Issue 7

S.K. Kudari et alii, Frattura ed Integrità Strutturale, 7 (2009) 57-64; DOI: 10.3221/IGF-ESIS.07.04 59 Figure 1 : Configurations of SENT and CT specimens used for plastic zone study. Notches were inserted in the fabricated specimen blanks by wire electro discharge machining (EDM). The notch length in each specimen was measured using the micrometer attached to the specimen stage of the Vickers microhardness testing machine. The configuration of the notch-tip was recorded at X200 with the help of a CCD camera attached to an optical microscope. The notch-tip root radii were then measured from these images with the help of an image analyzer. The notch radius measured was 180 µ m. One side of the specimen was finely polished around the crack-tip in order to facilitate measurements of microhardness and to observe the plastic spread near the crack-tip. This was done by successively grinding the specimen on finer silicon abrasive papers, followed by final polishing on a velvet cloth smeared with 0.25 µ m diamond paste. Plastic zone in each of the specimens was introduced by applying pre-selected monotonic tensile loads at a crosshead speed of 0.05 mm/min at room temperature (30 o C). A number of such tests were carried out on SENT and CT specimens at different tensile loads to introduce varied sizes of plastic zones. The applied tensile loads corresponded to stress ratio ( σ / σ y ) of 0.4-0.85 for SENT specimens and 0.3-0.75 for CT specimens, where σ and σ y are applied stress and yield stress of a material respectively. The applied stress σ for SENT and CT specimen have been computed by expressions cited in the earlier investigation of [11]. To evaluate the plastic zone size ahead of crack-tip (at θ = 0 o plane), a series of micro-hardness indentations were made ahead of each crack-tip with the help of a Vickers pyramid indenter using a load of 10 gm f for 15 sec duration. The distance between two successive microhardness indentations (during plastic zone estimation) was kept approximately 50 µ m so that there is no interaction between the deformation zones of the successive indentations. As the material is strain hardening type, because of large strain ahead of the crack-tip the hardness at the tip of the crack is expected to be higher and it gradually decreases as magnitude of strain reduces along the ligament ahead of crack-tip. The micro-hardness tests were terminated at a distance from the crack-tip where the hardness readings were found to have reached a saturation plateau equivalent to the average micro- hardness value of the material. F INITE ELEMENT ANALYSIS series of stress analyses by finite element method (FEM) have been conducted on single edge notched tension (SENT) and compact tension (CT) specimens (Fig.1) at various applied load steps. All the finite element computations were performed using general-purpose finite element code ANSYS [12]. Due to the symmetry of the specimens under Mode-I loading, only one half of the SENT and CT specimens were considered in the analyses. As specimen thickness is 2.7 mm, it considered to carry out plane stress FE analysis. Two-dimensional FE mesh was generated using eight-noded isoparametric quadrilateral elements, considering plane stress condition with specimen thickness input. The number of elements used for SENT and CT specimens were 1451 and 864 respectively. The load was applied in the form of pressure (uniform applied stress) on the surface parallel to the ligament for SENT specimen. But concentrated point loads were applied for CT specimen model to simulate pin-loading condition in this specimen. A