Multiaxial Laminates Research Articles

Photothermal NDE of UD/Epoxy-Based Carbon Fibre Reinforced Laminates for Quantitative Porosity Analysis

High strength and low density make epoxy-based CFRP a highly interesting construction material for the aerospace manufacturing industry. Porosity represents an unavoidable defect and significantly weakens strength values dominated by the matrix. To evaluate the quality of safety-relevant components, non-destructive evaluation and thus the characterization of porous structures is indispensable. Pulsed thermography represents a fast, large-area and non-contact testing method that enables efficient estimation of material parameters. In this work, the authors demonstrate the quantitative estimation of porosity by pulsed thermography on a multi-axial laminate fabricated from unidirectional Prepregs for the first time. The characteristic, extensive expansion of the pores in fiber direction, is addressed by the 3D microstructure characterization of Cone beam X-ray computed tomography data. Hence, the application of effective medium theories and thus the model based porosity estimation is enabled. After the investigation of the effect of pore expansion on the effective thermal diffusivity in 3D finite element simulations, the quantitative photothermal porosity estimation on a sample with a global volume porosity of Phi =1.51% is demonstrated. The accuracy of this fast and non-contact method for porosity estimation with pulsed thermography (Delta Phi =0.63%) is comparable to the standard ultrasonic method. Consequently, an efficient estimation of porosity for large, complex shaped UD/Epoxy composite components is enabled.

Intralaminar crack growth rates of a glass fibre multiaxial laminate subjected to variable amplitude loading

The literature shows that variable amplitude loading can be much more severe than constant amplitude loading in a fatigue damage context. However, the underlying physical explanations for this are still unknown. Therefore, this paper investigates if the local energy release rate at each intralaminar crack tip in a tension/tension fatigue loaded multidirectional laminate can be used to determine the crack growth rate of these cracks subjected to variable amplitude loading. The local energy release rate is investigated through a developed finite element model in which any number of cracks can be included to account for their interaction. The position of the cracks is determined based on previous experimental work. The experimental results from the previous work are compared to the numerical results in this work. At similar crack density levels the energy release rates increase in the variable amplitude tests compared to constant amplitude tests, but not enough to significantly impact the crack growth rate. It is concluded that less than 5% of the increased crack density rate and crack growth rate caused by variable amplitude block loading is due to increases in local energy release rate. The primary reason is more likely found on a microstructural level.

Bio-based hybrid cabin door of ultralight helicopter with variable-axis fiber design

This paper presents the development of an integrated technology concept for the production of semi-structural fiber composite aerospace components with a bio-based hybrid laminate structure. In this context, the high lightweight potential of a variable-axial hybrid fiber-reinforced laminates compared to a multiaxial laminate design is of particular importance. As a demonstrator, a double-shell cabin door based on the cabin door of the ultra-light helicopter CoAx 2D of EDM Aerotec GmbH was redesigned in hybrid bio-based mixed composite construction using selected finite-element method (FEM) simulation and optimization tools, manufactured and characterized. The obtained results illustrate that compared to the reference carbon fiber-reinforced polymer (CFRP) component, the developed bio-based hybrid composite with local tailored fiber placement (TFP) reinforcement has a 30% biomass content, exhibits comparable mechanical properties and significantly contributes to increase energy and resource efficiency significantly.

Prediction of the spring-in of cylindrically orthotropic media and cross-ply laminates

The equation for prediction of the spring-in angle of a cylindrically orthotropic segment is shown to be independent of all material properties except for the anisotropic coefficients of thermal expansion and a stress-free state is insured for the corresponding unconstrained deformation. In contrast, the complete cylindrical geometry is shown to provide constraint to thermal deformation and thereby induce thermal residual stresses in the form of a moment. The method of superposition is demonstrated whereby traction-free conditions yield stress-free cylindrical elements with corresponding angular displacements at the element free boundaries. The first derivation of the spring-in equation is attributed to Radford, in contrast to the widely accepted view that the equation was first developed by Spencer et al. Finite-element methods, combined with the superposition approach, further validate the accuracy of the Radford equation for cylindrically orthotropic segments and explore its limitations for multiaxial composite laminates.

Calculation of highly stressed components made of carbonfiber-reinforced polyamide-6

A procedure for the calculation of multiaxial laminates made out of unidirectional, continuous fiber-reinforced polyamid-6 (CF-PA6) is shown in this paper. The aim is to use the VDI 2014-3 guideline (Puck’s theory) for the calculation of thermoplastic composites and to validate the quality of the calculation results with experimental tests. First, the CF-PA6 material properties are determined by different mechanical tests on cylindrical specimens. These are then used to calculate multiaxial laminates with a programmed material routine based on VDI 2014-3. A comparison with experimental tests shows to what extent the test results on multi-layer composites match the calculation.

Hybridisation of CFRP by the use of continuous metal fibres (MCFRP) for damage tolerant and electrically conductive lightweight structures

Carbon fibre reinforced polymers (CFRP) offer superior weight-specific mechanical properties. However, their brittle failure behaviour limits the structural integrity and damage tolerance in case of impact and crash events. Furthermore, the electrical conductivity is insufficient for certain applications (e.g. lightning strike protection, signal transfer, grounding). Former material concepts tried to resolve these deficits by modifying the resin system, but could not prove sufficient enhancements. A novel approach is the incorporation of highly conductive and ductile continuous metal filaments into the CFRP, thus merging electrical and load-bearing functions within the metal fibres. In this context, the present study focuses on optimising the structural and electrical performance of such hybrid composites with shares of metal fibres up to 20vol.%. Fundamental experiments are carried out on unidirectional as well as on multiaxial (hybrid) laminates. Synchronous improvements of the electrical conductivity, the impact and penetration resistance, the notched properties as well as a pseudo-ductile behaviour can be verified. Additionally, the application of integrated metastable austenitic steel fibres for non-destructive health monitoring by measurement of deformation-induced phase transformations is introduced and discussed.

Simulating squeeze flows in multiaxial laminates: towards fully 3D mixed formulations

Thermoplastic composites are widely considered in structural parts. In this paper attention is paid to squeeze flow of continuous fiber laminates. In the case of unidirectional prepregs, the ply constitutive equation is modeled as a transversally isotropic fluid, that must satisfy both the fiber inextensibility as well as the fluid incompressibility. When laminate is squeezed the flow kinematics exhibits a complex dependency along the laminate thickness requiring a detailed velocity description through the thickness. In a former work the solution making use of an in-plane-out-of-plane separated representation within the PGD – Poper Generalized Decomposition – framework was successfully accomplished when both kinematic constraints (inextensibility and incompressibility) were introduced using a penalty formulation for circumventing the LBB constraints. However, such a formulation makes difficult the calculation on fiber tractions and compression forces, the last required in rheological characterizations. In this paper the former penalty formulation is substituted by a mixed formulation that makes use of two Lagrange multipliers, while addressing the LBB stability conditions within the separated representation framework, questions never until now addressed.

3D modeling of squeeze flow of multiaxial laminates

Thermoplastic composites are attractive because they can be recycled and exhibit superior mechanical properties. As part of a more general research effort, in this paper attention is paid to squeeze flow of continuous fiber laminates. In the case of unidirectional prepregs, the ply constitutive equation, when elastic effects are neglected, can be modeled as a transversally isotropic fluid, that must satisfy the fiber inextensibility as well as the fluid incompressibility. When laminate is squeezed, depending on the plies orientation and the lubrication conditions at their interfaces, the flow kinematics exhibits a complex dependency along the laminate thickness requiring one to describe the detailed velocity field through the thickness. Simulations at this scale are computationally intensive or almost impossible with standard FE techniques. In this work the squeeze flow of laminates is revisited from the perspective of the Proper Generalized Decomposition that makes possible extremely detailed 3D calculations with a computational cost characteristic of 2D (sometimes 1D) simulations.

Experimental Characterization of Multiaxial Glass Laminates for Front End Cab

This research paper deals with an experimental investigation of mechanical properties of prepared composite panels. Flexural behavior in three-point bending test is measured. Composites are prepared in order to satisfy requirements for transport industry. Totally, three material compositions are tested with different multiaxial reinforcing fabric and polyester resin. All samples are produced by low-pressure vacuum infusion mainly used for large-scale composite parts. Furthermore, fire resistance of prepared laminates is tested and appropriate classification into corresponding categories is also done. Experimental results proved suitability of prepared glass fiber composites for application in transport industry.

The biocomposite tube of a chaetopterid marine worm constructed with highly-controlled orientation of nanofilaments

The ultrastructure of the self-constructed tube housing of the bioluminescent marine worm, Chaetopterus sp. reveals that the bio-nanocomposite tube comprises of multiple non-woven plies of multi-axially oriented organic nanofilaments (ø 50-1100nm) cemented together by an unstructured organic matrix binder. The thin-walled, impermeable tubes are bio-inspirational for conventional pipe technology. Orientation distribution analyses revealed that the dominant orientation angles of nanofilaments in the tube were 0°, ±45° and ±65°, which correlate well with optimal winding angles for 'man-made' fibre reinforced composite pipes subjected to specific loading conditions. Such a use of high aspect ratio nanofilaments in multi-axial laminates would impart toughness and flexibility to the tube structure, and facilitate rapid tube growth. While the tube production mechanism is not entirely known at this stage, our time-lapse studies show that, contrary to generic assumptions in literature, the worm actively, rapidly and sporadically produces and expands the tube.

Analytical model to predict multiaxial laminate fracture toughness from 0° ply fracture toughness

The prediction of nominal strength is very important in the design and evaluation of materials especially polymer matrix composites. Various cohesive laws forms are successfully used in predicting the nominal strength of laminated composite structures. For composite structures, fracture toughness is dominated parameter when using cohesive laws to predict their nominal strength. In spite of complex reported models, this study propose an easy simple model to predict the fracture toughness of multidirectional composite laminates using the fracture toughness of the 0° ply ones. This model is mainly based on the geometry of fiber orientation and linear elastic fracture mechanics and uses the fracture toughness of the 0° ply obtained from compact tension test specimens. A good prediction is obtained by comparing the model results with experimental data which are obtained from center-cracked specimens manufactured using different lay-ups orientations and materials. POLYM. ENG. SCI., 54:234–238, 2014. © 2013 Society of Plastics Engineers

Stress Analysis in Single and Multi Fastened Single-Lap Composite Joints

The use of composite materials in many engineering structures has increased recently. The structures are jointed together by single or multiple bolted mechanical fastening to transfer the loads from one member to another. Applications of such joints require adequate knowledge of stress distribution in the vicinity of the boundary of pin hole. In this study, stress analysis is conducted with finite element method to study the stress distributions in the vicinities of both single-pin and multi-pin loaded holes of multi-axial warp knit fabric composite laminates. The perfect fit condition and contact with friction between the pin and hole without lateral constrain are assumed in the analysis. Effects of friction, stacking sequence, ratio of edge distance over hole diameter, ratio of width over hole diameter, number of pins, number of rows, number of column and hole patterns are evaluated. The results show they have a significant influence on joint performance.

Analysis of transverse lamina cracks in multi-directionally 0°, ±45° and 90° oriented reinforced laminates and a damage tolerance design of composite plates

Analytical approach to transverse lamina cracking of ply-groups in a multi-axially (0°, ±45° and 90° oriented) laminated composite material has been proposed in the present paper. Quasi three-dimensional stress and strain analysis of a ply-group with the through-the-width transverse lamina cracks was carried out by using finite differential total force method, assuming periodical arrangement of cracks along the longitudinal direction. Equivalent elastic moduli and Mode I and II energy release rates of the damaged ply group are evaluated. Effects of crack density, in-plane elastic moduli, ply-group thickness and fiber orientation on the equivalent elastic moduli and energy release rate are expressed as interpolation functions, which are applicable to general multi-axial composite laminates. Simple calculation procedure has been proposed for damage tolerance design of laminated composite structures.

A Novel Polyamide 12 Polymerization System and its Use for a LCM-process to Produce CFRP

The aim of this study was to investigate the scope of the recently developed polymerization molding system for polyamide 12 (PA12), especially with respect to the formation of carbon fiber-reinforced PA12 composites. Initial screening of PA12 composite formation was performed by means of an internal mixer in order to identify the suitable type of surface-treated carbon fiber (CF). The content of residual lauryllactam (LL) monomer reflected the influence of the CF-treatment on the polymerization molding involving anionic in situ polymerization of LL. The cryogenic fracture surfaces were analysed by scanning electron microscopy (SEM) in order to evaluate the adhesion quality between the components. Finally, a bench-scale polymerization molding process was established successfully for the fabrication of multi-axial laminates. Macromechanical tests and dynamic mechanical thermo-analysis (DMTA) indicated that the performance of the new polymerization molding process is very similar to that of state-of-the-art PA12/CF composites.

Mechanical Behaviour of Woven and Multiaxial Fabric Composites

Compression molded glass epoxy laminates (0.65 Wf) were prepared out of BID woven (reference material), multiaxial and tailored multiaxial/uni-directional hybrid fabrics. Results showed that the properties of the hybrid fabric laminates compared very well with those of the BID laminates both in longitudinal and transverse loading directions, with the former exhibiting superior impact resistance. Hybridisation with UD fabrics has helped in enhancing the multiaxial laminate properties in the transverse direction without affecting its properties in the longitudinal direction. The compression strength of the hybridised laminate has been found to be comparable with that of the BID laminate but higher in the longitudinal and transverse directions as compared to the multiaxial laminate.

Flow Instabilities during the Squeeze Flow of Multiaxial Laminates

This paper examines the effect that ply arrangement has on the processing characteristics of aligned long fiber reinforced composite laminates. More specifically, this study focuses on the difference between the consolidation squeeze flow behavior of unidirectional and multiaxial ply laminates. A commercially produced material tape composed of aligned carbon fibers in a thermoplastic PEEK matrix (APC-2) was used to construct the laminates. Using specially designed hot platens, constant platen closure rate squeeze flow-experiments were performed on 70 ply unidirectional [0°] laminates. Flow visualization techniques were then used to measure the velocity profile of the material during flow. Similar squeeze flow experiments were also carried out on macro-ply [014,9014,07] s and cross-ply [0°,90°]18 s laminates. Comparisons are made between the generated squeeze forces and macroscopic flow deformation of the three laminate configurations.

Reliability-based optimization of fibrous laminated composites

This paper is concerned with the optimum design of multiaxial fiber reinforced laminate systems under probabilistic conditions of loads and material properties. A multiaxially laminated composite is treated as a structural system with each ply contained in the composite as one element. The Tsai-Wu failure criterion is adopted as the limit state function of a unidirectional ply. It is assumed that the system failure occurs when any one of the plies in a laminate system fails. The multiple-check-point method is successfully applied to evaluate the system reliabilities of multiaxial laminates under probabilistic in-plane stresses. An optimization problem is defined to find the optimal number of fiber orientation axes, optimum orientation angles, and optimum ply ratios which yield the highest system reliability.

Tow Collapse Model for Compression Strength of Textile Composites

The unidirectional composite compression strength model based on microbuckling of fibers embedded in a rigid-plastic matrix was extended to multiaxial laminates and textile composites. The resulting expression is a function of matrix yield strength under the fiber constraint, fiber tow inclination angle, fiber volume fraction, and the area fractions of various sets of inclined tows. The analysis was verified by experimentation. Compression tests were conducted on laminated, three-dimensional triaxially braided and orthogonally woven composites using the IITRI test specimen. The laminate specimens were made up of AS4/3501-6 graphite/epoxy composite with (0)24, (0/30/0/ - 30)35, and [(0/90)6/0)]s stacking sequence. Textile composites were made of BASF G30-500 graphite fiber tows (tow size is 6K) and Dow Chemicals Tactix 123 matrix. Fiber preform architecture of braided and woven composites before resin consolidation was 0/ + 17 and 0/90, respectively and after consolidation it was about (7/ + 20) and (5/90/90), respectively. The analysis agreed reasonably well with the test data for all cases considered. The axial fiber/tow misalignment angle for laminated, braided, and woven composites were about 4, 7, and 5 degrees, respectively. The compression strength was found to be strongly dependent on the percentage of axial tows and its misalignment angle. A small variation in the off-axis fiber/tow orientation had marginal effect on the compression strength. Hence, the off-axis tow misalignment angle can be assumed to be same as the initial laminate or the tow orientation angle.

Application of lamination parameters to reliability-based stiffness design of composites

This paper is concerned with the optimum stiffness design of multiaxial fiber reinforced laminates under the probabilistic conditions of loads and material properties using the lamination parameters. The constraints are imposed on the in-plane stiffnesses or their functions with a prescribed reliability. The reliability analysis of the stiffness and the strains of the laminates under the probabilistic in-plane stresses is conducted by using the advanced first-order second-moment (AFOSM) method. Then, the optimum fiber orientation angles of multiaxial laminates are determined based on reliability and compared with those under the deterministic conditions. The effects of the probabilistic properties of the applied loads and the elastic constants of the ply material are discussed from the viewpoint of reliability and optimum design.

Optimum fiber orientation angle of multiaxially laminated compositesbased on reliability

This paper deals with the optimum design of multiaxially laminated fibrous composites under probabilistic conditions of loads and material properties. The first-ply-failure criterion is adopted, and the reliability analysis of biaxial, triaxial, and tetra-axial laminates under the probabilistic inplane stresses is conducted by a method based on the advanced first-order second-moment method. The optimum fiber orientation angles of the multiaxial laminates are determined based on reliability. The optimum fiber orientation angles are found to be much different