OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.6 Failure mechanisms and design criteria – Page 123
10.4.4 It is assumed that the reduction of strength with time can be described by one of the following
equations:
where,
− β log t
or
or σ(t) = σ(1) – β log(t)
or ε(t) = ε(1) – β log(t)
σ (t), ε (t) time-dependent stress or strain to failure
σ(1) , ε(1) scalar depending on the material, failure mechanism and on the environmental conditions at time
1 (units of time must be consistent in the equation)
β slope depending on the material, failure mechanism and on the environmental conditions
log denotes the logarithm to the basis 10.
10.4.5 It shall be documented that the material follows the equation in [10.4.4]. More details can be found in
Sec.4 [3.3] and Sec.5 [3.3]. If the long term behaviour of the material is different, the following equations to
calculate lifetimes may still be used, but the characteristic time to failure (see [10.4.7]) should be calculated by
a statistical analysis appropriate for the specific behaviour of the material.
10.4.6 The regression line described by the equation in [10.4.4] should correspond to the characteristic curve
as described in [3.11].
charact
10.4.7 The characteristic time to failure t γ ⋅σ
shall be extracted from the stress rupture curve (see
Sd
applied
also Sec.4 [3.3] and Sec.5 [3.3]) for each applied strain condition. The characteristic time to failure shall be
found for the applied strains ε applied multiplied by the partial load model factor γ Sd . Alternatively, the
charact
characteristic time to failure can also be found for an applied stain t ( γ ⋅ε
) , depending on what type
Sd
applied
of data is available. One of the following design criterion for stress rupture shall be used, depending what kind
of long term data are available:
with
γ Rd = 1 for a summation over various strain/stress levels, i.e. N>1.
γ Rd = 0.1 if the component is exposed to only one strain/stress level, i.e. N=1.
where t
ε j applied
σ j applied
t actual
t charact
N
j
t y
γ Sd
γ Rd
γ fat
{....}
γ
log
log
fat
t is a function of…
[ σ ( t)
] = log[ σ ( 1)
] ( )
[ ε( t)
] = log[ ε ( 1)
] − β log( t)
γ
γ
Rd
fat
t
γ
y
Rd
∑
j = 1
t
( )
{ σ applied }
j
Sd
<
{ } 1
γ σ
j applied
local response of the structure to the permanent static load conditions (max. strain)
local response of the structure to the permanent static load conditions (max. stress)
actual time at one permanent static load condition per year
N
N
t y∑
j=
1
t
charact
actual
characteristic time to failure under the permanent static load condition
the total number of load conditions
index for load conditions
number of years (typically the design life)
partial load-model factor
partial resistance-model factor
partial fatigue safety factor
t
t
or
actual
charact
10.4.8 A different γ Rd value may be chosen if it can be documented by experimental evidence. Load sequence
testing for the actual material on representative load sequences shall be used to document the use of a γ Rd in
the range of 1 > γ Rd > 0.1. The minimum is γ Rd = 0.1.
γ
Sd
{ applied }
j
γ ε
Sd
<
{ } 1
γ ε
j applied
Sd
DET NORSKE VERITAS AS