[Date RR # approved – ASTM to assign] Committee E08 on Fatigue and Fracture Subcommittee ...

175 Pages·2008·9.47 MB·English

[Date RR # approved – ASTM to assign]

Committee E08 on Fatigue and Fracture
Subcommittee E08.06 on Crack Growth Behavior

Research Report [RR # – ASTM to assign]

Interlaboratory Study to Establish Precision Statements for ASTM E647,
Standard Test Method for Measurement of Fatigue Crack Growth Rates

Prepared by:
Peter C. McKeighan
James H. Feiger
Dustin H. McKnight

ASTM International
100 Barr Harbor Drive
West Conshohocken, PA 19428-2959
ROUND ROBIN TEST PROGRAM AND RESULTS FOR
FATIGUE CRACK GROWTH MEASUREMENT
IN SUPPORT OF ASTM STANDARD E647

Prepared by

Peter C. McKeighan
James H. Feiger
Dustin H. McKnight

Southwest Research Institute (SwRI®)
San Antonio, Texas

Prepared for

ASTM International
Committee E08 on Fatigue and Fracture
Subcommittee E08.06 on Crack Growth Behavior
West Conshohocken, PA

February 2008

S O U T H W E S T R E S E A R C H I N S T I T U T E®
SAN ANTONIO HOUSTON WASHINGTON, DC
Acknowledgements

This effort would not have been possible without the input from all of the participants in
the round robin test program. The hard work provided gratis from each of the participants and
their labs is greatly appreciated. Furthermore, the United States Air Force is acknowledged for
graciously helping to provide the material for this program. MTS and Instron were kind enough
to supply most of the specimen machining with the remainder supplied by Southwest Research
Institute (SwRI). The Alcoa Technical Center provided machining of the notches in the
aluminum M(T) specimens. Finally, numerous ASTM Committee E08 individuals assisted in
this effort. Over the last decade, as this round robin evolved from idea to reality, excellent
support was consistently provided by John Ruschau (UDRI) and Steve Thompson (AFRL). Kind
acknowledgement is also extended to Ms. Loretta Mesa (SwRI) for preparing this manuscript
and the numerous ASTM members who reviewed this report prior to publication. Thanks to
everyone for making this collaboration a resounding success!

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TABLE OF CONTENTS

Section Page

Acknowledgements.........................................................................................................................ii

Executive Summary.....................................................................................................................viii

1.0 Introduction.............................................................................................................................1

2.0 Materials and Methods............................................................................................................3
2.1 Materials........................................................................................................................3
2.2 Round Robin Participants..............................................................................................4
2.3 Specimen Geometry.......................................................................................................5
2.4 Round Robin Rules........................................................................................................5

3.0 Analysis Methodology..........................................................................................................13
3.1 Interpolating Crack Growth Rate Data........................................................................13
3.2 Applying the Interpolation Approach..........................................................................14

4.0 Results...................................................................................................................................27
4.1 Final Matrix of Testing and Conditions.......................................................................27
4.2 The Big Six..................................................................................................................28

5.0 Discussion.............................................................................................................................43
5.1 Analysis Methodology.................................................................................................43
5.2 Interpolated Data for Each of the Six Conditions........................................................44
5.3 Interlaboratory Variability...........................................................................................46
5.4 Intralaboratory Variability...........................................................................................47
5.5 Variability Summary....................................................................................................47
5.6 Individual Laboratory Performance.............................................................................49
5.7 Statistical Anomalies?..................................................................................................50
5.8 Influence of Test Technique Variables........................................................................51

6.0 Summarizing and Concluding Remarks...............................................................................81

7.0 References.............................................................................................................................85

Appendix A – Supporting Guiding (Pretest) Documentation

Appendix B – Individual Lab Data Plots

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LIST OF TABLES

Table Page

2-1 Materials involved in the FCG round robin............................................................................8

2-2 Tensile properties of the materials involved in the FCG round robin....................................8

2-3 Round robin participants.........................................................................................................9

2-4 Specimen IDs for different materials and specimen geometries..........................................10

2-5 Generalized test conditions for the round robin testing including approximate lower
and upper bound envelope of crack growth rates and applied stress intensity factor
ranges....................................................................................................................................10

3-1 Tabulated values of average normalized growth rate (with standard deviation)
interpolated at various normalized ΔK levels.......................................................................16

4-1 Summary of round robin tests for each lab...........................................................................30

4-2 Specimen data summary for the 4130 steel material............................................................31

4-3 Specimen data summary for the thinner 2024-T351 aluminum material.............................32

4-4 Specimen data summary for thicker 2024-T351 and 7075-T6 aluminum materials............33

4-5 Primary test conditions for the different labs involved in the round robin...........................34

5-1 Processed statistics for the different labs testing condition STL-A and STL-B...................53

5-2 Processed statistics for the different labs testing condition ALX-A and ALX-C.................54

5-3 Processed statistics for the different labs testing condition ALX-B and ALN-A.................55

5-4 Laboratory-by-laboratory breakout for testing conditions STL-A and STL-B....................56

5-5 Laboratory-by-laboratory breakout for testing conditions ALX-A and ALX-C..................57

5-6 Laboratory-by-laboratory breakout for testing conditions ALX-B and ALN-A..................58

5-7 Influence of crack length measurement technique (DCPD and compliance) on
overall variability levels........................................................................................................59

5-8 Influence of loading method (constant amplitude versus K-control) on overall
variability levels....................................................................................................................60

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LIST OF FIGURES

Figure Page

2-1 Notch requirements for the different specimens in E647-05 (scanned from
standard)..............................................................................................................................11

2-2 Specimen geometries extracted from E647-05 illustrating specimen dimensions
for both C(T) and M(T) configurations (scanned from standard).......................................12

3-1 Sample set of fatigue crack growth rate data......................................................................17

3-2 Sample set of fatigue crack growth rate data with a subdivided ΔK in equal log
space increments (10 per decade)........................................................................................17

3-3 Interpolation strategy employed on either side of a ΔK increment.....................................18

3-4 Interpolation calculations for the data shown previously....................................................19

3-5 All of the data sets plotted on a common set of axes..........................................................20

3-6 All data sets with the interpolated average da/dN rate at each normalized ΔK level..........21

3-7 All data sets with the interpolated average and ±2 SD’s on FCGR da/dN..........................22

3-8 Typical error bound of the crack growth rate data at a given normalized ΔK....................23

3-9 Distribution of interpolated growth rates at a given normalized ΔK..................................24

4-1 Typical data sets from two of the participating laboratories (all data is plotted in
Appendix B)........................................................................................................................35

4-2 All fatigue crack growth associated with test condition STL-A.........................................36

4-3 All fatigue crack growth associated with test condition STL-B.........................................37

4-4 All fatigue crack growth associated with test condition ALX-A........................................38

4-5 All fatigue crack growth associated with test condition ALX-C........................................39

4-6 All fatigue crack growth associated with test condition ALX-B........................................40

4-7 All fatigue crack growth associated with test condition ALN-A........................................41

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LIST OF FIGURES (Cont’d)

Figure Page

5-1 Schematic approach of how spreadsheet-based analysis is organized................................61

5-2 Actual portion of the spreadsheet analysis for condition ALN-1........................................62

5-3 Comparing the interpolated data with the actual for condition STL-A in terms of
(a) growth rate and (b) variability.......................................................................................63

5-4 Comparing the interpolated data with the actual for condition STL-B in terms of
(a) growth rate and (b) variability.......................................................................................64

5-5 Comparing the interpolated data with the actual for condition ALX-A in terms of
(a) growth rate and (b) variability.......................................................................................65

5-6 Comparing the interpolated data with the actual for condition ALX-C in terms of
(a) growth rate and (b) variability.......................................................................................66

5-7 Comparing the interpolated data with the actual for condition ALX-B in terms of
(a) growth rate and (b) variability.......................................................................................67

5-8 Comparing the interpolated data with the actual for condition ALN-A in terms of
(a) growth rate and (b) variability.......................................................................................68

5-9 Average interlaboratory variability using (a) range extent ratio and (b) exponent.............69

5-10 Average intralaboratory variability using (a) range extent ratio and (b) exponent.............70

5-11 Relationship between inter- and intralaboratory variability using (a) range extent
ratio and (b) exponent..........................................................................................................71

5-12 Intralaboratory variability for conditions (a) STL-A and (b) STL-B..................................72

5-13 Intralaboratory variability for conditions (a) ALX-A and (b) ALX-C................................73

5-14 Intralaboratory variability for conditions (a) ALX-B and (b) ALN-A................................74

5-15 Laboratory performance for the different conditions (plot shows ranking method)...........75

5-16 Comparing the interpolated data with the actual for Lab Q data in terms of (a)
growth rate and (b) variability.............................................................................................76

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LIST OF FIGURES (Cont’d)

Figure Page

5-17 Anomalous Lab O data in terms of (a) growth rate (compared to interpolated) and
(b) variability.......................................................................................................................77

5-18 More typical Lab L data in terms of (a) growth rate (compared to interpolated) and
(b) variability.......................................................................................................................78

5-19 Effect of (a) crack length measurement method and (b) test control variability.................79

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EXECUTIVE SUMMARY

About the same time the ASTM fatigue crack growth standard (E647) was drafted, a round
robin program was held to quantify the variability observed when measuring fatigue crack
growth rate. Over the ensuing 35 years, technology has evolved considerably which may have
an impact on crack growth rate variability. This report details the results from an extensive
round robin performed using three materials (a steel and two structural aluminum alloys) and
encompassing a variety of specimen geometries and load ratios. In total, 141 fatigue crack
growth rate tests were performed during this testing. A systematic method was developed that
allowed quantifying growth rate (and associated variability in growth rate) at specific ΔK levels
for each test. The steel material exhibited the lowest level of interlaboratory (between labs)
variability. In general for steel, the growth rate variability observed is 1.9x whereas for
aluminum it is 2.4x. For 2024 and 7075 (the aluminum alloys utilized during testing), the mean
growth rate variabilities are 2.3x and 2.6x, respectively. The intralaboratory (within a given lab)
variability for steel is 1.5x, whereas for aluminum it is 1.65x. For 2024 and 7075, the mean
growth rate variabiltities are 1.6x and 1.7x, respectively. The interlaboratory variability ranged
from a low of 1.2x to typically 2.5-3.0x for all of the materials considered. The variability data
suggests that there is little statistical difference between that measured today and that
documented 33 years ago in a previous round robin assessment. The data suggests a slight
decrease in intralaboratory variability over this time period. Some influence of specimen
geometry was noted with M(T) specimens exhibiting variability levels that are 30-40% less than
similar C(T) specimens. A comparison between tests performed using DCPD and compliance as
the continuous, non-visual crack length measurement suggests that variability levels are 20% less
for DCPD when compared to compliance. Conversely, no discernable difference in variability
level was noted between different load control methods (constant amplitude versus K-control).

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1.0 INTRODUCTION

Thirty-six years ago, shortly after drafting the original version of the fatigue crack growth
(FCG) rate standard (ASTM E647 [1]), a round robin test program was performed to determine
the precision and bias of the test method. Since this time, gradual changes have occurred in the
body of the E647 test standard as a consequence of such things as servohydraulic control
capability improvements, test automation (few crack growth rate tests are performed manually
anymore), K-control test strategies and non-visual crack length technology.

The current ASTM E647-05 fatigue crack growth standard is a mature standard: the
version cited in Reference [1] totals 45 pages and cites 111 external references. However the
standard continues to evolve and embrace new approaches. For instance, a new method for
analyzing closure data (the ACR approach pioneered by Keith Donald at FTA) and a
compression load precracking methodology (spearheaded by Jim Newman at Mississippi State)
are both being drafted for likely inclusion in the standard.

This is not an easy test standard on which to perform round robin testing. Matters are
complicated by the flexibility that the current standard allows. For instance, a myriad of
different paths (i.e. specific test procedures) are allowed to generate FCG data for a given
material. Furthermore, a variety of nonvisual crack length measurement techniques are all
possible. Normally, in a round robin the test method is fixed and the single-valued output is then
assessed for variability. The absence of a single-valued result from FCG testing and the multiple
allowable paths seriously complicate a round robin test program for evaluating the E647
standard. Therefore, the intent of the round robin herein is to use whichever standard technique
desired to generate the FCG data for three materials (2024 aluminum, 7075 aluminum and 4130
steel). Different specimen geometries, specimen sizes and load ratios were perturbed during
testing. This report will describe the testing and the resulting data.

An extensive round robin like the one described in this report takes an enormous amount of
time to plan and execute. The initial discussions raising the possibility of a fatigue crack growth
round robin were held in the mid-1990s at the biannual ASTM committee meetings. Several
informal meetings were held during this period with likely participants working through the

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