Materials Science
Principia has an extensive practice in the area of materials science and engineering. The study of how materials fail forms a significant part of the technical foundation of our firm. We have broad knowledge of the application of materials in various industries such as automotive, aviation, medical, recreation, construction, and electronics. Broad knowledge of materials and their applications is important to determining how and why components fail, and for selecting the proper tools in their analysis.
Materials analysis when combined with solid mechanical engineering analysis and testing forms the foundation of the practice of failure analysis. Skills and experience in both fields are usually required to reach a conclusion about the root cause of a failure. We have the background, skills, and tools to study materials closely and evaluate their specific properties for compliance with requirements and specifications and contributors to the cause or causes of failure. The precise clues to the cause of a failure are transient at best, often disappearing with exposure to the elements, handling, or even well intentioned clean-up efforts. Principia engineers have studied and are trained in the latest documentation techniques including photo documentation, preservation of evidence, and chain of custody.
Metals are one of the most common engineering materials, and among them iron, steel, aluminum, copper, and their alloys make up the vast majority of those in use today. A fundamental understanding of their structure is necessary to properly incorporate them in engineering design. Some metallic alloys such as cast iron are brittle, while others such as low carbon steel are quite ductile; some are strong and stiff, and others like aluminum can be as strong as steel, but exhibit only 30% of the stiffness.
Metals have become one of the most widely used engineering materials in part because of their ease of processing. Types of processing include casting, rolling, forging, forming, powder metallurgy, welding and machining. The structures, and therefore the properties, are greatly affected by those operations. Over 98% of components begin as castings; the rest are made by powder metallurgy or other processes. After casting, many components are worked in some fashion such as rolling or forging. Many parts are then machined to their final dimensions, and may be welded together to form larger more complex components.
A fundamental understanding of the structure and processing of metallic components is necessary to properly analyze their failures. For example, parts made from ductile materials should exhibit significant deformation or stretching before they fail. If they do not, then the part may have been suffering from some type of long term progressive failure such as corrosion or fatigue. Principia engineers have the necessary fundamental understanding of the behavior of metals combined with significant experience examining broken parts to not just correctly identify the modes of failure, but to help avoid failures in the future.
Metals Example: Light Rail High Voltage Line Failure
This example involves the failure of an overhead copper high voltage line used to power a light rail train. Principia was hired to analyze the material and fracture surface to help determine the cause of failure.
The copper wire failed close to one of the overhang supports and an overall view of the fracture surface is shown below on the left. The fracture surface shows the characteristic necking and radial zone of a tensile fracture for a ductile material. Using a scanning electron microscope or SEM, a one thousand times magnification of the surface was taken and is shown below on the right. This magnified section of the fracture surface clearly shows the ductile dimpling consistent with a tensile fracture.
The fracture surface also showed a discolored section. Principia performed x-ray energy dispersive spectroscopy (EDS) on the surface and the discolored section showed a higher ratio of oxygen to copper (an oxide surface). This suggests that the discolored portion had been fractured for some time and exposed to the environment for a period of time prior to the final failure of the copper wire.
The definition of a polymer is “a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent bonds. The term is derived from the Greek words: polys meaning many, and meros meaning parts.” In engineering materials the more common term is plastics. The polymer molecule is composed of thousands of atoms. The spine of these molecules is a carbon chain of atoms, and the three different forms that this chain can take lead to the three basic divisions of polymers used as engineering materials:
- Thermoplastic – these have a linear structure, and the molecules slide by each other upon heating; it is the most common type with roughly 70% of the market.
- Thermosetting – these have a space network structure and are rigid.
- Elastomer – these have coiled structures and can exhibit elastic extension of as much as 1000%.
Compared to metals, polymers have only come into use recently as engineering materials. Vulcanized rubber was the first semi-synthetic polymer, and it was developed late in the 19th century. The first wholly synthetic polymer was Bakelite, and it was introduced in 1909. A description of the mechanical properties of polymers uses much of the same terminology developed for the characterization of metals such as tensile strength and stiffness. However, because polymers are often super-cooled liquids or glasses other properties must also be considered such as viscoelasticity, glass transition temperature, and creep. Metals also exhibit creep, but typically at temperatures much higher than their normal operating temperature. Parts made from plastic however operate at temperatures relatively close to their melting point, making them sensitive to creep or deformation under constant load.
Principia engineers are experienced with the analysis of the unique properties of polymers as differentiated from their metallic counterparts. Under the same conditions of loading and environment a polymer part may exhibit drastic differences in performance compared to metallic part. For example the metallic part may suffer corrosion while the polymer may be resistant to corrosion.
Polymers Example: Paint Transfer Analysis for Automobile Collisions
A great example of a common polymer is paint. In vehicle accidents, paint transfer can occur between the surfaces of the vehicles. When there is some question about the involvement of a vehicle in a particular incident, paint matching provides strong evidence about the vehicles involved in the incident.
This particular case involved a collision between a yellow Toyota pickup truck and a green Ford Ranger. There was some question as to the involvement of the yellow pickup truck in the accident. Principia’s inspection of the green Ford truck revealed yellow transfer marks on the front reflector. These yellow scuffmarks are shown in the figure below. Principia compared the yellow material from the reflector to a paint sample removed from the yellow truck. Fourier transform infrared spectroscopy was completed on both samples to determine the organic compounds in the layers of each paint sample. The results showed an exact match for all the paint layers, providing strong scientific evidence that the yellow truck did indeed make contact with the green truck.
 |
 |
Ceramic materials can be divided into two groups, conventional ceramics such as glass, structural clay, refractories and whitewares, and advanced structural ceramics that were developed for gas turbine, and electronics applications. Ceramics are characterized by their hardness, strength, resistance to wear, and resistance to high heat. Their drawback, however, at least for conventional ceramics, is their brittleness – only 2% the toughness of metals. Advanced structural ceramics are being developed specifically to address this problem.
To understand how ceramics are used in components, a different way of thinking about them compared to the way we think about metals is required. Instead of involving plastic flow on slip planes, as in metals, brittle fracture occurs on well defined cleavage planes, or by irregular fracture as in glass. Principia engineers have the training, fundamental background, and experience to design, analyze, test, and predict the performance of ceramic parts in actual use. We have examined and conducted design analyses on hundreds of ceramic parts over the years, including many that have suffered failures in service. Our experience includes conventional ceramics such as structural glass curtain walls, automatic fire sprinkler frangible glass bulb triggers, beverage containers, vitrified clay pipe, and concrete masonry units, as well as advanced ceramics used in computer chips, and on the Space Shuttle thermal protective system.
Ceramics Example: Automatic Fire Sprinkler Frangible Glass Bulb
The picture on the left below shows a glass bulb trigger removed from a residential automatic fire sprinkler. Normally the bulb is filled with liquid all but for a small, measured amount of gas. As the temperature rises, the liquid expands and the pressure inside the frangible bulb increases until the bubble is driven into solution. At this point a very small additional increase in temperature results in an extremely high increase in pressure fracturing the bulb at its prescribed preset temperature.
 |
 |
Normally when the trigger fires, the bulb breaks into many smaller pieces. The bulb shown in the picture is different. The sprinkler from which it was removed was inspected and found to have no liquid in the bulb. The sprinkler was disassembled, and the small tip was found broken. The picture on the right is a 100X magnification of the glass fracture surface showing the fracture initiation toward the lower left. To the upper right we see the compression hinge characteristic of a bending type load causing the fracture. This is not the type of failure we might expect from bulb overpressure. This was likely caused by high bending loads resulting from contact with the metal trigger seat at the time of assembly, or high inertial loads caused by mishandling.
The term composites came into common use late in the 20th century to describe a class of materials that found their way into the aerospace industry. Composite materials, however, have really been around since the first clay bricks were reinforced with straw; other examples of composite materials include steel reinforced concrete, and glass reinforced polymers, or fiberglass.
The most common of the modern composites is a graphite fiber reinforced epoxy polymer. In this application, parts of aircraft or spacecraft are built up in layers to form laminated sheets consisting of individual graphite fibers arranged side by side and adhered together with epoxy polymer. The sheets are cut and stacked to form the desired geometry, then heated in a vacuum to flow the polymer in, and around the layers; after curing, or cooling, the part becomes strong and stiff.
The most desired characteristic of modern composite materials is that they can be custom designed by arranging the different layers at different angles relative to each other. The strongest and stiffest orientation of an individual layer is parallel to the direction of the fibers. By custom designing a part, the engineer can put the strength or stiffness only where it is needed, thus saving on material cost and, most importantly, weight.
The design of composite structures requires a strong fundamental background in the mechanics of materials. Unlike metals, polymers, and ceramics, composite materials are not of uniform composition, and they exhibit different strength and stiffness when loaded in different directions. Because of this the designer must not only be familiar with the traditional methods of predicting stresses in parts, but must also be familiar with the internal mechanics of the material – how load is shared internally in the part among the different layers.
Principia engineers are trained in laminate plate theory which is the fundamental basis for modern composite material design. Properly evaluating the performance of a composite structure requires this background, as well as our years of experience examining, analyzing, and testing composite structures in the field. We have developed and written our own customized software for the design analysis of composite structures. We have used this code for many projects, and validated its output, comparing results to real-world testing.
As unique as composites are in their advantages, they are also unique in their drawbacks; compared to metals or polymers, they can be brittle, and they do not readily exhibit damage. Principia engineers have developed the unique skills and tools necessary to properly examine composite structures for damage. This includes non-destructive techniques such as acoustic emission, ultrasonic, thermal infrared, and x-ray, as well as destructive techniques such as burn off and dissection, and cross section mounting and polishing.
Composites Example: Fiberglass boom failure
 |
One of the most common composite materials in use is glass fiber reinforced polymer, more commonly known as fiberglass. This boom truck used in the maintenance of overhead power lines suffered a failure of its entire fiberglass boom dropping its bucket to the ground. |
 |
This schematic diagram shows the area of the boom where the fracture occurred. We would expect the “BOTTOM (when stowed)” side of the cross section to have primarily tension loads under static conditions, and the “TOP (when stowed)” side to have compressive loads. |
 |
The SEM photos on the left show that a portion of the cross section expected to have tensile loading at the time of the failure, actually suffered from compressive fracture. This suggests that the boom did not suffer its failure all at once, but actually had weakened over time prior to the final fracture. |
 |
Other damage was found to the boom, remote from the area of the fracture. The cross section shown at the left is from the outside surface away from the failure location. Cracking in the matrix is seen here. |
 |
A sample of the boom was heated in a ceramic crucible to temperatures high enough to ignite and burn the polymer matrix, but not high enough to damage the glass fibers (ASTM D2584). This is a common method to measure the weight percent of the reinforcement as well as determine the number and relative angles of the stacked layers of reinforcement. |
Principia uses the latest, most advanced tools to aid in the examination and analysis of failures. Much of what can be learned about the life of a broken part can be observed with the naked eye; however certain features such as fatigue striations, inclusions and other defects, or contamination can only be observed with the aid of modern examination tools. A sample listing of these tools are:
- Optical microscopy
- Scanning electron microscopy (SEM)
- Energy dispersive spectroscopy (EDS)
- Fourier transform infrared spectroscopy (FTIR)
We use metallographic techniques in both the traditional sense of examining the microstructure of steel and other metallic materials, but also for examination of composite materials to determine the sequence of layup for the many laminated layers. In both senses, the material requires significant expertise and time to properly mount and polish for examination.
Similarly, clues to the performance of the part are often hidden in the material structure itself such as a metal’s alloy composition or strength; these for example can be determined with tools such as hardness testing and inductively coupled plasma analysis.
Often destructive examination of a part is not allowed, or may cause damage to the part that is difficult to distinguish from the original failure. In these cases, Principia is familiar with many so called non-destructive techniques of examination such as:
- Ultrasonic
- X-ray
- Thermal image
- Acoustic emission
- Magnetic particle
- Dye penetrant
These tools are usually used to reveal flaws in parts that are not readily detected by an external examination. Because flaws can seriously weaken a part, and are difficult to detect, non destructive techniques are often employed in the manufacturing process to control quality and boost the performance of the final product. Principia engineers are familiar with these techniques, using them in the field to aid with the failure analysis of a component such as using thermal imaging to locate a delamination in a glass fiber reinforced epoxy pipe section, or helping a chip manufacturer find poor quality solder joints using ultrasonic emission and detection.
| 
