Chemical En

Extension of the indentation fracture toughness estimation method to very small length scales often requires the use of an indenting punch much more acute than the oft-used Vickers probe. Experimental results for very acute, sharp probes have motivated a new approach to the indentation fracture mechanics of radial crack development. An extension of the standard two-component (residual elastic–plastic+elastic contact) stress-field model of radial fracture is proposed, based on the concept that a sufficiently acute indenter can also act as a ‘wedge,’ prying open the surface-located radial cracks. In this, the first of a two-part series, a three-component wedging indentation model is constructed, and some general characteristics of the model are explored. In particular, the implications of the three-component stress field of the model for the description of radial crack development during load-unload indentation cycles of acute probes are considered. Explicit predictions of crack development are compared with the qualitative features of experimental observations, providing a basis for the quantitative comparisons in Part II.

ABSTRACT This paper developed 3D elastoplastic finite element analysis (FEA) models to investigate non-axisymmetric tubes neck spinning. The three-roller mandreless neck spinning process was simulated by commercial FEA software, MSC. Marc. The distribution of the transient von Mises stresses in the contact zone between the roller and the blank, and the equivalent plastic strains after spinning were obtained. The locations and causes of fractures were analyzed. The thickness distribution of the spun workpiece was investigated numerically and experimentally. It shows that during 3D non-axisymmetric tubes spinning, diametral elongation and fractures mostly occur at the opening end of workpiece, which considered as the primary location of defects. Fractures may also occur easily at the initial spinning area, which may be considered a secondary location of defects. The distributions of equivalent stresses and strains are non-uniform during both offset and oblique 3D non-axisymmetric tubes spinning. The maximum stress and strain values occur at the location where the biggest offset or inclination is induced, while the minimum values occur at the opposite location of 180° away from the maximum values. For the oblique tube, however, the strain distribution changes gradually along the axial direction, which is different from that of the offset tube. Thinning occurs during forward path spinning and thickening occurs during backward path spinning. Therefore, for multipath neck spinning processes, forward and backward paths should be applied alternately to avoid excessive thinning or thickening of workpiece.

Sudden displacement excursions during load-controlled nanoindentation of relatively dislocation-free surfaces of metals are frequently associated with dislocation nucleation, multiplication, and propagation. Insight into the nanomechanical origins of plasticity in metallic crystals may be gained through estimation of the stresses that nucleate dislocations. An assessment of the potential errors in the experimental measurement of nucleation stresses, especially in materials that exhibit the elastic–plastic transition at small ...

Nanoporous low-dielectric-constant films constitute a class of materials that are plagued by fracture concerns and are not amenable to traditional fracture toughness measurement techniques. An indentation fracture toughness measurement technique has been developed for these materials. The experiment utilizes nanoindentation in combination with cube-corner indenters which create flaws are on the scale of the film thickness, about a micrometer. Interpretation of experimental results are a far-reaching generalization of the traditional Vickers based indentation test used for ceramics at the mesoscale. Cube-corner indentation fracture is dominated by crack-wedging effects that are not important for Vickers indentation. Film-substrate elastic coupling is very important, and is manifested in three distinct ways. After film-substrate coupling phenomena are identified, they are combined with acute indentation fracture models to form a complete thin-film indentation fracture mechanics model....