Low Velocity Impact of Sandwich Composite Plates

Impact and compression after impact properties of plain weave carbon fiber sandwich composites were investigated in this study. Impact tests were conducted on different sample types to obtain information about absorbed energy and force. The different samples consisted of foam filled and hollow honeycomb cores with four layer carbon fiber sheets on one or both sides. The impact and compression after impact data provided valuable information to allow for comparisons between the different sample types. Also, the compression after impact tests were conducted in order to determine the reduction in compressive strength when comparing impacted to non-impacted samples. INTRODUCTION The use of woven fabrics as face sheets in composite panels is increasing due to interlacing of fiber bundles possessing high ratios of strain to failure in tension, compression or under impact loads. The core, which is typically a low strength, lightweight material, provides the distance between the face sheets required to significantly increase the overall stiffness. The high stiffness, lightweight nature of the sandwich composite makes it an ideal choice for the aerospace industry. The primary concern with carbon fiber sandwich composites is low and high velocity impact. After an impact, there is a drastic reduction in compressive strength due to delamination and core damage [1]. While several studies have been conducted on high velocity impact, studies on low velocity impact are of equal interest. Low velocity impacts can include situations such as tool drops and hailstorms. In this study, sandwich plates with different combinations of carbon fiber and cores were subjected to low velocity impacts at energies ranging from 5 J to those that caused complete penetration. The low impact energy samples were considered since even in the absence of fiber breakage, the laminate mechanical performance can be drastically affected [2]. After impact, compression tests were conducted to determine the remaining compressive strength. In-plane compression is the critical load for impact-damaged specimens, since strength reductions are the largest under this type of loading [3]. The different combinations of 100 mm x 100 mm samples used in this study are listed below. • Foam filled honeycomb core with a 4-layer carbon fiber sheet on one side (26 mm thickness, 58 g avg. weight) and on both sides (27 mm thickness, 75 g avg. weight) • Hollow honeycomb core with a 4-layer carbon fiber face sheet on both sides (27 mm thickness, 41 g avg. weight) • Foam core (24 mm thickness, 42 g avg. weight) The impact and compression tests were conducted in order to obtain energy, force, and compressive strength information, which can be used for comparisons. The energy and force data can be used, for example, to determine the benefits of utilizing foam core with a carbon fiber sheet on both sides compared to on one side. The compression after impact data can be used to determine how much reduction in compressive strength occurs as a result of different impact energies. SAMPLE CONSTRUCTION A hand-layup method shown in Figure 1 was used to construct the different sample types. The major components required for this method are a vacuum pump, vacuum bagging, spiral tubing and sealant tape. Polyurethane foam filled (FF) and hollow honeycomb (HH) were used as cores. The honeycomb structure was constructed out of craft paper. The foam filled and hollow honeycomb sheets were purchased from General Plastics. The hand-layup method provided high quality samples with minimal defects. New techniques were developed, which almost eliminated common problems of surface ripples and dry patches. In addition, a new method was designed for constructing samples with a hollow honeycomb core that eliminated epoxy pooling inside the cavities. This method also prevented the carbon fiber fabric from lifting into the cavities, which can occur as a result of the vacuum. Plain weave carbon fiber fabric from BGF Industries was used for this study. The carbon fiber fabric had the following properties. Yarn Type = 3 K Weight = 193 g / m Thickness 0.3048 mm Count = 12.5 x 12.5 The epoxy consisted of F-82 resin and TP-41 hardener, which was allowed to cure under a 600 mm Hg vacuum for a minimum of 8 hours. The cured properties of the epoxy, purchased from Eastpointe Fiberglass, are listed below. γ = 11.1 kN / m Tensile Strength = 63.6 MPa Compressive Strength = 131 MPa Cure Time = 9-12 Hours To create the samples with the FF core, a layer of epoxy was applied before each layer of carbon fiber was placed. Special care was taken to insure an adequate amount of epoxy was used in addition to being evenly spread out. After the four layers were placed, the vacuum bagging was carefully spread over the sample insuring no wrinkles would form when the vacuum was applied. Any wrinkles that form on the vacuum bagging will affect the surface finish of the sample. A roller and a rubber squeegee were used to remove the extra epoxy and trapped air. The HH core samples required three steps to construct. The first step involved creating two carbon fiber sheets with three layers each. Next, a carbon fiber layer was placed on top of the three-layer, hardened carbon fiber sheet. After the epoxy was spread out, the hollow honeycomb was positioned on top. A foam pad was placed on top of the hollow honeycomb in order to prevent the vacuum from reaching the cavities. This prevented the epoxy pooling and carbon fiber lifting. The other side of the sample was constructed in the same way, but the foam pad was not required since the other attached carbon fiber sheet prevented the vacuum from causing the above problems. To Vacuum Pump Vacuum Bagging Carbon Fiber Fabric / Core Spiral Tubing Vacuum Tubing Aluminum Sheet