Effect of Stress Risers on Tensile Tests Eileen O’Byrne-Hudson Mario Trinchero Jessica Enos Shawna 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 Steel  1045 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

Fracture Surfaces 0.002” fracture surface1/64” 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”

1/64”

0.002”

Interpretation of Results:  None of the samples exhibited the necking of the standard sample  Cracks 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 Subs Tech, Web. 5 Dec  "properties of 1045." metal supplies online Web. 5 Dec  Bureau of Labor Statistics, U.S. Department of Labor, Career Guide to Industries, Edition, Steel Manufacturing, on the Internet at (visited December 05, 2009 ).  Riveros, Guillermo A. "Numerical Evaluation of Stress Intensity Factors (KI) J-Integral Approach." Sep US Army Corps of Engineers, Web. 5 Dec  “Liberty Ship.” Wikipedia Web. 7 Dec  "Engineering Stress and Fatigue Experts - Summer 2007 News." Engineering Stress and Fatigue Experts - Home. Web. 07 Dec  "Stress concentration -." Wikipedia, the free encyclopedia. Web. 07 Dec  "De Havilland Comet -." Wikipedia, the free encyclopedia. Web. 08 Dec  "SS Schenectady -." Wikipedia, the free encyclopedia. Web. 08 Dec  Greenwood, John T., ed. Milestones Of Aviation. [Westport]: Hugh Lauter Levin Associates, Print.