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CROSS Safety Report

Failure of high strength studs

Report ID: 499 Published: 1 April 2015 Region: CROSS-UK

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Overview

A reporter raises concerns after several M12 threaded studs snapped not long after installation.

Key Learning Outcomes

For construction professionals:

  • Quality control and competent supervision on site can help to ensure the correct fixings are installed and are installed in accordance with the manufacturer’s requirements

For civil and structural design engineers:

  • Be aware that while grade 10.9 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

  • Chemical anchor applications should be designed in accordance with a European Technical Approval/Assessment (ETA) which specifically details how to approach stand-off fixings

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A reporter’s firm specified the use of M12 grade 8.8 threaded studs that were to be located in a tapped web of a parallel flange channel (PFC). This then supported an angle located a short but variable distance from the PFC. The stand-off arrangement was necessary due to a construction misalignment of the PFC and the original intent was that the angle should have been welded.

The installation proceeded normally, and the nuts were not over tightened. The firm then received reports that many of the studs were snapping overnight with the outer nuts falling onto the floor, and the angles being supported by part of the remaining studs and unsnapped studs (Figure 1). Further enquiries revealed that the studs actually used were grade 10.9 and bright zinc plated (BZP).

Image
Figure 1: fractured threaded stud

An examination of a failed stud surface revealed what appeared to be a uniform tension failure. It was concluded by the firm that this was likely due to hydrogen embrittlement of the material due to a poor manufacturing process.

The studs were thought to have been intended for use with chemical anchors. They were subsequently replaced with studs from another manufacturer and these have been satisfactory. The enquiries revealed that the studs were sourced from within the EU.

Expert Panel Comments

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The nature of the failure in this report would (without sight of a detailed metallurgical analysis) appear to be consistent with hydrogen embrittlement. This 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.

Hydrogen embrittlement is a rare phenomenon which is partly characterised by a delay in the failure (which makes it all the more worrying). Whilst grade 10.9 material is capable of performing satisfactorily, designers should beware that ‘over strength’ is not always a good thing if it is achieved at the detriment of ductility.

Whilst grade 10.9 material is capable of performing satisfactorily, designers should beware that ‘over strength’ is not always a good thing if it is achieved at the detriment of ductility

An internet search will produce several references to the phenomenon of hydrogen embrittlement for those requiring further information on the phenomenon. It seems unusual that such studs were intended to be part of a chemical anchoring system. Most carbon steel studs supplied for this purpose are manufactured to 5.8 or 8.8 steel rather than 10.9 grade which is not needed to match the strength of the resin bond.

Chemical anchor applications should be designed in accordance with a European Technical Approval/Assessment (ETA) which specifically details how to approach stand-off fixings.  The requirements placed on the manufacturer to obtain the ETA would ensure correct manufacturing processes were employed to remove any risk of hydrogen embrittlement and other manufacturing defects.

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