Tensile Creep of Cement and Concrete Composites: Monitoring by Means of 2D-Digital Image Correlation

Creep and shrinkage of Cement and Concrete Composites (CCC) are significant properties that need to be considered to use these materials in practice. Many previous scientific studies revealed CCC creep characteristics under sustained compression and shrinkage, using traditional test methods from design standards. Because of the complexity of experimental procedures, CCC creep in tension has not been studied as close. Furthermore, there is no unified standard that proposes applicable testing methods or specific testing apparatus. This study examines the suitability of 2D—Digital Image Correlation (DIC) to observe the creep deformations of specimens under tension. Ordinary Portland cement (OPC) mortar with 1% polyvinyl alcohol (PVA) fibres has been investigated in the research. Compact tension (CT) specimens 150 × 150 × 12 mm (with a notch) were used. Creep deformations under sustained uniaxial tension (applied loading corresponding to 60% of the ultimate strength) were measured. DIC images were captured using an entry/mid-level DSLR camera. Results show that DIC is suitable for studying uniaxial tensile creep of cement and concrete composites. Deformation of specimens in tension was similar to that measured using the conventional method (using surface-attached gauges).

Stability analysis of wooden arches with account for nonlinear creep

Introduction. The paper deals with the calculation of wooden arches taking into account the nonlinear relationship between stresses and instantaneous deformations, as well as creep and geometric nonlinearity, are considered. The analysis is based on the integral equation of the viscoelastoplastic hereditary aging model, originally proposed by A.G. Tamrazyan [1] to describe the nonlinear creep of concrete.Materials and Methods. The creep measure is taken in accordance with the work of I.E. Prokopovich and V.A. Zedgenidze [2] as a sum of exponential functions. The transition from the integral form of the creep law to the differential form is shown. The relationship between stresses and instantaneous deformations for wood under compression is determined from the Gerstner formula, and elastic work is assumed under tension. The solution is carried out using the finite element method in combination with the Newton-Raphson method and the Euler method according to the scheme that involves a stepwise increase in the load with correction of the stiffness matrix taking into account the change in the coordinates of the nodes with the sequential calculation of additional displacements of the nodes, which are due to the residual forces. The proposed approach for increasing the accuracy of determination of creep deformations at each step provides using the fourth-order Runge-Kutta method instead of the Euler method.Results. Based on the Lagrange variational principle, expressions are obtained for the stiffness matrix and the vector of additional dummy loads due to creep. The method developed by the authors is implemented in the form of a program in the MATLAB environment. Calculation examples are given for parabolic arches simply supported at the ends without an intermediate hinge and with an intermediate hinge in the middle of the span under the action of a uniformly distributed load. The results obtained are compared in the viscoelastic and viscoelastic formulation. The reliability of the results is validated through the calculation in the elastic formulation in the ANSYS software package.Discussion and Conclusions. For the arches considered, it is found that even with a load close to the instant critical, the growth of time travel is limited. Thus, the nature of their work under creep conditions differs drastically from the nature of the deformation of compressed rods.

Problems in the strength and stability of inhomogeneous structures of rocket and space hardware with account for plasticity and creep

Shell structures provide a compromise between strength and mass, which motivates their use in rocket and space hardware (RSH). High and long-term loads cause plastic and creep deformations in structural elements. RSH structures feature inhomogeneity: design inhomogeneity (polythickness, the presence of reinforcements, openings, etc.) and technological inhomogeneity (defects produced in manufacturing, operation, storage. and transportation, defects produced by unforeseen thermomechanical effects, etc.). These factors, which characterize structural inhomogeneity, are stress and strain concentrators and may be responsible for an early failure of structural elements and inadmissible shape imperfections. In inhomogeneous structures, different parts thereof are deformed by a program of their own and exhibit a different stress and strain level. In accounting for a physical nonlinearity, which is governed by plastic and creep deformations, the following approach to the determination of the stress and strain field is efficient: the calculation is divided into stages, and at each stage parameters that characterize the plastic and creep deformations developed are introduced: additional loads in the equilibrium equations or boundary conditions, additional deformations, or variable elasticity parameters (the modulus of elasticity and Poisson’s ratio). Successive approximation schemes are constructed: at each stage, an elasticity problem is solved with the introduction of the above parameters. Special consideration is given to the determination of the launch vehicle and launch complex life. This is due to damages caused by alternate high-intensity thermomechanical loads. The basic approach relies on the theory of low- and high-cycle fatigue. The plasticity and the creep of a material are the basic factors in the consideration of the above problems. This paper considers various aspects of the solution of RSH strength and stability problems with account for the effect of plastic and creep deformations.

Characteristics of Lightweight Concrete Based on a Synthetic Polymer Foaming Agent

The components of foamed concrete have a significant effect on its properties. Protein-based foamed concrete is used much more often. This study aims to assess the properties of foamed concrete with a density of around 500, 700, 800 and 1000 kg/m3 formed by using a synthetic polymer-based foaming agent. The distribution of pores, wet and dry density and compressive strengths were evaluated. In addition, the creep deformations of foamed concrete with different densities were measured. The difference in density of up to 170 kg/m3 for the highest densities was obtained. Foamed concrete with higher densities (700 and 800 kg/m3) showed similar characteristics of pores, which were different from those of samples with a density of 500 kg/m3. Compressive strength equal to 5.9 ± 0.2, 5.1 ± 0.2, 3.8 ± 0.3 and 1.4 ± 0.2 MPa was obtained for foamed concrete with a density of 500, 700, 800 and 1000 kg/m3, respectively. The obtained compressive strengths were higher than those found in the literature for the foamed concrete with the same densities. With increasing density, smaller creep deformations were obtained. Creep deformations were 509, 495 and 455 με for samples with densities of around 500, 700 and 1000 kg/m3 respectively. Deformation under long-term loading took place up to 90 days, regardless of the density of the foamed concrete.

Room Temperature Ferroelastic Creep Behavior of Porous (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ

The time-dependent deformation of porous (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) under constant uniaxial compressive stress at room temperature has been studied. Both axial and lateral stress–strain deformation curves clearly show the non-linear ferroelastic behavior of LSCF perovskite during compression. The ferroelastic characteristics of deformation curves such as coercive stress and apparent loading moduli decrease when the porosity of the samples increases. Ferroelastic creep deformations at applied stresses of 25 and 50 MPa demonstrate that stress and porosity are influencing factors on creep deformation, which increases with increasing stress and porosity. A negative creep or axial expansion and lateral contraction were observed in the sample with 35% porosity under 50-MPa constant compression stress.

Degradation of Mechanical Properties of Pine Wood Under Symmetric Axial Cyclic Loading Parallel to Grain

The mechanical properties of wood, respectively the elastic, plastic, and strength properties, depend on a large number of factors, due both to its structural and physical characteristics, as well as to the size, direction, nature, and speed of application of forces. Wood, generally considered to be a viscous-elastic material, has creep deformations over time under the effect of a constant load. In this study the behavior of pine wood samples was investigated due to its large utilization in different finished products, such as roof construction, furniture, outdoor applications, garden furniture, and toys. The paper aims to analyze the viscoelastic behavior of pine wood subjected to cyclically loading to traction-compression with different loads (1 kN; 1.5 kN; 2 kN), applied at different speeds (1 mm/min; 10 mm/min). It was observed that, at low speeds (1 mm/min) and low intensities of the applied force, it was possible to distinguish the three creep regions specific to wood: the primary area (primary flow), the secondary area, and finally the tertiary creep. As the force increases, the law of variation of the wood flow changes. The degradation of longitudinal elasticity modulus occurs with the increase of the number of cycles, so after 20 alternating symmetrical cycles of traction-compression of the pine wood samples, there is a decrease of its values by 35%.

Strip-Yield Modeling of Load-Time-Temperature Effects on Crack Growth in Engine Materials

Plastic and creep deformations around a crack front and in the wake of a moving crack under cyclic loading are implemented into the life-prediction code, FASTRAN (a strip-yield model). Creep deformations are modeled by stress relaxation around the crack-tip location, since the crack-front material is under displacement control due to the surrounding elastic material. Sinusoidal and trapezoidal loading are considered. A modified linear superposition model was used to compute the cyclic- and time-dependent damage, which was based on the stress-intensity-factor concept for creep-brittle materials. Application of the modified strip-yield model was made on two sets of test data on Inconel-718 alloy. The environments were laboratory air or helium gas. From the literature, the “environment” had been shown to be a major contributor to damage magnitudes. Thus, the time-dependent crack-growth constants were selected to match the test data. In addition, the effects of a small overload on time-dependent damage, and the effects of stress relaxation and varying temperatures on crack-opening stresses and cyclic crack-tip-opening displacements, were studied.