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9. Comparison of structural design in stainless steel and carbon steel

Stress-strain behaviour of carbon steel and stainless steel

The stress-strain behaviour of stainless steel differs from that of carbon steels in a number of respects. The most important difference is in the shape of the stress-strain curve. Whereas carbon steel typically exhibits linear elastic behaviour up to the yield stress and a plateau before strain hardening, stainless steel has a more rounded response with no well-defined yield stress (see Figure).

 

 

Stainless steel ‘yield’ strengths are generally quoted in terms of the 0.2% proof strength, i.e. the proof strength at an offset permanent strain of 0.2%.

 

Comparison of mechanical properties for stainless steel and carbon steel

Grade Design strength (N/mm2) Ultimate tensile strength (N/mm2) Young’s Modulus (N/mm2) Elongation (%)
Stainless steel
304 (1.4301) 210 520 200 000 45
316 (1.4401) 220 520 200 000 40
Carbon steel
S275 275 410 205 000 22
S355 355 490 205 000 22

No limitations on thickness in relation to brittle fracture apply to stainless steel; the limitations for carbon steel are not applicable due to the superior toughness of stainless steel. The austenitic stainless steel grades do not show a ductile-brittle impact strength transition as temperatures are lowered.
Stainless steels can absorb considerable impact without fracturing due to their excellent ductility and their strain-hardening characteristics.

Comparison of structural behaviour of stainless steel and carbon steel members

The main reasons for the difference in structural behaviour between carbon and stainless steel members are:

  • The stress-strain curve for stainless steel departs from linearity at a much lower stress than that for carbon steels
  • Stainless steels have greater ductility and a greater capacity for work hardening than carbon steels
  • The material modulus of stainless steels reduces with increasing stress, unlike that of carbon steels which is constant
  • The residual stresses arising from fabrication are higher in stainless steel than in carbon steels.

As a result of this, different buckling curves are required from those of carbon steel. This applies to:

  • local (plate) buckling for elements in compression
  • flexural, torsional, torsional-flexural buckling for members subject to axial compression
  • lateral-torsional buckling for beams with unrestrained compression flanges

Design guidance for structural stainless steel is available (see References).

 

Estimation of deflections

As the stiffness of stainless steel decreases as the stress level increases, deflections are greater that those for carbon steel members. It is therefore necessary to use a reduced modulus to predict the behaviour of members in which high stresses occur. This is explained in the article Calculating the deflections of stainless steel beams

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