Effects of Drilling Induced Heating During IHD of FRP Laminates

Abstract Background Incremental hole-drilling has been used extensively in composite laminates, however, the low thermal conductivity of GFRP laminates results in drilling induced heat build-up near the hole. This can lead to measurement errors due to post-cure shrinkage of the matrix, and additional thermal drift errors arising from the necessary data acquisition delay after each drilling increment to allow adequate heat dissipation before taking strain measurements. Objective Investigate the significance of drilling induced post-cure shrinkage and the effects of different bottom-surface thermal boundary conditions on the drilling induced heat dissipation in GFRP laminates during IHD. Methods IHD is performed on GFRP laminates, specifically an annealed $$[0_{8}]$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>[</mml:mo> <mml:msub> <mml:mn>0</mml:mn> <mml:mn>8</mml:mn> </mml:msub> <mml:mo>]</mml:mo> </mml:mrow> </mml:math> laminate and a $$[0_{2}/90_{2}]_{s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mo>[</mml:mo> <mml:msub> <mml:mn>0</mml:mn> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mn>90</mml:mn> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>]</mml:mo> </mml:mrow> <mml:mi>s</mml:mi> </mml:msub> </mml:math> laminate with different support configurations. The through-thickness residual stress distribution is determined using the integral computational method. The magnitude of drilling induced post-cure shrinkage effects and those of the different thermal boundary conditions are investigated. Results IHD on the annealed $$[0_{8}]$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>[</mml:mo> <mml:msub> <mml:mn>0</mml:mn> <mml:mn>8</mml:mn> </mml:msub> <mml:mo>]</mml:mo> </mml:mrow> </mml:math> specimens demonstrated that drilling induced post-cure shrinkage effects are not significant. The use of different thermal boundary conditions for the $$[0_{2}/90_{2}]_{s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mo>[</mml:mo> <mml:msub> <mml:mn>0</mml:mn> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mn>90</mml:mn> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>]</mml:mo> </mml:mrow> <mml:mi>s</mml:mi> </mml:msub> </mml:math> specimens demonstrated the necessity for good heat transfer out of the laminate to achieve accurate results. Conclusions Carefully performed IHD does not cause sufficient drilling induced heating to result in post-cure shrinkage of GFRP laminates. Experimental parameters and the thermal boundary conditions of the bottom surface must be carefully considered to ensure a successful measurement. A lack of good heat transfer out of the bottom surface of the specimen increases the required testing time and can produce unreliable results.