Tuesday, 24 November 2020

Plastic Part Failure – Part 1: Reasons

 


In this two part blog series we will shade some light into a very interesting and important topic: plastic part failure.

Parallel to this two part blog post series I made a presentation which can be watched here on YouTube:



The field of plastic part failure analysis is wide and we will focus first on the “why plastic parts fail”, together with showing the main reasons. After this we focus on the antidote – what can we do to prevent failure?

Why Plastic Parts Fail

In general, product failure is a costly business. There may be several consequences of part failure such as product liability which can result in significant settlements and penalties [3]. For example a manufacturer may be held liable if the product is defective. Furthermore, if the product is manufactured in a defective way and proper testing as well as inspection was not conducted, then product liability may be enforced too. There are several more reasons (missing of adequate labeling; missing instructions and warnings of the product).

“Nobody Wants to Air Their Dirty Laundry in Public”

In the past it was difficult to guide designers on plastic part failure. Plastic part failure was kept secret, since nobody wanted to air their dirty laundry in the public.

However, there was a study published by David Wright [1] which classified the causes of failure for  over 5,000 failed plastics parts. One of the amazing key findings was that the vast majority of failures were avoidable.

What was the big problem? The know-how on how to prevent failure was publicly known , however it was inadequately communicated along the plastic part manufacturing chain. This chain usually consists of specifiers, designers, processing companies, purchasing, and material suppliers. Designers might be aware of certain material differences and their impact on the overall part performance. Contrary, material purchasers might choose a cheaper material without knowing the aforementioned impact on the part performance.

Causes of Failure

Mr. Wright shows in his study two viewpoints on the causes of failure:

1.     Phenomenological causes of failure: in this viewpoint failures are attributed to a physical mechanism (Figure 1).

2.     Human viewpoint: in this viewpoint failures are attributed to human related decision making and execution (Figure 2).

Figure 1: Overview phenomenological causes of failure [1].

Figure 2: Overview causes of failure from the human viewpoint [1].

Figure 1 shows that environmental stress cracking (ESC) is the biggest cause of failure in plastic parts (30%), followed by static notch fracture (20%), and dynamic fatigue (19%). 

Interesting to see are the human caused failures in Figure 2. Here, material misselection and poor specification are with 45% by far the biggest reason for plastic part failure. The other reasons are fairly equally distributed.

There are several known cases where misselection and poor specification lead to catastrophes.

One of them was the space shuttle Challenger disaster from 1986 (Figure 3). 


Figure 3: Space Shuttle Challenger Disaster [2].

The space shuttle broke apart 73 seconds into its flight. All seven crew members were killed. The so-called “Roger Commission” was initiated, where Dr. Richard Feynman was part of the investigation. They found that the accident was caused by a failure of the O-ring sealing joint on the right solid rocket booster. The selected O-rings showed less resilience at 10°C. On the flight day it had 2°C and the seals were never tested at 10°C and below temperatures. Dr. Feynman presented the low resilience by putting the O-ring in ice water. He took it out and stretched them. The rings did not return to their original position.

This example highlights that material specification, selection, and testing are crucial points of having a proper function plastic part.

Polymer material selection as the antidote of plastic part failure will be discussed in the second part of this blog series.

Thank you for reading and #findoutaboutplastics

Greetings,

Herwig Juster

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Literature: 
[1] David Wright: Failure of Plastics and Rubber Products Causes Effects and Case Studies Involving Degradation, 2001, Rapra Technology Ltd.
[2] https://www.space.com/31732-space-shuttle-challenger-disaster-explained-infographic.html
[3] Jenny Cooper et.al. : Why Plastic Products Fail, Smithers Rapra Technology Ltd. 2010


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