Effect of fibre wrinkling to the spring-in behaviour of L-shaped composite materials

© 2014 Elsevier B.V. All rights reserved. To determine the amount of deformation resulting from fibre wrinkling at corner regions, a set of experiments have been conducted. As known in the conventional lay-up method, the prepregs are laid sequentially layer by layer on the mould surface. At the corner region of a female tool the radius decreases at the inner surface and the amount of wrinkles increase towards the top layer as the layers are laid up. In order to determine how much these wrinkles influence the dimensional stability of the manufactured parts, an alternative lay-up method is used. The amount of the wrinkles can be increased for the parts of same geometry by first stacking prepregs on a flat plate and then bending the whole stack to conform to the surface of the L-shaped mould. In this method, more wrinkling occurs on the inner surface of the corner regions as compared to the conventional lay-up procedure. It was found that fibre wrinkling decreases the spring-in values. The mechanism behind that observation is discussed with the help of a heuristic Finite Element Analysis (FEA). The conformation of the stacked prepregs on the mould was simulated by using FEA

3D finite element model for predicting manufacturing distortions of composite parts

A 3D finite element model has been developed for predicting manufacturing distortions of fibre-reinforced thermosetting composite parts. The total curing process is divided into three steps that correspond to the states that resin passes through during curing: viscous, rubbery, and glassy. Tool-part interaction properties were calibrated by modelling the distortion of a single ply part. For comparison, composite parts of various geometries (L-section and U-section), stacking sequences, thicknesses, and bagging conditions were manufactured. The full field thickness profile and full field distortion pattern were obtained using a 3D laser scanner, which reveals higher and lower resin bleeding and corner thickening locations. The effect of stacking sequence is also examined with the full field distortion pattern. It was found that the parts manufactured under the bleeding condition give higher spring-in and warpage values. The spring-in predictions were well matched to measurements of the manufactured parts.Bogazici University Research FundBogazici University [10020 15A60D3]; Istanbul Development Agency (ISTKA)Turkiye Cumhuriyeti Kalkinma Bakanligi [ISTKA/BIL/2012/58]The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Nuri Ersoy and Kenan Cinar acknowledge the support of the Bogazici University Research Fund and Istanbul Development Agency (ISTKA) under project codes 10020 15A60D3 and ISTKA/BIL/2012/58, respectively

3D finite element model for predicting manufacturing distortions of composite parts

A 3D finite element model has been developed for predicting manufacturing distortions of fibre-reinforced thermosetting composite parts. The total curing process is divided into three steps that correspond to the states that resin passes through during curing: viscous, rubbery, and glassy. Tool–part interaction properties were calibrated by modelling the distortion of a single ply part. For comparison, composite parts of various geometries (L-section and U-section), stacking sequences, thicknesses, and bagging conditions were manufactured. The full field thickness profile and full field distortion pattern were obtained using a 3D laser scanner, which reveals higher and lower resin bleeding and corner thickening locations. The effect of stacking sequence is also examined with the full field distortion pattern. It was found that the parts manufactured under the bleeding condition give higher spring-in and warpage values. The spring-in predictions were well matched to measurements of the manufactured parts

A Study on the OSH Performance Model in Coal-Fired Thermal Power Plants

Coal-fired Thermal Power Plants (CTPP) consist of many sections that are risky in terms of occupational safety. CTPP are also important part of electrical Energy Logistics systems. Then it is of great importance to examine which parts of complex systems such as coal-fired thermal power plants are more important in terms of occupational health and safety. CTPP are divided into nine main sections as coal parking area, ash and slag disposal facility, boiler maintenance service, mechanical workshop, measurement and control service, turbine maintenance service, technical office, tea and rest room and use of company vehicles section. In this paper, a weighting and ranking study has been conducted in terms of risk analysis of nine departments in CTPP. For this purpose, accident statistics according to years were obtained from an actively working CTPP, and the results obtained with AHP and the risk significance weight values were compared with these results. Since CTPP are very complicated and complex structures, it is of great importance to determine the risk levels of the departments more significantly and objectively from the point of view of OHS. From this point of view, CTPP were analyzed for the first time within the scope of OHS with the Analytical Hierarchy Process (AHP), which is a multi-criteria decision-making (MCDM) method. According to the findings obtained in our study, coal parking area, ash slag disposal facility and boiler maintenance service were determined as the most risky sections in terms of OHS. With this study, a more effective assessment can be made in terms of OHS for CTPP, so that work accidents will be reduced, energy production will be carried out in a safer and more sustainable way

Damage mode identification in transverse crack tension specimens using acoustic emission and correlation with finite element progressive damage model

© 2018 Elsevier Ltd In this study, damage progression in unidirectional composite specimens is investigated. Transverse Crack Tension specimens are used to stimulate damage in a predetermined progressive sequence. Acoustic Emission (AE) registration technique and its location detection capability is used to identify and locate the damage modes during the tension tests. The k-means++ algorithm is applied to cluster similar AE events and obtain reliable correlations between the damage modes and AE characteristics. Damage modes at the end of interrupted tests are identified under an optical microscope and correlated with locations of AE clusters. It is seen that matrix cracks have high amplitude and duration, whereas delaminations have low amplitude and mid-duration, and fibre breaks have high average frequency characteristics. A finite element analysis was performed to predict the progressive failure behaviour including intralaminar failure and delaminations. The correlations between the AE clusters and damage modes are validated with the finite element model

The association between placenta previa and leukocyte and platelet indices — a case control study

Objectives: Despite medical advances, rising awareness, and satisfactory care facilities, placenta previa (PP) remains a challenging clinical entity due to the risk of excessive obstetric hemorrhage. Etiological concerns gave way to life-saving concerns about the prediction of maternal outcomes due to hemorrhage. Our study aimed to detect an early predictive marker of placenta previa. Material and methods: Ninety-three pregnant patients diagnosed with PP and 247 controls were recruited for this retro­spective study. Platelet and leukocyte indices were compared between the two groups. Results: The groups were similar with regard to age distribution (31.2 ± 5.1 years [mean ± SD] in the PP group and 31.7 ± 4.2 years in controls), body mass index (BMI) (27.7 ± 3.6 kg/m2 in the PP group and 27.4 ± 4.6 kg/m2 in controls), and most characteristics of the obstetric history. Total leukocyte count, neutrophil count, and neutrophil-to-lymphocyte ratio were significantly higher in the PP group. Mean platelet volume (MPV) and large platelet cell ratio (P-LCR) values were significantly lower in the PP group as compared to controls, with regard to third trimester values. However, patients who were diagnosed postnatally with placenta percreta had lower MPV and P-LCR values than other patients with PP. There were no statistically significant differences between the two groups as far as first trimester values were concerned. Conclusions: Platelet and leukocyte indices in the third trimester of pregnancy may be valuable predictors of placenta previa and placenta percreta. More comprehensive studies are needed to address this issue

Investigation of transverse residual stresses in a thick pultruded composite using digital image correlation with hole drilling

© 2019 Elsevier Ltd Process induced residual stresses are one of the main sources of defects such as (pre) mature matrix cracking during pultrusion of fiber reinforced polymer composite profiles. Recently, comprehensive process models have been developed to understand and describe the underlying mechanisms of the residual stresses in pultrusion processes. The predicted stresses however have not been validated with experimental measurements which are necessary to verify the implemented material models and assumptions in the process models. A hole drilling method with digital image correlation (DIC) is used in the present work to measure the transverse strain relaxation due to material removal in a pultruded thick composite profile (20 × 20 mm) made of unidirectional glass/polyester. The corresponding residual stress state is back calculated using the measured strains in a finite element based numerical model. The estimated level of transverse residual stress in the core of the profile was found to be 6.1 MPa in tension

Do high frequency acoustic emission events always represent fibre failure in CFRP laminates?

© 2017 Elsevier Ltd When damage in carbon fibre reinforced composites (CFRP) is monitored by acoustic emission (AE), it is a common belief that high frequency AE events originate from fibre failure. This shows that this statement may not correspond to the reality, and matrix cracks can emit high frequency AE signals. Quasi-static tension of [−45 2 /0 2 /+45 2 /90 2 ] s laminates was monitored by AE, Digital Image Correlation (DIC) on the surface of the sample and in-situ optical microscopy on the sample's polished edge. Unsupervised k-means clustering algorithm was applied to the AE results. By comparison with the direct DIC and microscopic observations, the AE cluster with high frequency and low amplitude was found to correspond to directly observed matrix cracks

Multi-instrument in-situ damage monitoring in quasi-isotropic CFRP laminates under tension

© 2018 Digital Image Correlation monitoring of the surface strains, microscopic in-situ observations of the micro-damage on the specimen edge and Acoustic Emission (AE) are utilized simultaneously during tension tests of quasi-isotropic carbon fibre reinforced polymer composites. It is found that the cluster analysis for characteristic parameters of AE events (the main being the signal amplitude and frequency) does not unambiguously identify the type of damage which causes the event. With optical instruments, it is observed that the signatures of AE events depend on the position of the ply where damage happens and on the ply orientation (90° vs 45°). Robust evidences for the variations in AE characteristics of damage modes in different lay-ups are presented. AE events, originated from surface cracks, have high amplitude and low frequency, whereas AE events, originated from transverse cracks in the inner plies, have low amplitude and high frequency characteristics. Any conclusion for fibre breaks are not reached in this study. Therefore, measurements in this study rather point out that the AE events, which could be interpreted as fibre breaks because of their high frequency characteristics, as optical observations prove, correspond to other damage types in quasi-isotropic laminates

A Study on the OSH Performance Model in Coal-Fired Thermal Power Plants

Coal-fired Thermal Power Plants (CTPP) consist of many sections that are risky in terms of occupational safety. CTPP are also important part of electrical Energy Logistics systems. Then it is of great importance to examine which parts of complex systems such as coal-fired thermal power plants are more important in terms of occupational health and safety. CTPP are divided into nine main sections as coal parking area, ash and slag disposal facility, boiler maintenance service, mechanical workshop, measurement and control service, turbine maintenance service, technical office, tea and rest room and use of company vehicles section. In this paper, a weighting and ranking study has been conducted in terms of risk analysis of nine departments in CTPP. For this purpose, accident statistics according to years were obtained from an actively working CTPP, and the results obtained with AHP and the risk significance weight values were compared with these results. Since CTPP are very complicated and complex structures, it is of great importance to determine the risk levels of the departments more significantly and objectively from the point of view of OHS. From this point of view, CTPP were analyzed for the first time within the scope of OHS with the Analytical Hierarchy Process (AHP), which is a multi-criteria decision-making (MCDM) method. According to the findings obtained in our study, coal parking area, ash slag disposal facility and boiler maintenance service were determined as the most risky sections in terms of OHS. With this study, a more effective assessment can be made in terms of OHS for CTPP, so that work accidents will be reduced, energy production will be carried out in a safer and more sustainable way