Flatwise Tension Research Articles

EXPERIMENTAL AND NUMERICAL STUDY OF INCREASED CORE SURFACE AREA ON THE PERFORMANCE OF ADDITIVELY MANUFACTURED HONEYCOMB SANDWICH STRUCTURES

Abstract Sandwich composite structures have an efficient structural design that provides high bending stiffness and strength at low weight. However, these structures are plagued with weak core-to-facesheet bond strength. In this work, the effect of increasing the contact area between the composite facesheet and honeycomb core was studied. Sandwich panels were manufactured using two different honeycomb structures, a regular and a modified honeycomb, and their respective bond strengths were evaluated using the flatwise tension test (ASTM C297). The honeycombs were additively manufactured using selective laser melting (SLM) process. The modified honeycomb was designed to have a larger surface area while retaining the same relative density as the regular honeycomb. The facesheets were made out of carbon-fiber, while the core was additively manufactured using 304L stainless steel powder. Impact specimens were manufactured and tested for impact resistance. A finite element model was created to study the integrity of the sandwich structures subjected to localized impact damage. The finite element model of the damage resistance due to impact showed a good agreement with the experimental results. Samples with increased contact area showed higher impact resistance. The average impact strength of the modified samples was 41.3% higher than the average impact strength of the regular samples. Flatwise tension results showed that by increasing the contact area between the core and the facesheet the core-to-facesheet bond strength increased.

Face/core disbond fatigue growth in honeycomb cored aircraft sandwich elements under mixed mode flatwise tension loading

Disbond damage growth in honeycomb cored sandwich structures due to static and fatigue mixed mode loading is investigated numerically and experimentally. A two dimensional finite element model was generated using core homogenization and the Crack Surface Displacement Extrapolation mode separation method, integrated into a fracture mechanics based analysis sub-routine to predict face/core interface fatigue crack propagation. The Cycle Jump technique was furthermore applied to accelerate fatigue analysis. Mixed mode fatigue characterization testing was conducted using Double Cantilever Beam specimens loaded with Uneven Bending Moments, generating a relationship between crack propagation rates and energy release rate amplitudes as a modified Paris Law, measured at three mode-mixity phase angles. The measured Paris laws were subsequently used as input data for the numerical fatigue model. The numerical model was validated against CFRP/Nomex® Sandwich Tearing Test specimen tests with a propagating face/core interface crack yielding varying mode-mixities. The results from the validation showed good agreement between numerical predictions and experimental measurements.

Elastic Modulus and Flatwise (Through-Thickness) Tensile Strength of Continuous Carbon Fibre Reinforced 3D Printed Polymer Composites.

Additively manufactured composite specimens exhibit anisotropic properties, meaning that the elastic response changes with respect to orientation. Both in-plane and out-of-plane mechanical properties are important for designing purpose. Recent studies have characterised the in-plane performance. In this study, however, through-thickness tensile strength of 3D polymer composites were determined by printing of continuous carbon fibre reinforced thermoplastic polyamide-based composite, manufactured using a Markforged Two 3D printer. This paper discusses sample fabrication and geometry, adhesive used, and testing procedure. Test standards used to determine out-of-plane properties are tedious as most of the premature failures occur between the specimens and the tabs. Two types of samples were printed according to ASTM flatwise tension standard and the results were compared to determine the geometry effect on the interlaminar strength. This test method consists of subjecting the printed sample to a uniaxial tensile force normal to the plane. With this method, the acceptable failure modes for tensile strength must be internal to the structure, not between the sample and the end tabs. Micro-computed tomography (µCT) was carried out to observe the porosity. Surface behaviour was studied using scanning electron microscopy (SEM) to see the voids and the distribution of the fibres in the samples. The results showed consistent values for tensile strength and elastic modulus for Araldite glue after initial trials (with some other adhesives) to determine a suitable choice of adhesive for bonding the samples with the tabs. Circular specimens have higher tensile strength and elastic modulus as compared to rectangular specimens.

The quality control assessment of cyanate ester blend (EX-1515) resin based composites concerning hydrolysis

Cyanate ester resins are essential matrix materials for advanced composites demanding excellent mechanical properties and dimensional stability under varying hygrothermal and radiative environments. However, a critical processing complication arises due to moisture sensitivity during cure which compromizes mechanical and thermal properties. Therefore, a reliable evaluation technique is necessary to ensure the successful use of cyanate ester-based composites. In this report, quality control (QC) signatures of a cyanate ester blend (EX-1515) are evaluated in comparison to those of an unmodified single-component cyanate ester resin (RS3C). Varying degrees of EX-1515 hydrolysis, i.e., carbamate formation, were investigated via FTIR and correlated to decreases in glass transition temperature by DSC and changes in the thermal decomposition profile by TGA. Also flatwise tension (FWT) strengths were observed to decrease with increasing hydrolysis. These results on as-cured EX-1515 were analogous to what was previously reported on RS3C. However, with elevated thermal post-treatments, their thermomechanical properties displayed an opposite trend; While hydrolyzed RS3C exhibited even further progressive reductions in Tg with a post-cure, EX-1515 showed significant increases, likely due to a new polymerization reaction involving carbamates and additional chemical moieties in the blend formulation. Interestingly, the post-cure Tg increase did not translate to any improvement in mechanical properties. On the contrary, the strength showed a dramatic reduction due to thermal degradation of the hydrolysis by-products resulting in increased porosity. The QC signatures of EX-1515 proved to be more complex than RS3C. Therefore, it is necessary to consider multiple QC signatures with carefully chosen thermal treatments rather than relying on a single point Tg measurement without the full thermal history information of the cyanate ester material.

Proposal of an Interlaminar Fatigue Life Evaluation Method for CFRP Based on Inelastic Two-Scale Analysis

In this paper, a prediction method for interlaminar fatigue life of carbon fiber-reinforced plastic (CFRP) is proposed based on an inelastic two-scale analysis method combined with the Coffin-Manson law. For this, the flatwise tension test of a CFRP is simulated using the two-scale analysis method, and the distributions of microscopic viscoplastic strain occurring at resin parts in micro-structures (unit cells) are examined. Then, the maximum value of microscopic viscoplastic strain is applied to the Coffin-Manson law for the resin to predict interlaminar fatigue life of the CFRP.

Investigation into Mechanical Properties and Failure Mechanisms of Novel Sandwich Composite Material with Carbon Fibre / Epoxy Facesheets and PVC Foam Core

Sandwich composite materials have seen a significant increase in use across various engineering fields and applications due to their exceptional mechanical properties, including high specific strength and bending stiffness values. Because of their high strength properties, the use of sandwich structures is preferred in different industries. It is crucial to investigate material properties of novel composites in order to estimate their failure mechanisms under different loading types. In this study, a type of sandwich composite material consisting of Carbon Fibre/Epoxy Facesheets with two different dimensions and a PVC Foam Core was designed and fabricated. The selection of PVC foam for this study is motivated by its wide use in load bearing components in different industries. The sandwich composite material used for the experiments was manufactured by using vacuum-assisted resin infusion moulding process (VARIM) by IZOREEL in Izmir, Turkey. Failure styles of the structure in compression, shearing and bending were revealed by sandwich flatwise tension, core flatwise compression, and sandwich edgewise compression tests. Facesheet bending of manufactured composites have been investigated experimentally. Tensile strength of the facesheets and sandwich material have been presented separately. The detailed formulations and procedures are provided. The obtained results are discussed, and it is seen that maximum tensile and compression stresses in the three-point bending test depend on the direction of the loading with respect to different facesheet thicknesses.

Mechanical modeling of a stitched sandwich thermal protection structure with ceramic-fiber-reinforced SiO2 aerogel as core layer

Ceramic-fiber-reinforced SiO2 aerogel (CFRSA) composite was used as core layer to prepare a stitched sandwich thermal protection structure (SSTPS). Mechanical properties of the SSTPS were experimentally investigated and compared with that of CFRSA, including flatwise tension, flatwise compression, edgewise compression and shear. Research results showed that the SSTPS can greatly improve the mechanical properties of CFRSA. To further understand the non-linear, tension-compression asymmetric and transversely isotropic properties of the SSTPS, inner configurations were investigated by X-ray computed tomography and scanning electron microscopy. Mechanical models were established to predict the overall properties of the SSTPS through performance of each component, including theoretical model and finite element analysis (FEA) model. Mixed series-parallel spring models were constructed to theoretically predict the effective elasticity modulus of the SSTPS. Representative volume element (RVE) was selected for FEA modeling of the SSTPS, which can not only predict the equivalent elastic modulus of SSTPS, but also predict the nonlinear flatwise compression behavior. In order to verify whether the mechanical properties of large area SSTPS under complex stress can be represented by the properties of uniform materials through RVE analysis, four-point bending test and FEA modeling were carried out on a large scale SSTPS specimen. Results showed that when analyzing the macro bending behavior of large area SSTPS, the method of equivalent SSTPS to uniform material were of relatively high accuracy and efficiency.

Experimental and numerical investigation of T‐joints with multiwalled carbon nanotubes

AbstractCombined experimental and numerical methods were used to study the mechanical properties of multiwalled carbon nanotubes (MWCNTs) enhanced composite T‐joints in this article. Herein, an enhanced interface model was created based on the interface mechanical properties with MWCNTs obtained by a flatwise tension test and a single‐lap shear test. Then, the finite element model of composite T‐joints with MWCNTs was developed with a cohesive zone model to study the damage initiation, evolution, and final failure process of T‐joints. Meanwhile, a quasi‐static tensile test of the T‐joints was carried out to compare with the numerical analysis results. The study results showed that the out‐of‐plane tensile strength and the in‐plane shear strength of MWCNTs enhanced composite increased by up to 26.3%(@3 g/m2) and 23.4%(@2 g/m2), respectively. The properties of the filler, which was the weakest part of the T‐joints, can be enhanced by adding MWCNTs. The overall bearing capacity of the T‐joints was 18.8%(@2 g/m2 MWCNTs) increase under tensile loading. Predictions given by the FEM of composite T‐joints were in good agreement with the experimental results. This article provides a method for analyzing and predicting the complex composite structure enhanced with MWCNTs, which can be used for functional structure design.

Assessment on the ageing of sandwich composites with vinylester-based composite faces and PVC foam core in various harsh environments

The behaviour of moisture absorption and evolution of various mechanical properties for the sandwich composites immersed into two humid conditions: seawater with 30 °C (SWL) and purewater with 80 °C (PWH) have been examined by Ding et al. (2018). In this paper, three relatively harsh environmental conditions: salt-fog spray ageing (SFS), hygrothermal atmosphere alternating high low temperature (HHL) and solar radiation in combination with water vapour (SRW) are determined. Further studies on the ageing behaviour of previously selected sandwich composites constituted of thick ECR glass fibre reinforced vinylester composite faces bonded to closed-cell PVC were carried out under aggression of newly added three artificial environmental conditions. To this end, the specimens were prepared by VIMP and then exposed to the new three conditions for 1008 h. The moisture uptake before desiccation and after desiccation as well as various mechanical properties, including flatwise tension strength, flatwise compressive strength, in-plane shear strength, shear strength using short beam bending and edgewise compressive strength, were measured and compared with previous data from SWL and PWH. The weight change indicates that the specimens exposed to newly added environmental conditions absorb much less moisture and chemically bound water, meanwhile the mechanical characterization reveals that specimens exposed to the SRW deteriorate significantly in the faces-core interfacial properties.

Experiments on High Temperature Mechanical Properties and Stress Distribution Laws on Strain Isolation Pad

The tile is the main heat insulation part of a thermal protection system, and is attached to the outer surface of an orbiter through a strain isolation pad (SIP). In order to study its high temperature mechanical properties and stress distribution, experiments on flatwise tension, flatwise compression and flatwise shear are conducted in the environments of the room temperature of 23±2℃ and the 300℃ high temperature. The load-displacement curve, elasticity modulus and failure strain of the SIP are measured and calculated. Compared with the experimental results under the room temperature, the tension, compression and shear elasticity modulus of the SIP in the 300℃ high temperature environment fall by 9.68%, 9.24% and 8.73% respectively, and the tension and shear failure strain of the SIP in the 300℃ high temperature environment fall by 11.59% and 12.81% respectively. So, the high temperature causes the decrease of the stiffness and strength of the SIP. Its stress distribution is studied with the experimental results. Under the influence of tension, compression and shear force, the stress distribution of the SIP shows trends of low middle and high edge. Besides, shear stress and normal stress are produced under shear force, and the normal stress is tensile at one side and compressed at the other side. The above analysis shows that the edge of the SIP is high in stress and easy to damage.

Heat-resistant sandwich structure with carbon fiber-polyimide composite faces and a carbon foam core

This paper presents the processing and properties of a new heat-resistant sandwich structure, which consist of carbon fiber-polyimide composite face sheets and a carbon foam core. A novel phenylethynyl-terminated imide oligomer (TriA-X) was applied for both matrix resin and film adhesive. Mechanical properties of the sandwich structures were evaluated using flat-wise tension tests, and three-point bending tests. Bending strength of the sandwich panel was almost constant up to 250 °C. The bending rigidity of the sandwich panels showed good agreement with the numerical result by considering the temperature-dependent elastic modulus of CFRP and carbon foam. Thermal insulating properties of the sandwich structures were evaluated using infrared lamp heating tests in a vacuum chamber, resulting in good thermal insulating property. Heat transfer analysis of the sandwich structures was conducted using finite element method. Comparison with heating test results, the validity of the heat transfer analysis model was successfully demonstrated.

Hygroscopic ageing of nonstandard size sandwich composites with vinylester-based composite faces and PVC foam core

Abstract This paper aims to assess the hygroscopic behaviour of nonstandard size sandwich composites constituted of thick Electronical-Chemical Resistance (ECR) glass fibre reinforced vinylester composite faces bonded to closed-cell Polyvinyl Chloride (PVC) foam core in the service environment as a reference and basis for materials selection. In order to exhibit the change of sandwich composites properties in realistic environment, the specimens fabricated by Vacuum Infusion Moulding Processing (VIMP) were immersed in two humid conditions: seawater with 30 °C (SWL) and purewater with 80 °C (PWH) for 1680 h. The moisture uptake and various mechanical properties of specimens, including the flatwise tension strength, flatwise compressive strength, in-plane shear strength, shear strength using short beam bending and edgewise compressive strength were tracked periodically. The results from water absorption measurement indicate that the specimens immersed in two humid conditions reach saturation levels after immersion of 1008 h, and non-Fickian and Fickian diffusion occur simultaneously. The characterization on mechanical properties was mainly placed on the variation of PVC foam core and faces-core interface properties, and corresponding results reveal that the moisture absorption has insignificant effects on the PVC foam core properties but appreciably affects the composite faces and face-core interface properties, especially for the specimens immersed in PWH.

Experimental study of ultra-high temperature interlaminar tensile strengths of 3D-needled C/C composites using the V-shaped notched specimen compression method

In this work, interlaminar tensile (ILT) strengths and microscopic failure mechanisms of 3D needled C/C composites were studied at ultra-high temperatures (up to 2800 °C). Experiments were conducted using the V-shaped notched specimen compression method. The measured ILT strengths compared favorably to results obtained via flatwise tension and diametral compression tests. In addition, the high-temperature transverse displacement field was measured using digital image correlation and the temperature field was measured using an infrared thermal imager. The transverse displacement and temperature fields were uniform in the gage section. The results confirm the effectiveness of this method for measurements of 3D needled C/C composites. The ILT strengths of the 3D needled C/C composites were measured at 1200 °C, 1600 °C, 1800 °C, 2000 °C, 2400 °C, and 2800 °C. The ILT strengths increased with the temperature up to 1600 °C, and then decreased in the 1600–2800 °C range. The failure mechanisms were investigated via optical microscopy. The needling fibers were pulled out, which indicates that the ILT strengths were primarily dependent on interfacial strength. It is intended that these results will provide some practical guidelines for ultra-high temperature engineering applications.

High-Temperature Mechanical Properties of Strain Isolation Pad for Thermal Protection System

Flatwise tension, flatwise compression, and flatwise shear experiments were conducted to study the high-temperature mechanical properties of the strain isolation pad (SIP). No damage happened to the test specimens during the flatwise compression experiments. However, the failure mode of the test specimens was related to the thicknesses of the SIP and the load types in the flatwise tension and flatwise shear experiments. Besides, the elasticity modulus and failure strain of the SIP presented obvious anisotropic characteristics, and they reduced by and , respectively in the high-temperature environment as compared with the results in the room-temperature environment. The dispersion of the failure strain was smaller than the dispersion of the failure stress. So, the strain criterion was more suitable than the stress criterion for the strength of the SIP. The numerical analysis model was built to study the strength of the SIP under the simultaneous effects of tension and shear loads. The high-temperature results in the decrease of strength and the thicker SIP enhanced the strength. The strength calculated by the quadratic nominal strain criterion was slightly greater than that calculated by the maximum nominal strain criterion. As the load changed from pure shear to pure tension, the strength of the SIP decrease first and then increased.

Ageing of sandwich composites with E-glass fibre/vinylester skins and PVC foam core in synergistic environmental agents

The current work is concerned with ageing behaviour of sandwich composites constituted of E-glass fibre-reinforced vinylester bonded to PVC foam core being in service near the sea as a reference and basis for materials selection. In order to simulate the properties evolution of sandwich composites as real as possible, the sandwich composites were exposed to the two accelerated synergistic environmental conditions: solar radiation in conjunction with water vapour (condition A) and hygrothermal atmosphere alternating thermal cycling (condition B) for 1680 h, and the weight change and mechanical properties degradation were carried out. The results show that the sandwich composites lost weight in the condition A while gained weight in the condition B, and shear strength of sandwich composites fell finitely, revealing that PVC foam core was insensitive to the two ageing conditions. Meanwhile, the core fracture, interface-core fracture, interface debonding, and piles peel were observed as the increase of exposure time in the flatwise tension test, indicating debonding properties between skin and foam as well as between piles of composites skin were influenced by the two synergistic ageing conditions.

Influence of corrosion on the mechanical properties of hybrid sandwich structures with CFRP face sheets and aluminum foam core

Hybrid materials, such as sandwich structures consisting of an aluminum foam core and CFRP face sheets, offer high lightweight design potential. However, conductive pairing of two materials with different electrochemical potential bears the risk of galvanic corrosion inside an electrolytic environment. Due to the nature of the manufacturing process, the sandwich structures investigated within this contribution are susceptible to corrosion. Multiple specimen variants with differing foam core layers were exposed to a salt spray environment for up to 240 h. These corroded specimens were mechanically characterized using the flatwise tension test. Their mechanical properties were compared to reference specimens without corrosion. It was found that both open and closed cell sandwich specimens showed significant reductions in both strength and stiffness. Comparisons to alternative sandwich specimens consisting of a Nomex honeycomb core and CFRP face sheets, which underwent the same corrosion procedure, seem to prove that the mechanical degradation was indeed a result of corrosion, as the Nomex specimens did only show a significantly smaller decrease in strength, which probably resulted from material scatter superposed with water absorption. The erosion of the metallic core layer should be quantifiable by the determination of mass reduction. Therefore, a chemical cleaning procedure in order to remove any corrosion products from the specimen surface was applied. However, the procedure either led to a further mass decrease by chemical removal of the aluminum structure, or to mass increase by a chemical reaction of the detergent with the face sheets. Thus, complementary X-ray measurements by μ-CT were carried out. Specimens were scanned before and after corrosion, the respective volumetric densities were compared in order to verify material erosion. However, no significant change in volumetric density could be observed.

Experimental, analytical and numerical study of mechanical properties and fracture energy for composite sandwich beams

In this study, the mechanical and fracture behaviour of marine sandwich-structured composite that is manufactured by vacuum-assisted resin infusion process with a PVC foam-web core, and different thicknesses of upper and lower glass fibre-reinforced polymer face sheets used as outer and inner side of the hull have been examined. The properties of PVC core, face sheet material, and composite sandwich structures are presented. Failure modes include sandwich flatwise tension, core flatwise compression, sandwich edgewise compression, and face sheet bending of manufactured composites have been investigated experimentally for marine application. The analytical solution is derived to calculate the flexural rigidity, shear stresses and maximum tensile, compression stresses in three-point bending specimens for marine sandwich composites. 3 End Notched Flexure test method has been applied to calculate the strain energy release rate. Moreover, the finite element study is carried out by using a 2D model to calculate the fracture energy (J) value numerically and virtual crack closure technique. Reasonably good results are obtained between analytical and numerical methods.

Mechanical properties of carbon nanotubes based polymer composites

Abstract The objective of this paper was to understand the effect of carbon nanotubes (CNT) additives on the elastic behaviors of textile based composites. The materials consist of three phases namely, carbon fibers fabric, Epoxy matrix and carbon nanotubes. Different volume fractions of CNTs were used (0% as reference, 0.5%, 1%, 2% and 4%). A set of mechanical tests as Open Hole Tension, shear Beam Test and Flatwise Tension tests were performed. A damage initiation and cracks propagation in composite specimens were controlled. The experimental results show an increase the mechanical performance of the composite up to 2% of CNT additives. However, beyond this value, the material strength shows a significant decay.

Low cycle fatigue of CFRP laminated composites due to repeated out-of-plane loading

Fatigue of CFRP (Carbon Fiber Reinforced Plastics) laminated composites due to repeated out-of-plane loading was investigated using two testing methods. One is the four-point-bending fatigue test using L-shaped specimens and the other is the flatwise tension fatigue test. Interlayer-toughened unidirectional CFRP laminates were used as the specimens. Here, for reduction of the testing time, low cycle fatigue was focused on by setting cyclic loads not to be too small comparing with the static strength of the specimen. Fatigue fracture occurred in a brittle manner with almost no prior deformation in both tests. The fatigue life of the L-shaped specimen was much longer than that of the flatwise tension specimen. As the results of the fracture surface observation, it could be seen that the fracture at the CF (Carbon Fiber)/epoxy layer was predominant. However, regions of the toughened interlayers were also observed on the fracture surface at the curved section of the L-shaped specimen.

On mechanical behavior of looped fabric reinforced foam sandwich

Looped fabric reinforced foam sandwich (U-cor) is a new type of composites. Its loop in Z direction can be effectively embedded in the foam core, so that the interface mechanical properties can be apparently improved. In this paper, the micro-mechanical model of U-cor was built. A simulation of damage evaluation and strength analysis was carried out on this model by using ABAQUS software. Specimens were made to perform tests under flatwise tension and shear loadings. Experimental results reveal that the drawback of traditional 2D sandwich structures under flatwise tension and shear loadings is overcome. Agreement of results obtained by finite element method and tests verifies the correctness of the analytical method.