Papers by Author: Qing Qing Ni

Abstract: Barium titanate/graphene oxide/polyurethane (BTO@GO@PU) composite membranes for microwave absorption were designed and fabricated by mechanical-blending of BTO and GO in PU medium, followed by mold formation. The cross section morphology of the BTO@GO@PU membrane indicated that the BTO nanoparticles with 450 nm average diameter are successfully incorporated into the PU matrix. Mechanical tensile measurement showed that, as the amount of BTO nanoparticles increased from 5 wt% to 20 wt%, the elastic modulus of the corresponding membrane increased up to 23.0 MPa elongation with the elongation above 450 %. Microwave absorption property of the BTO@GO@PU membranes were evaluated by measuring its reflection loss in the frequency range of 0.1-18 GHz. With the addition of BTO up to 20 wt%, the maximum absorptivity of the composite reached up to 51 %. This is attributed to the dielectric loss of BTO nanoparticles.

Abstract: Carbon nanotubes (CNTs) reinforced natural rubber (NR) composite was fabricated in a solvent casting method. The tensile properties and electrical resistivities were investigated. As a comparison, carbon black (CB) reinforced NR was also prepared in the same method. The initial modulus of NR/3 wt% CNT composite was about 1.6 times as large as that of NR/ 3wt% CB composite. NR/CNT composite had a percolation threshold value of about 5 wt% CNTs. The morphology of fillers and composites were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The improved properties of NR/CNT composites were attributed to the excellent mechanical and electrical properties of CNTs and its formed network, which must be benefit to electromagnetic interference shielding properties of NR composites.

Abstract: Signals propagate on plate-like structures as ultrasonic guided waves, and analysis of Lamb waves has been widely used for on-line monitoring. In this study, the wave velocities of symmetric and anti-symmetric modes in various directions of propagation were investigated. Since the wave velocities of these two modes are different, it is possible to compute the difference in their arrival times when these waves propagated the distance from the vibration source to sensor. This paper presents an evaluation formulation of wave velocity and describes a generalized algorithm for locating a vibration source on a thin, laminated plate. With the different velocities of two modes based on Lamb wave dispersion, the method uses two sensors to locate the source on a semi-infinite interval of a plate. The experimental procedure supporting this method employs pencil lead breaks to simulate vibration sources on quasi-isotropic and unidirectional laminated plates. The transient signals generated in this way are transformed using a wavelet transform. The vibration source locations are then detected by utilizing the distinct wave velocities and arrival times of the symmetric and anti-symmetric wave modes. The method is an effective technique for identifying impact locations on plate-like structures.

Abstract: Electromagnetic shielding effect material is needed because electronic devices suffer electromagnetic interference. Otherwise, in many engineering designs such as antenna fairings, sonar cover and stealth aircraft, materials with good electromagnetic penetration are desired. High performance fiber-reinforced composites have high specific strength and mechanical properties, and there is therefore a need to develop an electronics enclosure with optimum shielding by using a combination of particular fiber reinforcements and a polymer matrix. This paper describes the development of high-performance fiber-reinforced composites that use four high strength fibers (super fibers), Dyneema SK60 (an ultra-high molecular weight polyethylene fiber), Zylon HM (poly-p-phenylenebenzobisoxazole fiber), Technora T-241J (aramid fiber) and Torayca T800HB (carbon fiber). These super fibers were fabricated by compression molding and their shielding effectiveness (SE) was tested. The results showed that the newly developed Dyneema fiber, Zylon fiber and Technora fiber composites exhibited low electromagnetic shielding properties of 1.3~2.3 dB at a frequency of 0.5~18 GHz. Furthermore, the Torayca fiber composite has high electromagnetic shielding properties of 10.2~20.7 dB at the same frequency. It is expected that these high-strength composites with optimum SE can be obtained by controlling the electromagnetic shielding properties from hybrid multi-fiber structures.

Abstract: The energy released from damages may give rise to small surface displacements and cause transient elastic waves in materials. Wave propagation of thin plates usually takes the form of lamb waves, and the modes of the lowest symmetric (S0) and anti-symmetric (A0) lamb waves are called the extensional and the flexural modes, respectively. In this study, a formulation, including the effect of shear deformation and rotary inertia, was derived to valuate the velocity of plate waves. The characteristics of plate wave propagation in thin laminated plates were investigated. Acoustic emission (AE) signals were generated by function generators and lead breaks, and propagated in laminated plates. These waves would be detected by broadband AE sensors because of their high sensitivity. With wavelet transform, AE waves were analyzed and the wave velocities of A0 and S0 modes were exactly calculated. Moreover, a simple method was presented to evaluate delamination in laminated plates. The length of delamination was predicted, and agreed well with the actual length.

Abstract: Carbon nanotubes (CNTs) reinforced natural rubber (NR) composites with the CNT contents of 1, 3, 5, 10 and 20 wt% were synthesized using a solution casting method. The morphology of the composites was observed by scanning electron microscopy and Fourier transform infrared spectroscopy. It was found that CNTs were well distributed into NR and there was an excellent interface between CNTs and natural rubber. The mechanical properties of the composites were investigated by dynamic mechanical analysis (DMA) test and tensile test. It was found that the increment of storage modulus of NR/CNT composites was about 0.42 MPa/wt% in the rubbery state which agreed well with the results of tensile test. The large modulus increment confirmed the reinforcement effect of carbon nanotubes.

Abstract: Based on Laminate plate theory, a formulation, including the effects of shear deformation and rotary inertia on the characterization of plate wave propagation, was derived. The characteristics of plate waves propagating were investigated and the influences of frequency, plate thickness and propagating direction were clearly known. A health monitoring system was built and the plate waves generated by lead break source were received by acoustic emission (AE) sensors. By the wavelet transform [1], the time-frequency domain of AE signal was derived. For a certain frequency, the first peak of the magnitude of wavelet transform indicates the arrival times of plate waves. The locations of lead break source and the delaminations of plate were compared with the predictions of theory.

Abstract: Polybenzimidazole (PBI) and vapor grown carbon nanofibers (VGNFs) nanocomposites were developed successfully by using ultrasonic mixing followed by hot compress. The contents of VGNFs used were 0.5wt%, 1wt%, 2wt% and 5wt%. The mechanical properties of neat PBI and PBI/VGNFs nanocomposites were discussed and the results were that the Young’s modulus, tensile strength, storage modulus and hardness were improved after adding VGNFs. Microscopic analysis showed that the dispersion of VGNFs in nanocomposites with a lower amount was considered to be uniform.

Abstract: There are exists positive thermal expansion property for almost all materials. However, in many cases, the material property with negative thermal expansion is requested for engineering applications. This work is to develop high performance fiber-reinforced composites with negative thermal expansion by using high strength polyethylene fiber Dyneema®, high strength PBO fiber ZYLON®, aramid fiber Technora ® and carbon fibers.

Abstract: For the short glass fiber reinforced high density polyethylene (HDPE), a high shearing stress-induced HDPE and MAH functional reaction method was introduced which was achieved by increasing the twin screw extruder rotation speed during melting extrusion process. The results show that the graft degree of consequent HDPE-g(b)-MAH product has been increased from 0.14% of the thermal-induced one to 0.77% by the high shearing stress-induced one when the rotation speed of the screw increases from 100r/min to 800r/min. Therefore, the mechanical properties of the GF/HDPE composites are greatly improved, namely, its tensile-strength, bending-strength and the notch impact absorbed work are increased from 21.43MPa, 15.49MPa and 12.82kJ/m2 to 30.18MPa, 29.72MPa and 19.12kJ/m2 respectively. The corresponding simulations through the commercial Moldflow soft show a good agreement with these experimental data.