Training Courses

to the detailed development of specific experimental techniques and their application. Emphasis by the rapporteurs on the more practical aspects encouraged lively discussion which made the conference particularly valuable. The conference led to a better understanding of the role of dynamic fracture toughness in controlling crack arrest which is of particular importance in pipelines and similar structures. However most of the emphasis thi~oughoutthe conference was on the prevention of crack initiationunder static loading conditions. This involves the selection of materials of construction which have sufficientlyhigh fracture toughness, such that their criticaldefect size at the operating stress level is larger by a considerable factor of safety than the largest size considered possible in the structure, taking account of the possibility of in-service crack growth from the biggest defect that would be allowed to remain in the vessel following non-destructive testing. Detailed examples of this approach to the assessment of nuclear reactor pressure vessel were given, and another example showed how it can be used to assess acceptability and safe operating conditions when a type defect has been found in service. R was pointed out that often two separate assessments were desirable, one covering conditions up to the time of thermal stress relief and another covering those existing at the proof pressure test and in service. However such detailed analyses could prove too expensive to make for each individual pressure vessel except where special expense or hazard existed. For most cases the method of application

of f r a c t u r e m e c h a n i c s was thought to be through the d e s i g n c o d e s and m a t e r i a l s t a n d a r d s . The type t e s t ing by f r a c t u r e m e c h a n i c s m e t h o d s of plate and w e l d m e n t s could p r o v i d e a r a t i o n a l b a s i s f o r s e t t i n g the r e q u i r e d s p e c i f i c a t i o n l e v e l of C h a r p y V-notch e n e r g y for quality c o n t r o l p u r p o s e s ; the a s s o c i a t e d f r a c t u r e m e c h a n i c s a n a l y s i s should be used by s p e c i fication c o m m i t t e e s c o n c e r n e d with s e t t i n g flaw s i z e acceptability standards. It was r e c o g n i s e d that such s t a n d a r d s a r e often s e t to give a s s u r a n c e that s a t i s f a c t o r y techniques have been used as well a s to e n s u r e ' f i t n e s s for p u r p o s e ' • N e v e r t h e l e s s t h e r e was s t r o n g evidence that shallow l i n e a r flaws w e r e in many c a s e s insignificant, no m a t t e r how long, since t h e i r s t r e s s - c o n c e n t r a t i n g s e v e r i t y was only that a s s o c i a t e d with t h e i r depth. In d i s c u s s i o n , s e v e r a l s p e a k e r s e m p h a s i s e d that the v a l i d i t y of a f r a c t u r e m e c h a n i c s a p p r o a c h was only a s good as the c e r t a i n t y of d e t e c t i o n of flaws e x c e e d ing the c r i t i c a l s i z e . This is not p r i n c i p a l l y a m a t t e r of s e n s i t i v i t y of n o n - d e s t r u c t i v e t e s t i n g techniques but r a t h e r of c o n f i r m i n g a l m o s t a 100% c o v e r a g e with a c e r t a i n t y of location, i d e n t i f i c a t i o n and r e p o r t i n g of r e l e v e n t d e f e c t s . It is a l s o n e c e s s a r y to e n s u r e that no f u r t h e r s i g n i f i c a n t d e f e c t s can a r i s e a f t e r i n s p e c tion by such m e c h a n i s m s as hydrogen c r a c k i n g , r e heat c r a c k i n g or handling d a m a g e . This c l o s e r e l a t i o n s h i p of f r a c t u r e m e c h a n i c s to inspection techniques was r e c o g n i s e d by the s u g g e s t i o n that a s i m i l a r c o n f e r e n c e would be a r r a n g e d on the P e r i o d i c I n s p e c t i o n of p r e s s u r e v e s s e l s in May 1972. R. W. Nichols

LETTER TO THE EDITOR The ndt of concrete D e a r Sir, S e v e r a l new techniques have been d e v e l o p e d during the r e c e n t y e a r s for the ndt of c o n c r e t e . Some of t h e s e w e r e r e v i e w e d in F e b . 1968 i s s u e of y o u r j o u r nal by L. J. I. Browne. Methods of m o i s t u r e m e a s u r e ment s u r v e y e d included the method of m i c r o w a v e a b s o r p t i o n . I should like to b r i n g to the notice of your r e a d e r s , a new method of m o i s t u r e m e a s u r e ment, which o v e r c o m e s s o m e of the d i s a d v a n t a g e s of the method of m i c r o w a v e a b s o r p t i o n . The method is b a s e d on the m e a s u r e m e n t of the d i e l e c t r i c constant at m i c r o w a v e f r e q u e n c i e s . The s e n s i n g e l e m e n t u s e d c o n s i s t s of a s p e c i a l l y d e s i g n e d m i c r o w a v e r e s o n a n c e cavity, whose r e s o n a n c e f r e quency depends on the d i e l e c t r i c constant of the m a t e r i a l within the e l e c t r i c a l field of the s e n s o r . When the s e n s o r is p l a c e d on a m o i s t m a t e r i a l i t s r e s o n a n t frequency changes. The shift In f r e q u e n c y is t r a n s f o r m e d into an e l e c t r i c a l c u r r e n t in a s i g n a l handling e l e c t r o n i c unR and is m e a s u r e d . Sharp r e s o n a n c e peaks of m i c r o w a v e s allow v e r y a c c u r a t e m e a s u r e m e n t s , which a r e independent of the d i e l e c t r i c l o s s e s and the conductivity of the m a t e r i a l . The

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method can be u s e d for m e a s u r e m e n t s on f l o o r s etc, as a c c e s s is r e q u i r e d only f r o m one s i d e . P r e l i m i n a r y t e s t s on c o n c r e t e have shown that t h e r e is a l i n e a r r e l a t i o n between the m o i s t u r e content of c o n c r e t e and the output r e a d i n g of the i n s t r u m e n t . A d e t a i l e d r e p o r t on the a p p l i c a t i o n of the method for c o n c r e t e and other building m a t e r i a l s will be p r e sented later. A c o m m e r c i a l v e r s i o n of the i n s t r u m e n t is a l s o a v a i l a b l e 1, and the s i z e and the shape of the r e s o n a n t cavity can be chosen a c c o r d i n g to the s i z e and the kind of m a t e r i a l to be studied. Y o u r s faithfully Dr J. B h a r g a v a Inst. for S t r u c t u r a l E n g i n e e r i n g & B r i d g e Building Royal I n s t i t u t e of Technology S-100 44 Stockholm 70, Sweden

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