Flatwise Tensile Strength Research Articles

Enhancing adhesive bonding and mechanical properties of composite sandwich panels through atmospheric plasma activation

AbstractThis study investigates the effect of atmospheric plasma activation (APA) treatment on the thermomechanical performance of adhesively bonded skins of composite sandwich panels. The investigations show that APA treatment enhances surface wettability through the water contact angle measurements, and adhesion between the skin and honeycomb core of the panels under mechanical tests. Specifically, APA treatment results in an 11% increase in flatwise tensile strength and a 12% enhancement in the impact strength of the sandwich laminates. Flatwise tensile tests reveal superior tensile strength and toughness in APA‐treated specimens, with fracture patterns suggesting more robust adhesion. Charpy impact test results confirm increased energy absorption, reflecting better mechanical resilience. Scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA) techniques are utilized to validate these findings. Overall, APA treatment proves to be an effective technique for enhancing the performance of composite sandwich panels, offering improved adhesion, strength, and impact resistance. This approach holds significant promise for advancing high‐performance composite materials in various engineering applications.Highlights APA treatment enhances wettability and adhesion in sandwich panels. Improved adhesive distribution between skin‐core after sandwich manufacturing. 11% increase in tensile strength and 12% in impact strength with APA. Better toughness and energy absorption via SEM and DMA support in APA.

Fracture analysis of composite-titanium adhesively bonded joints under mode-Ⅰ loading

In this paper, effects of adhesive, titanium surface roughness and manufacturing method on the fracture behaviors of composite-titanium adhesively bonded structures under mode Ⅰ loading were studied. Experimental results showed that the maximum GⅠc obtained from specimen bonded with tougher adhesive was approximately 276% higher than another adhesive. Higher flatwise tensile strength and roller peel strength of tougher adhesive resulted in stronger resistance for crack growth in tougher adhesive itself, leading to adhesive or adherend fracture mode. The titanium surface roughness only affected the fracture performance of specimen with cohesive fracture, and the GⅠc of specimen with higher surface roughness was improved by 20%. As for manufacturing method, due to the surface treatment of composite, the secondary and co-bonded specimen presented fiber pull-out and matrix failure, respectively. As a result, the lower fracture energy required for fiber pull-out failure resulted that the secondary bonded specimen present nearly 50% smaller GⅠc than co-bonded specimen. The findings can provide detailed reference for design of composite-titanium bonded structures.

Durability of sandwich structures with a maximized natural raw material basis: comparison of expanded polystyrene, cellulose foam and polylactic acid subjected to UV-rain aging

In this study, alternative core materials to commercial cork were searched for. Additionally, aging of new core materials was studied. Bio-based materials in sandwich structures can be useful for sports equipment, transportation, and furniture with much less impact on the environment in comparison with their synthetic counterparts. In this paper, sandwich panels made of fiber-reinforced polymer (FRP) skins and various sustainable core materials with a core thickness of 6 mm were studied. The core materials were: cork, expanded polystyrene (EPS), cellulose foam, and 3D-printed polylactic acid (PLA) honeycomb lattice. FRP composites made of flax fibre reinforcement and bio-based epoxy resin (30% bio-content) were used to manufacture the skins to compete the glass fibre reinforced rival composite. The experimental analysis of the panel performance focused on the out-of-plane behavior and aging due to conditioning in an ultraviolet (UV) irradiation-rain cabinet. The results showed that under mechanical and environmental loading, the sandwich composite with cellulose foam had comparable or even better mechanical performance under shearing load, including UV-rain effects, than structures with EPS cores. Failure localization was studied using digital image correlation (DIC). The 3D-printed PLA honeycomb sandwich structures had a high absolute flatwise tensile strength and shear strength but also greatest degradation by the UV-rain aging.

Experimental investigation on mechanical behaviors of composite sandwich panels with a hybrid facesheet

AbstractA multi‐functional composite sandwich panel was developed with a hybrid facesheet, which was composed of a quartz fiber reinforced plastic (QFRP) laminate and two copper layers. The other facesheet was made of QFRP laminate. The Nomex® honeycomb was used as the core material. The aim of this paper is to investigate the effects of the hybrid facesheet on the mechanical characterizations of the sandwich panel. Four mechanical performances of both composite sandwich panel with and without copper layers were considered, including the flatwise tensile strength test, the edgewise compressive strength test, the three‐point bending test and the tensile plate shear test. The experimental results show that all the four mechanical performances of the sandwich panel with copper layers were lower than the sandwich panel without copper layers. The flatwise tensile strength decreased by 26.1%. The edgewise compressive strength decreased by 9.5%. The shear strength obtained from the three‐point bending test decreased by 14.9%. The shear strength obtained from tensile plate shear test decreased by 13.7%.

Optimization of interfacial bonding properties between thermoplastic liners and carbon fiber‐reinforced composites by atmospheric‐pressure plasma and failure mechanism study

AbstractWeak interfacial bonding properties between thermoplastic liners and carbon fiber‐reinforced composites (CFRP) can easily lead to debonding failure, which is a major challenge in development of type IV composite hydrogen storage vessels. We optimized Box–Behnken design atmospheric‐pressure plasma process parameters for maximizing interfacial bonding properties between thermoplastic liner (PA11 liner) and CFRP using Design‐Expert software. From various designs, plasma process parameters, including time (t), nozzle‐to‐specimen distance (d) and gas flow rate (r) of treatment were selected for optimization to assess the effects of their interactions on climbing drum peel (CDP), flatwise tensile (FWT) and asymmetric double cantilever beams (ADCB). The CDP strength, FWT strength and strain energy release rate (GADCB) of untreated samples were 4.2 N·mm/mm, 0.26 MPa and 87.2 J/m2, respectively. After the plasma treatment, optimum process parameters of t = 200 s, d = 12.5 mm, and r = 700 L/h were used to achieve 33.8 N·mm/mm, 2.97 MPa and 1008.4 J/m2, respectively, which were 8.0, 11.4 and 11.6 times that of untreated PA11 surface, respectively. This method has the potential for guiding thermoplastic liner surface treatment of type IV hydrogen storage vessels before winding.

RETRACTED: Mechanical Properties Study on Sandwich Composites of Glass Fiber Reinforced Plastics (GFRP) Using Liquid Thermoplastic Resin, Elium®: Preliminary Experiments

Composite sandwich structures have been used in high performance applications such as wind turbine blades, due to their unique lightweight structure and superior mechanical properties. In the current study, a new liquid thermoplastic and thermoset resin were used to fabricate four different composite sandwich panels with two various foam types and densities. Composites made with epoxy resin are presented to comprehensively compare the mechanical properties of sandwich structures to elium resin. In the case of the mechanical properties and due to a new liquid thermoplastic resin, extensive comparisons of three-point bending, climbing drum peel, and flatwise tensile strength were investigated and compared with each other. The flexural and flatwise strength of sandwich composite increased by 53% and 75%, respectively, when using Elium resin. Then, the highest value was shown in the GF/PVC/ELIUM structure. The results revealed that Elium resin could be excellent in the case of mechanical properties to replace traditional resins to fabricate various composite structures and manage the challenge of recyclable composites. Elium resin can replace thermoset-based resins for the manufacturing of laminates and composites that are fully recyclable at room temperature with comparable mechanical properties.

Improvement of the structural, thermal, and mechanical properties of polyurethane adhesives with nanoparticles and their application to Al/Al honeycomb sandwich panels

Abstract Understanding the relationship between core/face adhesive interface and performance of MWCNT added nanoadhesives are essential for better designing and fabrication of aluminum honeycomb sandwich panels. In this study, we used multi-walled carbon nanotubes (MWCNTs) to prepare enhanced interfacial adhesion between honeycomb core and aluminum facesheet. MWCNTs are first introduced to isocyanate, and then MWCNTs/isocyanate blend poured into polyol to produce MWCNTs reinforced polyurethane (PU) nanoadhesives. The relationship between core/face adhesive interface and performance of MWCNT added nanoadhesives are investigated in detail in terms of thermogravimetric analyses (TGA), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, edge-wise compression tests, flat-wise tensile tests and three-point bending tests The experimental results showed that the MWCNT addition to PU adhesive led to enhanced thermal stability, flat-wise tensile strength and edge-wise compression strength for sandwich structures. This study gives a significant reference indicator on the fabrication of an improved core/face interface for obtaining sandwich panels with desired mechanical properties.

Fabrication, characterization and mechanical testing of carbon fiber sandwich composites with nanoparticle included polyurethane adhesives

In honeycomb core and composite face sheet sandwich panels, it is essential to understand the bonding characteristics of adhesive in relevance with its properties to observe synergistic effects of reinforcing nanoparticles such as multi-walled carbon nanotubes (MWCNTs). This study investigates the effects of MWCNT inclusion on polyurethane (PU) adhesive, which directly affects sandwich structures' structural and mechanical performance. MWCNTs are added to PU adhesive up to 0.2%, and their RAMAN spectroscopic analysis, Fourier transform infrared spectroscopy (FT-IR), thermo-gravimetric analyses (TGA) and differential-scanning calorimetry analyses (DSC) are evaluated. Aluminum honeycomb carbon-fiber-reinforced composite (CFRC) sandwich panels are fabricated using an out-of-autoclave manufacturing process. Carbon-fiber prepreg is used for top/bottom face sheets. Mechanical strength of face/core bonding evaluated as a function of MWCNT addition and core cell sizes. Manufactured sandwich composite structures are investigated for flat-wise tensile strength and three-point bending strength. Results show that MWCNT reinforcement to PU adhesive and lower cell size increases bending and flat-wise tensile resistances.

Ultrasonic Evaluation of Porosity in Out-of-Autoclave Carbon Fiber–Reinforced Polymer Composite Material

Ultrasonic longitudinal wave propagation is studied in out-of-autoclave (OoA) carbon fiber–reinforced polymer composite material with varying levels of porosity contents. A combination of cure pressures and a solvent is used to produce specimens with void contents in the range of 0% to 22%. Ultrasonic measurements are made in through-transmission mode, and the data is processed to study various aspects of wave interaction with porosity in OoA specimens. The specimens with a wide range of void contents have enabled the study of broader trends of ultrasonic center frequency, wave velocity, and attenuation with respect to porosity. Results show ultrasonic center frequency and wave velocity are decreased linearly as the void content increases. The relationship of ultrasonic wave attenuation can be approximated by a logarithmic relationship when considering the full range of void content studied. Strength measurements of specimens with varying void contents are made using the flatwise tensile (FWT) test. It is observed that the strength rapidly decreases with increasing porosity. Correlations made between FWT strength, ultrasonic wave velocity, and attenuation are best described by logarithmic relationships. The data shows a potential for inferring strength knockdowns due to the presence of porosity based on ultrasonic measurements.

On the tensile and compressive behavior of a sandwich panel made of flax fiber and agglomerated cork

Sandwich panels are used in buildings because of their light weight, thermal insulation, sound absorption, and better mechanical properties. Material scientists have been trying their best to replace synthetic materials as these are non-biodegradable. Stringent environmental policies, need for eco-friendly products, and problems in end-of-life disposal have made natural materials in sandwich panels the need of the hour. This research work attempts to replace the synthetic sandwich panels with natural sandwich panels without sacrificing the performance. The natural sandwich panels used in this work had agglomerated cork as the core and flax fiber as skin reinforcement. A vacuum-bagging technique was used to manufacture these panels with three core densities and three skin reinforcements. The performance of these panels was confirmed by conducting flatwise tensile, flatwise compressive, and edgewise compressive tests as per the American Society for Testing and Materials standards and the results are discussed in this article. The specific flatwise tensile strength of natural sandwich panels was 14–21% higher and the specific flatwise compressive strength was 2–10% higher than the synthetic sandwich panels. The edgewise compressive strength was found to be 32–34% lower than synthetic sandwich panels. The results suggest that natural sandwich panel could be a better alternative material for building applications.

Influence of temperature on flat-wise tensile strength of Al3003 honeycomb sandwich panels

Due to high specific modulus, the demand for lightweight structures made of sandwich panels is ever increasing in many Industrial sectors. Numerous research efforts have been taken by various researchers in this area in terms of weight and cost optimization. Sandwich panels consist typically of two thin face sheets and a lightweight thicker core and the core is joined with the face sheet by adhesive bonds. These panels find wide application in public transport industry, aeronautical sectors, tooling machines, serigraphy and building industries. The bond strength between face sheet and the core depends essentially on the method of manufacture and the geometrical characteristics of the core. Flat-wise tensile tests are recommended to evaluate the bond strength as well as maximum temperature to which the panels can be exposed during service. The results of tensile tests are used in selecting resin materials for engineering applications. Tensile properties frequently are included in material specifications to ensure quality. This paper discusses the experimental method for assessing the Flat-wise tensile tests and the results obtained for sandwich panels bonded with different resins.

A Numerical simulation of delamination caused by internal gas pressure for mid-density CFRP

Interlaminar debonding (delamination) is a major problem for carbon fiber-reinforced plastic (CFRP) when it is used in thermal protection systems subjected to rapid heating, such as in a re-entry situation. In this study, the possibility of avoiding delamination by controlling material porosity was investigated experimentally and numerically. CFRP specimens having various initial porosities (mid-density CFRP) were prepared and rapid heating tests (∼6 MW/m2) were performed, and the specimens were examined to determine whether delamination had occurred. To predict the delamination, heat transfer and pyrolysis gas-induced internal gas pressure growth were simulated numerically as a function of heating time. The parameters regarding temperature-dependent chemical reaction rate, orthotropic coefficients for gas permeability, and flatwise tensile strength were determined by individual experiments using the CFRP specimens. The coupled analysis of thermal transfer and pyrolysis gas flow following Darcy’s law was conducted by finite-difference form, employing the experimentally obtained parameters. When the gas pressure exceeds the flatwise tensile strength of CFRP, delamination is assumed to occur. The numerical results show good agreement with the experimental results, and it is concluded that delamination can be eliminated by using mid-density CFRP.

Thermally induced microcracks and mechanical property of composite honeycomb sandwich structure: Experiment and finite element analysis

Microcracking in composite honeycomb sandwich structure and its effect on mechanical properties are studied in this paper. A methodology is presented to study the extent of mechanical strength degradation of composite sandwich structure, subjected to thermal fatigue. The material under study is used for spacecraft structural applications. The test coupons were exposed to thermal cycling at elevated temperature as high as +150°C inside the oven and cryogenic temperature of −190°C by dipping in liquid nitrogen, which is comparable to the thermal environment experienced by spacecraft structures. After each thermal cycle, coupons were inspected for microcracks under an optical microscope at the cross section. The microcracks were then quantified using parameters like crack length and crack density with increase in the number of cycles. Flatwise tensile test was conducted on the coupons after every 10 thermal cycles, up to 60 cycles, to make a correlation between crack density and mechanical strength. It was observed that by increasing the number of thermal cycles, the crack density increases and the flatwise tensile strength decreases up to a specific number of cycles. Finite element analysis was performed to predict the possible location of microcracks formation and compared with experimental observation. Good correlation was observed.

The Comparison on Mechanical Bonding Properties of Untreated Coconut Fiber towards Synthetic Fiber for Fiberglass Boat Building

The development of high performance materials made from natural resources is increasing worldwide. The interest in natural fiber reinforced polymer composite materials is rapidly growing both in terms of their industrial applications and fundamental research. They are renewable, cheap, completely or partially recyclable, and biodegradable. The coconut fiber can be a potential candidate to replace the industrial core and foam which its application are worldwide and it is used to increase the thickness of the fiberglass boat. In this research, three types of testing panel are constructed by using 10 mm of Coconut Fiber, 3D Core Foam and Infusion Grooved PVC Foam as the sandwich core. The resin infusion method which is produce quality final products have been used. The findings be obtained by conducting two testing methods, for the Flatwise Tensile Strength testing, the specimens taken from Coconut Fiber product yielded a higher value of strength which is 3.005 MPa compared to the specimens taken from Infusion Grooved PVC Foam and 3D Core Foam which is 2.963 MPa and 1.264 MPa respectively. For the Flatwise Compressive Strength testing, the specimens taken from the Coconut Fiber product had higher value of compressive stress compared to the value of specimens taken from Infusion Grooved PVC Foam and 3D Core Foam which is 29.66 MPa, 2.58 MPa and 4.68 MPa respectively. This research has proved that the Coconut Fiber is quite suitable to become as one of the laminating schedule for the construction of the fiberglass boat hull.

Performance Evaluation of Out-of-Autoclave Sandwich Structures with K-COR and Nomex Core

Composite sandwich structures are being increasingly employed in the aerospace, marine as well as other applications which require high strength, high in-plane and flexural stiffness coupled with a reduction in weight. In the aerospace industry sandwich composites are usually manufactured using the autoclave process which is expensive and time consuming. A viable alternative is the Out-of-Autoclave (OOA) process whose main advantage is the low cost of manufacturing. One of the most dominant failure modes in sandwich composites is the debonding that occurs at the facesheet-to-core interphase. A well-formed adhesive bond at the skin to core interphase and improved adhesion strength are significant factors in preventing bond failure. In this paper honeycomb and K-COR sandwich composites are manufactured using the Out-of-autoclave process. The adhesive bonding strength in each of the manufactured panels is quantified by evaluating the flatwise tensile strength in accordance with ASTM standards. Core-facesheet debonding was observed in K-COR sandwich while core failure was observed in the Nomex sandwich structures. Finite element modeling and simulation of Nomex and K-COR sandwich structures subjected to flatwise tension is conducted using comprehensive three-dimensional finite element models. Cohesive damage modeling approach is used to simulate delamination failure in both Nomex and K-COR sandwich structures. Finite element results are compared with the experimental results for model validation. The ultimate flatwise tensile strength predicted by simulation was found to be in good agreement with the experimental results.

Out-of-autoclave processing and properties of bismaleimide composites

*This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The emergence of bismaleimide composites has fulfilled some of the increasing demand for higher temperature performance aeronautics and space exploration vehicles. This study examines and evaluates three bismaleimide matrix resins and two bismaleimide adhesives and reports on the processing properties of these resins and composites by out-of-autoclave–vacuum-bag-only oven processing. Measurements were conducted under various cure cycles to characterize and correlate thermal and viscoelastic properties of the materials. Specimens of all three aged matrix resins exhibited an out-time life up to 30 days when stored at room temperature. Solid and sandwich panels were fabricated with the out-of-autoclave–vacuum-bag-only process. Because of tooling limitations in industry practices, composite fabrication of these bismaleimides was restricted to a maximum 177℃ curing, followed by a free-standing postcuring at elevated temperatures in an oven. The adhesive foaming characteristics, composite resin/void content, flat wise tensile strength, and fracture surface features were evaluated. Due to the unique temperature limitations of this work, the resulting panel properties were not necessarily representative of manufacturer specifications or recommendations.

Evaluation of skin-core adhesion bond of out-of-autoclave honeycomb sandwich structures

Composite sandwich structures offer several advantages over conventional structural materials such as lightweight, high bending and torsional stiffness, superior thermal insulation and excellent acoustic damping. One failure mechanism in a composite sandwich structure is the debonding of the composite facesheets from the core structure. A well-formed adhesive fillet at the interface of the honeycomb core cell walls and the laminate is a significant factor in preventing bond failure. In the present work, honeycomb composite sandwich panels are manufactured using a low-cost vacuum-bag-pressure-only out-of-autoclave manufacturing process. CYCOM®5320 out-of autoclave prepreg is used for the facesheet laminates and FM® 300-2U film adhesive is used for the facesheet-to-core bond. The manufactured composite sandwich panels are of aerospace quality with a facesheet laminate void content of around 1%. In this study, adhesive fillet formation and adhesive mechanical strength are evaluated as a function of several different sandwich construction design variables. Both aluminum and aramid Nomex® honeycomb core materials are considered to study the effect of core cell size and core material. The effect of film adhesive thickness is studied. A process for reticulation of the adhesive is applied and studied. A quantitative investigation of the adhesive fillet geometry is carried out for all the panels. Manufactured panels are evaluated for flatwise tensile strength in accordance with test method ASTM C297. Optimized combinations of core material, core density, cell size and adhesive thickness are identified. Results show that the reticulation process improves fillet formation and increases flatwise tensile properties.

Preparation and performance of a novel polyimide foam

AbstractA new type of polyimide foam (PIF) was prepared and characterized based on a one‐pot process by the reaction of a first solution with different ratios of a second solution. The first solution was comprised of pyromellitic dianhydride (PMDA), N, N‐dimethyl formamide (DMF), methanol, water, surfactant, and catalysts, while the second solution contained polyaryl polymethylene isocyanate (PAPI). In the present study, the relationships among compositions, structures, and properties of PIFs were investigated. The results indicated that with the increase in the weight ratio of PAPI/(first solution), the foaming degrees of PIFs increased from 10.14 to 10.52 times and the apparent densities before postcure decreased from 15.96 to 14.51 kg/m3. The open cell contents, average sound absorption coefficients, and average cellular diameters of PIFs after postcure increased with increase in the weight ratio of PAPI/(first solution). The glass transition temperatures (Tg) of PIFs after postcure first increased from 287 to 299°C, then decreased to 292°C, and the 5% weight loss temperatures and 10% weight loss temperatures presented the same trend as well. The compressive and flatwise tensile properties scaled very well with the relative densities of the foams after postcure, with the highest compressive strength of 0.03 MPa and the highest flatwise tensile strength of 0.15 MPa. Copyright © 2011 John Wiley & Sons, Ltd.

Foam sandwich composites with cyanate ester based syntactic foam as core and carbon‐cyanate ester as skin: Processing and properties

AbstractFoam sandwich composites were processed using cyanate ester‐based syntactic foam as core and carbon fabric‐cyanate ester composite as skin. They were processed by a one‐step compression‐molding technique. The mechanical performance of the sandwich composites was evaluated in terms of flatwise tensile strength (FTS), flatwise compressive strength, and edgewise compressive strength. The dependency of these properties on the core composition was investigated. FTS initially increased with the increase in resin content of the syntactic foam core. However, higher resin content in the core led to a diminution in FTS due to high void content. The flatwise compressive strength and edgewise compressive strength and the corresponding moduli values showed an increasing trend with increase in resin content of the core despite the presence of voids at high resin content. The failure modes of the composites under different loading conditions have been examined. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

A study on metallic thermal protection system panel for Reusable Launch Vehicle

A Ni-based superalloy honeycomb thermal protection system (TPS) panel has been fabricated. And a curved Ni-based superalloy honeycomb sandwich has also been fabricated. The preliminary thermal insulation results of a fabricated Ni-based superalloy honeycomb TPS panel (the areal density of this panel is 6.7kg/m2 and total height is 32mm) indicate that the maximum temperature of the lower surfaces of the panel is lower than 150∘C when the temperature of outer surface is held at 650∘C for 30min. The flatwise tensile strength and compressive properties of a fabricated Ni-based superalloy honeycomb sandwich coupon was studied at room temperature. A multilayered coating has been developed on the surface of the superalloy honeycomb TPS panel for environmental protection and thermal control. The oxidation weight-change results show that the weight change of the Ni-based superalloy honeycomb sandwich with the oxidation resistant coating is extremely small at 1100∘C in air for 10h. The emittance layer of the multilayered coating imparts an emittance in excess of 0.85 during exposure at 850∘C, which was at least 14% greater than that of the substrate with oxidation resistant alone.