Mechanical properties and fracture
Check stress–strain curves, Young's modulus, 0.2% offset yield strength, true stress, hardness testing, fracture toughness, fatigue, creep and the ductile-to-brittle transition with multiple-choice questions, and practice strength-design calculations with explanations.
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Mechanical properties and fracture: key points
The stress–strain curve from a tensile test gives Young's modulus, the yield stress (or 0.2% offset yield strength), the tensile strength and the elongation. Young's modulus is set by interatomic bonding and hardly changes with heat treatment, whereas the yield stress varies greatly with microstructure. Make sure you also know the difference between engineering and true stress, and the principles of the Vickers, Brinell and Rockwell hardness tests.
In real fracture, the critical stress or crack length is estimated from the crack size and the fracture toughness K_IC using K = Yσ√(πa). Fatigue under cyclic loading, creep that progresses over time at high temperature, and the ductile-to-brittle transition that embrittles BCC metals at low temperature are also key phenomena that govern the life of components.
All Mechanical properties and fracture questions (12)
- Which statement about the Young's modulus of steel is correct?
- A round bar 10 mm in diameter carries a tensile load of 10 kN. Which value is closest to the engineering stress?
- A steel rod 2.0 m long with a Young's modulus of 200 GPa is loaded in tension to 100 MPa within its elastic range. Which value is closest to the elongation of the rod?
- For materials such as aluminum alloys that show no distinct yield point, which correctly describes how the 0.2% offset yield strength is determined?
- In a tensile test, the engineering stress was 400 MPa at an engineering strain of 0.20, before necking began. Assuming constant volume during plastic deformation, which value is closest to the true stress at this point?
- Which statement about the Vickers hardness test is correct?
- A steel plate with a fracture toughness K_IC = 50 MPa·√m carries a tensile stress of 200 MPa. Using K = Yσ√(πa) with a geometry factor Y = 1.0, which value is closest to the critical crack length a at which fracture occurs?
- When the fracture toughness of plates of the same material but different thickness is measured, what happens to the apparent fracture toughness K_c as the thickness increases?
- Which statement about fatigue S-N curves is correct?
- What is the stress amplitude of a cyclic stress with a maximum of +300 MPa and a minimum of −100 MPa?
- Take about 0.4 times the melting point (in absolute temperature) as the rough temperature above which creep can no longer be ignored. For aluminum (melting point 660 °C), which value is closest to this temperature?
- Which material shows a distinct ductile-to-brittle transition at low temperatures?