Simulation of Impact on Pneumatic

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Journal of Earth Science, Vol. 20,No. 5,p. 868878,October 2009 ISSN 1674-487X

Printed in ChinaDOI: 10.1007/s12583-009-0073-5

Numerical Simulation of Impact on PneumaticDTH Hammer Percussive Drilling

Bu Changgen* (), Qu Yegao(), Cheng Zhiqiang (), Liu Baolin()

School of Engineering and Technology,China University of Geosciences,Beijing 100083,China

ABSTRACT: The process of DTH (down-the-hole) hammer drilling has been characterized as a very

complex phenomenon due to its high nonlinearity, large deformation and damage behaviors. Taking

brittle materials (concrete, granite and sandstone) as impact specimens, the explicit time integrationnonlinear finite element code LS-DYNA was employed to analyze the impact process and the penetra-

tion boundary conditions of DTH hammer percussive drilling system. Compared with previous studies,

the present model contains several new features. One is that the 3D effects of DTH hammer drilling

system were considered. Another important feature is that it took the coupling effects of brittle materi-

als into account to the bit-specimen boundary of the drilling system. This distinguishes it from the tra-

ditional approaches to the bit-rock intersection, in which nonlinear spring models are usually imposed.

The impact forces, bit insert penetrations and force-penetration curves of concrete, granite and sand-

stone under DTH hammer impact have been recorded; the formation of craters and fractures has been

also investigated. The impact loads of piston-bit interaction appear to be relatively sensitive to piston

impact velocity. The impact between piston-bit interaction occurs at two times larger forces, whereasthe duration of the first impact doesnt change with respect to the piston velocity. The material proper-

ties of impact specimen do not affect the first impact process between the piston and bit. However, the

period between the two impacts and the magnitudes of the second impact forces greatly depend on the

specimen material properties. It is found that the penetration depth of specimen is dependent on the

impact force magnitude and the macro-mechanical properties of the brittle materials.

KEY WORDS: pneumatic DTH hammer, percussive drilling, LS-DYNA, brittle material, impact

force-penetration curve.

INTRODUCTION

Pneumatic down-the-hole (DTH) hammer drill-ing is a rotary percussive drilling technique widely

used in mining, exploration, water-well drilling, road

construction, and other drilling operations around the

world (Bu et al., 2006; Karanam and Misra, 1998).

This study was supported by the National Natural Science

Foundation of China (No. 50475056).

*Corresponding author: [email protected]

Manuscript received February 2, 2009.Manuscript accepted June 22, 2009.

When the DTH hammer works, it generates percussive

force to the bit to impact and shatter the ground andthe rotational torque rotates it to tear and cut the frag-

ments whilst the thrust force keeps it in contact with

the ground during bit advancement. In the meantime,

the drill cuttings and detritus in the form of fine parti-

cles and dust are brought from the hole to the ground

surface via an air flushing medium as shown in Fig. 1.

This drilling technique has a major advantage in that it

can rapidly and economically produce holes in hard

rocks for various construction and mining purposes.

In the pneumatic DTH hammer, a piston movingwith speed v0collides with a drill bit. A stress wave


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Numerical Simulation of Impact in Pneumatic DTH Hammer Percussive Drilling 869

Figure 1. Typical structure of pneumatic DTHdrilling system.

then begins to propagate through the drill bit towards

the rock and backwards through the piston from the

impact plane. The front end of the stress wave eventu-

ally reaches the rock interface, where the tungsten

carbide inserts mounted on the drill bit surface gener-

ate high point stresses. Depending on the drilling abil-

ity of the rock, a certain amount of energy will be dis-

sipated by the rock fragmentation. The remaining en-ergy will be distributed among the piston, drill bit and

other DTH hammer components according to their

mass, stiffness and geometric properties. How the

wave propagates in the piston, bit, rock and other

components is of paramount importance in the impact

process of the DTH hammer percussive drilling.

The mechanics of percussive drilling has been

analyzed numerically and experimentally since the

early 1960s.The pioneering works on theoretical and

experimental studies on the percussive drilling of rockwere done by Hustrulid and Fairhurst (Hustrulid and

Fairhurst, 1972a, b, 1971a, b; Fairhurst, 1961). They

investigated in detail the energy transfer in percussive

drilling, and thrust force requirements and some

comments were done for the design of percussive

drilling systems. Lundberg (1985, 1982, 1973a, b) set

up the stress wave equations for the case of

top-of-the-hole rock drilling in which a short piston

strikes a long bar containing different cutter shapes,

and carried out detailed investigations on stress wave

mechanics of percussive drilling and developed a mi-

crocomputer simulation program. Microcomputer

simulation studies (Lundberg, 1985, 1982) of percus-

sive drilling systems have shown that the predicted

values of impact stress, coefficient of hammer restitu-

tion and forces acting on the rock agree well with

theoretical results. A similar approach was adopted by

Stock and Schad (1992) to estimate the stresses at the

interface between the tungsten-carbide inserts and the

drill-bit body. Nordlund (1989) also studied the effects

of the thrust force on percussive drilling using ex-

perimental data and Lundbergs method. Chiang and

Elas (2000) developed a different method to solve the

impact of percussive drilling in terms of the impulse-

momentum principle, in which the solid bodies in

percussive drilling system were discretized into nodes

and elements, and the corresponding impulse momen-

tum equations were applied iteratively assuming that a

wave travels at the speed of sound in the medium.

Generally, the previous published works on per-

cussive drilling system impact were based upon the

solutions of the stress wave equation or the linear im-

pulse momentum principle. In these works, the

rock-bit interaction is usually modeled by a nonlinear

spring, while the piston and the bit are often simplified

as straight or cone-shaped bars. In fact, these simula-tion methods make use of a force-penetration curve to

model the rock-bit interaction, therefore, the validity

of the simulation results greatly depends on the avail-

ability and accuracy of this curve (Chiang, 2004). For

this reason, many researchers have been active in de-

veloping better and simpler methods to obtain accu-

rate force-penetration curves in rocks and other mate-

rials (Chiang, 2004; Carlsson et al., 1990; Pang et al.,

1989).

Actually, the piston and the bit in DTH hammerare usually thick and short, and have complex geo-


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Bu Changgen, Qu Yegao, Cheng Zhiqiang and Liu Baolin870

metric shapes. These simplifications mentioned above

in 1D elastic stress wave models or impulse momen-

tum equations may bring great error to the results as

they ignore the wave propagation attenuation and dis-

persion in piston and bit due to the radial inertia ef-

fects. Based on 3D axisymmetric finite element

method, Lundberg and Okrouhlik (2006, 2001) inves-

tigated the 3D effects on the efficiency of DTH ham-

mer drilling process. Chiang and Elas (2008) devel-

oped a more sophisticated finite element model to

simulate the energy transmission, the bit-rock interac-

tion, and the process of rock fragmentation in percus-

sive drilling. However, the effects on wave reflection

of local structures like spline and air slot are neglected

in these models. Furthermore, it should be pointed out

that bit-rock interaction conducted as a nonlinear

spring is a quasi-static method based on the measure-

ment of the penetration force on the rock, and can be

used as just an approximation for those bits with one

or two inserts. For bits with multi-inserts in DTH

hammers, the nonlinear spring parameters are difficult

to be obtained. Hence, the 1D wave model is restricted

to the DTH hammer and has some limitations. There-

fore, it is of great importance and interest to study the

percussive drilling process of DTH hammer in order

to achieve a better understanding of the percussive

drilling mechanism.

Taking brittle materials (concrete, granite and

sandstone) as impact specimens, this work employs

the explicit time integration nonlinear finite element

code LS-DYNA to analyze the impact and penetration

boundary conditions in pneumatic DTH hammer per-

cussive drilling system. The force-penetration curves

of concrete, granite and sandstone under DTH ham-

mer impact have been recorded and the formation ofthe craters and fractures has been investigated. The

simulated force-penetration curve is in fact the indica-

tion of the propagation of cracks and the formation of

chips. According to the simulated results, it is believed

that this numerical simulation method will contribute

to an improved knowledge of the rock fragmentation

process in DTH hammer drilling, which will in turn

help enhance mining and drilling efficiency through

the improved design of percussive drilling tools and

equipment.

THEORETICAL BACKGROUND AND

METHOD FORMULATION

LS-DYNA is used in the simulation of DTH

hammer percussive drilling system in the present

study. This computer code performs nonlinear tran-

sient dynamic analysis of three-dimensional structures.

LS-DYNA has a wide variety of analysis capabilities

including a large number of material models, a variety

of contact modeling options, a library of beam, plate,

shell, and solid elements and robust algorithms for

adaptively controlling the solution process (Hallquist,

2003). In the solution process, stress wave propaga-

tion and inertia effect are considered.Its principle al-

gorithm adopts Lagrangian formulation.

When a piston impacts a bit on a DTH hammer,

their contact is assumed to have no friction. The gov-

erning equations for both bodies are the following.

Equation of mass conservation

V=0 (1)

where Vrepresents the relative volume; denotes the

current density; and 0denotes the reference density.

Equation of momentum conservation

,ij j i if u + = (2)

where ij,j represents the Cauchy stress; fi represents

the body force density; and idenotes the acceleration.Equation of energy conservation

( )ijs ij

E V p q V= + (3)

where ij and p denote the deviatoric stresses and

hydrostatic pressure, respectively, as given in

( )ij ij ijs p q = + + (4)

where q represents the bulk viscosity; ijdenotes the

Kronecker delta (ij=1, if i=j; otherwise ij=0); and

ij denotes the strain rate tensor

1 1

3 3ij ij kk p q q = =

(5)

Based on the virtual work principle, equation (2) can

be expressed as a weak form of equilibrium equation

( ) ( )

( )

, d + d +

d = 0

i ij j i ij j i iv

ij ij j i

u f u v n t u s

n u s

+

where uifulfills all boundary conditions, and the in-

tegrations are over the current geometry. Application

of the divergence theorem gives

( )

( )

, d d

d

ij i j ij j iv

ij ij j i

u v n u s

n u s

+

= +

(7)

(6)


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specimen behavior under DTH hammer impact.

Figure 4 illustrates a general overview of the

Johnson-Holmsquist Concrete mode. The equivalent

strength component of the model is given by

( )

( )

* * *1 1 ln

NA D BP C = + +

(11)

The normalized equivalent stress is given by

*=/f C, where represents the actual equivalent

stress; and fCdenotes the quasi-static uniaxial com-

pressive strength;P*denotes the normalized pressure,

shown as P*=P/fC;

* denotes the dimensionless

strain rate, given by * 0 = ; represents the ac-

tual strain rate; 10 1.0s

= represents the reference

strain rate;D(0D1) denotes the damage parameter,

and the normalized largest tensile strength is given by

T*

=T/f C, where T represents the maximum tensilestress. Additionally A, B, N, C, and Smax denote the

material parameters, respectively as normalized cohe-

sive strength, normalized pressure hardening coeffi-

cient, pressure hardening exponent, strain rate coeffi-

cient and normalized maximum strength.

Figure 4. Diagram showing the equivalent strengthmodel.

Accumulated Damage Failure Model

The accumulated damage failure model for brittle

specimen is illustrated in Fig. 5. The Johnson-

Holmquist concrete model considered, owing to plas-

tic volumetric strain. The damage model is written as

p

f f

p p

D

+ =

+ (12)

where p and p represent the equivalent plastic

Figure 5. Diagram showing damage failure model

of brittle specimen. EFmin. minimum plastic strainof material fracture.

strain increment and plastic volumetric strain incre-

ment, respectively, during one cycle of integral com-

putation. The equation

( )2f f * *

p p p 1= + = +D

f D P T (13)

represents the plastic strain to fracture under a con-

stant pressure, where D1 and D2 represent damage

constants.

Equation of State (EOS)

EOS describes the relationship between hydro-

static pressure and volume. The loading and unloading

process of brittle specimen can be divided into three

response regions, as depicted in Fig. 6. The first zone

is the linear elastic zone, arising atPPcrush, where the

material is in elastic state. The elastic bulk modulus is

given by k=Pcrush/crush, where Pcrush and crush repre-

sent the pressure and volumetric strain arising in auniaxial compression test. Within the elastic zone, the

loading and unloading equation of state is given by

P=k (14)

where =/01; denotes the current density; and 0

denotes the reference density. The second zone arises

at Pcrush


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Figure 6. Diagram showing equation of state of

brittle specimen.

material. The brittle specimen has no air voids, and

thus fulfills the condensed material Hugoniot rela-

tionship. The pressure and the volumetric strain rela-

tionship is given by2 3

1 2 3P K K K = + + (15)

where ( ) ( )lock lock 1 = + represents the cor-rected volumetric strain; andK1,K2,K3are constants.

The tensile pressure is restricted to T(1D). To iden-

tify each material parameter in the constitutive law,

the tri-axial compression and high strain rate dynamic

tests must be performed on the brittle specimens. This

derives the brittle specimens EOS and the material

strength parameters.

In this analysis, the material parameters of brittle

specimens are presented in Table 2.

The finite element mesh schemes of the piston

mass, bit and impact specimens (concrete, granite and

sandstone) are shown in Figs. 7a7c, respectively. The

element type SOLID 164 (8-node hexahedron element)

in LS-DYNA is used in meshing. There are 10 332

elements and 14 280 nodes in the piston model,

62 542 elements and 28 464 nodes in the bit model. As

the impact specimen elements may fail during the

DTH hammer impact process, a finer mesh in some

specific regions of the impact specimen surface is

needed. Consequently, in this analysis, the impact

specimen is discretized into 478 652 elements and

308 464 nodes by SOLID 164 element.

Table 2 Material parameters of brittle specimens for the

DTH hammer impact analysis

Brittle specimenMaterial

constants

Material

parameters Concrete Sandstone Granite

A 0.79 0.79 0.79

B 1.60 1.60 1.60

N 0.60 0.60 0.60

C 0.007 0.007 0.007

Smax 7.00 7.00 7.00

Strength

constants

G(GPa) 14.86 31.14 34.13

D1 0.04 0.044 0.046

D2 1.00 1.00 1.00

Damage

constants

(pf+p

f)m 0.01 0.01 0.01

K1(GPa) 85 85 85

K2(GPa) -171 -171 -171

K3(GPa) 208 208 208

Pcrush(GPa) 0.016 0.034 0.05

Plock(GPa) 0.80 0.80 0.80

EOS

constants

crush 0.001 0.001 3 0.001

Figure 7. Finite element mesh model of JW150

DTH hammer percussive drilling system. (a) Finite

element mesh model of piston; (b) finite element

mesh model of bit and bit inserts; (c) finite element

mesh model of brittle specimen.


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Contact interfaces modeling

The contact interface type automatic surface to

surface is used to calculate the contact between the

piston and the bit. And an eroding contact algorithm

provided within LS-DYNA together with the inte-

grated failure model was used to simulate the impact

and penetration between the bit inserts and impact

area of the brittle specimens, in which way all failed

concrete elements are deleted and contact surfaces can

be automatically updated to the next layer of specimen

elements.

Initial condition and boundary condition

The piston mass is given an initial velocity v0

along the global coordinatey. All the nodes of bottom

surface of the impact specimen are set to zero dis-

placement constraints. In order to prevent artificial

stress wave reflections generated at the concrete

boundaries form reentering the model and contami-

nating the results, non-reflecting boundaries are used

on the exterior boundaries of the impact specimen

model, in which way the specimen model can be

treated as a half-space infinite domain.

RESULTS AND DISCUSSION

Wave Propagation and Crush Zone Formation in

Impact Specimen

The numerical analysis of the impact stage pro-

vides significant kinetic information of the penetration

process. Values for impact force, penetration velocity

and displacement of the bit inserts and for other rele-

vant physical and geometric quantities are provided at

each time increment.

The sandstone specimen is selected for numerical

modeling and simulation under the piston impact ve-

locity of 7.2 m/s. The time-history processes of impact

wave propagation in piston, bit, sandstone and sand-

stone crush zone formation are depicted in Fig. 8. Re-

ferring to Fig. 8, when the impact charge is initiated,

an elastic-plastic wave propagates in the piston and bit

outward from the surface of the initiation. However,

the sandstone surface is not immediately high stressed


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as the wave needs a little time to reach the contact

surface between inserts and sandstone.

When the impact waves reach the contact sur-

faces of inserts and sandstone, the bit acts on the

sandstone, and 18 high stress zones (10 bottom zones,

8 side zones), which correspond to the highlight zones

in Fig. 8, appear immediately beneath the tungsten

carbide inserts of the bit. Stress fields are radiated

outside the highly stressed zones and the stresses de-

crease rapidly with increasing distance from the

insert-sandstone contact points.

It is interesting to find that, although the sand-

stone immediately beneath the inserts is highly

stressed, its element does not fail primarily because of

the high confining pressure. And elastic-plastic zones

can just be seen from the surface of sandstone, seen in

Fig. 8g. As the stress intensity builds up with an in-

creasing impact load, the sandstone elements immedi-

ately beneath the inserts fail. The crushed zones

gradually come into being as some elements in the

high confining pressure zone fail. And big fragmenta-

tion zones on sandstone surface can be seen in Figs.

8h and 8i, respectively. Underneath the bit insert there

are three zones, i.e., a zone of disintegrated and partly

compacted sandstone fragments, a cracked zone and a

crushed zone.

Velocity Effects on Impact of Percussive Drilling

Process

The corresponding time histories of impact forces

are plotted in Figs. 9 and 10 for the sandstone speci-

men subjected to various piston impact velocities, i.e.,

6.5, 7.2, and 8.0 m/s, respectively.

The figures indicate that the impact loads of pis-

ton and bit appear to be relatively sensitive to pistonimpact velocity and the forces become larger as the

impact velocities increase. According to Fig. 9, it can

be found that the impact between piston and bit oc-

curred at two times larger forces, whereas the duration

of the first impact didnt change with respect to the

piston velocity. This can be explained by the fact that

the piston separates from the drill bit before the sand-

stone can reflect any stress wave back into the piston.

Another effect of the piston impact velocity can be

observed by varying the impact forces. The periodbetween the two impact times becomes shorter as the

Figure 9. The variation of impact force between

piston-bit interaction under different impact ve-

locities (sandstone).

Figure 10. Impact force of bit-sandstone interac-

tion under different impact velocities.

piston velocity increased.

As depicted in Fig. 10, all impact force history

curves show a sudden drop at one particular point,

which indicates the onset of sandstone fracture. It is

worthy noting that the fracture of the sandstone oc-

curred very suddenly during bit impact and resulted inmultiple crushed zones.

The impact force-penetration response is closely

related to the fractures in the sandstone induced by the

bit inserts. Figure 11 shows the simulated force pene-

tration curve and associated characteristics during the

sandstone fragmentation process induced by the DTH

hammer bit inserts. The area underneath the impact

force-penetration curve represents the impact energy

absorbed by the sandstone specimen.

As shown in Fig. 11, the force-penetration curvehas almost a linear shape in the initial loading stage,


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Figure 11. Force-penetration curves of sandstone

under different piston velocities.

i.e., the curve between point A and point B, which is

the linear elastic-plastic deformation stage of the im-

pact load. And little damage to the sandstone occurs in

this period.

With the increasing impact force, the

force-penetration curve attains its first peak value:

point B. As the impact loading increases, some of the

elements in the high confining pressure zone immedi-

ately fail beneath the inserts. Meantime, the first main

large chip occurs. As the elements immediately be-

neath the bit inserts fail, the bit inserts almost becomefree of constraints and its supporting forces from the

sandstone become weak. Therefore, the force-

penetration curve falls off to its trough at point C,

where the crushed zone comes into being. As the in-

sert displacement increases, these inserts reach the

new layers of sandstone elements. In this case

re-compaction behavior occurs in the crushed zone

immediately beneath the bit inserts, and the impact

force-displacement curve climbs, as illustrated in the

curve points form C and D.The large drop in the impact force occurs after

point D, and meantime the displacements of bit inserts

decrease, which indicates that a substantial part of the

sandstone has now been unloaded. The phenomenon

takes place because most of the impact energy of the

bit is released and eventually dissipated by sandstone

fracturing. However, the bit will bounce at a very low

velocity from the impact surface of sandstone as some

remaining kinetic energy still exists in the bit.

Specimen Material Effects on Impact of Percussive

Drilling

Figures 12 and 13 show the time histories of im-

pact forces when the bit impacts the concrete, granite

and sandstone with the piston velocity of 7.2 m/s,

demonstrating the differences in DTH hammer per-

cussive drilling process.

It can be seen from Fig. 12 that the concrete,

granite and sandstone predict almost the same results

for the impact forces between piston-bit interaction

before t=0.000 25 s. This means that the materials of

impact specimen do not affect the first impact process

between piston-bit interactions. However, the period

between the two times impacts and the magnitudes of

the second impact forces greatly depend on the speci-

men materials.

Based on the analysis, the impact force between

the granite specimen and bit interaction is clearly

higher than those of concrete and sandstone, as illus-

trated in Fig. 13. For the granite specimen, the magni-

tude of bit impact force is 691 939 N. However, for

the concrete and sandstone specimen, the magnitudes

of the bit impact forces are 459 621 and 290 085 N,

respectively. One can notice that when there is a larger

impact force of bit, then the shorter impact duration

between bit and specimen is. However, compared with

the area under the force-penetration curve, shown in

Fig. 14, which represents the energy absorption of the

specimens during the impact process, the concrete,

granite, and sandstone absorb similar energy quanti-

ties under the same impact velocity of 7.2 m/s. The

values of energy absorption of the concrete, sandstone

and granite under piston impact velocity of 7.2 m/s are

358, 357 and 333 J, respectively.

The recorded force-penetration curves associatedwith the fragmentation process of the concrete, granite

and sandstone specimen under impact velocity of 7.2

m/s are depicted in Fig. 14. In fact, the force-

penetration curves not only reflect energy absorption

by the impact specimens, but also indicate the propa-

gation of cracks, the crushing of micro-structural

grains and the formation of chips.

From the results, it can be seen that the

force-penetration curve of granite (triangular envelope


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loop) is very sharp due to granites hard properties,

and bit inserts generate small penetration depth in this


piston-bit interaction (different specimens: con-

crete, granite and sandstone).


bit-specimen interaction (different specimens: con-

crete, granite and sandstone).

Figure 14. Force-penetration curve (different im-

pact specimens: concrete, granite and sandstone).

case. When bit inserts impact the granite specimen,

they meet great stiffness support and the impact load-

ing increases immediately to a high level, and then the

elements in the high confining pressure zone immedi-

ately fail beneath the inserts. However, for those soft

specimens like sandstone and concrete, bit inserts

generate large penetration depth and the impact forces

seem to be very smooth. From the above-described

simulation results, it is easy to understand that the

penetration depth is dependent on the impact force

magnitude and the macro-mechanical properties of the

brittle materials.

CONCLUSIONS

The finite element program LS-DYNA was

adopted to study the impact performance of DTH

hammer percussive drilling process. The influencing

factors like piston impact velocity and specimen ma-

terial that affect DTH hammer impact process were

investigated. The physical process such as impact

penetration of specimen subjected to DTH hammer

impact was displayed visually.

It can be concluded from the simulation results

that the impact forces of piston-bit interaction appear

to be relatively sensitive to piston impact velocity and

the impact force becomes larger as the impact velocity

increases. The impact between piston-bit interactions

occurs at a two times larger force, whereas the dura-

tion of the first impact doesnt change with respect to

the piston velocity. In addition, the envelope loop

force-penetration curve of granite is very sharp due to

granites hard properties, and in this case the bit in-

serts generate small penetration depths. However, for

those soft specimens such as sandstone and concrete,

the bit inserts generate large penetration depth and theimpact forces seem to be very smooth.The simulated

force-penetration curve is in fact an indication of the

propagation of cracks and the formation of chips in

the specimen.

According to the simulated results, it is believed

that this numerical simulation method will contribute

to an improved knowledge of the specimen fragmen-

tation process under DTH hammer impact, which will

in turn help to enhance mining and drilling efficiency

through the improved design of DTH hammers andother percussive drilling tools.


11/11


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