Tuesday  |  Frick  |  10:20 AM–10:40 AM

#13260, Evaluation of the Maximum Allowable Hole Depth for IHD of GFRP Laminates

The maximum hole depth that ensures acceptable results when performing IHD of isotropic materials is well established and a final hole depth is specified in the ASTM standard. Limited investigation has, however, been conducted on the equivalent maximum hole depth in FRP laminates. The least-squares solution of the series expansion method benefits from an abundance of measured strain data, however, a depth is reached where the insensitive data at greater depths negatively impact on the stability of the least-squares solution. This not only leads to higher uncertainty in stress at greater depths but potentially at shallower depths also. A similar effect can occur in the case of the regularized fit of the integral method. It is therefore important to determine the maximum hole depth before the accuracy of the calculated residual stress distribution reduces to unacceptable levels. The effect of final hole depth on the accuracy of calculated residual stresses is investigated for a GFRP laminate of [0_2/90_2]_s construction using separate series expansion in each ply and also the regularized integral method. The same investigation is also performed on a GFRP laminate of [0_2/+45/-45]_s construction using series expansion only. Results indicate that the calculated stress distribution is dependent on the depth to which strain data is incorporated into the analysis and, in the case of series expansion, also on the relative position of the bottom of the hole within the deepest drilled fibre orientation. Neither computational method produces reliable results beyond a hole depth of about 1 mm which corresponds well with the limit specified by ASTM E837 for metallic materials.

Robert Reid   University of the Witwatersrand
Teubes Smit   University of the Witwatersrand