Waterjetting 20c - Critical Distances intro

I remember going down the New Orleans just after Hurricane Katrina, and being a member of one of the first inspection teams that drove down the Delta inspecting the damage. One of the things that has remained in my memory was that of driving past one of the large coal piles being stored outside one of the local power stations, and being surprised that – despite the wind and rain – it was still there. Some years previously I had visited several places that had build ultra-ultra-high pressure jet generating devices. (Not sure what else you call a system that produces jet impact pressure that begin to approach 1 million psi). The one that has struck in my memory was visiting the underground test for Dr. Bill Cooley’s water cannon. Built to advance the concept of using high pressure gas to drive a pulse of water at a rock target (this was during the days examining new ways to drive underground tunnels) the device fired a small slug of water through a very carefully designed nozzle to generate a jet measured to produce 500,000 psi on impact.

We stood around and watched as the device was loaded, charged and fired and were, I suppose, a little disappointed at the damage induced around the impact point. Only a small amount of fragments were produced from the shot, and as you can see from the figure above, there was not a lot of evident surface damage from the shots. Actually, for the amount of energy involved, the amount of material removed was quite significant and some of the pieces were several inches in size:

Figure 2. Fragments recovered after a single shot of the cannon.

Later analysis explained why the damage had not been greater. The problem arises because of the way that the pressure is generated to drive the jet. Most of the devices of the time, whether our own, others in the US, the UK or in Russia, used a gas driver to generate the driving pressure, either using smokeless powder (as we did) or storing gas volumes under pressure and then suddenly releasing the accumulated volume to provide the driver.

The problem with this approach is that the driving pressure is not sustained as the water moves down the relatively long nozzle, and the driving pressure on the surface is thus very transient. One recording of such from the UK showed a typical pulse for their unit:

Figure 3. Pressure pulse for the water cannon device developed in the UK.

Dr. Cooley’s cannon had a much shorter and more rapidly decaying pulse, and this was largely the reason that the damage that it induced was not greater. The high impact energy was able to generate large cracks into the surface as the jet penetrated, some of which coalesced with the surface and allowed fragments to release, but the energy pulse was not long enough at the high pressure and with inadequate volume to fill the cracks produced and pressurize them to cause rapid extension and result in larger volume material removal.

The relatively low pressure impact of the rain on the coal in the Gulf, and the high pressure impact on the rock in the test mine both saw the ability of the water to penetrate into the surface layers of the material. But in both cases the impact pressure on the water that permeated into the cracks within that surface was not enough in the secondary phase of the failure process, to sustain internal pressures within the cracks that would lead to large volume material removal.

Now, as I mentioned last time, there are ways to enhance the effect of an impact, by creating secondary surfaces for the rock to break to. The best illustration of this, perhaps is to consider two small (six-inch side) blocks of plexiglas. We drilled a small hole through to the center of each and placed a detonating cap in that hole. We then fired the detonator.

Figure 4. Block of Plexiglas after a detonating cap has been fired in the middle of the block.

Note that the block was big enough to contain the crack damage. But note also that there is very little material removed, because the cracks that were formed by the explosion were contained within the block and did not interact very much. Now consider what happened when we first drilled a circular relief slot around a similar hole and charge. (We based the radius of the cut on the results of the first shot, shown above). After the detonation, this is what we achieved:

Figure 5. Similar bock to figure 4 except that a relief slot was first cut into the block

Note that there are (for those of us interested in driving tunnels) several interesting improvements. Firstly the cracks that radiated out to the walls of the first block now stopped within the central isolated core, and the wall of the excavation is now smooth and undamaged. Secondly the interaction of the radiating cracks within the core all reached the relief slot, and broke the core into fragments that were liberated. And that this also broke to the back of the slot, which was left relatively flat and at the end of the relief slot, so that it would be easy to start a new drilling and breaking operation to deepen the tunnel without having to redrill any damaged zone at the end of the previous excavated section.

But, for the purpose of today’s exercise, note that in the first place – even though there were free surfaces at the edge of the block, they were too far away for the cracks to reach them, and effectively break out the material. It was only when the relief slot was moved closer to the exploding detonator that the central volume was broken out and the effective result that was desired was achieved.

The critical parameter is the correct assessment of the distance over which the interaction between the event and the free surface (or in the case of last time’s discussion between two concurrent jet cuts) will take place.

As I will discuss in later posts, sometimes the distance that is critical to effective jet use is only on the order of fractions of an inch, and if done correctly the result (as above) is impressive, if the distance is too great, then nothing happens. But we’ll talk more about this, in many applications, in the future.

Waterjetting 10b - Introduction to Abrasive use

In recent articles in this series I have written about the processes that occur as a high-pressure waterjet impacts on a surface and then begins to penetrate and cut into it. However, as I noted in the last post, one of the problems with using plain water as the cutting medium is that it can pressurize within the cut and exploit any surrounding cracks, to the point that the edges of the cut are cracked and fractured, often back up to the top surface of the material.


Figure 1. High-pressure waterjet cut along a sheet of Plexiglas, note the fracturing along the sides of the cut.

This is not usually desirable, and what is needed is a way of cutting into these materials, so that the cut edges remain smooth, and the risk of shattering around the cut line is much diminished. The way that is usually used for this is to add small amounts of a fine cutting abrasive into the waterjet stream, and use this to cut the slots in the material, with the water there to add cutting power.


Figure 2. Abrasive waterjet (AWJ) cuts through safety glass. Note that there are two sheets of glass with a thin plastic sheet attached between the two.

This can be of particular advantage if you are faced with trimming, for example, safety glass (as shown in Figure 2). Cutting and shaping this glass used to be a significant problem in the industry, since the presence of the plastic sheet, between the two glass layers meant that it was not always possible to get both to break to the same plane if scribed with a glass cutter. Failure rates of up to 30% were described, to the author, as common when the technology switch to AWJ took place. And with the abrasive in the water, the jet cuts through both layers without really seeing that there was a problem. (And complex contours can also be cut).

The combination of abrasive and high-pressure water has many advantages over existing tools. Among other things it removes the majority of the heat from the cut zone, so that in almost all cases the Heat Affected Zone (HAZ) along the edges of the cut disappears and the quality of the cut surface becomes, when properly cut, sufficient to require no further processing. This can lead to a significant savings in certain forms of fabrication.

There are many different ways in which abrasive can be added to a high speed stream of water, and Dr. Hashish illustrated some of these in the introductory lecture he gave at an early WJTA Short Course, as follows:


Figure 3. Some different ways of introducing abrasive into the cutting stream of a high-pressure waterjet (After Hashish, WJTA Short Course Notes).

The top three (a, b, c) involve mixing the abrasive and the water streams at the nozzle, while the fourth (d) is a relatively uncommon design that is used in cleaning surface applications, and the fifth has never been very effective in any trial that we have run. The sixth (e) technique has become known by a number of different names, but for now, to distinguish it from the more widely used Abrasive Water Jet cutting (AWJ) I will give it the acronym ASJ, for Abrasive Slurry Jetting. It has a number of benefits in different circumstances, and I will write more about it in future posts. In more recent alternative designs to that shown by Dr. Hashish the flow to the abrasive holding tank is more commonly through a diverted fraction of the total flow from the pump or intensifier.


Figure 4. Very simplified illustration of the circuit where abrasive is added to the flow from the pump/intensifier before the nozzle. Obviously the abrasive is held in a pressurized holding vessel – the optimal design of which is not immediately obvious.

When fine abrasive is added to a narrow waterjet stream, and that jet is moving at thousands of feet a second, there are a number of considerations in the design of the mixing chamber, and those will be discussed in future posts. But one early conclusion is that, if the jet is going to be small, then the abrasive that will be mixed with it will also have to be quite small, though – as will be noted in a future post – not too small.


Figure 5. The simplified and generic components of a mixing chamber that mixes abrasive with high-pressure water in an AWJ system.

There were a number of problems with the early systems, such as that shown in Figure 5, at the time that systems first appeared on the market, and I will write about some of these as the next few posts continue to focus on this subject.

There have been a number of different abrasives used over the years, and it depends on the needs of the job as to which is the most suitable in a given case. In some cases discriminate cutting is required, and so an abrasive can be chosen that will cut the desired layer on the surface, but not the material behind it. In other cases the target material is extremely tough, and so abrasive may be selected that will rapidly erode the supply lines and nozzle, but which can still prove economically viable in certain cases.


Figure 6. Various types of abrasive, that can include (from bottom left going clockwise) blasting sand, copper slag, garnet and olivine.

There are many different properties of the cutting system, and the abrasive which control the quality and speed of the resulting cut. Some of these will be the topic of the next few posts, others will be discussed in further posts at a more distant time, when we discuss different cutting applications and the changes in a conventional system that might be made to get the best results in those cases.

Abrasive properties are not just a case of knowing what the material is. There is a difference, for example, in cutting ability between alluvially mined garnet and that mined from solid rock. There is a difference between different types of the nominally same abrasive when it comes from different parts of the world, and there are differences when the shapes of the abrasive differ. Glass beads and steel shot cut in a different way that glass and steel grit, for example. So there is plenty to discuss as we turn to a deeper discussion of abrasive waterjet cutting.


Figure 7. Parameters controlling the cutting by an abrasive waterjet system. (After Mazurkiewicz)