Hydrogen Embrittlement Threshold Stress
Estimate the sustained-load threshold stress for a high-strength steel from its notched fracture strength and an F519-style threshold ratio.
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The engineering
High-strength steels (above roughly 1000 MPa / 145 ksi) can crack under a static load far below their yield strength once hydrogen is present — from plating baths, pickling, cathodic protection, or service. ASTM F519 screens plating and cleaning processes with notched round bars held at a fixed fraction of notched fracture strength; the highest load a specimen survives without cracking defines the threshold. This card scales an ultimate strength up through the notch stress-concentration to a notched tensile strength, then down by a threshold ratio to give a working threshold stress.
The threshold ratio is the number that matters and it is not universal: F519 sustained-load tests commonly key on 75% of NTS as the go/no-go bar, but a hydrogen-charged bar can fail well below that. Treat this as a screening estimate, not a design allowable — the real threshold drops with increasing strength, harder microstructure, and higher hydrogen fugacity, so verify with the actual F519 lot data for your process.
Where this math comes from
Hydrogen embrittlement of iron was reported by William Johnson in an 1875 Royal Society paper — steel wire pickled in acid lost ductility, then recovered it after baking. As alloy steels climbed past 1500 MPa for aircraft landing gear and fasteners in the mid-20th century, the problem turned from curiosity to crashes: cadmium-plated parts failing days after installation drove a hard look at plating chemistry.
The aerospace answer became the notched sustained-load specimen. ASTM standardized it as F519 for evaluating plating and coating processes, holding a stress-concentrated bar under constant load for 200 hours as an accept/reject screen. The test still governs qualification of electroplating, cleaning, and stripping lines for high-strength steel hardware.
- 1875William H. JohnsonFirst documents that acid-charged iron loses ductility and recovers after baking.
- 1960A. R. TroianoFrames hydrogen-enhanced decohesion and the threshold-stress behavior of high-strength steels.
- 1972R. A. OrianiQuantifies hydrogen trapping and lattice decohesion underpinning threshold stress.
- 2018ASTM InternationalMaintains F519 as the standard notched sustained-load screen for plating and coating processes.
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