REMAINING LIFE ASSESSMENT OF HYDRO TURBINE SHAFT

USING NUMERICAL CRACK PROPAGATION TECHNIQUE B.M. Sachin

*, S. Shamasundar, G. Chidanand, S. Pradeep

ProSIM R&D Pvt. Ltd., Bangalore, India *Corresponding author e-mail: info@pro-sim.com

ABSTRACT

Remaining life extension studies of hydro power plant components, systems, and structures

have to deal with the fatigue failure and fracture. This paper presents structural integrity

assessment studies of a hydro turbine shaft. Multiple sites of cracks were detected by NDT.

Operator had to be given a clear mandate to retire the turbine shaft or repair and reuse.

Towards such structural integrity assessment, authors have carried out finite element analysis

under operating loads, and fatigue crack propagation analysis to assess the remaining useful

life of shaft.

A finite element analysis (FEA) model of the shaft was created using commercial software

ABAQUS. Crack propagation under operating duty cycles was simulated using a

commercial 3D non-planar crack propagation software Zencrack. Fracture mechanics

characteristics such stress intensity factor (SIF), as k/n, da/dn are determined. The cycles for

fracture has been simulated using CTOD method. In conducting crack propagation analysis,

conventional FEM analysis, and X-FEM methods are seen to pose severe challenge for the

analyst in terms of excessive computational time and continuous user intervention to re-mesh

the crack tip region, to overcome the singularity issues. The crack block approach used in

Zencrack was found to elegantly solve the problems arising due to singularity at crack tip.

The FEA based fracture model with multiple sites was used to simulate the interaction effects

of propagating cracks and its effect in reducing the modulus. The hydro generator turbine

shaft was found to be safe till the next maintenance period.

Keywords: Stress intensity factor, structural integrity, Remaining life assessment, life

extension, crack growth.

1.0 INTRODUCTION

Structural integrity assessment of a hydel generator turbine shaft is presented in this paper.

Power plant of 120MW has been operating for last 30 years. Shaft was made of UNS S41000

material. Internal defects in the shaft are mainly due to processing (voids, porosity) and

material defects (inclusions). These flaws start growing and become noticeable in size with

increasing service life. The important question was, how long this shaft could be operated

safely.

During the maintenance schedule the turbine was detected to have multiple sites of noticeable

cracks. Operators concern was about the risk of continuing the shaft in operation. Authors

have done the crack growth analysis considering the multiple cracks to assess the remaining

life. An FEM analysis based fatigue crack propagation study was undertaken to study the

remaining life of the shaft. Crack propagation simulation enables one to predict the period of

sub-critical crack growth and hence the useful remaining service life of component.

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Once a fatigue crack propagation is developed, and validated, crack growth studies for a

variety of conditions can be conducted to assess the risk and reliability.

In the current analysis, authors have studied the effect of operational loads and the cyclic

loads due to plant start-up, and shut-down cycles.

2.0 OVERVIEW OF CRACK GROWTH ANALYSIS

Crack growth analysis is based on the calculation and use of fracture mechanics parameters.

Such parameters encapsulate and describe the local effect of the crack on a component. The

satisfactory calculation of fracture mechanics parameters is the most fundamental

requirement of crack propagation analysis. Having evaluated fracture parameters for a

particular crack in a component under a given load, the next task is to convert that

information into crack growth. This requires knowledge of the load history and appropriate

material crack growth data. The approach taken for integration of crack growth simulation

must be able to accommodate the evolution of an arbitrary crack shape that may be non-

planar. This shape evolution presents topologically difficult problems for the numerical

methods that are most often used to calculate fracture parameters.

2.1 Numerical Issues in Evaluating Crack Propagation

The numerical issues involved in crack propagation range from the basic solution method

(which essentially provides fracture parameter values) to the crack growth integration

technique.

There are three main approaches available to provide fracture parameters:

a) Closed form solutions

b) Boundary element methods

c) Finite element methods

Finite element method is a popular approach for tackling fracture mechanics problems. The

main advantage of finite element software codes is the flexibility they provide in terms of

non-linearity, ability to handle complex geometry, solution algorithms, and overall analysis

capabilities. The meshing and pre-processing issues are more difficult than for the boundary

element approach but non-linear analysis is possible without any additional meshing effort.

For practical industrial use, engineers need a flexible that is easy to use, and give more

valuable information on crack growth and propogation.

The main challenge for crack growth analysis is to relate fracture mechanics parameter,

topology and crack growth integration. This is effectively done using crack block approach in

ZENCRACK software. Using these crack blocks numerical singularities like SIF (K) can be

captured correctly and easily.

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Figure 1: Three key areas for crack simulation

2.2 Turbine Generator Shaft Crack Growth Analysis

Generator shaft is meshed differently for the stress analysis and for crack growth analysis.

For stress analysis it is modeled with tetrahedral elements in ABAQUS, as shown in Figure 2.

For crack growth model cracked location are modeled with Crack block elements from

Zencrack software. From the NDT the flaw detection sites were obtained. A study on the

location and size of the flaws was conducted. For the purpose of this paper two neighboring

cracks, are considered to show the interaction effects. Figure 3 shows two cracks of 60 mm

and 110 mm size at about 150 mm depth from the surface.

Figure 2: 3D FE model of the generator shaft

Figure 3: Two crack blocks considered at critical crack location

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2.3 Mesh Model for Crack Growth

The generator shaft is meshed with tetrahedral elements except in the region of the crack. The

cracked location is meshed with crack blocks as shown in Figure 4. Tie constraint are used to

define the contact between the crack blocks and the generator shaft.

For the analysis of generator shaft crack growth, first a hex mesh is generated at the location

of the crack. In the present study two cracks are considered which found to be largest in size

and the orientation of the defects.

Figure 4: Arrangement of the crack blocks.

3.0 RESULTS AND DISCUSSION

FEM analysis showed that the stresses in the shaft were very low, mainly arising due to

inertial effects and centrifugal forces. From the plant observation, the alignment of the shafts

and bearings were checked to be in order.

Figure 5: Stress values in crack (Peak stress found to be less than 70 MPa)

Crack front

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From the crack propagation analysis, authors have found the number of cycles required to

reach critical length and crack growth at various crack locations have been found. Figure 6

shows the graphs of sum of da v/s number of cycles for fatigue loading. From the crack

propagation study, it was found that, the cracks take practically a very long time to grow.

However, due to operational safety issues, life was extended by 2 years. Further usage of the

plant shaft was subject to inspection and further analysis.

Figure 6: Graph of Sum of da v/s Number of cycles

Figure 7: Graph of Sum of da v/s SIF

4.0 CONCLUSIONS

• During the plant maintenance schedule, multiple sites with noticeable crack sizes

were observed. The cracks had grown presumably between the previous maintenance

check and the current check.

• Shaft alignments, bearings were reported in tact.

• From FEM analysis, the stresses in the shafts were seen to be very low, mainly arising

due to centrifugal effects.

• From the above crack propagation analysis, it was seen that the shaft had quite long

time, and the cracks did not grow severely under operating and cyclic loads.

• Considering the risk factors due to operation, life extension was suggested for 2 years,

and further extension to be based on the record of plant observations and further

analysis.

cycles

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3. ZENCRACK user’s manual, Zentech International, U.K, pp.115, www.zentech.co.uk

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