Effect of Stress Risers on Tensile Tests Eileen O’Byrne-Hudson Mario Trinchero Jessica Enos Shawna...
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Transcript of Effect of Stress Risers on Tensile Tests Eileen O’Byrne-Hudson Mario Trinchero Jessica Enos Shawna...
Effect of Stress Risers on Tensile
TestsEileen O’Byrne-Hudson
Mario TrincheroJessica EnosShawna Enos
SRJC Engineering 45
December 9, 2009
Great Failures From Poor Design
The de Havilland Comet: Problems with explosive cabin decompression due
to poor window design Punch riveting around the windows and escape
hatches caused stress risers that induced failure
DH.106 in flightFailure around an escape hatch
Great Failures From Poor Design
Liberty Ships Ships had a tendency
to break in half when in cold waters and overloaded
Stress risers occurred where sharp corners on hatches met weld seams, and the welded design allowed cracks to propagate easily through the entire vessel
The S.S. Schenectady, A T2 tanker Liberty Ship
S.S. John W. Brown
1045 Steel1045 is a medium carbon steel is used
when greater strength and hardness is desired than in the "as rolled" condition. Extreme dimensional accuracy, straightness and concentricity combine to minimize wear in high speed applications. Turned, ground and polished bars can be machined asymmetrically with minimal distortion.
Properties of Steel Chemical Composition: C=0.45%, Mn=0.75%, P=0.04% max,
S=0.05% max Density : 7.872* 10³ kg/m³ Modulus of elasticity : 201 GPa Thermal expansion (20 ºC) : 11.7*10-6 ºCˉ¹ Specific heat capacity : 486 J/(kg*K) Thermal conductivity : 50.9 W/(m*K) Electric resistivity: 1.62*10-7 Ohm*m Tensile strength (hot rolled) : 565 MPa Yield strength (hot rolled) : 310 MPa Elongation (hot rolled) : 16% Hardness (hot rolled) : 84 RB
Testing of Steel with Fractures
This is an image of the stress at the tip of a fracture
Stress at the crack tip can be approximated by the formula
A very small crack radius will result in a “tip stress” many times that of the applied stress
Sample Preparation Each sample started as a standard 1045
tensile specimen, with a length of 2” and a diameter of 0.5”.
A “crack” was machined around the circumference of each sample.
All cracks have a depth of 0.005”. The three crack diameters are 0.002”, 1/64”
(0.0156”), and 1/32” (0.0313”). These diameters are at the very tip of the crack. The diameter of the smallest crack is a best estimation, as there were no tools available to measure the actual diameter.
Setup We used a standard
tensile testing setup for our tests. Top Left: Tester with
loaded sample; Bottom Left: Strain gauge on sample
Results
Stress-Strain Plot For 1045 Steel With Cracks
0
20000
40000
60000
80000
100000
120000
140000
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Strain (in/in)
Str
ess (
psi)
1/64".002" 1/32"Standard 1045
Fracture Surfaces
0.002” fracture surface 1/64” fracture surface
1/32” fracture surface
Standard 1045 fracture surface
o vs. m
The following graphs are of the maximum stress at the crack tip compared to the value of the applied stress.
1/32”Stress-Strain Plot of Nnormal Stress and Stress at Crack Tip for
1/32" Crack
0
50000
100000
150000
200000
250000
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Strain (in/in)
Str
ess (
psi)
NormalAt Stress Riser
1/64”Stress-Strain Plot of Normal Stress and Stress at Crack Tip for 1/64"
Crack
0
50000
100000
150000
200000
250000
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
Strain (in/in)
Str
ess (
psi)
NormalAt Stress Riser
0.002”Stress-Strain Plot of Normal Stress and Stress at Crack Tip for
0.002" Crack
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
500000
0 0.01 0.02 0.03 0.04 0.05 0.06
Strain (in/in)
Str
ess (
psi)
NormalAt Stress Riser
Interpretation of Results:
None of the samples exhibited the necking of the standard sampleCracks induced a brittle failure instead of
ductile
0.002” crack radius failed much earlier“Sharper” stress risers effectively make
material more brittle, indicating why rounded joints and seams are favored in many designs.
Work Cited Kopeliovich, Dmitri. "Carbon steel SAE 1045." Subs Tech- Substances and Technology. 16 May 2008.
Subs Tech, Web. 5 Dec 2009. <http://www.substech.com/dokuwiki/doku.php?id=carbon_steel_sae_1045>.
"properties of 1045." metal supplies online. 2009. Web. 5 Dec 2009. <http://www.metalsuppliersonline.com/Research/Property/metals/845.asp>.
Bureau of Labor Statistics, U.S. Department of Labor, Career Guide to Industries, 2008-09 Edition, Steel Manufacturing, on the Internet at <http://www.bls.gov/oco/cg/cgs014.htm> (visited December 05, 2009 ).
Riveros, Guillermo A. "Numerical Evaluation of Stress Intensity Factors (KI) J-Integral Approach." Sep 2006. US Army Corps of Engineers, Web. 5 Dec 2009. <http://chl.erdc.usace.army.mil/library/publications/chetn/pdf/chetn-ix-16.pdf>.
“Liberty Ship.” Wikipedia. 2009. Web. 7 Dec 2009. <http://en.wikipedia.org/wiki/Liberty_ship>. "Engineering Stress and Fatigue Experts - Summer 2007 News." Engineering Stress and Fatigue
Experts - Home. Web. 07 Dec. 2009. <http://www.stressandfatigue.co.uk/alpha_omega_news.html>. "Stress concentration -." Wikipedia, the free encyclopedia. Web. 07 Dec. 2009.
<http://en.wikipedia.org/wiki/Stress_risers>. "De Havilland Comet -." Wikipedia, the free encyclopedia. Web. 08 Dec. 2009.
<http://en.wikipedia.org/wiki/De_Havilland_Comet>. "SS Schenectady -." Wikipedia, the free encyclopedia. Web. 08 Dec. 2009.
<http://en.wikipedia.org/wiki/SS_Schenectady>. Greenwood, John T., ed. Milestones Of Aviation. [Westport]: Hugh Lauter Levin Associates, 1995.
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