Open Hole Compression Strength Research Articles

Manufacturing Autoclave-Grade Thermoset Carbon Fiber-Reinforced Polymer Aerospace Composites without an Autoclave Using Nanoporous Materials.

Composite laminates utilizing autoclave-grade carbon fiber-reinforced plastic (CFRP) prepreg were manufactured using a polymer nanoporous network (NPN) interlayer that generates capillary pressure in lieu of pressure from an autoclave. The polymer nanofiber NPN film is integrated into the interlaminar region and is shown to eliminate voids in a vacuum-bag-only (VBO) curing process. After a preliminary investigation of the effect of NPN thickness on the interlaminar region and performance, an 8 μm thick polymer NPN was selected for a scaled manufacturing demonstration. Combining the polymer NPN with "out-of-oven" (OoO) electrothermal heating of a carbon nanotube (CNT)-heated tool, a 0.6 × 0.6 m void-free plate is successfully manufactured. OoO cure enables an accelerated cure cycle, which reduces the cure time by 35% compared to the manufacturer-recommended cure cycle (MRCC). X-ray microcomputed tomography (μ-CT) reveals that the laminates are void-free and of identical quality to autoclave-cured specimens. An array of mechanical tests including tension, compression, open-hole compression (OHC), tension-bearing (bolt-bearing), and compression after impact, reveal that the accelerated NPN-cured composites were broadly equivalent, with some instances of improved properties, relative to the autoclave-cured parts, e.g., OHC strength increased by 5%. With reduced capital costs, energy consumption, and increased throughput, the facile polymer NPN-enabled out-of-autoclave (OoA) fabrication method is shown to be a practical and attractive alternative to conventional autoclave fabrication.

Experimental study of the interfacial micromechanics of multifunctional unidirectional carbon fiber reinforced polymer composite laminates with copper mesh

The mechanical properties of a unidirectional carbon-fiber reinforced polymer (CFRP) laminate with Cu mesh layers were investigated for use in aerospace applications as a multifunctional structural and electromagnetic interference shielding component. Nanoindentation was used to study the fiber/PEEK and Cu/PEEK interfaces. A data analysis methodology is presented to assess the interfacial strength from plots of hardness and modulus versus distance from fiber/Cu edge. The results indicated that annealing resulted in enhanced adhesion at the fiber/PEEK interface. Additionally, indents near the Cu/PEEK interface indicated poor interfacial adhesion. Bulk mechanical testing was performed to determine the effect of adding Cu mesh layers on the mechanical properties of the laminate. Additional Cu mesh layers decreased the tension, shear, and bending strength/modulus, while the open hole compression strength increased. This indicates that the Cu/PEEK interfaces weaken the laminate and lower the tension, shear, and bending strength by delamination.

Progressive damage analysis in an open hole compression of FRP laminates including fiber kinking and pre-existing damage

Progressive damage and strength analysis in an open hole compression (OHC) test is crucial in designing components made of fiber reinforced plastics (FRPs) composites. In this work, a continuum damage mechanics based 3D progressive damage model (PDM) incorporating LaRC05 failure criteria is developed. It considers identification of fracture and kink-band plane, shear non-linearity, in-situ strengths, and mixed-mode fracture in the formulation. The proposed PDM is validated through single element tests and applied to analyze the damage patterns and strength during open hole compression (OHC) tests of three different layups with two different hole sizes and a pre-existing damage. It is observed that fiber kinking in 0∘ plies initiates at the location of maximum in-plane shear stress and propagates due to a combined action of longitudinal compression and in-plane shear stress. The final failure of 0∘ dominant and quasi-isotropic layup is governed by fiber kinking in 0∘ plies, whereas matrix cracking in ±45∘ plies is responsible for the final failure of the shear dominant layup. Furthermore, OHC strength of the layups reduces by 15%–20%, when the hole size increases from 6.35 mm to 9.525 mm. In the presence of a pre-existing kink or matrix damage, fiber kinking initiates at locations of the pre-existing damage. A small pre-existing damage reduces OHC strength of the layups up to 7%–11%. The results predicted by the model match well with the experimental results in the literature.

Open-Hole Compressive Strength and Fracture Morphologies of Punched and Drilled CFRP Laminates

Open-Hole Compressive Strength and Fracture Morphologies of Punched and Drilled CFRP Laminates

Hygrothermal and temperature effects on load-carrying capacity and failure mechanism of open-hole laminates prepared with narrow tape prepreg of GFRP

The tensile and compressive properties of the open-hole laminates fabricated with glass fiber reinforced polymer (GFRP) under various temperature/humidity coupled conditions are important design parameters for GFRP structures in helicopter. In current work, a comprehensive experimental study about the effects of temperature and hygrothermal on the open-hole mechanical property of HS6/AC318 glass/epoxy multidirectional laminates was conducted. The load-displacement responses, fracture morphologies and failure mechanisms of quasi-isotropic GFRP laminates with a center hole under tensile and compressive loads at the five environmental conditions were compared, and the effects of the temperature and hygrothermal on the open-hole tensile (OHL) strengths and open-hole compressive (OHC) strengths of GFRP laminates were investigated. In addition, the relevancies between the OHL and OHC strength of open-hole laminates and the basic material properties of unidirectional laminates were explored, and some links between the OHL strength and interlaminar fracture toughness were found. The results showed that the OHC strength of open-hole laminates was directly proportional to the 0° compressive strength of unidirectional laminates. Based on the experimental data and analysis an empirical formula of the OHC strength at arbitrary temperature and humidity combined conditions was proposed.

Improved composite open-hole compression strength and trade-off with manufacturability controlled by stacking sequence effect and non-standard ply angles

The design of composite laminates requires consideration of part strength and manufacturability, both of which are controlled by ply angle and stacking sequence. This paper studies the open-hole compression strength and forming compatibility of laminates designed with standard angle (SA) and non-standard ply angle (NSA) layups using four different stacking sequence patterns. When increasing interply ply angle differences to provide greater interply lock-up, experimental test results show that dispersed stacking sequences improve failure strength of NSA laminates by up to 36%. Dispersed stacking patterns allow NSA laminates to consistently outperform reference SA laminates with increased failure strengths of up to 17%, indicating that this alternative design may produce stronger structural components. Results for stacking sequences of NSA laminates also reflect an inverse relationship between failure strength and formability . This implies an underlying trade-off between structural strength and manufacturing compatibility, which is worth considering when designing laminates for complex geometries. • Dispersed layups improve open-hole compression strength of non-standard angle laminates by up to 36%. • Non-standard angle laminates with dispersed layups can outperform standard angle laminates on open-hole compression strength by up to 17%. • Results show a trade-off between structural strength and manufacturing compatibility controlled by stacking sequence.

Investigating interfacial adhesion and open-hole compressive (OHC) strength correlations for CFRPs via variations in sizing and oxidative surface treatment

Increasing interfacial shear strength (IFSS) has been observed to improve fracture toughness within carbon fiber/epoxy laminates. However, a direct correlation between IFSS of carbon fiber surface treatments and open-hole compressive (OHC) strength has never been established. Here the effects of systematically altering electrochemical oxidation surface treatments (0, 2 and 3.4 Amps) and sizing deposition levels (Unsized, 1:15 and 1:20 dilution epoxy:water, respectively) to observe the correlation, if any, between microscale IFSS and laminate OHC strength. Increases in OHC strength of up to 30.2% relative to untreated and unsized fibers were observed and attributed to the surface treatments for sized fibers. No changes in OHC strength were observed when the surface treatment was altered for unsized fibers, suggesting a major role of the sizing in governing OHC strength. This is supported by OHC strength increases of up to 49.4% attributed to increased sizing deposition level.

A compact open-hole compression test fixture for composite materials

A new open-hole compression (OHC) test fixture for composite laminates is proposed as a promising alternative to existing test fixtures. It is tested on both standard angle (SA) and non-standard angle (NSA) laminates. As compared to ASTM D6484, the new test fixture increases the fiber continuity threshold from 17° to 27° (which is required for accurate testing of NSA), with 80% less material needed for coupons. The new fixture shows results consistent with ASTM D6484 on SA laminates, but are significantly different OHC strengths (up to 28%) on NSA laminates. The difference on NSA laminates may be due to the 17° fiber continuity threshold in ASTM D6484, suggesting a potential application of the proposed fixture in NSA studies. From this preliminary testing with NSA laminates, fiber continuity could be a critical factor for the laminate to benefit OHC strength improvement from flipped angle designs.

Experimental Investigation of Open-Hole Compression Strength of Carbon Epoxy Composite Material and Determination of Localized Strains Using Digital Image Correlation Technique

The biggest application of the fiber-reinforced composites are in the field of military and commercial aircrafts. Carbon fibers either alone or with the Kevlar 49 fibers, are widely used as main material in many aero plane wing, fuselage and empennage components. Composites materials have wide applications in different industries because of it has very different properties from the metals and polymers. In the drilling of Carbon/Epoxy Composites the cut surface quality is very much dependent on the drilling parameters set during drilling which further effect the strength of the hole during extension/compression loading. In this research, Carbon Fiber Epoxy Composite material is drilled with the standard carbide drill bit and Open Hole Compression (OHC) tests are performed on the Universal Testing Machine. The Digital Image Correlation (DIC) technique is used to find out the strain distribution around the hole during compression loading. From the experimental method and DIC, maximum strength of carbon epoxy composite is achieved by drilling at 1600-2400 mm/min in presence of notch. It was also observed that failure of the structure is dependent on the drilling feed rate and 1600-2400 mm/min was the optimized drilling range.

Experimental and numerical studies of the open-hole compressive strength of thin-ply CFRP laminates

Thin prepregs with thickness of less than 0.06 mm support flexible lamination design because it is possible to apply various stacking sequences. The objective of this study is to elucidate the effects of the layer thickness and stacking sequence on the open-hole compressive (OHC) strength for thin-ply carbon fiber reinforced plastic composite (CFRP) laminates. Details of layer thickness and stacking sequence effects on the OHC strength for the thin-ply CFRP laminates were evaluated experimentally. Furthermore, finite element analyses incorporating Hashin’s 2-D failure criteria and a cohesive zone model were conducted to evaluate the damage progression behavior. Results demonstrated that the numerical model was useful for predicting the OHC strength of thin-ply CFRP laminates with various stacking sequences.

Composite Laminate Design for Improved Open-Hole Compression Strength using Non-Standard Ply Angles and Customized Stacking Sequences Characterized by [D] Matrix

The conventional design of composite laminates underutilizes the large design space available due to stacking sequences and ply angles. This paper studies the use of non-standard ply angles and customized stacking sequences by placing higher-angle plies on the surface for reduced ply thickness and increased open-hole compression strength. Tests show non-standard angle designs at matched in-plane stiffness improve failure strength as compared to standard angle wing-skin laminates when subject to a small load misalignment. Customized stacking sequences improve the strength of both standard and non-standard angle designs, the improvement of which linearly decreases by having the lower ply angle on the surface. The results match predictions derived from the [D] matrix.

CFRP Thin-Ply Fibre Metal Laminates: Influences of Ply Thickness and Metal Layers on Open Hole Tension and Compression Properties.

Thin-ply laminates exhibit a higher degree of freedom in design and altered failure behaviour, and therefore, an increased strength for unnotched laminates in comparison to thick-ply laminates. For notched laminates, the static strength is strongly decreased; this is caused by a lack of stress relaxation through damage, which leads to a higher stress concentration and premature, brittle failure. To overcome this behaviour and to use the advantage of thin-ply laminates in areas with high stress concentrations, we have investigated thin-ply hybrid laminates with different metal volume fractions. Open hole tensile (OHT) and open hole compression (OHC) tests were performed with quasi-isotropic carbon fibre reinforced plastic (CFRP) specimens. In the area of stress concentration, 90° layers were locally substituted by stainless steel layers of differing volume fractions, from 12.5% to 25%. The strain field on the specimen surface was evaluated in-situ using a digital image correlation (DIC) system. The embedding of stainless steel foils in thin-ply samples increases the OHT strength up to 60.44% compared to unmodified thin-ply laminates. The density specific OHT strength is increased by 33%. Thick-ply specimens achieve an OHC strength increase up to 45.7%, which corresponds to an increase in density specific strength of 32.4%.

Effects of aging process and testing temperature on the open-hole compressive properties of a carbon fiber composite

The properties of T800 carbon fiber–epoxy composite specimens with a hole were studied in terms of mass change, scanning electron microscopy, glass transition temperature ( T g), heat-resistant temperature, Fourier-transform infrared (FTIR) spectroscopy, open-hole compressive strength at different temperatures, and stereomicroscopic observations after being subjected to hygrothermal aging and thermal-oxidative aging processes. FTIR spectra indicated that after hygrothermal aging at 70°C and 85% relative humidity (RH), chemical aging did not occur, whereas after thermal-oxidative aging at 190°C, the specimens exhibited chemical aging. The unaged specimens had a T g of 229°C and an extreme heat-resistant temperature T gmod of 184°C; after hygrothermal aging, the specimens had a T g and T gmod of 207°C and 143°C, respectively; and after thermal-oxidative aging, the specimens had a T g and T gmod of 252°C and 215°C, respectively. The effects of temperature on open-hole compressive strength were evaluated at room temperature of 23°C, 50°C, 100°C, 150°C, and 200°C. The compressive strengths of the specimens decreased after aging and with the increasing test temperature. At the highest test temperature, the unaged specimens, hygrothermal-aged, and thermal-oxidative-aged specimens retained over 73.7%, 65.5%, and 67.9%, respectively, of their compressive strength. Thus, the T800 carbon fiber–epoxy composite evaluated in this study exhibited good resistance to the effects of aging and high temperature. These results should be beneficial to the understanding of the long-term performance of composites.

Clustering effect on mechanical properties and failure mechanism of open hole high modulus carbon fiber reinforced composite laminates under compression

The clustering effect of open hole high modulus carbon fiber (M40J) reinforced composite laminates under compressive load is studied with digital image correction (DIC), acoustic emission (AE) and high-resolution digital camera. Three types of quasi-isotropic laminates [45 m/0m/−45 m/90 m]ns (m × n = 4) with different clustering levels (m = 1, 2 and 4) are manufactured and tested to investigate their mechanical properties and failure mechanism. The experimental results indicate that as clustering levels increases, the open hole compressive strength gradually decreases accompanied by failure mode transferring from brittle fracture to delamination. Four types of sub-critical damages are found in the progressive failure process: splitting, delamination, layers buckling and layers compressive damage. The layers buckling and layers compressive damage can result in final failure of laminates yielding failure mode of delamination and brittle fracture, respectively. Besides, the effect of sub-critical damages and layer thickness on both mechanical properties and failure mode are discussed consequently obtaining the qualitative relationships between open hole compressive strength and material mechanical properties. The ratio of inter-fiber strength to layer longitudinal compressive strength is the crucial factor affecting the mechanical properties, and it should be controlled within the range where the push-in failure mode occurs for better properties.

Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation

Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber-matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber-matrix level and predicting the compressive strength of laminates.

Effects of hygrothermal and thermal-oxidative ageing on the open-hole properties of T800/high-temperature epoxy resin composites with different hole shapes

T800/high-temperature epoxy resin composites with different hole shapes were subjected to hygrothermal ageing and thermal-oxidative ageing, and the effects of these different ageing methods on the open-hole properties of the composites were investigated, including analyses of the mass changes, surface topography changes (before and after ageing), fracture morphologies, open-hole compressive performance, dynamic mechanical properties and infrared spectrum. The results showed that only physical ageing occurred under hygrothermal ageing (70°C and 85% relative humidity), and the equilibrium moisture absorption rate was only approximately 0.72%. In contrast, under thermal-oxidative ageing at 190°C, both physical ageing and chemical ageing occurred. After ageing, the open-hole compressive strength of the composite laminates with different hole shapes decreased significantly, but the open-hole compressive strength after thermal-oxidative ageing was greater than that after hygrothermal ageing. Among the aged and unaged laminates, the laminates with round holes exhibited the largest open-hole compressive strength, followed by those with the elliptical holes, square holes and diamond holes. The failure modes of the laminates were all through-hole failures. The unaged samples had a glass transition temperature ( T g) of 226°C, whereas the T g of the samples after hygrothermal ageing was 208°C, which is 18°C less than that of the unaged samples, and the T g of the samples after thermal-oxidative ageing was 253°C, which is 27°C greater than that of the unaged samples.

Inverse method for estimation of composite kink-band toughness from open-hole compression strength data

Fiber-reinforced polymer matrix composite materials can fail by kink-band propagation mechanism when subjected to in-plane compressive loading. This mode of failure is especially prevalent in compressive loading of laminates with holes, cut-outs, or impact damage. Most of the successful models for predicting compressive strength of such laminates require “fracture” toughness associated with kink-band propagation under in-plane compression. However, this property is difficult to measure experimentally, limiting the use of such models in design practice. In this paper an inverse method is proposed to estimate the kink-band toughness of the laminate from its open-hole compression strength data, which is an easier property to measure experimentally. Furthermore, a scaling relationship is proposed to estimate kink-band toughness for other laminate configurations of the same material. Through various investigations, it is shown that the developed inverse method and scaling relationships can serve as effective and accurate tools in the prediction of open-hole compression and compression after impact strengths for multiple layups of different material systems.

Experimental and numerical investigation on the open-hole compressive strength of AFP composites containing gaps and overlaps

Laminated composite structures manufactured via the automated fiber placement process inherently contain process defects know as gaps and overlaps. These defects raise concerns when they are located on or near holes intended for mechanical fastening. This investigation attempts to predict the effect of automated fiber placement-generated defects on the open-hole compression strength by combining both experimental tests and numerical simulation. Tested open-hole compression specimens containing gaps and overlaps oriented at 0° or 90° and centered on or shifted near the hole show that, depending on their location, the gaps and overlaps may have negative, negligible, or positive effects on the open-hole compression strength. The better than expected effects are compatible with microscopic observations that clearly show the rearrangement of the plies during the consolidation process, which prevent the formation of deleterious discontinuities. Incorporating these observations in a numerical model, which simulates gaps and overlaps embedded inside the composite laminates, and applying a progressive failure analysis, confirms that the effects of automated fiber placement defects depend as much on their type as on their location relative to the hole center. Finally, the results obtained from a parametric study provided further explanation on the effects of automated fiber placement defects on the failure strength of perforated composite laminates.

Open hole compressive elastic and strength analysis of CFRE composites for aerospace applications

This paper presents an experimental and analytical study of hole-size effect on the compressive properties of woven carbon fiber-reinforced epoxy composites. The composite laminates were cut into the standard dimensions of compression after impact specimens with different center hole sizes using abrasive waterjet machine to avoid thermal and mechanical defects that were associated with conventional drilling processes. Damage development was monitored using instrumented specimens with 8-strain gauges. For the first time, the strain transverse to the hole edge thickness and perpendicular to the loading axis is measured, which has important contribution to interpreting the damage free at this position with the aid of scanning electron microscope images. In addition, a new analysis about increasing the slope (stiffening) of axial surface strain with the applied load was proposed. The predicted strain distribution in the vicinity of the hole has acceptable agreement with the experimental measurements. The compressive strength was measured and predicted by several two-parameter fracture models. Three-parameter fracture models are employed to improve the predicted compressive strength. The results showed that the critical stress intensity factor increased with the hole size and vice versa for the compressive strength and stiffness. An excellent agreement is found between the measured and predicted compressive strengths using three-parameter fracture models (R2≥99.5%). Damage initiation was effectively detected at about 77% of the ultimate stress using the instrumented specimens.

Progressive damage simulation of open-hole composite laminates under compression based on different failure criteria

The strength prediction of open-hole fibre-reinforced composite laminate under compression is very important in the design of composite structures. The modelling of fibre, matrix damage and delamination plays an important role in the understanding of the damage mechanics of laminate under open-hole compression. In this article, a progressive damage model for open-hole compression that is based on continuum shell elements and cohesive elements is established to model in-plane damage and delamination, respectively. The damage mechanics of sublaminate-scaled laminates with ply sequence [45/0/−45/90]ms and ply-level-scaled laminates with ply sequence [45n/0n/−45n/90n]s are investigated by our proposed model. The Tsai-Wu and Hoffman failure criteria are employed for the determination of matrix damage initiation. Compared with the experiments, the numerical results using the Tsai-Wu criterion exhibit better accuracy regarding open-hole compression strength prediction and failure modes simulation.