~*Happy New Year 2023*~
Thank you for using my blog!
~~*~~
Chapter 2
2.4 Microcrack formation in the shear zone
Traditional studies did not consider fracture (or crack) formation in analytical models as the cracks are usually not evident at the tip of the tool or at the root of chip during cutting experiments of ductile materials [29]. However, literature review reveals that the following three types of cracks can occur during micro-cutting as shown in Fig. 2.12. It is understood that:
· the cracks can occur at the end of shear plane,
· the microcracks can form in the middle of shear plane, and
· a gross fracture (or crack) can occur ahead of tool-tip, where material separation occurs.
Researchers have investigated the occurrence of microcracks or their accumulation leading to gross fracture along the shear zone during micro-cutting. Komanduri and Brown [32] have experimentally shown the presence of microcracks in micro-cutting of low carbon steel. Fig. 2.13a shows microcracks in the vicinity of tool-tip, whereas Fig. 2.13b shows microcracks in the middle of the shear zone. The microcracks (or smaller voids) tend to disappear after unloading of the chip and hence, the size and the number of microcracks observed in the shear plane were not very accurate. Zhang and Bagchi [67] stated that a micro-fracture occurs in the vicinity of the tool-tip during chip separation, which is based on the fact that ductile metals fail in three steps: nucleation, growth, and coalescence of micro-voids. Iwata and Ueda [68] observed the mechanism of microcracks (flaws) growth leading to form often a gross fracture in machining of ceramics. They have studied dynamic behavior of cracks in shear zone within the framework of fracture mechanics during micro-cutting of ceramics. Luong [45] reported the presence of microcracks during their experiments on specimens subjected to a loading similar to shear plane during metal cutting. But the quantification of density of microcracks was difficult on chip as reported by Komanduri and Brown [32].
In a pioneering work, Shaw [46] suggested that under the influence of unusually high shear stress and shear strain conditions in metal cutting, a localized fracture in the form of microcrack formation occurs along the shear plane (see Fig. 2.14). Discontinuous microcracks usually get initiated on the shear plane in the form of localized fracture (i.e. microcrack) during chip formation. In general, microcracks have been used to explain several aspects of material behavior during micro-cutting such as, chip segmentation process [68], microcrack coalescence process in the vicinity of the tool-tip [69], and the negative work hardening phenomenon observed due to microcracks [70]. Shaw [4] also suggested that the microcrack formation could contribute substantially to the size effect in micro-cutting.
Shaw [46] performed experiments using combined axial, compression and torsion loads to mimic the multi-axial state of stress that exists on the shear plane. The material used in their study was low carbon steel. The results of these experiments show that the failure strain increases as the compressive stress on the shear plane increases as shown in Fig. 2.15. A failure strain of the material is low when normal stress (s) on the shear plane is low. Fig. 2.15 shows that a downward trend observed in the shear stress beyond a certain value of strain is due to a gradual increase in the unsound internal area corresponding to microcracks formation resembling to internal necking during tensile tests. Shaw [46] proposed that the localized microcracks or voids will form along the shear plane, at a critical value of strain depending on the stress state on the shear plane. If the compressive stress on the shear plane is too high, the formation of microcracks will either be postponed to unusually high values of strain or likely to lead to re-welding of microcracks. Also, if the material being cut is brittle, microcracks grow into gross cracks thereby giving rise to a discontinuous chip.
As per the Shaw’s hypothesis, the microcracks may or may not lead to formation of gross fracture or chip separation depending upon the stress state in the shear zone. Nevertheless, the microcrack formation contributes significantly to the specific cutting energy and hence the size effect. However, the theory also suggests that the microcracks would get suppressed during metal cutting process and their number would vary in the process based on the cutting conditions. Moreover, literature [32, 45] reports that it is not possible to measure exact size and number of micricrocracks on shear plane during experiments on metal cutting due to their small size and unloading effect of quick-stop mechanism. In general, it is observed that there is no specific analytical or numerical formulation to estimate the number, the location and the contribution of the microcracks to the size effect during chip formation.
McClintock [71] stated that the growth of microcracks depends on stress triaxiality state parameter (h) given by Eq. 2.4:
A similar work was conducted by Rice and Tracey [72], who investigated the stress triaxiality effects on the growth of microcracks and observed that the growth rate is significantly affected by the hydrostatic and equivalent stress. In micro-cutting, similar multi-axial stress state exists as shown in Fig. 2.16.
2.5 Gross fracture phenomenon ahead of tool-tip
2.6 Workpiece microstructure effect
2.7 Conclusions from the literature review
2.8 Objective and scope of the research
