the improved spatial crack propagation paths are modelled for helical gear pair based ...on helical gear crack failure mode in actual working condition, in the direction along tooth width, the crack initiates in one end face of gear, and it propagates to tooth addendum or opposite end of the gear, meanwhile, along crack depth direction, the crack propagation path is modelled into straight line or curved line. Furthermore, the limiting line, to characterize the crack effect, is simulated as straight line or curved line. Unlike previous studies, the transverse and axial gear foundation stiffness are modified due to gear crack. In addition to transverse and axial gear tooth stiffness. Based on the improved mesh stiffness calculation method for cracked helical gear pair, the influence of crack parameters and crack types on the time varying mesh stiffness of helical gear pair are studied, and the analytical calculated results are validated against the simulated results of FE (finite-element) models.
•Initial toughness criterion generally gives better results for concrete of all grades.•FPZ length is much longer after PFZ is fully formed based on the nil SIF criterion.•The KR-curve obtained from ...the nil SIF criterion is lower at early fracture stage.•Difference from the two criteria becomes larger as concrete strength increases.•Different performance due the two cracking criteria on concrete is also elaborated.
In the analysis of mode-I crack propagation of normal strength concrete at a crack tip, the initial fracture toughness and nil-stress intensity factor (nil-SIF) are two distinguished and widely adopted types of crack propagation criteria. However, there is little information reported on the difference resulting from the two criteria when they are employed to analyze concrete with different strength grades. Aiming at this objective, three-point bending tests are carried out on notched concrete beams of five strength grades, i.e. C20, C40, C60, C80 and C100, and an arrange of initial crack length/depth ratios as 0.2, 0.3 and 0.4, to investigate initial fracture toughness, fracture energy and load–crack mouth opening displacement (P–CMOD) relationship. Meanwhile, the three-point bending tests are also conducted on notched concrete beams of four specimen heights, i.e. 60, 90, 120, and 150mm. The two aforementioned types of concrete crack propagation criteria are introduced to determine crack propagation and predict the P–CMOD curves of a series of notched concrete beams under a three-point bending test. It has been found that the P–CMOD curves calculated using the initial fracture toughness criterion show a better agreement with experimental results than the ones calculated using the nil SIF criterion. With the increase of concrete strength, the difference between the peak loads from experiment and those from analyses based on the nil-SIF criterion becomes increasingly larger than the scenarios based on the initial fracture toughness criterion. Therefore, it can be reasonably concluded that for the two types of concrete crack propagation criteria, the initial fracture toughness is more appropriate for describing the fracture behavior of concrete, especially for high strength concrete.
A new approach for concurrent multiscale modeling of three-dimensional crack propagation in concrete is proposed. A macroscopic model with homogenized elastic parameters is adopted in the regions ...where the material behaves elastically. For regions where cracks are expected to occur, a mesoscopic model based on a mesh fragmentation technique is used to represent the concrete as a heterogeneous three-phase material composed of mortar matrix, coarse aggregates and interfacial transition zone. In this technique, standard finite elements with high aspect ratio are inserted in between all regular finite elements of the mortar matrix and in between the mortar matrix and aggregate elements in order to describe the crack initiation and propagation process by using an appropriate tensile damage constitutive model. Coarse aggregates with regular shapes are generated from a grading curve and placed into the mortar matrix randomly, using the “take-and-place” method. Coupling finite elements are used for connecting the non-matching meshes corresponding to the macro and mesoscale regions, without increasing the total number of degrees of freedom of the problem. Realistic predictions of crack formation and propagation were obtained for different tests, replicating accurately the observed experimental patterns.
•An alternative 3D concurrent multiscale model for concrete members is proposed.•Crack propagation process is simulated through a mesh fragmentation technique.•3D crack propagation is represented without the need of tracking algorithms.•Coupling finite elements are used to couple 3D non-matching meshes.•Comparison with experimental results is performed.
•Fatigue crack propagation under cyclic loading is simulated by XFEM with the VNE and computational cost can be saved significantly.•The local refinement technique can improve the accuracy of the ...SIFs.•The proposed method has the advantage for the case where crack propagates to the boundary.•The condition number of the global stiffness matrix is alleviated by using only discontinuous branch enrichment function.
The extended finite element method (XFEM) with the local refinement technique using the variable-node element (VNE) is proposed to simulate the fatigue crack propagation under the cyclic loadings, in which the initially generated coarse mesh around the crack tip is refined after the crack propagation and the VNE is used to connect the refined elements and the adjacent elements. The local refine elements are used to improve the accuracy of stress near the crack tip and it is unnecessary to conform the discretization to the crack surfaces. The maximum tangential stress criterion is used to determine the crack propagation angle θc and the fatigue life is estimated by the Paris law. With this method, the condition number of the global stiffness matrix is alleviated by using the only discontinuous branch enrichment function and the crack increment size can be set flexibly and small enough to reproduce true crack path. Because only the refined mesh around crack tip is updated at each increment step during crack propagation, the simulation cost is much less than that of the method in which global fine mesh is required. Numerical examples of the crack propagation under the monotonic and cyclic loadings are given out and it is demonstrated that the proposed method can calculate the stress intensity factors (SIFs) and predict the crack paths more accurately with the local refine elements near the each crack tip. The proposed method is also suitable to simulate the crack propagation of the functional graded materials under the thermal loadings in the future works, in which the material non-homogeneity near the crack tip requires accurate modeling.
•Determination of crack tip position and FPZ in concrete based on CBM model and DIC technique.•Observation for the whole fatigue crack propagation process in concrete based on DIC method.•Discussion ...for the evolution laws of FPZ and non-cohesive crack.•Description for da/dN can be divided into descending and ascending segments.
A method to determine crack tip position and fracture process zone (FPZ) in concrete was proposed based on Bažant Crack Band Model (CBM) and digital image correlation (DIC) technique, which was described as: firstly, the critical crack opening displacement of concrete (wcr) was determined based on CBM and the basic mechanical properties; secondly, DIC method was used to obtain the displacement field on the surface of specimens; thirdly, the position where the crack opening displacement equaled to wcr was defined as the crack tip position. In the fatigue crack propagation tests, the whole fatigue crack was FPZ at the beginning of fatigue loading until the non-cohesive crack grew and the length of FPZ decreased. The length of FPZ tended to be stable when the number of loading cycles was close to half of the fatigue life. Fatigue crack propagation curves (a-N curves) were obtained and fitted by logistic function to reduce the influence of the data scattering. Fatigue crack propagation rate da/dN was separated into descending and ascending crack propagation functions based on the increment of crack Δa and stress intensity factor ΔK, respectively. The da/dN-Δa curves for the descending segment showed obvious scatter. Paris’ law was applied to describe fatigue crack propagation rate well for the ascending segment.
The introduction of an overload or underload within a constant amplitude loading fatigue test leads to a retardation or acceleration of the Fatigue Crack Growth Rate (FCGR). The understanding of the ...causes of these effects is essential in the context of variable amplitude fatigue loading, since in principle any loading history can be represented as a sequence of overloads and underloads. In the case of overload, along with some other minor causes, the residual stress changes at the crack tip and crack closure behind the tip can be thought of as the main factors that affect the fatigue crack growth rate. Whilst this has been recognised and accepted for many decades, controversy persists regarding the relative significance and presence of these two effects, and consensus is yet to emerge. The effect of crack closure, when the baseline loading ratio is high enough, can be inhibited so that the main cause of retardation becomes doubtless the residual stress present ahead the crack tip.
In the present paper we report our attempt to deconvolve the contributions of crack closure and residual stress on crack retardation following an overload. To accomplish this task we analyse the results of fatigue tests run at two baseline load ratios, namely R=0.1 and R=0.7. At the load ratio of R=0.7 the crack closure effect is not operative, as confirmed by Digital Image Correlation analysis of the crack flanks close to the tip, and post mortem fractographic analysis of crack surfaces. Therefore, for R=0.7 the compressive residual stress region created by the overload ahead of the crack tip is the sole mechanism causing crack retardation. Therefore, for R=0.7 the focus must be placed entirely on the strain field around the crack tip. To this end, line profiles along the crack bisector of elastic strain in the crack opening direction were collected at several stages of crack propagation past the overload using in situ Synchrotron X-ray Powder Diffraction (SXRPD) technique.
By performing comparison between the two loading conditions (R=0.7 and R=0.1), information was extracted regarding the role of residual stress alone, and then, by subtracting this effect for the R=0.1 sample, for crack closure alone. To enable this analysis, we propose a introducing the concept of equivalent effective stress intensity factor range, ∆Keq,eff proposed by Walker. Afterwards, the SIF range reduction ratio, β, which represents the “knock down” factor with respect to the steady state growth was assessed. It is in terms of these newly introduced parameters that the magnitude and extent of the overload-induced crack growth rate retardation can be plotted, fitted and decomposed into closure and residual stress effects, respectively. It is concluded that although the residual stress effect is present at all values of the load ratio R, its effect is relatively short-lived, whilst the closure effect that is dominant at low values of R causes longer range retardation.
▪
•Observation of crack opening reveals that high load ratio (R=0.7) inhibits crack closure during fatigue crack propagation.•The effect of residual stress-strain field generated during overload vanishes when the crack tip advances past the overload plastic zone.•When operative, crack closure mechanism has a longer lasting retardation effect on the crack growth rate than residual stress.
Continuous SiC fiber-reinforced SiC ceramic matrix (SiCf/SiC) composite tubes with/without external CVD-SiC coating were mechanically tested with a C-ring sample geometry. The hoop tensile strength ...of the tubes with the coating (∼169 MPa) was higher than those without coating (∼108 MPa). SEM observation showed the propagation of cracks in SiCf/SiC composite materials led to the debonding of fibers from the matrix and resulted in the delamination of fiber bundles. The propagation direction of the crack initiated in the CVD-SiC coating moved toward the center of the C-ring, deflected and extended along the boundary between the coating and SiCf/SiC composite. The CVD-SiC coating and the interface between the coating and SiCf/SiC composite could effectively protect the SiCf/SiC composite, thereby improving the hoop tensile strength and maintaining the integrity of the cladding tube.
