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Moment and Location of Rupture

02/26/2010 7:24 PM

with 20% elongation at rupture

I have a question here. Does the whole sentence mean "with 20% elongation at the moment of rupture " or " with 20% elongation at the location where rupture happens"? Pardon me if i have made some basic-knowledge error.

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#1

Re: rupture

02/26/2010 11:16 PM

The test specimen for this consists of a length of material with the middle section "thinned down" by machining. An initial segment of this reduced section is marked off. The ends are then chucked into a stretching device and pulled apart at a defined rate (not too fast). When the specimen breaks somewhere between the marked points, you measure how far apart the marks are and compare to the distance before stretching. This determines the percentage of elongation.

Thus the elongation is not just at the breakage point, but rather over a length that includes the breaking point. (That's a layman's way of putting it. A test lab tech could surely refine this explanation.)

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#2

Re: rupture

02/27/2010 1:24 AM

It would depend on context. You might find something like this:

"Testing found that the material experienced 4% elongation at yield, and 20% elongation at rupture." In that case, this means 20% elongation at the moment of rupture.

If someone were examining a failed pressure vessel and wanted to write that material at the site of the rupture was elongated by 20%, it would be more liketly that the wording would be something like: "The material was elongated by 20% at the site of the rupture."

Most of the time, I think you'd find the first usage. If it's about ordinary materials testing, its always the first usage.

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#3

Re: Moment and Location of Rupture

02/28/2010 2:34 AM

Elongation is determined in a tensile test, where a length (referred to as gauge length) of uniform cross-section material is subjected to a tensile load at a given rate. As the load increases the material will yield and necking occurs at a point then rupture at that point. The gauge length at rupture as compared to the original gauge length is the elongation at the moment of rupture over the length. The elongation at the location of rupture could be much higher( elongation after necking is localised here), but we are yet to have a standard definition for it.

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#4

Re: Moment and Location of Rupture

02/28/2010 8:43 AM

You have two yield readings:

Yield at initial materiel structure failure (usually in KPa or KPSI)

Ultimate yield, which occurs when the part ruptures (Usually in the same scale)

For brittle materiel, such as >T7 metal or compsites with a brittle matrix, Yield and Ultimate Yield will be close together, if not simultaneous. The structural failure is usually accompanied by a, "BANG."

You didn't mention if you witnessed the test or are just reading the report(s).

There are usually a set of pictures and graphic representations of the test, which show the test progression from initial setting to ultimate failure. This test is considered the "Gold Standard" for Metallurgy. The part is supposed to fail at the notch. If it fails elsewhere, there is usually a contamination, or alloy, problem.

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#5

Re: Moment and Location of Rupture

02/28/2010 2:08 PM

You ask "Does the whole sentence mean...", but you didn't provide the whole sentence!

Therefore, although Tornado's answer is probably the correct one, no one can be sure. A phrase removed from context is always subject to multiple interpretations. That context must include at least the entire sentence, and very likely, previous sentences and/or diagrams as well.

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#6

Re: Moment and Location of Rupture

02/28/2010 4:53 PM

I wouldn't know how to define "the elongation at the location at which rupture happens" - unless you are looking at the gap between two molecules that end up on different sides of the break; but that extension is at the whim of the experimenter, and never as small as 20%...
Were it possible, the objective would be to evaluate the greatest extension that is achieved before the break discontinuity is generated. For cases where this concept has meaning, the standard methods that are used get pretty close.

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#8
In reply to #6

Re: Moment and Location of Rupture

03/03/2010 9:18 PM

Wingman is a translator, not a test engineer. We are assuming that the elongation is in a test coupon, but that may not be the case. One could, I suppose, assess the elongation near a fracture to asses the degree of hydrogen embrittlement in a failed tank or pipe.

Break a file and the elongation near the break is negligible. Stretch a piece of mild steel to failure and the elongation near the rupture is much larger.

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#7

Re: Moment and Location of Rupture

03/03/2010 5:48 PM

Hi,

Elongation at rupture is a term which is "test standard related". During the final phase of a tensile test (of a ductile material), microcracks normal to the stress direction begin to develop and coalesce. This cause a local reduction of section, thus the local stress rise and this specimen part begins to stretch until rupture.

The elongation (once the specimen is broken) depends strongly on the initial length chosen (standard specified). For the same broken specimen, the longer the initial length the lesser rupture elongation. This is the main reason for stating L0 when specifying any rupture elongation. I think none of the terms are completely correct: There's no rupture elongation at the "location where rupture happens" The figure is an average result over the length chosen. And to speak about the "moment of rupture" isn't correct without specify the initial length.

So, I will translate just "elongation at rupture according to std... or for example with L0= 5 D0".

Kind regards

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