Foam-cored Structures Research Articles

Effect of core structure on the quasi-static bending behaviors and failure mechanisms of aluminum foam sandwiches at elevated temperatures

In this study, a novel three-step method involving liquid casting, hot-rolling, and foaming was employed to fabricate three types of metallurgical bonded aluminum foam sandwich (AFS): AFS with a uniform foam core (UAFS), AFS with a density-graded foam core (GAFS), and AFS with an interlayer plate (IAFS). The bending behaviors of different AFS structures in four cases were investigated using the digital image correlation (DIC) technique to examine the effect of foam core structure, loading direction, and loading temperature. The results showed that the initial peak load (Fp), plateau load (Fpl) and load oscillation decreased with increasing loading temperature. IAFS demonstrated the low sensitivity to loading temperature, an important reason was the addition of interlayer plate. The deformation mode involved interface separation and foam core detachment occurring in the interface vicinity beneath the indenter at the lower temperatures (≤300 °C) shifted towards well-bonded interface and foam core densification at the higher temperatures (400 °C and 500 °C). Different core structures and loading directions gave rise to four distinct modes of crack initiation and propagation. In addition, as the loading temperature increased, the strain localization effect was diminished, accompanied by hindering in crack formation. The fracture mode of core shear transitioned from brittle to ductile with increasing loading temperature.

Optimization of Free Vibration for Sandwich Foam Core

The structural dynamic features of a sandwich foam core structure with core and surface thicknesses are examined to increase the structure's resistance to vibration. The thickness of the core and surface of the sandwich foam core structure are defined as design variables in the optimization function of natural frequency parameters. The finite element analysis program FEA software was used for the analyses. The multi-objective optimization problem using RMS. The derived natural frequencies are compared with the outcomes of the experiments to validate the numerical model. The calculated natural frequencies are compared with the outcomes of the experiments to validate the numerical model. The structural optimization is then completed, using a sandwich foam core structure. The results show that the Aluminium Layer and foam core Thicknesses (m) are 0.0086 and 0,0357 m respectively

Experimental Investigation on the Use of a PEI Foam as Core Material for the In-Situ Production of Thermoplastic Sandwich Structures Using Laser-Based Thermoplastic Automated Fiber Placement.

Laser-based thermoplastic automated fiber placement (TAFP) is nowadays mainly used to produce pure carbon fiber-reinforced plastic (CFRP) structures. This paper investigates the feasibility of a novel application: The deposition of thermoplastic prepreg tapes onto a thermoplastic foam for the production of thermoplastic sandwich structures. Therefore, simple deposition experiments of thermoplastic PEEK/CF prepreg tapes on a PEI closed-cell foam were carried out. 3D surface profile measurements and peel tests according to DIN EN 28510-1 standard were used to investigate the joining area and bonding quality. The results show that a cohesive bond is formed between the deposited tapes and the foam core, however the foam structure in the area of the deposited tapes deforms in dependence of the process parameters, and increasingly with higher deposition temperatures. Due to the deformations that occur during tape deposition, the thermomechanical foam behavior under the TAFP process conditions was investigated in more detail in a subsequent study for an extensive parameter space using a simple experimental setup. Results show that for suitable process parameters, namely a short contact time and a high temperature, the foam deformation can be minimized with the simultaneous formation of a thin melting layer required for cohesive bonding. The inner foam core structure remains unaffected.

Bending Performance of Aluminum Sandwich Panel with Polyurethane Foam Core

Although rigid foam core structures have gotten a lot of attention, there have only been a few researches on foam reinforced sandwich panels with aluminum alloy A6061 sheets as face-sheets. In this research, the sandwich concept was applied to develop lightweight panels for roofing system. Analysis on the influences of core thickness, density, and foam layer arrangement on energy absorption, bending strength and displacement of sandwich panel under the quasi-static three-point bending test were investigated. Sandwich panel core is made of closed-cell polyurethane foam with densities of 40 kg/m3, 60 kg/m3, and 80 kg/m3. The quasi-static three-point bending tests were conducted in accordance to ASTM C-393 Standard and the polyurethane foam cores are design accordingly to the guideline of National Institutes of Standards and Technology (NIST). Load–displacement curves and mechanical properties are shown using data from experimental works. Results demonstrate that increased in thickness of the sandwich panel, also increased the bending strength, energy absorption and displacement. Furthermore, the sandwich panel with 50 mm thickness and 60 kg/m3 density foam core has the maximum bending strength.

Review of carbon fiber-reinforced sandwich structures

This paper first reviews the research progress of carbon fiber-reinforced sandwich plates with grid cores, truss cores, and foam cores, and the results can be summarized in the following four points. 1) The load-bearing capacity and energy absorption capacity of grid-core and foam-core structures under bending loads have been improved by toughening with short fibers and Z-pinned fibers. 2) The progressive buckling mode of the core layer under compressive loading can significantly improve the energy absorption capacity of a sandwich structure. 3) The compressive failure of a truss-core sandwich structure is closely related to the strength at the nodes. 4) Following a literature review, this paper discusses the existing problems in sandwich structures and the corresponding countermeasures, and the results can be summarized in the following two points. 1) After implementing the current toughening measures, the debonding is improved to a certain extent; however, this approach does not prevent debonding. 2) No method has been presented that can increase the strength of the end nodes in truss-core members. Therefore, it is encouraging that a trabeculae/honeycomb-core sandwich structure composed of fiber-reinforced proteins—mimicking the biological structure found in a beetle forewing—can effectively solve the abovementioned problems.

Experimental research on damage characteristics of CFRP/aluminum foam sandwich structure subjected to high velocity impact

In view of the wide demands of sandwich structure with porous material as core layer in many important fields such as protection engineering, aerospace, automobile manufacturing and ship, etc. In order to reveal the energy absorption and damage characteristics of high velocity impact aluminum foam core and CFRP/aluminum foam core structure, the quasi-static loading system, one-stage light gas gun loading system, the impact pressure testing system and the high-speed camera acquisition system were used to perform the quasi-static compression experiments of aluminum foam and the projectiles with different materials impacting on the aluminum foam core and CFRP/aluminum foam sandwich structure at the speeds of 100–300 m/s. The influences of impact velocity and projectile’ density on the damage characteristics of CFRP/aluminum foam sandwich structure and the attenuation effect of shock wave in sandwich structure have been studied. The experimental results showed that the curve of stress and strain has three typical zones similar to porous materials: elastic zone, yield platform zone and compaction zone under quasi-static compression loading. Due to its brittle and low tensile strength of aluminum foam material, the single aluminum foam plate has poor ballistic performance under the high-speed impact of projectiles with different materials. Therefore, closed cell aluminum foam needs to be combined with high strength panels to form sandwich structure to improve its ballistic performance.

Laser Forming of Metal Foam Sandwich Panels: Effect of Panel Manufacturing Method

Sandwich panels with metal foam cores have a tremendous potential in various industrial applications due to their outstanding strength-to-weight ratio, stiffness, and shock absorption capacity. A recent study paved the road toward a more economical implementation of sandwich panels, by showing that the material can be successfully bent up to large angles using laser forming. The study also developed a fundamental understanding of the underlying bending mechanisms and established accurate numerical models. In this study, these efforts were carried further, and the impact of the foam core structure, the facesheet and foam core compositions, and the adhesion method on the bending efficiency and the bending limit was investigated. These factors were studied individually and collectively by comparing two fundamentally different sandwich panel types. Thermally induced stresses at the facesheet/core interface were thoroughly considered. Numerical modeling was carried out under different levels of geometric accuracy to complement bending experiments under a wide range of process conditions. Interactions between panel properties and process conditions were demonstrated and discussed.

Failure mode maps for four-point-bending of hybrid sandwich structures with carbon fiber reinforced plastic face sheets and aluminum foam cores manufactured by a polyurethane spraying process

This work focuses on failure mode maps of sandwich panels exposed to bending load, which were produced using a polyurethane spraying process. This process allows for an automated production of sandwich panels omitting a separate bonding step of the face sheets to the core. The investigated sandwich panels consisted of carbon fiber reinforced face sheets in various configurations, and four different core structures of aluminum foam or Nomex honeycomb. After production, measurements of the pores inside the core foam structures, the fiber package thickness inside the face sheets, and the density homogeneity of the core structure were made using X-ray computed tomography. The failure mode maps were based on the individual mechanical properties of the face sheets and the core, determined by mechanical testing. The critical forces determining the failure modes were partially modified to fit the application on foam core structures and face sheets with a porous matrix. The verification of the failure modes was performed with four-point bending tests. Since all tested configurations of sandwich specimens were produced using the same process route, the applied models for the creation of the failure mode maps could be verified for numerous parameter combinations. Except for two parameters with inconstant properties, the failure modes determined by the failure mode maps matched the observed failure modes determined by the bending tests.

Full-field monitoring of resin flow using an area-sensor array in a VaRTM process

Conventional resin flow monitoring relies on point measurements even if using grid sensing, and it is difficult to estimate an accurate flow front or defect figuration. The current work investigates full-field monitoring of resin flow during a vacuum-assisted resin transfer molding (VaRTM) process using an area-sensor array. The squared area sensors are aligned as a matrix on a thin polyimide film without any un-sensing space; thus the film measures the full-field flow monitoring, and does not miss a dry spot that may occur anywhere on the film. To identify the precise flow front and dry spots, the impregnated area is estimated by minimizing the residual sum of squares between the measured and estimated impregnated area ratios. To demonstrate the applicability of the proposed method, the area-sensor array is applied to monitoring a VaRTM resin flow on glass fiber reinforced plastic (GFRP) and foam-cored structures. As a result, the precise figurations of the flow front and the dry spot were successfully estimated in real time.

Applications of laser tapping and laser ultrasonics to aerospace composite structures

Laser ultrasonics is a well known and mature non-destructive technique for inspecting polymer matrix composites used in aerospace. This technique uses a pulse echo interrogation mode, in which ultrasound is first generated by a pulsed laser and then detected by a second laser coupled to an optical interferometer. While very successful to find delaminations in laminates, difficulties are found for reliably detecting disbonds in honeycomb and foam core structures, particularly when the detachment occurs at the back skin. For this purpose, a novel technique called laser tapping is proposed. Laser tapping is based on the thermoelastic excitation by a pulsed laser of the top skin which bulges and is driven into vibration if it is detached from the material underneath. Laser tapping uses essentially the same hardware as laser ultrasonics but probes in a lower ultrasonic frequency range. Laser ultrasonics and laser tapping can then be advantageously used concurrently. This paper presents examples of applications of a combined laser ultrasonic laser tapping system to honeycomb structures with defects, including delaminations in the skin and skin detachments at the front side and the back side.

Failure of Sandwich to Laminate Tapered Composite Structures

Failure modes and load distributions in tapered, minimum-gauge composite sandwich structures were investigated. A total of 36 unidirectional graphite-epoxy/Rohacell foamcore structures were manufactured with varying taper angles and core thicknesses. The specimens were subjected to tensile, compressive, and bending loads. Experimental data were correlated with a three-dimensional e nite element model developed to determine the stress and the strain state within a tapered sandwich structure. Emphasis was placed on the root of the taper, where experimental observations suggested damage initiation.