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NDT in Aerospace - State of Art G. Tober, D. Schiller, EADS Airbus GmbH, Bremen, Germany Contact

0. Summary

NDT plays a major role in all life phases of an aircraft structure in order to ensure the defined quality level in components production and also of structural integrity in the service phase. Even the requirement of increasing economic efficiency in the operation of commercial aircraft at an acceptable price level for a new acquisition can be met by means of specially developed NDT inspection procedures.

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.

1. Introduction

In the aircraft industry, development activities are characterized by the striving for the highest possible operational safety and economic efficiency. From civil transport aircraft structure are expected:
• High operational safety (structural integrity)
• Suitable performance
• Long service lives (20 to 30 years and up to 60.000 flights)
• Long maintenance intervals

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:

• Optimized dimensioning
• Use of new materials and constructions
• Maximum exploitation of materials

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.

2. Task of Non-Destructive Testing Technology

The reliability of service of a civil passenger aircraft is of great importance. This importance is reflected also in the expense for quality assurance and therefore in the application of non-destructive testing during technology development, production and operation of an aircraft structure.

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

• damage tolerance concept

requires from the non-destructive inspection technology that all defects that are greater than

• the tolerable defect size

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

• will the smallest defects still be detected?
but
• will the tolerable defect size still be detected with a high degree of reliability?

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).

3. Main phases of an aircraft project

The project of major commercial passenger aircraft will have an average term of approx. 50 years, starting with the first ideas and ending with the retiring of the last series production aircraft from service with the operator. We subdivide this period into four phases (figure 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.

Tasks:


• Inspection of test components in order to determine their internal quality
• Follow-up of discontinuities of dynamic load tests
• Assistance with the recognition of discontinuities to enable specific examination with destructive testing methods
• Development and testing of new inspection methods

Objectives:


• Determining suitable non-destructive inspection methods
• What types of discontinuities occur with this technology, and what is their relevance for the practical employment of a component?
• Are there physical effects or methods that might be used as a basis for a non-destructive inspection method?

Applied methods:
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

• Neutron radiography
• X-ray tomography
• Holography / shearography
• Thermography
• Laser ultrasonic
• Ultrasonic phased arrays
• Ultrasonic with air coupling
• Acoustic emission

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.

Tasks:


• Inspection of components produced in manufacturing tests for internal quality
• Monitoring and documentation of dynamic structural compliance demonstration tests
• Tear down for dynamically loaded structures
• Selection of suitable inspection equipment and systems
• Elaboration and agreement of inspection methods for materials and semi-finished products with suppliers

Objectives:


• Definition of suitable inspection methods
  • for inspection of series production components
  • for in-service inspection
  • for semi-finished products at the suppliers'
• determination of reproducibility and reliability of the inspection methods

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.

Tasks:


• Inspection of materials and semi-finished products at the supplier's
• Inspection of safety-related components
• Local qualification of the inspection method at the implementing plant
• Automation of the inspection method
• Training, instruction of the inspection personnel

Objective:


• The inspections are carried out in accordance with non-destructive testing methods that are defined in detailed procedures and allow for the reproducible assurance of the defined quality level.

Applied inspection methods:


• The visual inspection is the most commonly used method.
• Ultrasound inspection, various techniques
  Tubing, panels, forged components at the supplier with pulse echo technique;
  automated method for monolithic CFRP components with US through-transmission, partly manually using pulse echo technique; adhesive bonded components, metal/metal or honeycomb sandwich with US through-transmission; thick welding seams with pulse echo technique.
• Penetrant crack inspection
  all machined components made from aluminium and titanium
• Magnetic-particle inspection (magnaflux)
  All machined components made from ferromagnetic steel
• Tapping test - manually or semi-automated, all sandwich components
• Resonance test (Fokker Bondtester)
  All metal/metal adhesive bonded joints, some sandwich components
• Eddy current inspection
  Electrical conductivity measurement for aluminium alloys to determine the heat treatment condition and to measure the layer thickness; for crack inspection only in special cases, e.g. for cold hardening of drill holes
• X-ray inspection with film and radiography
  Sandwich components, forged components, welding seams

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).

Task:


• Inspection of the defined areas in accordance with the instructions of the NTM
• Implementation of lead fleet inspections on specially selected components and zones

Objectives:
The inspection procedures specified in the Non Destructive Testing Manual (NTM) have been

• qualified and verified on the aircraft
• the applicable inspection equipment, probes and calibration standards are available worldwide
• the instruction must be designed in such a way as to suit the training level of the operators` inspectors

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.

• Eddy current
  Crack inspection on multi-layered rivet joints and hidden corrosion using low frequencies; inspection of surface cracks and hidden corrosion using high frequencies; rotating probes for drill hole inspection; conductivity measurements and paint layer thickness measurements
• Ultrasound
  For crack inspection of thick components, rivet joints and residual thickness measurements
• Resonance test with Fokker Bondtest
  On adhesive joints
• Tapping test
  On sandwich components
• Penetrate crack inspection
  Is not usually employed as the pretreatment on the aircraft can not be carried out within defined parameters
• X-ray inspection
  Is used only where it is inevitable now. The radiation protection requirements no longer allow work in other aircraft areas so that the maintenance activities are severely disturbed.
  Area of application is crack and corrosion inspection on metallic structure as well as water ingress in sandwich components.

Literature, references

1. 7th ECNDT, Copenhagen 1998, H.-J. Schmidt, B. Schmidt-Brandecker, G. Tober Design of Modern Aircraft Structure and the Role of NDI
2. American-European Workshop, Boulder, CO, G. Tober, W.-B. Klemmt How is Reliability Used? - Aerospace (European View Point)
3. 43th ATA NDT Forum, Atlanta, GE 1999, D. Schiller, D. Scherling, G. Wehmann Experience with convential and new NDI-Inspections on fiber reinforced plastic structures.
4. 45^th SPIE`s Meeting San Diego, CA 2000, W. Osten, M. Kalms, W. Jüptner, G. Tober, W.Bisle, D. Scherling, A shearography system for the testing of large scale aircraft components taking into account non-cooperative surfaces.
5. QNDE, Ames, Iowa 2000, W. Bisle, D.Scherling Improved Shearography for Use of Optical Non Cooperation Surface under Daylight Condition.

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