T.O. 33B-1-1 4-65 Table 4-6.  Eddy Current Reference Standards for Cracks. Type of Standard Advantages Disadvantages Recommendations Drilled Holes Ease of fabrication Relatively easy to obtain good dimensional tolerances Maximum response difficult to detect Negative and/or positive responses (with some instruments) Not recommended for reference standards. EDM Notches Good dimensional tolerances Response similar in phase to a crack Cannot be fabricated in the field Relatively expensive Best small crack standard for aluminum, titanium, and steel Sawed Notches Ease of fabrication in the field Dimensional tolerances cannot be precisely controlled in the field Sometimes satisfactory for large crack standards Can be used to check instrument operation and relative response Slots in Foil, Razor Cuts Not recommended for small crack standard Machined Notches Less expensive to fabricate than EDM notch Response varies with type of probe Cannot be fabricated in the field Large notches  not recommended for small crack standard Can be used to check instrument operation and relative response Induced Fatigue Crack Gives crack response High cost Hard to control size Critical applications when cost is justified 4.5.8 Evaluation Of Crack Indications. 4.5.8.1 Acceptance Rejection Criteria. In most cases, the depth of flaws detected by eddy current inspection cannot be directly measured.  In almost all cases, the eddy current signal of the flaw must be compared to the eddy current signal produced by the calibration standard. The relationship between response to the standard and the corresponding response to the defect size must be established prior to the test and should be considered an essential part of the calibration process.  Prior to the start of any test, the maximum   flaw   size   allowed   should   be   defined   by   the   test   specification   or   applicable   Technical   Order,   and   the calibration process should confirm that the test can be conducted with the required sensitivity. 4.5.8.2 Conditions Affection Flaw Evaluation. Inspection for cracks, measurement of conductivity, or hardness can often be complicated by the surface damage, and manufacturing processes.  Included in this category are scratches, gouges, pitting, and metal smearing.  Severe damage may  require  refinishing  of  the  area  prior  to  inspection,  inspection  at  a  lower  sensitivity,  or  selection  of  another  test method.