Electrical Testing and Evaluations

     As regular readers of our newsletter are probably aware, about two and a half years ago we started a new division, American Product Safety (APS), to focus on electrical testing and field evaluations of unlisted equipment. Since then, we've seen remarkable growth in APS, to the extent we now have two employees who focus full time on those activities, and we've recently made an arrangement with an electrical specialist in the Seattle area to provide services for us in that market. So we thought we'd take this opportunity to share with our readers a few of the projects APS has been working on lately.

      For those of you who may have missed our earlier newsletter article, let's start with a brief description of what a field evaluation entails and why it's important. First, why is it important? Well, for starters, federal and state law requires that all electrical equipment be either listed or field evaluated; and "all" means everything that uses electricity, from the toaster in your kitchen, to the computer and monitor on your desk, to the manufacturing equipment at an industrial plant.

     For many items, such as your toaster or your computer, a listing is far more practical than a field evaluation. Simply put, a listing means a product has been designed, manufactured, and tested in accordance with the appropriate standard, and the plant producing the product has the appropriate controls in place to maintain the quality of the product on an ongoing basis. Once a product becomes listed, the manufacturer can produce them by the hundreds or thousands and they'll all be considered to be in compliance with the standard's requirements

     So, what happens if the product is not listed? Well, then it must undergo a Field Evaluation, and that's where we come in. A field evaluation entails an inspection of one particular piece of equipment at its particular installation location to assure that it meets certain minimum requirements of the standard. The field evaluation process is substantially less involved than a listing, and it only applies to the one particular piece of equipment; so if there are multiple pieces of identical equipment, they all must be evaluated individually. Even so, it's much less expensive overall than a listing for equipment with only a few units. The following projects are a few examples of the kinds of field evaluations we've been working on.

Neon Horses

     Our first example is one that some of you may have seen, either in person or perhaps in a short article in the Oregonian a few months ago: Neon Horses. If you've driven down along the central Oregon coast lately at night, you may have noticed several neon tube, brightly-colored, horse-shaped display figures outside several towns. You guessed it — those are the neon horses we're talking about. A local Tillamook artist, Martin Anderson, creates them and leases or sells them to towns and businesses as an artistic means of drawing attention. And unlike real horses, which use oats and the like for energy, these use electricity, so they require a field evaluation in order to be installed.

     From an engineering standpoint, the neon horses are fairly simple devices, so it wasn't too much of a task to conduct a field evaluation on them. Of course, there were 13 of them spread out up and down the central coast from Florence to Lincoln City, so it took our division manager, Sandy Mikalow, the better part of a weekend to go from site to site and evaluate them. (And yes, Sandy did manage to get in a little sight-seeing and other activities during his trip!)

Injection Molding Machines

     Our next example involved a local industrial plant that had just purchased and installed eleven brand new injection molding machines. The machines were not listed, and thus, needed to undergo a field evaluation.

     The injection molding machines are used to make a variety of plastic toys and other items, ranging from yo-yos, to Halloween trick-or-treat buckets, to Humphrey Flyers (you know, those flying toy disks that look like Frisbees™, except they're not, because Frisbee is a trademarked name).

     Now as you might imagine, an injection molding machine is a bit more complicated than a neon horse. Each molding is about as big as a medium-size machine room and contains a conglomeration of wiring, switches, motors, heaters, hydraulics, electronics, and the like. And even though all eleven machines were identical, the law requires they all be individually evaluated, so this project took the better part of a week to complete.

Hydraulic Presses

     Our last example is a little different in that it's not a one-time field evaluation but rather an ongoing annual certification process. In this case, our client had a series of eleven large hydraulic presses, ranging in size from 80 to 800 tons capacity. The presses are used to form the housings for desk top computers, and as you can imagine, it's a good idea to keep your fingers and hands out of the way when the presses do their pressing.

     Not surprisingly, OSHA feels the same way about this, so there's a requirement that the presses be fitted with automatic safety devices capable of bringing the press ram to a full stop within 250 milliseconds once the safety device is actuated. On these presses, the safety devices are "light curtains", consisting of an array of optical beams, mirrors, and sensors which trigger if you reach your hand (or other valuable appendage) in toward the press ram and break the light beam.

     Our assignment was to test the function of the light curtains and emergency stops on all eleven presses. We did this using an oscilloscope, photocell, and a linear potentiometer.

     These are just an inkling of the types of projects APS handles. We look forward to sharing more interesting projects with our readers in future newsletters.

Sticking to the Basics:

Adhesion, Cohesion, and Bonding

     We have had many projects where a component has failed as a result of bonding problems between components. These problems can range from paint that doesn't stick, to braze or solder joints that fail to bond properly, to glue that fails to cure properly.

     Although the failures may seem widely disparate, many share a common cause(s), and the steps we go through to investigate them are remarkably similar. Fundamentally, the investigations can be summarized as a process of four tasks:

  (1) Analyze the components, including the adhesives, for their material composition.

  (2) Analyze for contamination on surfaces to be joined and/or within the materials themselves.

  (3) Investigate the influence of design and material selection on the failure.

  (4) Measure the physical strength of the components and assembly under different conditions.

     Any one of these steps can potentially solve the problem, but of course, we don't usually know ahead of time which one will, so the best approach is to be systematic and pursue them all; This is important because sometimes several different things will contribute to the problem, and only by reviewing all the test results together do we see the relative contributions of each.

     The following cases illustrate how this process works.

Silica Optical Components

     In our first example, a high tech client was experiencing serious difficulties with the silica optical components that were supposed to be attached to magnesium castings; namely, the optical components were literally falling off the castings!

     Now sometimes, we run into problems so subtle it's difficult to sort out whether the "problem" is even outside the range of expected manufacturing variance; this, however, was not one of those cases, as these failures were anything but subtle!

     The optical components were attached to the castings (or, rather, they were supposed to be attached to the castings) with an ultraviolet (UV) curing epoxy glue; this immediately suggested several avenues of investigation, including glue formulation, UV light intensity, and contaminants on the metal surface.

     Our examination showed the glue itself, which remained attached to the castings, was hard and well cured; this suggested the glue formulation and UV light intensity were probably not the source of the problem.

     At this point, we started looking for contaminants. The magnesium casting surfaces were examined and found to be very clean. Of course, this was not a big surprise because we'd already learned the bonds always failed at the interface between the glue and the glass. Epoxy contamination or some strange quirk of formulation or storage would likely have produced failures on both substrates.

     So it was on to an examination of the glass surface. An energy dispersive spectrographic analysis in our scanning electron microscope (SEM-EDS) showed a huge silicon peak on the surface of the glass. (Well, no surprise there — the components were glass after all.) No other elements showed up in more than trace amounts. However, SEM-EDS is not very sensitive to organic materials or elements of low atomic number, such as boron or lithium. So, to increase the sensitivity of our analysis toward organic materials, we rinsed the disbonded glass surfaces with solvent, hoping the solvent would dissolve any organic impurities that might be present. We collected the rinse solvent, evaporated off the volatile component, then examined the residue using FTIR spectroscopy. The analysis clearly showed silicone oil.

     Case solved! Well, almost. We still didn't know where the oil had come from. Luckily, the plastic trays in which the optical components had been shipped were also still available. Using the same rinse technique, we found silicone oil on the inside surfaces of the trays. Presumably, the silicone oil had been used as a mold release in manufacture of the trays.

     Once we had identified the cause for the failures, our client was able to solve the problem by cleaning the optical components in solvent prior to gluing them to the castings. Undoubtedly the client also mentioned the cause for his problem to the manufacturer and insisted that in the future the manufacturer use uncontaminated packaging materials.

Polyester-fiberglass Insulator

      Our next example is a little different in that it doesn't involve bonding between two separate components so much as it involves adhesion within the matrix of a composite material. Nevertheless, the investigative process is much the same as in the first example.

     In this project, the problem was the failure of a polyester-fiberglass insulator that was being used in an electrical distribution system. In this application, several hundred fiberglass insulator bars had been in service under a constant tension load for about 15 years when one of them failed by pulling apart at one end.

     The client was understandably concerned about whether this was simply an isolated occurrence, or whether the other bars still in service could be subject to the same failure mechanism.

     Several possible causes for the failure came immediately to mind: perhaps the failed bar had lost strength through weathering or some other environmental degradation, or perhaps the bar was defective in some way as manufactured, or perhaps the bar had been subjected to some unusually severe service stress.

     The first task, as always, was simply to look at the insulator bar for visual clues. The geometry of the insulator was about as simple as it gets — a long, rectangular bar with a 3/4-inch diameter hole at each end. Insert a steel pin in each hole to transmit the load, and presto! You've got an insulator bar.

     The failure occurred when the pin at one end of the bar had pulled completely through the material. The profile of the fracture (see figure at left) indicated it had occurred largely by internal shearing between the layers of fiberglass.

     Fortunately, only one end of the bar had failed, so we were able to conduct a series of tensile tests on the remainder of the bar. For comparison, we also tested an unused bar. The test results suggested that although the bar material may have been slightly weakened, overall it was still in pretty good condition as far as its overall strength was concerned. This suggested the material probably had not been degraded by weathering, a conclusion supported by a chemical analysis of the fiberglass-to-resin ratio, which showed no significant loss of resin.

     Although the tensile tests didn't reveal any significant strength differences between the new and used bars, a significant difference was observed between the field failure and the tensile test fractures. Specifically, the laboratory fractures were generally characterized by transverse fracture across the fiberglass sheets (see figure at right), characteristic of a tensile mode of fracture; in contrast, the field failure occurred principally as a delamination between the layers of fiberglass, indicative of a shearing mechanism.

     A clue about what was going on here was found around the bolt hole on the nonfailed end of the bar; namely, the bolt hole was elongated. At first, we suspected it was simple abraded, but a close visual examination showed the elongation was a result of stretching at the hole, rather than abrasion. These factors suggested long-term tensile creep of the resin was the mechanism by which the bar had failed.

     Creep is a time-dependant deformation process in which a material gradually stretches under the influence of a sustained, imposed load. Basically, the resin between the glass fibers was plastically flowing, allowing the fibers to slip laterally past one another without actually breaking. Once this progressed far enough, the pin simply pulled through the end of the bar.

MEI-C People

Mark Habel, engineering technician, and Jennifer Anderson were married on December 30, 2000 at the Portland LDS Temple. Mark and Jennifer met one year ago at a Millennium New Year's Eve party. Congratulations, Mark and Jennifer!