| 1. |
- Pakkam Gabriel, Vivek Richards, et al.
(författare)
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Effect of heat treatment and test temperature on transverse cracking in tensile loading
- 2024
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier. - 1359-835X .- 1878-5840. ; 181
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Tidskriftsartikel (refereegranskat)abstract
- Accumulation of transverse cracks in carbon fiber heat resistant polymer (with bismaleimide formulation) cross-ply laminates during tensile loading at elevated temperatures and after long heat treatment is analysed. Data shows that both the iso-thermal heat treatment and testing at elevated temperatures reduce the transverse cracking resistance. A two-parameter Weibull failure stress distribution model with scale parameter degrading with heat treatment and elevated temperature is used for crack initiation analysis. The degradation is described by polynomial expansion including interaction terms. Data shows that the scale parameter dependence on the heat treatment time and the test temperature is rather linear. The same expansion parameters have been successfully used for laminates with the same constituents but with a different layup and fiber content.
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| 2. |
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| 3. |
- Petkov, Valeri Ivanov
(författare)
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Characterisation of thermally induced degradation of high-temperature polymers and composites
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This project aimed to broaden the knowledge of high-temperature polymers and composites when exposed to elevated temperatures and an oxygen-containing atmosphere. The main accent has been on thermosetting polyimide resins reinforced with carbon fibers. When subjected to harsh atmospheric conditions, such as elevated temperatures and oxygen, polymer resins can undergo thermo-oxidative degradation, often resulting in weight loss and a surface layer with altered properties. High-temperature composites could experience such environments during operation. Therefore, it is crucial to understand how exposure to it could affect their performance. To simulate such an environment in the lab, the materials are aged in a controlled manner in a furnace or other equipment. The ageing of polyimide composites in this project was often performed at temperatures at or above 288 degrees Celsius for extended periods of up to 1500 hours.The first part of the project, and the first article, delved into the effect of different layups and thicknesses of the carbon fiber bundles on the thermo-oxidative behaviour of two composite materials made with the same thermosetting polyimide. Modelling the desorption during the initial stages of the ageing, showed that it exhibited a Fickian behaviour. X-ray computed tomography experiments were used to investigate the ageing behaviour of the materials and revealed that the satin weave composite formed a network of cracks, voids, and delaminations, that progressed with the ageing time, while the damage in the material made of thin plies was in the form of delaminations at the edges. The analysis of the tomographic datasets was performed using Otsu’s thresholding method for semantic segmentation of the defects within the materials.In an attempt to counter the crack formation on the surface of the satin weave composite observed during the first study, a new polyimide formulation was developed by the manufacturer. The amount of internal crosslinkers was reduced, aiming to increase the toughness of the resin after curing. The second article compares neat resin samples of the original and newly developed formulations with the help of a three-point bending test, differential scanning calorimetry, dilatometry, weight loss, light optical microscopy and nanoindentation experiments. Samples were aged up to 1500 hours in ambient air. The results showed that while there were hints of a slight increase in the fracture toughness of the new formulation, the glass transition temperature had decreased, compared to the original resin.The two formulations were further investigated and compared with the help of thermogravimetric analysis in the fourth paper. Experiments were performed in isothermal and non-isothermal conditions for more robust results. It was found that the thermal oxidation of the two materials follows an autocatalytic model. The study highlights the importance of using both isothermal and non-isothermal data in the pursuit of more precise and robust analysis and modelling of the thermal oxidation of high-temperature polymers. Based on the results, a diagram, predicting the weight loss at specific times and temperatures, was created for each material.An alternative way of studying crack formation within challenging polymer composite tomographic datasets was presented in the fourth article. Instead of using a thresholding method, such as thepreviously used Otsu’s in the first study, in this case, a deep learning model was applied to the datasets to follow the progressive micro-cracking within the composite during a series of thermo-mechanical loadings. In contrast to a global thresholding method, which segments all defects within the dataset, the deep learning model, Attention U-Net, made it possible to create a more straightforward and robust way of performing segmentation on transverse cracks. The model was compared to and outperformed both Otsu’s method and a conventional U-Net.The previously developed methodology for semantic segmentation and the obtained results on transverse cracks were applied in a practical case in the fifth article, where the developed damage prediction model assumes that transverse cracks in thick plies span through the whole width of the specimen. The tomography and deep learning methodology helped shed light on the nature of the cracks and showed that previous assumptions, based on edge observation with light optical microscopy, should be taken as a conservative estimation.
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| 4. |
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| 5. |
- Petkov, Valeri Ivanov, et al.
(författare)
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Semantic segmentation of progressive micro-cracking in polymer composites using Attention U-Net architecture
- 2024
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Ingår i: Tomography of Materials and Structures. - 2949-673X. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- The present study delivers a methodology for investigating the gradual damage development in a carbon fibre-reinforced cross-ply polymer composite during a sequence of thermo-mechanical loadings with the help of X-ray computed tomography. The procedure allows an in-depth analysis of the occurrence and nature of the multiple cracks that form within layers oriented perpendicular, or transverse, to the loading direction. This is achieved by using Attention U-Net architecture for semantic segmentation of the transverse cracks. The model shows promising results, through an ability to identify all the transverse cracks and reflect the damage progression. The described method provides a robust routine for analysing challenging polymer composite tomographic datasets.
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| 6. |
- Petkov, Valeri Ivanov, et al.
(författare)
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The Influence of Ethynyl In-Chain Crosslinkers on the Properties of 6FDA-Based Polyimides
- 2022
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 16:1
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Tidskriftsartikel (refereegranskat)abstract
- Two 4,4′-(hexafluoroisopropylidene)diphthalic anhydride-based thermosetting polyimide formulations with varied amounts of crosslinking sites were compared to understand the influence of crosslinking density on fracture toughness, glass transition temperature and thermal oxidative stability. The thermal and mechanical properties of both materials were investigated through a series of single-edge notched beams, differential scanning calorimetry, dilatometry, weight loss, light optical microscopy and nanoindentation experiments. It was found out that the reduced crosslinking resulted in slightly increased fracture toughness but decreased the Tg of the material. No significant difference could be observed in the thermal oxidative stability with the experimental techniques considered.
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| 7. |
- Petkov, Valeri Ivanov, et al.
(författare)
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Thermal oxidative aging of satin weave and thin-ply polyimide composites
- 2022
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Ingår i: Polymer Composites. - : John Wiley & Sons. - 0272-8397 .- 1548-0569. ; 43:5, s. 2615-2627
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Tidskriftsartikel (refereegranskat)abstract
- The weight loss and damage development of satin weave and thin-ply carbon fiber-reinforced polyimide composites are studied during high temperature (288 and 320°C) aging in air. Two distinct regions are identified in the weight loss measurements—a rapid initial desorption and a subsequent slower weight loss, which is attributed to matrix degradation. The initial desorption stage is modeled by combining material constitutive properties from literature, micro-mechanics and a one-dimensional Fickian diffusion model. Two different damage patterns are observed by using optical microscopy and X-ray computed tomography on specimens before and after aging. Crack clusters form a network in satin weave composites, while edge delaminations are observed in thin-ply laminates, which are known for their enhanced resistance to matrix cracking.
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| 8. |
- Saeidi, Kamran, et al.
(författare)
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Advanced Mechanical Strength in Post Heat Treated SLM 2507 at Room and High Temperature Promoted by Hard/Ductile Sigma Precipitates
- 2019
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Ingår i: Metals. - : MDPI. - 2075-4701. ; 9:2
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Tidskriftsartikel (refereegranskat)abstract
- Duplex stainless steel, 71 wt.% austenite, 13 wt.% ferrite and 16 wt.% sigma, was made upon heat treating of fully ferritic as-built selective laser melted (SLM) 2507 stainless steel at 1200 °C. Formation of sigma phase in the heat treated SLM 2507 was investigated using optical microscopy and scanning electron microscopy (SEM). The heat treated SLM 2507 demonstrated a yield strength of 686 MPa, ultimate tensile strength of 920 MPa and an elongation of 1.8% at room temperature with a brittle fracture morphology. Precipitation of sigma phase during heat treatment and slow cooling improved the mechanical and wear properties at high temperatures (1200 °C and 800 °C, respectively). The tensile strength and elongation of the heat treated SLM 2507 was measured 400 MPa and 20% as compared to casted duplex steel with 19 MPa and 30% elongation at 1200 °C. The 20 times higher mechanical strength as compared to casted duplex steel was attributed to sigma precipitates. Tribological behaviour of heat treated duplex SLM 2507 containing sigma at 800 °C showed very low wear rate of 4.5 × 10−5 mm3/mN compared to casted duplex steel with 1.6 × 10−4 mm3/mN.
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