slide serial no 1 © 2002 aea technology plc 3 rd european-american workshop on reliability of nde...

Slide serial no 1© 2002 AEA Technology plc

3rd European-American Workshop on Reliability of NDE and DeminingSeptember 10-13, 2002 in Berlin Paper 24 Wall and Burch

3rd European-American Workshop on Reliability of NDE and Demining, September 10-13, 2002, Berlin Germany

Computer Modelling and Simulation for Prediction of POD and Inspection Reliability Martin Wall and Steve Burch

UK National NDT Centre NNDTC

Paper 24 Thursday 12 September 2002



POD Trial – Conventional Approach

Why not model POD, PFI? - A Complementary approach

POD trial on airframes Sandia National Labs USA



Scope of Talk

Overview of POD modelling with Demonstration

• Background

• Basis of POD models

• Demonstration

• Where Models are useful

• Human reliability

• Conclusions



Modelling of POD is not new!• NDE Models have been used for many years as

part of the validation and qualification of NDE methods.

• ã v a approach [Berens] has been used for many years for evaluating POD in the US aerospace industry.

• Over the last 10 years increasingly sophisticated models have been developed for predicting POD notably at the UK National NDT Centre NNDTC and Iowa State University in the USA.

• NNDTC and BAM have separately developed signal/noise methods to predict POD. Simulation Models developed at CEA (Paper 25) could be suitable for adaptation.

• Current models run on standard PC's, produce simulated data and can be linked into CAD packages and probabilistic models.



POD Models developed at NNDTC

The UK National NDT Centre started looking in early 1980s at modelling POD. Models developed include:

• ESA/ESTEC Models Project 12228/96 Wall/Burch1997-2000

• Ultrasonic C-Scan• Radiography• Technique-Independent Model (image data)

• Ultrasonic TOFD Silk 1996 • Signal-Noise Models Wedgwood/Wall 1997

• Ultrasonic Pulse-echo Ogilvy 1989-92

• Radiography Windsor/ Wall 1990-95

• Eddy Current Holt 1995



Basis of a POD Model Calculation of single-point POD & PFI: No background signal



Basis - Single-point POD & PFI with background signal



Features of the POD Models

• Modular approach

• Model and simulator

• Give simulated NDE data and images

• POD curves and false calls (NFI)

• Choice of detection criteria

• Two independent methods of determining POD• Theoretical POD - From the Physical model• “ Spot the Ball” - POD trial using the Simulated data

• Run on a standard PC in real time



Modular Approach

Signal

Background, Noise

Detection Criteria

PODPFI

Use experimental or model-based inputs dependent on status of NDT technique



Detection Criteria – seek to reflect actual practice

• Single point POD & PFI • Probability that a single pixel exceeds the

reporting threshold ()• High false-call rates unless threshold set very

high

• Multi-point POD & PFI • Probability that a group of pixels exceed the

reporting threshold ()• Simulates automated inspection system• Reduced PFI without needing to set high

threshold

• Integral “POD” • approximation to visual detectability of a defect

(human inspector).

• “Spot the ball” - POD “trials” on the computer



Physical model overview

Inputs

Calculations

Component

details

Inspection

details

Modelling

Outputs Theoretical POD

Simulated images

“Spot the ball”



UT model: components

• User defines model region (or “space”)

• User builds up component from any number of rectangular regions added into this region

• Regions can be constant thickness or “wedges” in horizontal or vertical directions

• User shown image of component as it is built-up



UT model: basis (simplified)

• Setup model region & 2-D “map” of attenuation

• Add defects to attenuation image

• Derive transmitted amplitude “map”

• Convolve to represent ultrasonic beamwidth

• Add “white” noise

• Allow for data recording system dynamic range

• Calculate “theoretical” POD & PFI values

• Display modelled image on screen: available for “spot the ball” POD & PFI assessment



UT models: defects

• Through-wall location ignored

• Defects can be rectangular or elliptical.

• Defects can have any opacity between 0 & 1.

• Can model cracks, delaminations, single pores, inclusions & regions of excess resin.

• Added randomly to specified rectangular region of model. No overlaps!



POD Ultrasonic C-Scan DEMO

DEMO



Definition of Model Space - Example Windows Menu



POD Ultrasonic C-Scan Example 1

DEMO



POD Ultrasonic C-Scan Example 2



Simulated POD Trial - “Spot the Ball” POD

• Simulated images brought up on screen

• Can have zero to any number of defects. Range of defects sizes or types. Can constrain location for defects

• Operator clicks on cursor where they thinks there is a defect

• Data automatically analysed for POD and False calls

• By repeating can build up statistics. Analyse by grouping or ‘Hit-Miss’ method

• Compare with Theoretical POD from physical model. Gives second independent measure of POD



UT model: Example of output after “spot the ball” POD trial



X-ray model: Example of output after “spot the ball” POD trial –

Model POD to right



ESA RTM Component and Simulated C-Scan Image



X-ray model: basis

• Similar in concept to UT model: calculates modelled 2-D image (film radiograph) + POD & PFI values.

• Physical modelling of following processes in 2-D:

• Poly-chromatic beam of X-rays• Energy dependent absorption of X-rays in material• Scattering of X-rays within the component• Absorption of X-rays in the film• Unsharpness/spatial resolution due X-ray source size/film

characteristics.



Validation

• Comparison with experimental POD trials

• Comparison of NDE data and simulated images with experiment.

• By examining trends.

• Theoretical POD v “Spot the Ball” POD

Is the model consistent with the experimental results?



POD validation UT: Multi-point POD values

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7 8 9 10

Defect diameter (mm)

Mu

lti-

po

int

(2x

2)

PO

D

Multi-point POD (expt)

Multi-point POD (model)



Experimental v Model UT C-scan

Experiment Model



Model validation: UT signal amplitude

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16

Defect diameter (mm)

% d

rop

in

ult

ras

on

ic s

ign

al

Experiment (5MHz probe)

Model



Modelled radiograph of RTM component



Experimental radiograph of RTM component with added Al discs



POD Model Validation - Measured & predicted density differences

0

0.02

0.04

0.06

0.08

0.1

0.12

10 15 20 25 30 35 40 45

X-ray kVp

Def

ect

den

sity

dif

fere

nce

Measured

Modelled



Technique independent model overview

Calculations

Outputs

Inputs Expt. image

Identify defect

Statistical Model

POD & PFI

Interactive analysis



Technique independent model: example of application

Referencearea

Defectarea



Where is a POD Model useful?

• Application at Design Stage

• Assessment of Historical data

• Extension of existing data

• Parametric or ‘what-if’ calculations

• Optimisation of inspection

• Planning or in parallel with experimental trials. Alternative to trial?

• Validation and Qualification

• False call data (PFI, NFI)

• Simulated POD trials

• Optimisation of inspection and detection criteria

• Input to integrity assessments and economic models (IVM) Quick and cost effective



Current POD Model applications

• Fabrication Hydrogen cracking LPG Vessels

• Offshore Composite Repair Group (JIP)

• Railway line inspection

• Airframe inspection

• UT inspection offshore jacket structures

• Modelling radiography of steel weld containing rogue weld metal

• Models to be available via web internet – DTI MMS15 Interactive Knowledge Base (IKB) Base on NDE of composites.www.aeat.co.uk/ndt/mms15



Future Developments/ Exploitation

• Consultancy and model application

• Link to other design packages or CAD programmes.

• Extension to other NDE Methods and structures

• 3D implementation and visualisation

• Improved defect recognition modelling e.g. Neural Networks for automated defect detection (as NNXPOSE).



Conclusions

• Benefits in a modular approach to POD Modelling

• Own applications and advantages - quick, low cost, statistics, parametric studies, false calls, design...

• Modelling and simulation are complimentary to experimental measurements of reliability.

• Simulated trials in “ Spot the Ball” POD estimates offer a good halfway house between model and experiment.

• Deserves a higher profile as a compliment or alternative to experimental trials.



Human Reliability

• Empirical rules developed. Wall and Wedgwood ECNDT7

• Utilise experimental data like PISC III curves. Human error worse for small defects.

• Distinguish hand-to-eye coordination and interpretation of images

• PODActual = PODIntrinsic * HR Human Reliability Correction factor HR = H * ENV

• Normalise by threshold or 50% POD

Consistent with European/US model



NDE Simulation PISC III Action 7PCSimone ultrasonic simulator - Inspection Validation Centre (IVC)

AEAT Risley



Human Reliability POD PISCIII



Human Reliability - Long term operator fatigue - ultrasonic inspection 45o Probe PISC III



Other POD Model EXAMPLES



Example - Parametric VariationsDefect Tilt



Example - POD estimates for Historical Data Radiography 200mm thick forging

X-Ray Radiography 1970’s Thick Section pressure vessel forging



Example -Voids in steel plate XPOSE



Example - Modelled weld containing rogue metal

ESA models used to look at probability of detecting rogue metal in steel weld by radiography -Nuclear application



Complementary approaches

• Inspection Qualification

• Inspection Validation

• Performance demonstration

• Inspection Value Method (IVM)

• Inspection Simulators (e.g PISC III Action 7)

slide serial no 1 © 2002 aea technology plc 3 rd european-american workshop on reliability of nde...

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