ENGG 027
Engineering Materials
Dr. Saryn Goldberg
Lecture 20: Fracture & fatigue-limited design
Wednesday, November 8th, 2023
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Announcements
• HW 7 is due Monday 11/13
– After today you can work on problems 1-10
• Exam #3 (Ch 7-10) will be on Fri 11/17
– Last material on this exam will be covered Friday
– Exam will cover very limited material from Ch 7. See study
guide for details.
– Study guide and equation sheet are posted
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Outline and Goals for Today
Today we will cover:
– Fracture and fatigue-limited design
– Factors that contribute to fatigue resistance
– Materials selection for fracture and fatigue resistance
After studying material from class, you should be able to:
– Predict whether a material will fail due to a given crack size
– Select materials to prevent fracture or fatigue failure in a given
design
– Explain what it means for a design to be fail-safe
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Factors that contribute to fracture-limited design
Ys
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Fracture
toughness is a
function of
geometry and
loading
In all cases
c << w
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A polystyrene ruler has a width w = 25 mm and a thickness t = 4.6 mm.
A student supports the ruler as a horizontal cantilever, with 250 mm protruding
from the edge of a table, and hangs a weight of 10 N on the free end.
If the ruler has a 0.25 mm deep transverse scratch across its top surface near the
support of the cantilever, can it actually carry the load of 10 N without failing by
fast fracture? Assume that the fracture toughness of polystyrene is 0.7 MPa-m1/2
4.6
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Load-limited design requires a minimum K1C
Load-limited design
K1C >15 MPa-m1/2
energylimited
design
deflectionlimited
design
Other criteria for K1C
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Failure-limited design case study:
Pressure vessel
A truck-mounted propane tank was filled, driven to the driver’s
home where he parked it in the sun and went inside for lunch,
leaving the engine running. The tank exploded, killing one
person and causing significant property damage.
What caused the failure?
How could it have been avoided?
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What caused the failure?
Fracture toughness, K1C = 45 MPa-m1/2
10 mm crack found in longitudinal weld
Pressure release valve triggered at 1.5 MPa
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Yield before break
𝐾!"
𝜎#
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