2005 wagnoner buttlar paulino disk-shaped compact tension test

8/11/2019 2005 Wagnoner Buttlar Paulino Disk-shaped Compact Tension Test

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Fig. 1--Different fracture specimen geometries for asphalt

concrete:

a) Single-edge notched beam; 3,6-9 b) Semi-

circular bend; ]~ ]] c) Disk-sh aped comp act tension

Fig. 2--Example of a core obtained at a reflective crack

location in a pavemen t structure

current work is motivated by the need to characterize the

fracture behavior of asphalt concrete fro m standard cyl indri-

cal f ield and laboratory specim ens.

One prospect ive fracture test that can be performed us-

ing cyl indrical samples is the semi-circular bend (SCB) test

(Fig. l (b)) . T he SCB test has been used to obtain the fracture

toughness of asphalt concrete for use in est imating fat igue

cracking l~ and for invest igat ing the frac ture energy of as-

phal t concrete at low temperatures . l l

One concern wi th the SCB tes t a r rangement fo r asphal t

coi 'icrete is that the 150 mm diame ter of a s tandard laboratory-

compacted asphalt specimen leads to a relat ively short frac-

ture l igame nt i n the SC B specimen . Without conclusive e v-

idence of the s ize of the fracture process zone '4 of asphalt

concrete (the relat ively large grain s ize of the aggregates is

apparent in Fig. 2), the ini t ial l igament length should be as

large as possible to produce a rel iable fracture property and

fracture test . Also, the crack propagation with the SCB ge-

ome try creates an arching effect with high com pressive s tress

as the crack approaches the top edge. The high compressive

stresses could arrest the crack, which further reduces the ef-

fect ive l igament, creat ing potent ial ly inval id test ing results .

Other geometries , such as notched direct tension, 12 have

been used for obtaining fracture propert ies of asphalt con-

crete, but these geo metries have the sa me l imitat ion of being

restricted to non-standard labo ratory compactio n procedures.

Therefo re , the deve lopment o f a d i sk -shaped compact t ens ion

test (Fig. l(c)) was pursued in this investigation.

Disk shaped Com pact Tension Geome try for

Asphalt Concrete

The A ST M E399 Standard Test Metho d 13 describes the

D

W

a

d

C

R e c o m m e n d e d

D i m e n s i o n s m m )

150

l l 0

25

27.5

25

35

L _

ff

L

I +el -

i s

i.~-:-~

dd

Fig. 3~Re comm ende d DC T) geometry for asphalt

concrete

DC(T) specimen for obtaining the plane-strain fracture

toughness o f metal l ic specimens. A compa ct tension test has

previously been used to obtain the fracture toughness of as-

phal t concrete; 14 however, the spec imen geom etry w as o f the

standard rectangular-shaped form , which could not be easi ly

fabricated from field cores or s tandard laboratory com paction

methods.

The AS TM E399 specificat ion was used as a s tart ing point

in this s tudy for the developmen t of a DC(T) specime n geom -

etry for fracture test ing of asphalt concrete. A fter several i ter-

at ions in the laboratory, a specime n geome try that max imize s

l igamen t length but prevents specim en rupture at the loading

holes was developed. The resul t ing spec imen dimensions are

given in Table 1 along with the AST M E399 dim ensions. The

proposed specimen geometry is schematical ly displayed in

Fig. 3.

The d imens ions o f the p roposed DC(T) spec imen fo r as -

phal t concrete are within the tolerances given in the ASTM

E399 specificat ion, with the exception of the locat ion of load-

ing holes. During the develop mental s tages of the DC(T ) ge-

ometry, the loading holes were placed closer to the edges of

the specime n to create a longer l igament length. However, th is

geomet ry had a t endency to fa i l (45 0% of the spec imens) a t

the loading holes (see Fig. 4(a)) due to the geometrical con -

figurat ion with insufficient material betwe en holes and outer

boundaries . The loading holes were m oved towards the center

of the specimen to ensure sufficient material was present and

to reduce the chance of the specimen fai l ing at the loading

9 Society for Experimental Mechanics Experimental Mechanics 9

271


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Fig. 4--Crack growth in the initial and final specimen geom-

etry (a) init ial specimen geom etry with fai lure at the loading

points; (b) final specimen geometry with desirable fracture

plane

T BLE

1--COMPARISON OF DC(T) SPECIMEN DIMEN-

SIONS GIVEN IN ASTM E399 AND THOSE RECOM-

MENDED FOR ASPHALT CONCRETE

Dimensions in mm

Recommended for

ASTM E399 Asphalt Concrete*

D 150 150

W 111 110

max 56 50

Bmin 28

C 28 35

qb 28 25

d

31 25

amax 61 27.5

ami n

50

* Based upon experiments on asphalt mixtures with 9.5 mm

nominal maximum aggregate size and sma ller

holes. After modifying the loading hole locations (Fig. 4(b)),

over 80 specimens have been tested without a single failure

at a loading hole.

Because the DC(T) geometry is being developed as a prac-

tical engineering fracture test, the specimen thickness (B)

should reflect the typical pavement layer thicknesses present

in the field (25-100 mm). For test repeatability, it is often rec-

ommended that the smallest dimension of the specimen be at

least four times greater than the maximum aggregate size. 15

For this study, the maximum aggregate size in the mixture

was 9.5 mm. A specimen thickness of 50 mm was selected

for this study based upon these considerations. The influence

of specimen thickness on fracture energy has been observed

in Portland cement concrete 16 and deserves consideration for

asphalt concrete. However, the investigation of such an effect

was beyond the scope of this preliminary investigation.

The a~ ratio (see Fig. 3) for the present study was held

constant at 0.25, which is less than the ASTM range o f 0.45-

0.55. The ASTM specification has this recommendation to

ensure that the specimen meets the small-scale yielding cri-

teria. The 0.25 a~ ratio was selected to provide a larger

fractured face area, which should reduce the variability of

the test results caused by heterogeneity of asphalt concrete.

In order for the DC(T) geometry to be practical, the fab-

rication of the specimen should also be straightforward. The

specimen can be cut from a cylinder to the desired thickness

using a water-cooled masonry saw. Next, a template can be

used to mark the location of the loading holes, the flat edge at

the notch mouth, and the notch length. The specimen can then

be mounted on an X - Y table connected to a water-cooled

core drill with a 25 mm outside diameter core barrel to core

the loading holes. The use of a core drill was selected over

other methods because the core barrel produces a smooth

loading hole that is perpendicular to the specimen face. The

X - Y table was used to precisely locate the loading holes,

reducing the chance of eccentricity during loading. The flat

edge can be cut into the specimen using a water-cooled ma-

sonry saw. Careful consideration was taken in the fabrication

of the mechanical notch. The ASTM specification has the re-

quirement of a fatigue crack grown from the mechanical notch

tip to produce a sharp crack tip. The ability to produce a fa-

tigue crack of known length in asphalt concrete without dam-

aging the surrounding material is questionable and may not

be necessary if the width of the mechanical notch is narrow

compared to the aggregate structure. 9 To efficiently achieve

a narrow notch width, the mechanical notch was cut to ap-

proximately half of the intended length using a water-cooled

masonry saw with a notch width of 5 mm. Then, the mechan-

ical notch was finished using a handsaw with a carbide-grit

blade, producing a notch width of approximately 1 ram. The

two-step procedure was used to expedite the fabrication of

the notch and the entire notch could be fabricated using the

1 mm wide saw blade as well.

Experimental Procedure

An experimental design was developed to determine the

potential of using the DC(T) geometry for obtaining the frac-

ture energy o f asphalt concrete. A single mixture was used,

consisting of a 9.5 mm maximum aggregate size with a PG64-

22 asphalt binder. The PG64-22 asphalt binder, determined

by the American Association of State Highway and Trans-

portation Officials (AASHTO) MP1 Standard, 17 is a widely

used asphalt binder for the climate found in central Illinois.

The 9.5 mm maximum aggregate size mixture is typical of

a surface layer in an asphalt concrete pavement. Cylindrical

specimens were prepared in the laboratory following stan-

dard procedures using a gyratory compa cto r) 8 All specimens

were compacted to 934-1 of the theoretical maximum den-

sity of the mixture. From each gyratory-compacted cylinder

(115 mm high), two DC(T) specimens were fabricated with a

50 mm thickness following the procedures described above.

All testing was performed with an Instron 8500 servo-

hydraulic load frame with an environmental chamber capable

of controlling the temperature from 30~ to -30~ within

-t-0.1~ The loading fixture was fabricated following the

ASTM specification to ensure that the loading pins remained

frictionless throughout the loading. Load was measured with

a 10 kN load cell and the crack mouth opening displacement

(CMOD) with an Epsilon Model 3541-0020-250-ST clip-on

gage. Figure 5 shows the experimental setup with the loading

fixtures, specimen, clip-on gage, and crack detection wires:

one on the front (shown) and one on the back. The clip-on

gage was used as the feedback for the control loop to ensure

stable post-peak crack growth. The experimental design was

developed to detect the changes of fracture energy at various

temperatures (-20~ -10~ and 0~ and loading rates (10,

5, 1, and 0.1 mm mi n- 1). At each temperature and rate, three

replicates were tested to obtain the average fracture energy.

272 9 VoL 45, No. 3, June 200 5 9 200 5 Society for Experimental Mechanics


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Fig. 5--Experimental setup for the DC(T) specimen with

crack detection wire

Several proposed techniques were found in the literature

for determining the fracture energy but, for this study, the

fracture energy was computed as the area under the load-

CMOD curve normalized by the area of the fractured sur-

face (initial ligament length times specimen thickness). In

the computation of fracture energy, unavoidable electronic

noise in the sensors creates unreliable estimates of area un-

der the load-CMOD curve. To circumvent this problem, the

CMOD data were smoothed using linear regression through

the CMOD-time curve. The use of linear regression is valid

since the test was in fact controlled at a constant CMOD rate.

Test esults

For the DC(T) geometry to provide a useful fracture prop-

erty, the variability of the fracture energy should be within an

acceptable range. The repeatability of the test and the mea-

sured quantities (load and CMOD) influence the fracture en-

ergy, and understanding these curves provides insight into

the fracture process. Figure 6 shows the load~CMOD curves

for three replicates tested at -1 0~ and 1 mm min -1 CMOD

rate. For the three replicates, the initial stiffness and peak

loads are very similar. The softening or post-peak responses

of the replicates vary in shape and length of the softening

tail. Crack detection wires have been placed at the notch tip

(Fig. 5) to indicate when the macrocrack initiates, which typ-

ically occurs after peak load (see Fig. 6). However, it must

be acknowledged that microcracking and localized damage

occur throughout the loading, especially at peak load. The

aggregate structure could have an effect on the crack path

and thus the softening response. The crack path can traverse

through aggregates, between aggregate-aspha lt binder inter-

faces, or through the asphalt binder. The fracture energy as-

sociated with each of these paths is expected to be quite dif-

ferent. Since the aggregate structure for each test replicate is

different, the softening response will likewise differ for the

aforementioned reasons. The Ioad-CMOD curves of Fig. 6

suggest that while the aggregate structure does not signif-

icantly affect the load required for crack initiation, it does

lead to some scatter between replicates in the load-CMOD

response after peak load is reached.

The coefficients of variation (COVs) of the fracture en-

ergy at each temperature and loading rate were obtained for

comparison (Table 2). In general, the COV decreases as the

Fig. 6---Load--CMOD curve for three experimental results

(a/I /V= 0.25) at- 10 ~

temperature increases. The reason for the increase in vari-

ability with decreasing temperature could be that the material

is more brittle at lower temperatures, where defects (voids,

weak aggregates, etc.) significantly influence the fracture pro-

cess. Based upon visual observation of the fracture surface,

the crack appeared to travel around aggregates at the high-

est temperature (0~ whereas at the lowest temperature,

the crack front often traveled through aggregates. Asphalt

binder properties have a strong dependence on temperature,

where the modulus and strength generally increase as the

temperature decreases with the exception of binder strength

at very low temperatures.19 The COV from the DC(T) geom-

etry was compared to the data found in the literature for the

SE(B)Tand SCB It geometries. The COV determined for the

DC(T) geometry was within the range determined with the

SE(B) (3-28 ) and SCB (15-34 ).

With satisfactory repeatability o f fracture energy measure-

ments, the next step was to assess the ability of the DC(T) test

to detect the effects of temperature and loading rate on frac-

ture energy. Asphalt concrete shows a transition from brittle

to ductile fracture behavior as temperature increases. Fig-

ure 7 shows the Ioad- CMOD curves for a single loading rate

(1 mm mi n- I ) at the three test temperatures. The initial stiff-

ness of the material decreases with increasing temperature

(see Table 3) along with a change in the post-peak soften-

ing response. The curvature of the softening curve decreases

as the temperature increases, suggesting more load carrying

capacity, and thus more total energy is required to create the

fracture surface. In this case, more energy is spent on inelastic

deformation. The same phenomenon is seen for the loading

rates where the material appears more brittle as loading rate

increases. Table 2 shows the average fracture energy of three

replicates at all temperatures and loading rates. In general, the

fracture energy increases as the temperature increases, and

energy decreases with increasing loading rate. The current

analysis assumes the distribution of energy is dominated by

fracture energy, but creep strain dissipation, distributed ma-

terial damage, etc., will also consume energy. It is hoped that

current work to couple the experimental observations with

numerical analysis will lead to a truly fundamental material

9 Society for Experimental Mechanics Experimental Mechanics

9 273


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TABLE 2--AVERAGE FRACTURE ENERGY AND COV OF THE FRACTURE ENERGY AT VARIOUS TEMPERATURES AND

LOADING RATES fBASED UPON THRE E REPLICATES

Fracture Energy (J m -2) COV (%)

Rate Temperature (~ Temperature (~

(mm m n- 1 -20 -10 0 -20 -10 0

10 41 276 407 14.5 8.7 13.0

5 197 318 397 25.4 12.3 10.3

1 233 328 470 17.4 14.0 4.2

0.1 245 352 848 15.9 14.5 8.8

Fig. 8---Three types of crack paths observed from the tests

with the DC(T) geometry: (a) type I crack path; (b) type II

crack path; (c) type III crack path

Fig. 7--Load-CMOD curves for 1 mm/min loading rate at

three test temperatures

TABLE 3--INITIAL STIFFNESS FOR VAR-

IOUS TEMPERATURES AT 1 mm/min

LOADING RATE

Tempe rature Initial Stiffnes s

(~ (kN mm -1)

-20 60.4

-10 45.9

0 32.4

proper ty to be extracted from the DC T) tes t. Concurre nt tes t-

ing of s imilar materials with the SE B) tes t wil l be used to

val idate the analysis techniques, which are current ly under

development .

Discussion of Crack Trajectory

Although the tes t repeatabi l i ty and the t rends in the frac-

ture energy were sat isfactory, the observed deviat ions from

the mode I crack path were a cause for concern. Three dis-

t inct types of crack paths were observed during the tes t ing,

as shown in Fig . 8 . The mo de I or s traight crack path termed

type I ) was the second mos t comm on crack pa th and was a l so

the des ired c rack pa th . The m os t com mon crack pa th was the

type II path , where the crack ini t iates in a s traight path and

then deviates at a point along the l igament . The final crack

path , and also the least comm on, was the crack that in it iates

and propagates at an angle from the notch t ip . For el imina-

t ion of crack deviat ion, s ide grooves could be fabricated in

the specim en, but th is would require extra s teps in the fabri-

ca t ion p rocedure , wh ich wou ld be cumbe rsome and perhaps

impract ical . Table 4 shows the average crack deviat ion an-

gles for three repl icates at the temperatures and rates tes ted

during this s tudy. There are several possible causes fo r crack

turning and crack deviat ion, such as the presence of larger

and/or s tronger aggregates in the crack path . Other possible

factors include material heterog eneity and load eccentrici ty .

Statistical Analys is of Variance of Test Re sults

To support the observat ions that the fracture energy can de-

tect changes with tempe rature and loading rate, an analysis of

var iance ANOVA) was perfo rmed on the tes t da ta using SAS

Version 9.1 software. Two sep arate hypotheses w ere tes ted:

1) s tat is t ically different values of fracture energy can be ob-

tained with the propos ed DC T) tes t for the temperatures and

loading rates selected for th is s tudy; 2) fracture energy is not

correlated with c rack deviat ion angle. To tes t the hypotheses ,

the p-value, or the smallest s ignificance level at which the

observe d data w ould have caused reject ion, 2~ was used and

set at 0.05. For the fi rs t hypothesis , the p-va lue is muc h less

than the probabil i ty set 0 .05) as shown in Table 5 , indicat-

ing that s tat is t ical ly s ignificant d ifferences in fracture e nergy

can be de tec ted by the DC T) t es t s among the t es t t empera-

tures and loading rates s tudied. The second hypothesis that

the crack de viat ion does not affec t the fracture energ y is also

found to be t rue, as summarized in Table 6 . The correlat ion

betw een fracture energy and crack deviat ion is very low R =

-0.048) and the differences between the means do not d iffer

s ignificantly from zero p-va lue = 0 .7455).

Cohesive Zone Fracture Simulation of DC T)Test

To gain insight in to the fracture behavior of asphalt con-

crete, an in tegrated appr oach is being undertaken by cou pling

274 9 VoL 45, No. 3, June 200 5 9 200 5 Society for Experim ental Mecha nics


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TABLE 5--ANOV A TABLE FOR COMPARING FRACTURE ENERGY TO TEMPERATURE AND LOADING RATE

Source DF Type III SS Mean Square F-value P r > F

Tempem ture 2 751612.06 375806.03 216 .78


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X-Z.,

an d Wittmann, F.H., 'Thickness Effec t on Frac-

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9 20 05 Society for Experimental Mechanics Experimental Mechanics 9 7 7

2005 wagnoner buttlar paulino disk-shaped compact tension test

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