Page 67 - Ship Construction.DJ Eyres 6Ed

Ch07-H8070.fm Page 56 Wednesday, October 18, 2006 6:53 AM
56 Ship Construction
δ l l
P A

Ultimate tensile stress
Ultimate tensile stress
Proof stress
--- ใช้เพื่อการศึกษาเท่านั้น---
Yield stress
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี

Stress (P/A) MILD STEEL Stress (P/A) MATERIAL WITH
NO DISTINCT YIELD
POINT

Strain () Strain ()
δL δLδ l
δ l
l l
1% Strain
ึ
FIGURE 7.1 Stress/strain relationship of shipbuilding materials
Since stress is directly proportional to strain, the stress is equal to a
constant which is in fact the slope of the straight line part of the graph, and
is given by:

A constant = stress ÷ strain
This constant is referred to as the Modulus of Elasticity for the metal
and is denoted E (for mild steel its value is approximately 21 100 kg/mm 2
2
or 21.1 tonnes/mm ).
The yield stress for a metal corresponds to the stress at the ‘yield point’,
that is the point at which the metal no longer behaves elastically. Ultimate
tensile stress is the maximum load to which the metal is subjected, divided by
the original cross-sectional area. Beyond the yield point the metal behaves
plastically which means that the metal deforms at a greater, unpropor-
tional, rate when the yield stress is exceeded, and will not return to its
original dimensions on removal of the load. It becomes deformed or is
often said to be permanently ‘set’.
Many metals do not have a clearly defined yield point; for example,
aluminium having a stress/strain curve over its lower range which is a
straight line becoming gradually curved without any sharp transformation
on yielding as shown by mild steel (see Figure 7.1). A ‘proof stress’ is quoted
or the material and this may be obtained by setting off on the base some
percentage of the strain, say 0.2 per cent, and drawing a line parallel to the