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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