Measure crack Length at high temperature

[student85]

Hi, i'm a electrical student but recently i have been task to do a assignment which is material science related. I don't have any background for that, so i have quite a number of problem for it.

 

My assignment is to measure crack length ( micro-crack length / very small crack length) for aluminum at HIGH TEMPERATURE.

 

I have read up some methods example like using ultrasonic, DC potential, x -ray and so on. But the problem is i do not know which method is effective and simple. Is that any suggestion or information for me to read up. thanks

[studiot]

Ultrasonic testing is the cheapest. What is your budget?

What is the access to the specimen.

How high is 'high temperature' ?

 

 

[student85]

Actually, have not been brief with the budget matter. i was just being told to look for ways to measure micro-crack length. But i think most probably, i think they will ask me to use the equipments which are available in the lab.

 

By the way, what do u mean the access to the specimen.

 

I will be testing the sample at 100 to 110 degree Celsius.

 

thanks

[studiot]

By the way, what do u mean the access to the specimen.

 

 

Where is it? Where are the cracks in it? How big is it? Is it in the lab or part of an aircraft flying at MACH 2 or what?

 

I will be testing the sample at 100 to 110 degree Celsius.

 

 

Ultrasonic testing equiment relies on an acoustic coupling medium to the specimen. At this temperature you will have to look for a new one, perhaps a high temperature industrial grease.

[louis wu]

Are the cracks embedded or surface breaking?

[student85]

Sorry for not updating the post. There is changes for the project. Now, i just need to find way to measure micro- crack on the sample.

 

studio-> the micro-crack is on a aluminum rod sample. it is about length 310 mm, diameter 25 mm

 

louis wu-> i need to measure both surface and embedded micro-crack on the aluminum sample.

 

Currently, i only can think of X-ray methods. But, there is only little material regarding this technical on the websites. Is there any suggestion for me or any review for me to read up?

 

Thanks

[studiot]

If you have access to X-ray equipment I'm sure you have access to proper computers so that you can post a serious and considered description of the problem.

 

Until then you are wasting my time and yours.

[student85]

Basically i'm still at the research stage( might not be using x-ray methods), and i have yet to access any X- ray equipment.

 

And i have just confirmed that i have to measure both surface and embedded micro-crack.

 

Besides x-ray, please kindly advice do you have any better suggestion for the measurement

[Enthalpy]

For embedded cracks, you might consider ultrasound. In rods D=25 L=310, you can inject a pulse at one end and check the reflections.

 

It needs a clean source of pulse (or possibly a wideband continuous source) which implies good impedance matching at both ends. This needs ideas if you're to change the rod quickly. Or repel the rod's end with a magnetic pulse? Lithotriptors have such actuators. It can be the sensor as well, in a DC magnetic field.

 

If working with short pulses, which seems more common with the ultrasound community, you can stop listening when the echo from the remote end of the rod arrives. Then, it could be better to inject the (possibly shear) pulse at the rod's center rather than at one end.

 

The microwave community uses rather a reflectometer, which works on frequency-swept continous signals, and relies on directional couplers and heavy signal processing to detect faint echoes at precise locations and discard stronger ones, in your case from the imperfectly matched rod's remote end. If reflectometers already exist for ultrasound (I ignore it), use one; if not, it's too complicated to develop for your project.

 

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A different method depends on the cracks behaving non-linearly on sound - which I suspect but have never experimented. This method may be original, in which case you'd have to develop it.

 

It's essentially the way magnetic anti-theft stripes are detected on goods at store's gates: a very pure sine wave is emitted, and a received detects if a harmonic is produced by a non-linear item. The harmonic is chosen odd (3, sometimes 5) for magnetic stripes because their nonlinearity can be an odd function, but cracks may well bring an even nonlinearity.

 

Sine functions are used because analog electronics means can produce them extremely pure and filter them with a huge linearity and selectivity: my detector for magnetic anti-theft stripes had over 140dB linearity, which isn't attainable by an analog-to-digital converter, preventing any subsequent digital processing.

 

With this method, you might perhaps couple the (ultra-) sound by immersing the rod in a bath, provided the bath's surface behaves linearly enough. It needs linear transceivers as well, so check if ceramics achieve that.

 

Resonating the rod mechanically, for instance at its first bending frequency, or at some compression or shear resonance, would add filtering for free, and this would not rely on the transducers' linearity. Bending is less good to detect cracks at the rod's ends.

 

I used few kHz, with active filters and audio amplifiers, but higher frequencies enable passive filters (LC, lines, cavities...) after the power amplifier and before the harmonic amplifier. Use no magnetic core. Polypropylene and type I ceramic and air capacitors are linear.

 

Only a good analog electronics designer can do it. Good luck.

Marc Schaefer, aka Enthalpy

 

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Over 10 years ago a Belgian small company offered tiny X-rays tomographs with faint resolution. They may cost a bit, but would fit your D=25 rods, and development cost is zero for you. A tomograph will show you cracks, while a simple X-ray picture is difficult to intepret.

 

A different technique, which was recent 10 years ago as well, used sound propagation. Originally, a short laser pulse creates the sound in the target metal, and an other laser beam senses the target's movements at a different location, so no contact is needed. One funny idea is that the received reflected light is subtracted from itself with a delay, so that only the target's speed varying faster than the delay are observed, the rest being attenuated, and the reflection's intensity isn't essential.

[student85]

Thanks Enthalpy for your information and suggestion

 

Please advice the minimum length that can be measured by your suggestions

 

Besides that, i will like to check with you that is it possible to provide me websites for the above techniques that you mention. So that i can have better understanding for it.

 

Thanks

[Enthalpy]

The non-contact surveillance by laser was published long ago in a review, perhaps Optics and Laser Europe, as a working tool for metal sheet production. I won't find it again.

 

The acoustic non-linear method may well be new, and then it would need to be tried before anyone can tell how sensitive it is. From the 140dB signal cleanliness I got in the past, my intuition tells it would be sensitive. But not at the rod's ends, for which I see only a tomograph.

 

I like more and more the magnetic pulse to create a mechanical shock in your rod (or in a sheet or other shapes). It induces a clean signal without the time-consuming mechanical contact. But as the sensor, I prefer a capacitive one now, and I still ignore how to protect the sensor against the actuator. At a sheet we could put them on different sides. Maybe more to come as I find time.

 

Google microtomograph and microtomography:

http://en.wikipedia.org/wiki/X-ray_microtomography

http://www.skyscan.be/home.htm

and many more.

[Enthalpy]

After putting some figures at it, injecting a sound pulse in the rod by a magnetic pulse, and detecting the echo by a magnetic or electrostatic microphone would be seriously difficult - if any possible. Both the strong pulse and the faint signal stretch usual engineering values, but above all, both must be at the same end of the rod and operate within a short delay, meaning that the sensor's electronics would be dazzled by the actuator. This is what make the method more suitable to metal sheets, where the sensor can be at the opposite side of the sheet and the fields screened.

 

As a comparison, microtomography was nothing simple to develop, but now it exists. Just buy one.

 

If still considering acoustic methods, piezoelectric actuators and sensors (probably one actuator-sensor) are much more efficient and would reduce the huge factor between the emitted and received electric signals. Coupling with the metal rod could be made with a liquid in vacuum or an elastomer with at least a conical form before pressing it; the gap to the rod being wide and thin would need no seal to transmit pressure.

Edited September 30, 2012 by Enthalpy