Waterjet Technology – Crack Growth and Granite Sculpture

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

The last post in this KMT Waterjet Blog series showed that the main way in which waterjets penetrate into materials is by growing cracks that already exist within the material, and I used glass as an example to show that this was true.

It is this process during which water penetrates into cracks, and then comes under pressure, either by the impact of more falling water (say under a waterfall in nature) or because the water freezes and then thaws, that causes the cracks in the rock to grow under natural attack and the rock to slowly erode. As this happens, the cracks slowly grow and extend to the point that they meet one another, separating small pieces of rock from the solid.

Within the body of a piece of rock, the largest cracks that exist are normally at the boundaries of the grains of different minerals that make up the bulk of the rock. (Back in 1961, Bill Brace showed that the strength of a rock reduced as the square root of the increase in the grain size of that rock ).( Brace, W. F. (1961): Dependence of fracture strength of rocks on grain size. Bulletin of the Mineral Industries Experiment Station, Mining Engineering Series. Rock Mech. 76, 99± 103.) More recently, though still back in 1970, my second grad student, John Corwine, showed that it was possible to predict the strength of a block of granite knowing the size of its crystals.

Which makes a good time to tell a little anecdote. Back when I was doing my own doctorate at the University of Leeds (UK), we were looking at how waterjets drilled through rock and how that might be used to make a drill. We had already run some tests of different rocks that we placed under a nozzle, and gradually raised the pressure of the jet to see what pressure it took to make a hole in the rock. Tests on granite had shown that the jet (with a maximum pressure of just under 10,000 psi) would not drill a hole into those rock samples, and so the granite had been set aside. But, with the equipment just finished and yet having to go to lunch, I asked Dennis Flaxington, the lab technician helping me, to put a new sample into the rig so that we could run a test in the afternoon. When I came back I found that he had used a piece of granite. I made several disparaging remarks, at which point he noted that, having spent some significant time putting the rock in the apparatus, I should just go ahead and run the test (which normally took about 5 minutes) rather than being an unmentionable. And so we did, and, as I posted earlier, this is the resulting hole in the rock, which we were now able to drill right through in a process that took about half-an-hour.

9-inch thick block of granite drilled through by a 10,000 psi waterjet

Figure 1. 9-inch thick block of granite drilled through by a 10,000 psi waterjet at Leeds University. It took over 30 minutes. (Summers, D.A., Disintegration of Rock by High Pressure Jets, Ph.D. Thesis, Mining Engineering, University of Leeds, U.K., 1968.)

How could this now work, when a single jet clearly did not penetrate into the granite in the earlier tests? The answer is that as we moved the rock under the nozzle (we were slowly spinning the rock under the nozzle, and then raising the rock, since at the time there were no high-pressure swivels available for us to use) the jet passed successively over the edges of the different crystals in the granite. As it entered and pressurized these small fractures, the pressure in the crack was enough to grow the crack and remove individual crystals along the jet path. By starting at the center and taking successive passes around the axis, a large depression was cut into the surface, and the rock could then be raised, and a second smaller layer removed. Repeating this slowly removed the rock in front of the nozzle, and at the end of the test we had drilled through 9 inches of granite.

From this experience, over time we went on to cut, for a University, a lot of granite. Obviously, to cut at a competitive rate we had to cut at a higher pressure than just 10,000 psi. But, after showing that we could cut Georgia granite at a competitive rate in tests run at 15,000 psi down in Elberton, Georgia, Dr. Marian Mazurkiewicz and I led a group of our students in cutting 53 blocks of that granite to form the MS&T Stonehenge that now sits on the University campus.

Stonehenge at Missouri University of Science and Technology

Figure 2. View of the Stonehenge at Missouri University of Science and Technology, the vertical blocks are some 11 ft tall. The entire sculpture was cut by high pressure water jets operating at between 12,500 and 15,000 psi. (MS&T RMERC ).

Cutting commercially is not quite as simple as it might appear, since larger blocks such as those shown in Figure 2 will contain rock that varies quite significantly in properties as the cuts progress. In the Stonehenge case the rock came from close to the top of the quarry, and the cracks in the rock were quite well defined. Some fifteen years later, we were fortunate enough to be asked to cut a second sculpture, but this time working with the internationally acclaimed artist, Edwina Sandys. Edwina had designed a sculpture for the campus, the Millennium Arch, which required that we cut two figures from blocks of Missouri granite, and polish them to create one group, while using the original pieces as part of an Arch that would stand some 50 ft away.

The Millennium Arch at Missouri University of Science and Technology

Figure 3. The Millennium Arch at Missouri University of Science and Technology. (Each vertical leg of the Arch is some 15 ft long, and the figures removed and in the background, are 11 ft tall).

The vertical legs were first cut to shape, and then the figures cut out from them. In order to contain the crack growth to limit the amount of material removed the cutting lance had two jets inclined outwards and the lance was rotated at around 90 rpm, as the lance made repeated passes over the surface, removing between a quarter and half-an-inch of rock on each pass, until it had penetrated through the rock. It took 22 hours of cutting to isolate the female figure from the host block. The slot width was around an inch, and there was some significant difficulty in cutting this slot as the quality of the rock changed within the blocks being cut. (The problem was solved by raising the cutting pressure).

Partial cut for one of the figures of the Millennium Arch

Figure 4. Partial cut for one of the figures of the Millennium Arch, checking the depth.

This second sculpture illustrates both an advantage and a problem for the use of waterjets in cutting rock pieces. Use of the water gives a relatively natural look to the rock, although the vertical surfaces of the arch and the capstone were all actually “textured” to look natural using a hand-held lance at 15,000 psi. (The rock is a little harder than that from Georgia and most of the cutting took place at around 18,000 to 20,000 psi). But when the polished surfaces for the inside of the verticals and the isolated figures were prepared, the rough initial surface required much more time to grind and polish flat than a smoother initial cut would have needed.

Because water alone penetrates along crystal and grain boundaries in the rock, the surface left is relatively rough. This gets to be even more of a problem if waterjets are used to cut wood. Here the “grain” boundaries are the fibers in the wood structure. Thus when a relatively low pressure jet (10,000 pai) cuts into the wood, it penetrates between the fibers and the cut quality is very poor. One of the first things I have asked students to do, when given the use of a high pressure lance for the first time, was to write their name on a piece of plywood. Here is an example:

Student name written with a high-pressure jet into plywood

Figure 5. Student name written with a high-pressure jet into plywood. Note that areas of the wood around the jet path are lifted by water getting into the ply beneath the surface layer, and that part of the top ply between cuts is removed in places.

I thought about having you guess the student name, Steve, but this is one of the more legible ones. (Female students generally cut the letters one at a time and were more legible, male students tried to write the whole name at once).

There are many similar examples that I could use to illustrate that, while there are tasks where waterjets alone work well, when it comes to precision cutting, then adding a form of sand to the jet stream to provide a much more limited range to the cutting zone can give a considerable advantage, and so the field of abrasive waterjet cutting was born, and discussion of that topic will lead, in time, to a whole series of posts.

Waterjetting – Growing cracks and glass cutting

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

High pressure abrasive waterjets (AWJ) are able to cut glass with considerable precision, and maintain the accuracy of cut through thick material.

Glass Eagle

Figure 1. Cutting an Eagle from glass using AWJ (courtesy of KMT)

Because of this precision, and because the glass can be cut to leave very delicate webs between adjacent cuts, AWJ glass-cutting has been used to create art objects for a number of years. It is not as easy as it might at first seem, and Dr. Vanessa Cutler, an international leader in advanced cutting, has used the tool to create significant works of art. She has also written on the problems that can arise in cutting what often seems to be a simple, consistent material. (Noting, in passing that through the combination of computer control and memory it is easier at times to re-create works that break than would be the case with other tools for artistic creation). For the more mundane cutting world that comprises the rest of us, cutting glass is more likely restricted to simple activities such as cutting the parts for the windows of wood furnaces.

When the cuts are this simple, time can be saved by stacking two or more sheets of glass, one on top of the other, and cutting all of them at the same time. As one learns the parameters, thicker and greater numbers of plates can be stacked, and still successfully cut.

Cutting through four sheets of glass

Figure 2. Cutting through four sheets of glass simultaneously (courtesy of KMT)

However, if one gets too ambitious, and stacks too many plates then the lower plates may start to crack, often after the cut has started into the plate. As Dr. Cutler has noted in her new book “New Technologies in Glass”, cracks can also create problems for the unwary in dealing with internal stresses in the structure of the glass.

There can be several reasons for this, but it primarily goes back to the point I made in the introduction, about water pressure causing existing cracks and weakness planes to grow, as a way of removing material. There are two sorts of cracks that exist in glass, those created by the impact of the abrasive particles themselves, and those that were already present in the glass.

Micro-Photo - Cracks in Glass

Figure 3. Micro-photograph showing cracks growing out from the point where two abrasive particles struck a piece of glass. (This was adjacent to the main cutting path).

Micro-Photo cutting surface - Glass

Figure 4. Micro-photograph of the edge of the main cut by an AWJ on glass, showing that it is made up of the intersection of adjacent cracks created by the abrasive impact.

I’ll write about the mechanics of cutting glass in a later post or two, but for the moment I would like to write about the basics of crack growth from the point of view of cracks that already exist in the material. In large part this won’t be using waterjets alone to cut glass. Rather there are lots of other materials, particularly soil and rock, which have much higher crack densities, and longer cracks which make it easier to cut and remove material.

So, for the next four KMT Waterjet Blogs, I will focus the issues of cracks in materials including cutting glass, cutting granite and much more.

Waterjetting 1d – Not quite that simple!

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

When I first began the research on the applications of high-pressure water that was to be one of the major parts of my professional life, I must confess to a certain naïve innocence in regard to other folk’s work. One assumed that other folk had made similar mistakes to mine and then corrected them, so that when different systems were compared that the early, obvious, mistakes had not been made.

One of the first times I found that this wasn’t the case was when we were asked to go and demonstrate that high-pressure waterjet technology could economically cut granite, in quarries located in the heart of the Granite industry, in Elberton, Georgia. We were working with Georgia Institute of Technology (Georgia Tech) at the time and were asked if we could, at very short notice, go down to a couple of quarries and run a demonstration.

Back during my graduate studies I had found that Russian claims were true that said that it was possible, with a 10,000 psi jet pressure to cut through a rock with a compressive strength of 30,000 psi. (I’ll tell you how later)

9-inch thick block of granite drilled through by a 10,000 psi waterjet

Figure 1. 9-inch thick block of granite drilled through by a 10,000 psi waterjet at Leeds University. It took over 30 minutes. (Summers, D.A., Disintegration of Rock by High Pressure Jets, Ph.D. Thesis, Mining Engineering, University of Leeds, U.K., 1968.)

Knowing this, and having a suitable pump at The University of Missouri-Rolla (now Missouri University of Science and Technology), our group ran some tests at the RMERC (Rock Mechanics and Explosives Research Center) to get the angles right between the two jets that we were to use, and then, about a week later, we went down to Elberton and set up a system in the quarry.

Starting to cut a 1-inch wide slot in granite

Figure 2. Starting to cut a 1-inch wide slot in granite, pressure 14,000 psi, 90 rpm, linear cutting speed around 9 ft/min, areal cutting rate around 20 sq. ft./hour.( Raether, R.J., Robison, R.G., Summers, D.A., “Use of High Pressure Water Jets for Cutting Granite,” 2nd US Water Jet Conference, Rolla, MO., April, 1983, pp. 203 – 209.)

The trials demonstrated that high-pressure water could cut granite at commercial rates, we cut a slot some 11 ft long and about 2-ft deep, and, after a couple of days of work, we went home. Georgia Tech then went to one of our competitors who set up to run a similar test. We had been done in 2 days, it took them two weeks to cut a slot about 2 ft long and 6-ft deep. They were running a jet system at 45,000 psi, roughly 3 times the pressure of our system. Why did they do so badly?

Well it turned out that they connected their pumps to the nozzle through a very narrow length of high-pressure tubing, and we calculated (as later did they) that of the 45,000 psi being supplied at the pump, some 35,000 psi had been lost in overcoming friction between the pump and the nozzle, As a result they were trying to cut the granite with jets at a pressure of 10,000 psi effective pressure, and it was much slower than our system which retained most of the 14,000 psi from the pump to the nozzle. (Hilaris, J.A., Bortz, S.A., “Quarrying Granite and Marble using High Pressure Water Jet,” paper D3, 5th International Symposium on Jet Cutting Technology, Hanover, FRG, June, 1980, pp. 229 – 236.)

Now you may note that I said something about mistakes – it turns out that we had made an identical mistake a few years earlier and had added a second 10-ft length of narrow diameter tubing to the nozzle, and suddenly a system that had cut adequately with 10-ft of tubing did not work with 20-ft. The reason was that the pressure loss in the tubing was too great at the longer length, and the pressure fell below that required to cut into the rock. (But at the shorter length we were drilling the hard sandstone at 12-ft/minute).

It is a very simple mistake, and many folks have made it over the years. The system has to be designed from one end to the other to ensure that all the parts are properly sized for the systems that are to be used. (And I will refer to other cases such as that above as we go through this series.)

It is not just the diameter of the feed lines that is important. In 1972 it took, on average, 150 man-hours and about $2,000 for the U.S. Navy to clean a single ship boiler using chemicals and mechanical scrubbing and cleaning. An enterprising company showed the Navy that it was possible to use waterjet lances to clean the tubes. In the demonstration they cleaned a boiler in 10 hours, and it cost around $700. This being Government work, the Navy then arranged a competition to find the most effective contractor. Based on the performance of the system that had been used in the first demonstration they asked 5 companies to compete in cleaning boilers. The operating equipment was designated as having to operate at 20 gpm, at a pressure of 10,000 psi. The results were not even close, even with systems nominally the same.

Relative cleaning efficiency in areal percentage cleaned, of five competing systems in cleaning heat exchanger tubes in Navy boilers

Figure 3. Relative cleaning efficiency in areal percentage cleaned, of five competing systems in cleaning heat exchanger tubes in Navy boilers. (Tursi, T.P. Jr., & Deleece, R.J. Jr, (1975) Development of Very High Pressure Waterjet for Cleaning Naval Boiler Tubes, Naval Ship Engineering Center, Philadelphia Division, Philadelphia, PA., 1975, pp. 18.)

One of the differences between the competing systems, you won’t be surprised to hear, was that some had smaller feed hoses than others.

There are many different reasons that the various systems performed as they did. One of the aims of this series is to ensure that, should you be asked to engage in such a competition, you will know enough to follow the path of company A, rather than company E.

As systems have become more sophisticated the different factors that control the performance of the jets have increased in number. As a simple example, when abrasive particles are mixed with high-pressure water in streams of abrasive-laden waterjets at pressures that can run up to 90,000 psi in pressure, for high precision cutting of material, the factors controlling performance now include not only the delivery system for the water, but also that for the abrasive, the type of abrasive and the configuration of the nozzle through which that final cutting jet is created.

Again, when we were asked to compare the performance of these different systems we set up nominally identical test conditions under which to determine which nozzle system would perform better. If I were honest I would tell you that before the tests began I expected that the variation in performance of the systems would vary by perhaps 10% between the best and the worst. We were quite surprised by the result.

Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles

Figure 4. Comparative performance between 12 nominally similar abrasive waterjet cutting nozzles in cutting through steel at a standard speed, pump pressure, and abrasive concentration.

I use these last two figures to show that all the details of a high-pressure waterjet system are important, when it comes to optimizing performance. One of the reasons to write this series is to ensure that folk that use these systems in the future do not make the mistakes that we made, as we learned how to tune the systems from getting poor performance to the commercially viable rates that are achieved today.

Unfortunately much of the early research and tests that are the basis for this knowledge were performed before the Internet existed. As a result I will have to use references to books and papers (as above) rather than using the electronic references that are the more common habit now.

This concludes the basic introduction to the series, which will now focus on more specific subjects.

The Force of Water and Waterjet Technology – Force, Pressure and Flow Volume

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

Back in September of 2012, archaeologists found the remains of the last king of England to die in battle. It was Richard III, to whom Shakespeare gave the line “a horse, a horse, my kingdom for a horse!” He died because a blow to his head was focused down to a very high local pressure, even though the overall force of the blow might not have been that great.

Gothic Armor

Figure 1. Gothic Armor of the type worn by Richard III. (Detroit Institute of Arts)

On the other hand, in the middle of Hurricane Katrina, seven years ago, a barge broke through the levee wall above the Lower 9th Ward in New Orleans, LA (NOLA) and released a wall of water that swept through the district. The pressure of the water was quite low, but the overall force it exerted demolished the buildings in its path and swept them off their foundations for eighteen blocks back from the levee. In this case force, not pressure, was the cause of the damage.

Lower 9th Ward in New Orleans

Figure 2. The Lower 9th Ward in New Orleans after Katrina, as the water falls, it flows back into the Industrial Canal. The barge that broke through the levee is on the right. Most of the debris lined the second row of trees back from the levee. (Tulane)

It is this initial relationship between force and pressure and the role that each has to play in the use of waterjets to remove material that form the topic not only for today, but in a number of the posts that will follow. Waterjetting applications now cover a wide spectrum of different uses, and finding the best choice of pressure and flow (which combine to give power) will change from job to job, and hopefully these posts will help make the choice easier.

It is raining outside. As the water drops hit the soil, the water soaks into the soil by penetrating along the existing cracks that exist between the grains of the soil. After a short time the water fills this space, and as it continues to rain, the impact of new rain drops hit the thin wedges of water that now run down into the soil. Although at much lower forces the action is the same as when you hit a wedge driven into a log with a hammer. The wedge pushes the two walls on either side apart, and a crack grows. One of the key elements that give waterjet cutting its advantage is this transformation from an impact force into a pressure, and most particularly a pressure which is applied against all the surfaces with which the water is in contact. It is a point that will be repeated many times.

The stages of soil erosion

Figure 3. The stages of soil erosion – the white arrows in (b) and (c) show the small pressures that are exerted on the particles as additional raindrops keep falling on the water in the ground. This lifts the top two particles in (c) so that the flow of water will carry them away.

With the soil there is not that much material holding the grains together, and so as the rain continues, the soil grains begin to separate from those on either side. Water gets underneath the grains and starts to lift the individual grains free from the mass. Since most land is not flat, the water will now start to flow away under the continued rain, and as it does it carries some of the soil particles that have been freed. This is a simple explanation for the erosion that happens in fields, dirt roads, and other exposed surfaces as they weather. As materials get stronger this process can take much longer to be seen. A high quality stone will erode at the rate of perhaps an inch every thousand years, depending on local weather patterns. There are buildings and bridges built by the Romans all over Europe to prove that point. A weaker granite (and one thinks of the granite in the walls of the Basilica in St Louis as an example) may severely erode within a hundred.

Which brings up an important point: the performance of a waterjet stream is not just controlled by what happens upstream of the nozzle in the delivery system, but it is also affected by the material that it is hitting. And I’ll come back to that in future posts.

First, however, consider what happened during Hurricane Katrina in the Lower 9th Ward. When the barge broke through the levee wall and was carried into the district, it rode on a wall of water that was initially no more than about 30 ft high. We can make a very crude estimate of the pressure of the initial wall of water (neglecting any impact due to the speed at which it moved) based on the height of that wave. A cubic foot of water weighs 62.4 lbs. It sits on an area of 12 x 12 = 144 square inches, so that the pressure under that water is 62.4/144 = 0.43 pounds per square inch (psi). Since that is somewhat close to half-a-psi, as a very simple way of getting the pressure at the bottom of a column of water one can just divide the height in two, and call it psi instead of feet.

So, in the case of that wall of water the pressure at the bottom of the wall would be 30/2 – 15 psi. Since the pressure increases with depth, the average pressure over the height will be half of that, or 7.5 psi. That pressure, by itself, does not appear that powerful.

But when the wave hits a building that pressure is applied over the entire wall. So if the building is 40 ft long and 10 ft high, then the area that sees that pressure is 40 x 12 x 10 x 12 = 57,600 square inches. If that small (7.5 psi) pressure is applied over the whole area, then the force = pressure x area = 7.5 x 57,600 psi = 432,000 lb.

You can now perhaps understand why, when the wave hit the first rows of houses in NOLA that they almost immediately disintegrated, and were carried back as broken debris for about ten blocks.

Aerial view of the Lower 9th Ward

Figure 4. Aerial view of the Lower 9th Ward after the water had drained, and the levee had been replaced. For a sense of scale there is a school bus sitting partially under the barge, and that is the yellow dot at the end of the upper arrow. Each of the flat slabs to the left of the levee marks where a house stood. When we visited the site the house slabs were as shown, but there was still water – and some live fish, standing in the district. (Tulane)

This was a terrible disaster, but there are occasions, particularly in mining, where this terrible force, combining low pressure but high volume flow rates, can be harnessed to do useful work. Such flows are something that our ancestors have known for millennia, and were used as a way of mining from before the age of pumps, and l will tell how they did it in some later articles.

But in most cases we don’t have that amount of water, and the job is more often one where we want to precisely cut a hole, perhaps, in one of the walls of a building, rather than destroying the building. And we haven’t the patience to wait a hundred years to cut through a block of stone. So how do we speed it up? And so we come back to the death of King Richard.

Back in the day a foot soldier could make a bit of money in a battle by knocking a knight off his horse, and then holding him for ransom. The weapon that they used for this was generally known as a poleaxe. These come in various shapes, but one general idea was to have a hammer on one side of the long pole. Thus, by swinging the pole one could hit a knight with a force of say 50 – lbs. and this could knock him off his horse, allowing him – in the best of such worlds – to be captured alive and then ransomed.

Modern Reproduction of a poleaxe

Figure 5. Modern Reproduction of a poleaxe from about the time of the Wars of the Roses (Wallace Collection)

However that hammer head could measure about a square inch or two, and neither the force nor the pressure would have been enough to penetrate armor or a helmet of the type King Richard wore (Figure 1). To give the footman that advantage the design was changed to include a small spike in the center of the hammer.

Spiked Poleaxe

Figure 6. Spiked Poleaxe from about 1582, (Royal Armories, Leeds via My Armory.com)

Now when the force of 50-lb is applied through the hammer to the target it is not distributed over a square inch (giving a pressure of 50-psi). Instead it is focused down on a point that is less than a twentieth of an inch across. Total area of the circular point comes from pi x radius squared = 3.14 x 0.025 x 0.025 = 0.002 sq inches. Pressure applied through the spike to the helmet = 50/0.002 = 25,000 psi. That is enough for the spike to pierce through the metal helmet and the bone underneath, killing Richard III. Battle over, England had a new king, Henry VII, and the War of the Roses was over.

The intent of the two examples is to show how, in some circumstances, high volume flow rates at low pressure can do the most damage, and in others that much higher pressure applied over a much smaller area is the most effective. They are extreme examples but seek to illustrate the point, and in many cases neither extreme (highest pressure, lowest flow or lowest pressure, highest flow) will give the best answer.

Aluminum Cutting

Figure 7. Cutting 6” Aluminum with KMT Waterjet Streamline PRO Pump 60hp 90,000psi.

There are cutting conditions where operational concerns and benefits would argue that pressures of 90,000 psi, and flow rates around 1 gpm will be the best business choice. In other cases a flow of a thousand gpm, but at a pressure of 1,000 psi will be the most economic and viable way to remove soil, and weaker rocks like coal. This series is aimed, in part, at giving you the knowledge that will help you decide where, within that range, to make that balance, between flow and pressure.