Ultrasonic Testing of Structural Welds

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Cathode Ray Tube Screen,Approach ll, Angle Wedge and Transducer are Type A Approach 3 8 Type B Approach 3 8. collectively known as Search Unit,Figure 1 Basic ultrasonic weld testing scheme. the weld It is believed that the flattest TypeC Approach o Type D Approach 3 8. possible testing angle should be used The,Figure 2 Wedge noise and weld bead approach. basic reason for this choice is the fact that,typical production defects tend to be per. pendicular to the plate surface or are as, sociated with weld preparation surfaces The choice of flatter angles offers several.
other advantages First flatter angles result in more beam spread in the plate thick. ness Second flatter angles permit more of the weld volume to be inspected without re. sorting to bouncing the sound from the opposite plate surface This is advantageous. since bouncing causes definite errors in defect location 1. The flattest angle we consider practical for weld inspection is 75 deg Most tests. are limited to 70 deg Higher angles contain surface wave components which make in. terpretation difficult or impossible The angles generally used for particular thicknesses. are as follows,Thickness in Angle deg,2 to 3 70 or 60. 3 to 5 60 or 45, Steeper angles are used in heavier material for two reasons The long sound path. needed for flatter angles introduces more attenuation into the test than can be easily. compensated Beam spread associated with flatter angles in heavy sections becomes a. disadvantage reducing defect detection reliability. The phenomenon of mode conversion is,quite often discussed with regard to angle. choice Mode conversion is not related,specifically to any testing angle but only to. the angle of sound incidence upon a reflect,Tranamissivlty single interface.
ing surface The critical angle of incidence,Lucite glycerine 85 85. Lucite water 63 o5i is 30 deg Therefore detection of vertical. Lucile oil 53 87i,Augle beam testing tram1mlsslvity. reflectors using a 60 deg angle is question,1 Lucite glycerine steel 15 24i. able In practice the only real precaution,2 Lucite water steel. 3 Lucite oil steel,necessary when using 60 deg is the strict.
Overall transmission variation with surface variation avoidance of plate edges or vertically drilled. a 2 variation from a 101 dtrect luclte steel coupling condition. holes as calibration standards Actually 60,4 Lucite glycerine steel ht ar D 96. 5 Luette water steel sh ar 3 65 deg is a good testing angle because many. 6 Lucile oll steel shear 5 39,weld preparations involve 30 deg surfaces. 1 Quartz glycerine steel longitudinal 12 63,which as a lack of fusion are ideally de. De feel Angle Amplitude Plate Thickness Defect Length. No deg db In ln,a Defect Type Porosity,45 Not detected. b Delect Type Slag,Figure 3 Mode conversion effects 60 10.
Large Reflecting Area Smal Reflecting Area 60 17,Figure 4 Signal amplitude is a measure of de 60. 45 Not detected,fect reflecting area 100 70 1l,105 70 13 1. c Defect Type Lacko Penetration,58 70 19 12,Figure 5 Search unit trove I is a measure of de. fect length 42 70 1,e Defect Type Lack of Fusion,57 70 19 1. Defect l ocatiGn Chart 60 15,70 70 20 1,102 70 21 2.
Figure 6 Defect location can be accurately done, tected with 60 deg By the same token 45 deg is a poor angle for the detection of lack. of fusion for the same reasons These effects are shown in Figure 3 Some data relat. ing testing angle to defect detection reliability for actual defects are given in Table 2. When the pulse has traveled to the defect been reflected and returned through the. transducer it must be interpreted There are six basic types of information which can. Figure 7 Testing from both sides of weld reveals,defect orientation. Figure 8 Travel to and from the weld reveals de,fect height. be extracted Figures 4 through 9 depict,these information types We feel that the. first three types of information are of pri,mary importance in determining defect.
severity and pinpointing defect location, Rat Defect Pomily Defect for excavation The other three types deal. with parameters not easily quantified and, Figure 9 Pulse shape ind i cotes nature of defect should not be used as a basis for accep. tance or rejection f owever these types,of information are of great value to the. welding engineer or supervisor who must intelligently correct defective welding. conditions, In order to establish a basis for amplitude rejection it is necessary to develop data. on a great many defects It was found that conventional methods of test standardization. and amplitude read out were not sufficiently accurate Rather than attempt to judge the. height of signals on the screen an attenuator or calibrated gain control was used The. instrument is calibrated on an arbitrary nondirectional reference reflector such as the. 1 5 mm hole in the IIW block Defect amplitude is related to this reference by equating. all signal heights to that of the reference using an attenuator The difference in db is. now amplitude information CRT presentation vertical linearity deviations and visual. error are largely eliminated, Since CRT linearity limitations are eliminated the clipping or reject control can now.
be used not to clean up the screen but to decrease the vertical dynamic range pre. sented on the CRT This increases the accuracy of amplitude measurement using an. attenuator still further, Using this approach a gr eat many welding defects were measured as shown in Fig. ure 10 The attenuation within the material has been corrected by adding 2 db in after. the first inch of sound travel to all readings This value was obtained by making a great. many readings on various steel thicknesses and reflector types Figure 11 shows the. 10 O 1 l l 4 5 i I 1Zl 51l,signal size,in dKibels inches inches. 0 1 OJSU ll UIS 1 1 5 l l I 5 10 11,dtfH l ltngthin indtts inches. Figure JO Weld tests at 70 deg 120 defects, plate thickness 5 16 in to 5 in Figure 11 Results of some attenuation tests. Figure 12 Sensitivity standard,Figure 13 Distance calibration standard for angle.
beam testing,results of some of these tests Increased. attenuation is noted close to the search unit, for reflectors incorporating the cylindrical surfaces of drilled holes as compared with. flat bottomed holes drilled at the testing angle These geometrical effects apply only to. cylindrical surfaces and care should be taken not to use such standards for attenuation. measurements In all cases the attenuation is the slope of the curve. As was previously stated the IIW block 1 5 mm hole has been used as a sensitivity. standard Since different angles and resulting different path lengths introduce the geo. metrical effects a simple standard in which the distance to the reflector is l in for. 70 deg 60 deg and 45 deg has been developed Fig 12. Defect length is measured conventionally based on the point at which a 5 db drop in. signal is encountered, Defect location in cross section is accomplished using conventional geometrical. methods IIW block distance calibration surfaces may be used We have developed a. simple calibration standard for distance which gives calibration pulses at even inch in. crements This standard is shown in Figure 13 We prefer to calibrate for actual sound. path length We feel that the weld inspector is thus made more aware of the geometrics. with which he is working Having thus calibrated a chart such as Figure 14 can be used. for defect location To further simplify,this procedure we have developed a simple. retractable tape Fig 15 for each angle,Defect llist e AllNd of S t 5 a P t.
Figure 14 Defect location chart Figure 15 Retractable tape for defect location. which becomes part of the search unit,The tape is pulled out to the equivalent. path length At this point the defect is,directly under the tape end and the defect. depth is read from a secondary scale op,posite the observed path length dimension. As mentioned earlier it is necessary,to correct for defect location errors when. a bounce from the opposite plate surface,is used For angles greater than 45 deg.
a small l J value must be subtracted from,the apparent horizontal dimension sound. emission point to defect There is no,correction at 45 deg At 60 deg the cor. rection is about 1 e in at 70 deg about,Figure 16 Internally calibrated search unit. One of the pitfalls of ultrasonic weld,testing is the fact that the test is not. self calibrating There is no back reflec, tion or other continual reminder of testing uniformity Normal shop line voltages and.
battery powered unit voltage variations near the end of battery life can cause gradual. sensitivity changes which go unnoticed We use constant voltage transformers and volt. age drop alarms to protect us from these effects In addition we have developed an. internally calibrated search unit Fig 16 which provides a continual simple check of. amplitude and horizontal calibration stability A hole is drilled in the wedge so that a. small amount of sound reflected from the contact surface will be in turn reflected from. the hole This results in a small discrete signal on the screen which can be used to. monitor testing uniformity The reflector hole is threaded arid a screw inserted Mov. ing the screw changes reflector characteristics slightly The screw is used to compen. sate for slight wedge wear, The nature of the data recorded in Figure 10 makes possible an ultrasonic weld in. spection standard based on amplitude and length Most present weld quality require. ments can be interpreted in terms of amplitu e and length using such information The. data clearly indicate the factors which lend to ultrasonic weld inspection reliability in. detecting the more serious defect types Radiography has forced on us the necessity of. accurate defect identification since generally the most serious defects are most poorly. detected By its nature ultrasonic testing reflects defect severity directly Therefore. defect identification becomes much less important,Radiographic Ultrasonic. Defect Type Defect Severity,Lack of fusion,Lack of penetration. The most important part of the weld testing system is the inspector Use of ampli. tude and length rejection criteria has removed from him the onus of defect decisions. based purely on subjective observations However he must still have a detailed knowl. edge of ultrasonics and must be capable of working easily with complicated situations. in three dimensional geometry, We give each of our weld inspectors a four tofive weekclassroom course to this. end We have generally drawn on groups of radiographers qualified to 250 1500 or on. experienced draftsmen for our personnel, Our work in the area of ultrasonic weld testing over the past year has been in cooper.
ation with the Bureau of Public Roads We are developing a specification and a training. manual for the inspection of bridge weldments The publication and implementation of. this work should expand the use of ultrasonics as a weld testing tool. Generally this ork has bee lin ited to full penetratio1 butt welds in thickness s from. 1a in to 5 in We have also applied it to full penetration T welds in the same thickness. range It would be dangerous to attempt to apply the results of this work to other weld. configurations or thicknesses Partial penetration welds and backing strip welds can be. thus inspected but spurious reflections caused by these configurations must be taken in. to account We are extremely dubious as to the direct application of this information to. fillet welds, We hope that the result of our work will be greater use of ultrasonic weld testing. The use of amplitude and length rejection criteria will simplify interpretation Eventu. ally we may be able to treat particular areas in large welds more critically than. others depending upon the excellent defect location ability of ultrasonics to tell us. whether or not defects are in more highly stressed weld areas. Ultrasonic Testing of Structural Welds DEXTER A OLSSON Bethlehem Steel Corporation During the past five years Bethlehem Steel Corporation has de veloped and applied techniques for the ultrasonic testing of butt welds which provides an assurance of weld quality similar to that achieved with radiography Although there are no revolu tionary breakthroughs involved a number of significant

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