A novel test load frame for characterizing the buckling resistance of thick sandwich panels
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Alexandros ANTONIOU (GREECE), Nils ENGLISCH (GERMANY), Sara GHIASVAND (GERMANY), Malo ROSEMEIER (GERMANY)Abstract :
Wind turbine blades are typically constructed from thin-walled cylindrical and airfoil-shaped shells, primarily composed of fiber-reinforced polymers (FRPs). These shells, susceptible to buckling [1,2], are designed with sandwich panels for stability, incorporating thin FRP skins and core materials like balsa wood, polyvinyl chloride (PVC), or polyurethane (PU) foam for lightweight purposes. Resin-filled grooves and holes in the core material aid resin infusion during manufacturing and accommodate the curvature of the shells. In a study [3], the importance of realistically representing core materials, considering resin uptake in sandwich constructions for wind turbine blades, was emphasized. Reducing resin uptake offers notable benefits to the specific flexural strength of the core [4]. However, the challenge lies in the complex experimental validation required to assess the impact of resin uptake on buckling resistance, particularly in adherence to the new design standard for blade structures [5]. This standard advocates for experimental validation to minimize partial material safety factors during the design process.
In response to this need, our research introduces a novel load frame designed to characterize the buckling resistance of thick sandwich constructions [6]. This unique test frame accommodates flat plates with variable aspect ratios, spanning dimensions of 1.2m in height and 0.3m to 1.2m in width. All edges are simply supported, with modular rolling bearings for horizontal edges and 'knife edges' for vertical edges. The load frame can be mounted in a universal testing machine, see Figure 1.
To validate the load frame concept, a sandwich plate was fabricated using [±45]2 glass/epoxy skins and a one-inch-thick balsa wood core. The quasi-static testing of the structure until failure was conducted. During the experiment, deformation was closely monitored using a Digital Image Correlation (DIC) system and five pairs of strain gauges mounted on both sides of the plate (back-to-back), as shown in Figure 2a.
The back-to-back strain difference versus force recordings, obtained at the center of the plate in the load direction (0°) and transverse to the load direction (90°), clearly illustrate the bifurcation load, as shown in Figure 2b.
The critical buckling resistance was estimated for a simply supported plate considering an orthotropic buckling theory according to Wiedemann [7].
k=(p_xcr b^2)/(√(B_x B_y ) π^2 )=1/Φ_x +a_wm^2 (1/Φ_y -(H_1+H_2-2(v+v ̂ ))/((H_1 H_2-(v+v ̂ )^2 ) Φ_x Φ_y )) (1)
The buckling load will be further investigated analytically with different boundary conditions and with a finite element model of the panel test.
In response to this need, our research introduces a novel load frame designed to characterize the buckling resistance of thick sandwich constructions [6]. This unique test frame accommodates flat plates with variable aspect ratios, spanning dimensions of 1.2m in height and 0.3m to 1.2m in width. All edges are simply supported, with modular rolling bearings for horizontal edges and 'knife edges' for vertical edges. The load frame can be mounted in a universal testing machine, see Figure 1.
To validate the load frame concept, a sandwich plate was fabricated using [±45]2 glass/epoxy skins and a one-inch-thick balsa wood core. The quasi-static testing of the structure until failure was conducted. During the experiment, deformation was closely monitored using a Digital Image Correlation (DIC) system and five pairs of strain gauges mounted on both sides of the plate (back-to-back), as shown in Figure 2a.
The back-to-back strain difference versus force recordings, obtained at the center of the plate in the load direction (0°) and transverse to the load direction (90°), clearly illustrate the bifurcation load, as shown in Figure 2b.
The critical buckling resistance was estimated for a simply supported plate considering an orthotropic buckling theory according to Wiedemann [7].
k=(p_xcr b^2)/(√(B_x B_y ) π^2 )=1/Φ_x +a_wm^2 (1/Φ_y -(H_1+H_2-2(v+v ̂ ))/((H_1 H_2-(v+v ̂ )^2 ) Φ_x Φ_y )) (1)
The buckling load will be further investigated analytically with different boundary conditions and with a finite element model of the panel test.