Fracture Analysis of High Strength Steel Fastener Used in Separation System of a Satellite Launch Vehicle

Sushant K. Manwatkar; S.V.S. Narayana Murty; P. Ramesh Narayanan

doi:10.3139/147.110353

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Fracture Analysis of High Strength Steel Fastener Used in Separation System of a Satellite Launch Vehicle

Sushant K. Manwatkar
Sushant K. Manwatkar
Material Characterization Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum-695 022, India. e-mail: susarla.murty@gmail.com

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, S.V.S. Narayana Murty
S.V.S. Narayana Murty
Material Characterization Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum-695 022, India. e-mail: susarla.murty@gmail.com

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and P. Ramesh Narayanan
P. Ramesh Narayanan
Material Characterization Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Trivandrum-695 022, India. e-mail: susarla.murty@gmail.com

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Published/Copyright: February 25, 2016

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From the journal Practical Metallography Volume 53 Issue 3

Abstract

High strength 3.5Ni-1.5Cr-0.5Mo (AFNOR 35NCD16) steel fasteners are widely used for various space applications. In one such application, failure was observed in one of the M4X8L fasteners used in the band clip assembly of a separation system. The band assembly was preloaded to the specified value of load as per the assembly procedure and the fastener was torqued to the specified value of 2.3Nm with loctite. The fastener was found to be separated into two pieces at head-shank junction under assembly load conditions. Detailed analysis of the cracked fastener indicated the failure to be due to the hydrogen induced stress corrosion cracking (HI-SCC). This paper presents the details of the failure of the fastener.

Kurzfassung

Hochfeste Stahlverbindungselemente, 3,5Ni- 1,5Cr-0,5Mo (AFNOR 35NCD16), finden im Bereich der Raumfahrt vielfältig Anwendung. Bei einer solchen Anwendung wurde bei einem in einer aus Spannband und Bügeln bestehenden Baugruppe eines Trennsystems eingesetzten M4X8L Verbindungselement ein Versagen festgestellt. Das Spannband wurde entsprechend dem Montageverfahren mit dem spezifizierten Belastungswert vorgespannt. Das Verbindungselement wurde mit dem spezifizierten Drehmoment von 2,3 Nm unter Verwendung von Loctite angezogen. Im Lastzustand zerbrach das Verbindungselement am Übergang von Kopf und Schaft der Baugruppe in zwei getrennte Teilstücke. Eine eingehende Untersuchung des gebrochenen Verbindungselements ergab ein Versagen durch wasserstoffinduzierte Spannungsrisskorrosion (H-SpRK). Diese Arbeit beschreibt das Versagen des Verbindungselements im Detail.

Übersetzung: E. Engert

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