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Detection of defect on Aircraft Multi-layered Structure by Eddy Current technique Md. Mahi Uddin Khanz Biman-Bangladesh Airlines Contact

INTRODUCTION

Corrosion and fatigue cracks are always expected in aging aircraft structure. Corrosion is a critical problem of aircraft structure, which directly affect the airworthiness of an aircraft. Corrosion causes thinning of aircraft structure skins lead to a degradation of structural integrity. Fatigue cracks also occurs in multi-layered riveted joint especially in aluminum-alloy structures.

Inspections in critical areas and maintenance of its structure are essential to maintain a high degree of quality and reliability. To ensure the structural integrity and safety of aircraft, all operators are effectively maintaining an inspection program. In this inspection program nondestructive-testing inspection is playing a vital role.

Non-destructive inspection [NDI] is frequently used at certain interval in different critical areas of aircraft structure for detection of hidden small damage and to clarify the real condition of aircraft structure for appropriate repair and corrective action.

Although aircraft structures are assembled with different types of materials, considering the accessibility and cost 'Eddy Current' method is widely used for subsurface defect in aircraft structure. Damage tolerance and NDT reliability programs also proved eddy current inspection is superior to other NDT methods especially for detection of tight cracks and corrosion.

This paper will highlight some familiar eddy current techniques, generally used in aircraft structure, for detection of defect in multi-layered structure and fastener holes and two case studies.

DIFFERENT EDDY CURRENT TECHNIQUE

1. Defect and estimate corrosion loss on aluminum back surface
This inspection procedure is designed to detect and estimate material thinning caused by corrosion on the back surface or back surface interface of aluminum structures, where one surface is accessible for setting the inspection probe. This method utilizes the variable conductivity effect of the eddy current by the reduction of the conductive layer cross section caused by corrosion

But this process can not be used precisely measure the depth of corrosion or to make an estimate of the decrease in the material thickness of less than 10 percent.

Following items are used to carry out this inspection.

• Eddy current flaw detector with impedance plane display and frequency control between 3khz and 50khz.Preliminary instrument operating frequency may be adjusted considering the thickness of test item during calibration procedure from Table-01
• Eddy current flat surface probe with 'selected frequency' for application. Small diameter probes are preferred because they find small area of corrosion more accurately.
• Reference standard to be fabricated from panels representing the material, in the inspection area with basic thickness and specified material loss.

Thickness of aircraft structure [layer- 1] in inches	Thickness of reference standard [layer- 1] in inches	Test frequency range [kHz]
0.032-0.034	0.032	37-45
0.035-0.038	0.036	35-44
0.039-0.045	0.040	25-35
0.046-0.056	0.050	16-25
0.057-0.068	0.063	11-16
0.069-0.076	0.072	9-11
0.077-0.085	0.080	7-9
0.086-0.095	0.090	5.6-7
0.096-0.105	0.100	4.6-5.6
0.106-0.118	0.110	3.6-4.6
0.119-0.125	0. 125	3.2-3.6
Table 01: Reference standard and frequency selection for back surface corrosion loss for clad 2024-T3/T4 &7075-T6 AL alloys

2. Subsurface crack detection at damaged location
This inspection is designed [only aluminum skins with conductivity in the range of 29.5% to 38.5 %ICAS] to inspect for sub surface cracks in aluminum skin panels. Such cracking may result when skin is deformed from mechanical damage and straightening operations. This method only be used when access is limited to the panel surface on the opposite side of the suspected crack location.

Following items are used to carry out this inspection.

• Eddy current flaw detector with impedance plane display. Set operating frequency at approximate value has shown in Table-02 considering skin thickness of inspection area.
• 1/4-inch diameter spot probe [minimum frequency range 4 kHz-20kHz]
• Reference standard with EDM notch to be made from same material and thickness.

Skin thickness [in inches]	Frequency (kHz)
0.032	20
0.040	19
0.050	17.5
0.063	14
0.071	10
0.080	7
0.090	6
0.100	5
0.125	4
Table 02: Frequency values established using 7075-T6 AL material

3. Detection of cracks beneath external repair doublers
This procedure is used to detect fatigue cracks in the fuselage plating under external repair doubler and other repaired aluminium structures when specified. This procedure will also detect cracks in the outer row of fastener holes common to the external doubler and the fuselage plating. The crack must be oriented normal with the repair edge to be detected.

• Access to the external surface of the repair location is essential
• Eddy current flaw detector with- impedance plane display/frequency range settings
• Low frequency reflection/sliding probe [1khz-100 kHz]
• Stackable reference standard to be constructed to simulate the structure to be inspected and subject defects.
• Nonconductive straightedge, sandpaper and teflon tape

4. Open hole defect detection
This inspection procedure is designed to detect discontinuities, which initiate from or intersect the inner surface of holes in the subject to be inspected. Open hole eddy current is often used to detect defects which are too small to be detected with fasteners or hardware installed in the holes, or when surfaces are not accessible for inspection by other methods.

• Eddy current flaw detector with impedance plane display/meter display
• Hand held rotor and edd y current probes to fit the range of the hole diameters
• Reference standard must be fabricated with same material and contain the appropriate hole sizes representing the range of holes to be inspected

CASE STUDY

1. Inspection of horizontal stabilizer left and right side leading edge rib cap [multi layered structure] of DC10-30 aircraft.

Background
Ten instances of DC10 aircraft of horizontal stabilizer leading edge rib caps cracks at stations Xfs=166.161 and Xfs=241.289 reported by 4 DC10 operators. Manufacturer of the aircraft investigated the cause of crack and found the failure was fatigue. For safety measure manufacturer instructed for an 'eddy current inspection' of horizontal stabilizer left & right at rib caps stations Xfs=106.834, Xfs=166.161, Xfs=241.289 and Xfs=367.185 [Details location has shown in figure -01] to determine crack existence and continue the repeat inspection until preventive modification is accomplished.

Fig 01:

Fig 02: Reference standard assembly for horizandal stabilizer tee cap inspection

Inspection
Inspection carried out on 'Biman-Bangladesh Airlines' in its DC10 fleet, with following procedure, equipment, probes and reference standard.

1. Elotest B1 eddy current impedance plane flaw detector
2. Probe [Rohman PKA 34-18 with driver and receiver coil and frequency range200Hz-100kHz]
3. Reference standard assembly with EDM notch fabricated from 7075-T6 aluminum clad sheet [Details shown in figure-02]

Procedure


1.After proper cleaning calibrated the equipment [set frequency at1.5kHz] with probe and reference standard for null/balance and lift-off compensation.
Scanning accomplished in two phases for crack detection in
1. tee cap fastener hole and
2. tee cap flange radius.

2.Tee cap fastener hole


• Scan the probe [keeping the centerline of the probe in line with centerline of the fastener row] on the reference standard across a fastener hole 'without a notch' area. Adjust the horizontal-vertical and rotation control to achieve an impedance plane display.
  
• Scan the probe on the reference standard across a fastener hole 'with a notch' area and adjust equipment to achieve an impedance plane display of notch signal.
  
• Starting at one end of the inspection area after necessary null/balance, scan over each affected fastener row and observe CRT for crack indication.

3. Tee cap flange radius


• Place the probe on reference standard between fastener in an area 'without a notch' and null/balance the equipment and adjust lift off compensation.
• Scan the probe on reference standard across the notch located in the proximity of the tee cap flange radius to achieve impedance plane display.
• Scans at all station inspection area after necessary null/balance as the same manner done during calibration with reference standard and observe the instrument for crack indications.

Inspection findings


Aircraft	Inspection results	Crack location	Crack length
A/c-1	Crack indication 2 places	LH Stn Xfs=166.161	7mm,12mm
A/c-2	Crack indication 1 place	RH Stn Xfs=241.289	13mm
A/c-3	Crack indication 1 place	RH Stn Xfs=166.161	28mm
A/c-4	No crack	-------	-----

After analyzing the crack for safety and integrity of the aircraft preventive modification were accomplished in aircraft-1, 2 &3 and continued repeat inspection at 6000 cycles interval for rest of the aircraft.

2.Bolt hole inspection of DC10 aircraft pylon forward mount truss assemblies


Background
Twenty DC10 operators reported sixty incidents of crack in lower legs of pylon 1and 3engine forward mount truss assemblies. Detail location has shown in figure-03

Fig 03: Wing pylontruss assy. Inspection.

Fig 04:Reference standard assembly with EDM notch and CRT display.

The engine forward mount truss assembly on pylon 1,3 is made up of two truss fittings for fail safe design .The cracks initiated in the lower horizontal flange from the forward most attachment holes. On one aircraft, the cracks propagated into the vertical flanges and disconnected both lower legs of the truss assembly from the pylon box beam. The cracks in the lower legs of the other aircraft were only in the lower horizontal flanges of the truss fittings.
Investigation by the manufacturer revealed cracks were resulted from fatigue stresses.
To detect cracks manufacturer of aircraft provide instruction for visual and eddy current inspection of [typical pylons 1 &3] the lower legs of the engine forward mount truss assemblies for cracks and subsequent modification/ replacement.
Inspection carried out on 'Biman-Bangladesh Airlines' in its DC10 fleet, with following procedure, equipment, probes and reference standard.

1. Elotest B1 impedance plane flaw detector
2. Hand held mini rotor MR3HF and rotary probe [Rohman 7.8mm diameter]
3. Reference standard assembly with EDM notch was made from the similar material of the truss fitting, combination of three pieces [size 3 inch × 2.75 inch] one from 6AL-4V titanium plate annealed and two pieces from 4340 steel bar]. [Details shown in figure-04].

Fig 05: Inspection of holes

Procedure


1. Remove four forward horizontal flange attaching bolts [pylon 1&3].
2. Remove corrosion or burrs from surfaces of holes with a 5/16-inch diameter stainless bruss bore cleaner and clean surfaces of holes with 1,1,1 trichloroethane
3. Determine the condition of truss fitting with following technique
4. Attach probe assembly to Eddy current instrument and set operating frequency [520khz] and adjust null/balance and lift off compensation.
5. Insert probe to the reference standard assembly hole with EDM notch and calibrated the instrument to a traceable signal in CRT.
6. Insert probe from the bottom side into the hole and slowly feed through while observing the CRT for edge, interface, and hole crack indication.
7. Scan all four holes through the entire truss fitting thickness of 1 and 3 pylon. Inspection location has shown in figure-05.

Inspection findings


Aircraft	Pylon #1	Pylon #3	Remarks
A/c-1	No crack	Crack indicated at 1 hole	Repaired
A/c-2	No crack	No crack
A/c-3	No crack	No crack
A/c-4	No crack	No crack

CONCLUSION

In this paper a few familiar eddy current techniques with case studies were discussed for detection of crack in aircraft structure. Perhaps the aerospace industry is leading in the world for innovation of new materials and fabrication technique regularly to improve safety, efficiency and reduce cost. But with increasing demand of fast and reliable inspection requirement, there are innovating various NDT techniques for aircraft inspection. For easy and quick decision aircraft operator very often uses eddy current inspection during maintenance.

The automation, miniaturization of eddy current testing and test data evaluation permits mass inspection of similar parts at high rates with economics, which are not attainable by other commonly, used NDT methods.

However, with the adoption of the correct method for the specific application an operator will be able success fully to determine desired defects or structural characteristics.

References:

1. ASM Hand book-Vol-17 Nondestructive E/valuation & Quality control
2. DC10-SBA54-99, SB55-22
3. NDTSPM-Mcdonnel Douglas
4. Non-Destructive Testing-Barry Hull & V John

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