Waterjetting 27a - Cutting materials with internal stress

Safety glass, or toughened glass is typically designed so that, when it fails it will break into small pieces with few of the relatively sharp, and thus dangerous, fragments formed by ordinary glass. It is used in making shower doors, and automobile windows. As such is differs from laminated glass (which I will discuss in a later post in this section). The toughened glass is formed by quickly cooling the glass, after it has been heated. And one way to check if a sheet has been treated this way is to look at it through polarized sunglasses. Tempered glass will show a pattern. The reason for that, and for the rapidity of the breakup of the glass, is that the lines show the internal stresses that the glass treatment deliberately leaves in the material. (There is an interesting variation on this way to tell the difference using an iPhone.)

Figure 1. Broken pieces of tempered glass, showing the small fragments that result. (Floydglass)

Because the treatment puts the outer parts of the glass into compression, while the inner part is in tension once cracks start to appear in the glass, then the glass is designed so that these stresses will cause the cracks to grow, bifurcate and join in patterns that cause the glass to shatter into less dangerous fragments. But this creates a considerable problem if there is a need to reshape the glass after it has been heat-treated.

Note that this treatment is the opposite of the result where glass is annealed, where – by cooling the glass at a slow rate – the internal stresses are much reduced, but as a result, when the glass breaks the fragments can be more damaging.

Figure 2. Sheets of annealed glass, showing how it may break from impact. (ADMglass)

Annealed glass is, as a general rule, relatively easy to cut with an abrasive waterjet system provided that certain simple precautions are taken. However, when it comes to cutting tempered glass, one of the suggestions is to anneal it first, so as to get rid of the internal stresses. Unfortunately, in the process this also removes the benefits of the tempered treatment.

When one tries, without other treatment, to cut tempered glass the results are not pretty. Edgar Hernandez has posted a video of what can be expected to happen.

The problem goes back to the basic way in which waterjets, and abrasive waterjets work in cutting through material. Simplistically waterjet impact will penetrate the cracks that exist in a target surface; the following slug of water then pressurizes the water within the crack, causing it to grow. As cracks get longer it takes less and less pressure, either internally within the crack, or in the surrounding material, for that crack to grow catastrophically to failure of the piece. Where there are relatively few natural cracks in the material – as happens with glass – then abrasive is introduced into the waterjet stream, so that the impact of the small particles will form small cracks when they hit the glass surface. Normally those cracks are relatively small, and when first cutting into or piercing the glass the pressure of the jet is often lowered so that the particle speed is also lower and the crack length that the particles create is also small and localized around the impact point, so that the integrity of the whole piece is not threatened.

Figure 3. Cracks around the impact of single particles of abrasive onto glass.

The problem, from a cutting aspect, with tempered glass is that the internal stresses that are deliberately placed into the glass are designed so that cracks do not have to be very long before the concentrated stress at the crack tip (which increases with crack length) reaches a point where it will continue to grow at an increasing rate to failure of the piece. The longest cut we have made in tempered glass before it shattered was about an inch-and-a-half.

Because the stress in the glass is an inherent part of the nature of that particular type of glass there is no really effective way of cutting the material, after it has been tempered. If a particular shape is required then the glass should be cut to final shape before it is tempered, and care should be taken to ensure that there aren’t any large cracks or chips along the edge of the glass before it is then tempered.

Stress problems aren’t restricted, however, to trying to cut tempered glass. When cutting larger pieces of metal one can also run into problems from stresses that were left in the material after it was initially formed. Perhaps the most common of these is found where a partial cut allows a stressed part to lift slightly above the plane of the rest of the material. If the part is being cut in steps, the raised piece can then move into the path of the cutting nozzle as it moves back over the piece. This can have some unfortunate consequences for the nozzle and focusing tube (there goes bitter experience speaking again).

Other problems that can crop up come from the shifting of the piece in the plane of the part, but where the stress relief moves the edges so that subsequent cuts into the part no longer comply with the blueprint for the cuts, since the material has shifted. This shift can be a relatively small movement – depending on the level of stress that was captured in the material, but it can be enough to take the final part out of tolerance, and thus it never hurts to be sure of the stress condition of the piece before starting to cut.

I’ll return to this theme, but with a different illustration of stress effects next time.

Waterjetting 9c - Starting to make hole

This small sequence of posts describes the initial milliseconds during which a high-pressure waterjet penetrates into a target material. Because this work was largely developed using rock targets, most of the illustrations will be with that material, but the concept apples, to a degree, also with abrasive laden jets penetrating into materials such as glass.

For this post I am going to discuss just what happens with the jet being fired down onto the target surface, without either the nozzle or the target moving. Much of this work was carried out in the 1960’s in the UK, though I will begin with some tests that Dr. Bill Cooley carried out using his modification to a Russian hydraulic cannon that he redesigned so that it was capable of firing at 500,000 psi – and yes I have seen it fired at that pressure (I took the photo).


Figure 1. The Cooley Cannon ready to fire at 500,000 psi in an underground mine.

In order to see how effective different processes were in cutting into different materials the international scientific community that was developing waterjet technology at the time needed a method to compare the different approaches. The metric that was used was to define the Specific Energy as the amount of energy that it took to remove a unit volume of the target material. (And in time that will be subject of some more specific posts).

Bill’s cannon used stored gas that was suddenly released as a way of driving the water at the desired pressure, and measured the pressure indirectly by timing the break of two pencil leads in front of the nozzle. This gave the jet velocity, and pressure could be back-calculated from that value.

Dr. Cooley took results from his work and from other scientists working with similar devices, to produce the following graph.


Figure 2. Specific energy as a function of the impacting jet length, measured in nozzle diameters. (Cooley, W.C., "Correlation of Data on Erosion and Breakage of Rock by High Pressure Water Jets," Chapter 33, Dynamic Rock Mechanics, ed., G.B. Clark, 12th Symposium on Rock Mechanics, University of Missouri-Rolla, November, 1970, pp. 653 - 665.)

For those running a conventional cutting table the water orifice is around 10 thousandths of an inch in diameter. So what this graph is saying is that once the first thousand diameters of length (1000 x 0.001= 10 inches) has hit the surface, then the process starts to become significantly less efficient. If the jet is moving at 2,000 ft/sec that length arrives in around 0.0005 seconds. Why this rising inefficiency after that time, and how do we get around it?

Earlier in this series I mentioned that one of the tests to find the pressure at which a waterjet penetrates a target is to note the point at which, instead of the water hitting the surface, and flowing along it, it changed direction to flow back towards the nozzle. This is because, as the jet penetrates it makes a hole, and the only way out of that hole is back along the way the jet came. Unfortunately there is more water still coming down into the hole, and so the water leaving the hole (at the same volume flow rate) meets the water coming into the hole. The rapidly moving water going out is moving about as fast as that coming in, and so, as the hole gets deeper, so the pressure at the bottom of the hole gets less. This has been measured by a number of folk, but Dr. Stan Leach was the first, and produced this plot:


Figure 3. Depth at the bottom of a hole, as a function of the incoming jet pressure. (Leach, S.J., and Walker, G.L., "The Application of High Speed Liquid Jets to Cutting," Philosophical Transactions, Royal Society (London), Vol. 260 A, 1966,pp. 295 - 308.)

Because the holes were preformed of metal (to hold the transducer) and were sized to the nozzle diameter, this is not, as it turns out totally accurate, although it illustrates the problem.

It isn’t totally accurate because, as the illustration from the last two posts showed, the erosion occurs initially around the edge of the jet, rather than under it, and thus the hole created is about twice to three times the jet diameter, rather than being of the same size.


Figure 4. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

Nevertheless as the hole deepens the pressure at the bottom of the hole gets less, and after a while the jet penetration slows to almost a halt.


Figure 5. Penetration as a function of time (My Dissertation)

The sides of the hole, however, continue to erode, but from the bottom upwards so that, after a short while, the narrower entry hole starts to constrict the flow out, and pressure begins to build-up in the hole.

Remember that a waterjet works by growing existing cracks in the material. So that if there is a natural crack in the rock, which may be as small as a grain boundary, or the scratch made by an abrasive particle as it moves back out of a hole in glass, then the water entering that small crevice will pressurize the walls and cause the crack to grow. Often there is more than one, and the result can be, in rock:


Figure 6. Rock breakage around the jet impact point on a 1-ft block of sandstone (after Moodie and Artingstall Moodie, K., Artingstall, G., "Some Experiments in the Application of High Pressure Water Jets for Mineral Excavation," paper E3, 1st International Symp on Jet Cutting Technology, Coventry U.K., April, 1972, pp. E3 25 - E3 44.)

In rock that might not be such a bad thing, since in many cases the intent is just to break the rock out of the way, so that a tunnel can be created that folk can walk or drive through. But in the case of glass and other such brittle materials, where the object is just to make a very fine cut, with no side cracks, cracking the sheet is disastrous. This can be illustrated by the results when a jet was fired along the central axis of a 2-inch diameter core of granite. The escape of water into the cracks allowed the cycle to repeat several times, and the hole was, as a result, much deeper than it would have been if the cracking had not occurred.


Figure 7. 2-inch diameter granite core that split when a short jet pulse was fired into the core, along the axis.

And so, next time, I’ll write about some of the ways in which we can get around this problem.

Waterjetting 2a - growing cracks and glass cutting

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

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.

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.

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).

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.

But first a little demonstration you can carry out. Take a strip of paper, and cut a slit in it half way along the strip and half way through the paper. Now take both ends of the paper in your hands and pull them apart. This causes the cut (the crack) to grow through the paper and gives you two halves. If you do this a second time you should find that by stopping moving your hands you can stop the cut from growing all the way through the paper. Now repeat the process, but use a piece of paper that you have not cut a slot in. The amount of force you need to pull the paper apart is much higher, and I seriously doubt that one you get the tear (crack) to start that you can stop it before it goes all the way through the paper. (Remember this, and I'll come back to it a bit later in time).

The idea of putting cracks on the edge of packages to lower the force you need to tear them open can be found on the edge of lots of candy bars, packs of peanuts and other goodies in stores. The serrated edge acts as a series of cuts or cracks, that concentrate the force applied when you pull on the edges of the packet so that the package tears at a much lower force, and you can control the tear so that you don’t end up throwing all the contents around the room.

Figure 5. Serrations and tear at the top of a packet of honey

Now at this point you might say that there aren’t any cracks in glass, when we start to cut it. If the glass is very new this is true, but with all the chemicals in the air, and the dust that is carried in the wind, although glass can look clear, the surface actually contains a lot of very fine cracks.

John Field, one of the earlier investigators of high-pressure waterjet impact, showed this in one of those brilliant, yet simple demonstrations that, in this case, he carried out some forty-five-odd years ago. If waterjet impact grows surface cracks, and glass acquires surface cracks from damage through being out in the air, then if that surface layer is removed, then the underlying glass will have no cracks. So John took a glass slide, and etched off the surface of the lower half of the slide, by immersing it in acid. Then he fired a very high-speed droplet of water at the point on the slide where the acid etch stopped.

Figure 6. Impact of a high-speed droplet of water on glass. Above the dividing line the glass surface contains the micro-cracks and flaws that come with being exposed to the air over time. The lower section below the line has had these flaws removed. As can be seen the cracks only develop in the unetched part of the glass, where they grow pre-existing cracks, even into the side of the glass that was etched. (Field J.E. “Stress Waves, Deformation and Fracture Caused by Liquid Impact,” Phil. Trans. Royal Society, 260A, July 1966, pp. 86 – 93.)

In a single picture he captured the evidence that waterjets work by growing cracks (top half), and that without cracks there is no damage (bottom half). Understanding this opens up a whole vista of different applications, from the removal of soil from around pipelines underground (the new technology of hydro-excavation) to the removal of damaged concrete, while leaving healthy concrete in place (the developed field of hydro-demolition). And these, and other topics will be part of this series, as it moves forward.

But as John showed, not all the cracks a jet will grow can be seen, and as Vanessa found, they don’t have to be at the surface to create problems. One of her early pieces was entitled “p1.” Within it are an uncountable series of holes, drilled deep into the glass.

Figure 7. Detail of the glass sculpture "p1", by Vanessa Cutler.

One of the skills Vanessa has learned is in controlling the quality of the pierce, and its dimension, but initially there had to be a period of learning.

Figure 8. Single cracks growing out from partial piercings in a test piece during development (Vanessa Cutler).

And so, in the next sequence of posts the simple idea of growing existing cracks will be explored. Mainly, in the beginning, this will focus on cracks that are already there, and how to usefully make them grow. But in some cases we don’t want all those cracks to grow, and that will also come up, as this series continues.