Fatigue failure mystery, need help

[Azmio]

Hey guys,

I am not a material engineer. I need some help here,

Test condition 1

Engine part made out of high silicone Aluminum is subjected to X MPa of stress for 5 hours until it failed.

Test condition 2

Same engine part, subjected to 0.8X MPa of stress for 60 hours. Once the 60 hours of running time is done, the test continues with X MPa of stress up to its failure. Time will be recorded.

My question is, will it take less than 5 hours to fail the engine part if it is subjected to X MPa of stress?

I have referred to my old material eng. book, goodman diagram is handy but i still cannot relate the accumulation of different stress level in contributing to the final failure. Any help is highly appreciated cause I am not an expert in this field.

 

[metengr]

Based on the information in your post, I believe what you need is to understand the application of Miner's Law for predictive analysis of fatigue damage in components.

Summation of n(1)/N(1) + n(2)/N(2) +.... = 1

n(1) refers to number of cycles at a particular stress level
N(1) refers to number of cycles to failure at stress level n(1).



Most material engineering text books should have a section on fatigue damage along with a brief explanation of Miner's Law. If you need more information, consult a materials engineer.

 

[Azmio]

metengr

I dont have any access to a material engineer at this moment. Anyhow, based on this Miner's law, will it fail sooner than 5 hours for the test condition 2?

 

[metengr]

Can't say because you have not supplied cycles to failure in your fatigue tests. All you have supplied is time to failure.

 

[stanweld]

Unfortunately you have not provided the cycles to failure at the reduced load nor have you done so at the X load - only time. Was your failure in 5-hours based on one test or multiple tests? If it was based on one test, there is no way to acurately predict time to failure - only WAGs and gut hunches.

 

[israelkk]

Probably a student post

 

[CoryPad]

The time should be less, although this type of failure usually has large scatter. Averaged over enough samples, condition 2 should have a lower time/cycles to failure than condition 1.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

This could be true, provided test conditions had remained consistent. General conclusions about test results are difficult to quantify without following specific test methods, and proper analysis of data.

 

[Azmio]

We conducted a constant WOT durability test at maximum power and overspeed. The engine part was subjected to over 10e7 cycle and it failed. There were two similar failures happening at the same time at the same period of time.

Anyhow, I think Corypad has answered my question. Thanks buddy.