by K.L Kosanke
When firing by hand, the problems of steel mortars can easily be over-looked. However, even for relatively small electrically fired displays, steel mortars and the usual alternatives (paper and PVC mortars) have limitations that are difficult to cope with.
Steel mortars are heavy; when several hundred may be needed for a show, their weight can easily exceed 5000 pounds. This may necessitate the use of a large truck and must certainly be seen as undesirable by the crew that will need to handle them several times. Service life is long, but high initial cost (about $1/ pound) is another problem. Finally, even though steel mortars are quite strong, there is the possibility that a shell detonation may cause the production of dangerous mortar debris.
Paper mortars are lighter (about 1/3 the weight), cheaper (about 1/4 the cost) and produce less dangerous debris than steel mortars. However, the service life of paper mortars is generally quite short; probably about ten firings on average. Spiral tubes soon tend to have inner wraps peeling up which can bind a shell. All paper mortars will delaminate inner wraps if the mortars are even slightly damp when fired. Even treated paper mortars can be ruined by a single exposure to rain.
PVC mortars weigh and cost about the same as paper mortars. They are water proof and thus have a service life longer than paper. However, because the strength is less than steel mortars, so is their service life. The real problem with PVC mortars results from the combination of their only modest strength and the mechanism of their typical failure. Often a flowerpot and certainly a shell detonation will result in their destruction. Because PVC is relatively brittle, mortars over-stressed in this manner fracture and fragment. In addition, the fragments will usually have sharp edges and be pointed. This high velocity debris has the potential to do significant damage to nearby mortars and people.
A plastic mortar that retains all the good characteristics of PVC (low weight, modest cost, long service life) and eliminates the fragmentation problem would be near ideal. What is needed is a plastic that is strong, inexpensive and ductile. For the mortars to have sufficient strength requires a tensile strength approximately equal to that of PVC. For the mortars to be inexpensive requires not only a low base material cost but also that large amounts of pipe with the proper characteristics are already being produced commercially. Ductility is important because it results in a stress failure mode that might be described as bursting rather than fragmenting. Figure 1 is a series of sketches comparing the failure of less ductile (brittle) and ductile plastic mortars. Although fragments are still possible with ductile materials, often none will be produced. Also if any fragments are produced, they will have been stretched thin and will thus meet with greater air resistance, and will be slowed to harmless velocities sooner. Finally, if the material is somewhat flexible, fragments will tend to bend rather than penetrate on impact.
There are a number of promising plastics worth considering, but the one with the best overall characteristics is high density polyethylene (HDPE). This is basically the same material used to make plastic milk bottles; however, it is significantly stronger and is normally black in color (as used by the pipe industry). Table 1 is a subjective comparison of characteristics of steel, paper, PVC and HDPE mortars. As can be seen HDPE has characteristics that make it highly suitable for use as mortars in electrically fired displays.
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|| | | | Number of |
Mortar || | | | Fragments If | Danger of
Type || Weight | Cost | Strength | Over-stressed | Fragments
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Steel || High | High | High | Few | High
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Paper || Low | Modest | Low | Few | Low
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PVC || Low | Modest | Low | Many | Moderate
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HDPE || Low | Modest | Moderate | Few | Low
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In Table 1 note that HDPE's strength is listed as greater than PVC. This was
done even though the tensile strength of PVC is reported as exceeding HDPE.
When I initially considered HDPE, I assumed I would need to have special
extrusion dies made and produce the pipe myself. Commercially produced pipe
had wall thicknesses and pressure ratings that seemed inadequate for use as
mortars. However, during early tests to determine the necessary wall
thicknesses, I was pleasantly surprised by the unexpectedly high apparent
strength of HDPE. Mortars made of HDPE pipe, with pressure ratings of only
about 1/2 those for PVC mortars, consistantly were capable of withstanding
equal or greater stress than PVC mortars. The reason for this seems to be
related to their differing failure modes, and the nature of tests performed
to determine pressure ratings.
Figure 2 shows the approximate relationship between mortar strength and increases in mortar diameter due to internal pressure. The first portion of the graphs, when strengths are nearly constant, is characterized as elastic expansion of the mortars. Mortar strength falls slightly in this region because as it expands there is a slightly larger surface being exposed to the internal pressure. Mortars not stressed (stretched) further than their elastic limits (points EL in Figure 2) will regain their original diameter when the stress (pressure) is removed. Also they will not have suffered any permanent loss of strength. For PVC mortars the elastic limit is reached quite soon, and fracturing and fragmentation occurs when the elastic limit is exceeded. This results in the strength of the mortar falling essentially instantly to zero. For HDPE mortars, the elastic flexing region is larger than for PVC. Also, when the elastic limit is exceeded, the mortar does not fracture, rather there is a flow (inelastic stretching) of the material. However, unlike the elastic flexing region, now the mortar will no longer return to its original diameter if the stress is removed, and there will be a permanent reduction in its strength. The loss in strength is the result of the pipe walls stretching permanently thinner. In this region of irreversible stretching the strength of the mortar falls more rapidly, but not instantly as it did for the PVC mortar. Finally, after continued stretching, the mortar tears open and its strength falls to zero.
Pressure ratings for plastic pipe are a fraction (usually 1/2 to 1/4) the maximum pressure they can withstand for very long lengths of time when heated to their maximum operating temperatures. If the pipe material is elastic and deforms somewhat easily (desirable properties of mortars and mortar fragments), it may not stand up to pressure well over long periods of time. This is because as pressure is applied the pipe slowly expands a little; this causes the wall to stretch slightly thinner and exposes slightly more surface area to the pressure (i.e. the effective strength of the pipe is reduced). If the pressure is maintained, the pipe stretches further and weakens more. The process may continue until the pipe bursts. This does not mean that the pipe will not be able to successfully withstand a very short (fraction of a second) exposure to pressures many times greater than would burst the same pipe if sustained for many hundreds of hours. In comparison, a pipe that is more brittle, essentially unyielding under the effect of pressure, might withstand considerably higher pressures, until it fails catastrophically by shattering.
In a mortar, high lift pressures are sustained for only a small fraction of a second and are not uniform along the length of the mortar (i.e. they are high only below the shell being propelled). This sudden and non-uniform application of pressure may be considerably more stressful for a more brittle material like PVC than for one that is more ductile like HDPE. At any rate, this appears to be the case in practice, HDPE mortars (with rated pressures only 1/2 those of PVC) demonstrate equivalent or even superior strength in comparison with PVC mortars.
Below is a summary of the initial tests that were performed on HDPE mortars.
Test | Results
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Destructive test: Salute | Mortar was turned completely inside out and several
in a short mortar at | tears occurred at the end of the mortar where the
about 900F | salute was positioned. However, it appeared that no
| fragments left the mortar and the split ends of the
| mortars were thinned by stretching so that they were
| relatively flexible. Also, although the mortar was un-
| supported and above ground, it came to rest only
| three feet from its starting position.
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Destructive test: Salute | The mortar was turned completely inside out and a
exploded in a short | small number of pieces were torn from it. However,
mortar cooled to about | the pieces were all stretched quite thin resulting in
150F | their being relatively flexible and having large surface
| areas in comparison to their weight. It seemed likely
| that a person, if struck by a piece, might receive an
| abrasion but a serious puncture wound seemed unlikely
| Also it was difficult to imagine that adjacent mortars
| mortars could have been damaged by mortar fragments.
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Rapid fire test: Five 4" | The mortar successfully withstood the test even
canister shells with | though the temperature of the outside of the mortar
normal lift plus five | measured - after the test - rose to over 1500F.
50% over-lifted were |
fired from the same |
mortar in approximately |
5 minutes. The final |
shell flowerpotted. |
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Rapid fire test: Five 4" | The mortar successfully withstood the test.
canister shells 100% |
over-lifted were fired |
in 3 minutes. The fifth |
shell flowerpotted. |
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Strength test: One 4" | The mortar successfully withstood the test.
shell 200% over-lifted |
was fired from one of |
the above mortars. |
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Strength test: One 6" | The mortar successfully withstood the test.
spherical shell with |
normal lift and one 6" |
canister shell 50% over- |
lifted were fired. |
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Durability test: Ten | Inspection of inner mortar surfaces revealed a moder-
additional normally | ate build-up of black powder combustion products.
lifted 4" canister shells | After mortars were washed, inspection of inner mortar
fired from each of two | wall surface revealed essentially no deterioration.
mortars used in the |
rapid fire tests. |
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Final 4" test: Ten Addi- | The mortar successfully withstood the test. (Note, the
tional 4" canister shells,| mortar had previously been used to successfully fire
each 200% over-lifted, | 20 shells.) There were no visible signs of mortar de-
were fired in 6 minutes | terioration.
from a mortar cooled to |
300F.
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As a result of these initial tests, it seems clear that HDPE mortars have
both sufficient strength and service life to be used successfully as mortars
for electrically fired shows. In addition, their failure mode strongly
suggests they represent considerably less hazard to people and adjacent
mortars that do PVC mortars. Accordingly, we have decided to replace 100% of
our paper mortars (about 800) with ones made of HDPE.
Note that because HDPE mortars are quite resilient, it is possible to plug the mortars with wooden plugs stapled in place. Pre-drilling or screwing are not necessary as is the case with PVC mortars.
In conclusion we (Kosanke Services, Inc.) offer to share the experience we have gained with HDPE mortars with anyone wishing further information (phone 303-245-0692). This includes an offer to supply a limited number of test mortars for evaluation and a list of specifications and sources for HDPE pipe with the proper characteristics.
(Note: I attempted to have an expert from the plastic pipe industry review this article before its publication; I was not successful. Thus while I believe the technical background material presented in this article is accurate, I cannot guarantee it.)