Airbus defines a common measurement approach to structural testing
Structural engineers working on the development of modern commercial aircraft can benefit from sharing information, especially when they are located in a number of different countries.
Europe’s leading aircraft manufacturer, Airbus, is creating trans-European standards in structural testing as part of its intention to stay at the forefront of the technically demanding industry. By developing a consistent approach across its development program, Airbus will benefit from improved co-operation with teams of highly qualified engineers based in various locations able to share data and information to provide optimum designs.
The structural development of an Airbus aircraft is distributed over Europe. Each location has a separate responsibility for developing different structural and internal components. For example, the UK works on the wings and landing gear, France develops the forward fuselage and engine pylons, Germany the wing flaps and slats, fuselage parts and the vertical tail plane and Spain looks after the horizontal tail plane. Each of these components is subjected to a wide array of tests to ensure that the proposed design can meet all relevant safety criteria.
Comments Dr Karlheinz Stupperich, managing Airbus Structures Test Control Systems & Data Acquisition area: “The term trans-European standard means that we try to harmonise where possible and justified by business considerations. Within Airbus Structures Test we have chosen the HBM MGCplus system as the standard for data acquisition for static and fatigue structural testing.” By moving towards a common system, Airbus says there is a greater ability to share information and knowledge across sites.
Airbus now aims to replace its existing systems with new HBM equipment. A prime reason for the choice was the reliability of the hardware although there were a number of other aspects that appealed to Airbus Structures Test.
In particular, Dr Stupperich mentions the 24 bit analogue to digital (A to D) converters that give better signal resolution for every channel. The converters allow integration of the incoming signal and reduce noise as well as allowing the system to measure over the complete amplifier range. This makes tests faster to set up and ensures better than 1 microstrain resolution even at signal levels of ±160,000 microstrain that are unlikely to occur even in modern materials.
HBM’s equipment also enables the sampling speed to be changed and allows the samples to be used for triggers. “This helps us to reduce the amount of data we capture since we can cache the data around the trigger and thus only get the basic information that we need “, notes Dr Stupperich. He adds: “This is a very useful feature.”
To further improve the effectiveness of the equipment to Airbus, HBM’s engineers worked closely to develop a number of extensions to the standard software to meet Airbus ’ specifications. Says Dr Stupperich: “We are quite happy with the solution, although we want to develop some more extensions because of the useful feedback gained during the initial runs”. Hamburg uses over 1,000 channels to gather data from its static and dynamic structural tests and needed specific software extensions written for large channel usage.
The HBM system will be used at all Airbus internal Structures Tests. Dr Stupperich says; “We felt it would make all testing much easier and cheaper since the engineers need to be familiar with only one set of hardware and software. In addition, they can take parts of the system on one site to cover a temporary shortage at another site.”
Hamburg has three halls for different tests on aircraft components. In each experiment, hydraulic rigs are used to induce stress on the airframe that is measured using strain gauges. Most pieces are eventually tested to destruction even though the test results may exceed regulations set by the authorities, such as the Civil Aviation Authority.
Tests at Airbus can take between one hour and three years depending on the component under test and the aim of the examination. Each test looks at different materials and methods of construction to arrive at an optimum solution with every new project testing the component to the limits to ensure maximum safety.
Structural testing covers static, fatigue and dynamic testing. At the Hamburg facility measurements during structural tests are normally performed as static measurements where the loads to a specimen are increased step by step to a predefined level and at each step a measurement of all applied channels is taken. This is also done for fatigue testing where static measurements are taken after a block of simulated flights to see the influence on the specimen. It is rare for continuous measurements to be taken. HBM’s system has the advantage that the same hard- and software can be used for both static and some dynamic measurements at Airbus.
If a fracture happens then it is necessary to determine the exact point of breaking. Dr Stupperich remarks: “Nearly everything is broken to give interesting answers to the design office”. Occasionally damage is artificially introduced into a section to find the damage tolerance.
Normally the load at breaking point is not measured with a measurement device. Notes Dr Stupperich: “We know exactly when it breaks because our control systems will notice and shutdown the test. There is no need to take continuous measurements since we simply generate lots of useless data”. When conducting ultimate load tests the component is subjected to a force of 1.5 times the operational load. The load is then increased incrementally with measurements at each step until the specimen breaks.
Repeatability in configuring the equipment for different tests was a prime requirement and this was a principal reason for buying HBM’s system. Notes Dr Stupperich: “There needs to be a special set-up for the configuration and extension that allows repeatability when setting up different experiments with the same operational parameters where possibly different hardware might be used”.
Airbus needed to store configuration and measurement data together and the principal problem was in the new measurement systems. Airbus required an open and re-configurable system because every experiment operates differently and HBM’s ability to supply this was a key factor in winning the contract.
Airbus also required a solution to its calibration concerns. Normally equipment used in stress testing is returned to a certified laboratory for annual, or sometimes more frequent, calibration. Test centres that use a large number of channels can find it expensive and disruptive to schedules to send all of the equipment to a central location for these checks. They normally have their own calibration laboratories on site. To overcome this challenge, HBM has worked closely with Airbus to develop a calibration device that is planned for delivery later this year.