This paper reports on the design and application of a fatigue testing facility for drill pipes. A reversed bending load is applied using a rotating drill pipe in a four point static bend arrangement with the possibility of applying an additional static tension load. Drill pipes were tested with and without static tension at different cyclic bending stresses. The stress amplitudes ranged from 30 MPa to 230 MPa on the outside surface of the drill pipe. The test results show a consistent failure mode with a circumferential crack propagating in the groove of the first engaged thread of the pin, i.e. the male section of the joint. The fracture surface as well as ultrasonic monitoring of the sample during the testing suggest that the circumferential crack growth rate is fast, and final fracture occurs within a relatively small number of cycles as soon as a through wall crack has been formed. The S-N curve collected from a total of 18 samples shows a linear trend in the logarithm of the fatigue life versus stress amplitude

Berezovski, A.

Veidt, M.

University of Queensland eSpace

SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
Design And Application Of A Drill Pipe Fatigue Test Facility 
 
M. Veidt and A. Berezovski 
Division of Mechanical Engineering, University of Queensland, Brisbane, QLD 4072 
phone: (07) 3365 3621, fax: (07) 3365 4799, e-mail: m.veidt@uq.edu.au 
 
 
 
ABSTRACT: This paper reports on the design and application of a fatigue testing facility for  drill pipes. A 
reversed bending load is applied using a rotating drill pipe in a four point static bend arrangement with the 
possibility of applying an additional static tension load. Drill pipes were tested with and without static 
tension at different cyclic bending stresses. The stress amplitudes ranged from 30 MPa to 230 MPa on the 
outside surface of the drill pipe. The test results show a consistent failure mode with a circumferential crack 
propagating in the groove of the first engaged thread of the pin, i.e. the male section of the joint. The fracture 
surface as well as ultrasonic monitoring of the sample during the testing suggest that the circumferential 
crack growth rate is fast, and final fracture occurs within a relatively small number of cycles as soon as a 
through wall crack has been formed. The S-N curve collected from a total of 18 samples shows a linear trend 
in the logarithm of the fatigue life versus stress amplitude.  
1 INTRODUCTION 
The major component of any drilling operation is the drill string consisting of multiple drill pipes 
connected by threaded joints and providing the link between the surface of the well and the bottom 
of the assembly. Drill string failures are costly because of loss or damage of equipment and the time 
needed for complicated recovery operations. The most common reason for drill pipe failure is 
growth of fatigue cracks in the threaded regions due to reverse bending.  
 
 Drill pipes are manufactured to different lengths and diameters. This investigation focuses on 
NQ drill pipes, which are mainly used for directional exploration drilling of coal reserves. The NQ 
drill pipes are made from medium carbon alloy steel, which are cold drawn and subsequently heat 
treated to remove residual stresses. The essential drill pipe geometry and material characteristics 
are: outside diameter 69.85 (+0.25/-0) mm; inside diameter 60.32 (-0.25/+0) mm; yield strength 620 
MPa; and ultimate tensile strength 725 MPa. Figure 1 shows the details of the tapered parallel-
threaded male (pin) and female (box) ends of the pipes. 
 
 
Figure 1: Details of tapered, parallel-threaded male (pin) and female (box) ends of NQ drill pipe 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
 Drill pipes are exposed to extremely harsh environmental and loading conditions, e.g. [10]. 
Fatigue damage is caused when the pipe is subjected to large alternating stresses, for example when 
the drill pipe rotates in a curved segment (dog-leg) of the well bore, which may be the result of 
unintentional deviations or which are necessary for directional and horizontal wells, [4, 11]. In 
addition to bending, axial loads of either tension or compression are also acting, [6]. In vertical and 
moderate directional drilling the majority of the drill string is in tension due to the weight of the 
drill string and the application of bottom hole drill collars, which are used to prevent drill string 
buckling and excessive drill string vibrations and help providing the desired compressive force on 
the drill bit, [9]. 
 
 This paper reports on the design and application of a fatigue testing facility for NQ drill pipes. A 
reverse bending load is applied using a rotating drill pipe in a four point static bend arrangement 
with the possibility to apply an additional static tension load. 
2 EXPERIMENTS 
2.1 Test Rig and Procedures 
Figure 2 shows a drawing of the four point bend test rig. The main components are four roller 
bearings, a hydraulic bending actuator, two thrust bearings made in-house, a hydraulic tension 
actuator and an AC motor controlled pulley belt transmission system.  
 
 joint 
1: roller bearing 
2: thrust bearing 
3: bending actuator 
4: tension actuator 
5: belt drive pulley 
1075 mm 
600 mm 
1 1 
2 2 
3 
4 
5 
 
Figure 2: Drawing of four point bend test rig with major system components and dimensions 
 
 The facility is designed to operate at speeds up to 1000 rpm (corresponding to 1.44 million 
cycles in 24 hours). The number of cycles are measured using a photo-transistor sensor, and rotary 
end switches monitoring the displacement of the inner roller bearings are connected to the AC 
motor controller to automatically stop the experiment if excessive deflections occur. The design 
load for bending is 32 kN, which corresponds to maximum reverse bending stress amplitudes of 261 
MPa at the outside of the drill pipe in the region of the joint. The tension actuator can create static 
loads of 12 kN enabling the superposition of static normal stresses of up to 12 MPa.  
 
 In service, a so-called make up torque is applied to the threaded joints, which has to be larger 
than the torque applied to the drill string during operation. As a result, the joints are preloaded; the 
pin in tension and the box in compression. The make up torque has a considerable effect on the 
stress distribution in the threaded joint, [12], which, in consequence, changes the fatigue behaviour 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
of the drill pipe joint. Hence, a special loading apparatus was constructed, which enables the 
application of a controlled make up torque. According to American Petroleum Institute 
recommendations, [1], the specimens were preloaded by a torque, which creates a stress level at the 
outside of the pipe of 60% of the material’s yield strength. 
 
Commissioning tests of the system were performed using drill pipes instrumented with strain 
gauges. As an example, Figure 3 shows the results of bending and axial load calibrations and 
highlights the excellent agreement between the predicted and measured axial strains. 
 
 
Tension Calibration
0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000 1200 1400
Pressure [kPa] 
A
xi
al
 S
tr
ai
n 
[m
m
/m
] 
Predicted 
Measured 
 
Bending Calibration
0
100
200
300
400
500
600
700
800
900
0 100 200 300 400 500 600
Actuator Pressure [kPa] 
Ax
ia
l S
tr
ai
n 
[µm
/m
] 
Predicted
Measured
 
Figure 3: Results of a) bending load and b) tension load calibration 
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
2.2 Fatigue Testing 
A total of 18 fatigue experiments were conducted. The test plan was established based on a standard 
method to estimate the slope section of the S-N curve, e.g.[7]. The typical service life of a NQ drill 
pipe is in the order of 3000 hours, which corresponds to 18 million cycles, if the drill string rotates 
at a speed of 100 rpm. However, not all of these cycles are to be accumulated while the joint is 
passing through doglegs where reverse bending may create fatigue damage. As a result and based 
on manufacturers specifications, 3 million cycles were defined as an initial safe life limit for NQ 
drill pipes. Table 1 shows a summary of the experimental parameters and test results. The tests were 
carried out at rotational speeds between 500 and 800 rpm.  
 
Test Maximum Stress 
[MPa] 
Mean Stress 
[MPa]
Speed 
[rpm]
Cycles Failure 
1 236 0 550 656 complete break 
2 236 0 550 4420 complete break 
3 197 0 500 2280 complete break 
4 157 12 630 8238 complete break 
5 148 0 500 13408 complete break 
6 118 12 630 46180 complete break 
7 118 0 500 20524 complete break 
8 98 12 630 133214 complete break 
9 98 12 800 112661 complete break 
10 98 12 800 279719 complete break 
11 98 12 800 129602 complete break 
12 98 0 550 204610 complete break 
13 98 0 550 178518 complete break 
14 79 12 630 424380 partial break 
15 79 12 630 389558 complete break 
16 59 12 630 1043266 partial break 
17 59 12 630 1214349 partial break 
18 39 12 630 3000000 run out 
Table 1: Summary of experimental parameters and test results 
 
3 RESULTS AND DISCUSSION 
In all cases of complete specimen failure the threaded pin joint completely broke off at the last 
engaged thread as shown in Figure 4. The crack initiated on the outside surface of the threaded 
joint, in the corner of the last engaged parallel thread as shown in Fig. 1. It was found that the crack 
consistently initiated between 90º and 180º from the end of the helical thread. The box end of the 
joint sustains no visible damage on the outside surface of the joint. To inspect the inside surface of 
the failed specimens, the box joint was sliced along the axis of the drill rod. Once again there was 
no visible damage in this section of the joint. Based on the structure of the crack surface it is 
suggested that the crack initially propagates through the thickness of the pin joint and subsequently 
follows the corner of the square thread in both circumferential directions. Once the crack has 
reached the critical length, the remainder of the helical path is closed by plastic tearing in the 
direction of maximum shear, i.e. under 45º to the longitudinal axis of the drill rod. Based on the 
results of continuous non-destructive monitoring of the drill pipe during fatigue testing using 
structural wave ultrasonic techniques, it appears that this final stage of the fatigue process occurs 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
over a very limited number of revolutions, finishing with closing the 360º crack path by bridging 
the distance between the first and second thread with a final axial separation crack. 
 
crack initiation 
rough surface, 45º 
to longitudinal axis 
final separation 
crack 
 
Figure 4: Fatigue failure surface in threaded pin joint of the specimen. 
 
 Not all tests resulted in full fracture through the pin joint. In some cases the test was terminated 
before the threaded joint had completely separated. The cause being the activation of the rotary end 
switches due to excessive deflection of the drill pipe. Once the crack had propagated to a sufficient 
circumferential length, the open crack created considerable deflection, which in turn activated the 
stopping mechanism. Three drill pipes tested at lower stress levels failed in this mode. Once the 
drill pipes were removed from the testing facility and the make up torque released, a circumferential 
crack in the pin thread was clearly visible, which extended between 70% to 80% of the length of the 
first engaged thread.  
 
 Figure 5 shows the S-N diagram produced from the data points listed in Table 1. The first thing 
to note is the relatively small number of cycles needed to create fatigue failure of NQ drill pipes 
under reverse bending loads. Even for maximum stress levels of 100 MPa, which are very moderate 
compared with the material’s strength limits, the drill pipe joint consistently fail below 300,000 
cycles. Recommendations based on earlier studies, [2, 3, 5], use maximum allowable dog-leg 
severities measured in degrees per length to define operational limits to avoid drill string fatigue 
failures. A simple relation allows to convert dog-leg severities to stress amplitudes and it turns out 
that the recommended upper limits of dog-leg severities correspond to reverse bending stress 
amplitudes of approximately 150 MPa, which is clearly above the values resulting in early fatigue 
failure in the current investigation. A possible explanation is the large stress concentration factor 
that exists in square threaded joints. Finite element analyses of tapered threaded joints show that the 
highest stress concentration is in the last engaged thread of the pin joint and a stress concentration 
factor of 15 should be included [8, 12]. If this factor is applied to the nominal stress at the outside 
surface of the drill pipe and 30 MPa are assumed to be the endurance limit for 1 107 cycles, the 
effective fatigue stress is 450 MPa, which is in good agreement with reverse bending fatigue limits 
for alloy steels with tensile strengths of approximately 700 MPa. 
 
 The limited number of tests does not allow to draw any final conclusions of the influence of a 
superimposed static tension load corresponding to the suspended weight of a 180 m long drill string 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
has no measurable effect on the fatigue life. This is expected since the corresponding mean stress of 
12 MPa is small compared to the alternating stress.  
S-N Diagram for NQ Drill Pipes
0
50
100
150
200
250
100 1000 10000 100000 1000000 10000000
Number of Cycles 
S
tre
ss
 A
m
pl
itu
de
 [M
P
a]
 
 
Figure 5: S-N diagram of NQ drill pipes 
 
4 CONCLUSIONS 
This paper has reported on the design and application of a unique fatigue testing facility for NQ 
drill pipes. The test rig allows the application of combined reverse bending and static tension loads 
to the threaded joint of NQ drill pipes. The results of the fatigue tests show that:  
• single mode of failure exists, which is in agreement with predictions based on finite element 
analysis of pre-loaded tapered threaded joints. The crack initiates at the base of the last engaged 
thread of the pin joint. Once the crack has propagated through the thickness, two cracks 
propagate in opposite directions around the circumference of the joint.  
• fatigue failure occurs at low nominal stress levels. This is due to the considerable stress 
concentration in the pre-loaded thread. The effective stress levels agree well with fatigue limits 
predicted for alloy steels with similar tensile strength properties. 
• there exists a linear relation between the nominal stress amplitude and the logarithm of the 
number of cycles to failure in a stress range between 30 MPa and 230 MPa. 
• the superposition of a static tension load corresponding to the suspended weight of a 180 m long 
drill string has, as expected, no measurable influence on the fatigue life of the drill pipe 
specimens.  
 
ACKNOWLEDGEMENT 
The authors thank Jonathan Ramm, Asanka Basnayake and Law Tian We for collecting some of the fatigue 
test data, the CRC Mining (former CRC for Mining Technology and Equipment) for providing the Master of 
Engineering scholarship for Alex Berezovski, and Barry Allsop and Bob Gammie for their technical support 
in the design of the drill pipe fatigue test rig.  
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
REFERENCES 
[1] API, Recommended Practice for Drill Stem Design and Operating Limits, American Petroleum 
Institute, 15th edition, 1995. 
[2] Chen, W.C., Drillstring fatigue performance, SPE Drilling Engineering, 5: 129-134, 1990. 
[3] Dale, B.A., An experimental investigation of fatigue crack growth in drillstring tubulars, SPE Drilling 
Engineering, 3: 356-363, 1988. 
[4] Hansford, J.E., and Lubinski, A., Cumulative fatigue damage of drill pipe in dog-legs, Journal of 
Petroleum Technology, 1966. 
[5] Lubinski, A., Maximum permissible dog-legs in rotary boreholes, Journal of Petroleum Technology, 
173-194, 1961. 
[6] Macdonald, K.A., Failure analysis of drillstring and bottom hole assembly components, Journal of 
Engineering Failure Analysis, 1: 91-117, 1944. 
[7] Nakazawa, H., and Kodama, A., Statistical S-N testing method with 14 specimens: JSME standard 
method for determination of S-N curves, in: Current Japanese Materials Research, Vol 2, Statistical 
Research on Fatigue and Fracture, 59-69, Elsevier Applied Science, 1987. 
[8] Paslay, P.R., and Cernocky, E.P., Bending stress magnification in constant curvature doglegs with 
impact on drillstring and casing, in: 66th Annual Technical Conference and Exhibition of the Society of 
Petroleum Engineers, Vol. SPE 22547, Dallas, 1991. 
[9] Payne, M.L., Duxbury, J.K., and Matin, J.W., Drillstring design options for extended-reach drilling 
operations, Drilling Technology, 65: 99-107, 1995. 
[10] Starkweather, A.W. (Ed.), Drilling: The Manual of Methods, Applications, and Management, Lewis 
Publisher, (1997). 
[11] Short, J.A., Introduction to Directional and Horizontal Drilling, Pennwell Books, Oklahoma, 1993. 
[12] Tafreshi, A., and Dover, W.D., Stress analysis of drillstring threaded connection using the finite 
element method, International Journal of Fatigue, 15: 4429-438, 1993. 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 


Design And Application Of A Drill Pipe Fatigue Test Facility

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Design And Application Of A Drill Pipe Fatigue Test Facility

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