Figures 8 and 9 present the recommended curves for API grade C and D rods,respectively. API Krods can use the C rod curves will allow used, inspected rods to be used in current string dsigns provided that the allowable stresses are still below the derated values. If the derated stress is below this value,used rods should not be used for this application. It should be noted that derating the used rods will not show when the rod string should be replaced. It is recommended that inorder to determine when the entire string should be replaced,a full-cycle economic analyses needs to be done.In lieu of doing these full cycle analyses, it has generally been found that the entire rod string should be replaced after three related failures in the rod string in a 12 month period of time.