Open Hole Compression Research Articles

An MMF3 Criterion Based Multi-Scale Strategy for the Failure Analysis of Plain-Woven Fabric Composites and Its Validation in the Open-Hole Compression Tests

A modified micromechanics failure criterion (MMF3) based multi-scale analysis strategy was proposed in this article to analyze the failure behaviors of the plain-woven fabric composites. The finite-element (FE) representative unit cell (RUC) models of different scales were first established, and the RUC based stress transformation methods were developed. The micro-scale strengths of the constituents in the unidirectional laminate were achieved based on the tested macro-scale strengths. Under the micro-scale strength invariance hypothesis, the meso-scale strengths of the fiber tows from the plain-woven fabric composites were back-calculated first and were then validated and corrected with the assistance of tested strengths of the fabric laminates. With the micro-scale RUC and the calculated meso-scale strengths of the fiber tows, the micro-scale strengths of the constituents suitable for the plain-woven fabric composites were determined. The multi-scale analysis procedure for the plain-woven fabric composites was then established in providing a more direct failure observation at the constituent level. Open-hole compression specimens were tested according to the ASTM standard D6484, and the failure of the open-hole fabric laminate was simulated with the proposed multi-scale strategy. The numerical predictions were in good agreement with the experimental results, and the feasibility of the multi-scale strategy was validated.

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.

Machine learning assisted characterisation and simulation of compressive damage in composite laminates

A data-rich framework is presented to consistently characterise the macroscopic strain-softening response of laminated composites subjected to compressive loading. First, a highly efficient continuum damage finite element (FE) model is used to simulate compact compression tests of quasi-isotropic IM7/8552 carbon fibre-reinforced polymers in order to generate a large virtual dataset for training of machine learning (ML) models. Then, two ML methods, one based on theory-guided neural network architecture to solve the inverse FE problem, and one based on recurrent neural networks with Long Short-Term Memory (LSTM) architecture to solve the forward FE problem, are trained and predictive capabilities are compared. It is found that theory-guided ML for the inverse FE problem yields high loss values and is not applicable to compressive damage characterisation whereas a minimum number of 5,000 FE simulations are needed to train accurate LSTM models for the forward problem. Numerical calibration using the trained LSTM model is validated successfully against experimental data obtained from a wide range of compressive tests including size effect studies in open-hole compression tests and axial crush tests of composite tubes. The proposed strategy demonstrates the effectiveness and challenges of ML to reduce experimental efforts for damage characterisation in composites subjected to compressive loads.

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.

Time-dependent and Temperature-dependent Fatigue Strength under Open Hole Compression for Quasi-isotropic CFRP Laminates with Toughened Interlayer

Time-dependent and Temperature-dependent Fatigue Strength under Open Hole Compression for Quasi-isotropic CFRP Laminates with Toughened Interlayer

Experimental and numerical study on progressive damage and failure in composite laminates during open-hole compression tests

Progressive damages in the open-hole compression (OHC) tests of composite laminates were experimentally and numerically studied. In the experiment, the failure mechanisms were investigated via in situ microscopy observation, digital image correlation, X-ray radiography, and X-ray computed tomography. Three layups were tested to examine the dependence of progressive damages on the layups. Additionally, numerical simulation was conducted to comprehensively examine the failure mechanisms. In the numerical studies, the simulation scheme, considering the plasticity, kink-band failure, multiple intra-laminar cracks, and delamination, was developed. From the experiment and simulation, it is clarified that the kink-band is initiated and propagated by the combined stress states consisting of longitudinal compression and in-plane shear around the intra-laminar cracks. Therefore, for the high-fidelity OHC simulation in various layups, it is necessary to capture the interaction between kink-band and intra-laminar cracks by considering the combined stress state in the kink-band failure criteria and modeling the multiple intra-laminar cracks.

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.

Progressive damage modelling in carbon fibre-reinforced materials under various loading conditions: a comparison study

Carbon Fibre-Reinforced Materials (CFRM) are used in automotive, aerospace, building material and other fields. In application of CFRMs, various damage initiation criteria and evolution laws have been continuously proposed. In this study, Hashin's, Chang's and Linde's damage initiation criteria, and linear and exponential damage evolution laws were studied and the performances of their different combinations on simulating damage of CFRM were compared under the Open-Hole Tension (OHT), Open-Hole Compression (OHC) and low-speed impact conditions. Results showed good performance was made by (1) Hashin's damage initiation criterion + the exponential damage evolution law; (2) Linde's damage initiation criterion + the linear damage evolution law and (3) Chang's damage initiation criterion + the exponential damage evolution law. Using a Gross Correlation Index (GCI) to evaluate the overall performance of the six combinations, Chang's damage initiation criterion + the exponential damage evolution law ranks the best, revealing their wider applicability.

The effects of edge quality on the measured compression strength of quasi-isoptropic composite specimens and its practical implication to the development of allowables

This manuscript presents results from an experimental study that examined the effect of quality of edge finish of test coupons on the measured compression strength of quasi-isotropic carbon fiber laminates using test standard ASTM D6641. Different cutting and grinding tools were used to machine specimens to get them within the required dimensional tolerances, all of which caused microscopic damage to the specimen edge, mainly in the form of fiber pull-out. Results of compression strength testing showed that the fiber pull-out, despite being only on the order of 0.008 mm in depth, caused a significant reduction in measured compression strength. Upon polishing of the specimen edges to remove the microscopic “gouges” of fiber pull-out, the measured compression strength of the specimens increased. It was also found that open hole compression (OHC) and compression after impact (CAI) strength test results were not affected by fiber pull-out on the edges of specimens. The implications of these results on the development of pristine strength allowables is discussed and a more practical starting point at which to develop allowables is presented.

Influence of repass treatment on carbon fibre-reinforced PEEK composites manufactured using laser-assisted automatic tape placement

Laser-assisted automated tape placement (LATP) in-situ consolidation of thermoplastic composites offers an efficient solution for production of large, high performance composite structures. The present work investigates laser repass treatment as a method of enhancing surface roughness and mechanical properties of LATP processed CF/PEEK laminates. Three laminate stacking sequences were studied, viz. [0°]16, [−45°/0°/45°/90°]2s and [±45°]4s. Four repass treatments were performed, including single, double, perpendicular and tool-side repasses. The effect of these repasses on interlaminar shear strength (ILSS), open-hole compression (OHC), in-plane shear (IPS), density, fibre volume fraction, void content and surface roughness was investigated. Surface roughness tests confirmed the improvement of the surface finish due to repass treatment. However, insignificant differences in ILSS values were observed across all repass treatments, but a single laser repass did appear to improve the OHC and IPS performance. Autoclave treated samples were used as a benchmark to compare with LATP processed laminate properties.

Interrogating failure mechanisms of notched composites through a discrete crack modeling approach

The prediction of compressive failure of composite laminates is generally more challenging than tensile failure, due to more complex failure mechanisms and nonlinear effects. This work presents an extension of a previously developed numerical framework utilizing discontinuous solid-shell elements and enriched cohesive elements for the simulation of damage growth of open-hole laminates under compression. Unlike tension, compressive loading may result in inclined matrix cracks due to localized shear damage. An approximate but efficient modeling technology was adopted by formulating an assumed vertical cohesive crack with the inclined crack coordinate system. The proposed approach explicitly represents damage as discrete cracks and leads to more accurate prediction of brittle and push-out failure patterns for sublaminate-scaled and ply-scaled laminates, respectively, including size and lay-up effects. These failure events of open-hole compression are believed to be captured with discrete crack analysis for the first time.

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.

Open hole tension and compression behavior of 3D braided composites

AbstractThe damage tolerant behavior of 3D braided composites is evaluated in terms of the open‐hole notch strength test under tensile and compressive loading. The influence of fiber architecture is evaluated by comparing 3D braid with and without axial yarns as well as laminated composites based on ASTM quasi‐static tension and compression testing standard. The results show that the notched and un‐notched properties of the two types of 3D braided composites are similar under tension and compression loading. Comparing to laminates, both types of 3D braided composites demonstrated superior notch insensitivity than laminated composites. For the failure behavior of the 3D braided composites, under tension loading, crack of both notched specimens tends to propagate along the notch and finally render the specimen to failure. Clear cracks usually presented on the samples without axial yarns, while not for the specimens with axial yarn. Under compression loading, the two types of 3D braided composite are also macroscopically similar, showing transverse fracture. The modified Fabric Geometry Model is used to predict the tensile properties of 3D braided composite. The predicted and experimental results are shown to be in good agreement.

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%.

Damage characteristics of open-hole laminated composites subjected to longitudinal loads

The damage mechanism of open-hole composite laminates subjected to longitudinal loads was studied by a three-dimensional (3D) progressive damage analysis (PDA) model, which took two essential issues, fiber kinking and shear non-linearity, into consideration. Furthermore, a parameter, damage width, was introduced to indicate the damage extent of the specimen. A good agreement between the simulation and the experiment results demonstrates the effectiveness of the proposed model. Specifically, some interesting characteristics during the damage process were revealed by the finite element analysis. It was found that in open-hole tension (OHT) test, the damage appearing first was matrix tension damage, while fiber micro-buckling emerged first in open-hole compression (OHC) test. From the load- damage width curves of open-hole laminates, it was observed that as long as any fiber damage in 0° ply spreads to the outer edges, the specimen loses its loading capacity. However, this phenomenon, interestingly, cannot be found in 90° or ±45° plies. This characteristic pointed out that the damage of 0° plies in laminated composites can be used as an indicator to reflect the damage extent of composite structures.

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.

Measurement of Interlaminar Tensile Strength and Elastic Properties of Composites Using Open-Hole Compression Testing and Digital Image Correlation

Advanced polymeric composites are increasingly used in high-performance aircraft structures to reduce weight and improve efficiency. However, a major challenge delaying the implementation of the advanced composites is the lack of accurate methods for material characterization. Accurate measurement of three-dimensional mechanical properties of composites, stress–strain response, strength, fatigue, and toughness properties, is essential in the development of validated analysis techniques accelerating design and certification of composite structures. In particular, accurate measurement of the through-thickness constitutive properties and interlaminar tensile (ILT) strength is needed to capture delamination failure, which is one of the primary failure modes in composite aircraft structures. A major technical challenge to accurate measurement of ILT properties is their strong sensitivity to manufacturing defects that often leads to unacceptable scatter in standard test results. Unacceptable failure mode in standard test methods is another common obstacle to accurate ILT strength measurement. Characterization methods based on non-contact full-field measurement of deformation have emerged as attractive alternative techniques allowing more flexibility in test configuration to address some of the limitations inherent to strain gauge-based standard testing. In this work, a method based on full-field digital image correlation (DIC) measurement of surface deformation in unidirectional open-hole compression (OHC) specimens is proposed and investigated as a viable alternative to assessing ILT stress–strain, strength, and fatigue properties. Inverse identification using a finite element model updating (FEMU) method is used for simultaneous measurement of through-thickness elastic constants with recovery of the maximum ILT stress at failure for characterization of strength and fatigue S–N curves.

A high-efficient model for interface debonding analysis of carbon fiber-reinforced polymer composite

ABSTRACTDue to the difficulties in micro stress calculation of interface, most failure analysis models of macro composite structure mainly focus on the failure modes of fiber and matrix, which have not taken interface debonding into consideration. In this paper, a high-efficient failure analysis model is proposed to realize quick investigation of interface debonding as well as fiber and matrix failure in macro composite structure under complex stress condition. Stress amplification factors are adopted to realize the rapid calculation of micro stress at interface according to the macro stress of composite. Three-point bending tests with elaborate designed stacking sequences are carried out to extract the critical interfacial normal strength and shear strength used in this model. In the failure analysis of micro unit cell this model shows high accuracy in the prediction of interface debonding both in the failure modes and failure locations compared with cohesive zone model. To further inspect its capability in the analysis of macro composite structures, open-hole compression tests of quasi-isotropic laminates are carried out. Good agreements in the interface debonding mechanisms are obtained between numerical prediction and experimental results.

Analysis of Open-Hole Compressive CFRP Laminates at Various Temperatures Based on a Multiscale Strategy

In this paper, a multiscale analysis strategy was proposed to analyze the failure behaviors of open-hole compressive (OHC) CFRP laminates. Micro-level intralaminar failure was defined in the constituents (fiber and matrix) with a modified micromechanics failure theory. In the multiscale stress transformation, the effect of thermal residual stress was considered using constant thermal amplification factor. Meanwhile, macro-level interlaminar failure was defined with cohesive elements. Based on the simulated and experimental results, the sub-laminate scaled OHC laminates of the stacking sequence [45/0/−45/90]4s were studied at different temperatures. The established multiscale model showed good precision in the strength and failure mode predictions. Transverse throughout damage at the hole section led to the final failure. As the temperature increased, the damage process began at a lower load level and the strength of the laminates decreased significantly. Stiffness reductions and small load drops were more likely to occur before final failure. The differences in the delamination size among all interfaces tended to be smaller. Besides, matrix failure lagged under shear loading conditions if the thermal residual stress was neglected in the multiscale analysis.

Prediction of the ultimate strength of quasi-isotropic TP-based laminates structures from tensile and compressive fracture toughness at high temperature

This paper is intended to test the capacity of a simple model based on fracture mechanics concepts to predict the ultimate strength of notched hybrid carbon and glass fibers woven-ply reinforced PolyEther Ether Ketone (PEEK) thermoplastic (TP) quasi-isotropic (QI) laminates under different temperature conditions. In such materials, translaminar failure is the primary failure mode driven by the breakage of 0° and 45° oriented fibers in tension as well as the formation of kink-band in compression. Single-Edge-Notched Bending (SENB), Open-Hole-Tensile (OHT) and Open-Hole-Compression (OHC) specimens have been conducted at room temperature (RT) and at a temperature higher than the glass transition temperature (Tg). The Critical Damage Growth model derived from the Average Stress Criterion and Linear Elastic Fracture Mechanics (LEFM) have been applied to open-hole specimens to determine the critical damage zone from which the fracture toughness in tension (0° and 45° fibers breakage) KIctension and in compression (kink-band formation) KIccomp. are estimated. In Single Edge Notched Bending (SENB) specimens experience simultaneous tension/compression. From the estimation of KIctension and KIccomp., the ultimate strength of SENB specimens can be predicted. LEFM equations combined with the critical fracture toughness in tension give relatively accurate results, suggesting that failure is driven by fibers bundles breakage in tension.