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Fracture stress is only less than ultimate tensile strength in an engineering stress-strain diagram. This is because the material will experience a maximum stress before it undergoes necking. After necking, stress will decrease again until the material snaps.

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Ultimate stress is defined as force to fail divided by area of the section. Since in actual failure the section area is necked down and thus smaller, true stress to failure is actually a bit higher

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Q: Why is fracture stress less than ultimate tensile strength?
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Continue Learning about Engineering

When does Necking of a metal occur under tensile stress?

after ultimate tensile strength (UTS)


What is the tensile strength of plywood?

It is the ultimate strength of a material subjected to tensile loading. In other words, it is the maximum stress developed in a material in a tension test.


What is the greatest stress that material can resist before breaking?

The greatest stress that a material can resist before breaking is called the ultimate tensile strength. It is the maximum amount of stress a material can withstand without breaking under tension. Different materials have different ultimate tensile strengths, and it is an important property to consider for designing and engineering structures.


Is ultimate flexural strength and moment of resistance the same in a reinforced section?

No the moment of resistance is a defining parameter that can be used to calculate the stress in a cross section of a given material that is subject to flexural loading. The ultimate flexural strength is a numerical value of stress at which the material will crack, tear, rip etc. Think about ultimate tensile strength and the value of Young's Modulus. Young's Modulus is not defined at the point of 'necking' and therefore the ultimate tensile strength cannot be computed from Young's Modulus and Hook's Law, but the UTS is an empirically defined value.


What is the ultimate stress of mild steel?

Allowable stress would normally refer to design using Allowable Strength Design, also known as working strength design. In this the allowable stress is usually a fraction of the yield strength and can be different for uniform tension and bending. Typically mild steel has a yield strength of about fy=250MPa with allowable stresses in Tension, 0.6fy=150MPa Bending, 0.66fy=165MPa