This is a result of the high stress intensity caused by surface morphology of Ni tip.
The close-up in Fig. 3d shows the role of high stress intensity during indentation.
This leads to higher stress intensity factors, and concomitant decrease in strength, with decreasing thickness.
The rectangular web openings of 0.75d have a very high stress intensity compared to the other depth of openings such as 0.62d and 0.50d.
Mesh size and configuration is and important part of FE modeling, precise mesh refinement being necessary in regions of high stress intensity.
Without the influence of high stress intensity, the tendency of twinning formation during tip retraction is much stronger than that in the process of indentation.
This is not surprising if we take into account of the high stress intensity near steps of Ni tip during indentation.
However, deformation twinning will be retarded during indentation due to the high stress intensity caused by stepped surface of Ni tip.
However, the crack initiates much later in coupons stiffened using adhesive bonding method than its co-cured counterpart and thus shows higher stress intensity factor at initiation.
They are also limited in the amount of crack growth data that can be developed at high stress intensity values due to accumulation of plastic and/or creep strains leading to ratcheting in the specimen.
