PSI - Issue 20

Nikolay A. Makhutov et al. / Procedia Structural Integrity 20 (2019) 9–16 Nikolay A. Makhutov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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pipeline strength assessments. Local strains e max that are measured experimentally using the technique of grids and tensometry may essentially exceed ultimate elastic strains. Fracture may occur at the top of the crimp and at the place where the crimp slope is connected with the cylindrical part of the pipe. The threat of crimp formation consists in the fact that when the inner pressure is rising, stresses in the crimp zone increase more intensively than stresses in the smooth part of the pipeline, and either partially ductile fracture or completely brittle one becomes possible due to plasticity exhaustion in the crimp zone. 2. Analytical and experimental assessment of load carrying capacity The above considered changes of the geometry and mechanical properties in the crimp zone in general case is characterized by the processes of ultimate plasticity exhaustion and by the increase of local yield limit in the crimp zone. As the service time τ o f pipelines with crimps goes by and the working temperatures t increase the effects of aging are becoming more intensive, while the residual local plasticity of the metal in crimp zones is decreasing.

max c сr age e e e K , where K age is the aging fac tor, increasing from 1 to 1.6 with the growth of τ and t . c cr

To assess strength of the undamaged and damaged pipelines one can use the constitutive equations that determine the relationships between the intensity of true stresses σ i and strains e i in the form of linear and power laws for the elastic and elastoplastic regions:

i Ee for i

y ,

(1)

i

m

e

i

for

Y ,

(2)

i

i

y

e

y

where e y is the yield strain, e y =σ y /E ; m is a strain hardening exponent in elastoplastic region (for the steels 0≤m≤0.2 ). The intensities of stresses σ i and strains e i are expressed through principle stresses {σ 1 , σ 2 , σ 3 } and strains { e 1 , e 2 , e 3 } that are determined either by calculations or by experiment:

1

2 ) (

2 ) (

2

(

)

;

(3)

1

2

2

3

3

1

i

2

2(1 )

2 ) (

2 ) (

2

(

)

e

e e

e e

e e

;

(4)

1

2

2

3

3 1

i

3

where μ is the Poisson's ratio (μ =0.3 for elastic strains, and μ = 0.5 for developed plastic ones). Expressions (1) and (2) hold until the moment of the metal fracture in the neck during tension tests conducted on a smooth specimen (0≥ σ i ≥ S c ) where S c is fracture resistance in the neck region; 0≤ e i ≤ e c where e c is the true fracture strain in the neck region. According to Makhutov (2008) for structural pipeline steels it is possible to assume: (1 0.4 ), 1 ln , 1 c u c c c S e . (5) where ψ c is a relative narrowing in the specimen neck during static fracture. For low alloy pipe steels σ y =280 350MPa, σ u = 460- 580MPa and ψ c = 0.55-0.65. According to Makhutov (2005) the strain hardening exponent m can be defined using the expression: