Journal of the Korea Academia-Industrial cooperation Society (한국산학기술학회논문지)

The Korea Academia-Industrial cooperation Society (한국산학기술학회)
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Correction of the hardness measurement for pile-up materials with a nano indentation machine

파일-업 재료에 대한 나노 압입 시험기의 경도 측정값 교정

  • Park, Moon Shik (Department of Mechanical Engineering, Hannam University)
  • 박문식 (한남대학교 기계공학과)
  • Received : 2016.08.16
  • Accepted : 2016.12.08
  • Published : 2016.12.31
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Abstract

Measurements of the elastic modulus and hardness using a nano indentation machine rely on the equation for the fitted contact area, which is valid for only sink-in materials. For most soft engineering materials that involve pile-up behavior rather than sink-in, the contact area equation underestimates the contact area and thus overestimates the elastic modulus and hardness. This study proposes a correction method to amend erroneous hardness measurements in pile-up situations. The method is a supplemental derivation to the original hardness measurement with the known value of the elastic modulus. The method was examined for soft engineering metals, Al 6061 T6 and C 12200, via tensile tests, nano indentation tests, impression observations, and finite element analysis. The proposed technique shows reasonable agreement with the analytical results accounting for strain gradient plasticity from a previous study.

본 연구는 공업용 응용이 많은 알루미늄 또는 구리와 같은 재료를 나노 압입 시험기에 의하여 탄생계수 및 경도 값을 얻을 때 파일-업(pile-up) 현상이 생기는 경우 계측 값을 교정할 수 있는 방법에 대해 다룬다. 나노 압입 시험기에 의해 얻어지는 탄성계수와 경도의 측정치는 접촉면적의 피팅 (fitting) 식에 의존하게 되는데 이는 오로지 싱크-인(sink-in) 재료에만 유효하다. 그러므로 싱크-인이 아닌 파일-업인 많은 무른 공학재료들에 있어서는 그 접촉면적이 실제보다 적게 계산되고 따라서 탄성계수와 경도는 높게 계산된다. 본 연구에서는 이미 탄생계수를 알고 있는 파일-업 거동을 보이는 재료의 경우에 경도 값을 교정하는 방법을 제안한다. 이 방법을 경금속인 Al 6061 T6와 C 12200에 적용하기 위해 인장시험, 나노 압입시험, 압입자국 측정, 그리고 유한요소해석을 수행하였다. 압입 자국 측정과 유한요소해석을 흥하여 두 재료 모두 파일-업 거동이 발생하는 것을 알 수 있었다. 제안한 교정 방법은 싱크-인 접촉면적 값을 파일-업 접촉면적 값으로 늘려 주었고 경도 측정값을 낮추어 주었다. 교정된 경도 값은 별도의 연구에서 다룬 변형률 구배 소성을 고려한 유한요소해석 결과와 잘 일치하였다.

Keywords

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