Accurately measuring ultraslow motion

Jan. 26, 2006
Creep testers advance to the digital age.

John Churchill
Philip Barber
Virtech Inc.
Nashua, N.H.

This ATS creep-test machine features a Virtech extensometer (inset) with Heidenhain Specto photoelectric gauge. Replacing analog LVDTs with digital gauges permits automated testing along with higher reliability and accuracy.

Heidenhain Specto linear gauges photoelectrically scan an incremental scale on a glass substrate. They are insensitive to shock, vibration, and temperature changes and feature ±1- m accuracy.

When machine parts must withstand extreme temperatures — 2,000°F in a jet engine, for example — it is an understatement to say that they need to be made of remarkably durable materials. Engineers must reliably predict how components react in such critical applications. This is where highly sophisticated testing equipment comes into play.

For instance, parts subject to stress at high temperatures for long periods will undergo a slow and permanent deformation called creep. Creep tests are conducted at a constant high temperature and load that ultimately determine a material's creep rate. They are especially common in aircraft applications. It lets engineers, for example, determine how much a turbine blade in the hot section of a jet engine will elongate over time. With minimal clearances and extremely tight tolerances, creep properties tell when and if the blade will grow too long and lead to potentially catastrophic impact with the housing.

Technicians use extensometers to measure minute deformations in test specimens. They are located inside a creep test frame, along with an accompanying control system. Reliability of the test instrumentation is paramount because a single-specimen creep test can last weeks, months, or even years. So system failures cannot be tolerated, especially far into a test. And it is imperative that the equipment accurately measures the timing and magnitude of creep when the part begins to elongate and reach the end of its useful life. The system's embedded measurement components are of critical importance here.

Stringent part standards and certification requirements are in place for just these reasons. National and international bodies such as ASTM and ISO, as well as many aircraft, engine, and automotive manufacturers all require that parts used in extreme environments meet established standards for creep performance. For instance, ASTM E-83 is an extensometer grading specification and ASTM E-139 covers creep testing.

When the market for jet engines began to boom after World War II, so did the manufacture of creep-test frames. Early frames were simply a gantry with a seesaw-type lever arm at the top. Test specimens attached to a hinged rod on one end of the arm, and to the frame. The other end of the lever arm held metal weights that produced a tension load. Arms were set up with a 10:1 or 20:1 ratio, providing the required tension with relatively small weights. A small tube furnace wrapped around the specimen along with extensometer rods and, usually, a dial indicator to measure creep displacement.

Manual testing was the norm, with technicians checking temperatures and other variables regularly, often every half hour around the clock. As one might expect, the setup was labor intensive and prone to error.

The overall design of creep-test frames is still much the same today, while the controls and internal-measurement systems have substantially evolved. LVDTs within extensometers are now a common measurement choice.

LVDTs (linear-variable-differential transformers) are small, analog devices that measure position based on magnetic transfer. An LVDT transducer consists of a coil former or bobbin onto which three coils are wound. The moving element of an LVDT, commonly called the core, is a separate cylinder of magnetically permeable material, usually slightly smaller than the internal diameter of the coils.

The first coil, the primary, is excited with ac current, normally in the region of 1 to 10 kHz at 0.5 to 10V rms. The other two coils, the secondaries, lie on either side of the primary and are symmetrically wound in series but in opposite directions. The primary establishes a magnetic flux that couples through the core to the secondaries. When the core is in the central linear position, each secondary coil induces an equal voltage and the outputs cancel each other out. Displacing the core produces more magnetic flux in one secondary than the other, generating an imbalance output voltage that is a direct function of armature displacement. Electronics combine the information on the phase and magnitude of the output and convert the signals to position data.

Unlike digital gauges, this type of device has a number of analogbased error terms that vary with time and temperature. As a result, such transducers require calibration at setup and periodically thereafter — usually once or twice each year in an extensometer application. Because they are somewhat temperature sensitive, location in the test rig can be critical. They can act much like a thermometer because measurement readings vary with ambient temperature changes. And in creep-test applications, measurement range is limited to 2 to 5 mm. So most companies that use them must stock many different lengths of LVDTs.

Within the last few years, extensometer designs have under-gone a dramatic shift as miniature digital gauges became available. Their use in creep testers offers numerous benefits such as higher accuracy, temperature resilience, and standard reference marks.

For instance, Virtech has turned to the Specto and Metro gauges from Heidenhain Inc., Schaum-burg, Ill., for use in its creep-testing system. The gauges work by photoelectrically scanning an extremely fine incremental scale on substrates of glass or glass ceramic. This permits large measuring ranges, makes them insensitive to shock and vibration, and changes in atmospheric pressure or relative humidity have no influence on accuracy.

It also gives them a defined thermal behavior. Because temperature variations during measurement can result in measurement errors, the units use special materials with low coefficients of thermal expansion. This makes it possible to guarantee high measuring accuracy over a relatively large temperature range.

Graduations are scanned without mechanical contact or wear. Light passes through a reticle and over the scale onto photo-voltaic cells. These produce sinusoidal output signals with a small signal period. Interpolation by system electronics generates measuring steps in the nanometer range. The scanning technique, together with fine graduation lines and high edge definition, ensure quality output signals and small position error within one signal period.

Photoelectric scanning results in an incremental-counting measurement, so ascertaining position requires an absolute reference. A reference mark on the length gauges establishes a defined datum. It is permanently associated with exactly one measuring step, regardless of direction or traverse velocity.

Replacing LVDTs with digital length gauges in creep testers can, in many cases, significantly reduce operator workload. That's because it is no longer necessary to recalibrate or reset any correction curves usually required during tests with an LVDT. Eliminating recalibration with an optical-gauge system ensures a more reliable setup less prone to human error. This is a real advantage in tests where one or more LVDT recalibrations would have been required. In addition, the redesign hangs the miniature gauge from the extensometer for easy access and installation. The gauges can be installed or plugged in at any time depending on the application.

Heidenhain's digital Metro gauge has a ceramic substrate with a near-zero coefficient of thermal expansion. Tight graduations and interpolations result in higher resolutions and precision than previously possible in a creep test. Rated accuracy is ±0.2 m. The company's digital Specto gauge has a glass substrate in several output configurations and is available at a lower cost. Accuracy of the length gauge is ±1 m.

Most testing focuses on materials used in the hot sections of engines, although many types of general-purpose materials such as metals, ceramics, and plastics also undergo rigorous testing. Materials such as ceramics have a low creep and require high-accuracy measurements, while plastics have high creep and allow a bit more leeway. With optical gauges, both ends of the materials creep spectrum are now easy to measure. This type of technology allows a longer range of measurement, from 12 up to 100 mm on some gauges, thereby reducing the variety of gauges required. For instance, the LVDTs Virtech previously used had strokes of about 2.5 to 5 mm. The newer optical gauges have a 15-mm stroke.

Incorporating digital-measurement devices with other control elements has resulted in an unmatched, automated system. Virtech calls it the WIN CCS Creep/Stress Rupture Control System. Applied Test Systems (ATS) in Butler, Pa., currently manufactures creep-test frames and uses the Virtech system for automated control.

Pratt & Whitney became the first to use these digital creep test systems in their jet-engine manufacturing process. Many other manufacturers are now part of this trend, as well as some high-quality independent labs such as Dirats Laboratories, BodyCote Testing, Metcut Re-search, Stavely Services Materials Testing, and U.S. Inspections.

Today in the U.S., only about 20% of creep testers are equipped with automated digital-control systems. With hundreds of test facilities facing increasingly stringent certification requirements, digital technology is taking creep testing to a more-accurate level.

Applied Test Systems,
(724) 283-1212,
Heidenhain Corp.,
(847) 490-1191,
Virtech Inc.,
(603) 883-8118

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