In-plane Compressive Strength Research Articles

In-plane compressive strength and failure behaviors of composite sandwich structures for wind turbine blades

Composite sandwich structure, as the largest part of wind turbine blades, often experiences complex failure phenomena during full-scale blade testing and wind farm operation. In order to better understand failure behaviors and reveal the failure mechanism of sandwich structures for wind turbine blades, composite sandwich samples were designed, fabricated, and tested in consideration of the real laminate and core machining configuration, manufacturing process, and dominating load of wind turbine blades, In-plane compressive strength and failure behaviors were investigated by experimental and theoretic methods. It was found that the grooved with perforations and cut (GPC) machining configuration can improve in-plane compressive strength of sandwich samples more than that of the double grooved with perforations (DGP) configuration due to higher core shear modulus, the failure mode of sandwich samples shifted from global buckling for Plain configuration to a combination of core shearing and skin/core debonding at both sides for DGP configuration, and finally to skin/core debonding at the shallow groove core side for GPC configuration. The failure mode of sandwich samples with DGP core mainly transferred from global buckling to a combination of core shearing and skin/core debonding at both sides, and further to skin/core debonding at both sides alone with increasing foam density. All samples with GPC core exhibited skin/core debonding at the shallow groove side after slight buckling, which was driven by the asymmetric structure of the GPC core. Classical methods can reasonably predict failure loads of sandwich samples with Plain and DGP cores, but overestimate that of sandwich samples with GPC core.

Investigation into quasi-static compressive behaviors of several kinds of honeycomb like structures in three axial directions

The natural beesʼ honeycombs maintain long-term structural stability in harsh environments, employing a highly material-efficient approach. However, the reasons remain somewhat ambiguous. To clarify the stabilization mechanism and investigate quasi-static compression responses of double-layer ordered cellular structures, honeycomb, Tóth and single-layer cellular structures with a relative density (ρr) of 25.84 % were fabricated using 3D printing technology. Then, quasi-static compression experiments in three directions were conducted. Further, a numerical study was conducted to uncover the stabilization mechanism and effect of ρr on compressive behaviors. Results revealed that the stabilization mechanism was mainly attributed to bearing load priority of intermediate layer and its inhibition on formation of plastic hinges. A relative density of 5.17 % served as a transition point for deformation mode, beyond which honeycomb and Tóth structures exhibited stronger in-plane compressive strength at expense of less sacrificed out-of-plane compressive strength, below which they both exhibited more stable compressive curves compared to single-layer cellular structures, which were favorable for energy absorption. This study clarifies the stability mechanism of beesʼ honeycombs and addresses the lack on compression behaviors of double-layer ordered cellular structures. Moreover, it introduces two available bionic structures with controllable deformation modes to expand the application of single-layer cellular structures.

Ultrahigh-temperature mechanical behavior and failure mechanisms of SiCf/SiC composites

SiCf/SiC composites are potential candidates in the aerospace fields for application as high-temperature structural components. Their mechanical properties and failure behaviors at ultra-high temperatures (above 1473 K) are important to obtain systematically. This work performed the in-plane compressive, bending, and interlaminar shear tests at 1573 K and 1773 K in an inert atmosphere. The results showed dramatically that the in-plane compressive and interlaminar shear strengths increased with the temperature increment from 1573 K to 1773 K. In contrast, a reduction in flexural strength was noticeable. To reveal the failure mechanisms, fracture surface microstructures of these SiCf/SiC samples were characterized. It was found that the fiber surfaces for the samples tested at 1773K were rougher than the samples tested at 1573K. It meant that, with the temperature increasing, the fiber-matrix interfaces enhanced, which resulted in increasing compressive and interlaminar strengths. For the bending tests, serious delamination occurred when the temperature increased to 1773K, which induced the flexural strength reduction. The numerical simulations of bending samples were developed to reveal the mechanism of strength degradation as the temperature elevated.

Constructing orthogonally structured graphene nanointerface on SiC nanowires reinforced carbon/carbon composites to boost mechanical strength and strength retention rate

Carbon/carbon composites with higher mechanical strength and better reliability at elevated temperatures are urgently needed to satisfy the practical applications requirements. SiC nanowires (SiCNWs) modified C/C (SC-CC) composites have attracted an abundance of attention for their excellent mechanical performance. To further boost the mechanical strengths of composites and maximize the reinforcing efficiency of SiCNWs, we introduce orthogonally structured graphene nanosheets (OGNs) into SC-CC composites, in which OGNs are grafted on the SiCNWs via chemical vapor deposition (CVD) method, forming SC-G-CC composites. Benefiting from the nano-interface effects, uniform stress distribution, strong SiCNWs/PyC interfacial bonding and elevated stress propagation efficiency in the PyC matrix are achieved, thus SC-G-CC composites accomplish brilliant mechanical properties before and after 1,600 °C heat treatment. As temperature rises to 2,100 °C, SiCNWs lose efficacy, whereas OGNs with excellent thermal stability continue to play the nano-interface role in the PyC matrix. Therefore, SC-G-CC composites show better mechanical performance after 2,100 °C heat treatment, and the mechanical strength retention rate (MSR) of interlaminar shear strength, out-of-plane and in-plane compressive strength of SC-G-CC composites reach 61.0%, 55.7% and 55.3%, respectively. This work proposes an alternative thought for maximizing the potentiality of nanomaterials and edifies the mechanical modification of composites.

Transition of in-plane compressive strength and failure mechanism of 2D-C/SiC: Influence of off-axis loading angle and loading rate

In this study, the in-plane compressive behavior of two-dimensional plain-woven carbon fiber-reinforced silicon carbide composite (2D-C/SiC) was investigated using on- and off-axis compression experiments under both quasi-static and dynamic loading conditions. The failure mechanisms were elucidated through in-situ observations and microanalysis-based methods. As the loading angle increased from 0 to 45°, stress–strain curves exhibited stronger nonlinearity, and the in-plane compressive strength decreased by 50.2% and 41.1% under quasi-static and dynamic loading conditions, respectively. The failure mechanism shifted from fiber-dominant to interface- and matrix-dominant as the loading angle increased, which was responsible for the strength degradation and intensified nonlinearity in curves. A positive strain rate effect on the in-plane compressive strength attributed to the interface enhancement and multiple crack propagation was identified. Additionally, owing to the competition between the dynamic strengthening and damage aggravation effects, the strain-rate sensitivity factors increased with the loading angle up to 30° and then decreased at 45°.

Geometrical imperfection sensitivity on in-plane compressive modulus and strength of honeycomb core and deviations from isotropy

Honeycomb core sandwich may suffer from face/core debonding. Candidate debond fracture test specimens such as End-Notch Flexure (ENF) and Mixed Mode Bending (MMB) contain unbonded regions of the sandwich where the core is required to carry in-plane com-pressive load. In-plane modulus and compressive strength of honeycomb core are not pro-vided by core producers and must be estimated from unit cell models assuming perfect hexagonal cells. In this study it is shown that geometrical imperfections, such as deviations in the angle between cell walls and curved cell walls have a strong influence on modulus and strength because of deviations from transverse isotropy. A range of Nomex honeycomb cores were tested under in-plane compression. Modulus and strength ratios ( L/W) were found to vary from 0.3 to 3.24 and 0.96 to 1.8, respectively. Analysis based on unit cell models verified that deviations of the inclined cell wall angle, θ, from 30 ◦ is a major cause for orthotropy. Rounded corners and curved walls were analyzed using finite element analysis. Both principal elastic moduli of the core ( E L and E W) were reduced by the curvature while the Poisson’s ratios remained largely unaffected. Implications of the results for design of MMB and ENF fracture test specimens are discussed.

Damage sensitivity studies of composite honeycomb sandwich structures under in-plane compression and 4-point bending: Experiments and numerical simulations

Honeycomb sandwich structures, especially those with carbon-fiber panels and Nomex-honeycomb cores, are widely employed in automobiles and aircraft due to their excellent mechanical properties. Here, in-plane compression and 4-point bending experiments, as well as FEM simulations, are performed for a composite honeycomb sandwich structure with face-core de-bonding defects or impact damage to evaluate the damage sensitivity. It is found that the structure exhibits a higher sensitivity to impact damage than to de-bonding defects. Impact damage caused by an energy of 3 J–5.5 J can alter the failure mode under both in-plane compression and 4-point bending loads, and reduce in-plane compression strength by more than 50 % and 4-point bending strength by 15 %. In comparison, de-bonding defects of the same size can change the failure mode and decrease the strength by only 10 % under in-plane compression. The developed FEM model predicts all failure modes and residual strengths obtained from experiments. The simulation results show that the structural characteristics of producing concaves in the facesheet and core are the dominant contributors to structural failure. Our model, covering all geometric and structural details of the honeycomb core, provides a useful tool for studying the mechanical behaviors of such structures.

Analysis on the turning point of dynamic in-plane compressive strength for a plain weave composite

Experimental investigations on dynamic in-plane compressive behavior of a plain weave composite were performed using the split Hopkinson pressure bar. A quantitative criterion for calculating the constant strain rate of composites was established. Then the upper limit of strain rate, restricted by stress equilibrium and constant loading rate, was rationally estimated and confirmed by tests. Within the achievable range of 0.001/s–895/s, it was found that the strength increased first and subsequently decreased as the strain rate increased. This feature was also reflected by the turning point (579/s) of the bilinear model for strength prediction. The transition in failure mechanism, from local opening damage to completely splitting destruction, was mainly responsible for such strain rate effects. And three major failure modes were summarized under microscopic observations: fiber fracture, inter-fiber fracture, and interface delamination. Finally, by introducing a nonlinear damage variable, a simplified ZWT model was developed to characterize the dynamic mechanical response. Excellent agreement was shown between the experimental and simulated results.

Tensile and compressive behavior of Na-, K-, Ca-Montmorillonite and temperature effects

Molecular dynamics (MD) method was applied to simulate the nano mechanical behavior of Na-, K-, Ca-montmorillonite (MMT) under uniaxial tension and compression, and to explore the effects of the loading direction and temperature. The relation between nano mechanical properties of Na-, K-, Ca-MMT and the hydration was investigated, a comparison was made considering the bulk modulus, shear modulus, elastic modulus and Poisson’s ratio. There are significant differences for the nano mechanical behavior of montmorillonite in different directions, the in-plane tensile and compressive elastic modulus and strength are greater than those in perpendicular to the crystal plane. The nano mechanical properties of Na-MMT have a negative relation with temperature. The nano mechanical properties were found to decrease with the increasing hydration for the montmorillonite. The stiffness coefficients are also influenced by the hydration and interlayer cations. The tensile strength of Ca-MMT is smallest among three types of montmorillonite. The bulk modulus, shear modulus, and Young’s modulus of polycrystal Na-, K-, Ca-MMT are also decreasing with the hydration water layers, while Poisson’s ratio remains unchanged with the increasing hydration. By observing the nano structure change, the tension failure is caused by a slip plane due to bond breaks and the compression failure caused by the local buckling of the crystal sheet in x- and y- direction, while in z- direction, the failure is due to the separation of the interlayer water molecules in tension and bond breaks to form a shear band in compression.

Translaminar enveloping ply for CFRP interlaminar toughening

Developing low-cost and high-performance interlaminar toughening strategies for the advanced CFRP products has great engineering significances. Combining features on practical automated fiber placement and well-recognized 3D reinforcement technologies in the field of CFRP composites, herein, we alternately introduced a reinforcement ply in the manner of translaminar enveloping to generate interlocking and binding action into conventional lay-ups. Remarkably, more than 1700% for mode I (DCB) and 500% for mode Ⅱ (ELS) propagation fracture toughness improvements were experimentally investigated. The evolution of interlaminar toughening behaviors was specifically revealed. Multiple fracture energy absorption stages, successively including delamination tolerance, partial interface pull-out and through-thickness ruptures, reflect long-acting mode I toughening capacity. Under the multi-interface synergistic shear-delamination mechanism, the mode Ⅱ crack growth gets far more effectively suppressed. Meanwhile, the in-plane tensile and compressive strength could be maintained acceptably. We also gave insights into the technical superiorities of this original ply-level reinforcing work.

Evolution of microstructure and mechanical properties of Cf/SiC-Al composites after high-temperature oxidation

The evolution of micro-morphology and physical properties of Cf/SiC-Al composites prepared by precursor impregnation and pyrolysis (PIP) and high-temperature pressure infiltration processes after high-temperature oxidation (300–700 °C) was investigated. Nanoindentation method was used to characterize the hardness and elastic modulus of each component of the composites. The results demonstrated that with the increased of the oxidation temperature, the weight loss rate of the composites gradually increased, and in-plane compressive strength decreases gradually. The composites showed significant oxidation damage after the oxidation temperature exceeded 500 °C, and the nanoindentation hardness and elastic modulus of the Al alloy matrix were significantly reduced. After the oxidation treatment at 700 °C, the Al alloy matrix in the composites even melted and flowed out. The oxidative damage and compression fracture failure mechanisms of the composites after oxidation were also further discussed and analyzed.

Compressive property and shape memory effect of 3D printed continuous ramie fiber reinforced biocomposite corrugated structures

The present work aimed to study the quasi-static compression behaviors of 3D printed continuous ramie fiber reinforced biocomposite corrugated structures (CFCSs) with excellent shape memory effects. The in-plane compression test was conducted to evaluate the effects of cell shapes, fiber volume fraction (f v) and addition of fiber on the compression behaviors and energy absorption (EA) characteristics of the corrugated structures. The results showed that the compression property and EA capacity of the 3D printed CFCSs increased with decreasing f v and the addition of continuous ramie yarn. The 3D printed continuous ramie fiber reinforced biocomposite with inverted trapezoid cell shape corrugated structures (CFITCSs) outperformed other cell shapes in the compression strength and specific EA. The analytical model for the in-plane compression strength of CFITCSs was derived, and predictions were in good agreement with measurements. In addition, continuous natural fiber reinforced composite structure for shape memory was proposed for the first time. The shape recovery testing results demonstrated that 3D printed CFCSs had the potential to be a key element of lightweight programmable smart systems.

Geometry design and in-plane compression performance of novel origami honeycomb material

Origami is widely used for honeycombs to improve mechanical behavior. This research presents an origami-honeycomb material based on the assembly of Miura patterns. Firstly, the geometry stacking scheme and rigid folding mechanism are discussed. Secondly, the in-plane compression behavior of the proposed origami-honeycomb is experimentally and numerically investigated. Its performance is then compared with a hexagonal honeycomb. Both results show that the origami-honeycomb outperforms the conventional honeycomb in terms of in-plane compression strength and energy absorption behavior. Subsequently, deformation mode and energy efficiency ratio are obtained to illustrate the mechanical behavior. A parametric analysis is also conducted by employing finite element simulations. Finally, the analytical model of the proposed origami honeycomb is built to estimate the gather strain and plateau stress.

Simultaneously enhancing mechanical and tribological properties of carbon fiber composites by grafting SiC hexagonal nanopyramids for brake disk application

Extensive attention has been drawn to the development of carbon fiber composites for their application in brake disks due to the increasing demand for brake disks with high mechanical strength and better tribological properties. Herein, we design SiC hexagonal nanopyramids modified carbon/carbon (SiCNPs-C/C) composites, in which SiCNPs are radially grafted on the carbon fibers by the combined sol-gel and carbothermal reduction method, and pyrolytic carbon (PyC) matrix is deposited on nucleation sites including carbon fibers and SiCNPs by isothermal chemical vapor infiltration (ICVI). Benefiting from the special structure, SiCNPs-C/C composites exhibit superior mechanical and frictional performance. Compared with C/C composites, SiCNPs-C/C composites have 147%, 90.3%, 70.6%, and 117.9% improvement in the hardness, interlaminar shear strength, and out-of-plane and in-plane compressive strength, respectively, which is attributed to the optimized fiber/matrix (F/M) interfaces bonding and the enhanced cohesion strength of PyC matrix. In addition, the friction coefficient of SiCNPs-C/C composites increases by 25.5%, and the wear rate decreases by 38.0%. This work provides an optional design thought for the nanomaterials and enlightens the mechanical and frictional modification of composites in the field of the brakes.

Stitching Repair for Delaminated Carbon Fiber/Bismaleimide Composite Laminates.

Due to the excellent mechanical properties and heat resistance, bismaleimide matrix composite materials have been widely used in aircraft. However, they are susceptible to low-energy impacts, such as bird hits, gravel, tools falling, etc., which can easily result in delamination. The delamination can significantly reduce the compression performance of composites and become a potential hazard for aircraft in service. In this paper, a stitching method developed from the Z-pin manufacturing process was proposed to repair delaminated laminates. Firstly, the delaminated area was stitched by fiber bundles that were pre-impregnated with glue. Then, the fiber bundles threading through the laminate become the pins after the curing process, thus producing the bridging effect between delaminated layers. As a result, the in-plane compressive properties of the laminate are enhanced. The parameters, including the size, number, and position of the stitching hole, for the stitching repair were optimized, and the factors affecting the repair effect were discussed through both finite element analysis and experiments. The results showed that for a carbon fiber/bismaleimide composite plate with a circular delamination roughly 30 mm in diameter, the in-plane compressive strength can be recovered from 54.45% to 84.23% of the pristine plate, and the modulus was fully recovered.

In-plane compression failure mechanism of two-dimensional triaxial braided composite (2DTBC) material subjected to different load directions

Two-dimensional triaxial braided composite (2DTBC) often fails by external force from different directions in service. In this work, the in-plane compression failure mechanism of 2DTBC was studied along the axial (0°), off-axial (±60°) and perpendicular (90°) directions at quasi-static and high strain rate. The progressive failure process was recorded by infrared thermography with the final morphology using computer tomography technology. A three-dimensional mesoscale finite element model was established to simulate the initiation, accumulation and propagation of various damages. The results show that the in-plane compression strength, strain, energy absorption and failure mode of 2DTBC depends on whether there are main load-bearing (axial or braided) yarns directly transferring stress along load directions. The occurrence and development of localized damages can be captured by high-speed IR thermography and verified by final morphology. The in-plane failure mechanism of 2DTBC is induced by different dominating factors under different load directions, such as the out-of-plane movement of braided yarns, kinking effect at crimp positions, structure antisymmetry effect, buckling and shear instability. • Effect of main load-bearing (axial or braided) yarns during the in-plane compression. • Occurrence and development of damages captured by high-speed IR thermography. • Effect of yarns out-of-plane movement, kinking, buckling and shear instability.

Conceptual Multifunctional Design, Feasibility and Requirements for Structural Power in Aircraft Cabins

This paper presents a theoretical investigation into the potential use of structural power composites in regional aircraft passenger cabins and the corresponding challenges to widespread use, including fire resistance, long-term cycling performance, and cost. This study focuses on adapting sandwich floor panels with structural power composite face sheets, designed to power the in-flight entertainment system. Using a simple mechanical model to define the structural requirements, based on state-of-the-art laminated structural power composites, a series of electrochemical energy storage performance targets were calculated: a specific energy , a specific power , an in-plane elastic modulus , and in-plane tensile and compressive strengths . Significantly, the use of a distributed energy storage system offered a significant range of other mass and cost savings, associated with a simplified power system, and the use of ground-generated electrical energy. For an Airbus A220-100, the analysis predicted potential mass and volume savings of approximately 260 kg and 510 l and annual reductions in and emissions of approximately 280 tonnes and 1.2 tonnes respectively. This extended design analysis of a specific component highlights both the far-reaching implications of implementing structural power materials and the potential extensive systemic benefits.

Micro-damage initiation evaluation of z-pinned laminates based on a new three-dimension RVE model

Z-pinning is an effective method to increase the delamination resistance of composite laminate. To optimize the design of z-pinned reinforced composite material, the paper proposes a new three-dimension RVE model for z-pinned laminates based on the manufacturing process, which involves the evolution of microstructures inside the laminate. First, an insertion and curing simulation is conducted by the finite element method to obtain an initial three-dimension microstructure RVE grid. Then, use the Delaunay triangulation method to regenerate the grids of resin-rich zone, matrix layer and steel z-pin. Finally, the regenerated grid is transformed into a finite element RVE model. Local material properties and local fiber orientation are determined by the Chamis model and grid geometries, respectively. Periodic boundary conditions are defined to confirm deformation compatibility and continuous stress. The RVE model is used to investigate the effects of z-pinned process parameters on the in-plane compressive strength and micro-damage evolution of laminates, and is eventually verified by the experimental results.

Compression behavior and energy absorption of 3D printed continuous fiber reinforced composite honeycomb structures with shape memory effects

Three-dimensional (3D) printing is a potential rapid prototyping process that may replace traditional manufacturing processes to fabricate lightweight cellular structures with superior energy absorption performance. In the present work, continuous fiber reinforced composite honeycomb structures (CFRCHSs) with excellent shape memory properties were manufactured through a fused filament fabrication (FFF) technology, and their out-of-plane/in-plane compression behaviors and energy absorption characteristics were experimentally investigated. The results reveal that the failure process of the 3D printed CFRCHSs under in-plane loading is that the honeycomb cells collapse layer by layer along the loading direction，accompanied by the formation of a localized band. The crashworthiness analysis indicates that the 3D printed CFRCHSs outperform several competitive cellular topologies in the compression strength and specific energy absorption. A simplified analytical model for the in-plane compression strength of CFRCHSs was derived, and good agreement between measurements and predictions was observed. Additionally, the shape recovery tests demonstrate that the 3D printed CFRCHSs possess the potential as key elements of lightweight intelligent systems and adjustable energy absorbing devices.

Influence of strain rate and temperature on mechanical properties of carbon woven-ply PPS thermoplastic laminates under dynamic compression

High-strain-rate compression experiments are conducted on woven carbon fabric/polyphenylene sulfide (CF/PPS) composites in the in-plane and out-of-plane directions at different temperatures (23 °C, 95 °C and 125 °C) with a Split Hopkinson Pressure Bar (SHPB) apparatus. Macro-fracture micrographs were used to understand the failure modes. Experimental results indicate that the behavior of CF/PPS composites have strong strain rate and temperature dependence. The in-plane compressive strength and modulus increase approximately linearly with the strain rate, and decrease significantly with increasing temperature. The out-of-plane compressive modulus exhibits noticeable increase at elevated temperatures due to the viscoelastic characteristics of CF/PPS composite, whereas the out-of-plane compressive strength is not sensitive to the temperature. The fracture morphology images reveal that the main failure modes of CF/PPS specimens are delamination accompanied shear failure under in-plane compression loads, and warp fiber bundle breakage under out-of-plane compression loads.