Wilson, Daniel John
(2024)
Understanding the causes and effects of defects in advanced sheet moulding compounds.
PhD thesis, University of Nottingham.
Abstract
Discontinuous fibre reinforced composites such as Sheet Moulding Compounds (SMCs) have the potential to solve many of the problems inherent in conventional continuous fibre composites. Composed of shorter, randomly-orientated fibre tows, these materials are able to flow into complex geometries with minimal preparation and have rapid curing times. However, they come with their own disadvantages, including inferior mechanical properties and high variability. They also contain defects, defined as an imperfection in the material which results in a deviation from the design specification. The observed defects take the form of blisters and pockmarks on the part surface, and their causes and effects are currently not well understood. This research aims to investigate these defects to identify why they form and how they influence the mechanical performance of SMC parts.
Using optical microscopy, pockmarks and blisters were found to be voids at or near the surface of the part. Laser scanning of the charge and a form of augmented visual/tactile inspection of the moulded plaque were used to demonstrate that in flat plaques moulded from net-shape charges, blisters and pockmarks occur more commonly in areas with low initial charge thickness. This is because the flow of SMC from neighbouring regions of higher thickness transports porosity into those areas. More complex mould geometries, demonstrated using the so-called “waffle” tool, have other types of defects which are formed by different mechanisms. Most notably, resin is squeezed out of areas with steep slopes and low cavity thickness, causing dry, bumpy patches. This demonstrates that prepreg-based carbon/epoxy SMC flows poorly, possibly because of its high fibre volume fraction. More complex geometries might be more successfully moulded if an SMC with a greater resin content was selected.
Using ultrasonic C-scanning, parts were also found to contain non-visible subsurface defects, which can be seen on the scans as regions of high attenuation. These regions were studied using optical microscopy. In flat plaques, out-of-plane fibre waviness and wrinkling were identified in these areas of high attenuation, which also corresponded to areas of low initial charge thickness. Like the surface defects, these subsurface wrinkles are caused by the flow of SMC into low-thickness areas, which subjects the tows to an in-plane compressive load and makes them buckle. While surface blisters and pockmarks almost always indicate the presence of subsurface fibre wrinkles, there are many more such wrinkles which do not display on the surface. Surface features can also be indicative of subsurface defects in the more complex “waffle” parts. The dry, bumpy patches on the surface were found to correspond to areas of high ultrasonic attenuation and extend through the full thickness of the part, with optical micrographs observing internal voids where fibre has debonded due to low resin content. Extreme out-of-plane waviness and wrinkling, greater in magnitude than that observed in flat plaques and thought to result from SMC flow caused by the tool geometry, was also found, and is identifiable from the surface as a “rippling” effect.
Tensile tests were performed on a selection of coupons, using Digital Image Correlation (DIC) to measure local strains. In coupons taken from flat plaques, these local strains were observed to be highest where subsurface defects had been detected, and ultimate failure typically occurred in the same locations. Subsurface defects significantly reduce the tensile strength of parts moulded with more than one ply while having no significant effect on modulus. In low-thickness parts moulded from a single ply, tensile strength is lower irrespective of detectable defects, which have no significant impact on mechanical properties because regions of high unfavourable fibre alignment have a stronger influence. Tensile tests of coupons taken from the waffle parts showed that they have lower and more variable strength and stiffness than the flat plaques. This is because of the defects identified in these parts, of which dry patches are the most serious, leading to a tensile strength 80% lower than the datasheet value. In both flat and waffle parts, surface defects such as blisters, pockmarks, and surface-visible portions of dry patches did not consistently identify critical defects.
Because of its inability to detect all subsurface defects or determine which defects are critical, visual/tactile inspection was not found to be a useful means of Non-Destructive Testing (NDT) of SMC parts. Ultrasonic C-scanning, while more reliably able to detect defects with negative impacts on tensile strength, cannot be used to determine their criticality. Ultrasonic C-scanning is therefore also not an effective method of part quality assessment. While other methods like micro-CT scanning could provide more useful results, their prohibitive cost and test duration make them unsuitable for use in industry. It is therefore concluded that no viable NDT method currently exists for use with SMC parts. The properties of prepreg-based SMCs have been shown to be highly variable and unpredictable, and without an effective means of identifying critical defects, they cannot be considered ready for use in structural applications.
A basic charge generation and compression moulding computational model was developed, and has been used investigate the effects of changing tow dimensions on SMC charge thickness distribution and the behaviour of an SMC during moulding. A reduction of up to 22% in the standard deviation of a single-ply SMC charge’s initial thickness distribution can be achieved by halving the width of the tows. This reduced variation in thickness might lead fewer defects to form, while increasing the tow aspect ratio could improve the strength of the SMC. The compression moulding simulation supports the conclusion that surface defects like blisters and pockmarks form due to the movement of porosity into areas of low initial thickness by demonstrating how porosity flows into and accumulates in those areas. Although incomplete, the model is the beginning of a more detailed and useful tool for predicting the formation and effects of defects, which could be developed if current understanding was improved through further experimental work.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Harper, Lee T.
Warrior, Nick A. |
| Keywords: |
Discontinuous composite; Sheet moulding compound; Defect; Wrinkle
Waviness
Blister
Non-destructive testing |
| Subjects: |
T Technology > TS Manufactures |
| Faculties/Schools: |
UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering |
| Item ID: |
77601 |
| Depositing User: |
Wilson, Daniel
|
| Date Deposited: |
19 Jul 2024 14:14 |
| Last Modified: |
19 Jul 2024 14:14 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/77601 |
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