음향방출법을 이용한 금속복합재료의 기지재 항복이 손상에 미치는 영향 연구

Abstract

This study has focused on the damage of short fiber reinforced metal matrix composites (FR-MMCs) of which the failure is mainly governed by macro-damage mechanisms such as interfacial debonding, fiber breakages, and matrix cracks. Nucleation of damage and its subsequent accumulation was examined by resultant responses of mechanical variables and nondestructive parameters. Acoustic emission (AE) technique is a useful tool for the damage analysis of FR-MMCs because it can take the macro-scale damages by setting the AE threshold level. Damage variable was introduced based on the AE response. Among various AE parameters, ring-down-count was selected as an intermediary variable to be used for tensile and fatigue damage assessment. A series of laboratory experiments under tension and fatigue loading conditions were conducted. Experimental results of FR-MMCs were compared with those of Al-T6 (matrix-only case) to examine the effect of fibers and interface on damage characteristics. Deformation process and damage accumulation during tensile straining were examined by micrographic observations and AE parametric analyses. SEM fractographs of FR-MMCs showed brittle features such as interfacial debonding, and fiber breakages. Nucleation of the damage and its subsequent accumulation were analyzed by periodic load-drop (PLD) test with AE system. The conventional damage measure by elastic modulus degradation was well used for evaluating the ductile damage of matrix alloys (Al-T6), while failed to evaluate fiber-involved damages of FR-MMCs. Based on the Felicity effect and the AE count, a new damage variable was defined as a ratio of counts in load-drop period to total counts in normal loading period. The emitted signals during the load-drop period may attribute to the relative motions at the debonded interface

the motions are caused by the difference of elastic modulus between matrix and reinforcements. It has been proved that the new variable can estimate the damage accumulation of the fiber-involved damages of FR-MMCs successfully.In the second part, single-stress level fatigue life has been investigated from a viewpoint of nominal stress-life diagram. Three life prediction models (SD-LPM, CA-LPM, CD-LPM) were developed by monitoring resultant maximum strain and AE count. It has been proved that all new models satisfies the basic assumption of stress-life diagram, as a result, can predict the fatigue life better than conventional S-N curve and Basquin's equation. In the last part, we have defined a fatigue damage variable by AE count ratio and applied its accumulation curves to the fatigue damage assessment and the multi-stress level fatigue life prediction. FR-MMCs, prepared in this study, were found to be susceptible to interfacial debonding by matrix (Al-T6) yielding. Because of this nature, damage curves became different according to the applied stress level whether it was higher than the yield strength of matrix alloy or not. In consequence, the remaining life predictions by using conventional damage accumulation rules were far from the experimental data. A new damage accumulation rule, named “damage decomposition rule (DDR),” was proposed with reflecting the nature of damage curves. As compared with the experimental data, better predictions of two-stress level fatigue lives were found when using the DDR rather using other conventional rules. The characteristics of different cases of the DDR are described.