Proposed methods
Several methods to introduce a controlled and consistent delamination in a compos-ite specimen with a countersunk hole have been proposed in the first stage of the research, but only two of them were considered promising and selected for further analysis. These methods will be now described in detail.
3.1
“Shaped Indenter Damage Induction”
The first proposed method was called “Shaped indenter damage induction”. It aims to create a Mode II delamination pushing a properly shaped tool above the hole location, as shown in Figure 3.1.
Both sides of the specimen will be firmly clamped, except for a fraction of surface around the hole to allow the zone to bend. The involved mechanisms are inter-laminar shear force induced by bending and free edge effects on the internal surface of the hole. Different indenter’s shapes and different dimensions are considered:
• Sphere. Figure 3.2a, 3 dimensions. The radius of the spheres was chosen in order to obtain the contact in point A, B and C of Figure 3.3. The ratio between the depth of countersunk and straight region is a fixed parameter set to 0.5, so the diameter of the sphere will change depending on the thickness of the laminate.
• Cylinder. Figure 3.2b, 3 dimensions. The radius of the cylinder was chosen with the same criterion as the spherical indenter.
• Cone. Figure 3.2c, 1 dimension. The conical indenter reproduce in angle and dimension the countersunk part of the hole (opening angle set to 100◦). This shape was chosen to increase the contact area reducing stress concentrations.
(a) Spherical indenter (b) Cylindrical indenter (c) Conical indenter
Figure 3.3: Desired contact points
The other free parameter is the fixture. To increase the tendency to obtain a circular delamination, the specimen will be clamped on both sides, leaving only a circular area coaxial with the hole unconstrained, as shown in Figure 3.4.
Figure 3.4: Scheme of the constraint
Two diameters for this circular area were considered: 1 and 1.5 inches. The first was chosen because it matches exactly the dimension of the desired delamination, but the risk exists for the fixture to be too rigid, stopping the progression of the delamination before it reaches its edge. To avoid this issue a larger diameter was also chosen.
The greater concern about this method is on the risk of inducing unwanted mechanisms of damage before the initiation of the desired Mode II delamination, which has high activation energy. In particular the risk exists of creating multiple
cracks in the matrix due to shear and compressive stress which may affect the results of future bearing tests. Another issue is the possibility of creating damage at the contact zone with the indenter. Spherical and cylindrical indenters in particular represent the situation of greatest risk, having a reduced contact area.
3.2
“Two-Step Drill Indentation”
The second analyzed method was called “Two-Step Drill Indentation”. At first a small hole is drilled in the specimen through the entire thickness, then a hole of the actual bolt’s diameter is drilled co-axially with the first one, but only up to half way through the thickness (or in general to the point where the embedded delamination is desired) obtaining a counterbore hole. An indenter can be now used to push on the created flat surface to induce a pure Mode I delamination, as shown in Figure 3.5.
Figure 3.5: Scheme of the 2-Step Indentation method
The hole will be fully drilled and chamfered after the indentation. The flat surface in contact with the indenter will be removed after the procedure, so the risk of creating unwanted damage due to contact stress concentrations is negligible, however the risk exists of introducing additional damage during the subsequent machining process to complete the hole. Another source of uncertainty is the presence of bending that may induce a Mode II delamination in the lower half of the specimen, but supposedly the Mode I delamination, requiring less energy, will happen first and the procedure can be stopped before the initiation of other mechanisms of damage.
The chosen indenter is cylindrical and the fixture is identical to the previous method: the specimen is clamped on both sides, leaving a circular unconstrained area coaxial with the hole (Figure 3.4). Again, two diameters for this circular area are considered: 1 and 1.5 inches.
As previously stated this method presents few risks and seems more promising: the energy involved is lower and damage caused by contact does not represent an issue. In addition, it allows to better control the position of the delamination along the thickness by simply controlling the height of the counterbore part: theoretically, delamination should initiate at the bottom of the larger hole, as shown in Figure 3.5.
