fracture toughness of hdpe evaluated by fatigue and … · 2015-06-01 · recommended in metals...

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5463

FRACTURE TOUGHNESS OF HDPE EVALUATED BY FATIGUE AND

NOTCH PRECRACKING

Luis Fernando Iglésias1, Thais P. Sequeira

1, Marysilvia F. Costa

1, Celio A. Costa

1(*)

1Metallurgical & Materials Engineering Department (POLI/COPPE/UFRJ), Universidade Federal do Rio de

Janeiro (UFRJ), Rio de Janeiro, Brazil (*)Email: [email protected]

ABSTRACT

The present study compares the fracture toughness properties of high density polyethylene

(HDPE) when the precrack was opened by razor blade and by fatigue. Razor blades were

prepared with two different radius of curvature, while the fatigue precracking had the radius

of curvature close to zero, as demanded by fracture mechanics concept. In addition, side

grooved machined samples were evaluated with razor blade precrack opening. The KIC results

for the razor blade tests were 1.57 MPa.m1/2 and 2.42 MPa.m

1/2, depending on the radius of

curvature of selected razor blade. For the fatigue precracking, it was 0.57 MPa.m1/2. Likewise,

KIC results for side grooved machined specimens were 0.97 MPa.m1/2 and 1.67 MPa.m

1/2

respectively for selected razor blades. This discrepancy confirms that fatigue precracking

showed a much lower value for fracture toughness than the razor blade notch opening. It is

also proposed that fatigue precracking in polymers should be further investigated as a regular

procedure to be included in standards. Fractography investigation was realized and micro

mechanisms of precrack opening methods were discussed.

Keywords: Fracture Mechanics, polymer, HDPE, precracking methodology.

INTRODUCTION

Applications of polymeric materials have been increasing in structural components. For

instance, their applications in pipes as thermoplastic jackets or pressure barrier in flexible pipe

risers are quite common nowadays. Fracture mechanics has been largely used to measure the

fracture toughness of structural materials and, due to the use of polymers in high demand

applications, high concern has been raised about the correct use of this methodology for a

viscoelastic material. One topic that has been addressed recursively is the precrack opening

methodology [1,2,3], since a curvature radius close to zero is desirable.

In metals, precrack opening is easily conducted with controlled cyclic load and standards for

that are well stablished. For viscoelastic materials, as polymers, cycling becomes a difficult

task since creep/stress relaxations acts during the fatigue, and the determination of the glass

transition temperature (Tg) is very important [3]. To circumvent this problem, the ASTM

D5045 recommends the precracking method by a razor blade, which shall lead to a radius of

curvature with the same stress intensity factor as the one generated by fatigue. Care shall be

taken not to damage the material ahead of the notch, either by strain hardening or elevated

temperature due to friction.

Chan and Williams [2] have shown that the razor blade simulates neither an ideal crack plane

nor the sharpest possible crack tip; also, the blade can cause damage to the polymeric matrix

and, therefore, it shall lead to interference in fracture test results. However, fatigue precrack

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Mechanical Behaviours of Advanced Materials and Structures at All Scales

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opening methodology requires more controllable parameters as polymers are viscoelastic

materials and therefore precracking by fatigue has not been an easy process. Discussions

about the difference between fracture mechanics parameters and results using a razor blade

and fatigue precracking do not seem to have been addressed for HDPE at this moment.

The aim of this study is is to evaluate the KIC results between fatigue precrack and razor blade

notching opening conditions that lead to an acceptable crack considering ASTM D5045, as

recommended in metals standard (ASTM E399), with focus in a zero radius of curvature at its

tip. In addition, side groove notches will be notched in samples with razor blade precrack

opening in order to evaluate and compare this effect in KIC results. Finally, Scanning Electron

Microscopy (SEM) investigation was performed in specimens fractured surfaces and micro

mechanisms involved in precrack opening conditions were observed and discussed.

EXPERIMENTAL PROCEDURE

The material investigated was a commercial grade high density polyethylene (HDPE). This

material was selected because of its process simplicity, low cost and wide use and

applications. Specimens for fracture tests according to ASTM D5045 (Figure 1) were

machined with 25.5 mm thickness. Dimensions are shown in Figure 2-A and Figure 2-B. Side

grooves were introduced in some CT samples.

Fig. 1 - Specimens for fracture tests.

Fig. 2 - Dimensions of Compact Tension (CT). Figure 1-A shows specimens without side grooves (CT

specimens) and Figure 1-B presents side grooved specimens CT specimens.

Side

Grooves


Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores,

Fatigue precrack opening was performed by a servo

displacement-control with sinusoidal waveform and respective fatigue conditions: minimum

to-maximum load ratio R = 0.1, frequency f = 10 Hz (close to material’s glass transition) and

6000 cycles in stepwise fatigue crack opening mode. A small cut with a

side of the notch tip was necessary to reduce the plain stress state at border of the CT sample

and the tunneling effect [3].

Two different razor blades named 1 and 2 were used to open the precracks in the samples and

are represented in Figure 3. Note that blade 1 visually appears to be sharper than blade 2 and,

consequently, it has a lower value of curvature radius. Furthermore, precracks were opened in

side groove notched specimens using blades 1 and 2.

Fig. 3

Just after the precrack opening, either by fatigue or razor blade, dye penetrant liquid was

sprayed into the pre-cracks with specimens placed with the notch opening up for 15 minutes.

This technique marked the prec

CT precracked samples were tested per ASTM D5045 to verify if they match the plain strain

fracture toughness state. The tests were conducted in a MTS Landmark® machine. The

method used for measuring precr

it was performed by a Nikon profile projector Model 6C 12666 after fracture tests.

Final fatigue opened precracks mean length values were determined as 4.9 mm. For razor

blade opened precracks, a mean

specimens. These precrack dimensions are consistent with ASTM D5045 standard according

to specimen thickness (25.5 mm).

The fracture surface images obtained after fracture tests were obtained by SE

2000 FX model and precrack opening characteristics and micro mechanisms of crack opening

and propagation were observed.

(1)



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Fatigue precrack opening was performed by a servo-hydraulic MTS Landmark®, using

control with sinusoidal waveform and respective fatigue conditions: minimum

R = 0.1, frequency f = 10 Hz (close to material’s glass transition) and

6000 cycles in stepwise fatigue crack opening mode. A small cut with a razor blade at the end



in Figure 3. Note that blade 1 visually appears to be sharper than blade 2 and,


side groove notched specimens using blades 1 and 2.

. 3 - Razor blades used for precrack opening.


cracks with specimens placed with the notch opening up for 15 minutes.

This technique marked the precrack boundaries, making visual observation easier.



method used for measuring precrack lengths was adapted from ASTM D6068 procedure and



blade opened precracks, a mean value of 5.0 mm length was obtained in side grooved or not


to specimen thickness (25.5 mm).

The fracture surface images obtained after fracture tests were obtained by SE


and propagation were observed.

(1) (2)

hydraulic MTS Landmark®, using

control with sinusoidal waveform and respective fatigue conditions: minimum-

R = 0.1, frequency f = 10 Hz (close to material’s glass transition) and

razor blade at the end



in Figure 3. Note that blade 1 visually appears to be sharper than blade 2 and,



cracks with specimens placed with the notch opening up for 15 minutes.

rack boundaries, making visual observation easier.



ack lengths was adapted from ASTM D6068 procedure and



value of 5.0 mm length was obtained in side grooved or not


The fracture surface images obtained after fracture tests were obtained by SEM using a JOEL


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RESULTS

Samples had their precracks opened by a razor blade in CT specimens and CT side grooved

specimens and the results were compared with the data for a fatigue precrack procedure [3]

and with literature [1,4,5,6] concerning K

methodologies is shown in Figure 4

are presented in Figure 4-D. The razor blade crack is straight, while the fatigue crack has a

nail shape, where tunnelling can be observed. The K

0.0

0

200

400

600

800

1000

1200

1400

Axial Force (N)

(D) Load-displacement curves from K

Fig. 4 - Precracks front zones (Figures 4

Fatigue precracking

(A)

SEM analysis

(1)


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ts were compared with the data for a fatigue precrack procedure [3]

and with literature [1,4,5,6] concerning KIC toughness results. The crack front for these

methodologies is shown in Figure 4-A to 4-C and their respective curves of load

D. The razor blade crack is straight, while the fatigue crack has a

nail shape, where tunnelling can be observed. The KIC results are listed in Table 1 for HDPE.

0.5 1.0 1.5 2.0

Fatigue precrack

Razor blade precrack


(side grooved specimens)

Axial Displacement (mm)

displacement curves from KIC test record in 3 different precrack opening methods.

Precracks front zones (Figures 4-A to 4-C) and respective fracture toughness test record (Figure 4

Razor blade with side grooves

(B) (C)

SEM analysis

(2)


ts were compared with the data for a fatigue precrack procedure [3]

toughness results. The crack front for these

C and their respective curves of load-displacement

D. The razor blade crack is straight, while the fatigue crack has a

results are listed in Table 1 for HDPE.

2.5

Fatigue precrack



(side grooved specimens)

test record in 3 different precrack opening methods.

C) and respective fracture toughness test record (Figure 4-D).

Razor blade

with side grooves

SEM analysis

(3)



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In Table 1, the KIC results for razor blades 1 and 2 were similar to those reported in the

literature by Baterzak et al. [4] in similar conditions and it was 276% and 424% higher,

respectively, than the one calculated when fatigue precracking was the method used by

Sequeira [3]. Note that Ulmanu et al. [1] reported a KIC value using fatigue precrack opening

in stepwise fatigue mode with side grooves notches, frequency of 5 Hz and R = 0.1. This is

approximately 30% higher than the value obtained by Sequeira [3].

Literature reports other results in different test conditions. For example, Andreassen and Nord

have demonstrated that in impact conditions the KIC value was calculated as 1.66 MPa.m1/2

exihibiting a higher strain rate than the results presented in this work. Plati and Willians have

performed fracture tests in cryogenic conditions (T= -63oC) and KIC value was calculated as

1.00 MPa.m1/2. Note that in this condition, the material is closer to the glass transition. It’s

assumed that these results were obtained by razor blade precracking since the authors do not

present this specific information.

For curvature radius closer to zero, results are more conservative. Therefore, KIC calculation

depends tightly of precrack opening methodology and also by test conditions (temperature

and strain rate).

Table 1 - KIC results and selected methodology.

Precrack opening

Methodology

Calculated KIC

Toughness (MPa. m1/2)

CT sample

Razor blade 1 1.57

CT sample

Razor blade 2 2.42

Side grooved CT

Razor blade 1 0.97

Side grooved CT

Razor blade 2 1.67

Sequeira et al. [3]

(fatigue opening) 0.57

Ulmanu et al. [1]

(fatigue opening) 0.74

Baterzak et al. [4]

(razor blade opening) 2.05

Andreassen and Nord [5]

(impact condition) 1.66

Plati and Willians [6]

(cryogenic conditions) 1.00

Fractography investigation was conducted and images from specimens surfaces after fracture

tests and regions were selected for observation which are shown in Figures 4-A to 4-C. The

analysis allowed observing distinct micro mechanisms due to precrack nucleation and growth.

Three different regions were observed in Figure 4-A to 4-C for each type of methodology for

evaluating KIC: CT sample with precrack opening by fatigue (1); CT sample with precrack

opened by razor blade (2); side grooved CT sample with precrack opened by razor blade (3).

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Fractography analysis is presented in Figures 5-A to 5-F. The micro mechanisms involved in

precrack growth during fatigue cycling are the generation of beach marks and crazing process

followed by severe plastic deformation due to crack propagation.

Figure 5-A and 5-B exhibits the characteristics fractured surfaces. It can be noted the presence

of the tunneling effect because of a large difference in stress states along the notch/crack

front. At the notch borders there is always a plane stress state, while in the middle zone a

plane strain exists. In polymers, which are viscoelastic materials, this difference may be very

high and tunneling then happens with more intensity in samples borders than in metals and

this is not allowed for corrected KIC computation. Crazes can be observed during crack

propagation. After tunneling zone, severe plastic deformation by crazing deformation occurs.

It’s evident the presence of fibrils due to the coalescence of cracks and molecular

reorganization. Crack propagation continues by the rupture of fibrils and the creation of new

surfaces and cavities.

In Figures 5-C to 5-F, the characteristics due to razor blade crack opening in specimens with

or without side grooves are similar. It’s observed a smooth region corresponding to razor cut

followed by a cut zone as a consequence of machining process. Finally, during severe plastic

deformation, a plastic zone appears which is characterized by the formation of fibrils. Note

that the magnitude and size of cavities and fibrils just after the cut zone is higher than those

fibrils from fatigue crack tip. The formation of fibrils and cavities size was related to the level

of stress concentration, so as to, the smaller cavity size was present in the fatigue precracking,

and increased for side groove and razor blade.

In Figure 6-A, the fatigue precracked specimens, it is evident the transition region between a

smooth path and a plastic deformation zone characterized by fibrils and cavities. Figure 6-B

shows the region located in the center of the precrack. Note the existing paths distributed in a

non-homogeneous mode. This effect is expected as the specimen was cycled in stepwise

fatigue mode [3] and in order to maintain the R value constant during the test, the material

was reloaded whenever the material presented a relaxation behavior due to its viscoelastic

properties.

CONCLUSION

The present study showed that fatigue precracking can be a regular method for fracture

toughness tests. The fatigue precracking was opened in a fast and controlled manner, resulting

in the lowest KIC measured. The presence of side grooves in razor blade precrack method

increased the plain state of stress, but the result was higher than that the fatigue precracking.

The KIC measured by razor blade showed the highest value measured.

A precracking method to evaluate polymers needs to be further evaluated and better defined

in the standards, since the scatter in the results is large.

ACKNOWLEDGMENTS

The author would like to thanks to the Brazilian National Petroleum Agency (ANP), through

PRH 35, for supporting this research.



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Fig. 5 - Fractography analysis for fractured surfaces. Figure 5-A corresponding to fatigue precrack opened

specimen and Figure 5-B shows a higher magnification. Figure 5-C corresponding to razor blade precracked in

specimens without side grooves and Figure 5-D showing a higher magnification of this region. Finally, Figure 5-

E corresponding to razor blade precracked in specimens with side grooves and Figure 5-F exhibiting a higher

magnification of this zone.

(A) (B)

(C) (D)

(E) (F)

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Fig. 6 - In Figure 6-A, note the transition between the precrack fatigue zone and severe plastic deformation by

crack propagation where tunneling effect can be observed. In Figure 6-B, beach marks are shown in fatigue

precrack center region generated by stepwise fatigue opening mode.

REFERENCES

[1]-Ulmanu, V.; Draghici, G., Aluchi, V. Mechanics Testing High Density Polyethylene

(HDPE) Pipe Material with Compact Tension (CT) Specimens, Journal of Engineering

Studies and Research – Volume 17 (2011) No. 3.

[2]-Chan, M.K.V. and Williams, J.G. Plane Strain Fracture Toughness Testing of High

Density Polyethylene; Polymers Engineering & Science; pp 1019 -1026, October 1981.

[3]-Sequeira T., Costa M., Costa C. Fatigue Precracking Methodology for HDPE,

Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic

Engineering.

OMAE2014, June 8-13, 2014 (paper OMAE2014-24692).

[4]-Argon, A. S.; Bartezak, Z.; Cohen, R.E.; Muratoglu, O.K., “Novel mechanism of

toughening of plastics, advances in modeling and experiments,” vol. 42, n. 759, Washington

D.C., ACS, p. 2347, 2000.

[5]-Andreassen; Nord-Varhaug; Hinrichsen; Persson, “Impact fracture toughness of

polyethylene materials for injection moulding,” em PPS07EA, Gothenburg, 2007.

[6]-Plati, E.; Willians, J.C., “Effect of temperature toughness of polymers,” Polymer

Engineering Science, vol. 16, n. 915-920, 1975.

(A) (B)

fracture toughness of hdpe evaluated by fatigue and … · 2015-06-01 · recommended in metals...

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