A　collinear　wave　mixing　technique　was　developed　to　detect　micro-cracks　in　samples　by　measuring　the　mixing　of　two　ultrasonic　sinusoidal　waves.　The　bispectrum　was　used　to　process　the　nonlinear　response.　Experiments　were　conducted　to　investigate　the　influence　of　excitation　parameters,　such　as　driving　frequency　and　time　delay,　on　intermodulation　among　ultrasonic　waves　and　defects.　Mixing　components　were　tracked　for　varying　frequency　or　time　delay,　and　the　nonlinear　response　was　measured　at　fixed　frequencies　and　time　delay.　The　driving　frequency　was　found　to　strongly　affect　micro-crack　detection;　the　optimal　driving　frequency　corresponded　to　maximum　amplitudes　of　sum　and　difference　frequency　sidebands.　The　time-delay　dependence　of　the　amplitude　of　mixing　components　allowed　the　location　of　defects　throughout　a　sample.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.