Engineering ethics of the design of longevity

Engineering ethics is part of any engineer's life. Most every time their pencil meets the pad and every click of the mouse there is an ethical motive behind it. Most of the issues are not big, but small things throughout a design. The biggest ethical concern is with public safety, this is probably pretty obvious to most people. When a civil engineer is building a bridge everyone's life, who passes over and under the bridge, is indirectly in engineers hands.

An issue that has always revealed its face to me is that of engineers designing things to break. I would have my dad say 'why would they put that here, its just going to break from this' or 'this is the xth number of this part I have had to replace'. To me it seems crazy that a company could allow somthing to be desined to fail, so I never believed it.

Lokilech. "Stress Fracture of Aluminum"

After a few years in college I realized that it was a reality  Consumer products could be and were design to fail after a certain point. Sometimes it was for money others for safety. How could they be designed in this way? Most commonly by geometry and/or material selection. Not to be confused with bad engineering or poor manufacturing. One example of this is utilizing fatigue.

Fatigue - lowering of strength or failure of a material due to repeated stress, which may or may not be above the tensile strength.

Data can be represented in a stress vs number of cycles graph. As the number of cycles increases the max stress decreases. These charts are well defined for many different material. This means that a part can actually be designed to fail at a total number of cycles. In the automotive business the standard number of cycles required to withstand is usually around 500 million. It needs to be understood that theres graphs plateaus off at a certain stress. This means it is possible to design a part that could withstand an 'infinite' number of cycles, you could say it will last forever.

Schlichting, Austin. "SN plot for a brass alloy" 27 Feb 2013

This is where the dilemma lies. Should a company design parts that could last for the life of the machine or ones that will break after x amount of time? In some cases it might be a saftey hazard to have a part break, so an attached piece could be designed to break after an amount of use (that is well under the fatigue life of the dangerous part).

Fatigue Life - The total number of stress cycles that will cause a fatigue failure at some specified stress amplitude. 

If safety is not an issue, it is really up to the company on how these things are designed. I am more on the side of designing things to last for ages. I am willing to bet that there are lots of companies that do design stuff to fail at a certain point. They are probably pretty successful companies. The reasoning behind it is to force consumers to keep coming back to buy parts. The downside to this is consumers might think your product is cheap and might not buy the next new thing from the company. If things were built to last forever consumers would like the durability and linke the brand with quality  That single person may buy things from the company over their lifespan and may recommend it to others. Looking at the downside you might find that the one product does it job and never works resulting in no need to upgrade or purchase parts. To me, the ideal would be to design a product to last as long as the original owner is interested in it.

In the end it is the companies decision on how to design their own parts. If an engineer is on one side or the other they may want to carefully look at what a company is doing before acceping a job with that company.

Here is an example of digital fatigue testing.