Publisher’s version / Version de l'éditeur:
Technical Note (National Research Council of Canada. Division of Building
Research), 1970-12-01
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Vibration measurements at the National Defence building site
Ward, H. S.
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NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=efb7a38f-4dff-4c8e-bc88-08b7b73d401a https://publications-cnrc.canada.ca/fra/voir/objet/?id=efb7a38f-4dff-4c8e-bc88-08b7b73d401aDIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'If
E
C
JHI
N II CAlL
No.
555
NOTJE
PREPARED BY G. WilliaITls - Leir CHECKED BY GWS APPROVED BY NBH
PREPARED FOR Record Purposes
October 1970
SUBJECT IGNITION OF MAGNESIUM ALLOY COMPONENTS FOLLOWING
AIRCRAFT ACCIDENTS
In crashes of aircraft that incorporate cOITlponents of ITlagnesiuITl alloy, these cOITlponents ITlay be ignited by fuel burning on the ground, or by other ITleans. The Accident Investigation Division of the DepartITlent
of Transport has requested inforITlation on the tiITle-scale of this process.
I t was hoped to throw light on the sequence of events leading to or
follow-ing a helicopter accident, where ITlagnesiuITl oxide was stated to have been drawn into a gas turbine that would be expected to run down in at Inost severa] seconds after iInpact.
To ignite bulk ITlagnesiuIn it is not necessary to heat i t until ignition
occurs. I t is sufficient to raise it to a critical teITlperature estiITlated at
5850
C1. FroITl that point, oxygen diffuses to the ITletal and surface
COITl-'bustion takes place sufficiently to cause its teITlperatur,e to continue to rise, eventually reaching ignition at around 6300
C. This ignition point is slightly below the Inelting point 6500
C, so that ignition is rapidly followed by IneHing. TiITle lags froIn critical teITlperature to ignition up to over three hour s have been recorded2 .
The present probleITl, however, is not how slowly, but rather how quickly ITlagnesiuITl can be ignited froIn the sources envisaged. Thus the probleITl is one of heat transfer, where the teITlperature of ·the object under consideration is determined by the balance between rate of access of heat
2
-from the flames, and rate of los s, primarily by conduction, to other com-ponents in direct contact with the object.
I t was thought that simple experiments would be both quicker and more convincing than heat transfer calculations. For the heat source, it
seems plausible to assume that no diffusion flame of spilt fuel would be hotter than a premixed propane-air flame from a Meker burner. The ob-ject was a piece cut from 3/8 inch magnesium alloy plate to various sizes from 1/4 in. square to 1 in. square. It was supported by a pair of leads
ｦｲＶｮｾ a thermocouple in the piece of plate, so that the heat sink was mini-mal..
In several trials the minimum time to ignite a I-in. square from room temperature was 1 min. 10 sec., and for a 1/4-in. square, 25 sec. Addition of an inert heat sink would obviously lengthen these times, if it did not prevent ignition.
The material used was Alloy AZ31B3 which consists of:
Aluminum Manganese Zinc 2.5 to 3.5% 0.20% 0.6 to 1.4% Impurities subject to limits in the specification
Magnesium Balance to 100%
Alloys with these metals are stated4 to have oxidation rates higher than
pure magnesium at temperat.ures (420 to 4600
C) below the critical. This may make little difference if the minimum ignition time is governed by heat transfer from flame to metal as suggested above.
References
1
2
3 4
Mellor, A. M. and Glassman, 1. A physical criterion for metal ignition. Pyrodynamics,
i,
43-64 (1965).Ibid, p. 59.
American Society for Testing and Materials: Specification B90-66. Kubaschewski, O. and Hopkins, B. E. Oxidation of Metals and Alloys.
