A low design load drive shaft used to deliver power from an experimental exhaust heat recovery system to the crankshaft of an experimental diesel truck engine failed during highway testing. An independent testing laboratory analyzed the failure by routine metallography and attributed the failure to fatigue induced by a banded microstructure. Visual examination by NASA of the failed shaft plus the knowledge of the torsional load that it carried pointed to a 100 percent ductile failure with no evidence of fatigue. Scanning electron microscopy confirmed this. Torsional test specimens were produced from pieces of the failed shaft and torsional overload testing produced identical failures to that which had occurred in the truck engine. This pointed to a failure caused by a high overload and although the microstructure was defective it was not the cause of the failure

Reagan, J. R.

English

NASA Technical Reports Server

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I 1I 11 1 1111 11 1~~]lljllllrlllIf1]llmflI[ll~ ij ll Illllllil 
I I 3 1176 00501 6556 
DOE/NASA/1011-35 
NASA TM-82758 
I Failure Analysis of a 
I, Tool Steel Torque Shaft 
John R. Reagan 
National Aeronaut ics and Space Administration 
Lewis Research Center 
Work performed for 
U.S. DEPARTMENT OF ENERGY 
Conservation and Renewable Energy 
Office of Vehicle and Energy R&D 
ASA-TM-82758 198200033 11 
y 
u r'CI EY RE"<>'"ARCH CENTER 
L1BR\!(Y. NASA 
)1 
Prepared for 
Conference on Deformation , Fracture, Wear, and 
Nondestructive Evaluat ion of Materia ls 
sponsored by American Physical Society 
and Nat ional Bureau of Standards 
New Orleans, Louisiana, November 23~24 , 1981 
H'\M?10N, VIR31NII! 
I 
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https://ntrs.nasa.gov/search.jsp?R=19820003311 2020-03-21T11:30:52+00:00Z
NOTICE 
This report was prepared to document work sponsored by the United States 
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employees. makes any warranty . express or Implied. or assumes any legal liabi lity or 
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I 
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Failure Analysis of a 
Tool Steel Torque Shaft 
John R. Reagan 
DOE/NASAJ1011-35 
NASA TM-82758 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
Work performed for 
U.S. DEPARTMENT OF ENERGY 
Conservation and Renewable Energy 
Office of Vehicle and Engine R&D 
Washington, D.C. 20545 
Under Interagency Agreement EC-77-A-31-1011 
Conference on Deformation, Fracture, Wear, and 
Nondestructive Evaluation of Materials 
sponsored by American Physical Society 
and National Bureau of Standards 
New Orleans, Louisiana, November 23-24, 1981 
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FAILURE ANALYSIS OF A TOOL STEEL TORQUE SHAFT 
by John R. Reagan 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
ABSTRACT 
A low deslgn load drive shaft from an experimental diesel truck engine 
failed unexpectedly during highway testing. The shaft was driven by a 
turbine used to deliver power from an experimental exhaust heat recovery 
system to the engine's crankshaft. During design, fatigue was not 
considered a major problem because of the low operating cyclic stresses. 
An independent testing laboratory analyzed the failure by routine 
metallography. The structure of the hardened S-7 tool steel shaft was 
banded and the laboratory attributed the failure to fatigue induced by a 
banded microstructure. NASA was asked to confirm this analysis. Visual 
examination of the failed shaft plus the knowledge of the torsional load 
that it carried pointed to a 100 percent ductile failure with no evidence of 
fatigue. Scanning electron microscopy confirmed this. Torsional test 
specimens were produced from pieces of the failed shaft and torsional 
overload testing produced identical failures to that which had occurred in 
the truck engine. This pointed to a failure caused by a high overload and 
although the microstructure was defective it wa s not the cause of the 
fail ure. 
i 
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INTRODUCTION 
Any failure should be treated as a forensic study which encompasses all 
possible scenarios with a milt i-disciplined analysis. In the case presented 
the original analysis was conducted by an independent laboratory using 
metallographic analysis only, fractorgraphy and structural analysis were not 
investigated. 
Metallography examines metals, usually microscopically, to reveal its 
structure, hardness, and defects. It is a key tool when properly applied in 
failure studies. 
Fractography is the discipline of examining a broken surface and 
deducing from the macroscopic and microscopic appearance of the structure 
what type of stress/strain environment existed and whether this environment 
produced a ductile or brittle failure. 
Structural analysis will show when the loads needed to produce failures 
are, in fact, present. In this case structural analysis would consist of 
examining fatigue design values and torsional strength. 
BACKGROUND 
The failed shaft was turbine driven by an experimental 40 horsepower 
exhaust heat recovery system that delivered the power directly to the 
crankshaft of an experimental diesel truck. The truck was being highway 
tested at speeds below 50 mph. The exact time of the failure was unknown. 
Although the system had operated 20 000 miles it is conceivable that the 
shaft had failed earlier. The shaft was designed for infinite life. 
Physically the shaft was 1411 long and 1/211 in diameter with 3/4 11 
splines on each end. The shaft had failed at the smallest diameter just as 
shaft began its transition to the 3/4 diameter spline. The shaft was 
manufactured from (5-7) tool steel and heat treated to RC 56-58. Figure 1 
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VISUAL OBSERVATIONS 
1. The s~aft was' received in one long piece and several smaller pieces from 
which metallographic specimens had already been taken. 
2. Although 80 percent of the fracture surface was destroyed due to rubbing 
at the time of failure there was strong evidence that the failure had 
been completely ductile in nature. The fracture surface was totally 
perpendicular to the shaft. There was no evidence that smaller pieces 
had broken away. 
3. There was no evidence of fatigue "beach" marks. 
4. There was no observable permanent twist in the shaft. 
5. The independent laboratory report accompanying the shaft stated that the 
S-7 material failed due to a banded microstructure inducing fatigue. 
TEST PROCEDURE 
1. The shaft was Zyglo inspected for evidence of cracking. 
There was no evidence of any flaw being present outside the actual 
fracture. 
There was a secondary crack on the fracture surface. 
2. A portion of the fracture was placed in the 5.E.M. for examination. 
All areas surveyed showed micro-void coalescence which is indicative 
of ductile failure. Figure 2. 
3. A chemical analysis of the shaft was conducted. 
• The results showed the material to be 5-7 tool steel. There was a 
rather high residual nic~el content (0.22 percent), which is not 
regarded as 5ignificant to the failure. 
4. Hardness tests were conducted. 
• The shaft hardness was RC 56-57. 
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5. Sections of the shaft both transverse and axial were mounted for 
microstructure and Tukon hardness. The transverse specimen also included 
the fracture zone. 
The microstructure indicated a heavily banded structure. Tukon 
hardness indicated a possible 3 point difference in the bands 
(RC 52-55). There was no evidence of carburization or 
decarburization. Figure 3. 
6. A section of the shaft was macro etch to reveal ingot quality. 
The etched diameter did not show excessive inclusions or porosity. 
7. Torsional strength of the actual shaft was measured. 
Three specimens 211 long were cut from the shaft. A 7/16" hex was 
ground 1/2" long on each end taking care not to overheat the shaft. 
A center section approximately 1/211 long was cut to the diameter 
shown in Table 1. The specimen was mounted in a holder which 
prevented a bending moment from being applied to the test section 
while restraining one of the hex ends. Torque was applied to the 
other he x with an adjustable 400 foot pound torque wrench. Torque 
application was slowly incremented until failure occurred. Table 1 
details the Test data. 
DISCUSSION 
The maximum design conditions for this shaft was 40 hp at 3000 rpm. From 
standard design formulas this can be converted to torque by: 
Torqu€ 63 000 hp 
( i n- , b) = rpm 
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-- - ---- ---- ~--- - --- -- ---~- ----------" 
The maximum design torque was, therefore, 70 ft-lb. 
To find the maximum shear stress the shaft was subjected to a standard 
formula for elastic torsional stress. The formula being: 
Maximum Shear Stress = 16 Torque (in-l~) 
D· t 3 'IT lame er 
which shows the stress at the design torque of 70 ft-lb to be 35 000 psi. 
The values shown in Table 1 were also calculated from the above formula 
assuming no plastic deformation. (This assumption is particularly valid in 
this case because the high Rockwell hardness places the yield close to the 
ultimate strength.) The minimum shear strength far exceeds the design 
requirements. 
The numb~r two specimen failure occurred at the hex (probably due to 
bending of the torque wrench) and traveled in a spiral through the 
specimen. The diameter had been increased to better approximate the actual 
design conditions. 
The failure of the number one and number three specimens were 
indistinguishable from the actual failure surface. The surfaces were 
compared under the Scanning Electron Microscope (S.E.M.) and found to be 
predominately microvoid coalescence structures identical to the actual 
failure. 
Further assuming that a fatiguing action had occurred on the original 
failure and taking 200 000 psi as the actual shear strength of the shaft the 
final failure would have occurred through only a 9/32" diameter. The failed 
shaft showed the failure had taken place through a minimum of 5/16" and 
5 
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almost certainly through the entire 1/211 diameter. (A 1/32 difference in 
shaft diameters is significant.) 
The metallurgical structure of the number one and two specimens 
examined at the failure sight was shown to be heavily banded. The number 
three specimen was not examined metallographically. Figure 4. 
The banded microstructure could not be related to either the original 
failure or the test speciment failures. 
CONCLUSIONS 
The torsional strength of this material exceeded the design criteria by 
a factor of greater than five. 
looking at the design conditions of 40 hp at 3000 rpm and comparing 
them in standard sh~ft ing formulas, it wou ld seem that the shaft has 
adequate strength for all but the most severe loading conditions. 
Although the banding of the microstructure is undesirable there is no 
evidence to support that it caused or even contributed significantly to this 
failure. 
No evidence of fatigue was noted anywhere in the NASA analysis. 
The cause of this failure was over stressing of the shaft well beyond 
its design limits. 
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TABLE 1. - SPECIMEN DATA AND RESULTS 
[See discussion for explanations.] 
SpecimelJ Test Minimum Shear Type of Location of failure 
number section torque at stress at fail ure 
diameter, failure, failure, 
in. ft-lb psi 
1 0.378 180 200 000 Ductile Mid test section 
2 0.401 245 234 000 Britt 1 e Hex 
3 0.375 205 237 000 Duct il e Mid test section 
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1'1--1' --
t§ 
114" R 
TYP. 
14" 1 12" ---------j, \ • 
PLANE OF 
FAILURE 
<5/64 DIA. I ;~PLlNE 
112" DIA. 
Figure 1. - Configuration of torque shaft showing the location of the fail ure. 
Figure 2. - SEM micrograph at 1. 5 KX shOWing microvoid coalesence. 
Figure 3. - Banded microstructure at lOOX of fai led shaft showing fai lu re su rface and tukon hardness 
indentation s. 
Figure 4. - Typical banded structure at lOOX from No.1 and 2 test speci men. 
-- -----
1. Report No. I 2. Government Accession No. 3. Recipient 's Catalog No. 
NASA TM-82758 
4. Title and Subtitle 5. Report Date 
FAILURE ANALYSIS OF A TOOL STEEL TORQUE SHAFT 
6. Performing Organization Code 
778-34-12 
7. Author(sl 8 . Performing Organization Report No . 
John R. Reagan E-1077 
10. Work Unit No . 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
11. Contract or Grant No . 
Lewis Research Center 
Cleveland, Ohio 44135 13. Type of Report and Period Covered 
12. Sponsoring Agency Name and Address Technical Memorandum 
U. S. Department of Energy 
Office of Vehicle and Energy R&D 14. Sponsoring Agency ~Report No. 
Washington, D. C. 20545 DOE/ NASA/ 1011-35 
15. Supplementary Notes 
Prepared under Interagency Agreement EC-77-A-31-1011 . Conference on Deformation, Frac-
ture, Wear, and Nondestructive Evaluation of Materials sponsored by American Physical Society 
and Natural Bureau of Standards, New Orleans, LouiSiana, November 23-24, 1981. 
16. Abstract 
A low design load drive shaft from an experimental diesel truck engine failed unexpectedly during 
highway testing. The shaft was driven by a turbine used to deliver power from an experimental 
exhaust heat recovery system to the engine's crankshaft. During design, fatigue was not con-
sidered a major problem because of the low operating cyclic stresses. An independent testing 
laboratory analyzed the failure by routine metallography. The structure of the hardened S-7 
tool steel shaft was banded and the laboratory attributed the failure to fatigue induced by a banded 
microstructure. NASA was asked to confirm this analysis . Visual examination of the failed 
shaft plus the knowledge of the torsional load that it carried pointed to a 100 percent ductile 
failure with no evidence of fatigue. Scanning electron microscopy confirmed this. Torsional 
test specimens were produced from pieces of the failed shaft and torsional overload testing pro-
duced identical failures to that which had occurred in the truck engine . This pointed to a failu re 
caused by a high overload and although the microstr ucture was defective it was not the cause of 
the failure. 
17. Key Words (Suggested by Authods) I 18. Distribution Statement 
Torsion fatigue Unclassified - unlimited 
Failure STAR Category 26 
Failure analysis DOE Category UC-96 
Microstructure 
19. Security Classif. (of this reportl !20. Security Classif. (of this page) 21. No . of Pages 122. Price' 
Unclassified Unclassified 
, For sale by the National Technica l Information Service. Spr ingf ie ld. Virg inia 22161 
L 
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Failure analysis of a tool steel torque shaft

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