dÜrr ndt cr workshop iso 17636-2 weld test with x …œrr ndt cr workshop iso 17636-2 weld test...

Recommended Techniques

» There are many possible techniques for making good radiographs

using CR

» Following a defined procedure will simplify technique

development and shorten the time to a good image

» As an example, we will step through the process using

ISO 17636-2 standard

DÜRR NDT GmbH & Co. KG 2 September 6, 2017

Example

We want to test a steel weld plate of 18 mm thickness according the ISO 17636-2 Class B standard with X-ray


Acceptance specification according ISO 17636-2

» 7 Recommended techniques for making digital radiographs » NOTE unless otherwise explained, definitions of the symbols used in Figures 1 to 21 can be found in Clause 4

» 7.2 Choice of tube voltage and radiation source ( max KV)

» 7.3 Detector systems and metal screens ( min SNRN)

» 7.6 Source-to-object distance ( FFD)

» Annex B. Minimum image quality values ( IQI)

» B 4 Unsharpness ( SRb / BSR)


Step 1 – Physical Technique

» There are many different radiographic techniques:

• Single and double wall exposures

• Elliptical exposures

• Perpendicular technique

• Techniques for flat or curved samples


7 Recommended techniques for making digital radiographs

In the case of a flat plate, the recommended technique is perpendicular, with the radiation source in front of the object and the imaging plate on the opposite side


Step 2 – Maximum X-ray Voltage

» There are recommended maximum X-ray voltages for different materials and thicknesses

» Using the following table, we can determine the maximum X-ray KV

» For Imaging Plates (IP’s) with high structure noise (coarse grained) a reduction in Kv of about 20% is recommended

» Fine-grained IP’s can use the highest KV


7.2 Choice of tube voltage and radiation source

Max. 260 KV

WD 18 mm

1) We have a thickness of 18 mm, so we go up to curve 2 (steel)

2) We draw a line from the point of intersection to the left, to the KV


Signal-to-Noise Ratio normalized (SNRN)

Inspection class is determined by the normalized Signal-to-Noise Ratio (SNRN)

• SRNN takes into account the system Basic Spatial Resolution (SRB)

• Class A inspections are lower quality

• Class B inspections demand higher SNR and have more stringent image quality requirements

• SNR measurements must be made using specific techniques and have defined limits

Metal screens

• If metal screens are used, insure that there is intimate contact between the Imaging Plate (IP) and the metal screen.

• IP‘s are very sentitive to low energy backscatter, which must be controlled

• It is recommended that a Fe or Cu screen be used directly behind the IP, between the IP and any back lead screens

Step 3 – Detector systems and metal screens


7.3 Detector systems and metal screens

The Minimum SNRN

We need is :100 ?

If you measure the SNRN in the HAZ*, you must Multiply by 1.4

100 SNRN X 1.4 = 140 SNRN

We need a minimum

SNRN of 140

We are working with 260 KV under 50 mm thicknes for Class B

* HAZ = Heat Affected Zone


Step 4 - Source-to-object distance


7.6 Source-to-object distance

1) Focal spot size of this tube is d = 1 mm

2) The object-to-detector distance b for this test arrangement (from Figure 1) is the same as the plate thickness (WD =18mm), so b = 18 mm

The Minimum source-to-object distance for this example for Class B = 105 mm

• Source-to-object distance can be most simply determined by using the nomogram included in the inspection standard

• For our example we will assume:

– The focal spot of the X-ray tube is 1mm

– The plate is in direct contact with the imaging plate, thus the object-to-detector distance is the same as the plate thickness – 18mm


Step 5 – Image Quality Measurements

» Image Quality Indicators (IQI’S)

• Decide on the type of IQI that will be used

− Wire IQI

− Step and hole IQI

• Decide whether the IQI is to be placed on the source side or the detector side of the object

• Determine the correct IQI for the Inspection Class and the object thickness from the table in the relevant standard


Annex B. Minimum image quality values ( IQI)

This table is for wire IQI’s placed on the source side of the object when using the single-wall technique

Since our plate is 18mm thick, we need to see wire W13


Step 6 – Unsharpness

» Image unsharpness is a measure of the loss of definition caused by geometric factors

» It is caused because the radiation emitter is not a true point source

» The factors affecting unsharpness are source focal spot size, source-to-object distance and object-to-detector distance


Annex B. Unsharpness For this plate of 18 mm. we go to “Penetrated thickness” 12 <w>40 to see which Basic Spatial Resolution (SRb) we need. For our example we need a SRb better than 100 µm Dn refers to the wire pair that needs to be resolved, in this case D10


Step 7 – Determine Basic Spatial Resolution

» A reference image is required to measure the Basic Spatial resolution (SRb)

• The inspection target is not used - the penetrameter shall be placed directly on the IP

• The source-to-detector distance shall be 100cm ± 5cm

• The mean gray level in the image shall be >50% of the system maximum

• The measured SNR in the image shall be >70 for high-resolution systems


Unsharpness and Resolution

» Unsharpness can be calculated from the Basic Spatial Resolution (SRb)

» SRb can be measured using the EN 462-5 duplex wire penetrameter according to ISO 19232-5

» The Dn in the previous table refers to the duplex wire pair on the IQI


Basic Spatial Resolution Measurement » Basic spatial resolution is measured by evaluating the grey level dip in a line

profile drawn across the duplex IQI wire pairs

» A dip of >20% in grey level indicates that the wire pair is resolved

» This determines the SRb of the system


Step 8 – Choosing the Exposure Parameters

» To recap the acceptance specifications:

• X-ray max voltage = 260 KV

• Minimum SNRN must be 140 measured in the heat affected zone

• No lead screens are necessary

• FFD should be 105mm minimum

• We must see IQI wire 13

• We need a SRb of 100 µm


But we always try to be better than the minimum, so: » X-ray max voltage = 260 KV » Minimum SNRN must be 140 measured in the heat affected zone » No lead screens are necessary » FFD will be 500mm (this may affect the shot time) » We must see IQI wire 13 » We will aim for a SRb of 80 µm


• We will use the DÜRR NDT HR-IP imaging plate, because it can give us 63μm SRB

• We will use the DÜRR NDT HD-CR 35 scanner with the 12<W<40 scan mode, as follows:

– Scanning resolution 40μm – Laser spot size 50μm – Laser power 6 – PMT HV 620V – 3.000 RPM


dÜrr ndt cr workshop iso 17636-2 weld test with x …œrr ndt cr workshop iso 17636-2 weld test...

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