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 2
Example We want to test a steel weld plate of 18 mm thickness according the ISO 17636-2 Class B standard with X-ray 3
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) 4
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 5
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 6
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
7.2 Choice of tube voltage and radiation source 2) We draw a line from the point of intersection to the left, to the KV Max. 260 KV 1) We have a thickness of 18 mm, so we go up to curve 2 (steel) WD 18 mm 8
Step 3 Detector systems and metal screens Signal-to-Noise Ratio normalized (SNRN) Inspection class is determined by the normalized Signal-to-Noise Ratio (SNR N ) SRN N takes into account the system Basic Spatial Resolution (SR B ) 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 9
7.3 Detector systems and metal screens The Minimum SNRN We need is :100? We are working with 260 KV under 50 mm thicknes for Class B 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 * HAZ = Heat Affected Zone 10
Step 4 - Source-to-object distance 11
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 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-todetector distance is the same as the plate thickness 18mm The Minimum source-to-object distance for this example for Class B = 105 mm 12
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 13
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 14
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 15
Annex B. Unsharpness For this plate of 18 mm. we go to Penetrated thickness 12 <w>40 to see which Basic Spatial Resolution (SR b ) we need. For our example we need a SR b better than 100 µm Dn refers to the wire pair that needs to be resolved, in this case D10 16
Step 7 Determine Basic Spatial Resolution» A reference image is required to measure the Basic Spatial resolution (SR b ) 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 highresolution systems 17
Unsharpness and Resolution» Unsharpness can be calculated from the Basic Spatial Resolution (SR b )» SR b 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 18
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 SR b of the system 19
Step 8 Choosing the Exposure Parameters» To recap the acceptance specifications: X-ray max voltage = 260 KV Minimum SNR N 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 SR b of 100 µm 20
But we always try to be better than the minimum, so:» X-ray max voltage = 260 KV» Minimum SNR N 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 SR b of 80 µm 21
We will use the DÜRR NDT HR-IP imaging plate, because it can give us 63μm SR B 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 22