“Know your enemy!” Common material and design fails and why!

June 8, 2016

 

All too often I see cracked components on machinery and would like to share some insight as to why and how to eliminate the problem at it’s source.

 

Metal fatigue

 

Metal fatigue (partial or complete failure) is the result of excessive cycles applied to a material. This may be resulted by dynamic loading from lifting or lashing or even as simple as vibrations from driving down a dirt road. Materials such as steel are given a fatigue limit from factory and is an important consideration when designing products especially those in non stationary environments (things like engine & gearbox mounts, hydraulic/bull hose carriers etc are commonly overlooked areas).

Fatigue analysis can be performed with advanced FEA software but here’s a few things i’ve come across that might help in the design or repair phase.
 

Material grades! And their weldability…

 

This is a major and something I can’t possibly compile in this blog post however I cannot stress enough how important it is to follow the material data sheets (easily available!) from the manufacturer in regards to welding. With special focus on pre/contra and post heating/cooling methods you can go a long way into eliminating down-time and “warranty work”. Most common failure is the welding of 4140 Steel, It can be done! whether it be 4140 to 4140 or 4140 to another grade of steel the choice of electrode/wire and as mentioned heating requirements will ensure the joint is as good (as close as) the parent metal.

 

Gussets and Compensation plates (comp plates)

 

This is a big one and isn’t clean cut but definitely a worthy consideration. Gussets can be your best or worst friend and here’s an FEA to highlight the difference.

Note: the gusset on the left (the one EVERYONE uses!) localizes the load into a point thus often causing cracks. Gusset on the right (lap jointed i like to call it…) evenly distributes the load over an area resulting in minimal point loading. It’s vital to round the corners off though, like really round them off the more the merrier!

 This image shows a direct comparison to a top and bottom gusset (left) and a lap jointed gusset (right). Slightly more material required for the lap joint but easier to position and weld. More importantly the factor of safety difference 1.012 vs 1.343!!

 

The Yanks..

From working in the drilling industry there is no shortage of American built plant. Now i’m not here to compare levels of workmanship (we all know Australia wins :P) but one thing you may not have ever looked into is that they love “A36 steel”... What does this mean? This means that it’s around the 250MPa range (grade 250)....

This is fine from a showroom point of view but doesn’t do much for the above mentioned “fatigue limit” value(s) and some Australian terrain. Often the design is okay with this structurally but the material will yield prematurely compared to that of its common Australian counterpart AS G350. If you have a piece of plant that is failing in a particular area try replacing the section with some local Australian steel at grade 350.

 

Conclusion

So there you have it my two cents worth on metal fatigue and common structural failures. Please note the above is merely an observations over the years in the metal fabrication industry and are to be considered during design and repair and not solely relied on. Thanks for reading and if you would like to know more or have any questions or improvements please email me at moneill@rubixdesign.com.au

 

Cheers,

Matthew O'Neill

 

 

 

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