The UT For Aerial Propeller Blades With Lacquer Layer In Use

The UT For Aerial Propeller Blades With Lacquer Layer In Use

by Jizhen Xia,Shu Qing* and Lidong Chen*

NDT teaching research lab. in NANCHANG Institute of Aeronautical Technology

The article is first published on "5th International Symposium on Nondestructive Characterization of Materials^TM" (Karuizawa,Japan,May 27-30,1991)

* The Shu Qing and Lidong Chen were students of NDT speciality in NANCHANG Institute of Aeronautical Technology,graduated in 1990.

Abstract

The inspection for the fatigue crack of the aerial propeller blades,in the past,that is take the blades down from airplane,pass through to clear out the lacquer layer,then carry into ultrasonic testing by the Rayleigh wave.In this case,inspecting period is too longer and the working process is disadvantageous to raise the aero-utilization and economic benefit.The article proved by theory analysis and testing,for the aerial propeller blades in use,which is made of aldural,with lacquer layer about 90μm,that can use immediately ultrasonic testing by the Rayleigh wave for detecting fatigue crack with the depth about 0.3mm,so that simplified inspecting process.The article present the size,testing frequency,preferred incidence angle and the properest wedge shape of the ultrasonic probe,introduced the results and analysis related to testing.The article present an efficient method for inspection of the aerial propeller blades on site.

Introduction

For inspecting the fatigue crack of the aerial propeller blades,previous method is take the blades down from airplane,pass through to clear out the lacquer layer,then carry into ultrasonic testing by the Rayleigh wave.In this case,inspecting period is too longer and the working process is too many,so that is disadvantageous to raise the aero-utilization and economic benefit.The task's result is shown that can use immediately the Rayleigh wave to inspect the

fig.1

aerial propeller blades which is made of aldural,its lacquer layer(about 90μm) don't need clear out and don't need take down from the airplane,so that simplified the inspecting process greatly.

As well-known,the Rayleigh wave testing is an effective method used to detect defects on the surface and subsurface of the workpiece in UT technology.Rayleigh wave is a surface sound-wave propagated on the free interface of half-infinited solid medium.It is a nondispersion wave.On the solid medium,the particle's vibration trace is an ovalisation,the oval polarizing face is overlap with the plane constituted by the propagation vector and the perpendicular to surface of medium(when it is in the anisotropic body,it will have one angle to the plane).

The ovalisation's long-axis(perpendicular vibration element) is perpendicular to the medium's surface,its amplitude is highest in depth of 0.2λ from the surface.The ovalisation's short-axis(horizontal vibration element) is parallel to the medium's surface and the propagated direction,its amplitude is highest in depth of 0.4λ from the surface.The envelope of the particle's movement amplitude is present to exponential attenuation,its amplitude fall down to about 1/10 of highest amplitude in the depth of 1λ from the surface,therefore,the Rayleigh wave's effective detecting depth is regarded as within range of 1λ,usually,the sensitivest place is on the depth of 0.4λ from the surface.

At any directions of any surfaces,the Rayleigh wave's velocity always smaller than the velocity of volume acoustic wave which is propagated along the same direction in the infinited medium.

In the infinited medium,the acoustic velocity is:

C_L=(E/ρ)^1/2·[(1-σ)/(1+σ)(1-2σ)]^1/2; C_S=(E/ρ)^1/2·[(1/2(1+σ)]^1/2; C_R=C_S·[0.87+(1.12σ/1+σ)]

here: C_L--longitudinal wave's velocity;C_S--transverse wave's velocity;C_R--Rayleigh wave's velocity;E--Y-elastic modul;
ρ--medium's density;σ--medium's Poisson ratio.

The aluminum alloy's σ=0.34,so the Rayleigh wave's velocity of aluminum alloy is about 0.933C_S or 0.46C_L.

In the article,the problem is that the blade with lacquer layer about 60~90μm on its surface for protection.On this condition,can we excite the Rayleigh wave on the blade surface for inspection?

Take into consideration of that the Rayleigh wave is excited by longitudinal wave with oblique incidence to the interface and the incidence angle reaching or exceeding secondary critical angle(Rayleigh critical angle) on the interface of medium.

The lacquer layer will be damping the vibration of particles on the medium's surface.If the exciting power is large enough,the Rayleigh wave testing will be possible in the condition of air-thin film of lacquer layer-aluminum alloy.

Program

For quantitative evaluation and affirmance of the Rayleigh wave,and established the testing sensitivity,the calibration block is made of LC4(4# super-hard aluminum alloy) rolling plate,as shown in fig.2.
The block is treated with vitriol anodizing,it possess of a groove,then stage by stage spray the nitryl lacquer from 40μm,70μm,80μm to 107μm of layer thickness on the surface,and stage by stage testing correspondingly,so that obtain different detecting sensitivity for different lacquer layer's thickness corresponding with different incidence angle and wedge shape.
The detecting distance is 400mm from the groove,the beam propagating direction is vertical to groove's axis during testing.
The ultrasonic flaw detector is CTS-26 type,CTS-23 type and WT-2004 type manufactured by Chinese manufactory.The coupling medium is grease,machine oil and glycerine for testing.The ultrasonic transducer is a dismountable probe with 2.5MHz and Φ14mm dia.,its actual echo-frequency is 2.478MHz.When change incidence angle and wedge shape,only use the probe for assuring unification of the probe's sensitivity itself.

fig.2

In accordance with the basic theory of acoustics,the incidence angle to produce Rayleigh wave,exactly,that is:

α_R=sin^-1(C_L/C_R)

here:α_R--Rayleigh wave's critical angle(secondary critical angle);C_L--longitudinal wave's velocity in wedge;C_R--Rayleigh wave's velocity in aluminum alloy.

In condition of this article,the longitudinal wave in wedge made of organic glass will be penetrate lacquer layer into aluminum alloy.It can be proved that,for exciting Rayleigh wave in aluminum alloy,the lacquer film as middle medium is no effect on the incidence angle(i.e,its sound velocity can be omission).
In this article,the longitudinal wave's velocity of wedge made of organic glass is 2650m/s,the longitudinal wave's velocity of LC4 aluminum alloy plate is 6285m/s.
As C_R=0.46C_L,so the Rayleigh wave's velocity is 2890m/s,and the Rayleigh wave's critical angle is approximate to 66.5°(the Poisson ratio is approximate to 0.34).
In Rayleigh wave testing,ordinarily using incidence angle range is 62°~80°(organic glass wedge).For carrying comparison and choice in

fig.3

order to fix on optimal incidence angle for exciting Rayleigh wave,we fixed 10 incidence angle by optimum seeking method to test,they are:61.4°,62.6°,64°,65.2°,66°,67°,67.5°,68.4°,70° and 72.5°.There are 4 wedge shapes for comparison testing,as shown on fig.3.

Testing results

Testing is carring on the surface with the groove of the block.The probe's incidence point is distance 400mm from the groove and the beam propagation direction is perpendicular to axis of groove.

The testing is carring by different incidence angle and wedge shapes in 5 situation of mill finish,lacquer layer(40μm,70μm,80μm and 107μm),separately,using the same probe and instrument sensitivity.The contact pressure between probe and block is assured to constant by press block with 2Kg weight.It is affirmed that groove echo is Rayleigh wave by calibrating time-baseline on screen of instrument and using finger damping method.

Part testing results are shown in fig.4~6.


fig.4 The change of groove's echo amplitude along with different wedge shapes.CTS-26 type ultrasonic flaw detector,couplant is grease,lacquer layer's thickness is 107μm	fig.5 The change of groove's echo amplitude along with different wedge shapes.WT-2004 type ultrasonic flaw detector,couplant is machine oil,lacquer layer's thickness is 80μm

The testing resulte indicated,when using 67° incidence angle and wedge shape of fig.3(d),the Rayleigh wave testing is realized better to propeller blade with lacquer layer made of aluminum alloy.
For more checking,we use the probe to test on propeller blades of AH-24 type airplane(USSR) and 6 type primary trainer airplane(China) in site.It can detected satisfactorily,for example,the fine bruise mark,concave mark and scratch mark,and two defects under lacquer layer.
For the sake of contrast,we use too the 2.5MHz,6x6mm,
65° miniature probe of Rayleigh wave testing which is a commercial probe used in system of civil aviation at present.We use same instrument and couple condition for testing.When the testing is carry on these blades,the testing sensitivity of the miniature probe can not satisfy the requirement,obviously.This case may be concerned with that the size of piezoelectric crystal is smaller,so that the emission power no enough and the wedge shape is no good.
From these testing results,it indicated that the effect of wedge shape on testing sensitivity is greater.

fig.6 The change of groove's echo amplitude along with different incidence angle and lacquer layer thickness,the wedge shape is fig.3(d).CTS-26 type ultrasonic flaw detector,couplant is glycerine.

After analysis,we are regard as that,for anterior three shapes in fig.3,i.e,(a),(b),(c),because of existence of leading edge in front of incidence point by the geometry sound beam axis,so the echo within wedge may be interfere with incidence wave and lose energy for exciting Rayleigh wave when emiting ultrasonic,as shown in fig.7,here is existing effect of trigonal interference region.In addition,the contact pressure of the leading edge on workpiece surface will too increasing damping to vibration of particles on surface.The wedge possess of the shape in fig.3(d) will not the presence of the trigonal interference region,and not damping caused by leading edge,so the sound energy of concentrating on circumference of the sound beam axis will enter immediately into workpiece to excite Rayleigh wave.

fig.7 The effect of wedge's leading edge on emissing and incepting ability of Rayleigh wave

On the other hand,as consideration about aspect of incepting sensitivity,in the anterior three cases,the wedge is existing leading edge,so the groov's echo begin enter into wedge from leading edge,so it will cause the sound energy to disperse so that reducing the incepting sensitivity.In the fourth shape of fig.3,the echo is enter into wedge from incidence point immediately,this is a favourable place for incepting echo,so that raising the incepting sensitivity.

Conclusion

If use the piezoelectric crystal with larger size to raise emission sound-power,and use suitable incidence angle and wedge shape,then will can exciting Rayleigh wave on surface of blades made of aluminum alloy under certain lacquer layer's thickness for detecting fatigue crack and other defects in site.The testing results of this article indicated that ,use 2.5MHz frequency,incidence angle 67°,and the wedge shape in fig.3(d),this condition will can obtain satisfactory testing results.

Existent problem

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.For detecting this region,it need to use other method,for example,the transverse wave,but not description herein.