Plastics & Composites

A plastic component failure analysis should provide the answers to the following questions:

a. Which component failed first?

b. What was the mode of failure, How did it fail?

c. What were the trajectories of failed parts and the consequential damage ?

d. The root cause of the failure?

e. What are possible orrective actions ?

On-site investigation and reviewing operating conditions and maintenance records may be essential to determine the most likely sequence of events in a failure.

Microscopic analysis of the fracture surfaces can determine the nature of the fracture. Was it fatigue failure or simple tensile fracture? Where did the fracture initiate? Is the material pure or are there inclusions that act as stress concentrators? Or does the design itself indicate that unacceptable stress concentrations are being developed during normal use? Or does it appear that the failures occur when the mugs are mis-used (dropped or hit, for example)? It is often also useful to re-create the failure conditions, to force the mug to fail in the same way as the field samples, that is, to produce a fracture pattern the same as the ones in the field samples.

Fourier-transform infra-red spectroscopy can determine if the material in the mugs is as specified. A common problem in plastics that need to be flexible is loss of plasticizers during use, leaving the material increasingly brittle and commonly leading to failure. In this case, are all the original ingredients still in the material? Plastics often develop their desired properties by being manufactured under tight temperature regimes. Does it appear that this plastic has the properties it was designed to have? Controlled temperature testing can often simulate or stimulate changes that can guide the analyst toward answers to these questions. It is often informative to compare the failed mugs with today’s production to see if they are identical. If not, why not?

Most plastic components are a simple single-phase injection-molded plastic. A more complex situation occurs when the plastic is the continuous phase surrounding a discontinuous phase of filler or fibers, creating a multi-phase composite. Composites are common in our workaday world today. One of our MATCO scientists got in on the beginning of carbon fiber composites, making hand lay-ups of carbon fibers in the laboratory for spars of the America’s Cup yachts in the 1970’s! Since then carbon fiber composites have become quite common, found everywhere where exceptional strength and low weight are vital characteristics of the finished product. Tennis racquets and competition bicycle wheels are two high-end examples.

Failure of composite materials such as that from which the large collapsed acid tank on the right above was made usually occurs in a debonding of the interface in fiber-plastic (FRP or GRP, for example) or fiber-rubber ( steel or glass reinforced tires, for example). The usual question is: why did the bond fail? And the usual choice of answers is: improper surface preparation (foreign material at the interface interfering with the bonding), improper materials (usually having to do with the volatile components of the starting materials) or improper production procedures (ambient conditions out of specifications is a common problem). It is also possible that the use conditions exceeded the design specifications, as in when a tire hits the curb at high speed and a tire blows out. All of these “improprieties” have to be discovered or inferred from microscopic analysis, surface analysis or simulation of failure modes.

Of course there are other types of composite than fiber-plastic. The most common is concrete, where the cement paste is used to bind the aggregate and sand together to form a high-compressive strength material. Corrosion of included reinforcing is a major problem, but improper mixing and incorrect materials are also often to blame for concrete failures.

And there is at least one other important type of composite: those made from ceramic materials, usually designed for high temperature use. One of the favorite and most common of these is Corning Ware, actually composed of lithium aluminum silicate crystals and a glass binder. This produces a remarkable material which has a virtually zero coefficient of temperature expansion. Perhaps you remember photographs of Corning Ware dishes with a blow torch on one end and the other end sitting in a block of ice? From the oven to the table, it doesn’t crack. But most ceramic composites have other purposes, most commonly in uses in industrial furnaces. Here techniques borrowed from geology are useful in analysis of failures, using the petrographic microscope for transmitted polarized light examination. Inhomogeneties, improper production, out-of-design use or even poor specifications are all common causes of failures.

MATCO has a deep and broad background in all of these areas and can certainly help you move forward to solve problems in plastics and all forms of composites.

Our team is comprised of certified and experienced personnel from a variety of technical disciplines including: Polymer engineers, mechanical engineering, paint inspection, chemistry, metallurgy, and materials science. We also have a vast array of laboratory and field testing equipment. All of

MATCO's testing equipment is calibrated on a routine basis in accordance with both national and international standards, and is ready to be put to use at a moment's notice.

The MATCO team has experience working in many diverse fields and with many different clients. Some of our clients include Allegheny Energy, US Airways and AC Dellovade, GE, IBM, NASA, Valmont, and TVA.

Our team includes NACE Certified Corrosion / Cathodic Protection/ Materials Selection/ Design / Coating Specialists and Technicians (*) and other materials experts including:

Dr. M. Zee *                    Mr. Tom Thomas *

Dr. Don Gibbon               Mr. Ed Larkin *

Dr. George Bayer            Ms. Debra Riley *

Dr. Huiping Xu                 Mr. Marty Latona

Mr. Walter Gretz, PE       Mr. Antonio DiNunno