Nondestructive testing methods

How non-destructive testing methods contribute to aviation safety.

11.2015 | Text: Monika Weiner

Text:
Monika Weiner has been working as a science journalist since 1985. The geologist is especially interested in new develop­ments in research and technology, and in their impact on society.

Is it large, bulky or heavy? Engineers at the Fraunhofer Develop­ment Center for X-ray Technology (EZRT) in Fürth are used to dealing with oversized items. In their high-energy hall, which is equipped with an XXL computed tomography (CT) scanner, they examine objects every day that are too large for normal materials testing facilities. This includes such items as entire automobiles before and after crash testing, a T-Rex skull still half covered in sediment, and now, in connection with a feasibility study, even two exhibition pieces: a radial engine and a test engine on loan from MTU Aero Engines’ company museum. The images produced here are impressive, and not just in terms of aesthetics. “Even the 2D X-ray images show an incredible amount of detail. The resolution is high enough to see whether all of the components have been properly installed,” says Fraunhofer researcher Dr. Michael Böhnel. “The full 3D CT scan provides even more precise information. For instance, we can use it to determine the clearance on a fully assembled aircraft engine.”

Detecting defects tenths of millimeters across

“As attractive as the images are, XXL computed tomography is, un­fortunately, not suitable for use in routine production or main­tenance inspections,” explains Dr. Hans-Uwe Baron, Senior Manager, Non-Destructive Testing at MTU. “We don’t want to discover defects only after everything is already assembled. Right now, however, the imaging resolution is still too low for materials testing in the aviation industry.” Baron and his team work on different scales of magnitude. The defects they are looking for measure only several tenths of a milli­meter in size. Finding these tiny defects in large components is the future challenge for the researchers at Fraunhofer EZRT.

Achieving the highest quality standards through the highest precision

Minute hairline fractures or material inhomogeneities that are hardly perceptible with the naked eye can have devastating consequences in the aviation industry, says Baron. “With components that rotate at tens of thousands of revolutions per minute, a failure can cause an aircraft engine to explode.”

For 27 years, the mechanical engineer has been searching for defects both small and mini­scule. This is a constant challenge, as the job of locating defects is becoming ever more demanding. When he began his career, faults measuring 0.8 mm in size were still considered tolerable, while today the bar is set at 0.2 mm. But a lot has also changed in the intervening years, both in the sky and on the ground. Aircraft engines offer better performance today thanks to higher speeds and light­weight construction, and inspection officers also have entire arsenals of ultra-precise high-tech equipment at their disposal. In production and main­tenance operations, X-ray, ultra­sonic, thermo­graphic, magnetic powder, penetration, turbulence and corrosion testing are performed on a routine basis. One thing all of these techniques have in common is that they are “non-destructive”—they leave no trace of stress or damage in the inspected materials. This is particularly crucial in the aviation industry where, for one thing, the components are extremely expensive, so for economic reasons alone, there is much reluctance to destroy them. Another con­sideration is that random sample testing is not sufficient—all components must undergo testing during the production process. Only in this way is it possible to meet the highest quality standards.

Such standards are common at the inter­national level. Organizations such as the European Aviation Safety Agency (EASA) define exactly which components must be inspected, and how, when and by whom the inspection must be performed. All processes are regulated down to the last detail, and inspection officers at the various levels must be certified for the respective processes. “Of course we meet all regulatory require­ments,” says Baron. “At the same time, however, we try to design and, if possible, automate the processes to be as efficient as possible.”

NON-DESTRUCTIVE TESTING

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Diagnostics for penetrant testing.

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Diagnostics for penetrant testing.

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Visual inspection with magnifying glass and surgical light.

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Visual inspection with magnifying glass and surgical light.

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Thermographic testing of a CF6 rotor blade.

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Thermographic testing of a CF6 rotor blade.

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Testing titanium billets on an ultrasonic lab machine using phased array technology.

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Testing titanium billets on an ultrasonic lab machine using phased array technology.

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Eddy-current testing of a fan disk.

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Eddy-current testing of a fan disk.

The right test for each component class

The question of which tests a component must pass before being used in the aviation industry depends on the potential damage its failure could later cause. The designers separate the various air­craft engine components into classes 1 to 3, with the first class including parts whose failure would represent the greatest danger. For instance, a turbine disk, or blisk, that ruptures during flight poses a high risk, since the fragments can damage the wings or the fuselage.

Class 1 parts must therefore be tested more stringently than all the others. The MTU facilities have equip­ment for ultrasonic, X-ray, penetration and turbulence testing. The most laborious tasks are frequently carried out by robots, which can lift heavy parts, transport these parts to the next testing station, or scan their surfaces using the ultrasound probe. “Measure­ment tasks can be automated, as can some of the data analysis, but only test engineers can perform the final inter­pretation. They have the last word,” explains Baron.

Interaktivität

Overview of non-destructive testing methods in the AERO engine industry

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Ultrasonic testing
Penetration testing
Etching technology
Turbulence testing
Magnetic particle testing
Visual inspection
X-ray
X-ray CT
Thermographic inspection

Ultrasonic testing Ultrasonic testing

Penetration testing Penetration testing

Etching technology Etching technology

Turbulence testing Turbulence testing

Video Show as video

Magnetic particle testing Magnetic particle testing

Visual inspection Visual inspection

X-ray X-ray

X-ray CT X-ray CT

Thermographic inspection Thermographic inspection

Foreign body inclusions, pores and cracks are made visible using sound waves: Because the propagation of the soundwaves is affected by the density of the material, it is possible to determine whether or not the material is homogeneous using the signals that pass through it.

Fluorescent dyes make surface pores and cracks visible: Components are consecutively dipped into colorpigmented oil, removed, cleaned, sprayed with developer and illuminated with UV light.

Surfaces are etched using chemicals. The optical changes that occur in this process are material dependent.

Inhomogeneities become visible by altering the induction of an electric field.

Magnetic particles reveal disturbances in the magnetic field: If a metallic component is magnetized, the particles will arrange themselves parallel to the field lines. Cracks in the component will disturb the homogeneity of the field—the particles make this visible.

Inspection officer checks components.

Radiographic scanning of components produces two-dimensional images that make defects within the components visible.

Components are rotated inside an X-ray scanner, and afterwards the radiation is measured. The sum of the scans provides a 3D image. The three-dimensional images provide a look inside the components and make tiny defects visible such as those that can occur when drilling cooling channels into the turbine blades.

Measurement of the heat that disperses within a component: The heat is generated using two flash lamps directed at the component. A camera detects how much heat drains off. Irregularities in the heat drainage indicate poor bonding between the component and the surface coating.

Resolution from 0.4 mm, inspection depth up to 20 cm.

Defects from 0.2 mm are identifiable.

Resolution from 0.2 mm.

Resolution from 0.2 mm.

Resolution of 0.2 mm.

Resolution from 0.2 mm without magnifying lens.

Resolution from 0.2 mm, dependent on wall thickness.

Resolution from 0.1 mm, dependent on wall thickness.

Resolution from 0.5 mm, dependent on distance to surface.

Inspection for freedom from defects in base material.

Crack testing.

Locating chemical inhomogeneities.

Crack testing.

Testing of magnetizable steel components.

Testing of all components.

Locating foreign body inclusions and pores.

Measurement of wall thicknesses and internal geometries, such as rear panel tapping holes that can occur during laser drilling.

Inspection of bonding between metal and ceramic. This is a relatively new technique.

Component class 1 disks and blisks.

All component classes.

Component class 1 disks and blisks.

Drilling holes in component class 1 and 2 disks and casings.

All magnetizable components. Used especially in the maintenance of older engines with steel components of all component classes.

All component classes.

Cast components, welding seams, turbine blades with cooling channels in component classes 1 and 2.

Turbine blades with cooling channels and composite materials in component classes 1 and 2.

Heat insulation layers on casings and turbine blades as well as abradable linings in component classes 1 and 2.

Overview of all testing methods.

The require­ments for the second component class, which includes turbine blades and casings, aren’t quite as stringent. They routinely undergo penetration and X-ray testing, and in some cases even X-ray CT scanning. Engineers have largely automated the latter in recent years by having a robot trans­port the components into an X-ray chamber and, afterwards, sort out all of the components that deviate from the standard. Only these components need be reviewed by the inspection officer. A further process step entails the thermo­graphic inspection of turbine blades. This relatively new procedure is used to determine whether the ceramic coatings have adhered properly and whether the cooling channels are open.

The third class includes veneers, fasteners, and mounting fixtures that are not safety critical and that can be replaced during any routine check if failure is suspected. For these components, it is sufficient to conduct penetration testing and a traditional visual inspection—the latter, incidentally, is a require­ment for all class 2 and 3 components. The visual inspection is the only process that hasn’t yet been standardized, as the method in which it is carried out depends on the individual inspector. “It’s here that we expect the most innovation in the future, such as the use of digital inspection plans that specify the tasks and assist with documentation,” says Baron. The Industry 4.0 era has taken hold here, as well. Yet a truly radical level of automation and optimization, such as is common, for instance, in the auto­motive industry, will not soon materialize in aviation due to the industry’s high standards.

Insight Aviation industry and medicine both rely on extremely stringent safety and quality standards. It’s no surprise, then, that both the medical and materials inspection fields use similar imaging methods, such as X-rays, ultrasound and computed tomography. Images of a nine cylinder radial aircraft engine and a jet aircraft engine, both on loan from the MTU Museum, were created inside the XXL computed tomography scanner at the Fraunhofer Development Center for X-Ray Technology.

Zero defects? It’s only a question of perspective

So where do we go from here? “The trend toward ever more stringent fault tolerance specifi­cations will continue,” says Baron. At the same time, auto­mation is gaining ground, with ever greater speed and precision. Steffen Bessert at the Fraunhofer Institute for Nondestructive Testing (IZFP) offers his take on the current state of research: “Development trends are moving in the direction of thermo­graphy with active stimulation, as this is contact­less and can be automated. Minute defects are also increasingly being detected using CT technology, which is being correlated ever more with CAD design drawings. This will allow inspectors and designers to locate and classify defects with even greater precision in the future.”

And will there come a time when defects have been completely eliminated? “For technological reasons, there will likely never be such a thing as absolutely zero defects,” says Baron. “However, our ability to reliably pinpoint increasingly minute defects will continue to grow.”

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