Research Article Fracture Toughness Reliability in

Hindawi Publishing CorporationIndian Journal of Materials ScienceVolume 2013 Article ID 187802 4 pageshttpdxdoiorg1011552013187802

Research ArticleFracture Toughness Reliability in PolycarbonateNotch Sharpening Effects

A Salazar1 J Rodriacuteguez1 and A B Martiacutenez2

1 DIMME Departamento de Tecnologıa Mecanica Escuela Superior de Ciencias Experimentales y TecnologıaUniversidad Rey Juan Carlos CTulipan sn Mostoles 28933 Madrid Spain

2 Centre Catala del Plastic Universitat Politecnica de Catalunya CColom 144 Terrassa 08222 Barcelona Spain

Correspondence should be addressed to A Salazar aliciasalazarurjces

Received 10 October 2013 Accepted 27 November 2013

Academic Editors W Cantwell and A Kajbafvala

Copyright copy 2013 A Salazar et alThis is an open access article distributed under theCreativeCommonsAttribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The effect of the notch sharpening on the fracture toughness obtained under linear elastic fracture mechanics has been analyzedin an amorphous polycarbonate The samples for fracture characterization were sharpened via the traditional contact steel razorblade technique and the noncontact femtosecond laser ablation technique The values of the fracture toughness of the specimenssharpened through femtosecond laser ablation were lower than those measured on samples sharpened using a steel razor bladeMoreover the former was in plane strain state but the latter did not verify the size criterion The damage produced ahead ofthe crack tip through plastic deformation in the steel razor blade sharpened samples over the lack of damage in the femtolasersharpened specimens explains the differences in the fracture toughness It has been proven that there is a relationship between theplastic deformation at the crack front and the stress state This has been assessed through the application of a fracture criterion fordissipative systems with small scale yielding

1 Introduction

The influence of crack sharpness on the fracture tough-ness values of polymeric materials has recently caught theattention of the scientific community Several works haveappeared in the literature highlighting the role of the notchsharpening technique on the fracture parameters of eithersemicrystalline heterogeneous polymeric materials such asethylene-propylene block copolymers [1ndash3] or amorphouspolymers such as polycarbonate [4] In all these studiesthe values of the fracture toughness obtained using differentapproacheswere determined from samples sharpened via twoprocedures a noncontact technique based on the femtosec-ond laser ablation (femtolaser) and the traditional contacttechnique using a razor blade Interestingly both sharpeningtechniques resulted in similar initial crack tip radii forthese polymers and the fracture toughness determined fromthe samples sharpened via the femtolaser showed lowervalues than those obtained from the specimens sharpened

via the traditional razor blade technique independently ofthe material under study These differences reached in thecopolymer values up to 11 75 and 90 when the crackgrowth initiation parameters were computed under linearelastic fracture mechanics (LEFM) elastic-plastic fracturemechanics (EPFM) and Post-Yielding Fracture Mechanics(PYFM) conditions respectively [3] and radical changesin the J-R curves obtained under EPFM approximation inthe polycarbonate [4] The reason is that the femtosecondpulsed laser ablation is characterized by very rapid creationof vapor and plasma phases negligible heat conduction andthe absence of liquid phase preventing melting and thermaldeformations of the surrounding area [5]However the speci-mens sharpened using the razor blade always showed damageahead of the crack tip in the form of plastic deformationwhich seems to be the reason of the higher fracture toughnessvalues Moreover the abrupt differences between the fracturetoughness values especially under EPFM [3 4] and PYFM [3]obtained from samples sharpened via different techniques

2 Indian Journal of Materials Science

seem to indicate that there is a relationship between thenotching procedure and the stress

In the light of the previous results the aim of the presentwork is to analyze more deeply the possible relationshipbetween sharpening technique and the stress state developedahead of the crack tip during the fracture process For thatthe fracture toughness under LEFM will be determined inan amorphous polymer such as polycarbonate The fracturespecimens will be sharpened via the traditional razor bladeprocedure and the femtolaser technique The results will beanalyzed with the help of the fracture criterion for dissipativesystems with small-scale yielding behavior occurring underLEFM conditions [6]

2 Materials and Methods

The material under study is an amorphous polycarbonatesheet PC supplied by Nudec The glass transition temper-ature was 163∘C Five tensile specimens were tested at 23∘Cand at a crosshead speed of 1mmmin following the ISO527standard guidelinesThe correspondingmean values togetherwith the standard deviations of the Youngrsquos modulus and theyield stress were119864 = 194plusmn002GPa and 120590

119884= 603plusmn01MPa

respectivelyAt least five valid fracture tests were carried out on single

edge notch bend specimens with 5 times 10 times 44mm in size Aninitial straight-through slot with a length to width ratio of 05and terminating in a V-notchwith 015mm in root radius wasmachined The notch was sharpened via two procedures asfollows

S-Type It consists of sliding a fresh steel razor blade acrossthe root of the V-notch following the guidelines described byESIS [7]

F-Type The sharpening is produced through Femtolaser [5]using a commercial Ti sapphire oscillator (Tsunami SpectraPhysics) plus a regenerative amplifier system (Spitfire SpectraPhysics) based on chirped pulse amplification (CPA) tech-nique Linearly polarized 120-fs pulses at 395 nm with a rep-etition rate of 1 kHz were produced The scanning speed was130 120583ms Four passes were carried out with a pulse energyof 0008mJ The sharp length inserted by the femtolaser was500120583m

The total initial crack depth 1198860 to width ratio after sharp-

ening was 055The morphology and dimensions of the crack tip after

sharpening and the area behind it of the nontested specimensas well as the morphology of the fracture surfaces were ana-lyzed via scanning electron microscopy (Hitachi S-3400N)

The fracture tests were carried out at room temperatureand under displacement control at a cross-head speed of1mmmin using a three-point bending fixture with a loadingspan to width ratio of 4 mounted on an electromechanicaltesting machine (MTS Alliance RF100) The applied loadwas measured with a load cell of plusmn5 kN The mechanicalresponse of the PC at room temperature is linear and elastictill fracture so the ESIS TC4 protocol entitled ldquo119870

119862and 119866

119862at

Table 1 Fracture toughness values 119870IC and 119866IC of polycarbonate

119870IC (MPasdotm12) 119866IC (kJm2)S-Type 30 plusmn 02 44 plusmn 04

F-Type 41 plusmn 03 77 plusmn 11

slow speeds for polymersrdquo [7] was followed to determine thefracture toughness119870ICThe size requirement for plain strain119870IC is given by

119861 119886119882 minus 119886 gt 25(119870IC120590119884

)

2

(1)

where 119861 is the thickness and119882 is the width of the specimen

3 Results

Table 1 shows the fracture toughness 119870IC and the criticalenergy release rate 119866IC of S-Type and F-Type specimensFirstly both the 119870IC and 119866IC values of the F-Type samplesare lower than those from the S-Type specimens reachingdifferences of up to 37 It is worth noting that the frac-ture toughness values obtained from S-Type specimens areanalogous to those found in the literature [6] Secondly andattending to (1) the values of the F-Type samples are in planestrain state but those determined from the S-Type specimensdo not verify the size criterion

The inspection of the crack tip front of the nontestedspecimens after sharpening revealed the reason of these dif-ferences Figures 1 and 2 display the appearance of the cracktip and the zone ahead of it for F-Type and S-Type sharpenedsamples respectively The crack tip has been marked with anarrow Both sharpening techniques provided similar crack tipradii more accurately the F-Type specimens presented cracktip radii of 1 120583mwhile the S-Type samples of 15 120583mHoweverthere is a damage area ahead of the crack tip in the S-typesamples (outlined with dots) (Figure 2(a)) which was notobserved in the F-Type samples (Figure 1) This zone aheadof the crack tip in the S-Type sharpened samples revealedthe presence of plastic deformation caused during the notchsharpening (Figure 2(b)) Quantitative analysis of the damagearea at the surface of the virgin specimens provided zonesahead of the crack tip with lengths of 100 plusmn 20 120583m Thestresses generated when sliding along the notch root with thesteel blade may have surpassed the yield stress locally givingrise to local plastic deformation In contrast with the S-Typesharpened samples the F-Type sharpened specimens werecharacterized by complete absence of damage (Figure 1(b))

4 Discussion

Attending to the previous results there seems to be arelationship between the stress state and the influence ofthe notch sharpening technique The plastic zone size tospecimen thickness ratio is very much associated with thestress state In order to assess this the fracture criterion fordissipative systems with small scale yielding was applied [6]As first approximation the plastic deformation developed

Indian Journal of Materials Science 3

20120583m

(a)

2120583m

(b)

Figure 1 Micrographs of polycarbonate obtained via scanning electron microscopy of the crack tip front of the nontested F-Type sharpenedspecimens (a) panoramic view where the arrow points out the end of the crack tip and (b) detail of the crack front

20120583m

(a)

5120583m

(b)

Figure 2 Micrographs of polycarbonate obtained via scanning electron microscopy of the crack tip front of the nontested S-Type sharpenedspecimens (a) panoramic view where the arrow points out the end of the crack tip and the damage area has been outlined and (b) detail ofthe crack front

KC1

KC2

KC

B

Figure 3 119870119862

distribution across the crack front [6]

ahead of the crack tip just prior to the fracture process givesrise to a 119870

119862distribution across the crack front as shown in

Figure 3Assuming a bimodal distribution with plane stress 119870

1198622

fracture toughness over the surface zones and plane strain

1198701198621

fracture toughness in the centre area the measuredfracture toughness1198701015840

119862

can be attained through the expression[6]

1198701015840

119862

= 1198701198621+1198702

1198622

1205871198611205902

119884

(1198701198622minus 1198701198621) (2)

or in terms of energy

radic1198661015840

119862

= radic1198661198621+1198661198622119864

1205871198611205902

119884

(radic1198661198622minus radic119866

1198621) (3)

Identifying1198701198621

and1198661198621

with the fracture toughness and frac-ture resistance determined from the F-Type samples and 1198701015840

119862

and 1198661015840119862

with those obtained from the S-Type specimens theplane stress fracture values 119870

1198622and 119866

1198622 can be computed

via (2) and (3) (Table 2) Interestingly the plane stress fractureresistance119866

1198622 is analogous to the fracture values obtained in

PC films through the essential work of fracture methodology[8 9]


Table 2 Plain strain fracture toughness obtained from F-Typesamples 119870

1198621

and 1198661198621

fracture toughness obtained from S-Typespecimens1198701015840

119862

and1198661015840119862

and plane stress fracture toughness1198701198622

and1198661198622

119870IC (MPasdotm12) 119866IC (kJm2)Plane strainF-Type 119870

1198621

= 30 plusmn 02 1198661198621

= 44 plusmn 04

S-Type 1198701015840

119862

= 41 plusmn 03 1198661015840

119862

= 77 plusmn 11

Plane stress 1198701198622

= 53 1198661198622

= 143

5 Conclusions

The fracture toughness of the polycarbonate is stronglydependent on the sharpening notch technique The speci-mens sharpened using the traditional razor blade techniquepresented damage in the form of plastic deformation aheadof the crack tip which led to higher values of the fracturetoughness in comparison with those obtained from speci-mens sharpened via the noncontact femtolaser procedureMoreover the fracture toughness values determined from thefemolaser were in plane strain state but not the razor bladesharpened samplesThis evidences that there is a relationshipbetween the plastic deformation generated during the sharp-ening procedure and the stress state

Acknowledgments

The authors are indebted to Ministerio de Economıa y Com-petitividad of Spain for their financial support through theproject MAT2012-37762 and to NUDEC SA for the supplyof the PC sheets

References

[1] A Salazar J Rodrıguez A Segovia and A B MartınezldquoInfluence of the notch sharpening technique on the fracturetoughness of bulk ethylene-propylene block copolymersrdquo Poly-mer Testing vol 29 no 1 pp 49ndash59 2010

[2] A Salazar A Segovia A B Martınez and J Rodrıguez ldquoTherole of notching damage on the fracture parameters of ethylene-propylene block copolymersrdquo Polymer Testing vol 29 no 7 pp824ndash831 2010

[3] A Salazar J Rodrıguez A Segovia and A B MartınezldquoRelevance of the femtolaser notch sharpening to the fractureof ethylene-propylene block copolymersrdquo European PolymerJournal vol 46 no 9 pp 1896ndash1907 2010

[4] A Salazar J Rodrıguez and A B Martınez ldquoThe role ofnotch sharpening on the J-fracture toughness of thermoplasticpolymersrdquo Engineering Fracture Mechanics vol 101 pp 10ndash222013

[5] P Moreno C Mendez A Garcıa I Arias and L RosoldquoFemtosecond laser ablation of carbon reinforced polymersrdquoApplied Surface Science vol 252 no 12 pp 4110ndash4119 2006

[6] J G Williams Fracture Mechanics of Polymers Ellis HorwoodChichester UK 1984

[7] D R Moore A Pavan and J G Williams Fracture MechanicsTesting Methods for Polymers Adhesives and Composites Else-vier Amsterdam The Netherlands 2000

[8] A Al-Jabareen S Illescas M Sanchez-Soto and O O SantanaldquoTrabajo esencial de fractura de laminas de policarbonatomodificado mediante extrusion reactiva con PETrdquo Anales deMecanica de la Fractura vol 25 pp 275ndash280 2005

[9] T Barany T Czigany and J Karger-Kocsis ldquoApplication of theessential work of fracture (EWF) concept for polymers relatedblends and composites a reviewrdquo Progress in Polymer Sciencevol 35 no 10 pp 1257ndash1287 2010

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seem to indicate that there is a relationship between thenotching procedure and the stress

In the light of the previous results the aim of the presentwork is to analyze more deeply the possible relationshipbetween sharpening technique and the stress state developedahead of the crack tip during the fracture process For thatthe fracture toughness under LEFM will be determined inan amorphous polymer such as polycarbonate The fracturespecimens will be sharpened via the traditional razor bladeprocedure and the femtolaser technique The results will beanalyzed with the help of the fracture criterion for dissipativesystems with small-scale yielding behavior occurring underLEFM conditions [6]

2 Materials and Methods

The material under study is an amorphous polycarbonatesheet PC supplied by Nudec The glass transition temper-ature was 163∘C Five tensile specimens were tested at 23∘Cand at a crosshead speed of 1mmmin following the ISO527standard guidelinesThe correspondingmean values togetherwith the standard deviations of the Youngrsquos modulus and theyield stress were119864 = 194plusmn002GPa and 120590

119884= 603plusmn01MPa

respectivelyAt least five valid fracture tests were carried out on single

edge notch bend specimens with 5 times 10 times 44mm in size Aninitial straight-through slot with a length to width ratio of 05and terminating in a V-notchwith 015mm in root radius wasmachined The notch was sharpened via two procedures asfollows

S-Type It consists of sliding a fresh steel razor blade acrossthe root of the V-notch following the guidelines described byESIS [7]

F-Type The sharpening is produced through Femtolaser [5]using a commercial Ti sapphire oscillator (Tsunami SpectraPhysics) plus a regenerative amplifier system (Spitfire SpectraPhysics) based on chirped pulse amplification (CPA) tech-nique Linearly polarized 120-fs pulses at 395 nm with a rep-etition rate of 1 kHz were produced The scanning speed was130 120583ms Four passes were carried out with a pulse energyof 0008mJ The sharp length inserted by the femtolaser was500120583m

The total initial crack depth 1198860 to width ratio after sharp-

ening was 055The morphology and dimensions of the crack tip after

sharpening and the area behind it of the nontested specimensas well as the morphology of the fracture surfaces were ana-lyzed via scanning electron microscopy (Hitachi S-3400N)

The fracture tests were carried out at room temperatureand under displacement control at a cross-head speed of1mmmin using a three-point bending fixture with a loadingspan to width ratio of 4 mounted on an electromechanicaltesting machine (MTS Alliance RF100) The applied loadwas measured with a load cell of plusmn5 kN The mechanicalresponse of the PC at room temperature is linear and elastictill fracture so the ESIS TC4 protocol entitled ldquo119870

119862and 119866

119862at

Table 1 Fracture toughness values 119870IC and 119866IC of polycarbonate

119870IC (MPasdotm12) 119866IC (kJm2)S-Type 30 plusmn 02 44 plusmn 04

F-Type 41 plusmn 03 77 plusmn 11

slow speeds for polymersrdquo [7] was followed to determine thefracture toughness119870ICThe size requirement for plain strain119870IC is given by

119861 119886119882 minus 119886 gt 25(119870IC120590119884

)

2

(1)

where 119861 is the thickness and119882 is the width of the specimen

3 Results

Table 1 shows the fracture toughness 119870IC and the criticalenergy release rate 119866IC of S-Type and F-Type specimensFirstly both the 119870IC and 119866IC values of the F-Type samplesare lower than those from the S-Type specimens reachingdifferences of up to 37 It is worth noting that the frac-ture toughness values obtained from S-Type specimens areanalogous to those found in the literature [6] Secondly andattending to (1) the values of the F-Type samples are in planestrain state but those determined from the S-Type specimensdo not verify the size criterion

The inspection of the crack tip front of the nontestedspecimens after sharpening revealed the reason of these dif-ferences Figures 1 and 2 display the appearance of the cracktip and the zone ahead of it for F-Type and S-Type sharpenedsamples respectively The crack tip has been marked with anarrow Both sharpening techniques provided similar crack tipradii more accurately the F-Type specimens presented cracktip radii of 1 120583mwhile the S-Type samples of 15 120583mHoweverthere is a damage area ahead of the crack tip in the S-typesamples (outlined with dots) (Figure 2(a)) which was notobserved in the F-Type samples (Figure 1) This zone aheadof the crack tip in the S-Type sharpened samples revealedthe presence of plastic deformation caused during the notchsharpening (Figure 2(b)) Quantitative analysis of the damagearea at the surface of the virgin specimens provided zonesahead of the crack tip with lengths of 100 plusmn 20 120583m Thestresses generated when sliding along the notch root with thesteel blade may have surpassed the yield stress locally givingrise to local plastic deformation In contrast with the S-Typesharpened samples the F-Type sharpened specimens werecharacterized by complete absence of damage (Figure 1(b))

4 Discussion

Attending to the previous results there seems to be arelationship between the stress state and the influence ofthe notch sharpening technique The plastic zone size tospecimen thickness ratio is very much associated with thestress state In order to assess this the fracture criterion fordissipative systems with small scale yielding was applied [6]As first approximation the plastic deformation developed


20120583m

(a)

2120583m

(b)


20120583m

(a)

5120583m

(b)


KC1

KC2

KC

B

Figure 3 119870119862





1198622


1198701198621


119862


1198701015840

119862

= 1198701198621+1198702

1198622

1205871198611205902

119884

(1198701198622minus 1198701198621) (2)


radic1198661015840

119862

= radic1198661198621+1198661198622119864

1205871198611205902

119884


1198621) (3)


and1198661198621


119862

and 1198661015840119862


1198622and 119866







1198621

and 1198661198621


119862

and1198661015840119862


and1198661198622


1198621

= 30 plusmn 02 1198661198621

= 44 plusmn 04

S-Type 1198701015840

119862

= 41 plusmn 03 1198661015840

119862

= 77 plusmn 11


= 53 1198661198622

= 143

5 Conclusions


Acknowledgments


References

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




20120583m

(a)

2120583m

(b)


20120583m

(a)

5120583m

(b)


KC1

KC2

KC

B

Figure 3 119870119862





1198622


1198701198621


119862


1198701015840

119862

= 1198701198621+1198702

1198622

1205871198611205902

119884

(1198701198622minus 1198701198621) (2)


radic1198661015840

119862

= radic1198661198621+1198661198622119864

1205871198611205902

119884


1198621) (3)


and1198661198621


119862

and 1198661015840119862


1198622and 119866







1198621

and 1198661198621


119862

and1198661015840119862


and1198661198622


1198621

= 30 plusmn 02 1198661198621

= 44 plusmn 04

S-Type 1198701015840

119862

= 41 plusmn 03 1198661015840

119862

= 77 plusmn 11


= 53 1198661198622

= 143

5 Conclusions


Acknowledgments


References

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





1198621

and 1198661198621


119862

and1198661015840119862


and1198661198622


1198621

= 30 plusmn 02 1198661198621

= 44 plusmn 04

S-Type 1198701015840

119862

= 41 plusmn 03 1198661015840

119862

= 77 plusmn 11


= 53 1198661198622

= 143

5 Conclusions


Acknowledgments


References

















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

Research Article Fracture Toughness Reliability in