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NDT in Aerospace - State of ArtG. Tober, D. Schiller, EADS Airbus GmbH, Bremen, Germany
Experience has shown that the number of NDT inspections in the aeronautic industry has increased constantly. This trend will also continue in the third millennium, as securing of the operational safety of old aircraft and the manufacture of larger and larger aircraft can only be realized by using selected and appropriate inspection techniques.
All these aeronautic requirements have led to the development of branch-specific inspection techniques for extremely lightweight structures with selected new materials and material combinations. These are demanding economic inspection techniques with a coherent inspection philosophy and proven reliability.
This report explains NDT methods which are currently in use.
It is thus the aim of modern design philosophies to reconcile these contrasting features to the highest possible degree in the final product, i.e. the aircraft, first priority being the safety in any case. The fulfillment of these aims requires an extremely light weight construction with:
Currently, the damage tolerance concept is best suited to fulfill these requirements (1). This concept is based on the realistic assumption that any structure is faulty and defects are purposefully tolerated if they do not exceed certain defined dimensions.
The tolerable defect size depends on the materials used and the construction of the structure. For all safety-related components, such defect sizes are determined by means of fatigue strength tests of the individual components or a complete aircraft section, respectively. The results from such tests are then used as a basis to determine the time for the first inspection and the intervals for subsequent inspections. Any defect which exceeds these tolerance limits must be reliably detectable by non-destructive testing methods.
This makes the close integration of aircraft design, maintenance philosophy and non destructive testing clear.
The applied testing methods essentially depend on the used materials and structures.
The structure of a modern passenger aircraft consists at about 65% of the classical lightweight material aluminium. In addition to these aluminium alloys, also titanium alloys are used in a smaller percentage. Fibre reinforced plastics (GFRP, CFRP) are used in an increasing tendency, also for safety relevant components like flaps, vertical and horizontal stabilizers. Figure 1 shows the weight percentages of the materials of an Airbus A320 structure.
|Fig 1: Weight percentages of the materials of an A320|
All the materials are bonded, riveted, bolted or also welded together by aircraft specific construction methods whereby further materials are used for the joining elements.
The qualification quality of the applied testing procedures is essentially determined by the used safety concept.
The design philosophy in accordance with the
requires from the non-destructive inspection technology that all defects that are greater than
can be detected.
Thus, inspection methods must be defined in procedures that ensure that the components can be inspected in a reproducible fashion and that the measuring results can be evaluated in such a way that this defect size can be found. Within the scope of the damage tolerance concept, "finding" does not mean
For safety-related components, the requirement is that the probability of detection of the tolerable defect size is at least 90%. Demonstration thereof shall be provided with a statistical confidence level of 95%. This general requirement for NDT methods can only be met if they are integrated throughout the entire life cycle of an aircraft project and fulfills the tasks called for during the individual phases (2).
|Fig 2: Phases of a commercial aircraft project|
The tasks and objectives of the aircraft project are different for the individual phases. Accordingly, the tasks and objectives of the non-destructive testing technology vary as well. The activities and the methods employed are different particularly with respect to their implementation and their statistical assurance.
3.1 Technology / predevelopment
Within the scope of this phase, new materials, processes and constructions are developed for the project and examined with respect to their suitability.
The non-destructive testing activities are also determined by the project objective.
We are rather open to all options in this phase. In addition to the known and proven methods, new procedures are employed as well if they are promising with respect to providing comprehensive new information on the new technology.
It is state of the art practice for us to monitor and document load tests on components made from fiber reinforced plastics and acoustic emission methods. However, even highly
are examined for their degree of indicativeness.
As we do not always dispose of the respective facilities ourselves, we seek the cooperation with institutes where such facilities are available (4).
3.2 Development / design
In this phase, the materials, processes and constructions are further developed and defined for the project. Within the scope of the design and dimensioning process, the necessary quality levels (tolerable defect size) are defined as well.
The tasks and objectives of the non-destructive testing technology are defined, too, in accordance with the project objectives.
Applied inspection methods:
In accordance with the objectives, inspection methods that are still at the lab stage and which are not expected to reach maturity for employment throughout the manufacturing process or under in-service conditions or if it can be predicted that they will not be suitable are employed only on a limited basis.
Application and selection of the methods are carried out with regard to the future potential applications throughout manufacturing or in service.
|Fig 3: Shearography applied on the material GLARE|
Figure 3 shows a typical case of a change-over phase. Here a new inspection method, shearography, is tested on the material GLARE. Both the material and the inspection method have a high potential and probably will be applied on the next aircraft project (A3XX).
With inspection methods to be used throughout series production, automated inspection should be possible, and expensive facilities may prove economical in the long run. With inspection methods for in-service conditions, it is important to ensure that the specified inspection equipment is commercially available worldwide. The inspection procedures must describe the implementation of the tests and the interpretation of the results unambiguously so that the operators' inspectors can correctly apply them despite the radically different levels of training worldwide.
3.3 Series production
During this phase, the main objective of the project is to manufacture the components in keeping with the schedule and in an economically efficient way. The tasks and objectives of the non-destructive testing technologies reflect this objective.
Applied inspection methods:
3.4 Ultilization / operators
The objective of this project phase is to achieve structure integrity during operation. Thus, the NDT methods are also focused on the delivered aircraft (3).
The inspection procedures specified in the Non Destructive Testing Manual (NTM) have been
The instructions to be used shall be standardized to as far a degree as possible.
Applied inspection methods:
Aside from the visual inspection, the various techniques of the eddy current inspection are the most commonly used methods in our NTM today.
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