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Monday, 18 July 2016

BEGINNING OF NON DESTRUCTIVE EVALUATION (NDE).


           Beginnings of nondestructive evaluation:
  • Nondestructive testing has been practiced for many decades, with initial rapid developments in instruments in instrumentation spurred by the technological advances that occured during world war2 and the subsequent effort .during the earlier days ,the primary purpose was the detection of defects.As a part of safe life design,it was intended that a structure should not develop macroscopic defect during its life,with the detection of such defect being a cause for removal of the component from service.in response to this need ,increasingly sophisticated techniques using ultrasonic eddy currents,x-rays,dye penetrants,magnetic particles,and other forms of interrogating energy emerged.                                                                                                                                                                          In the early 1970s two event occurredwhich caused the major change in the NDT feild first improvement in the technology led to the ability ti defect small flaws.which caused more parts to be rejected even though the probability of component failure had not change .however the discipline of fracture mechanics emerged which enabledone to predict whether a crack of a given size will fail under a particular load when a materials fracture toughness properties are known other laws were developed to predict the growth rate of crack under cyclic loading with the advent of thes tools it become possible to accept structure containing defect if the sizes of those defects were known This formed the basis for the new philosophy of damage tolerent design component having known defects could continue in service as long as it could be established that those defects would not grow to a critical,failure producing size.
          A new challenge was thus presented to the nondestructive testing community.detection was not enough.one needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics based prediction of remaining life. The need for quantitative information was particularly strongly in the defense and nuclear power industries and led to the emergence of quantitative nondestructive evaluation .as a new engineering research discipline.A number of research programs around the world were started.such as the center for nondestructive evaluation at lowa state university (growing out of a major research effort at the Rockwell international science center): the electric power research institute in charlotte, North carolina; the fraunhofer institute for nondestructive testing in saarbrucken,Germany, and the nondestructive testing center in Harwell,England.