Sources and scope for this page: ISO 898-1:2013, covering the definition of proof load. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
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.
Common misuse
Treating proof load as a working limit. It is a test acceptance criterion with no design margin above it.