Meelis Pohlak

Weak formulation based Haar wavelet method for solving differential equations

The Haar wavelet based discretization method for solving differential equations is developed. Nonlinear Burgers equation is considered as a test problem. Both, strong and weak formulations based approaches are discussed. The discretization scheme proposed is based on the weak formulation. An attempt is made to combine the advantages of the FEM and Haar wavelets. The obtained numerical results have been validated against a closed form analytical solution as well as FEM results. Good agreement with the exact solution has been observed.

Modelling and optimal design of sheet metal RP&M processes

Purpose – To study the influence of process and product parameters on the properties of products in incremental sheet metal‐forming; to create models for process optimisation and to introduce an approach to incremental forming process optimisation.Design/methodology/approach – A new flexible sheet metal‐forming technique, incremental forming, has been studied. The technique can be viewed as a rapid prototyping/manufacturing technique for sheet metal parts. To analyse the process, an experimental study and finite element analysis were performed. For the optimal design of incremental forming process non‐linear mathematical programming was used. To estimate the limitations and main parameters of the process, a complex model was developed.Findings – Introducing optimisation procedures for the incremental forming process allows users to increase productivity and to assure quality.Research limitations/implications – As finite element analysis of the process is time‐consuming in real life situations, a future st...

Recycling of PA-12 in Additive Manufacturing and the Improvement of its Mechanical Properties

This study explores possible ways to make Additive Manufacturing (AM) a cradle-to-cradle process, that is, use the leftover from one process as the raw material for another process. The main goal of this study is to develop a set of new polymeric blends with innovative properties, suitable for using in 3-D printing of prosthetic limbs using Fused Deposition Modeling (FDM) technology. Sustainable acting is achieved by reusing polymeric material left over from Selective Laser Sintering (SLS) processes for making raw material for FDM processes. Test specimens of polyamide 12 (PA-12) in its virgin form and used- , un-sintered form alongside specimens of used PA blended with TPU, aramid, or graphite, were produced in a micro-injection moulding machine and then tested for their mechanical properties. This paper provides information about the differences in mechanical characteristics of these different material blends. An unexpected but positive finding was that the differences between virgin and recycled PA-12 are insignificant. The aforementioned additives influenced PA-12 by producing specimens that responded with predictable characteristics which is a significant accomplishment as it lays the groundwork for the next stages of the project.

An algorithm for localised and diffuse necking analysis

An analysis of localised and diffuse necking in orthotropic sheet metals is performed. Numerical algorithm covering different yield criteria and strain hardening laws is developed. The numerical algorithm proposed is based on plastic instability criterion derived in tensor notation. The capabilities of the instability criteria derived are improved using different anisotropic yield criteria and strain hardening laws. The obtained results are validated against experimental test results and compared with the results available in literature. The material parameters identification procedure for advanced yield criterion BBC2003 is treated. The Computer Algebra Systems (CAS-es) method is employed for material parameters identification. Simplicity and low computational cost of numerical analysis is achieved by the use of Hill and Swift instability conditions and empirical stress-strain relation. [Received 7 March 2007; Accepted 10 July 2007]

Design and Manufacturing of Variable Angle Tow Laminate

Variable angle tow (VAT) laminates have shown enhanced stiffness/strength performance compared to conventional straight fiber laminates. Employment of VAT allows utilizing variable stiffness design of composite structure, thus it widens the design possibilities. As a result, composite structure with improved mechanical characteristics can be manufactured. The main aims of the current study are to give an overview on methods and algorithms used for analysis and design of VAT laminates, and to develop technology and equipment for manufacturing laminate with improved structural performance. In order to improve the accuracy of the compaction process, a set of experiments were carried out using a simple testing device. For measuring the compaction force, a pneumatic cylinder, pressure regulator and digital manometer were used. The temperature of the consolidation area and the heat distribution were screened with the thermal camera. Infrared heater was used as a heating source. Material used in the experiment was carbon fiber reinforced polyamide.Findings show that in addition to the main parameters – the compaction force and temperature, there are many minor factors, such as the compaction wheel diameter, material and surface roughness of the compaction roller, the material and surface roughness of the mold and the pretension in the laminating tape and also the laminating speed, all influence the quality of the final product.Key words: Advanced Fiber Placement Technology, Automated Fiber Placement, Automated Tape Laying, Fiber Reinforced Composites, Laminates

Fatigue Performance of Semiconductor Strain Gauges in GFRP Laminate

Semiconductor strain gauges have good potential for embedded structural health monitoring (SHM) applications. Silicon mechanical and thermal properties are similar to glass fiber. In this paper the high cycle loading of specimens with embedded and surface mounted semiconductor strain gauges is conducted. The base material is glass fiber laminate. Two different matrixes are used: polyester and epoxy resins. Hygrothermal loading is introduced to weaken the sensor-matrix interface and to imitate real application conditions.

ISF process model development

Incremental Sheet Forming (ISF) process modelling is studied. Main attention is paid to safe manufacturing. Experimental, numerical and theoretical study is performed in order to determine the material formability and the forming load components. ISF strategies for determining the Forming Limit Diagram (FLD) are pointed out. The obtained experimental results have been found to be in good agreement with theoretical considerations. The effect of plastic anisotropy of metal sheets is considered in theoretical models and test design. The obtained results allow to predict tool and material failure in ISF process. [Received 5 June 2007; Accepted 5 July 2007]