Ultrasonic thermography or thermosonics is proved to be an effective non-destructive testing (NDT) method for inspecting carbon-fiber-reinforced polymer (CFRP) composites; however, the potential damages for the structure cannot be ignored, because of the contact vibration between the ultrasonic horn and the specimen. This work aims at developing a new excitation method for ultrasonic thermography-aircoupled ultrasonic excitation. CFRP laminates with impact damages are tested by air-coupled ultrasonic thermography, and the theoretical model of heat conduction is given. Results demonstrate good excitation performance for impact damages detection in CFRP composites. Moreover, the conventional ultrasonic thermography results are shown, and the prospect of air-coupled ultrasonic thermography is discussed.
:Wire arc additive manufacturing (WAAM) is used for fabrication of NiTi parts using a dedicated NiTi wire as the feedstock material. The microstructure, phase transformation characteristics and mechanical properties of the as-built parts are investigated. Experimental results show that the microstructure in each deposition layer is different as a result of the different thermal cycle conditions. Along the part height,the first deposited layer is larger equiaxed grains. As the heat input gradually accumulates, the grain growth tends to the finer equiaxed form, and there are columnar grains between two deposited layers. The as-built parts are completely austenite phase at room temperature. The Ni content of deposited layers which have higher hardness, wider transformational range and hysteresis compared to the as-received NiTi wire is 51.10 at.%. The tensile strenght of the as-built parts is of about 611.30 MPa with a corresponding fracture strain of 19.50%. The fracture surface with dimple fracture had good ductility. Additionally, it can be seen that the WAAM specimens exhibit good superelastic properties, evidenced by a low irrecoverable strain of 1.01% upon unloading in the first cycle. The plastic strain increases during the first 8 cycles, and reaches near a constant value of 2.68% .
In the hydraulic and pneumatic equipments, the elastomeric O-ring gaskets are widely used to ensure their sealing. The sealing capability of an elastomeric O-ring gasket depends upon the contact stresses that develop between the O-ring and the surfaces with which it comes into contact. In order to increase the strength and service life of the general elastomeric O-ring gasket, this gasket including metal skeleton is gradually applied to the actual product in recent years. In spite of the sealing performance of the elastomeric O-ring gasket has been investigated in many literatures, few information of it is known about the elastomeric O-ring gasket including metal skeleton. If any gasket degrades or fails, the overall operation and performance of the production will be affected. This paper aims to study the contact stresses occurred on the metal skeleton seal structure under the various interference fits with three sectional forms (trapezoid cross section, square cross section and circular cross section). Finite element analysis is used to predict sealing performance of an installed metal skeleton seal, providing a normal force against the sealing surface. Results gained show that the contact stresses occurred on the metal skeleton seal is larger than it generated on the general elastomeric O-ring gasket. Furthermore, the metal skeleton seal possessed trapezoid cross section can produce the maximum contact stress, which will show the best sealing performance among three sectional forms.
The influence of 60Co gamma-ray irradiation on the chemical compositions and properties of an epoxy encapsulant was investigated. The total irradiation dose varied from 0.1 kGy to 100 kGy with a fixed dose rate of 500 Gy/h. Fourier transform infrared spectra, X-ray photoelectron spectroscopy analysis and discoloration proved the occurrence of oxidation caused by the irradiation. Tg and density remained unchanged owing to the competing effects of irradiation-induced degradation and crosslinking. The tensile strength, impact strength and electrical strength did not decrease distinctly until the irradiation dose reached above 11 kGy. Rubbery coefficient of linear thermal expansion (CTE), relative permittivity and dissipation factor, on the contrary, were more sensitive to the oxidation and increased sharply when the irradiation dose was only 0.1 kGy. Our results suggested that the evolution of CTEs and dielectric properties could affect the long-term application and reliability of the epoxy encapsulant in the irradiative environment.
The change of dielectric properties of epoxy/anhydride systems during eleven months storing was investigated. Test systems were bisphenol-A epoxy cured by different content of methyl-hexahydrophthalic anhydride with a slow cooling process across Tg range included in their curing procedure to reduce residual stress. The results showed all the systems’ dielectric constant and dielectric loss angle tangent had a trend of down then up. The highest decreasing amplitudes were about 4% and 10% respectively. Possible explanations for these phenomena were proposed, and then some preliminary investigations were carried out like DSC and IR.
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