FEA STUDY OF A HIGH STRENGTH STUD FAILURE...2. Stud Test Trials In order to improve the reliability...

FEA STUDY OF A HIGH STRENGTH STUD FAILURE

Prof. Dr. Y Song CEng FIMechE, Xianwu Dai, Yi Li

(Chongqing Wangjiang Industrial Co., Ltd. Chongqing, China)

Abstract High strength stud bolted joins are widely used in wind turbine gearbox. The guideline of VDI 2230 is widely adopted in current design practice for calculation of the stud as in a static loading condition. However, the stud failure in wind turbine occurs a lot. A stud in a wind turbine gearbox with a high safety factor designed according to VDI 2230 and without any extra designated loading during operation has been found broken. Since the safety factor of stud is quite large, it is initially referred the stud failure to its material defect. A replacement was soon done but after just one month, the same position’s stud broke again. A system analysis of microscope and material of the stud failure section reveals the original fatigue crack and its developed failure, its a stress creep and relaxing happening during the operation. To explore the failure mechanism of the stud, this paper presents the findings through nonlinear finite element contact analysis by Ansys’ screw modelling method and Abaqus' nonlinear screw contact model. This study concludes that the current stud design method is not suitable for dynamics situation such as in a modern large wind turbine gearbox. If the local stress is over the yielding stress, its service life will be seriously affected. The local stress should be calculated by model FEA software. In the design practice, this study recommends the maximum local stress in the stud should be smaller than its yielding stress. The recommendation has been implemented in the thereafter design and there are no stud broken since then.

1. Introduction

High strength studs are widely used in wind-turbine (Figure 1: ) gearbox with planetary gear system. The ring gear and the housing are normally connected by high strength studs, show in Figure 2: . The studs have to be tensioned highly to resist the torque between the ring gear and housing. It has been found that the stud is prone to fail under the heavy loads in practice.

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A stud of a 2MW main gearbox used to connect the ring gear was spotted fractured at the first screw position after operating one year. The replaced new stud broke again after running only one month, and the fracture situation is similar to the previous one, as shown in Figure 3: The failure mode has been identified as fatigue failure. The crack initialled in the edge and extended into the mid-section and finally snapped broken (Figure 4: ). However, the designed safety factor based on VDI 2230’s calculation shows the stud has infinite fatigue life. This kind of practical failure is against the design theory. This study is trying to figure out failure causes and provide design guideline for future design.

2. Stud Test Trials

In order to improve the reliability of wind turbine gearbox, stud tension tests have been carried out. The stud’s property is shown as Table 1: A large amount of experimental data has been collected to study the factors resulting in the stud’s failure.

Testing sample from the broken stud has been analysed for its chemical composition. It has low nitrogen hydrogen and oxygen content of the material and is well qualified (Table 2: ) . The material is within the designed material specification (0,reference to GB/T 3077-2012). The material strength test also shows the mechanical properties are better than the specified value (Table 4: ). That means the material is all right and has no problem.

Figure 1: Wind turbine



Figure 2: The planetary ring gears and housing connection

Figure 3: The fractured stud

Figure 4: Stud fracture appearance

Table 1: Stud data



Nomenclature Data

Thread size M36×3

Strength grade 10.9

Material 40CrNiMo

Thread roll method SG

Table 2: Nitrogen Hydrogen and Oxygen Content

Element N O H

Value 57ppm 12ppm 2ppm

Table 3: Chemical composition

Table 4: Mechanical properties

Nomenclature Symbol

Rp0.2 Rm A Z KU2 KV2

（-40℃）

Unit MPa % J

Value 970 1080 16.5 56.5 87.5 86.5 82.5 30.4 30.8 30.8

In order to find out the stud’s pre-tension change under operation conditions. A maintenance check has been conducted on those remaining studs in gearbox. It is found that after two years’ operation, the pre-stress of studs in wind turbine gearbox under a 623kN preloaded tension has been significantly reduced, see Figure 7: . The maximum drop reached 20% of the initial preload. Those inspection

Element C Mn S P Si Ni Cr Mo

Value %

0.39 0.70 74 ppm

0.2 0.21 1.36 0.69 0.16



shows that the pre-stressed studs under dynamic operating condition, the pre-stress are reduced.

Figure 5: The number of gearbox stud

To confirm the preload stress decrease phenomena under operating conditions, a test has been carried out on pre-stressed studs. Strain gauges were applied on the studs to measure the stress amplitudes on a newly assembled gearbox (Figure 6: ). Test results showed that after nine hours’ running on a test bench under rated torque the stud shank’s stress decreased to varying degrees, shown as in Figure 8: . However, an identical gearbox in stock more than 3 years has been measured and did not show any pre-stress change. Gearbox stud running under rated load case conditions alternating stress maximum 6.3MPa, well the value calculated according to VDI 2230 the allowable fatigue amplitude is 47.8MPa.

Figure 6: Strain gauges applied on the shank of stud



Figure 7: Stud residual preload force after two year’s operation

Figure 8: Stress drops down after 9 hours running

Figure 9: The stress amplitude at rated load case

480

490

500

510

520

530

540

1 3 5 7 9 11 13 15 17 19 21 23 25

Re

sid

ua

l p

relo

ad

le

ve

l(k

N)

Stud No.

595

600

605

610

615

1 2 3 4 5 6 7 8 9 10 11 14 15 16 18 20 21 22 23 24 25 26

Stress assembly stress after running

Stu

d s

ha

nk

str

ess

(M

Pa

)

Stud No.

0

1

2

3

4

5

6

7

1 3 6 8 11 14 16 18 20 21 22 24 26

Stu

d s

ha

nk

str

ess

(M

Pa

)

Stud No.



The above test results confirmed that the pre-stress of stud decreased under dynamic operating conditions. The stud’s residual clamp load decreased with the operation time in wind farm. Since the pre-stress of stud in a static gearbox undergoes no stress losses, it indicates that this pre-stress declination may be the factor that causes or related to stud’s failure. The stud has undergone stress creep and show fatigue failure.

3. FEA of Threaded Coupling

The Linear FEA of Stud

Accurate stress analysis of threaded coupling usually is a difficult work due to the huge amount of contact elements involved. The release of ANSYS 15.0 makes this type of threaded stress analysis much easier. It has a new feature and the detailed 3D solid thread model is not required. The thread coupling model can be set up quickly by inputting the parameters and the results are accurate enough to 3D contact models.

According to the relevant functional description in ANSYS 15.0, for 3D parametric analysis, a threaded stud M36×3 at 623kN preload has been modelled. For the sake of shorten the computing time, only a single stud has been analyzed. A simplified 3D model in Ansys is shown in Figure 10: .

All the contact surfaces are set as frictional contact elements. This includes Nut-Front housing, Front housing-Ring gear, Ring gear-Mid, Bolts-nuts, Nut-Mid housing. The friction coefficient is of 0.15. The threaded connection part uses the latest features of bolt section from ANSYS 15.0. The settings of the parameters are shown in Figure 11: .

To obtain an accurate thread stress results, a fine mesh is used in the model. The mesh size is set to 2mm. The threaded connecting portion is refined into 1mm. The model mesh is shown in Figure 12: .



Figure 10: Simplified model for analysis

Figure 11: Bolt section feature settings

The stud in the model has been applied 623kN preload and its pre-tension stress is calculated and shown in Figure 13: . Figure 14: shows that the thread’s local maximum stress is as high as 1878.5MPa, located in the first screw tooth. Such a high stress has far exceeded the yielding strength of the stud material. It has definitely into plastic deformation. So, in order to get the accurate plastic deformation, the thread must be analysed using Elastoplasticity method.

The Elastoplasticity Finite Element Analysis of Stud

Elastoplasticity state analysis is the most commonly used research material nonlinear behavior. Description of linear elastic material behavior beyond the scope of the plasticity theory consists of three important concepts: (1) Yield criterion, it is determined that a given state of stress in the elastic range or plastic flow happened. (2) Flowing criteria describe plastic strain increment tensor related to the current state of stress and thus the formation of Elastoplasticity constitutive



relation expression. (3) Hardening guidelines to determine changes in the deformation of the development of yield criteria.

Thread contact is a highly non-linear problem, in order to accurately analyse the threaded portion; a very fine mesh is required. Since ANSYS 15.0 Bolt section feature is only available in 3D elements for elastoplasticity analysis and this requires huge amount of computing time. Instead to use the 3D model, a simpler but finer mesh 2D elements analysis model has been established, to reduce the computing time and to get the more accurate results.

The simplified 2D mesh elements model is shown in Figure 16: . The threaded portion of stud has been meshed with the super fine contact elements (see Figure 17: ), to obtain the accurate nonlinear elastoplasticity stress.

The material tensile curve has been obtained from the standard sample tensile test of stud and is shown in Figure 15: . The stud material plastic stage of the stress-strain curve is set by the data shown in Table 5: . Other parts material linear properties are set as standard steel material.

The same preload 623kN is applied to the stud. The calculated maximum contact stress of thread is 1014.7MPa. The maximum plastic strain is 0.0269. The stress and plastic strain contours can be seen in Figure 18: obtained in Ansys. To verify the accuracy of the analysis, a model in Abaqus is also build and analysed and an almost identical stress results has been obtained, shown in Figure 19: . The maximum stress and plastic strain are located in the first two screw teeth engaged.

Figure 12: Mesh effect



Figure 13: Stress nephogram

Figure 14: Stress nephogram at stud thread

According to stud material test, the stud’s yield strength is of 970MPa and ultimate strength of 1080MPa. It can be seen that under the preload of 623kN, the stud’s thread has been locally yielded and into plastic deformation.

So in practice, the stud’s first thread engaged has been in a plastic deformation condition, which greatly reduced its fatigue life than those calculated based on VDI 2230. Under dynamic operating condition, its local thread plastic deformation will crack with the time and then gradually extended until the stud is broken.



To avoid this kind of fatigue failure, the pre-stress of the stud has to be reduced. In practice, the maximum local pre-stress of the stud has to be smaller than the stud’s fatigue limits.

In our case, a replaced stud has been applied by a preload calculated. The preload has been decided by a nonlinear contact finite element analysis, with the condition that the maximum local stress should be smaller than stud’s yielding stress. The stud replaced has undergone for more than 2 years now and since then no any other studs have been found broken under this guideline.

Figure 15: Stud material tensile curve

Table 5: Stud material plastic stress-strain table

Stress（MPa） Strain 960 0 970 0.00092 980 0.002232 990 0.004692 1000 0.008684 1010 0.013228 1020 0.017769 1030 0.022126 1046 0.029932 1060 0.033388 1070 0.040545 1084 0.067547

0

10

20

30

40

50

60

70

80

90

0 0.1 0.2 0.3 0.4

Te

nsi

le f

orc

e (

kN

)

Stain



Figure 16: 2D Simplified Model

Figure 17: Super fine Element

Figure 18: Stud stress and plastic strain nephogram (ANSYS)



Figure 19: Stud stress and plastic strain (ABAQUS)

4. Conclusions

This study concludes that the current stud design method based on VDI 2230 is not suitable for dynamics situation such as in a modern large wind turbine gearbox. If the local stress is over the yielding stress, its service life will be seriously affected. The local stress of the stud should be calculated by modern FEA software. In the design practice, this study recommends the maximum local stress in the stud should be smaller than its yielding stress.

5. References

VDI 2230 Part 1 (2003) Systematic Calculation Of High Duty Bolted Joints; Joints With One Cylindrical Bolt.

GB/T 3077-2012 Technical conditions for alloy steels.

ANSYS, Inc., (2013) R15.0 Structural Mechanics Update, Pages 21-23.

Xianwu Dai, (2014) WF2000-HB-009 Concept of Stud Stress Test, CQWJ Internal Technical Report, Pages 4-7.

Xianwu Dai, (2014) WF2000-HB-010 Stud Preload Stress Test Report, CQWJ Internal Technical Report, Pages 4-9.



Yi Li, (2014) WF2000-HB-012 The Stress Distribution of Stud in Elastoplasticity State, CQWJ Internal Technical Report, Pages 2-8.

Ge Xiang, (2014) WF2000-HB-014 Report of Stud Service Stress on Test Bench”, CQWJ Internal Technical Report, Pages 4-12.

Physical And Chemical Test Center Of Ordnance Industry In The South-West Area Of China, (2013) The Stud Fracture Electron Microscopy Scanning Analysis Report, CQWJ Internal Technical Report.



FEA STUDY OF A HIGH STRENGTH STUD FAILURE...2. Stud Test Trials In order to improve the reliability...

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