ndt for delamination in graphite laminated structure

huangyhg

ndt for delamination in graphite laminated structure
do anyone has any research into detecting delamination in graphite laminate structure? i've tested using ultrasonic testing but the results doesn't seem promising. i've imparted a delamination in a laminate was this delamination was not detected using ultrasonic testing. the delamination was detected after a 500x magnification after cutting a cross section of the delaminated part. i've heard about shearography, but do not know how well it perform. can anyone help?
thanks!
sometimes a simple "tap test" will do.  this works on structures where a delaminated or debonded area that is too small to be heard when tapped is also too small to have a significant effect on the structure's strength.
your application may be more critical than this.  have you considered using x-ray examinations?
steven fahey, cet
ultrasonics is probably the most comonly used technology but there are several different techniques that can be used (through transmission, back-surface reflection, reflector plate, fluid coupling, etc.). heat pulse laser holography or thermal imaging is also very effective. a heat pulse is applied to a surface and after a short period the surface will be warmer over a delamination. this can be detected with thermal imaging. or the heat pulse will cause a slight warping of the surface over a delamination which can be visualized with laser holography.
there is a huge dependency between the type of defect to detect, size of the defect, and the method used for detection.  eg disbond vs delam vs void vs porosity
vs inclusion; each can have a preferred technique or multiple techniques for detection.
typical ultrasonic "a"-scan can see defects of .5x.5 in sq. area.  limitations are based on probe size.  most probes are .250" diameter. these involve using the backwall of the laminate and therefore the part geometry can severly impact the ability to see defects that are obvious elsewhere. small radii are much harder to see defects in than flat or slightly contoured areas.
thru transmission techniques that use one probe to pitch  and one to catch the soundwaves can see smaller defects but i have never seen one do better than .25x .25 in sq on a repeatable basis.
x-ray cannot detect a delamination except when there is an inclusion of a substantially different density. (sorry spar but ive been burned too many times by false positives) and ruined otherwise perfectly good parts.
c-scan - automated probes that use "through transmission" can resolve smaller defects .250"x.250" but even that is at the extreme end of their ability to see a delamination.
tap testing can detect larger defects but it takes a practiced ear and i would not let a novice do the task. but it is a quick and dirty test that is handy to have in your back pocket.
given that you had to use 500x to see the defect it sounds to me as if you have an a very small defect.   
good luck   
wongweiyew...
documents to start your education... available on-line with a service such as ihs...
sae arp 5089  composite repair ndt/ndi handbook
sae arp 5605  solid composite laminate ndi reference standards
sae arp 5606  composite honeycomb ndi reference standards
mil-hdbk-732a nondestructive testing methods of composite materials - acoustic emission
mil-hdbk-731 nondestructive testing methods of composite materials 鈥?thermography
mil-hdbk-733  nondestructive testing methods of composite materials 鈥?radiography
mil-hdbk-787 nondestructive testing methods of composite materials - ultrasonic
mil-hdbk-793  nondestructive testing techniques for structural composites
nas999  nondestructive inspection of advanced composite structures
usaf t.o. 33b-1-1  nondestructive inspection methods, basic theory [sometimes available at a usaf website... but it is mostly "down"]
sae book:  care and repair of advanced composites
and for the "human factors" involved...
sae air 4844b composites and metal bonding glossary
sae air 5279  composite and bonded structure inspector: training document
sae air 5278  composite and bonded structure engineers: training document
sae air 4938  composite and bonded structure technician/specialist: training document
regards, wil taylor
there are some reports on nde at:
dear wongweiyew:
i am with a company that manufactures shearography based systems for non destructive testing. shearography has some advantages over traditional ndt technologies especially if composite materials and/or joints are concerned. they have performed a wealth of contract measurements for customers. larger corporations, especially in aerospace or aircraft industry, bought their own equipment.
hi jim,
thanks for your recommendation. i've heard about laser shearograph before and had tried some test from a company called laser technology inc but they could not detect a flaw in our imparted delaminated test specimens.
thanks!