Impact ice interface shear stresses caused by blade bending and twisting

Abstract

The objective of this paper is to present approximate formulas to estimate the interlaminar shear stresses in an ice layer adhered to an airfoil. Three separate loading have been considered: stresses in impact ice on a rotating airfoil, interlaminar shear stresses caused by bending loads and interlaminar stresses caused by twisting. Following strength of materials methods, an approximate formula was developed to calculate interlaminar shear stresses at the interface between the impact ice and substrate. To evaluate the accuracy of this equation, stresses calculated from this formula are compared to finite element analyses. Both bending and torsional loadings are considered. In all cases, two layered composite elements were specified with an outer layer ice and ntroduction , i an aluminum inner layer. The evaluation of stresses due to impact ice on the leading edge of airfoils is a complex problem. Stresses are developed from aerodynamic pressure forces [l], bending and twisting of an airfoil, temperature variations as well as inertia forces. High inertia forces on ice are created from the rotation of helicopter blades [ Z ] and propellers and from the vibration motion caused by impact loading of deicing devices such as EIDI systems t31 . Stresses from aerodynamic loading are a function of air speed, angle-of-attack and ice shape. In Reference [l], stresses in the ice shape studied experimentally by Bragg [4] and analytically by Potapozuk [5] were calculated by finite element analyses. In this work it was concluded that interlaminar shear stresses can be significant at air speeds above Mach 0.45. Below that speed, these shear stresses are not significant and can be neglected. Figure 1 is a plot of the interlaminar shear stresses at Mach 0.6 at O o angle-ofattack. It should be noted that the maximum value is almost 9 psi and that the stress varies significantly at the interface. An approximate evaluation this maximum interlaminar shear stress for a particular ice shape at various airspeeds seams extremely complex. However, at airspeeds below Mach 0.45, this effect can be neglected.