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Ultrasonic Method for Testing Spot-Welds
Hajime Takada1;*, Takafumi Ozeki1, Rinsei Ikeda2 and Tomoyuki Hirose3
1Instrument and Control Engineering Research Department, Steel Research Laboratory,JFE Steel Corporation, Kawasaki 210-0855, Japan2Joining and Strength Research Department, Steel Research Laboratory,JFE Steel Corporation, Chiba 260-0835, Japan3Automotive Steel Section, Product Design & Quality Control Department, West Japan Works,JFE Steel Corporation, Fukuyama 721-8510, Japan
We developed a technique for nondestructively evaluating spot-welds based on through-transmission of Lamb waves. The nugget diametercan be evaluated by measuring the width of the zone where attenuated transmitted waves are observed. A particularly important feature of thedeveloped technique is that spot-welds with no weld metal can be reliably distinguished from spot-welds with weld metal. We also developed ameasuring system using array transducers. The measurement results using the system agree well with the nugget diameters measured by cross-sectional observation. [doi:10.2320/matertrans.M2010336]
(Received September 24, 2010; Accepted December 13, 2010; Published February 2, 2011)
Keywords: spot welds, nondestructive testing, ultrasound, probe array, nugget diameter
In the manufacturing plants for automobile bodies andelectrical appliances, the product parts are assembled byspot-welding at multiple points. When viewed exteriorly,spot-welds usually appear as small dimples. Typical spot-welding is performed by pressing 2 to 4 steel sheets betweentwo electrode tips several mm in diameter.
Spot-welds must be carefully inspected, as the quality ofthe welds directly influences the strength and durability of thewelded body. One conventional method for inspecting spot-welds is to check a cross-section of the welded metal.Another is to drive a cold chisel between spot-welded sheetsto determine if the sheets can be pried apart. These methodsare problematic, however, as both are destructive (breakingthe spot-welds) and can only be performed using spot-weldssampled from the products. For this reason, industries havelong awaited the development of a reliable nondestructivemethod for spot-weld inspection.
Engineers aspiring towards the nondestructive testing ofspot-welds have attempted to develop an ultrasonic testingmethod based on the decay technique16) and an electro-magnetic method (see Table 1). The conventional ultrasonictesting method, however, has two fatal shortcomings de-scribed as follows:
(1) Portions under the inclined surface in the spot-weldscannot be evaluated by the method based on the straight beamtechnique, because transmitted ultrasonic waves are severelyscattered in such portions.
(2) The decay may take the same state at different growthstages of the weld metal (nugget). In detail, the decayobserved with the spot-welds with little or no weld metal mayequal that observed with the spot-welds with large weldmetal. That is to say, the two different growth of the weld
metal mentioned above cannot be distinguished by themethod.
The conventional electromagnetic method is also imper-fect, because measuring results by the method are stronglyinfluenced by the shape of welds. In particular, there is noreliable means for distinguishing spot-welds with no weldmetal from spot-welds with weld metal. Neither the ultra-sonic nor the electromagnetic methods are reliable fornondestructive testing of spot-welds. In last 10 years ultra-sonic imaging technologies based on the straight beamtechnique have been applied to the nondestructive testing ofspot-welds.710) But they still have the problems mentionedabove.
Therefore, we recently developed a method that candistinguish between spot-welds with and without weld metalin the nondestructive testing of spot-welds. This paperoutlines this new method.
Table 1 Conventional techniques for spot-weld testing.
Method Technique Shortcomings
1. Portions under the inclined surface cannot be evaluated.
2. The decay may take the same state at different growth stages of the weld metal
Measuring permeability and conductivity
Measuring results are influenced by the shape of welds.
Growth of weld metal
Forming of nugget
*Eddy current testing
*Present address: Non-destructive Inspection Technology Department,
Instrument System Division, JFE TECHNO-RESEARCH CORPORA-
TION, Chiba 260-0835, Japan
Materials Transactions, Vol. 52, No. 3 (2011) pp. 539 to 546#2011 The Japan Institute of Metals EXPRESS REGULAR ARTICLE
2. Principle behind the Method for NondestructiveTesting of Spot-Welds
2.1 Ultrasound attenuation measurementFigure 1 outlines our recently developed nondestructive
spot-weld testing method. Ultrasounds (ultrasonic Lambwaves) are transmitted to a steel sheet at a sending pointoutside the weld metal of a spot-weld (nugget) and propagatetoward the weld metal as a certain propagation mode(mode 1). When the ultrasounds reach the edge of the weldmetal, the propagation mode of the ultrasounds is convertedinto other mode (mode 2) by reason of change in thicknessand the ultrasounds propagate toward the opposite edge ofthe weld metal as the propagation mode 2. At the oppositeedge of the weld metal, the propagation mode of theultrasounds is converted into the former mode (mode 1)and the ultrasounds propagate toward a receiving pointopposite to the sending point. Finally through-transmittedwaves are detected at the receiving point. The ultrasoundswith different fd values are treated as different modes in thedescription above. Here, f is frequency of the ultrasounds;and d is the thickness of the propagation medium.
The weld metal usually has a thickness-direction-orientedstructure, because the heat generated by spot-welding movestoward the electrode tips and then a temperature gradientoccurs in the thickness direction. Figure 2 shows a typicaloptical micrograph of weld metal.
As the weld metal has a coarse structure compared withthe crystal structure of the steel sheet to be welded, lowtransmissivity for ultrasonic waves (large attenuation) isobserved as shown in Fig. 1(b). In contrast, if there is no weldmetal in the spot-weld, high transmissivity for ultrasonicwaves (little attenuation) is observed as shown in Fig. 1(a),because there is no coarse structure in the propagation path.On occasions when two steel sheets are not jointed, through-transmitted waves with large amplitudes similar to thethrough-transmitted wave shown in Fig. 1(a) are observed.Figure 1(c) shows the relationship between the estimatedtransmissivity for ultrasonic waves and the growth of themeld metal. The estimated transmissivity with the developedmethod is a monotonic decreasing function of the weld metalgrowth. Accordingly, discrimination between spot-weldswith and without weld metal is easy to be carried out by
observing the amplitude of the through-transmitted wave.Whereas the estimated transmissivity with the conventionalmethod based on the straight beam technique may take thesame value at different growth stages of the weld metal.
No weld metal
Ultrasound (Lamb wave)
(a) Spot-weld with no weld metal
(b) Spot-weld with weld metal
:Mode 1:Mode 2
Growth of weld metal
Forming ofweld metal
(c) Estimated transmissivity as the function ofthe growth of weld metal
Fig. 1 Spot-weld evaluation method using the difference between (a)
ultrasound (Lamb wave) propagation in the spot-weld with no weld metal
and (b) that in the spot-weld with weld metal; and (c) estimated
transmissivity as the function of the growth of weld metal.
Fig. 2 Optical micrograph of typical weld metal.
540 H. Takada, T. Ozeki, R. Ikeda and T. Hirose
Therefore, discrimination between spot-welds with andwithout weld metal is not easy to be carried out by usingthe conventional method.
The ultrasounds propagating along the surface of thesteel sheet are attenuated proportionately with the pathlength of the weld metal, that is, the nugget diameter. Thenugget diameter can be measured by measuring ultrasoundattenuation. The accuracy of the ultrasound attenuationmeasurement, however, depends on the coupling betweenthe ultrasonic probe and the test object. A testing technique,in which the nugget diameter is evaluated by detecting theplanar zone where the nugget-induced attenuation of theultrasound is observed, is preferable.
The attenuation of ultrasonic waves can be altered by thescattering of ultrasound resulting from the step-like shapeformed in a spot-weld. Yet ultrasonic waves propagate inmany modes along a steel sheet. For this reason, it is possibleto use a propagation mode that minimizes the influence of thestep-like shape.
2.2 Ultrasound through-transmission time measure-ment
The thickness-direction-oriented structure also has aniso-tropy similar to transversely isotropic materials. Sinceultrasound velocity varies depending on the propagationdirection in the anisotropic medium, the region where theweld metal exists is distinguishable by measuring thethrough-transmission time.
The general linearly elastic constitutive equation foranisotropic medium is given by the following equation:11,12)
C11 C12 C13
C21 C22 C23
C31 C32 C33
where ij is the stress tensor; eij is the strain tensor; Cij is theelastic constant stiffness matrix. Considering the symmetry inthe transversely isotropic medium, eq. (1) is simplified as
C11 C12 C13
C12 C11 C13
C13 C13 C33
here, C66 C11 C12=2; z-direction is the thicknessdirection of the weld metal. The equation of motion is givenas follows:
where u; v;w is the displacement; is density.
The propagation of ultrasound satisfies the abovemen-tioned two equations. Then, let us assume the following planewave as a general solution:
u Ax exp2ik DL V t=;v Ay exp2ik DL V t=;w Az exp2ik DL V t=; 4
where k is the wave vector and k l;m; n; DL x; y; z;V is ultrasound velocity and is wavelength.
Accordingly, from eqs. (1) to (3), the following equationis derived:
A1 V2 A2 V2 A3 V2
A1 C11 l2 fC11 C12=2g m2 C44 n2;A2 fC11 C12=2g l2 C11 m2 C44 n2;A3 C44 l2 C44 m2 C33 n2; C13 C44 m n; C13 C44 n l; fC12 C11 C12=2g l m:
As the determinant on the left side must be zero to obtain anontrivial solution, the relationship between the ultrasoundvelocity V and the elastic constant stiffness coefficients canbe determined. Next, the values of the elastic constantstiffness coefficients can be obtained by measuring thevelocity of ultrasound that propagates along the specifieddirection. As a result, the velocity of ultrasound propagatingalong an arbitrary direction can be calculated based on thevalues of the elastic constant stiffness coefficients.
Using low-carbon cast steel as a representative sample oftransversely isotropic steel, the elastic constant stiffnesscoefficients measured with a solidified structure oriented inone direction are shown in Table 2. Figure 3 shows thevelocity curves calculated using the values in Table 2. Boththe velocity of shear (transverse) waves VSV, VSH and that oflongitudinal wave VL vary depending upon the propagationdirection.
The samples shown above are not results for the weldmetal of low-carbon steel. However, it is considered that theshape of the velocity curve is similar in the weld metal.Therefore, the through-transmitted waves that pass throughthe weld metal are distinguishable by measuring the through-transmission time. The region where weld metal exists isdistinguishable by measuring the through-transmission timeas well.
Table 2 Elastic constant stiffness coefficients.
Elastic constant stiffness
(C11/)/(m2/s2) 3:37 107
(C33/)/(m2/s2) 3:21 107
(C44/)/(m2/s2) 1:53 107
(C12/)/(m2/s2) 1:75 107
(C13/)/(m2/s2) 1:47 107
Ultrasonic Method for Testing Spot-Welds 541
3. Spot-Weld Evaluation Method Using PiezoelectricElement Array and Developed Measuring System
As mentioned in 2.1, the accuracy of the ultrasoundattenuation measurement depends on the coupling betweenthe ultrasonic probe and the test object. With this in mind, wedecided to study a testing technique in which the nuggetdiameter is evaluated by detecting the planar zone where thenugget-induced attenuation of the ultrasound is observed.However, mechanical scanning of the coupled ultrasonicprobe is unsuitable for the nondestructive spot-weld evalua-tion in a manufacturing plant. As an alternative, wedeveloped a method for linearly scanning the ultrasonicbeams using piezoelectric element arrays.
3.1 Developed measuring system using piezoelectricelement array
The appearance of the developed measuring system isshown in Fig. 4. Switching circuits interposed between anultrasonic pulser/receiver and the probe arrays (piezoelectricelement arrays) switch the connections between the piezo-electric elements in the probe array and the ultrasonic pulser/receiver. All received signals are subjected to A/D con-version and are handled by personal computer (PC). The
amplitude of the through-transmitted ultrasonic waves isdetected and displayed using a signal-processing applicationon a PC. As all the components are assembled in a plasticcase, the system can easily be handled and brought towherever it is needed.
Figure 5 shows the schematic depiction of measurementby using the developed piezoelectric element arrays (probearrays) and an appearance of the coupled piezoelectricelement arrays. Both the transmitting and receiving probearrays are equipped with a piezoelectric element array. Eacharray comprises 16 piezoelectric elements with a nominalfrequency of 10MHz, width of 0.5mm in the array direction,and element spacing of 0.1mm in the array direction.
The Lamb wave used in this method is the combination ofshear wave (SV wave) with a refraction angle of about 32
and longitudinal wave with a refraction angle of about 78.The waves might be called as partial waves.13) Referring toFig. 3, it is expected that the through-transmitted wave thatpasses through the weld metal is received later than thethrough-transmitted wave that does not pass through the weldmetal.
The system works via the detection of the through-transmitted wave moving between every element in both thetransmitting probe array and the receiving probe array. Thethrough-transmitted wave sent from the piezoelectric elementi (i 1; 2; . . . ; 16) in the transmitting probe array one by oneis received by all of the piezoelectric element (16 elements)in the receiving probe array one by one. The amplitude of thethrough-transmitted waves moving along the 256 (1616)paths in total is detected. In other words, the amplitude of thethrough-transmitted waves that take the oblique path is alsodetected.
The nugget diameter of a spot-weld is measured using theamplitude of the received through-transmitted waves along31 paths as shown in Fig. 6(a). The through-transmitted
Z (Thickness direction)
T (Transverse direction)
Z (Thickness direction)
T (Transverse direction)
(a) Longitudinal Wave
(b) Shear Wave
Velocity curve for isotropic steel
Velocity curve for isotropic steel
Fig. 3 Velocity curve of (a) longitudinal wave and (b) shear wave in
Fig. 4 Developed measuring system.
542 H. Takada, T. Ozeki, R. Ikeda and T. Hirose
waves propagating along the paths are recognized based onthe arrival time delay and then their amplitude and transittime are detected. As shown in Fig. 6(b), the amplitudeprofile of the received through-transmitted waves along thearray direction is determined by interpolation using theamplitude of the received through-transmitted waves de-scribed above. Then, the width of the portion where theamplitude was lower than the predetermined threshold valueis measured as the nugget diameter. In the transit timemeasurement, the transit time of the through-transmittedwave that propagates outside the weld metal is used as thereference time. The path where the transit time of a through-transmitted wave is longer than the predetermined time delaywith respect to the reference time is judged as having theweld metal.
Moreover, the amplitude of the through-transmitted wavesmeasured for all the paths is shown in a representation using amatrix. The brightness of each element indicated in thematrix is modulated according to the amplitude of thethrough-transmitted wave, where an indicated bright elementshows that the received through-transmitted wave has a largeamplitude. The matrix display can be used to determine thepositional relationship between the probe arrays and the spot-weld, as illustrated in Fig. 7. For simplification, the figure isdrawn with a total of 8 piezoelectric elements (64 paths intotal). The positions of the probe arrays are easily adjusted tothe spot-weld using the matrix display. Though the number ofindicated dark elements along the right-side-down diagonalin the matrix display shown in Fig. 7 corresponds to thenugget diameter, it is not suitable for the precise measure-
ment of the nugget diameter. The matrix display can be usedas an auxiliary function which shows the size of the nuggetroughly.
The measurement time per spot-weld is less than 3 s.
3.2 Test samplesThe spot-welded samples shown in Table 3 were used as
the test objects. The samples were prepared by stacking andspot-welding two steel sheets 0.7 to 2.0mm in thickness. Thenugget diameters were varied by adjusting the weld current.The samples 0.0 to 6.0mm in nugget diameter could be madeby the method described above.
Two samples were made under identical conditions foreach welding current. One (test sample) was used for theultrasonic measurement and the other (reference sample) wassectioned for the measurement of the nugget diameter(reference diameter). The measured nugget diameters areshown in the rightmost column of Table 3. Samples of 17 setsand 68 pieces were prepared in total.
3.3 Measuring results and discussionThe samples were measured on both surfaces along two
directions that intersect perpendicularly with each other.Accordingly, four measurements were made per sample. Thesame probe arrays as mentioned in 3.1 are used for all the
Ultrasound (Lamb wave)Steel sheet
(a) Measurement setup by using the developed probearrays
(b) Appearance of the developed probe array
Fig. 5 Schematic depiction of (a) measurement setup by using the
developed probe arrays; and (b) appearance of the developed probe arrays.
(a) Through-transmitted wave propagating along 31 paths
(b) Determination method by comparing wave amplitude with predetermined threshold value.
Fig. 6 Nugget diameter measurement by using (a) through-transmitted
wave propagating along 31 ultrasound paths and (b) determination method
by comparing wave amplitude with predetermined threshold value.
Ultrasonic Method for Testing Spot-Welds 543
measurements. Figure 8 shows the relationship between thenugget diameters DM measured using the abovementionedmethod based on attenuation measurement and the referencediameters DR determined by cross-sectional observationusing the samples shown in Table 3. As shown in Fig. 8, themeasured nugget diameters DM agreed with the referencenugget diameters DR to a precision of 0.5mm except forseveral samples. Overestimation tended to happen with thesamples that had no weld metal. The likely reasons for theoverestimation are as follows:(1) Coarse grains produced by high temperature during
welding attenuate the ultrasound.(2) It is possible that the actual nugget diameters of the
samples used in the nondestructive measurementmentioned above were different from the referencediameters to some degree. Exact estimation of theprecision is a future task.
Incidentally, as mentioned above, spot-welds with littleor no weld metal are harmful due to their poor strength andthey definitely must be rejected, so it is important not tooverestimate the nugget diameter. We therefore refined the
nugget diameter measurement method based on the attenu-ation measurement.
Figure 9 shows examples of B-scope representation usingthe signal received in the abovementioned 31 paths. Thehorizontal axis is the time and the vertical axis is theultrasound path shown in Fig. 6(a). As shown in Fig. 9, inthe sample with a reference diameter of zero, the timedifference between the through-transmitted wave along thecenter path and the through-transmitted wave along the sidepath is very short, because there is no weld metal to changethe ultrasound velocity. On the contrary, in the sample with alarge reference diameter, the time difference between thethrough-transmitted wave along the center path and thethrough-transmitted wave along the side path is long.Namely, the nugget diameter measured by attenuationmeasurement can be compensated by using the transit timemeasurement result. The path where the transit time of the
Receiving1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 T
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Fig. 7 Detection of misalignment using matrix display.
Reference nugget diameter, DR/mm
0 1 2 3 4 5 6 70
Fig. 8 Relationship between measured nugget diameter and reference
(a) No weld metal
(b) Large weld metal
Fig. 9 Comparison of B-scope between (a) a sample with no nugget and
(b) a sample with large nugget.
544 H. Takada, T. Ozeki, R. Ikeda and T. Hirose
through-transmitted wave is shorter than the predeterminedtime delay with respect to the reference time is judgedas having no weld metal, and then the nugget diameter isdetermined as zero.
Figure 10 shows the relationship between the nuggetdiameters DMC measured using the abovementioned methodwith the compensation based on the transit time measurementand the reference diameters DR. The spot-welds with no weldmetal are accurately distinguishable using the developedmethod.
It is expected that the nugget diameter can be measuredonly by using the transit time measurement. The orientationof the thickness-direction-oriented structure, however, ischangeable in some degree depending upon the spot-weldingcondition, so there is a possibility that the time delay arisenfrom the anisotropy of the weld metal changes dependingupon the spot-welding condition. Accordingly, the nuggetdiameter measurement by comparing the time delay with thepredetermined threshold value will not have sufficientaccuracy.
Referring again to Fig. 9, it was observed, as expected, thatthe transit time of the through-transmitted wave along thepath including weld metal is longer than that of the through-transmitted wave along the path including no weld metal.Thus, it is qualitatively confirmed that the velocity of theultrasound passing through the weld metal changes depend-ing upon the anisotropy of the weld metal.
For practical application, matching the coupled probearrays with curved surfaces is thought as a task to be cleared.This is the important future task.
We developed a method for nondestructively evaluatingspot-welds based on the measurement of ultrasound (Lambwaves) through-transmission, and drew the following con-clusions:
(1) The nugget diameter can be evaluated by measuringthe width of the zone where highly attenuated through-transmitted waves are observed.
(2) The spot-welds with no weld metal are accuratelydistinguishable by comparing the transit time of the through-
Table 3 Samples for nugget diameter measuring test.
Top side sheet Bottom side sheet Reference Nugget Diameter, DR/mm
tB/mmType A Type B Type C Type D
1 270 0.7 270 0.7 0.0 1.1 3.8 4.5
2 270 1.2 270 0.7 0.0 0.0 2.7 5.4
3 590 1.2 590 1.2 0.0 3.1 4.5 5.2
4 590 1.6 590 1.6 1.0 3.0 4.6 5.9
5 590 2.0 590 2.0 0.0 3.2 4.6 6.0
6 980 1.0 980 1.0 0.0 2.9 4.3 4.8
7 980 1.2 980 1.2 1.5 2.8 4.2 5.2
8 980 1.6 980 1.6 0.0 2.8 4.3 5.1
9 270 1.2 270 1.2 0.0 2.8 3.8 5.1
10 440 1.2 440 1.2 0.0 3.0 4.7 6.0
11 590 1.2 590 1.2 0.0 3.1 4.5 5.6
12 780 1.2 780 1.2 0.0 2.9 4.2 4.6
13 270 1.2 980 1.2 0.0 2.1 3.7 5.3
14 440 1.2 980 1.2 0.0 2.7 4.3 5.0
15 590 1.2 980 1.2 0.0 2.9 4.4 5.2
16 270 1.2 270 1.2 1.5 2.8 4.5 5.6
17 590 1.2 270 1.2 0.0 2.7 4.2 5.1
TST: Tensile Strength of top side sheet, TSB: Tensile Strength of bottom side sheet, tT: thickenss of top side sheet, tB: thickenss of bottom side sheet.
Reference nugget diameter, DR/mm
0 1 2 3 4 5 6 70
Fig. 10 Relationship between measured nugget diameter after compensa-
tion and reference nugget diameter.
Ultrasonic Method for Testing Spot-Welds 545
transmitted wave with the reference transit time of thethrough-transmitted wave that propagates outside the spot-weld.
(3) We developed a measuring system using a piezo-electric element array. The results of measurement by thissystem agreed well with the nugget diameters measured bycross-sectional observation.
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