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Sensing & Measurement

Non destructive evaluation

Coherent optical techniques such as shearography offer new inspection options to industrial manufacturers.

From oemagazine November 2002
31 November 2002, SPIE Newsroom. DOI: 10.1117/2.5200211.0004

Nondestructive evaluation (NDE) is a critical technology for improving the quality of a product in a cost-conscious production environment. NDE detects deviations of the required behavior of a material or a component to find bad parts, without altering or damaging the test piece. Using these techniques to improve the production process requires characterization of the faults and their influence on the component. This task is dependent on the material under test and on the complexity of the component, two parameters that have increasing importance in modern manufacturing.

Engineers typically perform NDE by "loading" the object with energy and measuring the reaction of the component under test; the load can consist of ultrasonic waves (ultrasound testing), x-rays (x-ray testing), penetrating waves, or even particles (neutron testing).1 In each case, the measured result is a change in the object—the behavior of the component material compared to a known sound material. Essentially, the test defines changes in the transfer behavior. The reactions of the sound material and the changed material have to be investigated before the test by an experimental study, by a computer simulation (for example, with finite element method, or FEM), or by a series of control tests.

Evaluation of the result is a key task. First, the defect has to be detected, which may occur by visual inspection or by automation. The application of neural networks and the knowledge-based method have been proven to be the most promising routes to automation of this process. After the detection of the defect, the system must make a decision about the use of the tested component.

optical test methods

Coherent optical methods are somewhat new to NDE. In these methods, engineers apply a mechanical or thermal load to the component under test and measure the resultant deformation. This implies the assumption that variations in the component under test with respect to a sound one induce local or global variations in strength or stiffness, which may influence the operational behavior.

The fundamental idea in a nondestructive test by means of coherent optical methods is to image the test object coherently, load the object in order to induce a deformation, and compare the image of the deformed object with that of the reference image. Holographic interferometry, for example, was used for the first industrial tire tester.2 Electronic speckle pattern interferometry (ESPI), which is better adapted to rough environments, is an image-plane holography but can be used with CCD cameras as light-sensitive material (detectors).

  

Figure 1. Similar to holography, shearography uses a sheared object wave for the reference wave to detect deformation (below) rather than displacement.

Shearography is another useful optically based NDE technology.3,4 This phase-sensitive method is comparable to holography, except that the reference wave is not an information-free wave but a sheared object wave (see figure 1). This modification is the source of shearography's advantage—the fringes are not related to the displacement of the object or parts of it but to the deformation:

 

In other words, the interference phase δ(P,P') is proportional to the partial first derivative of the displacement scaled by the sensitivity vectors eB and eQ and the shear Δx.5

NDE in aircraft manufacturing

Coherent optical NDE methods are particularly well suited to the testing of aircraft components. Shearography, in particular, will become increasingly important in the future since it is difficult to test the carbon-fiber reinforced plastics (CFRP) used in planes with conventional methods.6 As a result, German aircraft manufacturer Airbus collaborated with our group to develop a test system that may become standard in the future.

The first step was the selection of the load. In general, the operational load (the load the test piece experiences while in use, e.g., mechanical, thermal, etc.) seems to be the best choice for NDE since it only indicates severe defects related to component behavior during use. Very often, however, the complex operational load cannot be defined with the necessary accuracy. Furthermore, classification companies ask for the indication of any defect according to standard methods. To identify defects, it is necessary to choose the best adapted load for the given problem out of a number of deformation-inducing loads such as static mechanical loading (tensile, bending, or torsion); static mechanical loading by pressure; dynamic mechanical loading by impacts; dynamic load by vibrations; dynamic load by acoustic waves; thermal stressing; or thermal waves.

Figure 2. A shearography-based NDE system (upper right) for aircraft-component testing (upper left) produces shearograms (lower left) that undergo fringe processing to give defect indication (lower right).

For the aircraft test system, thermal loading by heat radiation proved to give the best results, considering the different possible environments for the test (see figure 2). We studied the reaction of the different defects experimentally using specially prepared components. This process involved a complex, varied investigation in the resulting shearographic nondestructive imaging system.

Figure 3. Shearography system clearly images flaws (left) and repairs (right).

After we concluded the initial investigations, our group developed a shearographic setup based on this data. The system consists of a CCD camera, a Michelson-interferometer-based shear component, and all necessary optics. We modify the original shearograms with fringe-processor software so that the system yields a defect indication that can be evaluated even by untrained personnel. This robust system was operated in an extremely harsh environment with good results (see figure 3).

In this type of system, the detected deformations are related to special defects by the experience of the technician operating the system. This leads to one of the main problems in all NDE applications: solving the inverse problem. The detected signal represents the reaction of a defect at any point or region inside the material or component. Both the type and the coordinates of the defect are unknown. Furthermore, different kinds of defects may sum up to the same signal at the surface, since the surface reaction is influenced by the integral over all of the internal stresses.

   

 

Figure 4. Virtual gauge at the interferogram (a) indicates the location of the on-fly strain measurement εy (b) taken by laser grating extensometer (c). The data also shows full-field in-plane displacement at any chosen moment during loading cycle (d).

A promising approach to solve more-complex tasks of NDE is the active change of test conditions. The procedure starts with arbitrary setup conditions that are usually based on past tests or special investigations of the problem. The evaluation of the resulting fringe pattern leads to a hypothesis about the defect. A simulation based on this hypothesis is compared to the measurement. If the two results match, it is fair to assume the simulation accurately describes the test—and more particularly, the defect. If they do not match, the procedure is repeated. 

new developments

Current trends in development of mass production of elements and machine parts based on new technologies such as laser processing and novel materials such as composites or nanomaterials, pose new, enhanced requirements for the industrial systems of full-field displacement and strain analysis. The quality of experimental data has to be sufficient for use in numerical analysis (for example, FEM) or in complicated modeling of fracture mechanics, fatigue processes, or residual stress distribution.

Optical methods fulfill the data quality requirements, but industrial applications place additional demands on sensitive optical equipment. Such optical systems must feature low sensitivity to environmental changes (especially vibrations), portability, minimal demand for user intervention, ease of use, compatibility with a loading machine, and automated data-acquisition capabilities.

There are several attempts to introduce various commercial instruments that meet industrial requirements. Most of them are based on ESPI or shearography based on ESPI. These commercial systems are extremely useful for solving a variety of problems, but they suffer from a high level of noise in the output data; in addition, decorrelation problems restrict the available range of continuous measurement. Finally, the available systems are not fully integrated with the loading machine.

Recently, researchers at the Institute of Micromechanics and Photonics (Warsaw University of Technology; Warsaw, Poland) and the Institute of Terratechnology proposed a novel type of automated laser-extensometer (LES) based on grating (Moiré) interferometry. Grating interferometry (GI) uses a high-frequency reflection-phase grating fixed to the plane surface of the object under load. Although GI requires this additional modification of the object surface, it has the advantage of producing interferograms with high interference contrast and low noise, while offering wide strain range, easy alignment and operation, and the ability to perform complete strain analysis in elastic and plastic regions.

The LES is fully integrated with a loading machine and performs several functional tasks, including on-line measurement of the local strain value at the point selected by the operator within the actual field of view. This data can be used to control the parameters of the loading machine. The machine also performs on-line acquisition of a series of interferograms according to an a priori chosen sequence of measurements (integration with the loading scheme—e.g., monotonic, cyclic, random); off-line analysis of the series of interferograms according to the selected path of the analysis process; and a wide range of visualization procedures on the base of interferograms and calculated results (including films showing changes in the displacement and strain fields).

The automatic acquisition and analysis of long sequences of measurements is especially useful for fatigue and fracture mechanics tests. Due to its design, LES is insensitive to vibrations and provides in-plane displacement maps in the x and y directions with 5-nm sensitivity and 0.01-mm spatial resolution.

Coherent optical testing, especially shearographic nondestructive testing, is a versatile tool for nondestructive evaluation because the method can evaluate stress-related deformations. In this sense, shearography matches the needs of testing modern inhomogeneous materials and compounds. In some cases—for example, tire testing—this method has been proven to be the only way of solving the problem. The developed systems match all needs for test in harsh environments performed by people untrained in coherent optics. The challenge that remains is to convince the classification associations that shearography is a competitive method with more conventional approaches, such as ultrasound or x-ray testing, which quite often fail with the new materials. oe 

References

1. W. Jüptner, W. Stadler, et al., Investigations in Non-Destructive Testing of GRP Tubes and Project Management of the Experimental Studies. BMVg Report Nr. T/RF52/RF520/42016, Bonn 1977 (German).

2. G. Brown, IEEE Rubber Plast. Ind. Conf., Akron, Ohio, June 1970.

3. Y. Hung, Opt. Eng. 21, pp. 391-395, (1982).

4. W. Osten, Kalms M., W. Jüptner, "Proc SPIE 3745, 1999, pp. 244-256.

5. Th. Kreis, "Holographic Interferometry," Akademie Verlag, Berlin.

6. W. Jüptner, Th. Bischof, "Nondestructive testing of fibre-reinforced plastics," Measurement 12, Elsevier (1993).

7. M. Kujawinska, L. Salbut, Proc. Fringe'01, Elsevier, 86-93 (2001).


photonics in 3-D

Besides their interest in non-destructive evaluation, Werner Jüptner and Malgorzata Kujawinska have another thing in common: both hold the distinction of being named to the International Order of Holoknights, founded by Hans Rottenkolber of Amerang, Germany (see oemagazine, June 2002, page 31). The honor is bestowed upon those who are deeply involved in optical metrology/holographic activities and for friendly support for holographic/optics community.

From the beginning of her study in optics, Kujawinska was fascinated by holography. Although she now works in the general field of optical metrology, she still tries to spend some of her time on holography. At the moment, for example, she is developing a model of holographic television based on digital holography principles.

Her fascination with holography shows itself in other projects as well. She and her doctoral students are investigating various aspects of 3-D imaging, such as the measurement of 3-D objects, monitoring their movements, morphing (for multimedia purposes), as well as interferometric tomography for determination of 3-D distribution of refractive index.

Currently, Kujawinska serves as head of the optical engineering division and deputy dean for scientific affairs at the Mechatronics Department, Warsaw University of Technology (Warsaw, Poland). In 1997, the Polish government honored Kujawinska with the title of National Professor?the highest scientific title in Poland. In addition, she is currently secretary of SPIE and an SPIE Fellow.

Despite all her professional honors, it is teaching that matters most to her. "My greatest achievement at the university is that a lot of young people want to work with me," Kujawinska says modestly. "I now have eight wonderful and smart doctoral students."

-Laurie Ann Toupin


necessity breeds great optoelectrical engineers

Many engineers make a name for themselves out of necessity. Bosses or coworkers ask, "Can you come up with a solution for this?" And they do.

Werner Jüptner, a professor at Bremer Institut für Angewandte Strahltechnik (Bremen, Germany) and the originator of digital holography, is no different. He began his career in the welding industry. One of the biggest problems in welding is quality control, Jüptner says. "Because conventional quality-control technologies were not reliable, my colleagues asked me to develop a new method based on holographic interferometry." So he did.

A friend familiar with his work suggested Jüptner apply for a research project that explored the use of holographic interferometry for the quality control of nuclear power-plant components, specifically evaluating defects in the pressure vessels of such plants. "The task was very exciting because we were doing such pioneering work in the area," says Jüptner.

As his reputation grew, he was asked to give a talk on opportunities of coherent optics for robotic sensors. While preparing for the talk, Jüptner looked for an idea for a 3-D sensor based on one CCD camera. "I found that the solution was well-known, in principle, by the invention of holography 47 years before by Gabor," he says. This request led to the development of digital holography. "This has to be my most far-reaching invention," he adds.

Jüptner is also proud of his affiliation with SPIE: He is a Fellow and has been a symposium and conference chair (see oemagazine, June 2002, page 30). "Scientists need to communicate with each other," he says. "As a member, I am happy to push the community in coherent optics metrology."

As for the future of optoelectronics in Germany, Jüptner is uncertain. "I would be a rich man if I could predict that!" he jokes. "At the moment we suffer an economic crisis in the whole world. However, Germany is excellently prepared in industrial fields?which I call middle technologies?in contrast to high tech. Therefore, I see economic growth in the future of my country, keeping us among the five strongest economies in the world."

-Laurie Ann Toupin


Werner Jüptner
Werner Jüptner is a professor at Bremer Universitat (Bremen, Germany) and executive director at the Bremer Institut für Angewandte Strahltechnik, Bremen, Germany. 
Malgorzata Kujawinska
Malgorzata Kujawinska is a professor at the Warsaw University of Technology, Warsaw, Poland.