@misc{oai:ir.soken.ac.jp:00001665,
 author = {Denk, Lars and デンク, ラース and DENK, Lars},
 month = {2016-02-17, 2016-02-17},
 note = {<b>1. Objective</b>
　Carbon-carbon (C/C) composite is at present the most mature high temperature material retaining its superior specific strength and specific modulus at temperatures above 2000℃. Despite its superior high temperature behaviour, the C/C composite has some important drawbacks which are their
<small>◆</small>　　high cost,
<small>◆</small>　　susceptibility to oxidation at elevated temperatures, and
<small>◆</small>　　not fully clarified fracture behaviour.
At present, the not fully clarified fracture behaviour prevents the usage of C/C composites in load bearing applications. The aim of this work is to contribute to a deeper understanding of the fracture behaviour in order to promote the usage of C/C composites in a broader variety of applications.

<b>2. Background</b>
　The notch insensitive behaviour of C/C s has been explained by Evans et al. [１] by the model so-called &quot;shear band&quot; formation. This concept is simply explained as: Tensile stress concentration leads inevitably to high shear stress at the stress concentration source, e.g. at a crack tip. Due to the C/C's low shear strength, shear damage extends. This shear damage starts at the crack tip and extends normal to the crack parallel to the loading direction as the tensile load increases. The shear damage near completely diffuses the stress concentration at the crack tip leading to notch insensitivity.
　According to Evans et al. [１], when the ratio of shear and tensile strength is sufficiently low, the shear band concept is effective for explaining notch insensitive behaviour of brittle matrix composites.
C/C composites in general have low ratio of their shear and tensile strength. Thus, Evans et al. have insisted that C/C composites should be categorized into materials exhibiting large shear bands.
However, this approach leaves several questions for applying to the tensile fracture of notched C/Cs:
　The shear bands were not observed at all in notched C/Cs with cross-ply and quasi-isotropic laminations by optical inspection in tensile tests, for example, carried out by Hatta et al. [2], leading to the question of the relevance of this concept.
　The present study first shows that the contribution of the shear band formation to the fracture toughness enhancement of laminated C/Cs is not so high as Evans et al. insisted. Then, other source mechanisms of toughness enhancement were explored.

<b>3. Fracture patterns of notched laminated C/Cs</b>
　Notched specimens of a uni-directionally reinforced C/C with fibres parallel to the loading direction (UD0) exhibit clear shear bands emanating from notch tips. In contrast, notched C/Cs with fibre orientation normal to the loading direction (UD90) show brittle fracture without shear bands.
Thus, in order to examine shear band formation and its contribution to toughening, fracture behaviours of notched and smooth laminated C/C s with a variable ratio between UD0 and UD90 were observed under tensile loads, and notch sensitivity of them was evaluated.
　The experimental results showed that the shear bands clearly observed when the ration of the UD0 and UD90 (<i>R</i>) of the C/Cs was larger than 0.6, but did not when <i>R</i> < 0.6. Thus, when <i>R</i> > 0.6, complete notch-insensitivity was resulted due to the large shear bands. In contrast, when <i>R</i> < 0.6, multiple small splits to the loading direction and low degree of notch insensitivity similar to quasi-isotropic (QI), or cross-ply laminates (CP) (0/90)<small>n</small> were observed.
Although the shear bands for <i>R</i> > 0.6 and multiple splits for <i>R</i> < 0.6 were parallel cracks extending to the loading direction, both the cracks were different at the following two points;
l) While the shear bands extended through-the-thickness, the splits remain only in the 0° layers. Thus, when <i>R</i> < 0.6, remaining 90°layers can transfer a limited amount of the concentrated load to the adjacent 0°ligaments.
2) The sear bands were formed by mode II due to shear stresses, but the splits by mode I due to tensile stresses.

<b>3. Tensile strengths of C/Cs</b>
　Tensile tests carried out using double-ends-noched(DEN) specimens with the CP lamination have revealed a surprising result. When the notch length <i>a</i> was small, the fracture stress with respect to the net sectional area, means, the net fracture stress at σ<i><small>f,net</i></small> of the notched specimens, was much higher than the tensile strength obtained using smooth specimens. This means that stress concentration sources lead in some cases to higher tensile strength. A concept like shear band formation, which focuses on the reduction of stress concentration cannot explain a tensile strength higher than that obetained using straight and un-notched specimens.
　For deep understanding of the strength enhancement behaviour, tensile tests with CP specimen having several holes were carried out. This configuration was chosen for two reasons. At first, this configuration allowed the observation of the strength-enhancing phenomenon under a wide variety of parameters. Secondly, the hole generates mild stress concentrations. The mild stress concentrations allowed many cycles of a non-linear FEM analysis before termination. Consequently, the process of initiation and propagation of the multiple splits from the stress concentration points could be studied numerically in detail. The conclusion of this comparison was that both the splits and strength enhancing phenomena have a major effect on the high toughness of C/Cs.
　In the above study, it was found that the strength enhancing effect at stress concentration sources is caused by damage of the fibre and matrix interface. In this case, the mechanism of rotation and straightification of the carbon fibres in the matrix was identified as source of the fibre-matrix de-bonding. The effect of strength enhancement by fibre-matrix de-bonding was then examined more extensively by application of various damage sources. The shear pre-load, fatigue loads, and slight
oxidation were found to yield the tensile strength enhancement effect, it was verified that the stregth enhancemnet was commonly caused by the interfacial damage between the fiber and matrix.

<b>5. Toughening mechanisms of C/Cs</b>
　Through the course of discussion, following conclusions on the toughening mechanisms of C/Cs
were obtained;
l) When the mode II toughness propagating parallel to the loading direction is sufficiently lower than mode I toughness normal to the load, shear bands are principal mechanism to yield complete notch insensitivity of C/Cs.
2) In other cases, multiple splits and tensile strength enhancement are identified as source of high toughness of C/Cs.
3) The tensile strength enhancement is caused by interfacial damage between the fiber and matrix.
The logical flow and conclusions as well as the constitution of the present thesis are summarized in the Figure 1.

<b>References</b>
[1] A. G. Evans and F. W. Zok, &quot;Review: The Physics and Mechanics of Fibre-Reinforced Brittle Matrix Composites, &quot; J. Mater. Sci., 29, 3857-3896, (1994).
[2] Y. Kogo, H. Hatta, H. Kawada and T. Machida, &quot;Effect of Stress Concentration on Tensile Fracture Behavior of Carbon-Carbon Composites&quot;, <i>J Comp. Mater.,</i>32 [13l, 1273-1294 (1998)., application/pdf, 総研大乙第197号},
 title = {The Effect of Damage on the Fracture Toughness of C/C composites},
 year = {}
}