Key Engineering Materials Vols. 353-358

Abstract: The interface strength of low-dimensional nano-components such as films and islands formed on substrates has been investigated in this project, and the focus is put on the mechanics of crack initiation from the free interface edge and propagation along the interface. The series of experiments elucidates the applicability of fracture mechanics concept on the structures. We proposed experimental methods for evaluating the initiation strength of an interface crack in submicron films and islands deposited on substrates. The initiation is governed by the singular stress field, and the criterion is prescribed by the stress intensity parameter. Using special loading apparatus built in a TEM, we developed a crack initiation method for nano-components and the role of plasticity on the delamination is clarified. Subcritical crack growth along an interface between submicron films under fatigue was also investigated by modified four-point bend method.

Abstract: Microstructural observations are presented for different metals deformed from low to high strain by both traditional and new metal working processes. It is shown that deformation induced dislocation structures can be interpreted and analyzed within a common framework of grain subdivision on a finer and finer scale down to the nanometer dimension, which can be reached at ultrahigh strains. It is demonstrated that classical materials science and engineering principles apply from the largest to the smallest structural scale but also that new and unexpected structures and properties characterize metals with structures on the scale from about 10 nm to 1 μm.

Abstract: There is an inherent relationship between the shape and the corresponding stress intensity factor (SIF) distribution of a crack. A typical inverse problem of linear elastic fracture mechanics about a crack, i.e. to predict the shape of a crack assuming that some information of SIF distribution is known, is presented. A finite-element based numerical procedure is used to determine the shape, correspondingly the SIF, of a mode-I planar crack based on a specified SIF distribution. The crack front is modeled using cubic splines, which are determined by a number of control-points. The crack front shape is achieved iteratively by moving control-points based on a gradientless algorithm. Numerical examples for planar cracks in through-cracked and surface-cracked plates with finite thickness and width are presented to show the validity and practicability of the proposed method. The SIFs obtained by present method are compared with the known solutions for cracks with same dimensions. The presented method is considered to be a promising alternative to the evaluation of SIFs and the prediction of shape evolution for fatigue cracks.

Abstract: The crack growth behavior and the fatigue life of welded members with initial crack in bridges under traffic loading were investigated. Based on existed fatigue experiment results of welded members with initial crack and the fatigue experiment result of welded bridge member under constant stress cycle, the crack keeps semi-elliptical shape with variable ratio of a/c during crack propagation. The calculated method of the stress intensity factor necessary for welded bridge member crack propagation was discussed. The crack remained semi-elliptical shape with variable ratio of a/c during crack propagation. The fatigue crack propagation law suitable for welded steel bridge member fatigue crack propagation analysis was deduced based on the continuum damage mechanics and fracture mechanics. The proposed fatigue crack growth model was then applied to calculate the crack growth and the fatigue life of existed welded member with fatigue experimental result. The calculated and measured fatigue life was generally in good agreement, at suitable initial conditions of cracking, for welded member widely used in steel bridges.

Abstract: Combining in-situ tensile tests with detailed observations of fracture surfaces of a two-phase TiAl alloy, the fracture process and fracture mechanisms of TiAl alloys are investigated. The results reveal that Cracks prefer to initiate and propagate along lamellar interfaces, which are the weakest link in the near fully lamellar microstructure. The interlamellar strength calculated is less than the translamellar strength. The tensile stress is the driving force for crack initiation and propagation. In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until final fracture. The process of a new crack nucleation with a bridging ligament formation decreases the crack propagation resistance rather than increases it.

Abstract: In this paper, the behavior of a finite crack in an infinite plate of functionally graded materials (FGM) with free boundary subjected to SH-waves is considered. To make the analysis tractable, it is assumed that the material properties vary exponentially with the thickness direction and the problem is transformed into a dual integrated equation with the method of integral transform. The dynamic stress intensity factor is obtained using Schmidt method. The numerical examples are presented to demonstrate this numerical technique for SH-waves propagating in FGM plate. Finally the number of the waves, the gradient parameter of FGM and the angle of the incidence upon the dynamic stress intensity factor are also given.

Abstract: A moving crack in a laminated structure with free boundary subjected to anti-plane shear loading is investigated in this paper. Using the bonding conditions of the interface between different media, all the quantities in our question have been represented with a single unknown function, and the problem is transformed into a dual integrated equation with the method of Fourier transform. The equation is solved using Schmidt method. Finally the numerical results show the relationships among the dynamic stress intensity factor and crack velocity, the height of different laminated material, shear moduli of different laminated material.

Abstract: Surface and depth morphology evodslutions of short crack propagation of 1Cr18Ni9Ti weld metal are investigated. In accordance with the previous effective short fatigue crack (ESFC) criterion, attention is paid on the formation zone of the dominant ESFC (DESFC), which resulted finally in the specimen failure, in micro-structural short crack (MSC) regime and then, the tip zone(s) ahead of the DESFC in physical short crack (PSC) regime. Results show that in MSC regime the surface ESFCs were imitated from the distributed randomly delta ferrite bounds separated from austenite matrix. The initiated ESFCs on surface propagated perpendicularly to loading axle. But in depth direction, the initiated ESFCs grew first similarly to the surface behaviour but lately, tended to be perpendicularly to the formation direction of the material columnar grain structure. When sizes of some longer ESFCs reached around the material maximum barrier size, coalescence occurred to form a true DESFC. In PSC regime the surface DESFC grew almost perpendicularly to loading axle. But in depth direction, it grew first perpendicularly to the formation direction of the columnar grain structure and then, tended to having 45 degree angle to the loading axle. Obviously, the evolutions were strongly affected by interactions between the load and the microstructures, especially, the columnar grain structure.

Abstract: In order to understand the grain size and porosity dependent mechanical behavior of porous, multi-phase nanocrystalline ceramics, each phase is treated as a mixture of grain interior and grain boundary, and pores are taken as a single phase. In conjunction with the secant-modulus approach and iso-strain assumption, Budiansky’s self-consistent method is extended to build a constitutive model for nanocrystalline ceramics with small plastic deformation. Based on the developed model, the predicted yield strength (σ0.2) values of porous, multi-phase nanocrystalline ceramics with different grain size and porosity are compared with experimental data in the literature, the comparison shows that the predictions are in good agreement with the published data. This suggests that the developed model is capable of describing the grain size and porosity dependent mechanical behaviors of nanocrystalline ceramics with small plastic deformation.