The effects on tensile, shear, and adhesive mechanical properties when recycled epoxy/fiberglass is used as an alternative for glass microballoons in fiberglass foam core sandwiches

The problem of this study was to determine whether fiberglass foam core sandwiches made with recycled epoxy/fiberglass have equal or better flatwise tension, shear, and peel (adhesion) mechanical properties when compared with composite sandwiches made with industry standard glass microballoons. Recycling epoxy/fiberglass could save money by: (1) reusing cured composite materials, (2) consuming less virgin composite materials, (3) spending less on transportation and disposing of unusable composites, and (4) possibly enabling companies to sell their recycled composite powder to other manufacturers;This study used three mechanical property tests, which included: flatwise tensile test, shear test, and peel (adhesion) test. Each test used 300 samples for a combined total of 900 sandwich test samples for this study. A factorial design with three independent variables was used. The first variable, filler type, had three levels: no filler, microballoon filler, and recycled epoxy/fiberglass filler. The second variable, foam density, had four levels: 3 lb/ft³, 4 lb/ft³, 5 lb/ft³, and 6 lb/ft³. The third variable, filler percentage ratio, had eight levels: 0%, 10%, 20%, 30%, 40%, 50%, 60%, and 70%;The results of this study revealed two primary conclusions. The first conclusion was that sandwich test panels produced with recycled epoxy/fiberglass powder were equal or significantly better in tensile, shear, and peel (adhesion) strength than sandwiches produced with hollow glass microballoons. The second conclusion was that sandwich test panels produced with recycled epoxy/fiberglass powder were equal or significantly lighter in weight than sandwiches produced with hollow glass microballoons

Effects of Low Velocity Impact on the Flexural Strength of Composite Sandwich Structures

The use of composite sandwich structures is rapidly increasing in the aerospace industry because of their increased strength-to-weight and stiffness-to-weight characteristics. The effects of low velocity impacts on these structures, however, are the main weakness that hinders further use of them in the industry because the damages from these loadings can often be catastrophic. Impact behavior of composite materials in general is a crucial consideration for a designer but can be difficult to describe theoretically. Because of this, experimental analysis is typically used to attempt to describe the behavior of composite sandwiches under impact loads. Experimental testing can still be unpredictable, however, because low velocity impacts can cause undetectable damage within the composites that weaken their structural integrity. This is an important issue with composite sandwich structures because interlaminar damage within the composite facesheets is typical with composites but the addition of a core material results in added failure modes. Because the core is typically a weaker material than the surrounding facesheet material, the core is easily damaged by the impact loads. The adhesion between the composite facesheets and the core material can also be a major region of concern for sandwich structures. Delamination of the facesheet from the core is a major issue when these structures are subjected to impact loads. This study investigated, through experimental and numerical analysis, how varying the core and facesheet material combination affected the flexural strength of a composite sandwich subjected to low velocity impact. Carbon, hemp, aramid, and glass fiber materials as facesheets combined with honeycomb and foam as core materials were considered. Three layers of the same composite material were laid on the top and bottom of the core material to form each sandwich structure. This resulted in eight different sandwich designs. The carbon fiber/honeycomb sandwiches were then combined with the aramid fiber facesheets, keeping the same three layer facesheet design, to form two hybrid sandwich designs. This was done to attempt to improve the impact resistance and post-impact strength characteristics of the carbon fiber sandwiches. The two and one layer aramid fiber laminates on these hybrid sandwiches were always laid up on the outside of the structure. The sandwiches were cured using a composite press set to the recommended curing cycle for the composite facesheet material. The hybrid sandwiches were cured twice for the two different facesheet materials. The cured specimens were then cut into 3 inch by 10 inch sandwiches and 2/3 of them were subjected to an impact from a 7.56 lbf crosshead which was dropped from a height of 38.15 inches above the bottom of the specimen using a Dynatup 8250 drop weight machine. The impacted specimen and the control specimen (1/3 of the specimens not subjected to an impact) were loaded in a four-point bend test per ASTM D7250 to determine the non-impacted and post-impact flexural strengths of these structures. Each sandwich was tested under two four-point bend loading conditions which resulted in two different extension values at the same 100 lbf loading value. The span between the two supports on the bottom of the sandwich was always 8 inches but the span between the two loading pins on the top of the sandwich changed between the two loading conditions. The 2/3 of the sandwiches that were tested after being impacted were subjected to bending loads in two different ways. Half of the specimens were subjected to four-point bending loads with the impact damage on the top facesheet (compressive surface) in between the loading pins; the other half were subjected to bending loads with the damage on the bottom facesheet (tensile surface). Theoretical failure mode analysis was done for each sandwich to understand the comparisons between predicted and experimental failures. A numerical investigation was, also, completed using Abaqus to verify the results of the experimental tests. Non-impacted and impacted four-point bending models were constructed and mid-span deflection values were collected for comparison with the experimental testing results. Experimental and numerical results showed that carbon fiber sandwiches were the best sandwich design for overall composite sandwich bending strength; however, post-impact strengths could greatly improve. The hybrid sandwich designs improved post-impact behavior but more than three facesheet layers are necessary for significant improvement. Hemp facesheet sandwiches showed the best post-impact bending characteristics of any sandwich despite having the largest impact damage sizes. Glass and aramid fiber facesheet sandwiches resisted impact the best but this resulted in premature delamination failures that limited the potential of these structures. Honeycomb core materials outperformed foam in terms of ultimate bending loads but post-impact strengths were better for foam cores. Decent agreement between numerical and experimental results was found but poor material quality and high error in material properties testing results brought about larger disagreements for some sandwich designs

Development Trend of Adhesive Joining of Aluminium Foams

Aluminium foams structures, due to its impact absorbing properties could be considered as passive safety systems in transportations which still have a great potential for development as a way to reduce deaths and injuries, which is also associated to the economic costs and social impacts associated with this problem. On the other hand, from an environmental standpoint, the use of advanced composite materials to this end can also represent an optimized level of energy efficiency. The impact energy absorption, with the use of a well-designed lightweight protection system, is directly related to the thermal efficiency and consumption of the engines, thus leading to a lower level of greenhouse gases sent to the atmosphere. Without developing manufacturing technologies, it can not be possible, that is why the joint technology should adapt to the recent, combinations of materials. The connection between aluminium foam to aluminium foam design is one way for the bonding established by adhesives. In this paper adhesive joining of aluminium foams were investigated for the base of a further research project

Manufacturing and Characterization of Highly Environmentally Friendly Sandwich Composites from Polylactide Cores and Flax-Polylactide Faces

[EN] This work focuses on the manufacturing and characterization of highly environmentally friendly lightweight sandwich structures based on polylactide (PLA) honeycomb cores and PLA-flax fabric laminate skins or facings. PLA honeycombs were manufactured using PLA sheets with different thicknesses ranging from 50 to 500 mu m. The PLA sheets were shaped into semi-hexagonal profiles by hot-compression molding. After this stage, the different semi-hexagonal sheets were bonded together to give hexagonal panels. The skins were manufactured by hot-compression molding by stacking two Biotex flax/PLA fabrics with 40 wt% PLA fibers. The combined use of temperature (200 degrees C), pressure, and time (2 min) allowed PLA fibers to melt, flow, and fully embed the flax fabrics, thus leading to thin composite laminates to be used as skins. Sandwich structures were finally obtained by bonding the PLA honeycomb core with the PLA-flax skins using an epoxy adhesive. A thin PLA nonwoven was previously attached to the external hexagonal PLA core, to promote mechanical interlock between the core and the skins. The influence of the honeycomb core thickness on the final flexural and compression properties was analyzed. The obtained results indicate that the core thickness has a great influence on the flexural properties, which increases with core thickness; nevertheless, as expected, the bonding between the PLA honeycomb core and the skins is critical. Excellent results have been obtained with 10 and 20 mm thickness honeycombs with a core shear of about 0.60 and facing bending stresses of 31-33 MPa, which can be considered as candidates for technical applications. The ultimate load to the sample weight ratio reached values of 141.5 N center dot g(-1) for composites with 20 mm thick PLA honeycombs, which is comparable to other technical composite sandwich structures. The bonding between the core and the skins is critical as poor adhesion does not allow load transfer and, while the procedure showed in this research gives interesting results, new developments are necessary to obtain standard properties on sandwich structures.This research work was funded by the Spanish Ministry of Science and Innovation (MICI) project number MAT2017-84909-C2-2-R.Lascano-Aimacaña, DS.; Guillén-Pineda, RM.; Quiles-Carrillo, L.; Ivorra-Martínez, J.; Balart, R.; Montanes, N.; Boronat, T. (2021). Manufacturing and Characterization of Highly Environmentally Friendly Sandwich Composites from Polylactide Cores and Flax-Polylactide Faces. Polymers. 13(3):1-13. https://doi.org/10.3390/polym13030342S11313

Interfacial reactions between alsi10 foam core and aisi 316l steel sheets manufactured by in-situ bonding process

Aluminum foam sandwiches (AFS) with AlSi10 foam cores and AISI 316L steel skins are manufactured by an in-situ bonding process. The precursor of the core foam was made with the powder compacted method. The precursor and skins, coupled together, were then heated up to the melting point of the Al alloy. The gas released by the blowing agent formed hydrogen bubbles in the melt. producing the foam. Such a porous structure was kept frozen at room temperature via cooling in cold water. To optimize the process conditions, some foaming experiments have been conducted with different holding times and temperatures. Such manufactured AFS were cut, chemically etched and studied with an optical microscope associated with image analysis software to get information about pores morphology in terms of circularity and equivalent diameter. The interface AlSi10-AISI316L has been characterized by SEM and EDX to investigate the bonding conditions between cores and skins. Finally, the AFS have been polished and etched to analyze the microstructure. Quasi-static compressive tests have been performed on the AFS. Obtained results showed that the interface formed during the foaming can be characterized by the inter-diffusion of alloying elements, as confirmed by the good quality of metallurgical joints

Master of Science

thesisGiven their high strength-to-weight and stiffness-to-weight ratios, sandwich composites continue to be considered for automotive applications. Thermoplastic materials, while difficult to bond, have an increased ease of manufacture and may be reprocessed, making them an attractive alternate to thermoset composites. This investigation focused on the evaluation of adhesives and surface treatments for both nylon and polypropylene thermoplastic composite adherends made from Towflex® pre-impregnated composite fabric. A manufacturing method was established for thermoplastic plates, which produced an acceptable surface finish without contaminating the bonding surface. Adhesives and surface treatments were evaluated using lap shear (ASTM D 3163) and cleavage (ASTM D 3433) test methods. The most promising adhesive/surface treatment combinations were selected for bonding of sandwich composites with two different core materials: balsa wood and polyurethane foam. Initial sandwich configuration testing consisted of flatwise tensile (ASTM C 297) and core shear (ASTM C 273) test methods. These tests provided insights into the sandwich properties and revealed any incompatibilities between the adhesive and the core. Follow on sandwich configuration evaluation consisted of edgewise compression testing, both statically (ASTM C 364) and dynamically. These tests determined the strength and ability of these sandwiches to absorb energy under two different types of loading

Manufacturing and quasi-static bending behavior of wood-based sandwich structures.

The quasi-static behavior of innovative wood based sandwich structures with plywood core and skins made either of aluminum or of fiber reinforced polymer (carbon, glass or flax composite skins) was investigated. The wood based sandwich structures were subjected to three point static bending tests to determine their strength and failure mechanisms. Two different manufacturing processes, namely vacuum bag molding and thermo-compression, were used to manufacture the structures. The influence of some aspects of the different manufacturing processes on the flexural behavior of wood based sandwich structures are discussed. It is shown that manufacturing processes influence strongly the static responses. Failure modes and strengths are investigated during quasi-static bending tests. Bending tests showed that the mechanical characteristics were very high compared to those of a reference sandwich that is currently used for civil aircraft floors. This new kind of structure is environmentally friendly and very cheap, and seems promising for the transportation industry in general

The projectile impact responses of the composite faced aluminum foam and corrugated aluminum sandwich structures: a comparative study

Thesis (Master)--Izmir Institute of Technology, Mechanical Engineering, Izmir, 2011Includes bibliographical references (leaves: 99-101)Text in English; Abstract: Turkish and Englishxii, 101 leavesThe projectile impact and energy absorption characteristics of the corrugated aluminum cored E-glass/polyester composite sandwich structures were determined at the impact velocities of 150 m/s. For comparison, E-glass/polyester sandwich structures cored with aluminum foam were also investigated. The test conditions were kept the same for each structure in order to identify the impact properties at the similar test conditions. The composite and the foam core composite sandwiches were produced by vacuum assisted resin transfer molding and the mechanical tests were performed on the composite and core samples based on ASTM. High strain rate tests were performed using a compression type Split Hopkinson Pressure Bar and drop weight test set-up. It was found that aluminum foam sandwich structures had higher ballistic limit and energy dissipating performance than corrugated aluminum sandwich structures; however, as the thickness of the face sheets increased the corrugated aluminum cores were observed to be more effective. The results showed that corrugated aluminum structures had the potentials to be used as core material in composite sandwich structures

Laser Forming of Metal Foam: Mechanisms, Efficiency and Prediction

This thesis deals with metal foam, a relatively new material whose tremendous potential has been identified early on. The material is an excellent shock absorber and also has a very high strength-to-weight ratio, properties that are highly desirable particularly within the aerospace and automotive industries. Despite the material’s immense potential, hardly any metal foam products have made it past the prototype stage. The reason is that the material is difficult to manufacture in the shapes required in industrial applications. Oftentimes, applications require sheets to be bent into specific shapes, yet bending is not possible with conventional methods. Laser forming is currently the only method that shows promise to bend metal foam panels to a range of shapes. In this thesis, the analysis of laser forming of metal foam was taken far beyond the experimental work that has been delivered thus far. A thorough analysis was performed of the thermo-mechanical bending mechanism that governs the deformation of metal foam during laser forming. This knowledge was then used to explain the effect of the process condition on the bending efficiency and the bending limit. Additionally, the impact of laser forming on the metal foam properties was explored. Experimental results were complemented by numerical results that were validated both thermally (using infrared imaging) as well as mechanically (using digital image correlation). Numerical models with different levels of geometrical complexities were used, and the effect of the model geometry on the predictive accuracy was explored. In the second half of the thesis, the aforementioned effort was extended to metal foam sandwich panels, in which metal foam is sandwiched between two sheets of solid metal. The material again has a high strength-to-weight ratio and excellent shock absorption capacity, while also being stiff and core-protective. Just like metal foam alone, metal foam sandwich panels are typically manufactured in flat sheets, and failure-free bending can only be achieved using lasers. The analysis was again initiated with the bending mechanism. It was revisited whether the foam core still follows the same bending mechanism, and how its deformation is affected by the interaction with the solid facesheets. This insight was then used to elucidate the bending efficiency and limit at different process conditions, as well as the impact of the process on the material performance. Additionally, the effect of the sandwich panel manufacturing method on the process outcome was investigated. This was achieved by contrasting two sandwich panel types with a different foam core structure, foam core composition, facesheet composition and facesheet attachment method. Lastly, three-dimensional deformation of metal foam sandwich panels into typical non-Euclidean shapes such as bowl and saddle shapes was explored. It was shown that a significant amount of 3D deformation can be induced. At the same time, it was discussed that the achievable deformation is limited to moderate curvatures, since only a limited amount of in-plane strains may be induced using laser forming. The aforementioned experimental efforts were again accompanied by numerical efforts. Sandwich panel models with different levels of geometrical complexity were used to study all aspects pertaining to the process, and the properties to the facesheet/foam core interface were discussed. Overall, the work in this thesis demonstrated that laser forming is capable of bending metal foam panels and metal foam sandwich panels up to large bending angles without causing failures, while maintaining the favorable properties of the material. Conceptual, experimental and numerical groundwork was laid towards a successful implementation of the material in industrial applications