Search

In order to enhance the large-scale and high-quality application of limestone powder (LP) and phosphogypsum in the construction industry. Therefore, this paper mainly studies the effect of LP on the physical and mechanical properties of phosphogypsum-based composites, and clear the mechanism of action from the micro level. Based on the results, the conclusions were as follows: LP has the effect of reducing the standard consistency water requirement and shortening the setting time of fresh phosphogypsum-based composites slurry; LP can increase the bulk density and reduce 24-hour water absorption of hardened phosphogypsum-based composites matrix; In addition, the strength of the matrix increases first and then decreases with the increase in the amount of LP and the recommended amount of LP is 5%-10%. Microscopic tests have shown that when the amount of LP is less than 10%, it can effectively reduce the porosity, refine the pore size, and play the role of a dense hardened matrix. In addition, the appropriate amount of LP particles can be adsorbed on the surface of dihydrate crystals, increasing the bonding force between the crystals, which is conducive to increasing the strength of the composite matrix.

The ultra-high toughness cementitious composites (UHTCC) was modified by different types of epoxy emulsion, the effect of the emulsion on working performance, mechanical properties and impermeability of UHTCC were studied. The result showed that compared with the control without epoxy emulsion, the type I and type II epoxy emulsion improved the working performance of UHTCC with low dosage. However, the type III epoxy emulsion exhibited an improvement effect on working performance of UHTCC only when the dosage was high. The addition of epoxy emulsion to UHTCC caused a decrease in compressive strength and flexural strength, but epoxy emulsion and hydration products formed an excellent energy absorption mechanism in the specimen and enhanced the elastic modulus of UHTCC. Type I and II epoxy emulsion had little effect on the tensile properties of UHTCC. Type III epoxy emulsion greatly increased the ultimate tensile strength of UHTCC, and the strain hardening phenomenon disappeared at low dosage level, however, the effect of strain hardening had been improved by increasing its dosage.

Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356 alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment. The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment.

The demand for low-cost, high strength-to-weight ratio components continues to drive the development of aluminum alloy castings for new applications. The automotive brake caliper, which has traditionally been made from cast iron, is one such component where aluminum alloys are being considered. The fatigue performance of the brake caliper is an important consideration in evaluating new designs. The use of aluminum alloys in cast components under cyclic loading conditions necessitates a design approach that, in addition to in-service loading, incorporates the impact of microstructural features on fatigue life. In this investigation, a through-process modeling approach has been employed to link a series of mathematical models describing the processing steps of (1) casting, (2) heat treatment, (3) machining, and (4) in-service performance for a preliminary design of a brake caliper made of aluminum alloy A356, Step (1) includes microstructural predictions (secondary dendrite arm spacing and maximum pore size), which are tracked through to the final component. The final lifing of the component combines the effects of these microstructural features with the complex stress state arising from the combined service loading and residual stresses. It was found that all three factors have a strong influence upon the component’s fatigue performance.

Fatigue strength optimization of cast aluminium alloys requires an understanding of the role of micropores resulting from the casting process. High cycle fatigue tests conducted on cast A3 56-T6 show that the pore size and proximity to the specimen surface significantly influence fatigue crack initiation. This is supported by finite element analyses (both elastic and elastic-plastic) which demonstrate that high stress/strain concentration is induced by pores which are both large and near to the specimen surface. A new pore-sensitive model based on a modified stress-life approach has been developed which correlates fatigue life with the size of the failure-dominant pore. The model prediction is in good agreement with experimental data.

Porosity is well known to be a potent initiator of fatigue cracks in cast aluminum alloys. This article addresses the observed scatter in fatigue life of a cast A356-T6 aluminum-silicon alloy due to the presence of porosity. Specimens containing a controlled amount of porosity were prepared by employing a wedge-shaped casting mold and adjusting the degassing process during casting. High-cycle fatigue tests were conducted under fixed stress conditions on a series of specimens with controlled microstructures (especially, the secondary dendrite-arm spacing), and the degree of scatter in the results was assessed. Stochastically, such scatter was found to be adequately characterized by a three-parameter Weibull distribution function. Large pores at or close to the specimen surface were found to be responsible for crack initiation in all fatigue-test specimens, and the resultant fatigue life was related to the initiating pore size through a relationship based on the rate of small-fatigue-crack propagation. With respect to the probabilities for the pores of various sizes and locations to initiate a fatigue crack, a statistical model was developed to establish the relationship between the porosity population and the resultant scatter in fatigue life. The modeling predictions are in agreement with the experimental results. Moreover, Monte-Carlo simulation based on this model demonstrated that the average pore size, pore density, and standard deviation of the pore sizes, together with the specimen size and geometry, are all of consequence regarding scatter in fatigue life.

A micro-scale analytical model for predicting size-dependent microhardness is presented. The indentation size effect is explained by dissipation energy associated with the contact surfaces. Micro-mechanics mechanism of this energy dissipation is studied via a plastic deformation analysis of micro-scale asperities on the contact surface under indentation. The analysis shows that the microhardness depends not only on the properties of the bulk material under test, but also the properties of the contact surface, such as the plastic behavior of the micro-asperities on contact surfaces. The dependence of microhardness on the roughness parameters of the contact surfaces is also revealed from the analysis.

Microscale contact between two solid surfaces induced by the liquid-bridging force is studied in the present article. Attention is focused on the interaction between the size of the contact area and the magnitude of the liquid-bridging force. The liquid-bridging force is related to the contact size and the other parameters, such as the filling angle, the contact angles, and the surface tension of the liquid, by solving the Laplace–Young equation. A singular integral equation for the coupled contact equilibrium problem is established and solved. Two approximate solutions for the same problem are also given and compared with the solution of the singular integral equation. Parametric study reveals the dependence of the semicontact width and the liquid-bridging force on the filling angle and the dimensionless group in the microscale contact.

The mixtures of mesogen-jacketed liquid crystalline polymers (MJLCP) and epoxy resin (E-51) have been prepared in a certain proportion. The category of the MJLCP is poly{2, 5-bis[(4-butoxyphenyl) oxycarbonyl] styrenes} (PBPCS). Methyl tetrahydro phthalic anhydride (MeTHPA) is served as a curing agent, and N, N-dimethylbenzylamine plays the role of catalyst. Then, based on the curing process, the modified materials have been acquired by casting molding. The effect of PBPCS's improvement has been researched through the mechanical properties test, dynamic thermal mechanical test and scanning electron microscope (SEM). The results indicated that PBPCS could apparently improve the mechanical properties of resin E-51. Compared with unmodified materials, the elongation at break and the tensile strength of PBPCS have been improved remarkably through modification. Both mechanical properties of the 4wt% PBPCS/E-51/MeTHPA were enhanced by 48% and 153% separately, and the characteristics of the 3wt% one were raised by 47% and 19% respectively. Also, the ductile fracture morphology of the resins was exhibited in SEM photograph clearly.