Robotic tow placement for local reinforcement of glass mat thermoplastics (GMTs)

A process has been developed where multiple yarns of commingled glass and polypropylene are heated and placed to a desired geometry. The placed unidirectional (UD) tow is then encapsulated by over-moulding with glass mat thermoplastics (GMTs). The effects of both force (20–80 N), with an equivalent pressure range of 8–46 bar, and the displacement rate (20–100 mm/s) during tow placement on void content and flexural modulus were investigated for nine placement conditions. Placement rate affected the tow quality measured before over-moulding. Void contents were reduced after over-moulding. The volume fraction of UD tow (vol. O° composite/vol. total composite) in the manufactured multi-material structure was 21.5%. Corresponding tensile tests showed a factor of 2 increase in modulus and a factor of 3 increase in strength relative to GMT. Tow placement rate had no significant effect on tensile properties after over-moulding. Tensile behaviour as a function of the UD tow volume fraction was modelled and predicted values agreed with the experimental results. Additionally, the predicted tensile performance of UD tow reinforced GMT has been compared with possible competitive materials using merit indices.     

The effect of crack tunneling on crack growth: experiments and CTOA analyses

This paper compares experimental crack-front shapes recorded at various stages of crack growth with area-average crack growth values during fracture tests conducted on 2024-T351 aluminum alloy plate. Crack-front shapes were determined by fracturing the specimen to a predetermined amount of crack growth and fatigue cycling the specimen for about 4000 cycles at a high stress ratio (P_min/P_max) to mark the crack-front location. For each shape, the area-average crack length was determined. The evolution of tunneling was used to create a calibration curve that could be used to adjust surface measured crack-length values, for a more representative comparison with analyses that use a straight crack-front approximation. The analysis compares much more favorably with the average crack growth than with the surface measured values near maximum load. However, the area-average technique tends to over correct crack growth near the crack initiation load. Crack tunneling results show that the area-average technique produces more representative crack-length measurements compared to optical based surface measurements.     

The influence of twins on the mechanical properties of nc-Al

The presence of grown-in twins in nanocrystalline-Al is found to enhance plastic deformation via twin-migration in which partial dislocations, emitted at the intersection of the twin boundary and the GB, travel through the entire grain. For samples without grown-in twins, full dislocation activity is observed as the predominant slip mechanism. These results are discussed in terms of generalised planar fault energy curves for the Al potential used in the present work. Simulations of 2D columnar nc networks are also performed to investigate the role of grain and grain-boundary-network dimension on the mechanical properties of nc systems.     

Super-twisting sliding mode differentiation for improving PD controllers performance of second order systems

Designing a proportional derivative (PD) controller has as main problem, to obtain the derivative of the output error signal when it is contaminated with high frequency noises. To overcome this disadvantage, the supertwisting algorithm (STA) is applied in closed-loop with a PD structure for multi-input multi-output (MIMO) second order nonlinear systems. The stability conditions were analyzed in terms of a strict non-smooth Lyapunov function and the solution of Riccati equations. A set of numerical test was designed to show the advantages of implementing PD controllers that used STA as a robust exact differentiator. The first numerical example showed the stabilization of an inverted pendulum. The second example was designed to solve the tracking problem of a two-link robot manipulator.     

A path conditioning method with trap avoidance

This work presents a sliding-mode method for robotic path conditioning. The proposal includes a trap avoidance algorithm in order to escape from trap situations, which are analogous to local minima in potential field-based approaches. The sliding-mode algorithm activates when the desired path is about to violate the robot workspace constraints, modifying it as much as necessary in order to fulfill all the constraints and reaching their limit surface at low speed. The proposed path conditioning algorithm can be used on-line, since it does not require a priori knowledge of the desired path, and improves the conventional conservative potential field-based approach in the sense that it fully exploits the robot workspace. The proposed approach can be easily added as an auxiliary supervisory loop to conventional robotic planning algorithms and its implementation is very easy in a few program lines of a microprocessor. The proposed path conditioning is compared through simulation with the conventional potential field-based approach in order to show the benefits of the method. Moreover, the effectiveness of the proposed trap avoidance algorithm is evaluated by simulation for various trap situations.     

E-mentoring: The effects on pedagogical training of rural teachers with complex geographical accesses

Considering multi difficulties that determine the labor of rural teachers who perform their teaching practices in semi-isolated contexts, it is necessary to provide them a supportive system which favors their pedagogical performances to benefit rural students’ education. The aim of this phenomenological study is to describe and analyze how e-mentoring can strengthen pedagogical performances of primary rural teachers with complex geographical accesses in Chile, exploring the subjective experiences of four couples of teachers and mentors that take place in this process by e-mail relationship. Results show the necessity of considering the accompaniment as a horizontal pedagogical assistance which can be influenced by the technological resources availability, identifying an adequate profile of e-mentor to influence teacher adherence to the process, such as his communicative style, empathy, pedagogical and cognitive skills. Finally, this investigation allows projecting a viable model to be applied as support for rural education with access to Internet resource.     

Effect of resin type and content on properties of composite particleboard made of a mixture of wood and rice husk

This study investigated the effect of resin type and content on the dimensional stability and mechanical properties of single-layer composite particleboards made of a mixture of wood particles (70 wt%) and rice husk particles (30 wt%). Two types of resin, urea–formaldehyde (UF) resin and phenol–formaldehyde (PF) resin, were used in the experiments at three different contents which were 8, 10, or 12 wt%. The dimensional stability of the samples was significantly improved by increasing the resin content. When the contents of the UF and PF resins increased from 8 to 12 wt%, the WA values of the samples decreased to18% and 33%, respectively. Similar results were also observed for the TS values. The UF resin bonded samples swelled two times more than the PF resin bonded particleboard. The mechanical properties of the PF resin bonded samples were better than the UF resin bonded samples. When the contents of the UF and PF resins increased from 8% to 12 wt%, the internal bond strength values of the samples increased to 21% and 41%, respectively. The bending strength and modulus of elasticity of the samples were not significantly increased by increasing contents of the UF and PF resins, except for the 12 wt% content.     

A parallel FFT on an MIMD machine

In this paper we present a parallelization of the Cooley- Tukey FFT algorithm that is implemented on a shared-memory MIMD (non-vector) machine that was built in the Dept. of Computer Science, Tel Aviv University. A parallel algorithm is presented for one dimension Fourier transform with performance analysis. For a large array of complex numbers to be transformed, an almost linear speed-up is demonstrated. This algorithm can be executed by any number of processors, but generally the number is much less than the length of the input data.     

The effect of fiber orientation angle in composite materials on moisture absorption and material degradation after hygrothermal ageing

This paper examines the effect of fiber orientation angle of composite material (T300/5208) on the phenomenon of humidity diffusion. Using an analytical method, humidity concentration through the thickness of plates of E-glass/epoxy and carbon/epoxy (T300/5208) composites are calculated. This method also predicts the saturation in humidity and the time to saturation.

Analytical formulations have been proposed [G. Verchery, Moisture diffusion in polymer matrix composites with cyclic environmental conditions, Developments in the Sciences and Technology of Composite Materials, in: A.R. Bunsell, J.F. Jamet, A. Massiah (Eds.), Proceeding of the 5th European Conference on Composites Materials (ECCM5), European Association For Composite Materials (EACM), Bordeaux, France (1992) 505; E. Adda-Bedia, W.S. Han, G. Verchery, Moisture diffusion in polymer matrix composites with cyclic environmental conditions, Textile composites in building construction (TCIBC 1992), in: P. Hamelin, G. Verchery (Eds.), Proceeding of the International Symposium Textile Composites in Building Construction, Pluralis, Paris, Part. 2, Lyon, France (1992a) 127; W.S. Han, E. Adda-Bedia, G. Verchery, Diffusion de l’humidité dans une plaque composite soumise à des conditions d’environnement cycliques”, Anales des Composites, Publication éditée par l’AMAC (1992) 29; E. Adda-Bedia, W.S. Han, G. Verchery, Simplified methods for prediction of moisture diffusion in polymer matrix composites with cyclic environmental conditions, International Journal of Polymers and Polymer Composites 6 (4) (1998) 189] to solve Fick’s equation which describes the kinematics of hygroscopic absorption in materials. The diffusivity in composites being a very important parameter, the present model is described in detail.

In addition, a study of the effects of environmental conditions and fiber orientation angle on saturation values and ageing degradation of E-glass/epoxy composites exposed to hygrothermal conditions is proposed.