Open access peer-reviewed chapter

Non-Steady First Matrix Cracking of Fiber-Reinforced Ceramics

By Huan Wang

Submitted: February 11th 2020Reviewed: May 29th 2020Published: December 23rd 2020

DOI: 10.5772/intechopen.93060

Downloaded: 63

Abstract

Matrix cracking affects the reliability and safety of fiber-reinforced ceramic-matrix composites during operation. The matrix cracking can be divided into two types, that is, steady state crack and non-steady state cracking. This chapter is about the non-steady stable cracking of fiber-reinforced CMCs. The micro stress field of fiber, matrix, and interface shear stress along the fiber direction is analyzed using the shear-lag model. The relationship between the crack opening displacement and the crack surface closure traction is derived. The experimental first matrix cracking stress of different CMCs are predicted.

Keywords

  • ceramic-matrix composites (CMCs)
  • first matrix cracking stress
  • brittle matrix
  • non-steady state
  • pre-existing defect

1. Introduction

Fiber-reinforced ceramic-matrix composites (CMCs) have greater specific strength and specific stiffness. It will decrease the weight of the aircraft structure when it is applied to the aircraft. However, there are some disadvantages like complex processing and preparation, expansive, and so on. Now, there are some models for first matrix cracking. The MCE model [1] is one of the most famous models which established the relation between A.C.K and crack theory. McCartney model [2] gives a detailed process about the numerical solution. Chiang et al. [3, 4] used a modified shear-lag model considering the matrix deformation and the fiber failure is also considered. This chapter is about the non-steady matrix cracking of fiber reinforced CMCs. We assume that the fiber is strong enough to keep intact when matrix cracking occurs, and the composites with interface debonding are susceptible to weak frictional resistance. The growth characteristics of short cracks are evaluated using the stress intensity method. We will do some analysis about the fiber-matrix stress and solve equations to get the closing traction distribution. Then, the matrix cracking condition is combined to obtain the critical matrix cracking stress. The final results will show how the cracking stress is related to the size of a pre-existing defect and prediction of the threshold stress. Differences between the MCE model and McCartney model are also analyzed.

2. Fiber-matrix stress analysis

All analyses come from McCarteny model [2]. By performing stress analysis, the influence of the fiber can be equivalent to applying a distribution of closing pressure p(x1) on the crack surface, and the influence of the applied stress can be evaluated by regarding the stress as a uniform opening pressure σacting along the matrix crack surface. Therefore, we can obtain the net pressure on the crack surface, σpx. And the relation can be assumed for the continuum model [1]:

px1=λux1=px1E1

where x1represents the location on the crack surface. According to the Sneddon and Lowengrub [5] and the force analysis, we can get the relation between the effective traction p(x1) and displacement distributions u(x1) as follows:

ux1=2π2x1att2x120tσpξt2ξ2dt0x1<aE2

And we can also get the corresponding stress intensity factor [6]:

K=2aπ0aσpξa2x2dxE3

To make the formula more simplified, we can get the following simpler formula [2]:

ux1=1π20aσpξlna2ξ2+a2x12a2ξ2a2x120x1<aE4

After making some substitutions, we can obtain the following equation [2]:

P2X=μ1X21π01Ptln1t2+1X21t21X2dt0X<1E5

here

μ=λ2a/πσE6
K=σπaYE7

here

Y=2π011PX1X2dXE8

To obtain the parameter λ, equating the energy availability for the continuum and discrete fiber models. For the discrete fiber model, the energy available per unit area for matrix cracking is [2]:

E=R6τEc1v22Vm2Em2Vf2Efϵ3E9

For the continuum model, the energy available per unit area for matrix cracking is [2]:

E=σu20u2pu2du2=4π3λ2Ec1v22ϵ3E10

By equating E = E, the parameter λis obtained:

λ=2VfVm2πτREfEcEm21/2E11

this is different with the parameter λ¯used by Marshall et al. [1].

λ¯=2Vf2πτREfVmEm1/2=VmEmEc1/2λE12

Now let us compare the parameter λwith λ¯. The parameter λis obtained through the energy balance considerations, while the parameter λ¯is gotten by doing some mechanical analysis of the discrete fiber model. Therefore, we can know that the parameter λcan account for the energy changes like stored energy and frictional energy dissipation.

What’s more, it is necessary to choose a more reasonable matrix cracking condition to obtain the final critical matrix cracking stress. Now two matrix cracking conditions will be on the list. It is noted that all the conditions are not on the physics ground. Firstly, it is about Griffith fracture criterion as follows [2]:

K2=2γEc/1v2E13

According to this, we can derive the cracking condition. Secondly, it is assumed that the matrix and composite stress intensities scale with the stress. So we can derive the relation as follows [2]:

KL=KcL=KcMEc/EmE14

And this chapter adopts the first kind of condition. Finally, the equations concerned with critical cracking condition in this chapter are derived as follows [2]:

a/a0=μc/Yμc23E15
σc/σ0=1/μcY2μc13E16

here, Y is a function only of μ.

To predict the threshold stress and obtain the relation between the cracking stress and the pre-existing defect, we should firstly solve Eq. (5) and obtain the effective traction distribution. Then, we can obtain many values of Y(μ) corresponding to a range of values of μ. According to the cracking condition, the curve related to the dependence of critical cracking stress on the pre-existing defect is generated.

3. Numerical solution to matrix cracking stress

The main problem is to solve the nonlinear integral Eq. (5). The Simpson’s integration formula is used here to derive the discrete form of the nonlinear integral equation which makes the substitutions. And finally, we can obtain the following discrete formulation [2]:

giN+122+μSigiN+1+1=μj=0jingjNgiNKij,i=0,.nN0E17

here

gi=gi/n,Si=2i/n21+i/n4,gi0=0,i=0,,n,

And,

Kij=8δjπn4lnSj+SiSjSij31+j/n42,ji,

With δ0=δn=13,δj=43ifjisodd23ifjis even,j=1,,n1.

And the corresponding discrete form of Eq. (6) for Y [2] is given by

Y=8πn2j=0njδjgj1+j/n4E18

When solving Eq. (15), the parameter μcan be valued like 0.1, 1, 10, 100 and so on. Then we can get the value of giN+1by solving a quadratic equation and using the starting value. What’s more, 0g1. After getting values of all giN+1, by comparing the two adjacent iterations until a group of solution meets the condition [2]:

1n+1i=0ngiN+1giN2<δE19

where δrepresents the accuracy and can be valued like 106,108.

In this chapter, the value of δis 106, and the values of μare 0.1, 1, 100.

This is the flow chart of the solution:

4. Results and discussion

By solving Eq. (15), we can get the curve of the distribution of effective tractions acting on the crack surfaces as in Figure 1. The horizontal axis represents the location in the crack surface, and the vertical axis represents the continuous effective traction. And it is simple that both the horizontal and vertical axes are in percentage terms. So this figure shows the closing traction distribution in a range of parameter μ. The distributions corresponding to different values of μare different. And larger the value μis, more load will the fiber support at the same position. When μcomes to 100, the fiber supports the applied load for most of the crack length. As is mentioned above, this figure gives the accurate result for Eq. (1). And according to this result, we can get the critical matrix cracking stress.

Figure 1.

The flow chart of the solution to the effective fiber tractions.

And the corresponding curve which shows the relation between the critical cracking stress and the pre-existing defect is gotten with the cracking condition in Figure 2. And the value of δis 106, and the values of n is 60. The result can be obtained by choosing different values of a/a0 and getting the corresponding value of μ. And the values σc/σ0varies with a/a0. a/a0represents the length of pre-existing defect, and σc/σ0represents the corresponding matrix cracking stress. Both a/a0and σc/σ0are standard forms. In this curve, it is noted that the critical stress decreases with the length of pre-existing defect increasing when the length of pre-existing defect is short. And the stress tends to be constant with the length rising when the length is over a value. When the length of pre-existing defect is below a value, the corresponding critical stress is decided by the length of crack, while the critical stress will be independent of the total pre-existing crack length when the length of the defect is over the characteristic distance (Figure 3).

Figure 2.

The distribution of effective traction acting on the crack surface.

Figure 3.

The critical stress for matrix cracking on the length of the pre-existing matrix crack.

To obtain the threshold matrix cracking stress by choosing a/a0, we choose the parameter μlike in the following table. It is shown that σc/σ01.331when a/a0. We can know that it is close to the theoretical value by calculation (Table 1).

  1. The condition that the fiber is strong enough to keep intact and it does not take into account the shear deformation in the matrix above the slipping region is considered. But obviously the fiber failure and the deformation above the slipping region influences the matrix cracking stress. This chapter gives the numerical solution. MCE model gives the approximate analytical solution [1] for short crack. And we can see that the two methods show the same trends, and the numerical solution is lower than the analytical solution. It is noted that the MCE model assumed the cracking condition instead of deriving the condition. And the result of MCE model do not establish the threshold matrix cracking stress below which is impossible to make the matrix crack. The model will be valid when the ratio a/R is large enough, as the fiber radius R was not quoted. It is not possible to determine that part of the curve in Figure 5 satisfying the validity condition a/R > 10. So it is of vital importance to derive numerical results. As Figure 1 shows, the distribution of effective tractions px1/σincreases with the parameter μ rising at the same position. By changing the value of Vf, we can finally change the standard value σ0. As a result, the value of matrix cracking stress changes. And the relation between the matrix cracking stress and Vfis shown in Figure 4. The parameters of three kinds of material are listed in Tables 24. They are SiC-glass ceramic, SiC/borosilicate, and C/borosilicate. And the formula can be derived according to Eqs. (6) and (11):

u0.1110100100015002000
σc/σ02.520381.5825941.3556361.3328821.3310291.3310121.331025

Table 1.

Values of the critical cracking stress for various values of μ[2].

Figure 4.

The distribution of effective tractions with different values of parameter V f in three kinds of material. (a) SiC-glass ceramic. (b) SiC/borosilicata. (c) C/borosilicata.

ParameterEf/GPaEm/GPaKICmMPamR/μmτ/MPa
Value20085282

Table 2.

The parameters of SiC-glass ceramic.

ParameterEf/GPaEm/GPaKICmMPamR/μmτ/MPa
value400630.77706–8

Table 3.

The parameters of SiC/borosilicata.

ParameterEf/GPaEm/GPaKICmMPamR/μmτ/MPa
value380700.75410

Table 4.

The parameters of C/borosilicata.

μ=8τaVf2EfEfVf+EmVmRVm2Em2σE20

The relation that the parameter μincreases with the Vfrising can be gotten by taking the derivative of the equation to determine its monotonicity. As a result, the distribution of effective tractions px1/σacting on the crack surface rises with parameter Vfincreasing. The parameter a represents the matrix cracking length, and σrepresents the applied stress. And as we all know, the distribution of effective traction which is equivalent to the effect of fiber traction can make the fiber ends displace to be rejoined. So the greater value of px1/σ, the harder the matrix to crack which means the value of the matrix cracking stress is bigger. And the parameter τis in positive correlation with the parameter μ. In the same way, the matrix cracking stress will increase with the parameter τrising. The curve can be seen in Figure 5 with three kinds of material which show the same tendency.

Figure 5.

The distribution of effective tractions with different values of parameter τ in three kinds of material. (a) SiC-glass ceramic. (b) SiC/borosilicata. (c) C/borosilicata.

5. Conclusion

McCartney model gives a more reasonable parameter λwhich is gotten by equating the available energy of fiber discrete model and continuum model, while the MCE model do some stress analysis of fiber discrete model to get this parameter. So the McCartney model can explain the energy changes. McCartney model derived rather than assuming the matrix cracking condition. The cracking condition is gotten through the Griffith fracture criterion in McCartney model. MCE model assumed that the matrix and composite stress intensities scale with the stresses. And McCartney gives the threshold stress to verify the theoretical value.

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 License, which permits use, distribution and reproduction for non-commercial purposes, provided the original is properly cited.

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Huan Wang (December 23rd 2020). Non-Steady First Matrix Cracking of Fiber-Reinforced Ceramics, Safety and Risk Assessment of Civil Aircraft during Operation, Longbiao Li, IntechOpen, DOI: 10.5772/intechopen.93060. Available from:

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