Figure 11 shows a schematic for the compliance calibration testing of the RDCB test piece, subjected to pure Mode I loading at the crack tip.
Figure 11: Compliance calibration test plot for the mode I RDCB test piece
Testing at low displacements, so as to avoid crack growth during calibration testing, is performed on five separate test pieces, each with different crack length. Test piece compliance is measured using suitable data reduction software. From the resultant plot of crack length versus cube root of test piece compliance, the following relationship is derived from the linear regression fit:
(8)
The same effective procedure is performed on samples tested under Mode II and Mixed-Mode I + II loading, with gripping and loading applied as shown in Figure 8.
After measuring crack length and test piece compliance for each of the five test pieces, a plot of a3 versus C is plotted, shown in Figure 12.
Figure 12: Compliance calibration test plot for Mode II + Mixed-Mode I + II test pieces
From a linear regression fit applied to these data, the following relationship is derived:
(9)
Test pieces are deflected at constant displacement at a fixed frequency to induce crack propagation. During cyclic fatigue testing, three measurements are recorded:
Cycle number - Peak cyclic load - Test piece compliance
From this data the following equations are used, dependent on mode of loading, to determine the maximum value of G during the chosen cycle:
(10)
(11)
The recorded test piece compliance values are substituted into either Equation (1) or (2) to calculate the associated crack length. The crack growth rate between two subsequent measurements is calculated using Equation (5).
(12)
Next: Part 3 Crack Growth Testing – Non-Ambient Conditions
