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CROSS Safety Report

Bolt failures due to hydrogen embrittlement

Report ID: 490 Published: 1 April 2015 Region: CROSS-UK

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Overview

A reporter recalls how, in the 1970s while working on a bridge refurbishment project, several new zinc electro-plated bolts failed suddenly, and often shortly after installation.

Key Learning Outcomes

For civil and structural design engineers:

  • Be aware that while higher grade material may be capable of performing satisfactorily, overstrength may be at the detriment of ductility due to hydrogen embrittlement

  • Hydrogen embrittlement is a phenomenon that afflicts particularly high strength steels as a result of poor process control during plating and is not known to affect lower strength steels such as 8.8 grade, itself regarded as high tensile steel

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A reporter recalls how, in the 1970s, while working on a bridge refurbishment project, several new zinc electro-plated bolts failed suddenly, and often shortly after installation. It was found that hydrogen embrittlement was the principal cause.

A secondary cause identified was the shape of the shoulder of the bolts which was too abrupt. their report was prompted by recently published reports on hydrogen embrittlement as the reason for some bolt failures and points out that this is not a new problem.

Expert Panel Comments

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This issue appears to be linked to the plating and echoes some other cases where embrittlement has been linked to galvanising of high strength materials. All bolts have numerous stress concentrations in them at changes in section and these have to be accommodated by local ductility.

Risk factors are galvanising, high strength, hardness, and bigger diameters (thick steel). Care should always be taken when using higher than normal strength materials. These may not behave in the same way as more typical strength materials, giving rise to problems which the designer may not have considered.

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