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Re: cracked frame

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  Re: cracked frame Richard Smith <null@void.com> - 2023-11-18 09:44 +0000
    Re: cracked frame "Jim Wilkins" <muratlanne@gmail.com> - 2023-11-18 09:40 -0500

#13854 — Re: cracked frame

The Nolalu Barn Owl <&#103&#111&#114&#100&#105&#101&#64&#110&#111&#108&#97&#108&#117&#46&#111&#110&#46&#99&#97> writes:

> On Wed, 14 Dec 2005 11:40:14 GMT, "Steve Mackie"
> <stevemackie@ns.sympatico.ca> wrote:
>
>>>   Fishmouths & shear ties are a way to blend stiffness between a box
>>> section
>>>
>>>   and a channel... which minimizes stress concentrations...   Metal
>>
>>You just seem to be stating the same thing over and over. I have never used 
>>anything but plain rectangular plates and they have always met stress 
>>requirements. I want to read into the theory to see if changing the way I do 
>>it is worth it.
>>
>>Steve 
>>
>
> I have looked and looked for information on the NET about fish mouthed
> re-inforcing plates and there isn't much to be had.
>
> Basically you can use a plate cut to form what looks like an open fish
> mouth on each end.
>
> ------
>>    <
> ------
>
> You can also use a diamond shaped plate to give the same effect.
>
> To weld the frame directly from the top to the bottom makes it likely
> to crack again for reasons which I am not qualified to try and
> explain.  After over 30 years of working as a Millwright, I can say
> that once a crack has been welded it quite often cracks there again so
> a plate is used to bridge the repaired area and this plate is welded
> with stitch-welding so as not to make a continuous weld.  This way, if
> one stitch was to crack it won't carry over to the next weld.  The
> fish mouth (or diamond) allows you to place welds that are not in a
> direct line from top to bottom.
>
> Since my explanation is wanting, and I am just a DIY repairman, I have
> cross posted this message to a welding NG to draw in some expert
> opinions for you - and for me :)
>
> -- 
> Gordie
>
> There are tongues speakers who speak in an unknown language that actually
>  exists among the peoples of the earth. I recall one such incident in which a
>  tongues speaker got up and spoken in some unknown language. As it
>  happened, there was a priest in the audience who just happened to speak the
>  language this women was speaking as an unknown tongue. The priest asked
>  her if she knew what she was saying. The women had no idea so the priest
>  told her: "You were speaking my native language and what you were saying
>  was cursing God."
> http://www.saint-mike.org/Apologetics/QA/Answers/Faith_Spirituality/f0404120221.html

This looks to be a fatigue cracking matter...

Geordie seems to know from practical experience what does and doesn't
work to avoid fatigue cracks or how to repair them when it happens.

Steve - you've read and all seems fine and from that basis you
disbelieve and experienced practitioner.

Steve - you could do Finite Element Analysis modelling to see the
stress concentrations.  FEA for the general region of the structure.
Here's a somewhat comparable example of me probing a feature I met
while actually out there as a welder
http://weldsmith.co.uk/tech/finiteelement/1609_beam_tjn_sim/160927_beam_w_column_w-wo_stiffplts.html
What I would bet is that you will see theory concurring with what
Geordie is telling you already.  Steve - you'd benefit from "treading
the path" and seeing how where you come from can plug you into the
deep knowledge out there.

I take it you are proposing some feature with an abrupt "right-angled
in every way" change in thickness and possibly direction.
That will be highly fatigue-prone.

Steve I think you are being given a chance to learn and you are not
seeing it.

As best I can tell from message trail so far...

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#13856

"Richard Smith"  wrote in message news:lycyw78kfc.fsf@void.com...

I take it you are proposing some feature with an abrupt "right-angled
in every way" change in thickness and possibly direction.
That will be highly fatigue-prone.

Steve I think you are being given a chance to learn and you are not
seeing it.

As best I can tell from message trail so far...

--------------------------------

I vaguely remember learning in Strength of Materials class that the diagonal 
weld across the beam web reduced stress concentration. Maybe the reason for 
both diamond and fish-mouth plates is that one is scrap from cutting the 
other.

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