Steel Founders' Society of AmericaResearch Report No. 94B

Fatigue and Fracture Toughness

Published by the Carbon and Low Alloy Steel Technical Research Committee Steel Founders' Society of America

DR. JOHN M. SVOBODATechnical and Research Director

MAY, 1983

Climate Temperature (Part II)

Cast Steels at Room or Low

Of Five Carbon or Low Alloy

Fatigue and Fracture Toughness of Five Carbon or Low Alloy Cast Steels at Room

or Low Climatic Temperatures

(Part II)

By: Ralph L. Stephens

Table of Contents

Chapter Page

Preface ........................................................................................................ iSummary and Conclusions ......................................................................... ii High Cycle Axial Fatigue of Smooth Specimens at Room Temperature and -50F (-45C) ..................................................... 1 7.1 Test Procedures .................................................................................. 17.2 Test Results and Discussion ............................................................... 17.3 Summary and Conclusions ................................................................. 3References ................................................................................................. 3

Threshold, Kth, and Near Threshold Fatigue Crack Growth Behavior at Room Temperature and -50F (-45C) ................................... 48.1 Introduction ........................................................................................ 48.2 Test Procedures .................................................................................... 48.3 Test Results and Discussion ................................................................. 58.4 Summary and Conclusions ................................................................... 9References ................................................................................................... 9

Variable Amplitude Fatigue Crack Initiation and Growth Calculations at Room Temperature and -50F (-45C) ............................ 109.1 Introduction ......................................................................................... 109.2 Mathematical Models .......................................................................... 10

9.2.1 Fatigue Crack Initiation .............................................................. 109.2.2 Short Crack Growth .................................................................... 119.2.3 Long Crack Growth ............... .................................................... 11

9.3 Comparison of Experimental and Calculated Fatigue Lives ................ 129.4 Summary and Conclusions .................................................................. 14References................................................................................................ 14

Integration of Fatigue and Fracture Behavior .......................................... 1510.1 Introduction ...................................................................................... 1510.2 Room Temperature........................................................................... 18

10.2.1 Monotonic Tensile Test .......................................................... 1810.2.2 Fracture Toughness ................................................................. 1810.2.3 Constant Amplitude Axial Fatigue .......................................... 1810.2.4 Constant Amplitude Fatigue Crack Growth ............................. 1910.2.5 Variable Amplitude Fatigue .................................................... 19

10.3 -50F(-45C) Temperature ............................................................... 1910.3.1 Monotonic Tensile Test ......................................................... 1910.3.2 Fracture Toughness ................................................................ 1910.3.3 Constant Amplitude Axial Fatigue ......................................... 2010.3.4 Constant Amplitude Fatigue Crack Growth.............................. 2010.3.5 Variable Amplitude Fatigue ................................................... 20

10.4 Summary and Conclusions .............................................................. 21

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8

9

10

PREFACE

The original SFSA research project 111 (Dec., 1981) was extended in order tocomplete the needed fatigue properties of five common carbon or low alloy caststeels. The original report was also incomplete, since the variable amplitude fatigue crack initiation and growth calculations could not be properly made with- out the extended fatigue research. A proper integration of all the fatigue behavioralso could not be made without the extension data. This report includes theresults of the extended research, the fatigue life calculations, and the integration. This report has been written assuming the reader is well acquainted with the original report of Dec., 1981. Repetition of the first report has been omitted, ex- cept some integration has been made for better continuity. The new chapters have been labeled 7 through 10 in order to provide referencing to the previous chapters.

This fatigue research report includes:

Chapter 7: the extension of smooth specimen axial low cycle fatigue behavior from about 105 or 106 reversals to 2 x 107 reversals at both room temperature and - 50 F ( - 45 C). Specifically, fatigue limits, Sf, and fatigue ratios Sf/Su for the fully-reversed condition, R = - 1, were obtained in addition to completing the entire - 2Nf curves.

Chapter 8: the extension of region II fatigue crack growth behavior to threshold and near threshold regions with da/dN ranging from about 4 x 10-7 to 4 x 10-9inlcycle (10-8 to 10-10 m/cycle) at room temperature and -50F (-45C). Specifically, threshold stress intensity factors, Kth, for R = 0.05 and R = 0.5were obtained in addition to completing the entire da/dN - K curves.

Chapter 9: the fatigue crack initiation and fatigue crack growth life calculations for the compact keyhole notch specimens using the T/H and mod T/H variable amplitude load histories at room temperature and - 50 F ( - 45 C).

Chapter 10: the integration of the total fatigue and fracture behavior including fracture toughness, low and high cycle fatigue, fatigue crack growth under cons- tant amplitude loading, and fatigue crack initiation and growth under variable amplitude loading.

Prof. Stephens, his students and his staff are to be commended on a major con- tribution to the understanding of the behavior of cast steels. The Carbon and Low Alloy Technical Research Committee wishes to express its appreciation on behalf of the Society for this fine work.

Dr. John Svoboda Technical and Research Director

by direction of theCarbon and Low Alloy Technical Research Committee

R. C. Maxton, Chairman

D. C. HarschC. Brown E. J. LenarK. MurphyW. ScottE.K. Weber A. K. Zaman

i

SUMMARY AND CONCLUSIONS

A comprehensive research project to obtain room temperature and low climatic temperature [principally - 50F ( - 45 C)] fatigue and fracture properties of five representative carbon or low alloy cast steels was performed at The University of Iowa during 1979 to 1982. The five cast steels were SAE 0030, SAE 0050A, C-Mn, Mn-Mo and AISI 8630. The metallurgical structure of the first two steels was ferrite-pearlite while the later three steels were tempered martensite. The ultimate strengths varied from 72 to 166 ksi (496 to 1144 MPa) which indicates awide range in ultimate strengths. The following fatigue or fracture properties were obtained along with SEM fracto-graphic analysis:

1. Constant amplitude axial smooth specimen low cycle and high cycle fatigue using strain or load control with approximately 102 to 2 x 107 reversals to failure.

2. Constant amplitude fatigue crack growth rates from 10-4 to 4 x 10-9 in/cycle(2.5 x 10-6 to 10-10 m/cycle) which includes the log-log linear region, near threshold and threshold behavior.

3. Variable amplitude fatigue crack initiation and growth behavior using a keyhole notched specimen.

4. Complete Charpy V notch energy and NDT temperatures. 5. Plane stress fracture toughness and R-curves. 6. Plane strain fracture toughness using the J-integral.

In addition to the above, fatigue life calculations were made for the variable amplitude loadings using the constant amplitude fatigue properties and several life prediction models. An integration of the different test results was made and a complete summary of the properties at the two principal test temperatures are given in Tables 10.1 and 10.2.

The monotonic tensile properties and the fracture properties were quite varied for the five cast steels, while the fatigue resistance was somewhat similar. For in-stance, at room temperature, the smooth specimen fatigue limits varied from 28.5 to 42.4 ksi (196 to 293 MPa) for the five cast steels, the notched fatigue limits (not a requirement of this project)varied from 18 to 21 ksi (124 to 147 MPa),the threshold stress intensity factors for R = 0 varied from 7.4 to 9.3 ksi in(8.1to 10.2 MPa m) and the variable amplitude reversals to several different fatigue criteria varied by factors between 1.6 and 5.3. These are not large differences.

All five cast steels were suitable for room temperature fatigue design situations and can be best ranked in the following general order: 8360, Mn-Mo, C-Mn or 0050A and then 0030. At -50F (-45C) the fatigue resistance generally increased for most of the different tests with the cast steels except for 0050A. This steel showed substantial decreases i