Published: 18 September 2015

Dynamic stress intensity factor of a finite crack based on a fractional differential model

Runtao Zhan1
Zhaoxia Li2
1, 2Department of Engineering Mechanics, Southeast University, Jiangsu Key Laboratory of Engineering Mechanics, Sipailou 2, Nanjing, 210096, China
Corresponding Author:
Zhaoxia Li
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Abstract

Fractional differential constitutive models are introduced for transient problem of Mode III finite length crack in a viscoelastic medium. The basic equations which govern the deformation behavior are converted to fractional wave-like equations. Integral transform method reduces the problem to Fredholm integral equation of second kind. Dynamics stress intensity factors of Mode III finite crack based on fractional differential constitutive are obtained by numerical solution of Fredholm integral equation.

1. Introduction

The increasing use of polymers and concrete demands an understanding of the response of cracked viscoelastic body to vibrational load. Rosakis et al. discovered cracks in a polymer, loaded under shear, propagate faster than the speed of shear waves [1]. Motivated by the experimental observations, it is desirable to make a study of cracks, loaded dynamically, in viscoelastic materials. Computation of the stress intensity factor (SIF) is one of the key tasks in fracture analysis. Dynamic stress intensity factors (DSIFs) of cracks imposed by dynamic load in viscoelastic materials have been explored by some researchers [2-6].

Previous work on DSIFs is mainly concerned with viscoelastic materials whose equation of motions are given in the form of wave equations, and constitutive equations are given in the form of various models of integer linear viscoelasticity, which are ‘‘integer differential” viscoelastic model and variations of them, however, they are not sufficiently accurate for more complex materials, i.e. some polymer. Recently the “fractional differential” models have been investigated for viscoelastic materials by a number of researchers [7, 8].

DSIFs of Mode III semi-infinite crack in a viscoelastic medium have been obtained by four parameters Bagley-Torvik fractional differential constitutive models [9]. This paper deal with the mode III finite length crack in a viscoelastic medium based on the fractional differential constitutive models.

2. Formulation of the problem

The basic equations of finite Mode III crack in viscoelastic medium consist of equation of motion as shown in Fig. 1.

Fig. 1Finite Mode III crack in a viscoelastic medium

Finite Mode III crack in a viscoelastic medium
1
σxzx+σyzy=ρ2uzt2,

Strain-displacement relations:

2
εxz=12uzx, εyz=12uzy,

and constitutive equations defined by extending the one-dimension Bagley-Torvik fractional differential model to two-dimension [10, 11]:

3
1+b1Dασxz=2μ1+b2Dβεxz,
4
1+b1Dασyz=2μ1+b2Dβεyz,

where α, β (0 <α, β< 1) is the order of the fractional derivative and b1, b2 (b1, b2 0) are parameters. Obviously, when b1=b2= 0, Eqs. (3), (4) are equal to the linear elastic constitutive relationships.

There are various definitions of fractional integration and differentiation, such as Grunwald-Letnikov’s definition, Riemann-Liouville’s defintion, Caputo’s definition [12-14]. For the purpose of this paper, the Caputo’s definition of fractional integration and differentiation will be used, taking the advantage of Gaputo’s approach that the initial conditions for fractional differential equations with Caputo’s differentiation take on the same form as for integer-order differential equations [13, 14]:

5
Dηft=ηftη=1Γn-ηatfnτ(t-τ)η+1-ndτ, n-1<Reηn, nN,

where the parameter η is the order of fractional differentiation.

The Laplace transform of f(t) and its inverse formula are defined by:

6
Lft=f*p=0fte-ptdt, ft=12πic-ic+if*peptdt.

The Laplace transform of the Caputo’s fractional derivative can be written as [14]:

7
LDηft=pηLft-k=0n-1fk0sη-k-1.

Taking the derivation of the Eqs. (3), (4) with respect to x, y, we have:

8
1+b1Dασxzx =μ1+b2Dβ2uzx2,
9
1+b1Dασyzy =μ1+b2Dβ2uzy2.

The fractional wave-like equation can be obtained [9]:

10
1+b2Dβ2uz=1c22uzt2+b12+αuzt2+a,

where c=μ/ρ.

For the finite length crack, the boundary conditions are:

11
uzx,0,t=0, x>a, t>0,
12
σyzx,0,t=-τHt, x<a, t>0,
13
σijx,y,t=0, x2+y2, t>0,

where τ is the amplitude of the anti-plane shear load, H(t) is Heaviside unit step function and crack length is 2a.

The initial conditions are:

14
uz(x,y,0)=0, uzx,y,ttt=0=0.

3. Dynamic stress intensity factors

Taking the Laplace transform of the Eq. (10) with respect to t, we have [9]:

15
2uz*=ξpc2uz*,

where ξ2=1+b1pα/1+b2pβ, ξ>0.

The Fourier transform and its inverse transform is:

16
f¯s=-fxeisxdx, fx=12π-f¯se-isxds.

Taking the Fourier transform of the Eq. (15) with respect to x, we have:

17
d2u¯z*(s,y,p)dy2=ξpc2+s2u¯z*s,y,p.

The solution to the Eq. (17) by applying the boundary condition Eq. (13) is:

18
u¯z*s,y,p=Cs,pe-γy,

where γ2=ξp/c2+s2, (γ> 0).

The Laplace transform to Eqs. (11) and (12) gives by using the symmetry:

19
u*zx,0,p=0, x>a,
20
σ*yzx,0,p=-τp, 0<x<a.

Taking the inverse Fourier cosine transform of the Eq. (18), we have [15]:

21
uz*x,y,p=2π0Cs,pe-γycossxds.

Eqs. (19) and (21) can be applied to lead to:

22
0C(s,p)cos(sx)ds=0.

The Laplace transform to Eq. (9) with applying the Eq. (2) leads to:

23
1+b1pασyz*=1+b2pβuz*y.

From Eq. (20) and (23), we find:

24
0γC(s,p)cossxds=πτ1+b1pα2μp1+b2pβ.

Eqs. (20) and (23) form a pair of dual integral equations. The function C(s,p) is defined by:

25
Cs,p=πτ1+b1pαa22μp1+b2pβ01ζΦ3*ζ,pJ0saζdζ.

The dual integral equations can be solved by a Fredholm integral equation of the second kind [15]:

26
Φ3*ζ,p+01Kζ,η,pΦ3*η,pdη=ζ,

where:

27
ζη120s2+ξpac212-sJ0sζJ0sηds=Kζ,η,p,

where J0 is a zero order Bessel function. Fredholm integral equation Eq. (26) is treated by the composite Simpson of rules to obtain Φ3*(1,p), the numerical solution to Eq. (26) can be found in Appendix Φ3*(1,p) is function of c/ξpa which can be seen in Fig. 2.

Fig. 2Solution of Fredholm equation for finite Mode III crack

Solution of Fredholm equation for finite Mode III crack

The DSIF of cracked viscoelastic body can be found [15]:

28
K3t=L-1Φ3*1,pξpτπa=L-11+b2pβ121+b1pα12Φ3*1,ppτπa.

From Eq. (1), the viscoelastic DSIF is equal to elastic DSIF when 1+b1pα=1+b2pβ.

The Miller and Guy numerical inverse Laplace transform is used for Eq. (1).

4. Numerical results

Let K3#(t)=K3/τπa. Numerical results for the dynamic SIF were obtained for 4 cases:

– Case a: b1=b2=8, β= 0.5, α= 0.25, 0.5, 0.75.

– Case b: b1=b2= 8, α= 0.5, β= 0.25, 0.5, 0.75.

– Case c: α=β= 0.5, b1= 8, b2= 4, 8, 12.

– Case d: α=β= 0.5, b2= 8, b1= 4, 8, 12.

Fig. 3Viscoelastic DSIF time history for different cases

Viscoelastic DSIF time history for different cases

a) Case a

Viscoelastic DSIF time history for different cases

b) Case b

Viscoelastic DSIF time history for different cases

c) Case c

Viscoelastic DSIF time history for different cases

d) Case d

As shown in Fig. 3(a)-Fig. 3(d), DSIFs are proportional to the parameters β, b2, and inversely proportional to the parameters α, b1.

As shown in Fig. 3(a)-Fig. 3(b), The fractional differential order have strong effects on the viscoelastic DSIF after DSIF curves reach the peak of time history curve. When fractional order α>β, the curve is negative gradient, while when α<β, the curve is negative gradient after the peak of DSIF time history.

As shown in Fig. 3(c)-Fig. 3(d), parameters b1, b2 have little effect on the gradient of the DSIF curves after peak of DSIF curve.

5. Conclusion

Bagley-Torvik fractional differential model are introduced to obtain a fractional wave-like equation for finite Mode III crack. Bifurcation phenomenon of viscoelastic DSIF can be observed by choosing appropriate fractional differential order α, β. Parameters b1, b2 have some effect on amplitude of viscoelastic DSIF curve but little effect on the gradient of viscoelastic DSIF after the peak of the DSIF curve.

References

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About this article

Received
Accepted
24 July 2015
Published
18 September 2015
SUBJECTS
Fault diagnosis based on vibration signal analysis
Keywords
crack
fracture mechanics
dynamics
fractional calculus
wave