More details are required to speculate on potential causes of the cracking you are describing. One must look for failure patterns to aid in this determination. Where the failures are occurring (on random tubes, near supports or baffles, clustered about shell inlets or outlets, on outer u-tube rows or inner u-tube rows, etc,) will narrow down the possibilities. For example, tube failures near a tube sheet in a condensing zone may indicate inadequate venting. ASME Code, Section V, "Nondestructive Examination" is a helpful reference and describes what NDE can disclose. Test methods covered by this standard include acoustic emission (AE), electromagnetic (eddy-current) testing (ET), leak testing (LT), magnetic particle testing (MT), liquid penetrant testing (PT), radiography (RT), ultrasonic testing (UT), and visual testing (VT). Another excellent source of information for your specific situation is the reference book, "Tubular Heat Exchanger Inspection, Maintenance, and Repair" by Andreone and Yokel (McGraw-Hill, 1998). I suggest you obtain copies of both of these references to facilitate finding a resolution to your problem and plan a path forward.