investigating the implementation of usaf damage tolerant risk analysis with a structural health...

Investigating The Implementation Of USAF Damage Tolerant Risk Analysis

With A Structural Health Monitoring System

Jeff Tippey,MS Electrical Engineering, UTA Department of Electrical Engineering

Thesis BoardDr. George V. Kondraske, Professor of Electrical Engineering, UTADr. Dale L Ball, Materials Science, UTADr. Venkat Devarajan, Professor of Electrical Engineering, UTADr. Qilian Liang, Associate Professor Of Electrical Engineering, UTA

Road MapIntroductionPrimary ObjectiveWhat Are We Trying To Improve?Specific ObjectivesThe ProcessDefining Metrics: Probability Of Detection (POD) And Probability Of Failure (POF)Elastic-Wave Based Structural Health Monitoring (SHM) SystemsSMART Layer® Piezoelectric SHM SystemEstimating POD for SMART Layer® SHM SystemEddy-Current SHM SystemEstimating POD Of Eddy-Current SHM SystemUSAF Structural Risk ModelUSAF inspection And Repair ModelEvaluation Study Of USAF Structural Risk Methodology Using SHM Inspection SystemConclusion

Aircraft fleet maintenance is resource intensiveUltimately a trade off between:

Why is this important?1978 study, National Bureau of Standards and Battelle

Laboratories [1,2] Estimated total cost associated with material fracture/failure

in the US to be over $88 Billion Dollars (approximately 4% of US GDP in 1978) To put into perspective, 2010 US GDP was approximately 14.72 Trillion

US Dollars, 4% of GDP = $589 Billion [3]

Study concluded substantial savings could be achieved by a better understanding of a structure’s reliability over its lifetime

Introduction

Cost&

Resources

Safety

Introduction To SHM

Structural Health Monitoring (SHM) uses non-destructive inspection sensor systems to quantify the health of structures [1]SHM has the potential to [1] Decrease lifecycle costs of aircraft Decrease operation risks Improve aircraft fleet health and readiness

Two most commonly used SHM systems Piezoelectric (elastic-wave based) SHM

systems Eddy-current SHM systems

Introduction ToUSAF Structural Risk

Methodology

USAF became first organization to require damage tolerant design based on fracture mechanics [1]Methodology used to evaluate airframe structure safety [4]MIL-A-83444 specifies aircraft frames must be designed assuming cracks are present at all critical locations in a structure [1,5]The cracks affect the strength of the structure and grow over time, ultimately leading to failureUSAF Structural Risk Methodology uses probabilistic models of structure’s material properties and cyclic loading to quantify the risk of failure of an aircraftCurrently, frequent visual and periodic magnetic-optical imaging (MOI) inspections are used to keep risk at acceptable levelsThe cost associated with these inspections is high

To investigate the combination of the USAF Structural Risk Methodology with SHM systems for a risk analysis of a cargo aircraft.

Primary Objective

Old Process:

New Process:

Primary Objective (cont’d)

What Are We Trying To Improve?

What improvements are we looking for? Cost - visual inspections and MOI inspections are

manpower intensive and high cost Reliability – visual inspection quality is hard to predict

[6] Aircraft Readiness – MOI are extremely accurate but

require long aircraft downtime

What could the proposed system provide? Cost – High up front cost for SHM system but would

reduce the resources needed for future maintenance Reliability – A well designed commercially available SHM

system could be more reliable Aircraft Readiness – Inspection does not require

substantial amount of aircraft time

Primary Objective: To investigate the combination of the USAF Structural Risk Methodology with SHM systems for a risk analysis of a cargo aircraft.

Supporting Objectives1. Define probability of detection (POD) and probability of

failure (POF)2. To investigate using commercially available SHM

systems using different NDI techniques (piezoelectric and eddy-current SHM system)1. Discuss the function of a piezoelectric and an eddy-current SHM

systems2. Calculate/estimate POD for a piezoelectric and eddy-current SHM

systems

3. Discuss USAF Structural Risk Methodology4. Perform risk analysis of cargo plane using piezoelectric

and eddy-current inspection5. Determine single-flight probability of failure (SFPOF) for

the airframes lifetime for each system

Specific Objectives

Process

Defining Metrics: Probability Of Detection (POD)Probability of Detection (POD) is the probability that the SHM system will detect a crack of size, a, on the structurePOD is a probability and is always <= 1

0.00 0.50 1.00 1.50 2.00 2.50 3.000.0

0.1

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POD

Crack Size (inches)

PO

D

Defining Metrics: Probability Of Failure (POF)

Fracture is defined as when the structure can no longer support the load it is intended to support.

Fracture occurs when applied stress produces a stress intensity factor greater than the fracture toughness of the material [7-9]

Probability of Failure (POF) is the probability that fracture of the structure occurs during the aircrafts operation

This can also be viewed as the probability that the maximum stress in a flight will exceed the critical stress of the material

A function of material properties and mission loading properties of the aircraft in flight

Elastic-Wave Based Structural Health Monitoring (SHM)

SystemsElastic-wave SHM systems, also called piezoelectric SHM systems, are one of the most commonly used SHM systems [10] Uses propagation of elastic waves through a material and compares to a baseline to detect damage presentPiezoelectric sensors measure the elastic waves through the piezoelectric effect where mechanical vibrations cause change in electrical polarization of the material, resulting in a voltage change

Piezoelectric Sensor System [11]

Have two modes of operation [12] Passive Mode Active Mode

Elastic-Wave Based Structural Health Monitoring (SHM) Systems

(cont’d)

A commercially available system by Accelent embeds piezoelectric sensors on a thin layer of material called the SMART Layer® [13]The SMART Layer® manufacturing process utilizes printed circuit techniques to connect large number of sensors in the layer without requiring individual connections to be made manually [13]Provides a wider area of structural coverage than systems where each piezoelectric element is installed individually [13]

SMART Layer® Piezoelectric SHM System

[14]

Piezoelectric Sensors

SMART Layer® Piezoelectric SHM System

(cont’d)SMART Layer® system has two active piezoelectric SHM analysis methods

Direct Path Imaging - determines the total amount of energy in each sensor path and creates a 2-D mapping of the structure.

Reflection Based Analysis - utilizes the wave velocities of the S0 and A0 Lamb wave modes in each sensor/actuator path to extract the reflectance of each signal and generate a resulting diagnostics image

Direct Path Imaging – Better at distinguishing several cracks in close proximityReflection Based Analysis – Better representation for single cracks

Estimating POD for SMART Layer® SHM System

Currently, there is no standard way to calculate POD for integrated piezoelectric SHM system installed on metallic sheets

Metallic sheets frequently crack linearly, whereas a composite would form voids in the material

Difficult to accurately predict linear cracks effect on elastic waves

Instead, we performed an experiment on a representative model of our system and used Hit/Miss Analysis to determine PODCut out piece of cargo plane


(cont’d)

SMART Layer® Test Setup

Cargo Plane With Test Pieces Shaded

Test SetupSensor Paths

Used Dremel tool to artificially create 1inch linear cracks on the test article and then used SHM system to inspect structure


(cont’d)


(cont’d)

Baseline Measurement

Direct Path image for crack 1 RBA image for crack 1

Initial Direct Path image for crack 2 Direct Path image of crack 2 with increased sensitivity

. RBA analysis image of the crack 2


(cont’d)

Crack NumberDirect Path

( Y=Detected N=Not Detected)

RBA( Y=Detected N=Not

Detected)

1 Y Y

2 N Y

3 Y Y

4 Y Y

5 Y Y

6 Y Y

7 Y Y

8 Y Y

9 N Y

10 N Y

11 Y Y

12 Y Y

Took data from experiment and performed Hit/Miss Analysis using DOD Handbook [15]Tried to stay conservative due to small sample sizeUsed Standard Deviation of 1.0 inch Minimum detectable flaw size 0.07 inch 50% detection size of 0.95 inch


(cont’d)

,


(cont’d)

0.00 0.50 1.00 1.50 2.00 2.50 3.000.0

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1.0

Piezoelectric SHM System POD

Crack Size (inches)

PO

D

Use surface mounted periodic field eddy-current meandering winding magnetometer (MWM) arrays to detect cracks and monitor crack growth in real-time [16]Allows for inspection of surface cracks and subsurface cracks [17]

Eddy Current SHM System

Wanted to determine through experiment, but were unable due to limited resourcesDecided to use POD data from [18,19]

Standard Deviation of 0.04 in Minimum Detectable Flaw Size of 0.02 in 50% Detection Size of 0.06 in

Calculated POD curve using method in [15]

Estimating of POD for Eddy-Current System

Estimating of POD for Eddy-Current System (cont’d)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.200.0

0.1

0.2

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Estimated POD Of Eddy-Current System

Crack Size (inches)

PO

D

United States Air Force (USAF) Structural Risk

Model

USAF ensures structural integrity of its fleet based on a structural risk model where cracks initially assumed on the structure are grown over time/usage [7, 9, 20]Structural risk analysis is a fleet management tool that is used for decisions regarding the timing and extent of inspection, repair, or replacement maintenance on structures [7,9, 20]In a fatigue environment, the strength and stress relationship is dynamic because the strength of the structure degrades as defects grow on it [7,9]The model includes probabilistic models of material properties, operational loads [7,9]

Fracture of a structure occurs when an applied stress creates a stress intensity factor that exceeds fracture toughness of the material (7, 9, 20)

USAF Structural Risk Model (cont’d)

= stress intensity= maximum normal stress allowed in the critical plane

a = the crack size= fracture toughness of the material= A geometry dependent factor

Probability of failure (POF) can then be defined as the probability that the max stress encountered during flight is greater than the maximum allowable stress [7,9, 20]


In the case of POF as a function of the growth of a single crack the relationship can be expressed as

This is the defining relationship between stress intensity factor, stress, and crack size [7,9]

In order to calculate POF, probabilistic models of crack size, fracture toughness, and stress exceedance are used [7,9]

The POF can then be expressed as [7,9]


= crack size probability density function= fracture toughness probability density function = the exceedance probability density function= the critical stress for the given crack size and fracture

toughness

Fracture toughness modeled as normal probability density functionMaximum stress during flight is modeled as Gumbel Type 1

In the case of calculating POF for a given crack size, it can be expressed as [7,9]


This is the analytical solution however this is vulnerable to errors because f(a) cannot be determined explicitlyTo get around this, a change of variable is used such that F(a) is used rather than f(a)Then we take advantage of the fact that for any uniform random variable u on the interval (0,1) x = F-1 (u) has the distribution function F(x)

The POF can be expressed in a simplified manner as [7,9]

POF(a) can then be expressed as [7,9]


Currently we are calculating the POF as a function of crack size but we want single flight probability of failure (SFPOF) [7,9]

There are two cases to solve for a < ac

a > ac

Interpolated Region

Extrapolated Region

Extrapolated probability that crack exceeds critical size


Case 1: a < ac

Interpolated Region

Interpolated Probability that the crack is larger than the critical

crack size

Case 2: a > ac

USAF Structural Risk Methodology uses a probabilistic model to account for inspection and repair of a structure

For example, during an inspection several cracks were found and repaired

However, a crack is only found if the inspection system can resolve it

The quality of the repair is accounted for by a probabilistic model of the initial crack size for the subsequent calculationsThe entire process is broken down into two components

The inspection process The repair process

USAF Inspection And Repair Model (cont’d)

The inspection process can be expressed as


P = The percentage of cracks that is found during the inspection

POD(a) = The inspection systems probability of detecting a crack of size a

f(a) = crack size density function before maintenanceIn our case, POD(a) would be the POD that was calculated for the piezoelectric SHM system and the eddy-current SHM system

The quality of the repair is expressed as an equivalent repair crack size distribution fR(a)

Define fbefore and fafter as representing the crack density function of the fleet before and after maintenance, then


Evaluation Study Of USAF Structural Risk Methodology

Using SHM Inspection System

For the study, a log-normal initial flaw size distribution was used with a mean, µ, equal to 3.616*10-1 and a standard deviation of 0.7227

0.0E+00 2.0E-01 4.0E-01 6.0E-01 8.0E-01 1.0E+00 1.2E+00 1.4E+00 1.6E+00 1.8E+00 2.0E+000.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Initial Flaw Size Distribution

Initial Crack Size (inches)

Cum

ula

tive P

robabilit

y D

istr

ibuti

on

The fracture toughness of the material was modeled as a normal distribution with mean of 62 ksi*in1/2 and a standard deviation of 6.2 ksi*in1/2



Piezoelectric SHM System



The system is able to keep risk below an acceptable levelThe inspection period can be increased to yearly and the MOI inspection could be eliminated

Eddy-Current SHM System



The system is able to keep risk below an acceptable levelThe inspection period can be increased to > 1yr and MOI could be eliminatedTheoretically it would work, however the eddy current system would be extremely expensive to cover the entire plane

The current USAF maintenance and inspection plan for cargo aircraft includes frequent visual inspections and periodic MOI inspectionsThese inspections take time and require a lot of resourcesTo address this issue, we have proposed using a structural health monitoring system to perform the inspection of the structureIn order to test this idea, we estimated the POD for two common SHM systems, a piezoelectric SHM system and an eddy-current system

Piezoelectric SHM system – We performed an experiment on a model of the system

Eddy-Current – We used published POD data

We performed a USAF Structural Risk Analysis using the SHM systems for the inspections

Conclusion

From the results, it is evident that using an SHM system with the USAF Structural Risk Analysis is a viable solutionWe were able to keep risk below an acceptable level for both systems, while increasing the inspections intervals, eliminating the need for visual inspections, and eliminating the need for MOI inspections

Conclusion

Opportunities For Future Work

Conclusion

1. Optimizing the design by developing realistic cost functions for maintenance and inspection processes

2. Integrate the aircraft and the SHM system together so that the inspection process can be completely automated

3. Calculation of POD for an integrated SHM system – there was no standard method/process to calculate this. Current systems rely heavily on laboratory testing, however the ability to calculate or approximate would be useful during design

4. Using multiple SHM techniques on the system to get wide coverage area using piezoelectric and high-resolution “hot spots” with eddy current system

[1] Shantz, C. (2010). Uncertainty Quantification In Crack Growth Modeling Under Multi-Axial Variable

Amplitude Loading. Vanderbilt University.

[2] R.P. Reed, J. S. (n.d.). The economic effects of fracture in the United States.

[3] CIA Website. https://www.cia.gov/library/publications/the-world-factbook/geos/us.html, accessed on

March 30, 2011.

[4] J. Gallagher, C. Babish, J. Malas. “Damage Tolerant Risk Analysis Techniques for Evaluating the Structural

Integrity of Aircraft Structures”. 11th International Conference On Fracture, March 20-25, 2005.

[5] A.F. Grandt, J. (2004). Fundamentals of structural integrity: damage tolerant design and nondestructive

evaluation. John Wiley & Sons, Inc.

[6] A. Coppe, R. H. (2008). A Statistical Model For Estimating Probability Of Crack Detection. IEEE

[7] P. Hovey, A. B. (1998). Update Of The Probability Of Fracture (PROF) Computer Program For Aging

Aircraft Risk Analysis. Air Force Material Command.

[8] P. Hovey, J. G. (1983). Estimating The Statistical Properties Of Crack Growth For Small Cracks. 18, 285

-294.

References

https://www.cia.gov/library/publications/the-world-factbook/geos/us.html

References[9] Berens, A. (1996). Applications Of Risk Analysis To Aging Military Aircraft. 41, 99-

107.

[10] S. Bo-Lin, S. B.-F. (2008). New Sensor Technologies In Aircraft Structural Health

Monitoring. Conditional Monitoring And Diagnosis. Beijing, China.

[11] Omegadyne Inc. http://www.omega.nl/prodinfo/loadcells.html, accessed on March

26, 1011

[12] S. Beard, C. L. (2007). Design Of A Robust SHM System For Composite Structures.

Industrial and Commercial Applications of Smart Structures Technologies , 6527, 1-

9.

[13] S. Bearda, A. K. (2005). Practical Issues In Real-World Implementation Of Structural

Health Monitoring Systems. Smart Structures and Materials 2005: Industrial and

Commercial Applications of Smart Technologies , 5762, 196-203.

[14] Composites World Website,

http://www.compositesworld.com/articles/structural-health-monitoring-composites-g

et-smart

, accessed March 30, 2011.

[15] Department of Defense. (2009). Nondestructive Evaluation System Reliability

Assessment. United States of America.

[16] N. Goldfine, D. S. (2002). Surface Mounted And Scanning Periodic Field Eddy-Current

Sensors For Structural Health Monitoring. Aerospace Conference Proceedings (pp. 6-

3141- 6-3152). IEEE.

http://www.omega.nl/prodinfo/loadcells.html

http://www.compositesworld.com/articles/structural-health-monitoring-composites-get-smart

http://www.compositesworld.com/articles/structural-health-monitoring-composites-get-smart

[17] R. Grimberg, L. U. (2005). 2D Eddy Current Sensor Array. NDT&E International ,

39, 264-271.

[18] Peil, U. (2003). Life-Cycle Prolongation Of Civil Engineering Structures Via

Monitoring. Structural Health Monitoring , 64-79.

19] Sindel, R. (1998). Zur Untersuchug Von Systemen Von Ermaudungrissen Bei Der Inspektionsplanung.

Technological University, Munich, Germany.

[20] P. Hovey, J. G. (1983). Estimating The Statistical Properties Of Crack Growth For Small Cracks. 18, 285-

294

References

My wife – AmandaMy parents – Darold and KarenMy Thesis Board

Thesis Advisor – Dr. Kondraske Dr. Liang, Dr. Devarajan

Dr. Dale BallAnn LewistonEd Kolesar

Acknowledgements

Backup Slides

investigating the implementation of usaf damage tolerant risk analysis with a structural health...

Documents

uta slide

risk of failure

high slide

aircraft fleet health

primary objective slide

health of structures

cost visual inspections

aircraft readiness moi