Description:Field of Invention
The present invention pertains to increase the mechanical properties of stitched foam composite structures which is widely used in the various engineering field such as industries, marine, aerospace by replacing traditional materials.
Background of the Invention
Composites lead to the achievement of lighter and stronger material. Many traditional materials are replaced by composites due to their high strength and lightweight properties. Manufacturing of composites has been continuously increasing for many engineering applications. There are many composite materials like fiber reinforced composites, fiber volume fraction, sandwich panels, metal matrix composites, ceramic matrix composites etc. these composites are formed in order to increase their properties such as strength, stiffness, corrosion resistive, resistance to wear, thermal conductivity, temperature resistance, high damping property, acoustic insulation, aesthetics etc. Composites are heavily used in the automotive sector, aerospace sector, sports, transportation. When it comes to design, they are flexible and can be molded into most intricate components. There are many techniques to manufacture composite materials, such as wet layup, spray lay-up, compression molding, injection molding, pultrusion, filament winding, vacuum bagging process. One of the best examples of composites are sandwich composites which have been used/tailored in specific areas. Sandwich composites consist of two thin skins and lightweight core material [US3435470A].
Foam core sandwich Composites are widely used in structural applications. The core materials are not strong enough to suppress plane shear, buckling instability which causes the structure to reduce its strength and stability. Through thickness reinforcement is a promising attempt to overcome these defects. Through thickness reinforcement can be achieved by stitching, 3D weaving, and Z-pinning techniques. Due to interlacing several layers of fiber bundles, 3D weaving is a slow production method. Z pinning is an expensive process and it is only suitable for the aerospace market [US5664518A].
The mechanisms of sandwich composite materials life are more complicated than metals, as the assembly of fiber, resin, and foam are of non-homogeneous materials. Some studies show that the bending strength of the stitched composite is improved by 50% compared with the non-stitched composite. The strength of stitched composite is improved by increasing the stitching thread diameter and decreasing the stitching thread distance [US5800749A].
Stitching is a promising technique for the fabrication of composites, and it is more effective through thickness direction. It basically involves high strength yarn, made from fiberglass, the stitching is done through the layers of composites using a normal stitching needle. Stitching is a low cost technique, it improves the stiffness and strength of composites. Stitched composites and the utilization of stitching through the thickness reinforcement have been studied for many years for structural applications. Stitched methods are improved or developed over the past few years. Advanced stitching machine was designed under the ACT programme by NASA in order to stitch large, thick, complex wing structures. Altin-Nahtechnik used tufting by inserting the thread under an angle of 45° and 90°, and developed the one-sided stitching technique, which is similar to the one-thread chain stitch method, by using two needles penetrating from one side of the fiber preform [US4065820A].
Polyurethane foam is used in this composite, much of the earlier work in the study on sandwich composites focused on honeycomb core, but low surface area, sensitive to hot and humid environment are some of the problems faced while using honeycomb cores. The most commonly used core material is polyurethane foam which is a thermoset to achieve reasonably high thermal tolerance [US4263247A]. In this study, a customized orthogonal stitch has been proposed to reinforce the foam core sandwich panels. Stitch-bonded sandwich panels were impregnated in a vacuum bagging process and subjected to three point bending tests to evaluate their structural performance.
Detailed Description of the Invention
E-Glass Fiberglass is a lightweight material that is commonly used in industrial, marine, and aerospace applications. In this study, the composite material utilises E GLASS FIBER with 0.15mm thickness and a density of 180g/m^2. The polyurethane foam with 5mm thickness used as core material. The glass fiber with dimensions of 250 x 250mm was cut and placed three layers of fiber on top and bottom portions of polyurethane foam of the same dimensions which is 250 x 250 mm. Lines are marked on glass fiber by leaving space of every 1 cm vertically and horizontally respectively. Using an interlock stitching process, for every 1cm interlock stitching is performed. As it has high strength and flexibility, fiber yarn was used in the stitching process. The stitching has been done through thickness i.e. through transverse direction. This stitch bonded sandwich structure is impregnated with epoxy resin (Araldite ly 556), hardener (aradur hy 951) with a mixing ratio of 1:10 in a vacuum bagging process and cured for 3-4 days. During resin impregnation, Gaps caused by sewing needles are filled by resin.
Similarly, for unstitched composite E GLASS FIBER (dimensions:250mm x 0.15mm x 250mm) of three layers is placed on top and bottom portions of polyurethane foam (250mm x 5mm x 250mm). Hand lay up technique is used, the layup process involves manipulating each ply into shape by hand and then firmly stuck to the previous layer or mold surface leaving no air pocket between plies. It is impregnated with epoxy resin (Araldite ly 556), hardener (aradur hy 951) mixing ratio of 1:10 and cured for 2-3 days.
Three point flexural test is performed using universal testing machine. The specimen has been subjected to loading until failure occurs according to ASTM D7264. Sample dimensions of stitched foam composites is 192 mm (specimen span length) x 13 mm (specimen width) x 6.17mm (specimen thickness) is placed on three point bending set up and applied load gradually. Similarly, the test is performed on unstitched foam composite with dimensions of 192 mm (specimen span length) x 13 mm (specimen width) x 6.32mm (specimen thickness).
The bending tests of stitched and unstitched foam composites are performed. Even though the same dimensions are considered, two composites show huge differences in results, as many studies stated that stitched foam composite has more advantages than unstitched composite, this has been proven experimentally by this study. The properties such as flexure, stress, tension and compression are improved or degraded by 20% by stitching through transverse direction.
From the analysis, it was observed that performance of the unstitched sandwich is much lower than the proposed stitched foam sandwich. The flexural strength of stitched composite is almost 40% higher than the unstitched composite, this clearly shows that unstitched composite has high strength and can be used at various engineering applications. for stitched composite load at peak is greater than unstitched composite, it proves that it can withstand more load compared to unstitched composite.
Brief description of Drawing
In the figures which are illustrate exemplary embodiments of the invention.
Figure 1 Stitched foam composite (a) Top view (b) Side view
Figure 2 Unstitched foam composite (a) Top view (b) Side view

4 Claims & 2 Figures , Claims:The scope of the invention is defined by the following claims:

Claim:
1. A composite-based stitched foam sandwich structure comprising:
a) A set of face sheets at the top and bottom surface of the sandwich structure.
b) A foam core is present in between the top and bottom face sheets.
c) The face sheets and foam core are rigidly attached to each other by using synthetic resin and interlock stitching.
2. As mentioned in claim 1, the face sheets are brittle and protects the inner core material from mechanical degradation.
3. According to claim 1, the increase in thickness of the structure without affecting the strength to weight ratio provides better flexural rigidity for the structure.
4. As mentioned in claim 1, the interlocking stitching between the face sheets and core provides better interlaminar shear strength and avoids delamination.