A Probabilistic Approach to Investigate the Size and Boundary Condition Effects on the Fracture Response of Brittle Materials Loaded in Diametral Compression

The focus in this work is toward an investigation of the fracture response of brittle materials with different specimen size loaded in diametral compression using different boundary conditions. The compacted zone underneath the loading points is assumed to be limited and only responsible for the load transition to the rest of the material. Therefore, the theory of elasticity is used to define the stress state within a circular specimen. A tensile failure criterion is used, and the final load capacity is related to the formation of a subsurface crack initiated in a probabilistic manner in a region in the vicinity of the loaded diameter of the specimen. This process is described by Weibull theory, and it is assumed here that the growth of the subsurface crack occurs in an unstable manner. Therefore, the assumption in Weibull theory that the final failure occurs as soon as a macroscopic fracture initiates from a microcrack is fulfilled. The concept of disk effective volume used in Weibull size effect is presented in a convenient way that facilitates the application of the model to transfer the tensile strength obtained from different methods such as three point bending and Brazilian test. The experimental results for Brazilian test on a selected hard rock are taken from the literature and a fairly close agreement is obtained with the model predictions.

Mechanical Properties of Al6061 Processed by Equal Channel Angular Pressing

The severe plastic deformation (SPD) is an effective approach for producing bulk nanostructured materials. The Equal Channel Angular Pressing (ECAP) is the most efficient SPD solution for achieving ultra-fined grained (UFG) material as billet undergoes severe and large deformation. The process parameters of ECAP (Channel Angle, angle of curvature, friction, number of passes, etc) influences major impact on the properties. In present work, the ECAP process is performed by pressing a specimen through a die consisting of two intersecting channels meeting at an angle φ and outer corner meeting at an angle ψ. Experiments with a circular specimen of Al6061 were conducted to investigate the changes in mechanical properties upto 2 passes. 3-D finite element simulations were also performed using metal forming software FORGE to study the evolution of strain in the specimen during the ECAP process. Simulation results were investigated by comparing them with experimental measured data in terms of load variations. The present work clearly shows that ECAP caused accentuated increase in Al6061 hardness and tensile strength during multi-pass processing. This study is beneficial in developing high quality, high strength products in manufacturing industry on account of its ability to change microstructure of materials.

Modeling Flexforming (Fluid Cell Forming) Process with Finite Element Method

In this paper, the flexforming process is modeled by finite element method in order to investigate the operation window of the problem. Various models are established using explicit approach for the forming operation and implicit approach for the unloading one. In all analyses the rubber diaphragm has been modeled revealing that the modeling of this diaphragm is essential. Using the material Aluminum 2024 T3 alclad sheet alloy, three basic experiments are conducted: Bending of a straight flange specimen, bending of a contoured flange specimen and bulging of a circular specimen. By these experiments the effects of blank thickness, die bend radius, flange length and orientation of the rolling direction of the part have been investigated. Experimental results are compared with finite element results to verify the computational models.

Deformation and Curvatures in Sheet-Metal in the Bulge Test

The displacements and strains of a circular specimen subjected to hydraulic pressure are studied and are compared with theoretical results predicted by previous investigators. By proposing a concept called prolateness, which gives an indication of the deviation of a local surface from a perfect sphere, it is found that the shape of the deformed bulge is not as simple as it has often been thought to be—the prolateness of a bulge is never constant at any deformation stage. Results also show that, between the beginning and the end of the forming process, there are the initial, stable and unstable regimes of deformation.

Combined Mode Fracture Toughness Measurement by the Disk Test

The disk test in which a circular specimen with an internal crack is subjected to diametral compression is used to investigate combined mode I and mode II fracture. The stress intensity factors in the disk test are calculated numerically by means of the boundary collocation procedure and the dislocation method. Special care was taken to analyze the effect of the compression anvils. This method has the advantage, of allowing successive measurement of mode I, mode II and the combined mode fracture toughness under the same conditions. Some kinds of graphite, plaster and marble are examined to obtain the fracture toughness values, KIC, KIIC and the combined mode fracture criterion.