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·Aeronautics and Aerospace
NDT Approach and multi-sensors tools for the Inspection of aeronautic WeldsGuillaume Ithurralde, Didier Simonet , Jean-Pierre Choffy , Laurent Bernard
(Aerospatiale Matra CCR,France)
From a NDT point of view ,these new welded conceptd are tested to identify the specific discontinuities.Their occurence is connected to process parameters and conditions.Their influence on mechanical resistance is evaluated.The operating welding limits are determined.The NDT method are selected and criteria are defined.
Ability for on-line automation is an important requirement to choose NDT systems.It leads Aerospatiale Matra to evaluate multi-sensor technologies,because they allow to inspect the welds as fast as they are produced.These multi-sensor are selected to meet the addressed problem : charged couple device (CCD) camera for computer-aided visual inspection,multi-coils eddy current probes for the detection of surface defects,ultrasonic phased arrays for inner discontinuities...
These multi-sensor techniques are fast tools to investigate and improve welding processes.Then they can be used as online production testing,whenever it is necessary.Their interest can be extended to any linear inspection.
To evaluate these new concepts, several welded demonstrators have been produced at the Corporate Research Center (CCR) of Aerospatiale Matra. Below are presented both the associated non destructive testing approach for process finalization, and the multi-sensor systems evaluated for on-line defect tracking.
2.1 Aerospace welds features
Most of aerospace welded assemblies are made of aluminum. Few are made of titanium alloy and stainless steel. Thickness ranges between 2 mm and 150 mm. No edge preparation is observed. Various configurations and processes are tested : T-joints, butt-joints; laser beam and electron beam, friction stir... They generate specific discontinuities, which only some are noxious.
The type, size, density and position of defects depend on mechanical loading. Fatigue loaded assemblies are calculated according to the damage tolerance principles.
2.2 NDT approach
|Fig 1: NDT approach for new welding processes|
Welding optimization is an interactive process implying different professionals working together (see figure 1). First all the discontinuities specific to the materials, the geometry and the welding process are identified. They are connected to process parameters and conditions, to eliminate most of them, and to determine the operating welding limits. Mechanical tests are performed to differentiate "inoffensive" discontinuities from defects. Expertise, consisting in metallography and fractography, enable to correlate process monitoring signals, NDT signals and real discontinuities. Criteria regarding the type, the dimensions and the position of the defects are deduced from the calculation.
For obvious economic reasons in production, process monitoring is preferred to NDT, especially if it is off-line. Anyway, it has to prove its capability to detect all the identified defects definitely. It can be plasma emissions analysis for laser welding, press force control for friction stir welding, etc. Ideally, there is a feedback from the process monitoring system to the welding device, in order to correct any failure on-line (see dotted arrow on the production line #1, figure 1). Otherwise it works like an alarm system and it is combined to NDT (see production line #2, figure 1).
In this case, NDT methods and NDT systems that can detect all the potential defects are selected. The choice is made considering signal to noise ratio and probability of detection, equipment acquisition cost, necessary education level, and aptitude for on-line automation. These requirements lead Aerospatiale Matra to evaluate:
3.1 Charged coupled device(CCD) cameras for computer -aided visual inspection
CCD cameras and laser diodes are used for computer-aided visual inspection. A low cost laboratory system was developed by Aerospatiale Matra CCR to take pictures and measure the positions of blind holes simulating craters down to 0.5 mm diameter at 2 m/min (see figure 2). It can be reconfigured at will, for example to track undercuts, lacks of penetration, or any shape defect.
|Fig 2: Craters tracker principle.Measured seam profile.||Fig 3: Examples of typical multi-coils EC probe and C-SCAN of an aluminium parts|
CCD lines are also tested for problems that require high speed acquisition.
3.2 Multi eddi-current probes for the detection of surface defects
Multi-coils eddy current probes and systems can be used directly on flushed seems to track superficial defects, sub-surface defects, and even inner defects in thin welds. Experiments showed that it is possible to detect artificial semi-elliptic notches down to 1.5x0.2x0.1mm, electro- discharge machined (EDM) in a 7075 plate, at 3 m/min (see figure 3).Probes design (frequency, coils positioning and linking, etc.) have to be carefully defined to differentiate defects signals from conductivity variations between the weld, the heat affected zone (HAZ) and the base metal.
3.3 Ultrasonic phased arrays for inner discontinuities
Ultrasonic phased arrays are interesting for inner discontinuities in welded assemblies of any thickness. They give the opportunity to take advantage from electronic focusing, electronic steering and/or electronic scanning.
|Fig 4: Different linear array probe orientation usinf electronic scanning.|
|Fig 5: Ultrosonic C-SCANS obtained respectively with LW and SW,in pulse echo mode|
For volumetric inner defects, a linear phased array working in pulse-echo mode is sufficient. The orientation does not matter. A normal incidence generating longitudinal waves (LW) provide data easy to interpret. An oblique incidence generating shear waves (SW) provide more accurate results.
For planar inner defects like cracks, the acoustic beam has to be perpendicular in pulse-echo mode. Any preferential orientation induces appropriate probe tilt. When defect orientation is unpredictable, electronic beam steering is a solution, for example with a 2D matrix array. Tandem configurations and Time of Flight Diffraction (ToFD) technique are also possible with phased arrays, as well as with single elements probes.
Finally, dynamic electronic depth focusing is well-adapted to the NDT of thick welds containing small defects in pulse-echo mode.
At Aerospatiale Matra, NDT people are involved as early as the process feasibility study. They work together with designers, metallurgists, process professionals, etc. to identify discontinuities, connect their occurrence to process parameters and conditions, determine their influence on mechanical resistance, define acceptance criteria, alarm levels, and operating welding limits...
As long as process monitoring cannot guaranty welds perfect health in production, NDT is necessary. Multi-sensor techniques are tested because they are able to inspect welds as fast as they are produced, thanks to one-line scan and real-time results display. They speed up process optimization step, and their on-line carrying-out is easier. Three complementary methods have been selected by Aerospatiale Matra CCR. Charged coupled device (CCD) camera for computer-aided visual inspection are devoted to external defects. Multi-coils eddy current probes are efficient for the detection of superficial and sub-surface defects. Ultrasonic phased arrays are interesting for inner discontinuities. Automatic diagnosis would be a plus to enable a "NDT feedback loop" to the welding device.
Ultrasonic phased arrays and multi-coils eddy current are also considered for maintenance NDT purpose, to track in-service defects, like fatigue cracks for example. They are also tested for other linear inspections in production: thermoplastic welds, composite bending radius, etc.
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