ES

TA

KÄ

GO

N

atural vibrations of elastic stepped plates with cracks

Tartu 2013

ISSN 1024–4212ISBN 978–9949–32–341–8

DISSERTATIONES MATHEMATICAE

UNIVERSITATIS TARTUENSIS

86

ESTA KÄGO

Natural vibrations of elastic stepped plates with cracks

DISSERTATIONES MATHEMATICAE UNIVERSITATIS TARTUENSIS 86

DISSERTATIONES MATHEMATICAE UNIVERSITATIS TARTUENSIS 86

ESTA KÄGO Natural vibrations of elastic stepped plates with cracks

Faculty of Mathematics and Computer Science, University of Tartu, Tartu, Estonia

Dissertation has been accepted for the commencement of the degree of Doctor ofPhilosophy (PhD) in mathematics on June 17, 2013, by the Council of the Insti-tute of Mathematics, Faculty of Mathematics and Computer Science, Universityof Tartu.

Supervisor:Prof. Jaan LellepUniversity of TartuTartu, Estonia

Opponents:Prof. Evgeny BarkanovRiga Technical UniversityRiga, Latvia

Prof. Aleksander KlausonTallinn University of TechnologyTallinn, Estonia

Commencement will take place on August 28, 2013, at 12.15 in Liivi 2-403.

Publication of this dissertation has been granted by the Estonian Doctoral Schoolof Mathematics and Statistics.

ISSN ISBN

Copyright: Esta Kägo, 2013

www.tyk.ee

University of Tartu Press

1024–4212 1

Order No. 278

1978–9949–32–341–8 1(print)ISBN 978–9949–32–342–5 (pdf)

Contents

List of original publications 6

1 Introduction 7

1.1 Historical review of literature . . . . . . . . . . . . . . . . . . . . 71.2 Aim of the dissertation . . . . . . . . . . . . . . . . . . . . . . . 91.3 Structure of the dissertation . . . . . . . . . . . . . . . . . . . . . 10

2 Free vibrations of stepped plate strips with cracks 11

2.1 Formulation of the problem . . . . . . . . . . . . . . . . . . . . . 112.2 Basic equations . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Solution of the equation of motion . . . . . . . . . . . . . . . . . 132.4 Local compliance of the plate strip . . . . . . . . . . . . . . . . . 152.5 Determination of natural frequencies . . . . . . . . . . . . . . . . 16

3 Free vibrations of stepped plate with cracks 21

3.1 Statement of the problem and governing equations . . . . . . . . . 213.2 Additional flexibility due to the crack . . . . . . . . . . . . . . . 223.3 Determination of eigenfrequencies . . . . . . . . . . . . . . . . . 25

4 Natural vibrations of stepped plate with cracks on elastic foundation 27

4.1 Formulation of the problem . . . . . . . . . . . . . . . . . . . . . 274.2 Equations of motion . . . . . . . . . . . . . . . . . . . . . . . . . 27

References 30

Summary 37

Kokkuvõte (Summary in Estonian) 38

Acknowledgements 39

Publications 41

Curriculum vitae

5

109

2

List of original publications

I J. Lellep, E. Kägo, Vibrations of stepped plate strips with cracks, Proc. of

the Int. Conf. on Mathematical Models for Engineering Science, (2010),WSEAS Press, 244–249.

II J. Lellep, E. Kägo, Vibrations of elastic stretched strips with cracks, Inter-

national Journal of Mechanics, 5(1) (2011), NAUN, 27–34.

III E. Kägo, J. Lellep, Free vibratons of plates on elastic foundation, Procedia

Engineering, 57 (2013), Elsevier, 489-496.

IV E. Kägo, J. Lellep, Vibrations of a cracked anisotropic plate, Proc. of the

Int. Conf. on Optimization and Analysis of Structures (accepted).

V E. Kägo, J. Lellep, Vibrations of anisotropic stepped plates with cracks,Journal of Sound and Vibration (submitted).

VI E. Kägo, J. Lellep, Vibration of a cracked plate on an elastic foundation,(manuscript).

Author’s contribution

The author of this dissertation is responsible for majority of the research in allphases (including writing, simulation and preparation of images) of the papersI–VI. The solution procedure was developed in co-operation with the supervisor;the statement of the problem belongs to the supervisor.

6

1 Introduction

1.1 Historical review of literature

Generally speaking, all the materials used in the mechanical engineering and tech-nology, can be divided into homogeneous and non-homogeneous materials. Themost of non-homogeneous materials can be treated as composites, consisting oftwo or more ingredients incorporating various constituents have wide range ofspecific strength and others material moduli. Among various composites the uni-directionally reinforced materials seem to have the most favorable combination ofhigh specific modulus and strength. Unidirectional composites can be modeled asanisotropic quasi-homogeneous materials having different moduli in the directionof fibers and in the transverse direction, respectively (see Jones [26], Daniel andIshai [10]).

Due to the growing interest in non-destructive testing techniques and vibra-tion monitoring of structures and machines there is an emerging demand for thevibration analysis of structural elements with flaws and cracks. It was alreadyrecognized long ago that the presence of surface flaws or intrinsic cracks in amachine element is a source of local flexibility, which in turn influences the dy-namic behavior of the whole system. This leads to an important idea to modelcracks as equivalent elastic springs, which was first used to quantify the relationbetween the applied load and the strain concentration in the vicinity of the cracktip by Irwin [24] in the early 1960s. Later Rice and Levy [71] extended the ideato rectangular plates with part-through cracks.

However, these initial works did not focus on the vibration analysis of thestructures. At the same time the basic approaches to vibration analysis initiallystudied the isotropic plates without cracks and steps employing the classical the-ory with Kirchhoff-Love assumptions; see Leissa [38]. Subsequent studies havegeneralized the basic theories of vibration analysis to the anisotropic plates andthe plates with cracks and steps.

The first study to combine vibration analysis with spring model of cracks wasundertaken by Rizos et al. [72] in 1990. The authors developed a method toidentify the crack location and magnitude in cantilever beams. Liang et al. [50],Dimarogonas [12] and Chondros et al. [8, 9] extended this approach to severalcracked structures.

An important approach to identify crack size and location was introduced byLiang et al. [49]. The novel idea was to use the measurements of natural vibrationsto detect cracks. This idea was later extended by Nandwana and Maiti [58]. DeRosa [11] investigated the influence of cracks on the natural vibrations of steppedbeams with flexible ends. Prestressed beams with fixed ends were studied by Ma-

7

soud et al. [54]. However, all previously mentioned methods of natural vibrationsconsidered only a single crack. The effect of multiple cracks on the natural vibra-tions of uniform beams was studied by Lin et al. [52] making use of the transfermatrix method.

Zhu et al. [85] extended the work by Leissa [38] to the free vibration analysis ofthin isotropic and anisotropic rectangular plates, while Gutierrez and Laura [16],Xiang and Wang [81], Li et al. [46] have modeled analytically an isotropic steppedplate with varying boundary conditions.

The well known Lévy method was used by Gorman et al. [14] for the vibra-tion analysis of a plate with two opposite simply supported edges and arbitraryboundary conditions at the other two edges. The Navier method is used by Kantet al. [31] to analyze the vibration of a plate with all four edges simply supported.The Rayleigh-Ritz method was used by Nallim et al. [57] to model an anisotropicplate without steps and by Laura et al. [37] to model an anisotropic stepped plates.Ramamurti et al. [66] have applied the generalized Rayleigh-Ritz method to de-termine the natural frequency of cracked cantilevered plates.

Recently, more analytical and numerical methods for studying the vibration ofanisotropic rectangular plates have been proposed. Anisotropic plates were stud-ied by Huang et al. [23] and by Bui et al. [6] using an efficient mesh free-method.Exact solutions for free vibrations of rectangular plates are proposed by Wu et

al. [80] using Bessel functions. Free vibrations of orthotropic rectangular platesare investigated by Jafari and Eftekhari [25] with mixed Ritz-differential quadra-ture method and Paiva et al. [63] using the boundary element method. A vibrationanalysis of thin rectangular plate has been undertaken by Li and Yuan [48]. Theauthors used a Green quasifunction method to obtain the solution of the free vi-bration problem of clamped thin plates. Wang et al. [78] used the discrete singularconvolution algorithm to analyze free vibrations of a simply supported anisotropicrectangular plate. Natural frequencies for Lévy plate are studied by Park et al. [64]by using a harmonic response estimation method.

Further results on the application of finite element method for dynamic anal-ysis of a thin rectangular plate with a crack have been obtained by Krawczuk et

al. [34,35]. Liew et al. [51] employed the decomposition method to determine thevibration frequencies of cracked plates. An experimental study of natural frequen-cies of clamped rectangular plates with cracks has been described by Maruyamaand Ichinomiya [53]. They investigated experimentally the effect of length, po-sition and inclination angle of a crack on the natural frequencies. However, itshould be noted that the work by Maruyama and Ichinomiya clarified the vibra-tion of a plate with penetrating crack which is different from part-through cracksat the corners of the re-entrant parts of the steps. The latter is the focus of this

8

dissertation.

In recent years the investigations of structures resting on elastic foundationhave gained large importance. The vibration and bending problems of plates onelastic foundations are often faced in the practical engineering of structures. Theexamples of the plates resting on elastic foundations are road surfaces, airport run-ways, aircraft parking areas, railway tracks and building foundations. In this studywe use the well known Winkler model [67], which was originally developed forthe analysis of railroad tracks. The Winkler elastic model is the simplest exampleof the continuous elastic foundation.

The vibration analysis of thin structures on elastic foundations has been car-ried out by various researchers. Let us consider some of recent results and researchmethods in this area. Pengcheng and Peixiang [65] studied vibrations of the plateson elastic foundation using the multivariable spline element method. A few yearslater Cheung et al. [7] developed a finite strip method for the natural vibrationanalysis of stepped plate on an elastic foundation. At the same time Huang andThambiratnam [22] proposed a procedure incorporating the finite strip method to-gether with a spring system to analyze plate resting on elastic supports and elasticfoundation.

Hsu [20] investigated the eigenvalue problems of cracked hinged–hinged andcantilevers Bernoulli–Euler beams resting on elastic foundation using the differen-tial quadrature method. A year later Hsu [21] proposed the differential quadraturemethod for the rectangular plate. Also the finite strip method was complementedby Hatami et al. [17]. Authors provided the exact finite strip method and usedthe method to solve the vibration and stability problems of the axially movingorthotropic plates on the elastic foundation.

Recently, Li and Yuan [47] adopted the quasi-Green’s function method to ana-lyze the free vibration of a clamped thin plate resting on the Winkler foundation.Motaghian et al. [55] proposed a novel mathematical approach to find the exactanalytical solution of the problem of natural vibrations of plates resting on par-tially elastic foundation with certain boundary conditions. The natural vibrationsof the cracked cantilever beam resting on elastic foundation is analyzed by Nassaret al. [59] using the differential quadrature method in the case when the beam ismade of a functionally graded material.

1.2 Aim of the dissertation

The primary goal of this study is to develop a method for finding natural vibrationsof elastic structures with steps and cracks. To the best of our knowledge this wouldbe the first analytical method that has this feature. The goal is to have a methodfor both, for isotropic and anisotropic structures, with the cracks located at the

93

re-entrant corners of the steps (see Fig. 1). It is worthwhile to mention that inthe following we will use the term "crack" instead of a "crack-like defect" for theconciseness sake.

The secondary goal of the dissertation is the application of the proposed methodto different plate structures in order to investigate the effect of cracks on the natu-ral vibrations.

crack

Figure 1: Side view of stepped plate with crack at re-entrant corner.

1.3 Structure of the dissertation

The dissertation has been organized as follows. Section 1 contains a historic back-ground of vibration analysis, the aim and the structure of the dissertation. In Sec-tion 2 our method is described in detail and applied to isotropic plate strips thatare stepped and have cracks. In Section 3 the method is extended to anisotropicplates for determination of frequencies of natural vibrations. Finally, Section 4extends the approach to anisotropic plates resting on the elastic foundation.

10

2 Free vibrations of stepped plate strips with cracks

The objective of this section is to develop the basic concepts for the analysis of thefree vibrations of plate strips. A review of the papers by Lellep and Kägo [39,40]is presented herein.

2.1 Formulation of the problem

Let us consider natural vibrations of a plate strip subjected to the in-plane tensionN (Fig. 2). Let the dimensions of the strip in x and y directions be l and b,respectively.

y

x

b

la1 a2 an

NN

(a) Dimensions of the plate strip

z

x

h0

h1

hn

a1 a2 an l0

(b) Side view of the stepped plate strip

Figure 2: Plate strip

It is assumed that the thickness h(x, y) = hj is piecewise constant for x ∈(aj , aj+1), where j = 0, . . . , n. The quantities aj and hj (j = 0, . . . , n) are

11

given constants whereas a0 = 0, an+1 = l.

12

In the present study like in Rizos et al. [72], Chrondos et al. [9], Dimarog-onas [12] the effect of crack propagation in the body during vibrations is disre-garded.

The material of plates is considered as a linear elastic material. Both, homo-geneous elastic plates and these made of non-homogeneous composite materialsare studied.

The aim of this section is to elucidate the sensitivity of natural frequencies onthe crack parameters and geometrical parameters of the plate strip.

2.2 Basic equations

In the present case of the plate it is reasonable to assume that the stress-strain stateof the plate depends on the time t and coordinate x only, provided the stressesmean generalized stresses (bending moments and membrane forces). However,due to the tension N applied at the edge of the plate, in-plane forces have to betaken into account. If, moreover, the inertia of the rotation is not neglected theequilibrium equations of a plate element can be presented as (Reddy [70])

∂Mx

∂x= Qx

∂Qx

∂x= −N

∂2W

∂x2+ ρhj

∂2W

∂t2− Ij

∂4W

∂x2∂t2

(1)

for x ∈ (aj , aj+1). In (1) W = W (x, t) stands for the transverse deflection cor-responding to the point with coordinate x at the middle plane of the plate whereasMx is the bending moment and Qx is the shear force. Herein

Ij =ρh3j12

(2)

where ρ stands for the density of the material. According to this theory the mem-brane force in the direction of the axis x Nx = N in the present case.

Eliminating the shear force Qx from (1) one easily obtains

∂2Mx

∂x2+N

∂2W

∂x2= ρhj

∂2W

∂t2− Ij

∂4W

∂x2∂t2(3)

for x ∈ (aj , aj+1) where j = 0, . . . , n.It is well known that (Reddy [70], Soedel [74])

Mx = −Dj∂2W

∂x2(4)

where Dj = Eh3j/[12(1 − ν2)]; j = 0, . . . , n.

Substituting (4) in (3) yields

Dj∂4W

∂x4−N

∂2W

∂x2= −ρhj

∂2W

∂t2+ Ij

∂4W

∂x2∂t2(5)

for x ∈ (aj , aj+1) where j = 0, . . . , n. Here W is the transverse deflection,

Ij =ρh3j12

, Dj =Eh3j

12(1 − ν2), (6)

E and ν are elastic moduli and ρ is density of the material. The equation (5) willbe considered as the equation of motion for the segment (aj, aj+1); j = 0, . . . , n.It can be solved accounting for appropriate boundary conditions.

In the case of a free edge of a strip the boundary conditions are

∂2W

∂x2= 0,

∂3W

∂x3= 0, (7)

for the case of the clamped edge

W = 0,∂W

∂x= 0, (8)

and for the simply supported edge

∂W

∂x= 0,

∂2W

∂x2= 0. (9)

Let at the initial moment

∂W

∂t= 0, W = ϕ(x) (10)

where ϕ is a given function.

2.3 Solution of the equation of motion

It is reasonable to look for the general solution of (5) in the form

W (x, t) = wj(x)T (t) (11)

for x ∈ (aj , aj+1) where j = 0, . . . , n.Differentiating (11) with respect to variables x, t and substituting in (5) one

easily obtainsDjw

IVj T −Nw′′

j T = −ρhjwjT + Ijw′′

j T (12)

134

for x ∈ (aj , aj+1) where j = 0, . . . , n. Here prims denote the differentiation withrespect to the coordinate x and dots with respect to time t.

Separating variables in (11) yields

DjwIVj − (Ijω

2 +N)w′′

j + ρhjω2wj = 0 (13)

for j = 0, . . . , n andT + ω2T = 0 (14)

where ω stands for the frequency of natural vibrations. Evidently, the solution of(14) which satisfies according to (10) initial conditions T (0) = d, T (0) = 0 hasthe form

T = d cosωt (15)

where d is a constant.The equation (13) is a linear fourth order ordinary equation with respect to the

variable wj . The characteristic equation corresponding to (13) is

Djr4j − (Ijω

2 +N)r2j + ρhjω2 = 0 (16)

From (16) one easily obtains the roots

rj = ±

√

√

√

√

Ijω2 +N

2Dj±

√

(Ijω2 +N)2

4D2j

−ρhjDj

. (17)

Introducing the notation

(r2j )1 = −λ2j

(r2j )2 = µ2j

(18)

one can present the general solution of (13) as

wj(x) = A1j cos λjx+A2j sinλjx+

+A3j sinhµjx+A4j cosh µjx (19)

which holds good for x ∈ (aj , aj+1), j = 0, . . . , n. Here A1j , . . . , A4j standfor unknown constants of integration. These will be determined from boundaryconditions and requirements on the continuity of displacements and generalizedstresses.

However, it appears that the quantity W ′ cannot be continuous at x = aj ac-cording to the model of distributed line springs developed by Rice and Levy [71];Dimarogonas [12], Chondros et al. [8].

14

2.4 Local compliance of the plate strip

Let us consider the influence of the crack located at the cross section x = a onthe stress-strain state of the strip in the vicinity of the crack. For the concisenesssake we shall study the case when n = 1 and thus in the adjacent segments to thecrack the thickness equals to h0 and h1, respectively. Let h = min(h0, h1).

According to the distributed line spring method the slope of the deflection hasa jump

Θ = w′(a+ 0)− w′(a− 0) (20)

at the cross section x = a. The angle Θ can be treated as a generalized displace-ment corresponding to the generalized stress Mx. Thus

Θ = CMx(a) (21)

or

C =∂Θ

∂Mx(a)(22)

where C is the local compliance due to the crack. It is known in the linear elasticfracture mechanics that (see Anderson [1], Broberg [4])

Θ =∂UT

∂Mx(a)(23)

where UT is the extra strain energy caused by the crack. Combining (21)–(23)one obtains

C =∂2UT

∂M2x(a)

. (24)

According to the concept of the distributed line spring 1/C = K , where K standsfor the stress intensity coefficient. It is known in the fracture mechanics that (seeAnderson [1])

KM = σM√πcFM

( c

h

)

. (25)

In (25) c is the crack depth and

σM =6Mx(a)

bh2, (26)

provided the element involving the cross section x = a is loaded by the bendingmoment Mx only. Here the function FM is to be approximated on the basis ofexperimental data [76].

If the element is loaded by the axial tension N then the stress intensity coeffi-cient

KN = σN√πcFN

( c

h

)

(27)

15

where

σN =N

bh. (28)

In the case of a combined loading the stress intensity coefficient

KT = KM +KN . (29)

Note that (29) holds good under the condition that (25)–(28) refer to the commonmode of fracture (see Anderson [1] and Broek [5]).

In the present case this requirement is fulfilled, KM and KN regard to the firstmode of the fracture. It was shown in the previous studies (Lellep et al. [43], [41],[45]) that in the case of loading by the moment

KM =E′h2b

72πf(s)(30)

where E′ = E for plane stress state and E′ = E/(1 − ν2) in the case planedeformation state.

Here s = c/h and the compliance

C =72π

E′h2b

∫ s

0

sF 2M (s)ds (31)

whereas

f(s) =

∫ s

0

sF 2M (s)ds. (32)

The function FM was taken in the studies by Dimarogonas [12]; Rizos et al. [72]as

FM = 1.93 − 3.07s + 14.53s2 − 25.11s3 + 25.8s4. (33)

According to the handbook by Tada et al. [76] the function FN can be approxi-mated as

FN = 1.122 − 0.23s + 10.55s2 − 21.71s3 + 30.38s4. (34)

2.5 Determination of natural frequencies

In the case when the plate has a unique step the deflected shape of the plate canbe presented according to (19) as

w(x) = A1 sinλ0x+A2 cos λ0x+A3 sinhµ0x+A4 cosh µ0x (35)

for x ∈ [0, a] and as

w(x) = B1 sinλ1x+B2 cos λ1x+B3 sinhµ1x+B4 cosh µ1x (36)

16

for x ∈ [a, l].

Arbitrary constants Ai, Bi (i = 1, . . . , 4) have to meet boundary requirementsand intermediate conditions at x = a. The latters can be presented as (Lellep,Roots [43])

w(a− 0) = w(a+ 0)

w′(a− 0) = w′(a+ 0)− pw′′(a+ 0)

h30w′′(a− 0) = h31w

′′(a+ 0)

h30w′′′(a− 0) = h31w

′′′(a+ 0)

(37)

where according to (29), (30)

p =Eh3

12(1 − ν2)KT. (38)

It is worthwhile to mention that the third and the fourth equality in (37) expressthe continuity of the bending moment and the shear force, respectively, when pass-ing the step at x = a. It is known from the solid mechanics that these quantitiesmust be continuous (Soedel [74]).

Boundary conditions (8) at x = 0 admit to eliminate from (35) the unknownconstants

A4 = −A2,

A3 = −A1

λ0

µ0

.(39)

The intermediate conditions (37) with boundary requirements (8) at x = l leadto the system of six equations which will be presented in the matrix form. Thecontinuity of the deflection leads to the equation

A1

A2

B1

B2

B3

B4

⊤

×

sinλ0a− λ0

µ0sinhµ0a

cos λ0a− cosh µ0a− sinλ1a− cos λ1a− sinhµ1a− coshµ1a

= 0. (40)

175

According to the second relation in (37) one has

A1

A2

B1

B2

B3

B4

⊤

×

λ0(cos λ0a− cosh µ0a)−λ0 sinλ0a− µ0 sinhµ0a)λ1(pλ1 sinλ1α− cos λ1α)λ1(sinλ1α− pλ1 cos λ1α)

−µ1(cosh µ1α+ pµ1 sinhµ1α)−µ1(sinhµ1α+ pµ1 cosh µ1α)

= 0. (41)

The continuity requirements imposed on the bending moment and the shear force,respectively, lead to the equations

A1

A2

B1

B2

B3

B4

⊤

×

−h30λ0(λ0 sinλ0a− µ0 sinhµ0a)−h30(λ

20 cos λ0a− µ2

0 coshµ0a)h31λ

21 sinλ1a

h31λ21 cos λ1a

−h31µ21 sinhµ1a

−h31µ21 coshµ1a

= 0 (42)

and

A1

A2

B1

B2

B3

B4

⊤

×

−h30λ0(λ20 cos λ0a− µ2

0 cosh µ0a)h30(λ

30 sinλ0a− µ3

0 sinhµ0a)h31λ

31 cos λ1a

−h31λ31 sinλ1a

−h31µ31 coshµ1a

−h31µ31 sinhµ1a

= 0. (43)

The boundary conditions (7) can be expressed as

A1

A2

B1

B2

B3

B4

⊤

×

00

−λ21 sinλ1l

−λ21 cos λ1l

µ2 sinhµ1lµ2 coshµ1l

= 0 (44)

and

A1

A2

B1

B2

B3

B4

⊤

×

00

−λ31 sinλ1l

−λ31 cos λ1l

µ3 sinhµ1lµ3 cosh µ1l

= 0. (45)

18

The system (40)–(45) is a linear homogeneous system of algebraic equations.It has a non-trivial solution only in the case, if its determinant ∆ equals to zero.The equation ∆ = 0 is solved up to the end numerically. Let us consider someexamples. The results regarding to cantilever plate strips are presented in theFig. 3 and Fig. 4. Here the natural frequency is plotted versus the location of thestep a = αl. It can be seen from Fig. 3 and Fig. 4 that the frequency of a crackedstructure is always less than that of a strip without defects.

200

300

400

500

α

ω

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

s=0.0s=0.2s=0.4s=0.6

Figure 3: Natural frequencies of stretched strips; γ = 0.5.

19

200

250

300

350

400

450

500

α

ω

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

s=0.0s=0.2s=0.4s=0.6

Figure 4: Natural frequencies of strips; γ = 0.7.

20

3 Free vibrations of stepped plate with cracks

The aim of this section is to determine the eigenfrequencies of the plate and tostudy the sensitivity of free vibrations on the crack location and depth. This sec-tion is based on the papers Kägo and Lellep [29, 30].

3.1 Statement of the problem and governing equations

Consider a thin stepped rectangular plate of anisotropic material, as shown inFig. 5. The plate is simply supported at all edges and the plate has width b, lengthl. Assume that the thickness h = hj for x ∈ (aj , aj+1), where j = 0, . . . , n. Letus introduce the notation

hj = γjh0, (46)

where 0 < γj ≤ 1. The parameters hj , aj , γj will be treated as given constants.

x

y

h0 h1 hn

l

b

a1 a2 an

Figure 5: Stepped plate with crack

It is assumed, that the plate has part-through cracks [19] at the corners of there-entrant parts of the steps. Hence according parameters for the cracks are thecrack position

aj = αj l, (47)

where 0 ≤ αj ≤ 1 and the crack length

cj = sjhj , (48)

where 0 ≤ sj < 1.The differential equation of the vibration of the anisotropic plate is (Reddy

[69])

D11

∂4w

∂x4+ 2(D12 + 2D66)

∂4w

∂x2∂y2+D22

∂4w

∂y4= q − I0

∂2w

∂t2. (49)

216

In (49) w(x, y, t) is the deflection of the plate, Dij stand for flexural stiffnesscoefficients, I0 = ρhb is the moment of inertia, ρ is the density of the materialand q is the load intensity. Since we study free vibrations of the plate one has totake q = 0.

The function w(x, y, t) is presented in the form

w(x, y, t) = X(x) sin

(

kπy

b

)

cos(ωt). (50)

Substituting (50) into (49) we obtain

D11XIV − 2 (D12 + 2D66)

(

kπ

b

)2

X ′′ +D22

(

kπ

b

)4

− I0ω2 = 0. (51)

If the material is an unidirectionally reinforced fiber composite then the con-stants Dij take the form

D11 =E1h

3

12(1 − ν12ν21), (52)

D22 =E2h

3

12(1 − ν12ν21), (53)

D12 + 2D66 =ν12E2h

3

12(1 − ν12ν21)+

2G12h3

12. (54)

Here E1 and E2 are Young’s moduli, ν12 and ν21 are Poisson’s ratios, hi are theplate thicknesses, where i = 0, . . . , n, and G12 is shear modulus (Herakovich[18]). Hence, solving the characteristic equation of (51) the solution of the fourthorder equation can be expressed as

Xj(x) = A1j sinλjx+A2j cos λjx+A3j sinhµjx+A4j cosh µjx (55)

for x ∈ (aj , aj+1), j = 0, . . . , n. Here A1j , . . . , A4j are integration constants,which are to be determined using the boundary and continuity conditions.

However, it appears that the quantity X ′ cannot be continuous at x = aj ac-cording to the model of distributed line springs developed by Rice and Levy [71];Dimarogonas [12], Chondros et al. [8].

3.2 Additional flexibility due to the crack

The weakening effect of cracks, flaws, notches and different types of defects wasrecognized a long ago. The relationship between the additional compliance C andthe stress intensity coefficient K of a cracked beam was caught by Irwin [24]. On

22

the other hand, the stress intensity coefficient is coupled with the energy releaserate computed for the infinitesimal change of the crack length.

In the case of anisotropic bodies with eventual fracture modes I and II theenergy release rate is (see Nikpour [61], Nikpur and Dimarogonas [62])

G =−A22

2Im

(

µ1 + µ2

µ1µ2

)

K2I +

A11

2Im(µ1 + µ2)K

2II

+A11 Im(µ1µ2)KIKII . (56)

In (56) A11, A22, A12 stand for elastic constants for an anisotropic material(see Herakovich [18], Reddy [69])

A11 =1

E1

(

1−E2

E1

ν212

)

,

A22 =1

E2

(

1− ν223)

,

A12 = −ν12E1

(1 + ν23) .

HereIm(µ) = y

if the complex number µ = x+ iy.The complex numbers µ1, µ2 in (56) are the roots of the characteristic equation

A11µ4 − 2A16µ

3 + (2A12 +A66)µ2 − 2A26µ+A22 = 0. (57)

In the following we shall confine to the unidirectionally reinforced composites.Moreover, let us confine to the case of cracks of the first mode. Let the stressintensity coefficient and the bending moment applied to the crack at x = aj beKj and Mj , respectively. It is well known in the linear elastic fracture mechanicsthat for isotropic materials

Kj =6Mj

bh2√πcjF (58)

whereh = min(hj−1, hj)

Following the paper [3] we can state that in the case of the orthotropic materials

Kj =6Mj

bh2j

√πcjF (sj)Y (ξ), (59)

23

provided hj < hj−1 and

Y (ξ) = 1 + 0.1(ξ − 1)− 0.016(ξ − 1)2 + 0.002(ξ − 1)3. (60)

Here

ξ =

√E11E22

2G12

− ν12

√

E22

E11

and F is a shape function which must be determined on the basis of experimen-tal data. It is known in the fracture mechanics that the energy release rate; thegeneralized force P and the compliance C are coupled as (see Broek [5])

G =P 2

2b

dC

dc. (61)

Specifying the last formula for the crack located at x = aj and taking into accountthat the generalized force P = Mj one obtains

M2j

2b

dCj

dcj=

36M2j A

h4jbπcjF

2(sj)Y2(ξ) (62)

where

A =A22

2Im

(

µ1 + µ2

µ1µ2

)

. (63)

The solution of (62) which meets the natural initial condition (Cj = 0 whensj = 0) can be presented as

Cj =72π

h2jf(sj)AY

2 (64)

where

f(sj) = 1.8624s2j − 3.95s3j + 16.375s4j − 37.226s5j + 76.81s6j

− 126.9s7j + 172.5s8j − 143.97s9j + 66.56s10j (65)

It was shown in the previous section that the slope discontinuity at x = aj

Θj =∂w

∂x(aj+0, t)−

∂w

∂x(aj−0, t) (66)

can be calculated asΘj = CjMj (67)

where Cj is given by (64) and Mj = Mx(aj , t). Since Mx is continuous withrespect to x one can select

Mj = −(

D11

∂2w

∂x2+D12

∂2w

∂y2

)

x=aj+0

. (68)

24

3.3 Determination of eigenfrequencies

In the case when the plate has an unique step the deflected shape of the plate canbe presented according to (55) as

w(x) = A1 sinλ0x+A2 cos λ0x+A3 sinhµ0x+A4 cosh µ0x (69)

for x ∈ [0, a] and as

w(x) = B1 sinλ1x+B2 cos λ1x+B3 sinhµ1x+B4 cosh µ1x (70)

for x ∈ [a, l].Arbitrary constants Ai, Bi (i = 1, . . . , 4) have to meet boundary requirements

and intermediate conditions at x = a. The latters can be presented as (Lellep andKägo [40])

w(a− 0) = w(a+ 0)

w′(a− 0) = w′(a+ 0)− pw′′(a+ 0)

h30w′′(a− 0) = h31w

′′(a+ 0)

h30w′′′(a− 0) = h31w

′′′(a+ 0)

(71)

where p is the stiffness parameter defined according to (64)–(68).It is worthwhile to mention that the third and the fourth equality in (71) express

the continuity of the bending moment and the shear force, respectively, when pass-ing the step at x = a. It is known from the solid mechanics that these quantitiesmust be continuous (Soedel [74]).

The obtained system is a linear homogeneous system of algebraic equations. Ithas a non-trivial solution only in the case, if its determinant ∆ equals to zero. Theequation ∆ = 0 is resolved up to the end numerically. In the following figuressome examples are shown. The results for simply supported plates are presentedin Fig. 6 and Fig. 7. Fig. 6 corresponds to the case h1 = 0.5h0 whereas Fig. 7 isassociated with h1 = 0.7h0

257

3.0

3.5

4.0

4.5

5.0

5.5

α

ω

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

s=0.0s=0.2s=0.4s=0.6

Figure 6: Natural frequencies of an anisotropic plate; γ = 0.5.

4.0

4.5

5.0

5.5

α

ω

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

s=0.0s=0.2s=0.4s=0.6

Figure 7: Natural frequencies of an anisotropic plate; γ = 0.7.

26

4 Natural vibrations of stepped plate with cracks on elas-

tic foundation

The aim of this section is to consider the effect of the elastic foundation on theeigenfrequencies of the plate. Additionally, we study the sensitivity of free vibra-tions on the crack location and depth. This section is based on the papers by Kägoand Lellep [27, 28].

4.1 Formulation of the problem

We consider an elastic stepped rectangular plate (Fig. 8) made of anisotropic ma-terial with dimensions and crack locations like in previous section.

x

y

h0 h1

l

b

a

(a) System of coordinates

l

h1

h0 a

x

z

(b) Influence of the foundation

Figure 8: Geometry of the plate

4.2 Equations of motion

According to the classical thin plate theory given by Reddy [69] the differentialequation of the free vibration of thin plates on the Winkler foundation (Fig. 8) canbe expressed as follows [47]

D11∂4w∂x4 + 2(D12 + 2D66)

∂4w∂x2∂y2

+D22∂4w∂y4

+ κw − ω2ρw = 0, (72)

27

where w(x, y, t) denotes the function of the mode shape, κ is the elasticity coeffi-cient of the foundation, ω is the natural frequency, ρ is the mass density.

The transverse displacement w can be written in the form

w(x, y, t) = X(x) sin

(

kπy

b

)

eλt, (73)

where k is the wave number of the kth mode in y-direction (k = 1, 2, . . . ), b isthe width of the plate, λ is a complex number.

Substituting (73) into (72) one obtains

D11XIV − 2 (D12 + 2D66)

(

kπ

b

)2

X ′′ + (74)

+

(

D22

(

kπ

b

)4

+ κ− ω2ρ

)

X = 0.

The solution of equation (74) can be expressed as

Xj(x) = A1j sinλjx+A2j cos λjx+A3j sinhµjx+A4j cosh µjx (75)

for x ∈ (aj , aj+1), j = 0, . . . , n. Here A1j , . . . , A4j are integration constants,which are to be determined using the boundary and continuity conditions [40].

However, it appears that the quantity X ′ cannot be continuous at x = aj ac-cording to the model of distributed line springs developed by Chondros et al. [8],Rice and Levy [71], Dimarogonas [12].

The crack emanating from the re-entrant corner of the plate affects the vibra-tional behavior of the structure. The influence of cracks on vibrations of the platewas described in the previous section.

Some examples are presented in Fig. 9 and Fig. 10. These results regard toanisotropic plates on elastic foundation, which are clamped at two opposite edgesand other two are free. Fig. 9 corresponds to the case h1 = 0.5h0 and Fig. 10 isassociated with h1 = 0.7h0. The length of the crack is the same in the both cases.

28

3.5

4.0

4.5

5.0

5.5

6.0

6.5

α

ω

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

κ = 0κ = 400κ = 800

Figure 9: Natural frequencies of a stepped plate on elastic foundation; γ = 0.5,s = 0.6.

5.0

5.5

6.0

6.5

α

ω

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

κ = 0κ = 400κ = 800

Figure 10: Natural frequencies of a stepped plate on elastic foundation; γ = 0.7,s = 0.6

298

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36

Summary

In the current dissertation free vibrations of elastic plates and plate strips arestudied. It is assumed that the plates are weakened by crack-like defects and havepiecewise constant thickness. The cracks are considered as stationary surfacecracks which are located at the re-entrant corners of the steps and which have notfully penetrated the plate thickness.

By combining the theory of elastic plates and the theory of the linear elas-tic fracture mechanics a new method for determining the natural frequencies ofelastic structures is developed in the dissertation.

The dissertation is based on the six papers of the author (three of these arepublished during the last three years). The dissertation consists of the reviewpaper of the obtained results, the copies of the papers, the list of literature and CVof the author.

The review paper consists of the historical review of the literature (Section 1)and of three main sections.

In the second section of the paper natural frequencies of plate strips subjectedto the axial tension are studied. The material of the plate strips is assumed to bea pure elastic material and the hypothesis of Kirchhoff are assumed to hold good.A refined version of the classical bending theory is employed.

The influence of cracks on the vibrational characteristics is taken into accountaccording to the model of distributed line springs. The latter uses the stress inten-sity coefficient known in the elastic fracture mechanics.

In the subsequent sections the developed method is used for determination ofthe natural frequency of free vibrations of anisotropic plates with and without anelastic foundation. The influence of geometrical and material parameters on thevibration of the plates resting on the elastic foundation has been analyzed.

This solution can be used in nondestructive testing.

3710

Kokkuvõte

Pragudega elastsete astmeliste plaatide omavõnkumised

Käesolevas dissertatsioonis vaadeldakse isotroopsete ribade ja anisotroopseteplaatide omavõnkumisi. Vaatluse all olevad plaadid on tükiti konstantse paksuseganing nõrgestatud pragudega.

Antud dissertatsioon põhineb autori kuuel teaduslikul publikatsioonil. Kolmneist publikatsioonidest on avaldatud kolme viimase aasta jooksul.

Väitekiri koosneb kokkuvõtvast ülevaateartiklist, publikatsioonide koopiatest,kirjanduse ülevaatest ja autori elulookirjeldusest. Ülevaateartikkel omakorda koos-neb ajaloolisest kirjanduse ülevaatest ja kolmest põhiosast koos kokkuvõtvate jä-reldustega.

Esimeses põhiosas vaadeldakse plaadi ribade vabavõnkumisi erinevate rajatin-gimuste korral, juhul kui plaadi ribale mõjub telgsuunaline tõmbejõud. Eeldame,et plaadi ribad on elastsest materjalist ja kehtivad Kirchhoffi hüpoteesid.

Teises põhiosas on vaatluse all anisotroopsed plaadid ning kolmandas põhi-osas anisotroopsed plaadid elastsel alusel. Mõlemal juhul vaadeldakse ülesanneterinevate rajatingimuste korral.

Plaatide ja plaadi ribade geomeetrilisteks iseärasusteks on astmelisus ja paksu-se muutumise kohtades asuvad praod. Praod on stabiilsed ja konstantse pikkuse-ga. Prao mõju plaadi omavõnkumisele võetakse arvesse lokaalse järeleandlikkusekoefitsiendi abil ning pinge intensiivsuse koefitsiendiga, mis tuleneb purunemis-mehaanikast.

Kõikidel juhtudel on omavõnkumiste uurimiseks kasutatud analüütilist meeto-dit, mis põhineb klassikalisel plaatide teooria ning purunemismehaanika võrran-ditel ja kriteeriumitel.

38

Acknowledgements

I would like to thank my supervisor Professor Jaan Lellep for his advices andsupport during my doctoral studies. I also like to thank my colleagues in theTallinn University of Technology and in the University of Tartu. Special thanksgo to my family and friends, especially to Jaak Simm for their continuous supportand encouragement.

39

Publications

11

Curriculum vitae

Name: Esta KägoDate of birth: 02.06.1982Citizenship: EstonianE-mail: esta.kago@ut.ee

Education

1997 - 2000 Võru Kreutzwald Gymnasium2002 - 2005 University of Tartu, Faculty of Mathematics and Computer

Science, Bachelor of Science (Mathematics)2005 - 2007 University of Tartu, Faculty of Mathematics and Computer

Science, Master of Science (Mathematics)Since 2007 University of Tartu, Faculty of Mathematics and Computer

Science, doctorate studies, Mathematics

Research work

The main field of reasearch is studing of vibrations of elastic cracked plates andplate strips with analytical-numerical methods. Results have been published in 3papers and other 3 are submitted for publication. Results have been presented atthe five conferences:

"The Third Finnish-Estonian Mathematical Colloquium", Tartu, Estonia(2009),

The Int. Conf. "Mathematical Models for Engineering Science", Puerto dela Cruz, Spain (2010),

The Int. Conf. "Optimization and Analysis of Structures", Tartu, Estonia(2011),

17th Int. Conf. "Mechanics Of Composite Materials", Riga, Latvia (2012),

11th Int. Conf. "Modern Building Materials, Structures and Techniques",Vilnius, Lithuania (2013).

10928

Elulookirjeldus

Nimi: Esta KägoSünniaeg: 02.06.1982Kodakondsus: EestiE-mail: esta.kago@ut.ee

Haridus

1997 - 2000 Võru Kreutzwaldi Gümnaasium2002 - 2005 Tartu Ülikool, matemaatika-informaatikateaduskond,

loodusteaduse bakalaureuse kraad matemaatikas2005 - 2007 Tartu Ülikool, matemaatika-informaatikateaduskond,

loodusteaduse magistri kraad matemaatikasAlates 2007 Tartu Ülikool, matemaatika-informaatikateaduskond,

doktoriõpe matemaatikas

Teaduslik tegevus

Peamine uurimisvaldkond on elastsete pragudega plaatide ja plaadi ribade võnku-miste uurimine analüütilis-numbriliste meetoditega. Tulemused on publitseeritudkolmes teadusartiklis, lisaks on kolm artiklit esitatud publitseerimiseks. Tulemus-tega on esinetud viiel teaduskonverentsil:

The Third Finnish-Estonian Mathematical Colloquium, Tartu, Eesti (2009),

The Int. Conf. Mathematical Models for Engineering Science, Puerto de laCruz, Hispaania (2010),

The Int. Conf. Optimization and Analysis of Structures, Tartu, Eesti (2011),

17th Int. Conf. Mechanics of Composite Materials, Riia, Läti (2012),

11th Int. Conf. Modern Building Materials, Structures and Techniques, Vil-nius, Leedu (2013).

110

DISSERTATIONES MATHEMATICAE UNIVERSITATIS TARTUENSIS

1. Mati Heinloo. The design of nonhomogeneous spherical vessels, cylindrical

tubes and circular discs. Tartu, 1991, 23 p. 2. Boris Komrakov. Primitive actions and the Sophus Lie problem. Tartu,

1991, 14 p. 3. Jaak Heinloo. Phenomenological (continuum) theory of turbulence. Tartu,

1992, 47 p. 4. Ants Tauts. Infinite formulae in intuitionistic logic of higher order. Tartu,

1992, 15 p. 5. Tarmo Soomere. Kinetic theory of Rossby waves. Tartu, 1992, 32 p. 6. Jüri Majak. Optimization of plastic axisymmetric plates and shells in the

case of Von Mises yield condition. Tartu, 1992, 32 p. 7. Ants Aasma. Matrix transformations of summability and absolute summa-

bility fields of matrix methods. Tartu, 1993, 32 p. 8. Helle Hein. Optimization of plastic axisymmetric plates and shells with

piece-wise constant thickness. Tartu, 1993, 28 p. 9. Toomas Kiho. Study of optimality of iterated Lavrentiev method and

its generalizations. Tartu, 1994, 23 p. 10. Arne Kokk. Joint spectral theory and extension of non-trivial multiplica-

tive linear functionals. Tartu, 1995, 165 p. 11. Toomas Lepikult. Automated calculation of dynamically loaded rigid-

plastic structures. Tartu, 1995, 93 p, (in Russian). 12. Sander Hannus. Parametrical optimization of the plastic cylindrical shells

by taking into account geometrical and physical nonlinearities. Tartu, 1995, 74 p, (in Russian).

13. Sergei Tupailo. Hilbert’s epsilon-symbol in predicative subsystems of analysis. Tartu, 1996, 134 p.

14. Enno Saks. Analysis and optimization of elastic-plastic shafts in torsion. Tartu, 1996, 96 p.

15. Valdis Laan. Pullbacks and flatness properties of acts. Tartu, 1999, 90 p. 16. Märt Põldvere. Subspaces of Banach spaces having Phelps’ uniqueness

property. Tartu, 1999, 74 p. 17. Jelena Ausekle. Compactness of operators in Lorentz and Orlicz sequence

spaces. Tartu, 1999, 72 p. 18. Krista Fischer. Structural mean models for analyzing the effect of

compliance in clinical trials. Tartu, 1999, 124 p.

111

19. Helger Lipmaa. Secure and efficient time-stamping systems. Tartu, 1999, 56 p.

20. Jüri Lember. Consistency of empirical k-centres. Tartu, 1999, 148 p. 21. Ella Puman. Optimization of plastic conical shells. Tartu, 2000, 102 p. 22. Kaili Müürisep. Eesti keele arvutigrammatika: süntaks. Tartu, 2000, 107 lk. 23. Varmo Vene. Categorical programming with inductive and coinductive

types. Tartu, 2000, 116 p. 24. Olga Sokratova. Ω-rings, their flat and projective acts with some appli-

cations. Tartu, 2000, 120 p. 25. Maria Zeltser. Investigation of double sequence spaces by soft and hard

analitical methods. Tartu, 2001, 154 p. 26. Ernst Tungel. Optimization of plastic spherical shells. Tartu, 2001, 90 p. 27. Tiina Puolakainen. Eesti keele arvutigrammatika: morfoloogiline ühesta-

mine. Tartu, 2001, 138 p. 28. Rainis Haller. M(r,s)-inequalities. Tartu, 2002, 78 p. 29. Jan Villemson. Size-efficient interval time stamps. Tartu, 2002, 82 p. 30. Eno Tõnisson. Solving of expession manipulation exercises in computer

algebra systems. Tartu, 2002, 92 p. 31. Mart Abel. Structure of Gelfand-Mazur algebras. Tartu, 2003. 94 p. 32. Vladimir Kuchmei. Affine completeness of some ockham algebras. Tartu,

2003. 100 p. 33. Olga Dunajeva. Asymptotic matrix methods in statistical inference

problems. Tartu 2003. 78 p. 34. Mare Tarang. Stability of the spline collocation method for volterra

integro-differential equations. Tartu 2004. 90 p. 35. Tatjana Nahtman. Permutation invariance and reparameterizations in

linear models. Tartu 2004. 91 p. 36. Märt Möls. Linear mixed models with equivalent predictors. Tartu 2004.

70 p. 37. Kristiina Hakk. Approximation methods for weakly singular integral

equations with discontinuous coefficients. Tartu 2004, 137 p. 38. Meelis Käärik. Fitting sets to probability distributions. Tartu 2005, 90 p. 39. Inga Parts. Piecewise polynomial collocation methods for solving weakly

singular integro-differential equations. Tartu 2005, 140 p. 40. Natalia Saealle. Convergence and summability with speed of functional

series. Tartu 2005, 91 p. 41. Tanel Kaart. The reliability of linear mixed models in genetic studies.

Tartu 2006, 124 p. 42. Kadre Torn. Shear and bending response of inelastic structures to dynamic

load. Tartu 2006, 142 p.

112

43. Kristel Mikkor. Uniform factorisation for compact subsets of Banach spaces of operators. Tartu 2006, 72 p.

44. Darja Saveljeva. Quadratic and cubic spline collocation for Volterra integral equations. Tartu 2006, 117 p.

45. Kristo Heero. Path planning and learning strategies for mobile robots in dynamic partially unknown environments. Tartu 2006, 123 p.

46. Annely Mürk. Optimization of inelastic plates with cracks. Tartu 2006. 137 p.

47. Annemai Raidjõe. Sequence spaces defined by modulus functions and superposition operators. Tartu 2006, 97 p.

48. Olga Panova. Real Gelfand-Mazur algebras. Tartu 2006, 82 p. 49. Härmel Nestra. Iteratively defined transfinite trace semantics and program

slicing with respect to them. Tartu 2006, 116 p. 50. Margus Pihlak. Approximation of multivariate distribution functions.

Tartu 2007, 82 p. 51. Ene Käärik. Handling dropouts in repeated measurements using copulas.

Tartu 2007, 99 p. 52. Artur Sepp. Affine models in mathematical finance: an analytical approach.

Tartu 2007, 147 p. 53. Marina Issakova. Solving of linear equations, linear inequalities and

systems of linear equations in interactive learning environment. Tartu 2007, 170 p.

54. Kaja Sõstra. Restriction estimator for domains. Tartu 2007, 104 p. 55. Kaarel Kaljurand. Attempto controlled English as a Semantic Web language.

Tartu 2007, 162 p. 56. Mart Anton. Mechanical modeling of IPMC actuators at large deforma-

tions. Tartu 2008, 123 p. 57. Evely Leetma. Solution of smoothing problems with obstacles. Tartu 2009,

81 p. 58. Ants Kaasik. Estimating ruin probabilities in the Cramér-Lundberg model

with heavy-tailed claims. Tartu 2009, 139 p. 59. Reimo Palm. Numerical Comparison of Regularization Algorithms for

Solving Ill-Posed Problems. Tartu 2010, 105 p. 60. Indrek Zolk. The commuting bounded approximation property of Banach

spaces. Tartu 2010, 107 p. 61. Jüri Reimand. Functional analysis of gene lists, networks and regulatory

systems. Tartu 2010, 153 p. 62. Ahti Peder. Superpositional Graphs and Finding the Description of Struc-

ture by Counting Method. Tartu 2010, 87 p. 63. Marek Kolk. Piecewise Polynomial Collocation for Volterra Integral

Equations with Singularities. Tartu 2010, 134 p.

11329

64. Vesal Vojdani. Static Data Race Analysis of Heap-Manipulating C Programs. Tartu 2010, 137 p.

65. Larissa Roots. Free vibrations of stepped cylindrical shells containing cracks. Tartu 2010, 94 p.

66. Mark Fišel. Optimizing Statistical Machine Translation via Input Modifi-cation. Tartu 2011, 104 p.

67. Margus Niitsoo. Black-box Oracle Separation Techniques with Appli-cations in Time-stamping. Tartu 2011, 174 p.

68. Olga Liivapuu. Graded q-differential algebras and algebraic models in noncommutative geometry. Tartu 2011, 112 p.

69. Aleksei Lissitsin. Convex approximation properties of Banach spaces. Tartu 2011, 107 p.

70. Lauri Tart. Morita equivalence of partially ordered semigroups. Tartu 2011, 101 p.

71. Siim Karus. Maintainability of XML Transformations. Tartu 2011, 142 p. 72. Margus Treumuth. A Framework for Asynchronous Dialogue Systems:

Concepts, Issues and Design Aspects. Tartu 2011, 95 p. 73. Dmitri Lepp. Solving simplification problems in the domain of exponents,

monomials and polynomials in interactive learning environment T-algebra. Tartu 2011, 202 p.

74. Meelis Kull. Statistical enrichment analysis in algorithms for studying gene regulation. Tartu 2011, 151 p.

75. Nadežda Bazunova. Differential calculus d3 = 0 on binary and ternary

associative algebras. Tartu 2011, 99 p. 76. Natalja Lepik. Estimation of domains under restrictions built upon gene-

ralized regression and synthetic estimators. Tartu 2011, 133 p. 77. Bingsheng Zhang. Efficient cryptographic protocols for secure and private

remote databases. Tartu 2011, 206 p. 78. Reina Uba. Merging business process models. Tartu 2011, 166 p. 79. Uuno Puus. Structural performance as a success factor in software develop-

ment projects – Estonian experience. Tartu 2012, 106 p. 80. Marje Johanson. M(r, s)-ideals of compact operators. Tartu 2012, 103 p. 81. Georg Singer. Web search engines and complex information needs. Tartu

2012, 218 p. 82. Vitali Retšnoi. Vector fields and Lie group representations. Tartu 2012,

108 p. 83. Dan Bogdanov. Sharemind: programmable secure computations with

practical applications. Tartu 2013, 191 p. 84. Jevgeni Kabanov. Towards a more productive Java EE ecosystem. Tartu

2013, 151 p. 85. Erge Ideon. Rational spline collocation for boundary value problems.

Tartu, 2013, 111 p.

114