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Technical Discussions
Chirag
India, Joined Aug 2015, 16

Chirag

India,
Joined Aug 2015
16
13:53 Oct-19-2015
Back wall echo higher than front wall echo

Hi,

I was just trying out a few things with the olympus phased array scan. I was testing CFRP laminates and noticed that in most of the laminates the back wall echo is higher than the front wall echo. is this possible? And the distance between the front wall and back wall echo should give me the thickness (i think) but i have noticed an offset of around 1.5 mm. can anyone let me know why?

Thanks and Regards
Chirag

2
 
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Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
15:44 Oct-19-2015
Re: Back wall echo higher than front wall echo
In Reply to Chirag at 13:53 Oct-19-2015 (Opening).

You didn't indicate what probe you are using, but if it includes a straight plastic "wedge" as is usually the case, then what you see with respect to amplitude ratios is normal. Plastic wedges or delay lines represent a fairly close impedance match to typical carbon fiber composites, so there is relatively little reflection from the front surface when well coupled, and if the composite isn't too thick then there is relatively little signal loss across the sound path in the test material. Thus, the backwall echo from an air boundary (with essentially 100% reflection of energy) can easily be larger than the front surface echo from a plastic/composite boundary where there is a close impedance match.

Regarding the apparent thickness measurement error, you haven't provided any information about how you calibrated your instrument, and incorrect velocity/zero calibration is the most common cause of an offset. Other possibilities include significant phase distortion or localized velocity variations in the test material, but those are less likely.

3
 
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chirag
India, Joined Aug 2015, 16

chirag

India,
Joined Aug 2015
16
15:53 Oct-20-2015
Re: Back wall echo higher than front wall echo
In Reply to Tom Nelligan at 15:44 Oct-19-2015 .

Hi Tom, thanks for your reply.. the probe i used was straight plastic wedge. so in case i didnt use a plastic wedge i would get a higher front wall reflection right?
I dont think the calibration is done for the instrument. I will calibrate and then check if the error comes again.. Thanks again!!

1
 
 Reply 
 
Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
16:04 Oct-20-2015
Re: Back wall echo higher than front wall echo
In Reply to chirag at 15:53 Oct-20-2015 .

If you do not use a straight wedge or delay line, you will not see a distinct front surface reflection since it will occur within the excitation pulse ringdown envelope.

2
 
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