It is very common to investigate failures where the cause is the use of an “inferior” material. As an example, the client wants to perform a failure analysis of a widget that exhibits cracking. After performing the analysis, we conclude that the mode of failure is a stress overload event, or that the widget cracked due to chemical degradation. Then, the questions are: Did it fail due to an inferior design while it was used in its expected environment and service conditions? Or did it fail because it was misused/abused? The answers to these questions could be quite complex in some situations due to the unknowns of the service life for the widget. However, for simplicity, let us assume that the widget was used in a controlled environment where the customer was certain it was not misused. Then the next question is; Was the widget under-designed? Without reviewing the geometry and material of the widget, this is yet another unanswered question. In this case, design refers to the combination of geometry and material selection. Is this a problem of material or geometry? Eventually, it can be both, since you could have a widget designed with the best geometry, but it failed due to the use of a poor material. Conversely, you could have a widget manufactured from a great material, but still have a faulty geometry that can lead to failure. You would design the geometry for the material or select the material for the geometry, but eventually the process must be concurrent.
A different situation arises when the customer tells you that this widget has been in the field for five years with no failures. Once you start investigating, you discover changes in the customer’s supply chain, such as a new resin supplier or molder. Material testing of good and bad parts quickly shows that the new material is inferior, or a geometry checkup reveals that it has changed at the failure location. Once this conclusion is reached, it is easy to point the fingers at the manufacturer. However, in many occasions the problem is a deficiency in specifications. In this particular example, even further investigation reveals that the prints, drawings and documents used to specify the widget refer to materials or geometry details in generic terms. As an example, the drawing indicates that the material of the widget is polypropylene (PP). The problem is that there are hundreds of PP resins available in the market. Your overseas manufacturer assures your purchasing department that they can mold this widget at 30% off the cost of the previous manufacturer. Unbeknownst to you, their quote assumed a PP resin with inferior properties than the current PP used to mold the widget. From the new manufacturer’s perspective, they have met your specifications. Remember, that the drawing specifies “polypropylene.” The type of PP resin to be used was not properly specified. The new manufacturer quoted the widget assuming the cheapest PP in the market, which ended up being inferior in quality, and produced widgets that underperformed.
The above material specification examples can be extended to geometrical specifications. A typical specification as shown in drawings is “Round all corners to 0.04 in.” The designer should be careful with this general statement. First, critical regions of the part that are exposed to elevated and continuous stress may need a radius much larger than 0.04 in. to avoid premature failure. Second, just because this statement is there, you should not assume that it will be followed. It only takes one edge to not be rounded for premature cracking and failure to occur. Therefore, the final geometry of the part should be double-checked. This should also be accomplished every time the mold is replaced or refurbished.
These are just a few examples of problems arising from insufficient specifications in a drawing or documentation. The designer would be in a better position to be as specific and detailed in the drawing as possible to reduce the chances of future problems. Especially, when years later, any of the suppliers in the supply chain could be replaced. In an ideal situation, when specifying a material, the designer should detail the resins(s) to be used. This is the same resin that was used throughout testing and validation of the widget. Many times, a resin callout is followed by the expression “… or equivalent.” However, what does an equivalent material mean? Since the meaning of equivalent is vague, this is another specification that could cause future problems. Another method commonly used is to specify a material based on one or two short-term properties. If you have been a reader of this newsletter, you are familiar with long-term properties, creep failure and environmental resistance of plastics. Materials with comparable short-term mechanical properties can have dramatically, different long-term behavior. By the same token, the chemical resistance of the widget would be unrelated to short-term properties. Therefore, beware of specifying materials solely based on short-term properties.
In summary, communication along the supply chain is of extreme importance. Language and cultural barriers need to be managed to assure effective communication. The details about material and design should be specific in the prints and documentation. If more details are needed, a callout to a quality control document can be made where approved specific requirements, material resins and suppliers are listed.
