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나노인덴테이션을 이용한 입자강화 금속복합재료의 잔류응력측정

나노인덴테이션을 이용한 입자강화 금속복합재료의 잔류응력측정
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Metal matrix composites (MMCs) are materials where reinforcements with superior properties are added in lightweight metallic matrix. A lot of researches have been progressed for their application as high-performance structural material. However, some studies reported that residual stress is developed due to misfit of coefficient of thermal expansion (CTE) between matrix and reinforcement, and this stress greatly affect mechanical properties of MMC. Therefore, it is important to consider residual stress in MMC before its structural application. Among the many residual stress measurement techniques, nanoindentation has great power because this technique is possible to test in micro/nano scale and overcome the disadvantages which other conventional methods have. For measuring residual stress by indentation, ‘stress-free reference’ is inevitably required. But general methodology for determining stress-free reference of particle reinforced metal matrix composites is not suggested yet. In this study, residual stress on particle reinforced metal matrix composites (PR-MMC) surface was measured by nanoindentation after quenching. Required stress-free references for matrix and particle are suggested respectively. After the test, discussions were done for validity of suggested stress-free references and explanation in possible causes of test result. ‘Stress relaxed micro pillar’ by laser machining was suggested as stress-free reference of MMC matrix. For 2024 aluminum alloy, the matrix material in this study, its thermo-mechanical properties are vulnerable to be changed, direct stress relaxation method was taken. On the other hand, three requirements for stress relaxation at pillar were presented. Resultant pillar satisfied the three requirements. Also, test results showed evidence of stress relaxation without mechanical properties change. Therefore, micro pillar suggested in the research is valid. ‘Particle + acryl resin composites’ was suggested as the reference for particle. Acryl resin fixed the particles so mechanical polishing and nanoindentation on surface became possible. Residual stress occurred in the particle + resin composite is expected to be very small or negligible, because the material had fabricated through ordinary specimen mounting process. However, large variation in the result from reference and hardness difference between two regions is considered to effect of particle. Also, complex crack behavior on SiC particle contributes the error on result. Further investigation is needed. Compressive residual stress state can be measured with regular cracks. Compressive residual stress -114±53MPa at MMC matrix and compressive residual stress -427±266MPa at particle were measured. This result was explained via characteristics of residual stress development by quenching and effect of volume fraction of reinforcement. Rapid cooling (quenching) develops two types of residual stress: 1) residual stress from thermal gradient (compressive residual stress at surface and tensile at core) and 2) residual stress due to coefficient of thermal expansion (CTE) misfit between matrix and reinforcement (compressive at reinforcement and tensile at matrix). Actually measured result is combination of these two stresses. Volume fraction of MMC in this research was 20%, so second type of stress has small contribution. In result, compressive residual stresses at both matrix and particle were understood.
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