·Table of Contents
·Aeronautics and Aerospace
Contribution of Modelling Tools to aeronautics NDTOdile Pétillon
(Aerospatiale Matra CCR, France)
A wide range of tools has been developed in order to better understand and control the NDT parameters for ultrasonic, radiography or eddy currents. Though these tools are not those appropriate for aeronautic structures, especially when composite materials are concerned, it already becomes evident that they can provide help through at least two axes:
The evaluation of NDT modelling tools is currently underway at Aerospatiale Matra CCR and has covered till now a number of applications. Main concerns of the aeronautic industry and the potential contribution of modelling are discussed and illustrated.
2.1 AERONAUTIC CONTEXT
A large range of participants is involved in the development of aeronautic structures beginning with the design office, the stress department, industrialisation people as far as production and aftersales people. NDT developments have to cope with the whole cycle stream and it is currently quite difficult to share information on NDT concerns before the design and the sizing have been achieved. Yet design and sizing are strongly linked to NDT; for instance design has an influence on the accessibility and sizing requires information on the method performances that will be used for the damage tolerance design. As long as no loop is available to optimise the different parameters at the same time, it will be impossible to trim the developments in order to make the testing easier and as a result the part cost lower.
Modelling NDT methods provide a good basis for allowing an early evaluation of part inspection. Emerging tools can take reliably into account the geometry, material properties and representative defects. At the design stage, such tools could offer a prediction of part inspectability. They could also give an approximate value of the defect size that could be detected and the cost of the foreseen method. With this information, an inclusive solution could be discussed during the conception phase.
In a later stage, modelling tools can be used as a platform for preparing the inspection procedure, covering the transducer choice and test parameters. They can help as well to evaluate the sensitivity to different defects or to set-up a thorough training.
2.2 SELECTION OF TOOLS
Considering the different needs aforesaid, Aerospatiale Matra CCR compiled its requirements. The main concerns are as follows:
A survey showed that a large number of NDT modelling tools are already available. Two approaches can be spotted; one is based on finite element solving and the other uses a more analytical process. Both approaches have advantages and drawbacks that have been weighted up taking the requirements into account. A finite element approach enables the simulation of very complex situations (heterogeneous material, complex defects, etc.). However, this type of tools implies a mesh definition and long calculation time. On the other hand, the parameters of analytical models are mainly physical, thus maybe easier to handle, and calculation takes less time. The drawbacks are linked to the facts that it is not possible to represent too complex situations and that the final results could have a lower precision.
Considering the expected output of simulation tools, it appeared that a system providing an easy approach and quick calculation was the best solution, allowing for a possible lower precision, and Aerospatiale Matra decided on a ultrasonic tool developed by the French atomic energy commission(CEA). The software has now been used for 2 years on different applications. Typical examples will be reviewed.
In the first investigation phase, the field map by itself helps to make the transducer choice, to define the focal laws for phased-array transducers or to evaluate the inspectability of a part. The need for a full scan arises only when it becomes necessary to evaluate a detection threshold or to simulate a full inspection.
3.1 PROCEDURE DEFINITION
A very simple and recurring operation deals with the definition of testing parameters for new standard samples. These parts are usually quite simple, with a flat shape and simulated defects, such as flat bottom holes (FBH). Till recently, these parts were used in a first stage to develop the testing procedure following an experimental approach. In a second stage they are a reference with which results on real parts are compared. Using an experimental approach the definition of the testing parameters can take a few hours for an uncommon configuration. Using a simulation approach took less than 1 hour in total for the case illustrated in Figure 1. The part is representative of a thick electron beam weld and small diameter FBH defects have been machined in the weld area at various depths.
|Fig 1: simulation of an inspection on a standard part|
|Fig 2: inspection of a weld with a phased array transducer|
Recurring use of the model concerns as well the definition of focal laws for Aerospatiale Matra CCR phased-array system. All transducers have been entered in the database and a simple computation provides adequate amplitude and time parameters for each case to be studied. In the case of the weld inspection presented in Figure 2, the use of the model enables to visualise the focal point and to calculate its size thus the acquisition step. It also enables to check if the full weld (molten area) can be inspected in one shot. In this example, it appears that the focal point doesn't cover the upper and lower sides of the weld and, in order to keep a high sensitivity, two focalisation depths should be used.
3.2 ANALYSIS OF CURRENT PRACTICES
The system has also been used as an analysis tool to verify already established procedures for complex situations. In the example shown in Figure 3, the whole procedure has been simulated, including the calibration step, and the answer of the smallest defect to detect has been computed and compared to a calibration signal. The modelling results enabled to better control the test parameters.
|Fig 3: inspection of a fitting|
It can be seen in this example that the software doesn't handle yet CAD inputs. The geometry in the area of interest had to be simplified and defined through a specific interface.
Several developments are under study in order to tackle the major limitations of the simulation tool. The first point addressed concerns the set-up of a direct link to CAD files. It shall enable the reading and handling of structures as defined in the various CAD environments used within Aerospatiale Matra.
The second action deals with the simulation of ultrasonic waves in composite material. Indeed, composite parts represent an important amount of current and future structures, and a major challenge for improving NDT methodology.
Aerospatiale Matra CCR and CEA have launched these developments within the frame of a running Brite project. Aerospatiale Matra CCR is currently widening its investigation of modelling tools towards eddy current and radiography methods.
The current evaluations are mainly focused on the precision of the model for different part geometry, material and defect characteristics. The positive results shall allow a more extensive use of these tools in the future; the possibility to calculate a probability of detection for a given procedure out of a simulated database would mean important gains for NDT. The activity in the field of modelling being quite high currently, most of the questions should be answered more quickly than expected.
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