In　recent　years,　nonlinear　ultrasonic　techniques　have　been　developed　rapidly　for　defect　detection　and　characterization　at　its　early　development　stage.　In　this　paper,　non-collinear　shear　wave　mixing　is　used　for　fatigue　crack　detection　and　characterization,　exploring　how　orientation　and　size　may　be　determined.　This　is　achieved　through　measuring　the　amplitude　of　the　generated　longitudinal　wave　from　a　crack　of　interest.　This　amplitude　is　a　function　of　the　interaction　angle　and　frequency　ratio　of　two　incident　shear　waves　and　the　resulting　amplitude　parameter　space　is　termed　the　nonlinear　fingerprint　of　the　crack.　Numerical　analysis　of　the　nonlinear　interaction　between　two　incident　shear　waves　and　cracks　was　performed　using　two-dimensional　finite　element　models　to　explore　a　broader　parameter　space　than　possible　experimentally.　It　is　shown　that　the　interaction　angle　which　leads　to　the　maximum　generated　longitudinal　wave　amplitude　is　related　to　the　orientation　angle　of　the　crack.　To　confirm　these　conclusions　the　model　was　validated　using　experimental　measurements　of　vertical　fatigue　cracks　grown　using　3-point　bending　tests　from　initiation　notches.　The　polarity　flipping　method　was　used　to　improve　signal　to　noise　ratio　in　such　measurements.　It　is　shown　that　there　is　a　good　agreement　between　the　experimentally　measured　fingerprints　and　those　simulated　using　finite　element　methods.　Finally,　an　approximate　method　for　sizing　fatigue　cracks　was　introduced　that　uses　multiple　non-collinear　measurements　along　the　crack　length　to　determine　its　extent.　As　expected,　the　measured　sizes　from　the　proposed　method　indicate　greater　crack　lengths　than　seen　with　linear　ultrasonic　phased　array　images　due　to　the　closed　fatigue　cracks　being　undetectable　with　linear　arrays.