The load a fastener must withstand without permanent deformation.
Proof load sits between yield and the working load, and it is the acceptance criterion for production testing — a fastener that passes has not yielded. It is not a safe working load and it is not a breaking load.
ISO 898-1 defines proof load for each property class, and it is the figure a testing house verifies rather than the tensile strength.
Treating proof load as a working limit. It is a test acceptance criterion with no design margin above it.
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