Monotonic and Cyclic Loading/Unloading Tensile Behavior of 3D Needle-Punched C/SiC Ceramic-Matrix Composites

In this paper, monotonic and cyclic loading/unloading tensile behavior of four different 3D needle-punched C/SiC composites are investigated. Under tensile loading, multiple micro parameters of tensile tangent modulus, tensile strength, and fracture strain are used to characterize tensile damage and fracture behavior. Under cyclic loading/unloading, multiple damage micro parameters of unloading residual strain, tensile peak strain, hysteresis loops width, hysteresis loops area, unloading and reloading inverse tangent modulus (ITM) are used to describe the tensile damage evolution. After tensile fracture, fracture surfaces were observed under a scanning electron microscope (SEM). Damage of matrix cracking, interface debonding, fibers fracture and pullout in different plies is observed. Relationships between composite tensile mechanical behavior, damage parameters, and micro damage mechanisms are established. When the fiber volume fraction along the loading direction increases, the composite initial tangent modulus, tensile strength and fracture strain increase, and the unloading residual strain, peak strain, hysteresis width and hysteresis area decrease. For Types 1–4 3D needle-punched C/SiC composite, the fiber volume lies in the range of 25.6–32.8%, the composite initial tangent modulus was in the range of 161.4–220.4 GPa, the composite tensile strength was in the range of 64.4–112.3 MPa, and the composite fracture strain was in the range of 0.16–0.25%.

Cement-Stabilized Waste Sand as Sustainable Construction Materials for Foundations and Highway Roads

In this study, cement-treated waste sand as a by-product material produced from Al-Ahsa quarries (Saudi Arabia) was experimentally tested and investigated as a base course material for the foundation of structures and roads. The study aimed to use the waste sand as a construction material by improving its strength, bearing capacity, and stiffness. The waste sand was mixed with different percentages of Portland cement content (0, 2, 4, 6, and 8%) at the maximum dry density and optimum water content of the standard Proctor compaction conditions of a non-treated sample. Unconfined compressive strength and California Bearing Ratio (CBR) tests for different curing times were conducted. X-ray diffraction (XRD), laser-scanning microscopy (LSM), and X-ray spectroscopy (XPS) were used to explore the microstructure and composition of the treated sand. The results showed that the compressive strength, initial tangent modulus, and CBR of the treated sand increase with the increase in cement content and curing time. Furthermore, good correlations were established among the strength, initial tangent modulus, and CBR. Based on the obtained results, cement-stabilized waste sand is a potential material for use in construction. This is expected to save the environment and reduce the cost of road construction.

EXPERIMENTAL STUDY ON COMPRESSIVE YOUNG’S MODULI AND TENSILE YOUNG’S MODULUS OF FLY ASH CONCRETE

Initial cracks due to volume changes at an early age affect the durability of concrete structures, so numerical simulations are often conducted in order to predict cracks. Such prediction requires some mechanical properties of early age concrete. Tensile Young's modulus is directly dependent on the prediction of tensile stress and is one of the important input data for FEM analysis. However, direct tension test for tensile Young's modulus needs a unique apparatus and specimen, and such test is not suitable for evaluating Young's modulus at early ages of concrete. The present study compared tensile Young's modulus with compressive Young's moduli of Fly ash concrete. Compressive Young's moduli used in this study were secant modulus and initial tangent modulus. In addition, linear modulus taken from a regression line of a compressive stress-strain curve in the range of stresses less than the splitting tensile strength was also evaluated. It is found that the secant modulus, which is generally used as Young's modulus in Japan was clearly smaller than the tensile Young's modulus, which means that, tensile stresses evaluated using a secant modulus might be underestimated. On the other hand, linear modulus and initial tangent modulus were almost equal to the tensile Young's modulus. This result indicates that tensile stresses can be evaluated using Young's modulus obtained from a compression test with general apparatus and specimens.

Calculation of Tangent Modulus of Soils under Different Stress Paths

During excavation of foundation pit, soils of different sites may undergo different unloading paths. This study focuses on the tangent modulus of soil under different stress paths and provides theoretical basis for the deformation calculation of soil. In this paper, conventional triaxial compression test and K0 consolidation unloading test were conducted with mucky soil and silty clay to obtain the deformation characteristics of soils under different stress paths. Experiment results show that the soil samples exhibit distinct stress-strain characteristics under different stress paths, but they all show nonlinearity. The initial tangent modulus increases as the consolidation confining pressure intensifies. Then, based on the test data, the power function relationship between initial tangent modulus and confining pressure under unloading was verified. Simultaneously, a hyperbola function to express stress-strain relation of soils under the K0 consolidation unloading condition was proposed and proved. Finally, the formulas of the tangent modulus under K0 consolidation unloading were established referring to the derivation of that in Duncan–Chang model. The theoretical calculation results agree well with the test results. It can expand the use of the Duncan–Chang model and improve its application to engineering practice.

Experimental Research on Compression Characteristics of Compacted Loess

A series of compression tests were conducted on the samples taken from compacted soil site in Lulling, in order to investigate the compressibility of compacted loess in different water content and different compaction energy. Through statistics analysis of the testing data, the stress-strain curve of compacted loess could be expressed with Hyperbolic form. And the relation between initial tangent modulus and water content change coefficient was founded by introducing the water content change coefficient. The result shows that at the same compaction energy, the initial tangent modulus decreases and the ultimate strain increases as water content increases; while at the same water content change coefficient, the initial tangent modulus increases and the ultimate strain decreases as compaction effort increases.

The influence of aging on the shear strength behaviour of two fine-grained soils

A series of undrained triaxial shear tests was conducted on normally consolidated remoulded black cotton soil and kaolinite, in order to examine the influence of aging on their shear strength behaviour. It was found that apart from an increase in shear strength and brittleness, both soils exhibited an increase in their angles of shearing resistance when aged for a month. The cohesion intercept was found to reduce on aging. The initial tangent modulus increases very significantly (to the extent of even 10 times) especially for black cotton soil after 30 days of aging.