Definition

Acoustic Emission (AE) refers to generation of transient elastic waves during rapid release of energy from localised sources within a material. The source of these emissions in metals is closely associated with the dislocation movement accompanying plastic deformation and with the initiation and extension of cracks in a structure under stress. Other sources of AE are: melting, phase transformation, thermal stresses, cool down cracking and stress build up, twinning, fiber breakage and fiber-matrix debonding in composites.

AE Technique

The AE technique (AET) is based on the detection and conversion of high frequency elastic waves emanating from the source to electrical signals. This is accomplished by directly coupling piezoelectric transducers on the surface of the structure under test and loading the structure. The output of the piezoelectric sensors (during stimulus) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by suitable electronics. AET can non-destructively predict early failure of structures. Further, a whole structure can be monitored from a few locations and while the structure is in operation. AET is widely used in industries for detection of faults or leakage in pressure vessels, tanks, and piping systems and also for on-line monitoring welding and corrosion. The difference between AET and other non-destructive testing (NDT) techniques is that AET detects activities inside materials, while other techniques attempt to examine the internal structures of materials by sending and receiving some form of energy.

Types of AE

Acoustic emissions are broadly classified into two major types namely, continuous type and burst type. The waveform of continuous type AE signal is similar to Gaussian random noise, but the amplitude varies with acoustic emission activity. In metals and alloys, this form of emission is considered to be associated with the motion of dislocations. Burst type emissions are short duration pulses and are associated with discrete release of high amplitude strain energy. In metals, the burst type emissions are generated by twinning, micro yielding, development of cracks.

Kaiser Effect

Plastic deformation is the primary source of AE in loaded metallic structures. An important feature affecting the AE during deformation of a material is ‘Kaiser Effect’, which states that additional AE occurs only when the stress level exceeds previous stress level. A similar effect for composites is termed as 'Falicity effect'.

AE Parameters

Various parameters used in AET include: AE burst, threshold, ring down count, cumulative counts, event duration, peak amplitude, rise time, energy and rms voltage etc. Typical AE system consists of signal detection, amplification & enhancement, data acquisition, processing and analysis units.

Sensors / Soure Location Identification

The most commonly used sensors are resonance type piezoelectric transducers with proper couplant. In some applications where sensors cannot be fixed directly, waveguides are used. Sensors are calibrated for frequency response and sensitivity before any application. The AE technique captures the parameters and correlates with the defect formation and failures. When more than one sensors is used, AE source can be located based by measuring the signal’s arrival time to each sensor. By comparing the signal’s arrival time at different sensors, the source location can be calculated through triangulation and other methods. AE sources are usually classified based on activity and intensity. A source is considered to be active if its event count continues to increase with stimulus. A source is considered to be critically active if the rate of change of its count or emission rate consistently increases with increasing stimulation.

AET Advantages

AE testing is a powerful aid to materials testing and the study of deformation, fatigue crack growth, fracture, oxidation and corrosion. It gives an immediate indication of the response and behaviour of a material under stress, intimately connected with strength, damage and failure. A major advantage of AE testing is that it does not require access to the whole examination area. In large structures / vessels permanent sensors can be mounted for periodic inspection for leak detection and structural integrity monitoring. Typical advantages of AE technique include: high sensitivity, early and rapid detection of defects, leaks, cracks etc., on-line monitoring, location of defective regions, minimisation of plant downtime for inspection, no need for scanning the whole structural surface and minor disturbance of insulation.

AET Limitations

On the negative side, AET requires stimulus. AE technique can only qualitatively estimate the damage and predict how long the components will last. So, other NDT methods are still needed for thorough examinations and for obtaining quantitative information. Plant environments are usually very noisy and the AE signals are usually very weak. This situation calls for incorporation of signal discrimination and noise reduction methods. In this regard, signal processing and frequency domain analysis are expected to improve the situation.

A few Typical Applications

• Detection and location of leak paths in end-shield of reactors (frequency analysis)
• Identification of leaking pressure tube in reactors
• Condition monitoring of 17 m Horton sphere during hydro testing (24 sensors)
• On-line monitoring of welding process and fuel end-cap welds
• Monitoring stress corrosion cracking, fatigue crack growth
• Studying plastic deformation behaviour and fracture of SS304, SS316, Inconel, PE-16 etc
• Monitoring of oxidation process and spalling behaviour of metals and alloys

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I've replaced plugs on quite a few 5.4Ls now (the 4.6L with plug wires is similar) and once you've done a set they really are not as bad as they look. Contrary to what some people will say, you don't have to remove the fuel rails. The Coil On Plug (COP) assemblies will come out past the fuel rail. I take an old piece of seat foam and put it on top of the radiator support to the engine to allow me to lay on it without hurting my tummy. It makes the job way less painful.

Start by removing the cover over the throttle body (the black plastic cover that says "5.4" on it). There are three 10mm head bolts that hold it on. Next remove the air intake tube from the throttle body to the air filter housing. You loosen the hose clamps at either end of it, disconnect the connector on the AT (about half way up the air intake hose), the pull out the small hoses that go into the air intake tube near the throttle body. Next remove the brace from the power steering reservoir to thermostat housing. There are three 8mm or 5/16" head screws that hold it on. Now you should be able to see the COPs.

To remove the COPs you can use a 7mm or 9/32" wrench or nut driver or socket, extension and ratchet or all of the above. If you turn the fuel injectors to the side it will give you more room to work with the COPs. Unplug the connector on each COP by pressing the tab in and pulling on the connector. After you're done that just twist and pull the COPs out. A couple of the COPs on the driver's side and #4 on the passenger's side are a bit hard to get at but with some patience they will come out.

After you've removed the COPs take a blow gun and blow out the spark plug holes. Don't be surprised if there is rust and junk in them. Next you can actually remove the plugs. Use a combination of extensions, swivels (universal joints), sockets and ratchets to get at them. Whatever works best for you is good.

On the harder ones to get at I usually use a socket with a 4" extension, then a swivel, then a long extension, then the ratchet. The plugs are way down in the holes which is why I use the extension then the swivel. The swivel makes it easier to clear the firewall.

Set the gap on the new plugs to whatever it says on your emissions decal on the radiator support....usually .052-.056". Apply a small amount of anti-seize to the threads only on the spark plug. You can use a piece of vacuum hose or fuel hose over the end of the plug to get it started in the hole. Carefully start the plugs in their holes. If you can't get them most of the way in by hand with the hose take a look and see why not. Cross threaded plug threads are no fun! The plugs are to be tightened to 13 lb-ft. which is just hand tight with a short ratchet. Don't over tighten them! The threads in the aluminum heads have enough problems as it is. After that just put everything back together in reverse order. Apply some dielectric grease to the plug boots as well to help seal them.

I've done enough of these that I can replace the plugs in approximately 45 minutes but don't be surprised if the first time you do it takes a few hours.

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Thinking of buying used backhoes? It could be that you're trying to save some money or that its need may not be that vital to warrant a new equipment. However, buying a used one doesn't really have to mean that you need to buy the first one that you can lay your eyes on. You still have to consider their quality and even the service that can be offered by the supplier.

If you're thinking of purchasing used Caterpillar equipment or backhoe, you can take note of the following tips:

1. Check the essential parts of the Caterpillar equipment. Used backhoes should still consist of three parts. These include the tractor, loader, and the bucket, which can be found at the back end of the tractor. If one of them is missing or not functioning properly, ask the supplier if they can provide the part and how much it may possibly cost you. Of course, it doesn't hurt to inquire if you can get the missing part for free.

2. Conduct an inspection first before you buy used backhoes. Consumers tend to overlook the importance of having a thorough inspection. It doesn't mean that it's Caterpillar equipment it's not prone to wear and tear. In the same way, it doesn't mean that when the supplier says they're still in good working condition, the used backhoes are. Just to make sure that you're still making a good investment on your purchase-it's still your money anyway-check the equipment yourself. Take note of its hydraulics as well as mechanical and electrical issues. As much as possible, the parts should not be close to deterioration; otherwise, you could save on the purchase but spend a lot on the maintenance costs.

3. Bring an expert with you. This is necessary when you don't know how to inspect a used backhoe yourself. You may want to bring someone who knows Caterpillar equipment too well. Most definitely, the supplier can provide you with one, but to avoid any biases, you can select your own. In the end, though, the inspector can only provide you with recommendations, and the final decision rests upon you. Make sure that you can take note of everything that he's going to say.

4. Make sure that you can expect good customer support. If there's one thing that a used backhoe is prone too, it would be downtime, and with that, you absolutely need excellent customer support. Make sure that the supplier can extend the right kind of assistance to you. It should be able to give you the necessary parts as well as the overhaul just in case the entire Caterpillar equipment breaks down.

This also means that you should also know the company's policies when it comes to maintenance. How do they manage Caterpillar equipment breakdown? How long does it take for them to repair the used backhoe? How much would you spend for the maintenance? Can they offer discounts for a period of time?

Thinking of buying used backhoes? The best choice will be a Caterpillar equipment, and you can get it at Cleveland Brothers. They can also extend an exceptional customer support, where you can look forward to easy change of parts. You can also search through their website of the available backhoes that you can buy.

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