Characterization of Ductile Crack Propagation by Fractal Energy Dissipation Rate

Because of its geometry dependence and loss of physical meaning, the incremental crack resistance curve cannot characterize ductile fractures with large crack extensions and plastic deformations. Therefore, the energy dissipation rate R is employed to overcome these deficiencies, even though specimen size effects still exist. In the study, considering the fractal crack path and concomitant plastic dissipation in the fractal domain, a scale‑invariant energy dissipation rate, γp*, is proposed in the context of renormalization group theory. Some experiments in the literature have validated this approach. The fitted fractal energy dissipation rate is independent of the specimen size and initial crack length; moreover, as the specimen size increases, progressive fractality vanishing is found consistently with geometrical multifractality

Hardening Cohesive/Overlapping Zone Model and Fractal Approach to Ductile Fracture

Most of fracture parameters (K, G, J-integral, COD) break down, especially for ductile materials and large crack extensions. In addition, even at small scale yielding, K, G, J- integral and COD are limitedly used to distinguish the onset of unstable fracture, without any concern regarding the pre-onset and post-reaction. On the other hands, the specimen size and geometry dependences of K/G/J-integral/COD are beyond the capability of the classical Fracture Mechanics where most of methodologies rely on only a single-parameter to characterize the structural integrity. In essence, K/G/J-integral/COD are based on the same underlying idea of energy released per crack extension. Thus, when crack extension is accompanied by the other irreversible energy dissipation, neither J-integral nor COD/CTOA is able to describe the failure processes (crack onset, growth and catastrophic fracture). Moreover, K/G/J-integral/COD are by definition global parameters in nature. Their average field characters are unable to describe some mechanical behaviour inherent to material heterogeneity and disorder. As a result, it is no wonder that the applications of K/G/J-integral/COD are severely limited, and even invalid as observed in substantial experiments. The cohesive zone model and fractal approach are the cornerstone in this thesis. It is essential that a proper fracture theory should basically take into both the stress intensification and material disorder at different scales into account. A Hardening Cohesive/Overlapping Zone Model is proposed for the analysis of complex mechanical phenomena in fracture of metallic materials. It is assumed that the unique correspondence between uniaxial tension/compression and ductile fracture can be established. As a result, the advantages of the cohesive crack model are kept in order to simulate the stress and strain concentrations. Most metals and alloys are not homogeneous. Thus, macroscopic level behaviours are essentially originated from a microscopic one, as well as taking into account local fluctuations due to disorder. Especially, fracture is quite sensitive to disorder, and sometimes mean field theory without consideration of disorder is not able to provide accurate predications. The fractal approaches to scale effects are adopted to introduce the inherent effects from material disorder at different scales. The scale-independent cohesive and overlapping laws with respect to the classical stress–strain relations can be presented. Consequently, fractal approach can avoid the limitations of a classical mean-field approach, where disorder is simply averaged in an elementary representative volume. In addition, energy brittleness number sE is proven to be a characteristic parameter to evaluate the fracture instability not only in quasi-brittle materials but also in metallic materials under certain conditions

Experimental evidence and numerical simulation of size effects on the ductile fracture of metallic materials

The results of experimental tests investigating the size effects on single-edge-notched metallic specimens loaded in three-point bending are presented. Five different specimen scales were tested, with dimensions varying within the range 1:16. The samples were subjected to a fatigue pre-cracking to pro- duce a sharp crack stemming from the notch root and, then, a quasi-static loading process was carried out up to the complete failure, in order to capture also the post-peak response. Notable size effects on the overall behaviour were obtained, with a variation of the fail- ure mode from plastic collapse to ductile fracture and brittle failure by increasing the specimen size. An inter- pretation of the obtained size effects on ductile fracture is proposed based on numerical simulations carried out with a finite-element model that combines the cohesive method and the J 2 plasticity to take into account all the possible mechanisms for energy dissipation. The best- fitting of the experimental results is obtained by scaling the mechanical properties with the specimen size, thus proving the need of considering size-dependent con- stitutive laws to correctly predict the ductile fracture

Fatigue Characteristics of 7050-T7451 Aluminum Alloy Friction Stir Welding Joints and the Stress Ratio Effect

The fatigue crack initiation and growth characteristics in 7050-T7451 aluminum alloy butt joints subjected to different stress ratios and owing to friction stir welding (FSW) were investigated using fatigue tests for stress ratios of 0.1, 0.3, and 0.5. The difference between the fatigue crack initiation in the base material (BM) and FSW joints, related to coarse secondary phases, was explored using scanning electron microscopy (SEM). Accordingly, Al23CuFe4, Al7Cu2Fe, and Al2Mg3Zn3 were the preferred joint crack initiation locations, whereas Mg2Si was the major fracture initiation point of the parent material, and cracks tended to propagate along dense, coarse secondary phases, becoming more pronounced for larger cracks. In addition, as the stress ratio increased, non-Mg2Si phase fracture initiation points appeared in the BM. Meanwhile, the quantity of non-Mg2Si phases in the joints continued to increase, and the crack initiation sites became increasingly concentrated in the TMAZ-HAZ region

Experimental investigation of out-of-plane constraint effect on fracture toughness of the SE(T) specimens

In the study, the experiments are performed to determine thickness effect on critical crack tip opening displacement (δ_IC) of the single-edge tension (SE(T)) specimen with side-groove. The applicability and thickness sensitivity of several fracture toughness estimation procedures are also investigated by the experimental data. Referring to the results by the double clip gauge method, it is found that the critical crack initiation toughness decreases significantly as specimen thickness increases until the thickness-to-width ratio equal to 4, beyond which thickness effect becomes relatively weak. Accordingly, a dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint into account for SE(T) specimen. The further analyses based on the plastic zone size confirm the result as well

THE REMOTE-SENSING IMAGE FUSION BASED ON GPU

Along with computing capability of the Graphic Processing Unit (GPU) getting more powerful, GPU is widely applied to the general purpose computing not just restrict to graph manipulation. Remote sensing image data can be parallel processed in many of the image fusion arithmetic. The fusion arithmetic in spatial domain is mapped to the SIMD computing way on GPU. This paper realized the product fusion, ratio fusion, high-pass filtering fusion and weighted fusion using GLSL in spatial domain, and accelerated the speed of fusion computing using RTT (Render to Texture) technology. This paper focused on the arithmetic in transformed domain, realized IHS transform fusion and DWT (Discrete Wavelet Transform) fusion. The IHS forward transform and inverse transform are mapped to two fragment shading processes, parallel computing and outputting of the 3 component in both transform processes is realized using MRT (Multiple Render Targets) technology. 2D DWT is divided into two steps of 1D DWT. An indirect address texture is created for every transform step and the transform of each level is based on the result stored on the texture of the last level. A FBO is set for every image to be fused to restore intermediate data and to do data exchange. The result shows that for the same fusion algorithm, the fusion images are identical using the two different methods, but the processing velocity in GPU implementation is obviously faster than the CPU implementation, and with the fusion algorithm getting more complicated, the fusion images getting bigger, the advantage of the velocity is more obvious in GPU implementation. 1