|NDT.net - March 2003, Vol. 8 No.3|
In everyday ultrasonic testing applications the calibration and reference blocks are used. Furthermore all obtained testing results are given with reference to the particular characteristic of these blocks. Beside worldwide known codes, rules and technical specifications the International and European standards define requirements for calibration blocks for ultrasonic testing.
Within this paper the state of the art is presented and the comments are given regarding to the quality requirements given by standards. Acoustical, geometrical and surface characteristics of standard ultrasonic testing calibration blocks are discussed considering their impact on the consistency of ultrasonic testing results. Analysis is based on the results measured on the calibration blocks taken from industry.
To provide range setting and sensitivity setting for ultrasonic testing applications European standards (EN) designate EN 12223 (1) and EN 27963 (2). These two standards specify calibration blocks No. 1 and No. 2. and are identical to International standards ISO 2400 (3) and ISO 7963 (4) respectively. Both ISO standards had been prepared by Technical Committee TC 44, Welding and allied processes, while CEN Technical Committee TC 138, Non-destructive testing, accepted both ISO as European standards. Calibration blocks No.1 and No.2 are commonly designated V1 and V2.
This paper is going to discuss dimensional, acoustical and surface requirements given by these two standards. While taking into consideration practical features, main goal is to ensure traceability, comparability and consistency of ultrasonic testing results using these calibration blocks.
Specification for calibration block No.2, ISO 7963 (4) was issued in 1985 what was 13 years after issue of specification for calibration block No.1, ISO 2400 (3). Anyhow, requirements given by ISO 7963 (and identical EN 27963) are not so comprehensive as those given by ISO 2400 (and identical EN 12223). There are also several articles in ISO 7963 which are spurious and ambiguous from the technical point of view, here quoted: "block is very much smaller" ; "geometry is much simpler" ; "Ra is not very different from Rz " ; "fine structure" ; "good homogeneity".
Here is the comparison of requirements for two main characteristics of calibration blocks:
Both standards for calibration blocks V1 and V2 refers to ISO 468 (5), an International standard which specifies surface roughness parameters, but for block V2 in ISO 7963 there is a serious mistake when put in writing that "Ra is not very different from Rz". Beside that, specified surface roughness of the calibration block V2 is not declared clearly and undoubtedly. Only values for surface quality are given on the Figures in ISO 7963, without precise specification which parameter of surface roughness it should be. Contrary, in ISO 2400 (EN 12223) it is clearly stated that the faces and edges shall be machined to an Ra value not greater than 0,8mm.
To ensure between V1 and V2 regular microstructure which mostly impacts amplitude responses, both standards indicate detailed procedure of the heat treatment of the block. While we have faced enough subjects to discuss, which are related to dimensional, surface and velocity requirements, this paper will not pay attention on the variations of amplitude responses and transfer loses. This topic will stay on for future investigations and measurements.
ISO 2400 (EN 12223) in Annexes precisely describes procedures for attenuation and velocity measurements. There is no doubt about set up for both measurements, even accuracy for velocity measurement is called for: 0,1 %, what means uncertainty of ± 3 m/s for transverse and ± 6 m/s for compressional waves. This imposes tolerances, in other words uncertainty for dimensional and time of flight (TOF) measurements:
Attenuation coefficient for transverse waves shall not exceed 0,05 dB/mm @ 4 MHz. All measurements of acoustical parameters were carried out according to the given requirements.
Surface roughness and topography were measured with stylus instrument Perthometer S8P. Dimensions were measured by means of CMM. For TOF measurements digital oscilloscope was used with sampling rate 100 MSamples/second.
Despite clear and comprehensive instructions how to measure this parameters, we have encountered one irrationality. As it is stated in ISO 2400 (EN 12223), at the areas where velocity is going to be measured the variations in thickness over the area of the probe shoe (contact region) must not be greater than 0,01 mm. To get satisfactory TOF measurements it is quite clear that variation in thickness over the probe contact area has to be uniform and therefore limited to a certain value. But this requirement could be interpreted also in another way: just to avoid causing systematic error in TOF measurements, the maximum gap in contact region between probe and object has to be limited. Therefore, the maximum variation of 0,01 mm in surface topography is allowed at the contact region. Contact region is probe shoe area that is 10 mm to 15 mm in diameter as given in standard.
To get an impression about real situation in everyday use of calibration blocks V1 and V2, we have taken 3 of each from industry, from different companies.
Calculated values for velocities of transverse and compressional waves are within required tolerances. First step was to perform check of the variations in thickness over the probe shoe area where the TOF measurements were taken. In general results of this check were not greater than 0,01 mm, what matches the requirement given by standard, but the analysis of surface topography has shown that certain areas exist where this requirement is not fulfilled.
Figures 1 and 2 present topography of calibration block V2 marked #2. Analysing distribution of lines, one can indicate areas not greater than 15, or even 10 mm in diameter (like area indicated with circle on the Figure 1), where shape (relief) differences in topography approach even 0,016 mm.
|Fig 1: Topography of calibration block No.2.|
|Fig 2: Isometric topography of calibration block No.2, #2|
Same result was found on two of three calibration blocks V1. Block #4 has variations up to 0,02 mm while probe is put on block thickness 25 mm, and block #6 has variations up to 0,03 mm while probe is on thickness 100 mm. Surface topographies of calibration block V1 #6 are shown on figures 3 and 4.
|Fig 3: Topography of calibration block No.1, #6.|
|Fig 4: Isometric topography of calibration block No.1, #6.|
Figure 5 presents shape of the contact surface of the V1 block #4 at the thickness of 100 mm,. This block satisfies required tolerances for parallelness and dimension while the same surface for itself is convex. The range of variations all over the surface is 0,14 mm. Therefore standard must also specify requirement for surface flatness.
|Fig 5: Isometric topography of calibration block No.1, #4.|
The greatest deviation from the parallelness, reaching almost 0,28 mm is shown on the Figures 6 and 7. Among six examined calibration blocks, only one of them, V1 calibration block #5 (shown on Figures 6 and 7) was outside dimensional tolerances. Results of dimensional measurements are given in Table 1. Cells in table with values out of tolerances are indicated.
|Fig 6: Topography of calibration block No.1, #5.|
|Fig 7: Isometric topography of calibration block No.1, #5.|
|calibration block||mark #||Zmin||Zmax||arithmetic mean Z|
|#1 bottom side||12,4231||12,5302||12,4835|
|#2 top side||12,5034||12,5411||12,5170|
|#2 bottom side||12,4990||12,5758||12,5330|
|#3 top side||12,4714||12,4825||12,4760|
|#3 bottom side||12,4715||12,52177||12,4907|
|#4 top side||25,0446||25,0677||25,0533|
|#4 bottom side||25,0619||25,0837||25,0749|
|#5 top side||25,1072||25,4780||25,3189|
|#5 bottom side||25,0794||25,3404||25,2158|
|#6 top side||25,9898||25,0195||25,0033|
|#6 bottom side||25,9658||25,0305||25,0097|
|Table 1: Surface thickness values for calibration blocks|
Measured surface roughness parameters are given in Tables 2 and 3. Analysing calibration blocks V2, values of Ra which are out of the required tolerance we have found only on the radii reflection surfaces (R20 and R50). It was surprising for us that this was found on the very new calibration block wich has certificate of conformance according to EN 27963.
Situation of surface roughness was worser with calibration blocks V1. Two of three calibration blocks (#5 and #6) completely do not satisfy requirement Ra < 0,8 mm. Third one as in the case of block V2 do not satisfy only on the radii reflection surface R100.
|mark#||parameter||top surface t=12,5mm||bottom surface t=12,5mm||scale surface||R25 surface||R50 surface|
|Table 2: Surface roughness values for calibration block No.2.|
|mark#||parameter|| top surface|
|scale surface||R100 surface|
|Table 3: Surface roughness values for calibration block No.1|
|© NDT.net - email@example.com|||Top||