The UT for aerial propeller blades in use on site

by Jizhen Xia,Rongzhen wang*,Weidong Ren* and Jianlin xiong*

NDT teaching research lab. in NANCHANG Institute of Aeronautical Technology
The article is first published on "13th WCNDT(World Conference on Non-Destructive Testing)" (Sao Paulo,Brazil,18-23 October,1992)
* The Rongzhen wang,Weidong Ren and Jianlin xiong were students of NDT speciality in NANCHANG Institute of Aeronautical Technology,graduated in 1991.
Abstract
The article researched the method on UT for the area under deicer of aerial propeller blade made of aldural on site.The method is using transverse wave(shear wave) to detect the fatigue crack with depth approx 1mm on the blade body's surface under deicer.
Introduction
The article is a sequel of "The UT For Aerial Propeller Blades With Lacquer Layer In Use" which was published on the "5th International Symposium on Nondestructive Characterization of MaterialsTM" (Karuizawa,Japan,May 27-30,1991).In this article,it is further to research the method on UT for the area under deicer of aerial propeller blade with lacquer layer's thickness approx 0.09mm on site.
In foregoing article,it has proposed using larger size's piezocrystal for obtaining larger transmiting power,using proper incidence angle(67¡ã) and proper wedge shape,and testing frequency 2.5MHz of the ultrasonic transducer,and proper inspecting process.This method is using the Rayleigh wave to detect fatigue crack with depth approx 0.3mm on the blade body's surface under lacquer layer by contact method.
For bigger airplane propeller blades,it usually fitted deicer on upper of the blade's air-ingress edge.It is stuck together with blade.It will cause greater damping to the blade surface,so that the Rayleigh wave can not enter into blade surface under the deicer.
In this article,it is attempting to detect the area under deicer of aerial propeller blade with lacquer layer's thickness approx 0.09mm on site.
The deicer is mainly consist of the glass-fibre,adhesive layer,electrothermal element,stainless steel piece for protection,so it is a multiple structure adhered to surface of the blade body.This multiple-adhesive-layer formed biggest damping to blade's surface,so that the Rayleigh wave is incapable to enter into the area under deicer.
Therefore,according to conventionality at present,that is take the blade down from the airplane,then take down the deicer and clean out the lacquer layer,then to detect by the Rayleigh wave.After finishing the inspecting work,then again to stick the deicer to blade,and again to spray-paint for the blade.In this case,the inspecting period is too longer and the working process is disadvantageous to  increase the airplane's utilization of capacity and the economic benefit.
In this article,it is using the transverse wave and successfully detected the artifical crack with depth approx 1mm which area is under deicer and the blade is with lacquer layer.This method is don't need to take down the deicer from the propeller blade.
Testing process
On a propeller blade of AH-24 plane(USSR),uncovering two piece of the deicer,then notching V-shape grooves(1x10mm) of each surface on the area of blade body,then again to stick the two piece on quondam place.The blade will be used as sample.
It is using angle beam transducer with different incidence angle,frequency 2.5MHz,¦µ14mm dia. piezocrystal and higher sensitivity to detect on the sample.


fig.1
Because of the blade surface is a torsional surface and the blade body is a variability section,so that the transverse wave's propagation property is variable in blade body.By repeatedly testing,the relation between echo-amplitude(dB) of the direct echo(groove at the back) and the reflected echo(groove at the detecting surface) and the incidence angles is shown in Tab.1,the testing method is shown in fig.1.
Tab.1
incidence angle 34¡ã 40¡ã 41¡ã 42¡ã 43¡ã 45¡ã
on a surface:
the echo amplitude of groove at the back (dB) 11 11 10
the echo amplitude of groove at the detecting surface (dB) 10 6 15 14 13 6
on other surface:
the echo amplitude of groove at the back (dB) 6 10 11
the echo amplitude of groove at the detecting surface (dB) 8 12 12
note:
1. The blade is made of aluminium alloy LY11(11# hard aluminum) or LY16(16# hard aluminum) with the CL=6260m/s,
CS=3080m/s;
2. The wedge is made of organic glass with the CL=2620m/s.
Conclusion
By repeatedly testing,it shown that using angle beam transducer with frequency 2.5MHz,¦µ14mm dia. piezocrystal and the incidence angle 41¡ã~43¡ã,it can well to detect artifical groove with depth approx 1mm under deicer of propeller blade with lacquer layer thickness approx 0.09mm.Of cource,this method just only primary testing,it need awaiting further research and testing for actual application.