Internal hydrogen embrittlement of pre-cracked, cadmium-plated AISI 4340 high strength steel with sustained load tests and incremental step-loading tests

•Threshold stress intensity for onset of internal hydrogen embrittlement (IHE) is studied.•The incremental step-loading method is used on pre-cracked compact samples.•Compact samples are less sensitive to IHE than circumferentially-notched samples.•Hydrogen is located in or close to the coating for embrittled Cd-plated samples. Cadmium-plated high-strength steel parts in aerospace are subject to hydrogen embrittlement (HE). Standard 200-h sustained-load tests (SLT) detect embrittlement on plated parts; however tests relying on fracture mechanics can improve interpretability and reduce testing times. We measured the invariant threshold for the onset of HE on compact tension samples with the incremental step-loading method. No HE was observed after plating conditions deemed embrittling per SLT, unless a 200-h preload was applied. We argue that hydrogen is located in or close to the coating, and that the compact samples are too thick to allow a uniform hydrogen profile across the thickness.