Simulation and experiment analysis of relationship between voids and permeability of composites

Due to the significant weight reduction advantage, aerospace composite tanks have become the focus of international competition of spacecraft. However, the permeation of small molecules at low temperature caused by the internal voids of composites is one of the key technical problems which restrict the engineering application of composite tanks. In this study, a combination of theoretical research and experimental research was adopted. By changing the curing pressures of the composites’ autoclave process, laminates with different porosities were prepared and tested for low-temperature permeability. Based on the grayscale processing and median filtering methods, a permeability prediction model with the true voids morphology of composite materials was established. Based on the random medium theory, the random voids model of composites was built to study the effect of void size and shape on the permeability of laminates. The results showed that the finite element analysis of composites, permeability based on the real voids morphology model was in good agreement with the experimental results, which proved the feasibility of this method. The permeation rate of laminates increased with the raised of porosity, and the internal porosity of the laminates could be diminished by improving the curing pressure so as to reduce the permeation of the composite components. Under the condition of a certain porosity, the laminates with small, stripe voids had a higher permeation rate than the laminates with large, circular voids.

The Effect of Curing Pressure on Shear Bond Strength of Zirconia to Resin Cement

Background: Nowadays, the esthetics demand is continuously increasing; therefore, metal-free materials are widely used, like a zirconia-based ceramic, which is conveniently fabricated via computer-aided design and computer-aided manufacturing (CAD/CAM) system for restorations from single to full mouth rehabilitation. Objective: This study evaluated the effect of pre-curing pressure on the shear bond strength of zirconia to the resin cement. Methods: A total of sixty-three sandblasted cylindrical zirconia mounted in autopolymerizing resin were randomly assigned to three groups; Group 1: no treatment (control), Group 2: negative pressure, and Group 3: positive pressure to resin cement after resin cement application and resin composite columns bonded to zirconia. Thirty-three of the samples were stored in distilled water at 37 °C for 24 hr before the shear bond strength test for thirty samples and three samples were cross-sectionally cut for interfacial observation with FESEM. Another thirty samples were thermocycled for 5,000 cycles in distilled water at 5°C to 55 °C before testing. The shear bond strength and failure mode were evaluated. Examination of the bonding interface was also done. Results: The results were analyzed using two-way ANOVA. The means of shear bond strength of non-thermocycle of the control group were 8.01 ±1.74 MPa, 9.10 ±1.90 MPa, and 9.14 ±2.58 MPa, whereas that of thermocycle group were 5.71 ±0.84 MPa, 5.53 ±0.68 MPa, and 5.68 ±0.77 MPa in zero pressure group, negative pressure group, and positive pressure group, respectively. It showed no statistically significant differences in shear bond strength in all pressure groups (p > 0.05). The pre-curing pressure did not influence the shear bond strength of the zirconia and resin cement. Conclusion: There was no difference in the shear bond strength between the pressure groups and the no treatment control group. The positive and negative pressure did not influence the shear bond strength of the zirconia and resin cement.

Mechanical and Microstructural Characteristics of Calcium Sulfoaluminate Cement Exposed to Early-Age Carbonation Curing

: The early-age carbonation curing technique is an effective way to improve the performance of cement-based materials and reduce their carbon footprint. This work investigates the early mechanical properties and microstructure of calcium sulfoaluminate (CSA) cement specimens under early-age carbonation curing, considering five factors: briquetting pressure, water–binder (w/b) ratio, starting point of carbonation curing, carbonation curing time, and carbonation curing pressure. The carbonization process and performance enhancement mechanism of CSA cement are analyzed by mercury intrusion porosimetry (MIP), thermogravimetry and derivative thermogravimetry (TG-DTG) analysis, X-ray diffraction (XRD), and scanning electron microscope (SEM). The results show that early-age carbonation curing can accelerate the hardening speed of CSA cement paste, reduce the cumulative porosity of the cement paste, refine the pore diameter distribution, and make the pore diameter distribution more uniform, thus greatly improving the early compressive strength of the paste. The most favorable w/b ratio for the carbonization reaction of CSA cement paste is between 0.15 and 0.2; the most suitable carbonation curing starting time point is 4 h after initial hydration; the carbonation curing pressure should be between 3 and 4 bar; and the most appropriate time for carbonation curing is between 6 and 12 h.

Effect of CO2 Curing on the Strength of High Strength Pervious Concrete

This study was conducted to assess the effect of CO2 curing on the compressive strength of high strength pervious concrete. The factors studied to evaluate compressive strength of concrete on CO2 curing pressure, curing time, and age of specimen at testing. Three Aggregate sizes, three CO2 curing pressures, three CO2 curing time, and three testing ages were used in this investigation. The research tried to produce a high strength pervious concrete and use carbon dioxide for curing to find out whether it could enhance the compressive strength. The results show that the compressive strength of the control group increases rapidly and its 90-day compressive strength closed to 60 MPa. The 1-day compressive strength has a major impact after CO2 curing and their strength decreased by about 0% to 50% as compared to the control group. However, it is observed that there is only slight difference in relationship between modulus of elasticity and compressive strength obtained from 100 by 200mm cylinders with CO2 curing.

Effects of Curing Temperature and Pressure on the Mechanical Properties of Gasket Material Used for Polymer Electrolyte Membrane Fuel Cells

The long-term mechanical stability of the gasket material is critical to sealing and electrochemical performance of the polymer electrolyte membrane (PEM) fuel cells. In this paper, the silicone rubber material, which is being considered as gasket material for PEM fuel cells, was fabricated at different curing temperatures and different curing pressures. Effects of the curing temperatures and curing pressures on the mechanical properties of the silicone rubber material were investigated. The tensile test results show that tensile strength of the specimen cured at the curing temperature of 160 was larger than that for the specimens cured at the curing temperature of 150 or 170 under the same curing pressure. The test results of the compression stress-strain, compression set and compression stress relaxation show that the curing temperature and curing pressure affected significantly the compression elastic modulus, compression set rate and compression stress relaxation behavior. It is found that the silicone rubber material cured at the curing temperature of 160 under the curing pressure of 10MPa had good compression mechanical properties compared to the materials cured at the other curing temperatures and curing pressure in this work.

Variation of voids and inter-layer shear strength of advanced polymer-matrix composites at different pressures with high-pressure microwave

Due to its rapid and volumetric heating capabilities, microwave heating has been conceived as an alternative cost-effective route for curing advanced polymer-matrix composites. Different curing pressures were applied to the pre-impregnated laminates of advanced polymer-matrix composites in a given high-pressure microwave curing process. The variation of voids’ shapes in the advanced polymer-matrix composites was revealed, and the shear failure mechanism of advanced polymer-matrix composites with different porosity was analyzed. During the shear failure process of the advanced polymer-matrix composites, sounds ceaselessly occur. The experiment results of the optical digital microscopy, the scanning electron microscope, and the inter-laminar shear properties test showed that in high-pressure microwave curing process, pressure was not only the main factor that affects the porosity and the mechanical properties but also influences the bonding strength of the resin and the carbon fiber interface significantly. There is a threshold value for the effect of curing pressure on porosity and inter-laminar shear strength of advanced polymer-matrix composites. The internal relationship among porosity, inter-laminar shear strength, and curing pressure of advanced polymer-matrix composites could be summarized as a piecewise power function.

Influence of Curing Pressure on Unconfined Compressive Strength of Cemented Clay

The soil-cement columns are generally installed and cured in the soft clay layers under confining pressure. The strength of the soil-cement columns may be influenced by confining pressure during curing period. In this study, the main objective was to study the influence of curing pressure on unconfined compressive strength of cemented clay. A series of unconfined compression tests was performed on a cement admixed clay sample cured under pressure values of 0 kPa (atmospheric pressure), 25kPa, 50kPa and 100 kPa using a typical unconfined compression equipment. The test samples with values of cement content of 0.5, 1.0 and 2.0 percent were cured for 28 days.The stress-strain curves obtained from all tests show a peak value of stress. The unconfined compressive strength or peak stress obviously increased with increasing cement content for all curing pressure conditions. It can be observed that the strength of samples gradually increased with curing pressure for cement content of 0.5 percent. For cement contents of 1.0 and 2.0 percent, the strengths of samples cured under pressures of 25 kPa dramatically increased from the strength of samples cured without pressure (0 kPa), however, the strengths of samples for curing pressures of 25, 50 and 100 kPa were not clearly different.

Analysis of the Mechanism for the Appearance of Bubbles in Rubber during Vulcanization

Appearance of bubbles in the rubber has been observed in real time by using X-ray imaging method in SPring-8. The behavior of diameter and quantity of bubbles has been measured for the samples which have the various contents of cure agent and moisture. The babbles appear after 30 to 50 sec from releasing curing pressure at the place where there are no any feature in the X-ray images pixel size is 0.5 μm. The diameter of bubbles increases linearly with time at first, and then it grows up to be closer to the limit diameter finally. This final diameter changes with cure time and amount of cure agent. And the quantity of bubbles changes with amount of moisture. The result shows the one of origin of bubbles is moisture, and even if the moisture content changes, the size of moisture particles does not change, the quantity of particles changes. The crosslink density participates in whether this particle changes to a bubble or keeps that state. That also participates in the final diameter of bubble. X-Ray experiments were performed at BL19B2, BL46XU in the SPring-8 with approval of the Japan Radiation Research Institute (JASRI) Proposal No. 2014A1571,2014A1572).