Complex Lay-ups Research Articles

Influence of process parameters in AFP fiber steering on cylinders: Constant curvature paths

Automated Fiber Placement (AFP) is a flexible manufacturing technique which allows the manufacturing of complex layups where steered paths are necessary to achieve the surface coverage. However, steering tows using the AFP process introduces challenges in terms of quality and productivity due to steering induced defects. The quality of AFP layup is highly dependent on the geometry and the curvature of the path as well as the manufacturing process parameters such as layup temperature, head speed and compaction force. Understanding how these parameters affect the process is important to achieve the desirable quality of the final part. In this paper, several paths are steered at different radii of curvature (1270, 635 and 318 mm) on a cylindrical tool using different combinations of process parameters. The quality of the obtained layup is assessed through image processing of acquired profilometry scans. In-plane tow displacement and out-of-plane tow deformations are captured over the length of each placed course. Using numerical techniques, the defect occurrence is correlated to the input process parameters. Results show that the path curvature is the main feature driving defect formation. The remaining process parameters are ranked according to their influence.

Experimental investigation and damage simulation of large-scaled filament wound composite pipes

This paper investigates the progressive collapse of filament wound Toray T700/Epotech X4201 composite pipes with various layups via a combined experimental and numerical study. The experimental program examines the flexural behavior of full-diameter T700/X4201 pipes under four-point bending and the tensile behavior of filament wound coupon specimens cut from the pipes. The tests show that the composite layup significantly influences the deformation, flexural resistance, and failure mechanisms of the composite pipes. The CFRP pipe with the optimal complex layup [(90/±15/90/±453)5/±453] design has higher ultimate resistance in bending than other designs with layups of [±45]28 and [90]56. Coupon tests show similar effects of composite layup on the tensile behavior of coupon specimens. The numerical effort evolves a three-dimensional progressive damage model to predict the failure in the CFRP coupons and pipes. The damage model incorporates a nonlinear in-plane shear behavior of the T700/X4201 composite material prior to damage initiation and uses a continuum damage mechanics approach to model the matrix and fiber damage progression in the composite pipes. The combined experimental-numerical work provides a more complete understanding of the bending behavior of large-scaled composite pipes, showing distinct failure modes for filament wound pipes with different layups. The load-displacement curves, ultimate bending resistance and failure modes of the composite pipes with different layups are accurately captured using the proposed damage model, demonstrating the capability for the failure analysis of large-scaled composite pipes such as composite risers in offshore applications.

Effects of defects on laminate quality and mechanical performance in thermoplastic Automated Fiber Placement-based process chains

Automated Fiber Placement of thermoplastic unidirectional tape materials offers several advantages over conventional organosheets, such as enhanced part performance through tailored fiber architecture, and economic and ecological benefits due to scrap reduction. Because material is cut perpendicular to the feeding direction in state-of-the-art machine technology, triangular gaps and overlaps occur when geometrically complex layups are fabricated. Their effect on part properties is unknown for thermoplastic materials. This study investigates the influence of various defect configurations on laminate quality and mechanical performance for different consolidation processes. Analysis of microsections prepared from post-consolidation specimens shows out-of-plane undulations in defect areas. The undulation extent is quantified by angle and deflection. Tensile and compressive testing is performed. Gaps reduce ultimate tensile and compressive strength significantly for variothermal press and autoclave consolidation. Digital-image-correlation-based strain measurement during tensile testing shows strain concentration in the defect area for these specimens. Specimens consolidated in an isothermal stamp forming process show no comparable stress concentration, as well as no reduced ultimate strength. Specimens containing overlaps generally show a better performance in terms of ultimate strength compared to those containing gaps. Even though no full factorial design of experiments was used, the results obtained from this study can be used as a baseline for sector-boundary design strategies. The definition of defect-specific knock-down factors would be a next step towards the solid engineering of thermoplastic Automated Fiber Placement parts.

A Nonlinear Method for Characterizing Discrete Defects in Thick Multilayer Composites

Discrete defects in thick composites are difficult to detect for the small size and the structure noise that appears in multilayer composites. In this paper, a nonlinear method, called recurrence analysis, has been used for characterizing discrete defects in thick section Carbon Fiber Reinforced Polymer (CFRP) with complex lay-up. A 10 mm thick CFRP specimen with nearly zero porosity was selected, and blind holes with different diameters were artificially constructed in the specimen. The second half of the backscattered signal was analyzed by recurrence analysis for areas with or without a defect. The recurrence plot (RP) visualized the chaotic behavior of the ultrasonic pulse, and the statistical results of recurrence quantification analysis (RQA) characterized the instability of the signal and the effect of defects. The results show that the RQA variable differences are related to the size of blind holes, which give a probable detection of discrete geometric changes in thick multilayer composites.

Mesh morphing methodology for strength predictions in composites

This work presents a methodology based on mesh morphing techniques for transfer of high-fidelity X-ray computed tomography (CT) data, including manufacturing defects information, into finite element (FE) models for failure prognosis in composite structures. An IM7/8552 carbon/epoxy hat-section element with complex layup is considered. Failure initiates at the location of seeded defects, including in-plane and out-of-plane fiber waviness, representative of irregularities that could occur during manufacturing of composite parts with complex layup and geometry. The ability of the method to efficiently transition high fidelity CT data into structural models for assessment of the effects of manufacturing defects is essential for condition-based structural substantiation. Stress-based failure criteria are considered for prediction of matrix-dominated failure under quasi-static loading. FE results are compared with experimental data, including correlation with CT-based post-failure damage information.

Size-dependent behavior of laminates with curvilinear fibers made by automated fiber placement

AbstractVariable stiffness laminates can be manufactured using curvilinear fiber paths. A curvilinear fiber path is generally defined based on the plate size and has a curvature that is dependent on the plate size. In practice, however, the fiber path must satisfy manufacturing constraints, such as the minimum turning radius imposed by the automated fiber placement machine, thereby limiting the possible amount of fiber steering. In this work, we studied the effect of the plate size on the structural properties of a plate manufactured with curvilinear fibers. We considered four plate sizes, which were designed by a constant curvature fiber path. We optimized the plates for both maximum buckling load and in-plane stiffness. The results showed that the in-plane stiffness of the plate was not controlled by the plate size, whereas the buckling load was highly affected by the curvature of the fiber path. Hence, the potential of a buckling load increase reduced for plate sizes smaller than the minimum turning radius. In addition, for a given maximum curvature of the fiber path, the influence of a complex layup on the buckling load was marginal.

A study on the failure mechanisms of carbon fiber/epoxy composite laminates using acoustic emission

The complex failure mechanisms that are commonly considered as the distinctive characteristic of composites are being amenable to nondestructive test advance. This research adopts the acoustic emission technique to study the failure mechanisms and damage evolution of carbon fiber/epoxy composite laminates. Effects of different lay-up patterns and hole sizes on the acoustic emission response are studied to set up the mapping between the failure properties and the acoustic signal features such as the energy, counting and amplitude. Moreover, the microscopic properties of different composite specimens after fracture are watched and analyzed by scanning electron microscope (SEM). Based on the mapping conception, the controlling microscopic failure mechanisms of composites including the splitting matrix cracking, fiber/matrix interface debonding, fiber pull-out and breakage as well as delamination are identified. It is expected the influence of complex lay-up patterns and sizes on the damage and failure properties of composites is represented by creating true mapping based on the acoustic emission technique.

Prediction of crack onset strain in composite laminates at mixed mode cracking

Failure process of continuous fiber reinforced composite laminates in tension usually starts with appearance of intralaminar cracks. In composite laminates with complex lay-ups and/or under combined loading, intralaminar cracks may develop in plies with different reinforcement directions. A necessary part of mixed mode cracking models is the criterion of failure. For propagation-controlled fracture it is usually formulated in terms of energy release rates and their critical values of the particular composite material. Intralaminar fracture toughness of unidirectionally reinforced glass/epoxy composite was experimentally determined at several mode I and mode II ratios. It is found that the crack propagation criterion, linear in terms of the energy release rates, reasonably well approximates the test results. The determined mixed mode cracking criterion was applied to predict intralaminar crack onset in cross-ply glass/epoxy composite under tensile loading. The predicted crack onset strain values agree with test results at small off-axes angles of the cracking ply (on-axis and 15° off-axis loading), but underestimate crack onset at larger reinforcement angles with respect to the loading direction. The discrepancy is likely to be caused by the deviation of linearity in laminate response before cracking onset in these laminates, related to non-linear shear characteristics of unidirectional plies. The applicability of strength-based fracture criterion for initiation-controlled cracking is discussed.

Design Optimization of Composite Plates: Part I—Design Criteria for Strength, Stiffness, and Manufacturing Complexity of Composite Laminates

Design criteria are essential for describing the objectives of optimization. Strength, stiffness, and manufacturing are addressed. The mechanical criteria, for strength and stiffness, provide with the magnitude by which the thickness of a given laminate must be scaled in order to just meet all requirements. For first-ply-failure strength, a quartic failure criterion was derived from the quadratic Tsai-Wu criterion, for unsymmetric and symmetric laminates subject to mixed loadsets. For stiffness, maximum displacement constraints can be selectively set on all components and modulus. Combined strength and stiffness control is also analyzed. While developed for single elements, these criteria are also suited for designing entire laminated plates. Assessing manufacturing complexity without undertaking a detailed cost analysis is achieved by an index which interprets numerically qualitative issues such as simple and complex layups. The new manufacturing complexity index takes into account variations in thickness, the steepness of plydrops and the number of zones across a structure.

Acoustic emission during tensile testing of SiC-fibre-reinforced BMAS glass-ceramic composites

One of the problems preventing the industrial application of ceramic-matrix composites is the lack of an efficient method to detect and discriminate among types of damage occurring during service. With this in mind, the mechanical response, damage development and acoustic emission activity during monotonic tensile testing of a BMAS glass-ceramic matrix reinforced with SiC fibres have been investigated. Damage initiation and propagation were easily detected and evaluated using the acoustic emission technique. Comparing the acoustic emission activity characteristics in simple lay-ups with those of more complex lay-ups allowed discrimination between matrix microcracking, matrix macrocracking accompanied by interface debonding, and delamination cracking. In this way, the paper contributes to the development of the acoustic emission technique for in situ monitoring of damage development in ceramic-matrix composites.