Ultrasonic Measurement of Weld Penetration The use of pulse-echo techniques to determine weld pool dimensions is investigated BY D. E. HARDT A ND J. M. KATZ ABSTRACT. The automatic production of high quality welded joints requires a means of measuring weld quality in real time and a feedback control strategy for regulating that quality. Penetration is a good first order indicator of weld integri ty, and most efforts at weld quality con trol have been concentrated on penetra tion. In this work, a technique for using ultrasonic pulse-echo measurements to determine weld pool dimensions is exam ined. Although current research in nonde structive evaluation indicates that size and shape of discontinuities can be inter preted from the time history of an ultra sonic reflection, the thermal gradients caused by welding cause sufficient distor tion in the ultrasound reflection to pre clude use of these techniques for initial studies. In a more straightforward approach, a geometric optics framework is developed for the estimation of ultra sonic transit times between an ultrasound transducer and a stationary, hemispheri cal weld pool in a rod. Experiments were conducted to verify these predictions by performing mea surements of ultrasonic reflections from machined hemispheres and from weld pools in long rods. The results show good agreement between the measurements performed on the cylindrical rods and the geometric optics predictions for both machined surfaces and weld pools. Introduction The automatic production of high qual ity welded joints requires the side-by-side development of techniques for position- D. F. HARDT and J. M. KATZ are with the Laboratory for Manufacturing and Productivi ty, Massa chusetts Institute of Technology, Cambridge, Massachusetts. ing the welding torch (seam tracking) and for regulating welding parameters in real time to obtain a desired level of weld quality. The work described in this paper concentrates on weld penetration as a measure of weld quality using ultrasonic pulse echo techniques to measure the penetration of a weld pool in real time. Several researchers have considered techniques for measuring weld penetra tion for feedback control. In 1976, Vro- man and Brandt (Ref. 1) used a line scan camera to measure the width of the top side of a weld. Since for a given geome try and fixed welding conditions the depth to w idth ratio of a weld remains somewhat constant, regulating the top side weld pool width regulates penetra tion. The results reported were not con clusive and indicated the need for further study of this approach. In a refinement of this technique, Richardson et al. (Ref. 2) recently reported the use of video mea surement methods to control the topside width of GTA welds. The unique aspect of this wor k is that the pool is viewed directly from above by putting the optical axis in line with the electrode. In a more direct approach, Nomura ef al . (Ref. 3) have used photodetectors placed along the back side of the weld ment to measure the back bead width of th e weld. As the weld goes from partial to full penetration, the infrared radiation from the back side of the weld goes through a step transition. In this way, photodetectors can be used to detect a full penetration weld. Garlow (Ref. 4) and Reiff (Ref. 5) have used a simple photo- transistor to measure the back side weld bead width in GTA welds with reason able accuracy. This measurement was used in a closed loop controller that quite accurately regulated the back bead width. However, back side weld bead sensing has an inherent drawback in that it is difficult or impossible to conveniently locate a sensor on the back side of the weldment for many weldment configura tions. In addition, back side sensing does not provide useful measurements when knowledge of partial penetration is desired. Hardt and Zacksenhouse (Ref. 6) demonstrated that weld pool size could be determined by measuring the resonant frequency of a full penetration pool. The pool is modelled as a dynamic mass spring system, and they show that the natural frequency of this system is a function of the size of the weld pool. The existence of this frequency dependence on weld pool size has been verified experimentally, and the present work is attempting to show that arc voltage fre quency measurements can be used to determine the weld pool natural frequen cy. Renwick and Richardson (Ref. 7) have also observed a pool resonance in the case of a partially penetrated weld. All of the weld pool measurement techniques mentioned so far share the difficulty that they are attempting to mea sure variables that are not single valued indicators of penetration. In an attempt to provide a means of directly measuring the desired weld pool dimensions, the concept of using ultrasonic pulse echo techniques to directly measure weld pool dimensions was developed, based upon established technology for other applica tions. In this paper, the techniques avail able for ultrasonic measurement and the implications of application to in-process welding are discussed. This is follow ed by a critical experiment where the existence of reflections from a weld pool are con firmed, and some rudimentary depth measurements performed. Ultrasonic Methods for Defect Measurement Ultrasonic testing has been used effec tively as a nondestructive evaluation WELDING RESEARCH SUPPLEMENT | 273-s
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
8/10/2019 Ultrasonic Measurement of Weld Penetration
Fig. 4 — Transmission of ultrasound through a flat ended rod
Weld Pool Reflections in a
Cylindrical Rod
In order to measure the dimensions of
a weld pool using ultrasound, i t is neces
sary to determine how the presence of a
weld pool reflects an ultrasound pulse
and then how the dimensions of the
reflecting weld pool can be isolated from
a time trace of the ultrasound pulse. To
th is end, a s impli f ied we ldme nt geom etry
(a cyl indrical rod) combined with a ray
optics wave analysis was employed. (The
results of Schmitz (Ref. 20) are encourag
ing with regard to the uti l i ty of this
approach.)
The case of ultrasonic weld pool
dimensional measurement lends itself to
the geometric optics approach, s ince the
locat ion o f the weld poo l is known. By
physically connecting the transducer
mount ing s truc ture to the weld ing torch,
it can be guaranteed that the transducer
wil l track the weld pool at al l t imes. Using
the ray tracing techniques of geometric
optics, it is possible to reconstruct a
surface pro f i le o f the weld poo l f rom
ultrasonic t ime traces. Important
con
cepts for such a reconstruction are dis
cussed below.
Cons ider the conf igura t ion shown in
Fig. 3 where an ultrasound transducer is
clamped to the end of a cyl indrical rod. A
GTA welding torch is placed above the
rod end oppos i te to the t ransducer, and
a weld pool is established in the rod end.
In this configu ration , the ultrasound trans
ducer wil l be directly v iewing the weld
poo l as i t fo rms. Ho wev er, be fore
con
sidering the reflections of ultrasound
f r o m a w e l d
p o o l ,
it is well to look at the
ultrasonic reflections from the end of a
plain cylindrical rod of length L, and
diameter d.
In Fig. 4, an ultrasound transducer wit h
maxim um b eam angle is sho wn placed o n
the end of the rod (posit ioned such that
its center is aligned with the rod axis and
that the geometry is axial ly symmetric).
This wil l a l low th e use of a tw o dim en
sional model of the ultrasonic ray paths.
In order to simplify the discussion, it is
assumed that Lftanf/y)) is less than half the
rod diameter so that the ultrasound pulse
wi l l reach the oppos i te rod end before
the beam spreads out enough to reach
the sidewalls of the rod.
The ultrasound beam can be described
by a series of rays traveling down the
length of the rod. The rays wil l make an
angle 0 with the axis of the rod such that
9 is less than th e absolut e value o f th e
transducer beam angle. For an arbitrary
ray with angle 9, the t ime required for
ultrasound to travel from the transducer
to the opp osite ro d end is given by:
t i = L / ̂ cos(9) where c-\ is the lo ngitudi
na l wave speed.
The rod end is a bounda ry betw een a
solid and air so refraction from the rod
end will not take place. From Snell's law it
can be shown that the longitudinal pulse
is reflected at the same angle as the
incident pulse while the reflected shear
pulse has an angle given by: 9
r t
= sin
(c
t
/Ci
s in 9) where c
t
is the shear wave
speed. The radial posit ion at the rod end
is given by:
r = L tan 9
(1)
It is possible for ultrasound to return to
the transducer along several different
paths,
and Fig. 5 depicts four possible
Fig. 5 - Return paths o f reflections from a flat ended rod: A — longitudinal wave return path without sidewall reflection; B - longitudinal return path wit
sidewall reflection; C —return path with one she ar wave reflection; D —return path with multiple shear wave reflections
WELDING RESEARCH SUPPLEMENT | 275-s
8/10/2019 Ultrasonic Measurement of Weld Penetration
T h i s r e p o r t w a s p r e p a r e d i n r e s p o n s e t o a r e q u e s t f r o m t h e S u b g r o u p o n T o u g h n e s s o f t h e P r e s s u r e
Vesse l and Bo i le r Code Commi t tee o f the Amer ican Soc ie ty o f Mechan ica l Eng inee rs . I t s pu rpose i s to
p r o v i d e a g e n e r a l f o r m a t f o r a s s e s s i n g e l a s t i c - p la s t i c f r a c t u r e i n a f r a c t u r e - c o n t r o l p l a n f o r s t r u c t u r a l
s t e e l v e s s e l s o r m e m b e r s 2Vz in . o r less in th ick nes s . A gene ra l ove rv iew o f the s ub je c t o f e las t i c -p las t i c
f r a c t u r e i s p r e s e n t e d .
P u b l i c a t i o n o f t h i s r e p o r t w a s s p o n s o r e d b y t h e S u b c o m m i t t e e o n F a i l u r e M o d e s i n P r e s s u r e V e s s e l
M a t e r i a l s o f t h e P r e s s u r e V e s s e l R e s e a r c h C o m m i t t e e o f t h e W e l d i n g R e s e a r c h C o u n c i l .
The p r i ce o f WRC Bu l le t in 291 i s $12 pe r copy , p lus $5 fo r pos tage and hand l ing . Orde rs shou ld be sen t
w i t h p a y m e n t t o t h e W e l d i n g R e s e a r c h C o u n c i l , R o o m 1 3 0 1 , 3 4 5 E . 4 7 t h S t . , N ew Y o r k , N Y 1 0 0 1 7 .
WRC Bullet in 290
December , 1983
Factors Affecting Porosity
in
Aluminum
Welds—A
Review
by J. H. Devletian and W. E. Wood
T h e e m p h a s i s o f t h i s r e p o r t i s o n t h e v a r i o u s f a c t o r s a f f e c t i n g t h e p o r o s i t y i n a l u m i n u m w e l d s
d e p o s i t e d b y t h e G T A W a n d G M A W p r o c e s s e s . A t o t a l o f 9 4 t e c h n i c a l p a p e r s w e r e r e v i e w e d a n d
e v a l u a t e d i n t h e p r e p a r a t i o n o f t h i s r e p o r t t h a t s u m m a r i z e s t h e s t a t e o f t h e a r t o n t h e c a u s e s a n d
f o r m a t i o n o f p o r o s i t y in a l u m i n u m w e l d m e n t s .
P u b l i c a t i o n o f t h i s r e p o r t w a s s p o n s o r e d b y t h e A l u m i n u m A l l o y s C o m m i t t e e o f t h e W e l d i n g R e s e a r c h
C o u n c i l .
The p r i ce o f WRC Bu l le t in 290 i s $1 2 .0 0 pe r cop y , p lus $5 .00 fo r pos tage and hand l ing . Ord e rs s hou ld
b e s e n t w i t h p a y m e n t t o t h e W e l d i n g R e s e a r c h C o u n c i l , R o o m 1 3 0 1 , 3 4 5 E . 4 7 t h S t r e e t , N e w Y o r k , N Y
1 0 0 1 7 .
WRC Bullet in 287
September ,
1983
Welding of Copper and Copper-Base Alloys
by R. J. C. Dawson
T h i s I n t e r p r e t a t i v e R e p o r t d i s c u ss e s t h e c u r r e n t s t a t u s of f u s i o n w e l d i n g t e c h n o l o g y f o r c o p p e r - b a s e
m a t e r i a l s o f m a j o r i n d u s t r i a l i m p o r t a n c e . C u r r e n t w e l d i n g p r a c t i c e s f o r e a c h g r o u p o f c o p p e r - b a s e
m a t e r i a l s a r e d i s c u s s e d . L i t e r a t u r e r e f e r e n c e s a n d s u g g e s t e d f u r t h e r r e a d i n g is a l so p r e s e n t e d .
P u b l i c a t i o n o f t h i s r e p o r t w a s s p o n s o r e d b y t h e
Interpretative Repo rts Com mittee
o f th e
Welding
Research Council.
The p r i ce o f WRC Bu l le t in 28 7 is $1 2 .0 0 pe r cop y , p lus $5 . 00 fo r pos tage and hand l ing .
O r d e r s s h o u l d b e s e n t w i t h p a y m e n t t o t h e W e l d i n g R e s e a r c h C o u n c i l , R o o m 1 3 0 1 , 3 4 5 E a s t 4 7 t h S t r e e t ,