Characterization and Modeling of Mechanical Behavior of Sandwich Composites and 3D-printed Polymers at Meso and Nano Scales
| dc.contributor.advisor | Lu, Hongbing | |
| dc.contributor.advisor | Haas, Zygmunt | |
| dc.contributor.advisor | Griffith, D. Todd | |
| dc.contributor.committeeMember | Qian, Dong | |
| dc.contributor.committeeMember | Li, Wei | |
| dc.creator | Cao, Dongyang | |
| dc.date.accessioned | 2023-09-07T15:04:04Z | |
| dc.date.available | 2023-09-07T15:04:04Z | |
| dc.date.created | 2022-08 | |
| dc.date.issued | August 2022 | |
| dc.date.submitted | August 2022 | |
| dc.date.updated | 2023-09-07T15:04:06Z | |
| dc.description.abstract | This dissertation covers the topics of mechanical characterizations of sandwich composites and 3D printed polymers which can potentially be used in the wind turbine blades. Chapter 1 is studying the localized viscoelastic properties at the skin-core interphase of a sandwich composite using a viscoelastic nanoindentation technique. The skin-core interphase stiffness is one of the most important design factors in the sandwich composite. However, the stiffness distribution on the interphase remains unclear. Chapter 2 is studying the effects of resin uptake on the mechanical properties of sandwich composites under bending conditions. Improper resin uptake could lead to the resin starvation issue in the matrix or interphase and decrease the stiffness of the skin. So, defining an adequate range of the sandwich composite is essential in the design and optimization of the sandwich composite. And the effect of resin uptake on the sandwich composite has rarely been studied. Chapter 3 is studying the bonding quality of the displacement-controlled resistance welded; adhesive-bonded slender 3D-printed PLA beams through three-point bending experiments. The 3D printed polymer beams (e.g.; PLA, ABS, etc.) can act as the core material of the sandwich composite. However, due to the limitation of the building volume of the conventional 3D printer, a good bonding technique is needed to extend the volume of the structure with satisfactory structural integrity. How sufficient the 3D printed polymer is bonded together needs further investigation. Chapter 4 is studying the in-plane and out-of-plane shear strength and stiffness of the PVC foam core, balsa core, and 3D printed core sandwich composites by three- point bending and four-point bending tests. The core material of the sandwich composites plays a critical role in the structural integrity of the wind turbine blade. The 3D print process possesses a lot of advantages compared with the traditional manufacturing routine. However, whether the 3D- printed engineering core has the potential to replace the conventional core materials needs further attention. Chapter 5 is studying the annealing effect on the flexural strength and modulus, and the bonding quality of adhesive bonded, and thermoformed 3D-printed PLA beams and 3-inch chords by three-point bending experiments. The Fused Deposition Modeling (FDM) possess is one of the suitable manufacturing processes for the small-scale and thermoplastic wind turbine blades. However, what types of 3D printing parameters are suitable for thermoplastic wind turbine blade fabrication is still not clear. Also, the engineering structure fabricated by the FDM process usually needs to strengthen the deposited layer adhesion to increase its durability. Whether the annealing treatment is beneficial to the durability still needs further investigation. Debonding at the core–skin interphase region is one of the primary failure modes in sandwich composites under shear loads. As a result, the ability to characterize the mechanical properties at the interphase region between the composite skin and core is critical for design analysis. This work intends to use nanoindentation to characterize the viscoelastic properties at the interphase region, which can potentially have mechanical properties changing from the composite skin to the core. A sandwich composite using a polyvinyl chloride foam core covered with glass fiber/resin composite skins was prepared by vacuum-assisted resin transfer molding. Nanoindentation at an array of sites was made by a Berkovich nanoindenter tip. The recorded nanoindentation load and depth as a function of time were analyzed using viscoelastic analysis. Results are reported for the shear creep compliance and Young’s relaxation modulus at various locations of the interphase region. The change of viscoelastic properties from higher values close to the fiber composite skin region to the smaller values similar to the foam core was captured. The Young’s modulus at a given strain rate, which is also equal to the time-averaged Young’s modulus across the interphase region was obtained. The interphase Young’s modulus at a loading rate of 1 mN/s was determined to change from 1.4 GPa close to composite skin to 0.8 GPa close to the core. This work demonstrated the feasibility and effectiveness of nanoindentation-based interphase characterizations to be used as an input for the interphase stress distribution calculations, which can eventually enrich the design process of such sandwich composites. Resin uptake plays a critical role in the stiffness-to-weight ratio of wind turbine blades in which sandwich composites are used extensively. This work examines the flexural properties of nominally half-inch thick sandwich composites made with polyvinyl chloride (PVC) foam cores (H60 and H80; PSC and GPC) at several resin uptakes. We found that the specific flexural strength and modulus for the H80 GPC sandwich composites increase from 82.04 to 90.70 (𝑘𝑘𝑘𝑘 ∙ 𝑚𝑚) ⁄ 𝑘𝑘𝑘𝑘 and 6.03 to 7.13 (𝑀𝑀𝑘𝑘 ∙ 𝑚𝑚) ⁄ 𝑘𝑘𝑘𝑘, respectively, with 11.0 % resin uptake reduction, which stands out among the four core sandwich composites. Considering reaching a high stiffness-to-weight ratio while preventing resin starvation, 32 to 38 % and 40 to 45 % resin uptakes are adequate ranges for the H80 PSC and GPC sandwich composites, respectively. The H60 GPC sandwich composites have lower debonding toughness than H60 PSC due to stress concentration in the smooth side skin-core interphase region. The failure mode of the sandwich composites depends on the core stiffness and surface texture. The H60 GPC sandwich composites exhibit core shearing and bottom skin-core debonding failure, while the H80 GPC and PSC sandwich composites show top skin cracking and core crushing failure. The findings indicate that an appropriate range of resin uptake exists for each type of core sandwich composite, and that within the range, a low-resin uptake leads to lighter blades and thus lower cyclic gravitational loads, beneficial for long blades. Fused deposition modeling (FDM) is often used in additive manufacturing of materials such as thermoplastics and metals. Due to the size limitation in the building volume of an FDM system, fusion joining two or more FDM printed parts allows upscaling to manufacture larger structures by the assembly of printed subcomponent parts. In this work, a displacement-controlled joining process was used, rather than a force-controlled process previously reported. The bonding quality of the resistance welded, adhesive bonded slender polylactic acid (PLA) beams was investigated using three-point bending experiments. The slender beams were fabricated by FDM with a 0.3 mm layer thickness. Several infill patterns were explored, and it was determined that the 3D-triangular infill pattern gives the highest flexural properties. Three types of metal mesh, namely 30% (open area fraction)/ 0.11 mm (open size) Ni-Cu, 34%/0.07 mm Ni-Cu, and 36%/0.25 mm Co-Ni, were used as the heating elements. The micrographs of the PLA slender beams resistance welded by the three types of metal meshes show that there are no voids formed in the interphase region. Process parameters were varied, including the power output, mesh opening size, wire diameter, wire resistivity, initial joining pressure, displacement rate, and total displacement traveled. The mechanical properties of the resistance welded beams are compared with those of the corresponding beams printed continuously on FDM. Optimum process parameters were determined for the configuration investigated. In general, a smaller opening size, smaller wire diameter, and higher wire electrical resistivity are preferred in the resistance welding under a low current to reach a higher bonding quality. A higher wire diameter with a larger opening size yields a higher flexural modulus. The flexural strength does not depend on the types of materials used for the Joule heating meshes; in addition, it does not depend on the mesh opening area fraction and wire diameter. The beam samples joined by Ni/Cu mesh (34%/0.07 mm Ni-Cu metal mesh) possesses 96%, 94%, and 88% of the flexural strength, modulus, and maximum allowable strain, respectively, of the one continuous FDM printed sample; this sample gives the flexural properties closest to the continuously printed sample. The adhesive, cohesive, substrate and opening crack failure modes were captured for the resistance welded, adhesive bonded, and continuously printed slender beams. The substrate failure mode is the most desired failure mode which correlated to the highest bonding strength. Surface strain concentration was found in the bonding region. The normal strain dominates when the flexural load is applied to the continuously printed sample. Whether the 3D printed artifacts with the Fused Deposition Modeling (FDM) process remains further investigation in structural engineering applications, especially in wind turbine industries. In this work, the effect of resin uptake on shear strength and stiffness of compression-molded H60 PVC foam core, end-grain balsa core, and PLA lattice core sandwich composites were systematically studied through three-point and four-point bending tests. The surface strain and failure modes were investigated by DIC and fractured images. The skin/core bonding quality is shown to affect the in-plane and out-of-plane shear strength and stiffness of the 3D printed lattice core sandwich composites. The PLA filament is chosen for the 3D printed core material with sufficient strength to weight ratio and cost-effectiveness. The out-of-plane and in-plane shear strength and stiffness of two types of core sandwich composites were investigated by three-point bending and four-point bending tests. In the 3D printed core sandwich composites, based on the information on specific out-of-plane shear strength and stiffness, the optimized resin uptake regime of PLA core sandwich composites is from 20.43% to 22.86%. In the PLA core sandwich composites, the specific out-of-plane shear strength and stiffness increase from 19.22% to 20.43% of resin uptake, and then decrease from 22.86% to 25.79%. Both low and high amounts of resin in the interphase region of the 3D printed core sandwich composites lead to low bonding strength. The in-plane shear strength and stiffness of three types of core sandwich composites were computed based on the out-of-plane shear strength and stiffness. The specific in-plane shear stiffness of 3D printed and raw balsa core sandwich composites are similar. The UV light curing epoxy-based agent coated balsa core sandwich composites possess 8.19% and 15.16% higher specific in-plane shear stiffness than 3D printed core sandwich composites within all resin uptake regimes. The UV light curing epoxy-based agent coated balsa core sandwich composites possess 9.81% and 17.44% higher specific in-plane shear stiffness than 3D printed core sandwich composites within an optimized resin uptake regime. The 3D printed core stands out among conventional PVC core materials and is similar to conventional balsa core materials in shear and specific shear strength and stiffness. As a result, this work presents some new and important findings that support the greater use of additive manufacturing of core materials in applications such as wind turbine blades. A few subjects still need to be studied in the future regarding the PLA lattice core sandwich composites, including the performance under high cycle fatigue, debonding toughness in the interphase region, and strategies for the interphase stiffness enhancement. Small-scaled additively manufactured wind turbines give a solution for power generation in rural areas and large-scale wind turbines performance prediction by on-site construction, hence reducing the construction error and impact on the climate change. This work mainly investigates the effects of annealing on the structural performance of 3D printed short beams and 3-inch chords (smooth, adhesive bonded, thermoformed) by three-point bending experiments. The effect of vertical shells, horizontal shells, bed temperature, nozzle temperature, and printing resolution was systematically studied through three-point bending experiments. The 3D printed beams with 2 vertical shells and 4 horizontal shells possess the highest specific flexural modulus (4.81 × 10 3 ± 37.24 𝐺𝐺𝐺𝐺𝐺𝐺 (𝑘𝑘𝑘𝑘 𝑚𝑚 3⁄ )⁄ ). The 3D printed beams with 3 vertical shells and 3 horizontal shells possess the highest specific flexural strength (105.17 ± 1.17 𝐺𝐺𝐺𝐺𝐺𝐺 (𝑘𝑘𝑘𝑘 𝑚𝑚 3⁄ )⁄ ). The 60 °C bed temperature and 215 °C nozzle temperature achieve the best-deposited layers adhesion quality. The 0.6 mm nozzle diameter is an adequate resolution to 3D print a small-scale wind turbine blade with satisfactory efficiency and complexity. The effect of the annealing time on the 3D printed short beams and 3- inch chord were studied by three-point bending experiments. The 70 °C with 0.5 hours is an adequate annealing plan for a constant temperature annealing treatment due to a relatively small shrinkage under a unit heating time by a higher temperature for the smooth and adhesive bonded type 3D printed structure. For the thermoformed 3D printed structure, no annealing treatment is needed for maximizing the strength to weight ratio, annealing treatment is suggested for maximizing the modulus to weight ratio. The thermal buckling and edge wrapping were captured under the 70 °C/0.5 hour annealing treatment for 2 vertical shells and 4 horizontal shells 3D printed beams. So, a sufficient thickness of the vertical side should be achieved to encounter the thermal buckling and edge wrapping. Annealing treatment is not suitable for the 3-inch chord due to low flexural strength, flexural stiffness improvement, and a high level of volumetric shrinkage on the lengthwise. | |
| dc.format.mimetype | application/pdf | |
| dc.identifier.uri | ||
| dc.identifier.uri | https://hdl.handle.net/10735.1/9842 | |
| dc.language.iso | en | |
| dc.subject | Engineering, Mechanical | |
| dc.subject | Engineering, Materials Science | |
| dc.subject | Engineering, Industrial | |
| dc.title | Characterization and Modeling of Mechanical Behavior of Sandwich Composites and 3D-printed Polymers at Meso and Nano Scales | |
| dc.type | Thesis | |
| dc.type.material | text | |
| local.embargo.lift | 08/01/2023 | |
| local.embargo.terms | 08/01/2023 | |
| thesis.degree.college | School of Engineering and Computer Science | |
| thesis.degree.department | Mechanical Engineering | |
| thesis.degree.grantor | The University of Texas at Dallas | |
| thesis.degree.name | PHD |
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