Failure Analysis of a Komatsu Excavator´s Revolving Frame Joining Bolts

Ignacio Bezanilla, Raimundo Echeverría, Ricardo Mera, Francisco Sahli, Eugenio Zegers

Students of ICM35032 Mechanical Behavior of Materials at PUC School of Engineering

Jorge Ramos, Associate Professor Mechanical and Metallurgical Engineering Department

Lecturer of ICM35032 Mechanical Behavior of Materials

PUC School of Engineering

Santiago Chile

ABSTRACT

This paper presents the failure analysis of two joining bolt from a Komatsu excavator revolving

frame that has undergone facture by fatigue. The bolt ´s material corresponds to a quenched and

tempered 41xx steel series. The bolts we subjected to tensile and compressive loads at a rate of

82 cycles per hour for about 9500 hours before failure. The excavator bucket was exchanged to

increase its loading bucket volumetric capacity from 1,6 to 2,1 m3 in order to remove a higher

amount of mineral. This decision made the revolving frame to experience a larger cyclic

momentum that increased the stress amplitude over the bolts making them more likely to fail by

crack growth. Numerical analysis done using ANSYS 11 and Beasy v10.r12 supported the assumed

fatigue failure hypothesis.

Introduction

Failure by low cycle fatigue is not an uncommon event to see happening in engineering practice,

nonetheless careful design of mechanical components, especially of sophisticated equipment such

as excavators, is conducted by the manufacturers as to avoid such unwanted events. However, if

nominal operation ranges are over exceeded by the end user, then fatigue events may be

activated finalizing in catastrophic failure of the component.

In this paper we present the failure analysis of two bolts that join the revolving frame of a

Komatsu PC 450-7 excavator to the platform of the track rollers. The joining bolts are designed to

work under tensile-compressive stress cycles. However, exchange of the bucket size from 1,6 to

2,1 m3 overloaded the revolving frame structure above its admissible load levels. Figure 1 shows a

photograph of the Komatzu 450-7 excavator in field operation. The size of the base of the

revolving frame is 1,6 meters in diameter.

Figure 1. Image of the Komatzu 450-7 excavation in field operation.

The bolts failed by progressive fracture after about 9500 hours of operation under this overloaded

condition. Figure 2 illustrates a schematic diagram of the excavator with its arm fully extended,

reaching a length of 12 meters. At the bucket (2,1 m3), the maximum load, when fully loaded with

gravel, corresponds to approximately 4,2 tons. Therefore, a maximum flexion moment over the

revolving frame and thus over the joining bolts corresponds to 50,1 ton-m. In total there are 32

bolts joining the revolving frame to the track rollers, half of them work under compression loads

and the other have under tensile loads. It is assumed that all bolts take on the same load

magnitude.

Figure 2. Diagram of excavator dimensions and maximum work load: A: 12 m C: 4,2 ton. A two dimensional simplified freebody diagram of the excavator arm and revolving frame is shown

in Figure 3. Notice that the load distribution at the revolving frame is both tensile and compressive

and assumed to be of the same magnitude.

Figure 3. Freebody Diagram of crane arm fully extended

Failure Hypothesis

It is estimated that the bolts have experienced an average of 82 load cycles per hour, failing after

780000 cycles. The failure hypothesis corresponds to low cycle fatigue, as beach marks can be

observed over the fractured surface as shown in Figure 4, discarding the possibility of failure by

static load only. The bolt material integrity is also sound without presence of defects. Fracture

occured at the thread of the bolt starting inside its trough.

Figure 4. Fractured bolts, showing fracture surface with beach marks as well as some percent of

plastic deformation.

In Figure 5 one can observed a martensitic structure with some small amounts of ferritic grains,

signaling that the material was quenched to increase its hardness and strength. Tempering process

was done after quenching in order to increase its toughness.

Figure 5. Microstructure of the bolt material a) no etching, showing metallic inclusions type 4-B

and 4-D mostly oxides b) Nital etching, corresponding to tempered martensite, as fine cementite

carbides have precipitated at the martensite grains.

Chemical composition of the bolt material was obtained by EDS analysis and is shown in Table 1,

indicating that the bolt material corresponded to a Cr-Mn steel, from the 41xx series family.

Chromium in this case helps preventing corrosion while Mn increases strength. On the other hand,

Table 2 shows the results from the uniaxial tensile test. The ultimate tensile strength is slightly

higher than that of martensitic carbon steel.

Table 1. Chemical composition of the bolt material

Element % Weight

Cr 0,82

Mn 1,24

Fe 97,93

Table 2. Uniaxial tensile test results

Maximum tensile force (N) 39840

Maximum displacement (mm) 0,2

% Deformation 7,98

Ultimate tensile strength (MPa) 1278

Tensile (MPa) Ref. Martensite 1100

Figure 5 illustrates a SEM micrographs of the fracture surface at low magnification (40x) indicating

the initiation crack zone at the bottom of the thread, corresponding to a sub millimeter crack size.

Figure 5: SEM micrograph indicating the location where the crack nucleated and started to grow.

Figure 6 illustrates SEM micrographs of the fracture surface at both high magnification (500x)

signaling that plastic deformation has occurred at the surface, and at low magnification (40x)

indicating the presence of beach marks spaced every 1 millimeter.

Figure 6: a) SEM image of a torn zone at the fracture region showing an irregular and plastically

deformed surface at 500x magnification; b) SEM image of beach marks over the fractured surface

at 40x magnification.

Table 3 presents two Charpy impact tests values obtained on each of the two bolts bulk material,

indicating slightly lower energy dissipation than in pure martensite.

Table 3: Charpy impact tests results on each bolt in kg-m and martensite reference value

Bolt 1 Bolt 2 Martensite

4,3 4,0 5,5

4,3 4,3

Linear Elastic FE modelling of Bolts

A 3D FE model done using ANSYS Workbench 11 was done on the geometry of the bolt including it

threaded feature. An static tensile load of 33000 N (corresponding to 105 MPa) was applied at the

bolt head corresponding to the load from a full 2,1 m3 bucket. In the case of a 1,6 m3 bucket the

applied static load at the bolt head was 23000 N. Figure 8 shows the minimum safety factor values

under both loading conditions, of 3,29 and 2,38 respectively. However, at the trough of the thread

the safety factors reduces to a minimum of 0,82 after a Goodman fatigue analysis and the bolt

would fail at 323000 cycles under such scenario.

(a) (b) (c)

Figure 8. FEM linear elastic results of the bolt. (a) Safety factor for 2,1 m3 capacity bucket (b) safety

factor for 1,6 m3 capacity bucket (c) Safety factor after Goodman fatigue analysis.

Fatigue Analysis by Paris Law

A standard crack growth analysis using Paris Law was carried out, considering in this case a

constant average geometric factor. An failure crack size of cf = 14,3 mm was measured from beach

marks observation at the fracture surface, thus the average geometric factor was computed as

.

The estimated number of fatigue cycles corresponded to 780000 and the maximum tensile

amplitude max = 105 MPa was considered for an alternating tension-compression load. Paris

parameters used accounted to m = 3,4 and A = 2,8 x 10-12. A fracture toughness value under plane

strain conditions of KIc = 100 MPa*m0.5 was also selected. From this analysis an initial crack size of

0.0015 mm would have been enough to propagate the fracture front up to its critical size when

using the 2,1 m3 capacity bucket.

Fracture Analysis using Beasy

A 2D model of the bolt in cross section was implemented in Beasy v10r12 software as seen in

Figure 9. This in order to calculate crack growth propagation, stress intensity function (SIF) as well

as crack growth values all versus number of loading cycles.

Bucket 1,6 cubics

Min=3,29 Min=2,38

Bucket 2,1 cubics Min=0,82

Figure 9. 2D model of bolt implemented in Beasy v10.12r including boundary

conditions.

The von Misses stress analysis gave a distribution shown in Figure 10 after an applied load of 3000

N (ten times lower that the predicted maximum load of 33000 N), this considering an elastic

modulus E of 200 GPa and a Poisson ratio of 0,3.

Figure 10. Von Misses stress distribution over the bolt cross section for a 3000 N load.

Using fatigue properties from NASGRO database available in version 12.8r, specifically:

Threshold SIF: 190 MPa-mm0.5 and K1C: 3800 MPa mm0.5 and introducing a line crack at the

trough of the thread of 0,1 mm, Beasy is able to determine a crack growth path as illustrated

in Figure 11.

Figure 11. Crack initiation and propagation after 60000 cycles at a load of only 3000 N with a

initial crack of length of 0,1 mm and a failure crack of length 3,8 mm.

Figure 12 illustrates the calculated SIF as a function of number of cycles to failure, the

increasing value is in correct agreement to what is expected, as crack size also increases

almost exponentially with cycle number to failure as seen in Figure 13. Nonetheless, the

failure crack size predicted by Beasy of 3,8 mm is lower than the actual 14 mm failure crack.

Figure 12. SIF as a function of loading cycles.

Figure 13. Crack size growth versus number of cycles to failure.

0

500

1000

1500

2000

2500

3000

3500

4000

000 10.000 20.000 30.000 40.000 50.000 60.000 70.000

SIF

(M

Pa m

m^

1/2

)

Cycles

SIF vs. Cycles

0

1

1

2

2

3

3

4

4

5

00 10.000 20.000 30.000 40.000 50.000 60.000 70.000

Cra

ck s

ize (

mm

)

Cycle

Crack Size vs. Cycles

From, the latter results, a 2D Beasy model of the bolt predicts failure at a load 10 times lower than

the estimated maximum load of 33000 N when an initial crack is located inside the trough of the

bolts thread, reaching the critical crack size (3,8 mm) only at 60000 cycles versus the 300000 cycles

predicted by ANSYS fatigue analysis. These life cycle results underestimate the 780000 cycles that

actually took place after failure. The actual crack path predicted by Beasy is not exactly identical to

the real path that took place on the bolt.

Conclusions

There is no integrity problem from the actual material and fabrication process of the bolts.

The overload caused by an increase in bucket capacity was the main reason for the catastrophic

failure of the two bolts.

Beach marks observed at the fracture surface of the bolts indicate that a low cycle fatigue process

took place with a critical crack size of 14 mm.

3D numerical analysis using ANSYS FEM showed that under a 33000 N load, fatigue would occur

inside the trough of the bolts thread (lowest safety factor below 1).

2D numerical analysis of the bolts using Beasy showed that under a 3000 N load and an initial

crack at the trough of the bolts threads would be enough conditions to initiate a crack growth

propagation through the bolts bulk, reaching a critical size of 3,8 mm.

2D numerical analysis of Beasy predicts a total number of cycles to failure of 60000 cycles against

the 780000 cycles that actually took place before the bolts fractured.