Key Engineering Materials Vols. 324-325

Abstract: Stress intensity factor K is analyzed by measuring distribution of sum of the principal stresses around slit tip by an infrared stress measuring device. Under the biaxial tensile stress, both K-values of the isotropic materials with slit angle 0° and 45° show there is no influence of the slit angle at Mode I. The tolerance of the measured K-values of the test specimens which are compared with measured K-values by photoelastic method are around 10-20
and these increase as applied cyclic load increases. These tolerance can be reduced up to about 8% by modifying measurements. The effectiveness of the thermoelastic stress measurement method to KI value analyzing under biaxial tensile stress was confirmed.

Abstract: In this paper the rather complex 3D fatigue crack growth behaviour in a SEN-specimen under anti-plane shear loading is investigated by the aid of the programme ADAPCRACK3D and by application of a recently developed 3D fracture criterion. It will be shown that the computationally simulated results of fatigue crack growth in the FE-model of the specimen are in good agreement with experimental findings for the development of two anti-symmetric cracks, which originate from the two crack front corner points, that is where the crack front intersects the two free side surfaces of the laboratory SEN test-specimens. Consequently, also for this case with a rather complex 3D crack growth of two anti-symmetric cracks, the functionality of the ADAPCRACK3D-programme and the validity of the proposed 3D fracture criterion can be stated.

Abstract: Very-High-Cycle Fatigue (VHCF) is the phenomenon of fatigue damage and failure of metallic materials or structures subjected to 108 cycles of fatigue loading and beyond. This paper attempts to investigate the VHCF behavior and mechanism of a high strength low alloy steel (main composition: C-1% and Cr-1.5%; quenched at 1108K and tempered at 453K). The fractography of fatigue failure was observed by optical microscopy and scanning electron microscopy. The observations reveal that, for the number of cycles to fatigue failure between 106 and 4×108 cycles, fatigue cracks almost initiated in the interior of specimen and originated at non-metallic inclusions. An “optical dark area” (ODA) around initiation site is observed when fatigue initiation from interior. ODA size increases with the decrease of fatigue stress, and becomes more roundness. Fracture mechanics analysis gives the stress intensity factor of ODA, which is nearly equivalent to the corresponding fatigue threshold of the test material. The results indicate that the fatigue life of specimens with crack origin at the interior of specimen is longer than that with crack origin at specimen surface. The experimental results and the fatigue mechanism were further analyzed in terms of fracture mechanics and fracture physics, suggesting that the primary propagation of fatigue crack within the fish-eye local region is the main characteristics of VHCF.

Abstract: Two phase metals during cyclic loading can suffer from non-uniform load or strain sharing between the phases due to elastic/plastic anisotropy. This can strongly influence the fatigue damage and crack initiation behavior. In this study, the fatigue damage and crack initiation behavior of an austenitic-ferritic duplex stainless steel with anneal/quenched and aged conditions has been studied by both experimental investigations and simulation using multi-scale material modeling. It was found, both experimentally and via simulations, that the material damage and crack initiation start in the ferrite phase in the material with the anneal/quenched condition and in either the ferrite or austenite phase in the material with the aged condition, mainly in the weakest phase if the deformation hardening is considered.

Abstract: A special specimen was created cutting a rectangular notched area from the surrounding of the upper left corner of a wide body aircraft door. This part of the aircraft skin is made of different layers with variable thickness and material (titanium or aluminum). Then a fatigue traction load was applied and some notches were cut in the different layers in order to speed up the crack initiation and reproduce a realistic crack scenario. Such through cracks were monitored during their propagation along the specimen width, in order to have available for the simulation a realistic initial scenario and experimental propagation data useful for the correlation with the simulated crack path and growth rates. In particular an innovative DBEM modelling approach was devised, using a commercial code (BEASY), capable of explicitly modelling the different test article layers with their rivet connections even in a two-dimensional approach. The results of the simulation show a satisfactory correlation with the experimental crack path and growth rates even for such a complex problem: three different panels (one skin with two doublers), made of different materials, each one with a variable thickness and connected through numerous rivets (whose shear stiffness is taken into account for the simulation).

Abstract: In this paper, the problem of torsional buckling of a thin-walled edge cracked column, under pure bending, is studied. To the authors’ knowledge, this subject has not been widely investigated in the literature. A cracked member can buckle in the plane orthogonal to the crack and out of plane triggering the Mode-III crack opening. Analytical expressions for the characteristic equation are derived for an I-beam. The influence of the crack length and position on the buckling load is investigated.

Abstract: In this study, it was made clear that the non-linear notch mechanics is useful not only in the case of uniaxial tension but also in the case of biaxial tension. The difference of both cases is as follows. In the former the plastic strain field near a notch root is determined by εp y0,FEM (plastic strain at a notch root) and ρ (notch root radius) alone, but in the latter case it is determined by εp y0,FEM, ρ and stress ratio k=σxn/σyn.

Abstract: The present paper describes a numerical method which is able to calculate the stress intensity factor in two dimensional heterogeneous materials under mechanical and thermal loads. The proposed method uses two hybrid element formulations to model the second phase heterogeneities of the material and the crack tips. The method was used to analyse alumina-zirconia ceramic materials, and the effects due to the zirconia t→m phase transformation and the thermal stresses, which develop during the cooling stage of sintering, were taken into account in calculating the stress intensity factor.

Abstract: The purpose of this study is to investigate the wear characteristics of yttria-stabilized zirconia coating as a function of effect on post-spray heat treatment. We concerned that the residual stress is due to during cooling from a high deposition temperature. We will focus on the tribological wear performance as a magnitude of residual stress. The effect of residual stress as post-spay heat treatment temperature will be discussed. The plasma-sprayed 8%Y2O3-Zirconia coating is studied to know the relationship between phase transformations and wear properties after post-spray heat treatment. Wear tests is carried out with ball on disk type on 50 N, 70 N, 90 N under room temperature. The transformation of phase and magnitude of residual stress are measured by x-ray diffraction method (XRD). Tribological characteristics and wear mechanism for post-spray heat treatment is observed by SEM.

Abstract: The problem of an edge-bridged crack terminating perpendicular to a bimaterial interface in a half- space is analyzed for a general case of elastic anisotropy bimaterials and specialized for the case of orthotropic bimaterials. The edge crack lies in the surface layer of thickness h bonded to semi-infinite substrate. It is assumed that long fibres bridge the crack. Bridging model follows from the assumption of “large” slip lengths adjacent to the crack faces and neglect of initial stresses. The crack is modelled by means of continuous distribution of dislocations, which is assumed to be singular at the crack tip. With respect to the bridged crack problems in finite dissimilar bodies, the reciprocal theorem (ψ - integral) is discussed as to compute, in the present context, the generalized stress intensity factor through the remote stress and displacement field for a particular specimen geometry and boundary conditions using FEM.