Abstract
Matrixinclusion composites are known to exhibit interaction among the inclusions. When it comes to the special case of inclusions in form of flat interfaces, interaction among interfaces would be clearly expected, but the twodimensional nature of interfaces implies quite surprising interaction properties. This is the motivation to analyze how interaction among two different classes of microscopic interfaces manifests itself in macroscopic creep and relaxation functions of matrixinterface composites. To this end, we analyze composites made of a linear elastic solid matrix hosting parallel interfaces, and we consider that creep and relaxation of such composites result from microsliding within adsorbed fluid layers filling the interfaces. The latter idea was recently elaborated in the framework of continuum micromechanics, exploiting eigenstress homogenization schemes, see Shahidi et al. (Eur J Mech A Solids 45:41–58, 2014). After a rather simple mathematical exercise, it becomes obvious that creep functions do not reflect any interface interaction. Mathematical derivation of relaxation functions, however, turns out to be much more challenging because of pronounced interface interaction. Based on a careful selection of solution methods, including Laplace transforms and the method of nondimensionalization, we analytically derive a closedform expression of the relaxation functions, which provides the sought insight into interface interaction. The seeming paradox that no interface interaction can be identified from creep functions, while interface interaction manifests itself very clearly in the relaxation functions of matrixinterface materials, is finally resolved based on stress and strain average rules for interfaced composites. They clarify that uniform stress boundary conditions lead to a direct external control of average stress and strain states in the solid matrix, and this prevents interaction among interfaces. Under uniform strain boundary conditions, in turn, interfacial dislocations do influence the average stress and strain states in the solid matrix, and this results in pronounced interface interaction.
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Abbreviations
 a :

Coefficient of ordinary differential equation in γ
 a _{1} :

Radius of first interface family
 a _{2} :

Radius of second interface family
 A _{ I } :

Relaxation capacity associated with characteristic time τ_{relax},I
 A _{ II } :

Relaxation capacity associated with characteristic time τ_{relax},II
 :

Thirdorder straintodislocation downscaling tensor, quantifying the concentration of macroscopic strain into the average dislocation encountered across the interfaces; j = 1, 2 refers to the two interface families
 :

Thirdorder influence tensor describing the influence of macroscopic stress on the dislocation of interfaces; j = 1, 2 refers to the two interface families
 b :

Coefficient of ordinary differential equation in γ
 :

Thirdorder influence tensor describing the influence of interface traction on the macroscopic stress; j = 1, 2 refers to the two interface families
 :

Thirdorder influence tensor describing the influence of interface traction on the macroscopic strain; j = 1, 2 refers to the two interface families
 c _{ j } :

Constant used in Laplace transformation method, related to dislocation of interface family j = 1, 2
 C _{ j } :

Integration constant used in the eliminationnondimensionalization method, related to the dislocation of interface family j = 1, 2
 C̄_{1} :

Constant appearing in relaxation function
 C̄_{2} :

Constant appearing in relaxation function
 \({\mathcal{C}_j}\) :

Domain of interface belonging to interface family j = 1, 2
 :

Fourthorder elastic stiffness tensor of solid
 :

Inverse of
 :

Fourthorder homogenized stiffness tensor of matrixinterface composite
 :

Inverse of
 d _{ j } :

Interface density parameter of interface family j = 1, 2
 D _{ j } :

Integration constant used in the eliminationnondimensionalization method, related to dislocation of interface family j = 1, 2
 D̄_{1} :

Constant appearing in relaxation function
 D̄_{2} :

Constant appearing in relaxation function
 e _{ j } :

Constant used in Laplace transformation method, related to the dislocation of interface family j = 1, 2
 :

Unit base vectors of Cartesian coordinate system
 :

Secondorder tensor of macroscopic strain
 E _{ xz } :

Shear component of
 E _{ s } :

Young’s modulus of the solid
 j :

Index for interface phase (family)
 :

Symmetric fourthorder identity tensor
 :

Deviatoric part of
 :

volumetric part of
 J _{ xzxz } :

Creep function
 \({\mathcal{L}}\) :

Laplace transformation operator
 \({\mathcal{L}^{1}}\) :

Back transformation operator from Laplace space to time domain
 \({ \mathcal{N}_j }\) :

Number of interfaces per unit volume of a matrixinterface composite; j = 1, 2 refers to the two interface families
 :

Outward unit normal at any point of the boundary of the volume element representing the composite material
 :

Unit vector oriented orthogonal to the interface family j = 1, 2
 R _{ xzxz } :

Relaxation function
 s :

As subscript: index for solid phase
 s :

As variable: Laplace space parameter
 :

Interface traction vector acting on interface phase j = 1, 2
 :

Fourthorder morphology tensor for 2D interface inclusion (“sharp crack” morphology)
 T _{ j,x } :

Shear component in xdirection, of traction vector
 T _{ j,y } :

Shear component in ydirection, of traction vector
 x, y, z :

Cartesian coordinates
 :

Position vector
 γ:

Macroscopic engineering strain (related to E _{ xz })
 γ_{ j } :

Strainlike variable related to dislocation of interface family j = 1, 2
 γ_{ j,h } :

Solution of homogeneous differential equation in γ_{ j }
 γ_{ j,p } :

Particular solution of nonhomogeneous differential equation in γ_{ j }
 γ_{ j } :

Laplace transform of function γ_{ j }
 δ:

Kronecker delta
 Δμ :

Loss of effective stiffness during relaxation test
 :

Secondorder tensor of microscopic linear strain
 η _{ i,j } :

Viscosity of interface family j = 1, 2
 η _{ j } :

Differential equationrelated “viscosity constant” related to interface family j = 1, 2
 μ _{ s } :

Shear modulus of isotropic solid matrix
 μ _{ j } :

Differential equationrelated “stiffness constant” related to interface family j = 1, 2
 ν _{ s } :

Poisson’s ratio of isotropic solid matrix
 :

Displacement vector
 :

(average) Dislocation vector of interface phase j = 1, 2
 [[ξ]]_{ j },_{ x } :

Shear component of
 [[ξ]]_{ j },_{ z } :

Normal component of
 :

Secondorder tensor of microscopic Cauchy stresses
 :

Secondorder tensor of macroscopic Cauchy stresses
 Σ _{ xz } :

Shear component of
 τ_{creep},j :

Characteristic creep time related to dislocation in interface family j = 1, 2
 τ_{relax},I :

First characteristic relaxation time of material system comprising two interface families
 τ_{relax},II :

Second characteristic relaxation time of material systme comprising two interface families
 Ω :

Volume of the matrixinterface composite
 Ω _{ s } :

Volume occupied by the solid phase
 ∂:

Partial derivative
 ::

Secondorder tensor contraction
 \({\dot{\bullet}}\) :

Partial derivative with respect to time (“rate”), of quantity “•”
 \({\otimes }\) :

Dyadic product
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Shahidi, M., Pichler, B. & Hellmich, C. How interface size, density, and viscosity affect creep and relaxation functions of matrixinterface composites: a micromechanical study. Acta Mech 227, 229–252 (2016). https://doi.org/10.1007/s0070701514299
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DOI: https://doi.org/10.1007/s0070701514299