ABSTRACT - WIT Press€¦ · Localized Damage 519 3-1 b 2-Notch depth 1 mm o A D 4 mm # A • Fiber content 15wt % 30wt % 40wt % 01234 1/p (mm*') Figure 7 : Relation between Omax,f

Evaluation of notched tensile strength of

CFRPEN based on linear notch mechanics

H. Nisitani," H. Goto*

" Faculty of Engineering, Kyusyu University,

6-10-1 Hakozaki, Higasi-ku, Fukuoka, 812, Japan

* Polymer Research Laboratory, Idemitsu Petrochemical Co.

Ltd., 1-1 Anesaki-kaigan, Ichihara 299-01, Japan

ABSTRACT

Notched tensile strength of short carbon fiber reinforced plastics were charac-terized. The materials used are short carbon fiber reinforced polyethemitrile(CFRPEN), in which the short carbon-fibers were randomly combined in aPEN resin matrix with a fiber content of 15%, 30% and 40% (by weight).Tensile tests of a strip with double edge notches were carried out for a widerange of notch root radii and depths under the condition of constant specimenthickness. The results were discussed based on linear notch mechanics. Thenominal stress for fracture in the sharply notched specimens having the samenotch depth and the same minimum section is nearly constant independent ofthe value of notch root radius. This is due to the fact that in those specimens thestable crack growth can be recognized before final fracture. The fracture ofCFRPEN specimen is controlled by the maximum elastic stress and the notchroot radius alone, independent of the other geometrical conditions.

INTRODUCTION

Fiber reinforced plastics (FRP) are being used increasingly for engineeringapplications by reason of the higher specific strength and specific modulus.Especially short fiber reinforced plastics molded by injection mold are in highdemand, because unnecessary of cutting work reduces the cost of components.To increase the demand further, it is important to investigate the fracture criteriaand the prediction method of fracture strength in consideration of stress concen-tration for the requirement of reliability in the component.

In this paper the notched tensile fracture criterion and the effect of fibercontent on the notch sensitivity were discussed based on the linear notch me-chanics, which was proposed by one of the authors [1].

Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533

514 Localized Damage

MATERIALS AND EXPERIMENTAL PROCEDURE

The materials used are carbon fiber reinforced polyethernitriles (CFRPEN) inwhich the short carbon-fibers having a diameter of 7//m and mean length of100/zm were randomly combined in a PEN resin matrix. Fiber content is15%, 30% and 40%. The mechanical properties of the materials used are tab-ulated in Table 1. Figure 1 shows the specimen configurations of CFRPEN.

Tensile test of a strip with double edge notches were carried out for a widerange of notch root radii and two notch depths under the condition of constantspecimen thickness. It was performed at room temperature under a constantcross-head speed of 0.5mm/min. The initiation and propagation processes ofcracks were observed successively by the replication technique. The observa-tions of fracture surfaces were also carried out by a scanning electron microscope.

Table 1 : Mechanical properties of PEN and CFRPEN.

Fiber

CTR

E

0

content (%)

(MPa)

(GPa)

(%)

0

135

4.4

3.8

15

200

19

1.6

30

218

22

1.4

40

230

25

1.0

cr B : Ultimate tensile strength, E : Young's modulus, 0 : Elongation

s. ,\O'CN

1

1 1

U

e*70 J

220 ^

1

VI06«— 4

3

Detail at A

t =1,4

p=0.2, 0.3, 0.5, 1,2,4

Figure 1 : Specimen configurations of notched CFRPEN.


250-1

200

?150-

•100-

50-

w-2l

250-n

200-

« 150-

100-

50-

(a) Fiber content is 15%.

250n

200-

150-

100-

50-

3

Kl

T-r-rq5

(b) Fiber content is 30%.

I = 1

2 3 4

Ki

(c) Fiber content is 40%.

Figure 2 : Relation between the stress for causing brittle fracture af and the stress concentration factor Kt for CFRPEN. <§



RESULTS AND DISCUSSION

Figure 2 shows the relation between the nominal stress causing brittle fracture<7f and the stress concentration factor Kt in the CFRPEN plate specimens havingvarious notch depths and notch root radii. There is no one to one correspondencebetween <7f and Kt, and the effect of notch depths is clearly seen on the <7f - Ktrelation. The relations of <7f -Kt for CFRPEN are to be classified into two parts,namely, a part where <jf decreases with increasing Kt and a part where fff isnearly constant independent of Kt, as shown in Figure 2. This is due to the exist-ence of stable crack growth in the sharply notched specimens (Figure 4). Asimilar phenomenon can be seen in the fatigue tests [2] of notched steelspecimens.

According to the concept of linear notch mechanics, there should be one toone correspondence between Kt • (Jf ((/max ) and I/ p , independent of the val-ue of notch depths and the other geometrical conditions. Figure 3 supports itclearly. Figure 3 shows the relation between the maximum elastic stress forfracture, cr max,f, and the reciprocal of the notch root radius, I/ p . It can beseen from Figure 3 that all experimental data of each material fall approximate-ly on a unique characteristic curve, independent of the notch depths. Therefore,it can be concluded from the above results that the occurrence of fracture inCFRPEN is controlled by the maximum elastic stress (7 max and the notch rootradius p alone, independent of the other geometrical conditions. By usingFigure 3 as the master curves for the brittle fracture of these materials, we canpredict the limiting stress for brittle fracture of an arbitrarily shaped platespecimen having the same thickness. This proves the usefulness of linear notchmechanics.

CFRPEN

_____— PEN|3)

200

100

Notch dcplh1 mmoAD

4 mm#A

•

Fiber conlcnl15wt%30wt%40wt%

1 2 3 4 51 / p (mm -i)

Figure 3 : Relation between the critical elastic maximum stressfor CFRPEN.

<7f and II p


Localized Damage 517

100/Zin ,100

(a) OkN 4kN(p - 4 mm, t = 1 mm)

The load for final fracture was 4.1kN

(b) OkN 2.8kN(p = 0.2 mm, t = 1mm)

The load for final fracture was 3.2kN

Figure 4 : Successive observations of crack initiation and stable crack growth(a) In the case of p > p Q, (b) In the case of p < p Q.

Figure 4 shows the results of successive observations using the plasticreplica method. It is obvious from Figure 4 that in the case of p > p Q manymicro cracks initiate over a wide region and no crack propagates in a stable way,even just before fracture, and, on the other hand, in the case of p < p$ a crackpropagates in a stable way to some degree, where p$ is the notch root radius atthe branch point.

Figure 5 shows SEM fractographs in the case of p < PQ. Many fiberpull-out can be seen in the region of 1. A fracture surface such as 1 is equiva-lent to the zone of stable crack growth and the one such as 2 is equivalent to thezone of unstable fracture. Figure 6 shows SEM fractographs in the case of p >PQ. A fracture pattern such as 3 is very similar to the one such as 2. It suggeststhat a crack can not propagate in a stable way in the case of p > p$. The stablecrack growth before final fracture seen in the region of 1 in Figure 5 is the rea-son that the stress causing brittle fracture a f is nearly constant in the sharplynotched plate specimens independent of the stress concentration factor Kt.

Figure 7 shows the relations between <7max,f /VB and IIp which areuseful to discuss the effect of fiber content on the notch sensitivity. It can beseen from Figure 7 that <7max,f /^B ^ nearly independent of fiber content in theregion of p > /OQ>resin* where the /OQ, resin ̂ the notch root radius at a branchpoint of matrix resin [3]. In the region p < £o,resin> the specimen of 30% fibercontent is most insensitive to the notch .



Notch root

Crack propagation

(a) Enlargement of (T) (b) Enlargement of

Figure 5 : SEM fractographs in the case of p < p$ (p = 0.2mm, t = 1mm).

Notch root

Crack propagation »̂ Enlargement of (3)

Figure 6 : SEM fractographs in the case of p > p$ (p = 4 mm, t = 1mm).


Localized Damage 519

3-1

b 2-

Notch depth1 mmoAD

4 mm#A

•

Fiber content

15wt %30wt %40wt %

0 1 2 3 41/p (mm*')

Figure 7 : Relation between Omax,f /0"B and

CONCLUSIONS

1/P CFRPEN .

The static tensile tests were carried out to investigate the fracture criterion ofnotched CFRPEN specimens . The results are summarized as follows.(1) The relation of <jf - K* is to be classified into two parts, namely, a part

where <jf decreases with increasing K* and a part where af is nearly con-stant independent of K*. The phenomenon of the nearly constant af shouldbe attributed to the existence of the stable crack growth .

(2) The occurrence of fracture in CFRPEN is controlled mainly by a max and palone, independent of the other geometrical conditions.

(3) Linear notch mechanics is very useful for analyzing the static tensile fracturebehavior of notched CFRPEN specimens.

(4) Notch sensitivity of CFRPEN is almost independent of fiber contents with-in the range of our experiments.

REFERENCES

1. Nisitani, H., "Linear Notch Mechanics as an Extension of Linear FractureMechanics.', (Ed. Sih, G. C. and Nisitani, H.), pp. 25-37, Proc. Role ofFrac. Mech. in Modem Tech., Fukuoka 1987, North-Holland, 1987.

2. Nisitani, H., "Effect of Size on the Fatigue Limit and the Branch Point inRotary Bending Tests of Carbon Steel Specimens.', Bull. Jpn. Soc. Mech.Eng., Vol. 11, No 48, pp 1947-1957, 1968.

3. Nisitani, H., Y-H. Kim, Y. Yamaguchi and H. Noguchi, "A Study on theStatic Tensile Strength of Notched CFRP Specimens.', Trans. Jpn. Soc.Mech. Eng., (in Japanese), Vol. 57, No.539, A, pp 1643-1647, 1991.


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ABSTRACT - WIT Press€¦ · Localized Damage 519 3-1 b 2-Notch depth 1 mm o A D 4 mm # A • Fiber content 15wt % 30wt % 40wt % 01234 1/p (mm*') Figure 7 : Relation between Omax,f