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RE:Charpy Impact Test

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There is a difference in ASME code and the AASHTO bridge design code
requirement.

ASME UG84 allows anywhere from 0 to 10 degree F difference between the
test temperature and the minimum design metal temperature provided the
yield point of the steel is less than 55 ksi.
ASME UCS-66 provides impact testing exemption based on a combination of
min design temp and thickness.

AASHTO and ASTM A709 allow 70 degree F shift in the minimum service
temperature and test temperature provided the yield point of the metal
is less than 65 ksi..

This difference is based on the loading rate. The loading rate in the
bridge is considered intermediate rate (1/1000 in/in/sec) between the
static (1/100000 in/in/sec) and impact loading ( 10 in/in/sec).
(typical strain rates described in the test results from US Steel
Research, AISI Project 168) .

If you design a bridge and have a normal steel with Charpy V-notch value
of say 15 ft-lb at test temperature of 32 degree F, this steel is
considered proper for zone 2 area where the minimum operating
temperature is about -1 to -30 degree F. Do this steel have the required
toughness to use in area where the minimum temperature is 70 degree?

A-515 and A-285 are used mostly in pressure vessel application while
A-53 is for steel pipe. The Nil Ductility temperature (NDT) of these
metals and the exemption clause in UCS-66 may not be applicable for A53.
______________________________________________________

Subject: Re: Charpy Impact Test
From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
To: "SEAOC Newsletter" <seaoc(--nospam--at)seaoc.org>

>1. The test temperature can be well above the minimum operating
>temperature and is dependent on the strain rate of the application
>(slow, moderate, impact).  For a strain rate of 1/1000 in/in/sec as in
>bridges, the difference is about 70 degree F. If you have the CVN value
>at test temp of 32 degree, i.e. it is good for min operating
temperature
>of -38 degree. This value can be used for application that required
test
>temperature at 70 on the safe side ( min operating temp is 0 degree).

I'll quibble with this. The ASME Code (UG-66) classes A-53 with A-515 gr
60 and A-285 Gr A and B which have NDT above 32F. Your test value at 32F
may indeed reflect the values at -38F because 32F is very near the lower
energy shelf, anyway.

Moreover you shouldn't go extrapolating Cv data to different strain
rates. The 70 deg temperature shift reflects the effect of different
testing speeds. If you're trying to qualify a material for toughness,
the
Charpy data shouldn't be fiddled to reflect another loading rate.
Besides, ssuming that the loading rate equals the strain rate is
incorrect, since the loading rate doesn't reflect the strain rate in
front of a propagating crack. If you're looking for fracture toughness,
A-53 is the wrong choice.

>(ASTM STP 466, Impact Testing of Metal, 1970). Not for A53
specifically,
The correlation includes A-36 with chemistry that overlaps that of A-53
so it'll probably be as good for A-53 as for anything else.


Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw