Stiffness of the testing machine

The significance of the pop-ins in the nickel steel COD and i -curves is the subject of some debate. Pop-ins represent rapid crack extensions followed by arrest. These arrests may be related to material toughness characteristics, but may also be the result of a drop-off in applied load due to the stiffness of the testing machine. The fracture toughness results developed in this work are intended to supplement the designer s basic understanding of the materials and aid in the selection of a particular material for a specific application. These tests show that the performance of a structure fabricated with the nickel steels will be dependent on the HAZ behavior. This information must be blended with additional design, fabrication, and economic factors before final material selection. 

In this last case the stability depends on the stiffness of the testing machine, since elastic energy stored in the spring can be used for crack propagation. 

The local friction image (Figure 4.a), shows the initial situation turns to high friction with some stick-slip phenomena, inducing contact vibration, due to the low longitudinal stiffness of the test machine. The low fnction effect of the tribofilm is found to last about 25 cycles. 

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