Processing of thick section epoxy powder composite structures

The use of epoxy powder as the primary matrix in thick fibre-reinforced composite parts is investigated. The characteristics of three epoxy powders are assessed using several experimental techniques, focusing on their curing behaviour. At least one epoxy powder is shown to have advantageous characteristics for manufacturing thick-section composites. Material models are developed which can describe the processing behaviour (cure kinetics, viscosity change, etc.) of an epoxy powder. The cure kinetics model makes use of an additional rate constant to better describe the rate of cure at both high and low temperatures. The chemorheological model is based on an existing model for toughened epoxies. A one-dimensional simulation tool for manufacturing thick-section composite laminates is developed in MATLAB. The simulation tool employs a resin flow model for vacuum-bag-only prepregs to describe the infusion process and subsequent thickness change. This thickness change is coupled to a model for through-thickness heat transfer which can be solved numerically for various thermal boundary conditions. The model is used to explore the suitability of epoxy powders for the manufacturing thick-section composite structures. The aforementioned simulation tool is validated against experimental results for thick-section composite laminates. The experiments are carried out using a modified heated tool and test apparatus which apply known thermal boundary conditions. A linear variable differential transformer is used to measure the thickness change of each laminate during testing, while thermocouples are used to measure the temperatures at various positions within each laminate. The results of the tests show good agreement with the one-dimensional simulation tool. Additional simulations are performed to investigate the influence of material format, thickness change, and heating methods. Methods for reducing thermal and cure gradients are explored also. A method is outlined for implementing the process models within commercial finite element software, Abaqus FEA. User subroutines for heat transfer and thermal expansion are used to define the various process models. One-dimensional simulations are validated, and a convergence study is performed on time step size and element size. Simulations show the effect of in-plane heating for glass-fibre and carbon-fibre laminates, and the processing of a wind turbine blade root section is investigated. Overall, it is shown that thick-section composite structures can be manufactured using a low-cost commodity epoxy powder from the coating industry, and that these structures do not suffer from the risk of uncontrolable thermal events

Die Wirksamkeit von Radiowerbung für Employer Branding bei der Gewinnung von technischen Fachkräften eines regionalen mittelständischen Unternehmens

The subject of this paper is the effectiveness of radio advertising with a focus on attracting technical specialists from regional medium-sized companies. This is done on the basis of BDT Media Automation GmbH, a medium-sized B2B company from Rottweil. For this purpose, the target group is first determined and analyzed. Subsequently, five hypotheses are formed in view of the employer branding concept and an extensive literature research regarding radio advertising. These hypotheses will be examined in the empirical part of this paper by means of a study with 31 human resources managers and recruiters, and their accuracy will be tested. After evaluating the data, the results of the study are presented in order to provide BDT as well as other medium sized companies with recommendations for action regarding the effectiveness of radio as a communication tool for employee recruitment

Productivity, Zombie Firms and Exit Barriers in Portugal

Productivity growth is slowing in OECD countries, coupled with increased misallocation of resources. A recent strand of literature focuses on the role of non-viable firms "zombie firms" to explain these developments. Using a rich firm-level dataset for Portugal, we explore the role played by zombies in firm dynamics and the misallocation of labour and capital. We confirm the results on the high presence of zombie firms, which are significantly less productive than their healthy counterparts and drag down aggregate productivity. Higher zombie presence is associated with lower growth of viable firms, stifling intra-sectoral capital reallocation. Portugal has shown one of the largest reductions in barriers to exit and restructuring of all OECD countries and is therefore particularly suited for an assessment of the extensive margin effects of these policy changes. We show that a reduction in exit and restructuring barriers promotes a more effective exit channel and fosters the restructuring of the most productive zombies. The results highlight the role of public policy in addressing zombies' prevalence, fostering a more efficient resource allocation, and promoting productivity growth.publishersversionpublishe

Obtaining 2D surface characteristics from specular surfaces

Today's surface appearance measures often ignore the inherent two-dimensionality. This paper proposes a method to acquire and assess the appearance of larger specular surfaces in 2D. First, we describe a deflectometric setup to obtain a gradient field of the surface microstructure. Hence, we propose an areal measure based on the angular power spectrum, as defined in ISO 25178, to characterize the waviness of coated surfaces in relevant scales. To verify the validity of this measure, we compare it with an 1D industry standard appearance measurement system (wave-scan). While our method shows the same characteristics when mapped to the wave-scan values, we observed differences between both systems. These are mainly caused by the different measurement principles and the resulting information of the surface