Development and Fatigue Life Characterization of Composite Sandwich Structures Based on Aluminum Honeycomb Core / Carbon Fiber-Epoxy Face Sheets

Summary

Sandwich structures are used in maritime, automotive, aviation, space and defence industries due to their properties such as low weight, high bending moment, resilience and high energy absorption capacity. Sandwich structures can be produced from lightweight core material and thinner yet resilient outer shell materials. The examination of fatigue behaviour of sandwich structures has become important due to expanding application areas. In this study, sandwich structures containing an aluminium honeycomb core with carbon fiber reinforced epoxy composite shells were developed. Carbon fiber reinforced epoxy composites were produced using the vacuum infusion method with unidirectional carbon fabric arranged in [0°/90°] orientation. Sandwich structures were laminated by bonding the aluminium honeycomb core to the carbon fiber reinforced epoxy composite shell using an adhesive. Static bending behaviour of sandwich structures was determined by three-point bending tests. Fatigue tests were applied to obtain fatigue damage modes and low-cycle S-N curves of these structures.

Introduction and Objectives

Sandwich structures find application in a wide range of fields such as satellite systems, aviation, maritime, automotive, wind energy systems and transportation due to their low weight relative to high stiffness and strength [1,2]. Sandwich structures are formed by strongly bonding thin composite or metallic outer shells with high elastic modulus to lightweight, relatively thick and porous core material in honeycomb or foam form [3,4]. The facing material carries the bending load and provides a hard outer surface resistant to wear, while the lightweight core material carries the shear stress resulting from external loads [5]. Due to their wide application areas, characterization of the fatigue behaviour of sandwich structures is important. Although the behaviour of these structures under fatigue loading is not fully understood, there are a certain number of experimental studies in the literature. Boukharouba and his team examined the fatigue behaviour of sandwich structures with carbon fiber reinforced polymeric composite facing and nomex honeycomb core. In the aforementioned study, they investigated the loss of resilience of specimens under fatigue loading [6]. Belouettar and his team studied the fatigue behaviour of sandwich structures with aluminium facing and nomex honeycomb core. In this study they found the fatigue damage modes and resilience losses of the developed structures [1]. Chemami and his team studied the fatigue behaviour of sandwich structures with glass fiber reinforced polymeric composite facing and PVC foam core. The researchers characterized the fatigue behaviour of these structures with Wöhler curves in their study [7]. Coskun and Turkmen studied the fatigue behaviour of sandwich structures with carbon fiber reinforced polymeric composite facing and aramid core. In the study, the researchers examined the effect of changes in test frequency values on the fatigue life and resilience losses of specimens [8]. In this study, sandwich structures containing carbon fiber reinforced epoxy outer shell and aluminium honeycomb core were developed. Static bending behaviour of the developed sandwich structures was examined. Load-displacement curves and damage modes of the structures under bending load were obtained. Fatigue behaviour of the developed sandwich structures was characterized for different load levels. Low-cycle Wöhler (S-N) curves and fatigue damage modes of sandwich structures were obtained.

Experimental Studies

In this study, carbon fiber reinforced polymer matrix composite material was used as shell material for sandwich structures. Unidirectional carbon fabric was used as the reinforcement element, and epoxy resin was used as the matrix material. The composite shell material was produced using the vacuum infusion method. Carbon fabrics were placed in the mould in [0/90] orientation. Each outer surface contained four layers of fabric. Aluminium in honeycomb form with hexagonal cell content was used as the core material of sandwich structures. The thickness of the core material is 21 mm. For the production of sandwich structures, the produced composite shell material was bonded to the aluminium honeycomb core material using an adhesive. The produced sandwich panels were left to cure at room temperature for 24 hours, and after this process the panels were subjected to post-curing at 40°C for 24 hours. Static three-point bending tests were performed in accordance with ASTM C393 standard to determine the bending behaviour of sandwich structures. The bending test setup is shown in Figure 1. Bending tests were performed on a universal mechanical testing machine at a speed of 6 mm/minute. Four specimens were tested and load-displacement curves of the specimens were obtained.

Figure 1: Bending test setup applied to sandwich structures

Load-controlled fatigue tests were applied using a three-point bending fixture on the MTS servo-hydraulic testing machine. Load ratio (R) of 0.1 and frequency of 2 Hz were selected as test parameters. Fatigue tests were performed taking 80%, 70%, 60% and 50% of the maximum bending strength obtained from the results of the static three-point bending tests. Low-cycle fatigue behaviour and fatigue damage modes were characterized.

Results and Discussion Static Bending Test Results

Figure 2 shows the load-displacement curve obtained as a result of the static bending test of four specimens. Three different conditions were observed in the load-displacement curves. These conditions indicate the damage modes of sandwich structures. Primary damage was observed to occur in the core region at the point where the linearity of the load-displacement curves ended. In this case, the primary damage mode was considered as core damage. Secondary damage was observed in the interfacial region between the core and shell material. This damage occurred in the second linear region seen in the load-displacement curves. Final damage was observed in the composite shell material, and it was determined that the carbon fiber reinforced polymer matrix composite material was damaged due to compressive stresses on it. An abrupt drop was seen in the load-displacement curves at the moment composite shell damage occurred. All damage modes observed under static bending load are shown in Figure 3.

Figure 2: Load-displacement curves of sandwich panels under static bending load

 

Figure 3: Damage from static bending tests of sandwich structures; a) core damage, b) interfacial damage, c) composite shell damage.

The values of load parameters used for fatigue tests were determined taking into account the values obtained in the static bending test.

Fatigue Test Results

Fatigue tests were applied for four different load levels (80%, 70%, 60% and 50% of maximum bending strength) and damage modes were examined for each load level. In addition, low-cycle S-N curves of the developed sandwich structures were obtained. Fatigue damage modes for different load levels are shown in Figure 4.

Figure 4: Fatigue damage modes for different load levels; a) 80% of maximum bending load, b) 70% of maximum bending load, c) 60% of maximum bending load, d) 50% of maximum bending load.

Whereas the primary damage mode under static bending load was core damage, the primary damage mode under fatigue load was observed as core-composite shell interfacial damage for load levels of 80%, 70% and 60%. Damage was observed at the end sections of the specimen for load levels of 80% and 70%, while it was detected in the mid-section of the specimen for the 60% load level. Core damage followed the interfacial damage and was considered as the secondary damage mode under fatigue load. A different damage condition was observed for the 50% load level. At the 50% load level, damage was observed simultaneously in the core and core-composite shell interface. All these damage conditions are shown in Figure 2. Primary damage occurred at 1100 cycles for the 80% load level, at 1800 cycles for the 70% load level, at 6500 cycles for the 60% load level and at 11500 cycles for the 50% load level. This data is presented in Table 1. The low-cycle S-N curve for the specimens is given in Figure 5. Table 1: Fatigue damage cycles for different load levels Prof. Dr. Metin Tanoğlu - Mechanical Engineering - Engineering Faculty, İzmir University of Economics Research Asst. Mehmet Deniz Güneş - Mechanical Engineering - Engineering Faculty, İzmir University of Economics Research Asst. Serkan Kangal - Mechanical Engineering - Engineering Faculty, İzmir University of Economics Mehmet Ziya Okur - Innoma Innovative Materials Inc.

Figure 5: Low-cycle S-N curve of sandwich structures

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