Compressive Size Effect Research Articles

Discrete modeling of deterministic size effect of normal-strength and ultra-high performance concrete under compression

Size effect of heterogeneous granular materials under tensile loading is a well known phenomenon and the subsequent size effect laws are widely available in the scientific literature. On the contrary, size effect in compression is scarcely studied, which, especially for concrete, is more significant for its brittle failure mode in practical applications. This paper investigates the deterministic size effect in compression of concretes including a normal strength concrete (NSC) and an ultra high performance concrete (UHPC). The Lattice Discrete Particle Model (LDPM) is selected to determine the compressive strength of a broad range of sizes utilizing experimental-based calibrated data sets. The simulated tests include both cylinders and prisms with the diameter/width range from 150 mm to 600 mm for normal strength concrete and from 50 mm to 200 mm for UHPC. The investigated aspect ratio ranges from 1 to 16. It is found that the simulated strengths show no size effect under low friction loading, and the magnitude of size effect decreases with rising aspect ratio under high friction loading. The size effect laws available in the literature are not able to fit the obtained compressive strength data and a new compressive size effect law formula named ComSEL3D is proposed in terms of diameter and aspect ratio. The correlation by ComSEL3D for the simulated strengths is above 0.99, which shows its excellent ability in describing and predicting the deterministic size effect behavior in compression for both concretes in cylindrical and prism shapes. The ComSEL3D also outperforms the available formulas in the literature in matching the experimental results of various concretes under compression.

Size effect on compressive behaviors of waste fiber-reinforced recycled aggregate concrete

The compressive size effect of conventional concrete is well known, but there is limited knowledge of the fiber-reinforced recycled concrete. A total of 270 specimens with the different cube and prism sizes, water-cement ratios, compressive strength grades, the replacement rates of recycled aggregates, and volume fractions of waste fibers were tested until failure to study the size effect behaviors of waste fiber-reinforced recycled concrete subjected to axial compression. The results showed that as the replacement rate of recycled aggregates increases, the size effect degree of the cube and axial compressive strength increases. With the increase of the volume fraction of waste fibers, the size effect degree of compressive strength decreases first and then increases, with the optimum volume incorporation of 0.12%. The higher the strength grade of concrete is, the more obvious the size effect of strength is. For large-scale components, no incision correction of Bažant and Carpineri’s fractal size effect theory is more suitable for the analysis of the size effect of waste fiber recycled concrete. It is essential to understand the compression size effect of waste fiber-reinforced recycled concrete so that the laboratory investigation results can be applied to real full-scale structures subjected to compressive loading.

Size effects in compression in Electron Beam Melted Ti6Al4V diamond structure lattices

In experimental work on the mechanical properties of stochastic metal foams, the consensus is that a minimum of six pores along each direction are required to give representative mechanical properties. This theory is tested for another porous metal, regular lattices, built using repeating unit cells of the diamond structure (a tetrahedral structure, in a cubic formation) by Electron Beam Melting (EBM) from grade 5 Ti6Al4V. Samples with different numbers of unit cells are made, using 3 different sets of EBM manufacturing conditions, and tested in compression. In all cases, a minimum of four unit cells are needed to ensure that size-independent mechanical properties are measured. Small changes in manufacture lead to large differences in properties.

Thickness scaled compression tests in unidirectional glass fibre reinforced composites in static and fatigue loading

Thick laminates can be found in different wind turbine blade sections, such as the root or the spar caps. However, static and fatigue design of thick laminate components is based on thin coupons tests (around 4 mm thick) and safety coefficients determined by the guidelines and standards.The aim of this work is to study the size effect in compression for unidirectional glass fibre reinforced composites. Scaled static and fatigue compression tests were performed on glass/epoxy laminates of 4 mm, 10 mm and 20 mm thick. A series of scaled coupons were designed taking into account the influence of the manufacturing process and the final testing conditions. While static tests on laminates with different thicknesses showed no significant change in the static ultimate stresses, fatigue tests showed a decrease in the fatigue life for increasing thicknesses as well as wider intervals of confidence. In addition, fatigue data were compared with the reduced fatigue capacity reported by the design guidelines showing reserve factors larger than 2.0.

Influence of aggregate size on the compressive size effect according to different concrete types

The present tests prepared 9 ready-mixed concrete batches to examine the influences of the concrete type and the maximum aggregate size (da) on the size effect in compression. For each concrete mixing, the aspect ratios of specimens with circular or square sections were 1.0 and 2.0, while the lateral dimensions of specimens varied from 50mm to 400mm. A comprehensive database including the 1492 normal-weight concrete (NWC) and 363 lightweight concrete (LWC) specimens was also established in order to trace the size-effect trend in LWC through the comparisons with the modern prediction models proposed by Bažant, Neville, Kim and Eo, and Sim et al. The present study clearly showed that the size effect and the slope of the size effect became stronger with the decrease of the unit weight of concrete. The influence of da on the size effect observed in the current NWC specimens was smaller than the predictions obtained from the models of Bažant and Kim and Eo; while it was found to be negligible for LWC. Further, the refined model of Sim et al. confirms that the gain of the compressive strength of concrete due to the increase of da is insignificant, especially for LWC.

Size effect in compression of single-crystal gold microparticles

Single-crystal Au microparticles on a sapphire substrate were deformed under compression. Most of microparticles yield with a large strain burst, and there is a strong dependence of the yield strength on microparticle size. With the help of molecular dynamics simulations and finite-element analysis we conclude that the deformation is dislocation nucleation-controlled and that the stress levels reached at the onset of plasticity approach the theoretical shear strengths of Au. A significant size effect is identified in both the experimentally measured and computed strength of the microparticles; the smaller microparticles yield at higher compressive stresses. We propose a stress-gradient nucleation model relating this size effect to stress gradients along the slip plane.

Tensile and compressive behavior of tungsten, molybdenum, tantalum and niobium at the nanoscale

In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of 〈0 0 1〉-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log–log slope of strength vs. nanopillar diameter, is −0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (−0.44). In tension, however, Ta and Nb show higher size effect slopes (−0.80 and −0.77) as compared with W and Mo (−0.58 and −0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.

Effect of Specimen Size on the Dissipated Energy Density in Compression

The size effects in compression on drilled cylindrical concrete specimens obtained from a unique concrete block over a large scale range (1:19) are analyzed. The experimental results show scale effects on dissipated energy density rather than on the compressive strength. A theoretical explanation for such a phenomenon is presented, assuming a noninteger physical dimension of the subdomain where dissipation occurs. A comparison between experimental and theoretical values is discussed and a renormalization procedure to obtain a scale-independent constitutive law is presented.

An Assessment of Compressive Size Effect of Plane Concrete Using Combination of Micro-Plane Damage Based Model and 3D Finite Elements Approach

In recent years, the material behavior dependence of laboratory concrete specimens built with the same concrete mixture under the same load conditions to their geometrical sizes is well established. This phenomenon which is observed not only in concrete but also in most quasi-brittle materials such as rock, ceramic or composite materials is now called as size effect. Many of the existing structural analyzing codes are not able to consider this important feature of concrete structures especially under compressive loadings. However we know that the main purpose of concrete application in structural members is to resist compression. The aim of this study is to show the ability of author's recently developed 3D finite elements code equipped with the proposed author's newly micro-planes damage based model for considering of compressive size effect of plane concrete. To do so, two different sizes of cubic concrete specimens are modeled with mentioned code under the uniaxial compressive test and their fracture mechanisms, pre-peak and post-peak strain-stress paths are investigated. Obtained results reveal the good coincidence with experimental evidences. In fact, the combination of proposed micro-planes damage based model and developed presented 3D finite elements technique creates a powerful numerical tool to capture and predict precisely strain localization and fracture mechanism in the specimens and consequently to assess properly the compressive size effect of plane concrete in analysis and design.

Size effect of concrete members applied with flexural compressive stresses

In this study, two types of special experiments are carried out to understand flexural compressive strength size effect of concrete members. The first type is an ordinary cylindrical specimen (CS) with a fully penetrated and vertically standing plate type notch at the mid-height of the specimen, which is loaded in compression at the top surface (e.g., in the parallel direction to the notch length). The second type is a general double cantilever beam (DCB), which is compression loaded in axial direction (e.g., in the parallel direction of the notch). For CS, an adequate notch length is taken from the experimental results obtained from the compressive strength experiment of various initial notch lengths. The trial tests to select the effective initial notch length show that CS with an initial notch length approximately greater than four times the maximum aggregate size fails without an additional increased load and in stable manner under Mode I failure mechanism. Therefore, the initial notch length to the maximum aggregate size ratio of 4.0 is used for all size specimens. For DCB, the eccentricity of loading points with respect to the axial axis of each cantilever and the initial notch length are varied. In both specimens, the compressive loads apply flexural compressive stresses on the crack tip region of the specimens. These two types of specimens fail by Mode I crack opening mechanism. By testing 3 geometrically proportional size specimens for CS and DCB, the experimental datum for flexural compression size effect of concrete are obtained. Using the obtained flexural compressive strength size effect datum, regression analyses are performed using Levenberg-Marquardt's least square method (LSM) to suggest new parameters for the modified size effect law (MSEL). The analysis results show that size effect is apparent for flexural compressive strength of specimens with an initial notch. For CS, the effect of initial notch length on flexural compressive strength size effect is apparent. For DCB, flexural compressive size effect is dependent on the eccentricity of loading points with respect to the axial axis of the cantilever beam. In other words, if DCB specimen is applied with greater tensile stress at the crack tip, the size effect of concrete becomes more distinct. The results show that the flexural compressive strength size effect of initial notch length variation of DCB exists but directly dependent on the loading location. This is due to the fact that the sizes of fracture process zone (FPZ) of all DCB specimens are similar regardless of the differences in the specimen slenderness ratio, but the flexural compressive and tensile stress combinations resulting in stress concentration at the crack tip region has direct effect on size effect of concrete members.

Microcrack propagation, coalescence and size effects in compression

The strength size effects of brittle materials in compression are studied in the framework of the Boundary Element Method, including the coupling influences of crack interaction, propagation and intersection. Crack interaction coefficients are obtained in the form of algebraic expressions. The curved branching cracks are simulated through a step by step propagation process. Some intersection cracks and edged cracks are checked by the proposed model with acceptable accuracy. With different specimen sizes, crack lengths and proportional crack configurations, the strength size effects are obtained. It is found that the crack interaction, intersection and finite sized boundary have remarkable influence on the loading capacity of the brittle specimen, and that the strength size effects due to crack length and global scaling are not closely connected with the classical stress-singularity power of LEFM.