Experimental studies of indentor misalignment for indentation test on soft tissues

Effects of indentor misalignment of skin indentation test were investigated numerically using finite element analysis and experimentally on soft tissues in vitro using an ultrasound indentation apparatus. Earlier numerical results showed the stress distribution was significantly influenced by the misalignment, but the resultant force transient of the indentation response only was slightly affected by the indentor misalignment. Experimental results demonstrated the indentation responses could be influenced by the misalignment, especially when tissue thickness was comparable to the indentor diameter. This effect decreased as the thickness increased, and became almost negligible while the thickness was larger than double of the indentor diameter. The effects due to misalignment could also be significantly reduced by adopting some amount of pre-load. The amplitude of the ultrasound signal reflected from the bottom interface of the tissue obviously decreased as the misalignment degree increased, while the time shift of the signal increased slightly.