Why was it failed?

If the damage is caused by mechanical reasons such as breakage, wear and deformation, the correct answer will always be "Because the tension (stress) exceeds the strength." It is the job of the failure analyst to find the root cause of this.

70m wind turbine that collapsed due to a storm (Burglind) near the village of Volksdorf, Stadthagen, Lower Saxony / Germany.

When starting a failure analysis, I always convey to my customers who apply to me for analysis that the human factor can be a strong factor.

So who writes your material specifications?

As I have seen so far, except for some aviation, space and defense industry companies, these specifications are written by mechanical engineers who are designers.

I'm definitely a fan of using industry-wide accepted standards. If a part is being designed to operate in an impact environment, a mechanical property requirement must be established that has been validated as adequate by some form of competent calculation or computer modeling. Determining test locations and methods is also important for the design function (best accomplished, in my opinion, by a designer and a materials engineer working together).

Recently, while examining the damage mechanism of a defense industry part made of SAE 4140 material with a diameter of 4 mm, we noticed this. The desired hardness was 40-45 HRC and the heat treatment supplier report confirmed this.

Do you think the Rockwell C method can be applied on such a small scale?

The answer is of course no! In the worst case scenario, if there was a distance of 2.5 times the diameter of the trace to be created from the edge to the inside, the part was cut and bakelite and the hardness was checked close to the middle point. Neither the heat treatment supplier nor the defense industry access control engineer had implemented this practice.

So what did we do? After cutting the part and baking it, we scanned the hardness of the material from the surface to the inside using the Vikers method and a 1kg weight.

The hardness we found was over 500 HV1. We can think of the Rockwell C equivalent as 48HRC.

Due to ignorance, the heat treatment supplier and input quality control had tested the small diameter cylindrical material on its surface and obtained an incorrect result. Their conclusion was that while the material was OK, according to a simple measurement we made, the part was RED and this caused the damage.

Who do you think wrote this heat treatment and control specification?

Who writes your specifications?