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The relation of the occurrence of airframe acoustic emissions to aircraft manoeuvre are reported for Avro CF-IOO upper forward wing trunnions. Periods of excessive noise are found when the airframe load is changing during entry to and exit from sustained-G manoeuvres. During constant-G periods, the airframe noise level is reduced by a factor of more than one hundred. These quiet periods provide a suitable signal-to-noise level for the in-flight detection and monitoring of slow, stable crack growth in cornmon airframe materials, even in a noisy load transfer component such as the wing trunnion studied here. Simultaneous in-flight acoustic emission measurements in symmetrically-located airframe components are also reported. The ratio of the number of recorded event counts in a cracked component to that in an uncracked component during the same flight is found to increase linearly with the crack face area for through crack lengths in the range 0-5 rnm.

Appl i e d P h y s i c s L a b o r a t o r y The Johns Hopkins U n i v e r s i t y L a u r e l , MD 20707 Abs t rac t S o l i d samples w i t h s u b s u r f a c e s t r u c t u r e , p r i n c ip a l l y v e r t i c a l c r a c k s , have been s t u d i e d u s i n g b o t h p h o t o a c o u s t i c and photothermal-optical-beamd e f 1 e c t i on imaging. Q u a n t i t a t i v e comparison o f images i s presented. It was found t h a t t h e t r a n sverse, peaked-on-normal , o p t i c a l -b e a m -d e f l e c t i on image most r e a d i l y sensed m i c r o s c o p i c cracks. I . I n t r o d u c t i o n Focused l a s e r beams can be used t o l o c a l l y h e a t a metal1 i c sample v i a p h o t o t h e r m a l c o n v e r s i o n . W i t h t h e use of s h o r t -f o c a l -l e n g t h l e n s e s such as microscope o b j e c t i v e s , t h e h e a t i n g (pump) l a s e r beam w a i s t can be as s m a l l as a few micrometers. Scanning o f t h e pump beam r e l a t i v e t o t h e sample s u r f a c e makes p o s s i b l e t h e i n v e s t i g a t i o n o f t h e thermoel a s t i c p r o p e r t i e s o f m i c r o s t r u c t u r e n e a r t h e h e a t e d s u r f a c e . D e t e c t i o n o f l o c a l v a r i a t i o n s i n t h e r m o e l a s t i c p r o p e r t i e s can be accomplished b y meas u r i ng t e m p e r a t u r e va r i a t i ons u s i ng p h o t o t he rmal o p t i c a l -b e a m d e f l e c t i o n (PTOBD), e.g., o r by measuring v a r i a t i o n s i n t h e g e n e r a t i o n o f e l a s t i c d i s t u r b a n c e s u s i n g a p i e z o e l e c t r i c t r a n s d u c e r a t t a c h e d t o t h e sample, e.g. Since t h e l i m i t i n g f a c t o r i n d e f i n i t i o n / r e s o l u t i o n i s n o t t h e t h e r m a l d i f f u s i o n l e n g t h [l-31, f e a t u r e s o f t h e o r d e r o f t h e pump beam o r s m a l l e r i n s i z e can be i n v e s t igated, even a t r e l a t i v e l y -l o w pump-laser-beam m o d u l a t i o n f r e q u e n c i e s .I m p o r t a n t a p p l i c a t i o n s o f t h e PTOBD and photoa c o u s t i c (PA) t e c h n i q u e s i n c l u d e s e n s i n g and c h a ra c t e r i z a t i o n o f m e t a l l i c i n t e r f a c e s . F o r example, t h e s p a t i a l e x t e n t o f t h e v a r i a t i o n o f p h y s i c a l parameters i n t h e neighborhood o f t h e g r a i n boundary i n h i g h -p u r i t y , c a r e f u l l y p r e p a r e d b ic r y s t a l s i s s u f f i c i e n t l y s m a l l t o r e q u i r e h i g hd e f i n i t i o n i m a g i n g i n s t r u m e n t a t i o n and t o s e v e r e l y t e s t t h e u l t i m a t e d e f i n i t i o n c a p a b i l i t i e s o f PTOBD and PA imaging [4,5]. s t a n d such i n t e r f a c e s , a sample w i t h a p r e p a r e d i n t e r f a c e has been f a b r i c a t e d f o r use i n comparison o f PTOBD and PA images. W e r e p o r t t h e r e s u l t s o f i n v e s t i g a t i o n s o f t h a t sample and another, a c o m p a c t -t e n s i l e sample w i t h a c l o...

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