Detection of Corrosion Damage on Aircraft Fleet

Structure

EBB 405Failure Analysis & Non-Destructive Testing

Title:

BEARING

SLIDING DOOR

Eddy Current Technology

the alternative current flowing through the coil-

generates a magnetic field around the coil

coils is place closely to an electrically conductive materials-eddy current

induced

any defects or flaw in the conductive materials will disturbs the eddy current

circulation-defect signal read by the impendence

Primary magnetic fields

Secondary magnetic fields

based on the electromagnetic induction and the circular electric currents in materials

• Single coil• Raster scan

Eddy current technology

allows electronically driving and reading several eddy current

sensors that locate side by side in the same

probe

Eddy current array

technology

• Ultrasonic phased arrays use a multiple element probe whereby the output pulse from each element is time delayed in such a way so as produce constructive interference at a specific angle and a specific depth.

• These time delays can be incremented over a range of angles to sweep the beam over the desired angular range.

• The main advantages of phased array in NDE are: i. Ability to sweep a range of angles ii. Ability to display the image in real time for the swept angles iii. Ability to focus

Ultrasonic Phased Arrays

PHASED ARRAY UT (PAUT) MANUAL UT Automated UT (AUT)

PAUT displays images in real time showing the depth and location of indication relative to the probe.

PAUT simultaneously takes data from a range of angles and reconstructs an image in real time

PAUT image is easy to comprehend as it gives a display of the ultrasound superimposed on the test piece

Using an encoder with the PAUT probe, all raw A-scan data can be stored.

Manual UT produces a single A-scan at a specific angle. Manual UT evaluation requires plotting the indication using the refracted angle, metal path and surface distance.

Manual UT is limited to a single refracted angle.

There is no data storage capability in manual UT.

AUT reconstructs the test piece cross-section (B-scan) after taking data using a single refracted angle and scanning it back and forth on the test piece.

AUT always requires either a 2-axis scan or multiple probes to reconstruct the image.

AUT requires significantly less inspection space for scanning compared to PAUT

AUT store raw A-scan data that can be replayed for analysis

Probe Elements

Transmitted Beam

Time Delay

A linear phased array (PA) transducer usually consists of 64 elements that can be independently driven. By incorporating suitable time delays, it is possible to steer the beam to any required angle as shown in figure below

Beam Steering in Phased Array

Beam Focusing in Phased Array

Two layer aircraft fuselage

Sample Preparation

Dimensions of Corrosion Damage

SI

NoDamage Type Damage Size

Immersion

in

Electrolyte

Material

removed by

EDM

1. Chemically eroded 40 mm x 20 mm1,2,3 and 4

hrs-

2. Chemically eroded 40 mm x 20 mm2,4,6 and 8

hrs-

3.EDM notch and

chemically erodedØ 20 mm 30 minutes 0.2 mm deep

4. EDM notch

18 mm and 8 mm

square notches

Ø 18 mm and Ø 8

mm cicular notches

-

5 %, 10%,15%

of 1 mm and

1.6mm deep

•Experiments have been carried out using RDTech Omniscan MX equipment (Fig.

a).

•The equipment can be used for both ultrasonic phased array and Eddy current

array by changing the acquisition module and probe.

•Ultrasonic linear phased array probe fitted to an encoder (Fig. b) is used which

consists of 64 elements.

•The eddy current array probe (Fig. c) which has 16 elements is balanced on a

damage free area for calibrating the equipment.

•ECA scan results shown is lift off compensated horizontal component containing

the phase information thereby differentiating the corrosion damage at various

depths.

•Higher the intensity of the image, more amount of material is lost due to corrosion.

Experimental Setup

Fig. a: Omniscan MX Equipment

Fig. b: Phased array probe with encoder

Fig. c: ECA probe with encoder

The experiments are carried out for:

Detection of corrosion damage in the single layer

Detection of corrosion damage in simulated two layer aircraft fuselage structure

Results and Discussion

Detection of corrosion in single layer

Eddy Current Array result Ultrasonic Phased Array result

Eddy Current Array result Ultrasonic Phased Array result

containing corrosion damage created by immersing the sample in the electrolyte for 30 minutes and a circular EDM notch of 0.2 mm deep (as shown in the image from left to right)

ECA scan result for a specimen with corrosion damage of 40 mm X 20 mm at four locations created by immersing the sample in electrolyte for 1, 2, 3 and 4 hrs

Detection of corrosion in two layer fuselage lap joint

Eddy Current Array result Ultrasonic Phased Array result

Eddy Current Array result Ultrasonic Phased Array result

ECA scan result for a specimen with damage in the bottom of the top (BOT) layer. The corrosion damage is two EDM square notches of 18 mm and 8 mm with 5% and 10% material loss of the first layer thickness respectively. The ECA result has three features which includes two EDM square notches and a false indication at upper left due to the bulging in the structure. The vertical cursor lines measures the distance between the EDM notches which is 35 mm.

Detection of corrosion in two layer fuselage lap joint

The scan result does not reveal the corrosion damage at the bottom layer due to the effect of scanning

Eddy Current Array result

Eddy Current Array result

Conclusions

Ultrasonic Phase

Array• Defects within first layer

Eddy Current

Array

• Detect size volumetric 5% of layer thickness