Thick Composite Laminates Research Articles

Integrating parametric HFGMC and isogeometric RZT[formula omitted] for multiscale damage modeling of composite structures: A numerical and experimental study

In this research effort, a novel multiscale analysis scheme is proposed for damage modeling of composite laminates, sandwich structures, and stiffened plates relying on capabilities of the parametric HFGMC and isogeometric RZT{3,2} formulations. The Ramberg Osgood (RO) model is incorporated into the micromechanics model to reflect polymer matrix material nonlinearities on the overall homogenized composite behavior. Carbon fibers are assumed to behave in a linear transversely isotropic manner. The higher order RZT{3,2} theory employed at the macro level facilitates efficient applicability of the model to thick composite laminates and soft core sandwiches. On the other hand, it generates all three-dimensional stress components and thus ensures dimensional consistency between micro and macro levels. Numerical discretization and prediction of RZT{3,2} kinematic variables are enabled by performing NURBS based isogeometric analysis (IGA) thereby enhancing modeling efficacy to a significant degree. Soft core plasticity and failure in the composite are evaluated at the macro level through the RO model and Hashin criteria, respectively. Applicability of the method is presented for thin and thick flat composite and sandwich laminates; and further extended to stiffened plates via developing a multipatch formulation. A comprehensive validation of our analysis is conducted by comparing the results with established benchmarks from the literature, experimental data, and three-dimensional finite element method (3D-FEM) simulations. Initially, a moderately thick, simply supported square laminate under transverse loading is examined, a common verification benchmark. Then, results from standard mechanical tests, including tensile, shear, and four-point bending tests on thin laminates, followed by experiments on moderately thick sandwich structures subjected to four-point bending, are presented. Finally, the analysis is extended to a stiffened plate under uniform pressure, demonstrating the method’s accuracy and applicability across diverse structural configurations.

Delamination bridging response of Z‐pins under mixed mode loading: The influence of carbon and Kevlar Z‐pins

AbstractThis paper presents the effect of varying the type of Z‐pin on the delamination bridging performance of unidirectional composite laminates. Carbon and Kevlar Z‐pins were inserted in thick composite laminates using the pre‐hole insertion Z‐pinning process and their bridging performance characterized across the different mode‐mixity range. The bridging response indicated that Kevlar Z‐pin experienced load reduction driven by interfacial friction (Stage III), which was absent in the Carbon Z‐pin. When Mode II dominated the bridging crack, Kevlar Z‐pin showed partial pullout and fiber shear cracking, while the Carbon Z‐pin exhibited clean transverse fracture. Although the bridging load obtained by the Carbon Z‐pin was twice that of Kevlar Z‐pin, the Kevlar Z‐pin provided a higher interlaminar fracture energy. The superior bridging performance of Kevlar Z‐pin is correlated with the enhanced ductility. Moreover, the resin‐rich area was involved in deformation and provided the extra axial load of Z‐pin.Highlights The comparison of mixed‐mode bridging mechanism between Kevlar Z‐pin and Carbon Z‐pin. Reveal the variation of multi‐directional load and energy consumption with mixed‐mode angle in thick laminates. Detailed scanning electron microscope observation to understand failure characteristics of ductile Z‐pin. Evidence that the extra axial load of Z‐pin provided by resin‐rich area.

Thermal runaway criterion for thick polymer composites

An analytical solution has been developed for the curing of thick polymer composite laminates that shows how the temperature profile responds to arbitrary changes to the material properties and process parameters and that curing with slow reactions and a low exotherm temperature is impossible if the Damköhler number is above a well-defined limit. The thermal runaway criterion can be recast as a criterion for the maximum allowable thickness of the laminate. The thermal runaway criterion was found to agree well with some results for thick laminates from the literature, but the peak temperature in the laminate was underpredicted for stable conditions. The model has a constant that can be adjusted to improve the peak temperature prediction, but more validation data is needed before the model can be optimized to simultaneously predict the peak temperature and thermal runaway with high accuracy.

Influence of fibre orientation parallel to impact direction on the impact response of unidirectional glass/epoxy composite: Experimental investigation on confinement and hybridisation

In a high-velocity impact phenomenon, the damage is localised, and for thick composite laminates, most of the projectile’s kinetic energy is absorbed through out-of-plane compression and shear mechanisms. In conventional composite laminate, as the fibre orientations are in the in-plane of laminate, the out-of-plane compression and out-of-plane shear of laminate are utilised. In composites, the out-of-plane compressive strength is significantly lower than the in-plane (fibre direction) strength. The present study investigates the impact response of unidirectional (UD) glass/epoxy composite targets with fibres oriented parallel to the impact direction. The lack of research on the impact behaviour of unidirectional glass/epoxy composites with fibre orientation parallel to the impact direction demands further experimentation and understanding of failure mechanisms. In this study, firstly, the impact response of UD glass/epoxy composites with fibres oriented parallel to the impact direction (GFID) was investigated and compared with conventional cross-ply (GFCP) and quasi-isotropic (GFQI) glass/epoxy laminates. Secondly, to address the issues of GFID splitting on impact, GFID was confined in both the hoop and normal-to-binder directions. Thirdly, the impact response of various hybrid GFID with and without confinement with GFCP as backing was evaluated. GFID under out-of-plane punch load fails by splitting, and its compressive participation depends on the impact velocity. GFID under the hoop and normal-to-binder direction confinement showed better specific impact energy absorption relative to pure GFID. The experimental results show that GFID wounds with carbon fibre used as a facing with GFCP as a backing provide better impact resistance at HVI conditions than conventional laminates of the same thickness. These findings suggest that the combination of GFID and GFCP targets can be used to have better impact resistance in various applications.

Static and Fatigue Characterization of Large Composite T-Bolt Connections in Marine Hygrothermal Environments

Fiber-reinforced polymer composites have been highlighted as ideal candidates for structural applications in marine renewable energy devices, such as tidal turbines and wave energy converters. It is well understood that harsh marine environments can cause strength degradation of composite laminates, which has been extensively researched at the coupon scale; however, no research has investigated how this translates into larger-scale composite structures. This paper presents a subcomponent-scale study which investigates the effects of hygrothermal aging and subsequent static and fatigue characterization of thick composite T-bolt connections as part of a large, multilaboratory materials research effort. Of the glass-reinforced epoxy and vinylester-epoxy matrix composites tested, both showed measurable static strength degradation (4–36%) after being hygrothermally aged, even though the composite specimens were only partially saturated with water. Under tension–tension fatigue loading, the epoxy specimens performed very well in their dry states but exhibited significant degradation after hygrothermal aging. In comparison, the vinylester-epoxy specimens had much shorter fatigue lives in their dry states but exhibited no degradation after hygrothermal aging. Overall, this research demonstrates that hygrothermal aging can have significant effects on the ultimate strengths and fatigue lives of even partially saturated thick composite T-bolt connections, indicating that degradation of the outer plies on thick composite laminates can have pronounced effects on the whole structure. It discusses the challenges of building an understanding of the effects of harsh marine environments in large-scale composite structures.

Application of Acoustic Metamaterials in Pulse-Echo Ultrasonic Evaluation of Thick Hybrid Composite Laminates

Significant challenges exist in inspecting thick composite laminates for manufacturing defects and operational damage. This is due to acoustic attenuation and impedance mismatch at the interface between the different composite layers. An innovative concept for enhancing ultrasonic testing of such composite laminates is introduced in this study. The proposed solution exploits the ability of acoustic metamaterials to cloak virgin composite. Herein, we show that by incorporating carefully designed metamaterials in a pulse-echo ultrasonic testing setup, the position and size of a delamination in a thick hybrid composite laminate can be determined accurately.

Sensitivity Analysis and Multi-Objective Optimization Strategy of the Curing Profile for Autoclave Processed Thick Composite Laminates.

To mitigate the risk of manufacturing defects and improve the efficiency of the autoclave-processed thick composite component curing process, parameter sensitivity analysis and optimization of the curing profile were conducted using a finite element model, Sobol sensitivity analysis, and the multi-objective optimization method. The FE model based on the heat transfer and cure kinetics modules was developed by the user subroutine in ABAQUS and validated by experimental data. The effects of thickness, stacking sequence, and mold material on the maximum temperature (Tmax), temperature gradient (ΔT), and degree of curing (DoC) were discussed. Next, parameter sensitivity was tested to identify critical curing process parameters that have significant effects on Tmax, DoC, and curing time cycle (tcycle). A multi-objective optimization strategy was developed by combining the optimal Latin hypercube sampling, radial basis function (RBF), and non-dominated sorting genetic algorithm-II (NSGA-II) methods. The results showed that the established FE model could predict the temperature profile and DoC profile accurately. Tmax always occurred in the mid-point regardless of laminate thickness; the Tmax and ΔT increased non-linearly with the increasing laminate thickness; but the DoC was affected slightly by the laminate thickness. The stacking sequence has little influence on the Tmax, ΔT, and DoC of laminate. The mold material mainly affected the uniformity of the temperature field. The ΔT of aluminum mold was the highest, followed by copper mold and invar steel mold. Tmax and tcycle were mainly affected by the dwell temperature T2, and DoC was mainly affected by dwell time dt1 and dwell temperature T1. The multi-objective optimized curing profile could reduce the Tmax and tcycle by 2.2% and 16.1%, respectively, and maintain the maximum DoC at 0.91. This work provides guidance on the practical design of cure profiles for thick composite parts.

Numerical simulation of inductive heating in thermoplastic unidirectional cross-ply laminates

Thermoplastic Composites can be re-melted allowing them to be joined via welding. This is an attractive alternative to conventional methods that are used to join thermoset composite parts such as mechanical fastening and adhesive bonding. In this work the inductive heating of uni-directional (UD) plies of thermoplastic carbon fiber reinforced polymer (CFRP) laminates is investigated. The focus is on developing a numerical electromagnetic and thermal simulation model that captures the main processes involved in eddy current generation and heat generation, in particular in the interface areas of the UD plies. A measurement technique has been developed to obtain the electric properties of the ply material. Furthermore, to support the modelling of both the induction heating equipment and work piece a field measurement of the magnetic field surrounding the coil and work piece has been developed. Inductive heating experiments were carried out on several thick composite laminate plates with different ply lay-ups to compare and validate the electro-magnetic-thermal simulation model. The measured surface temperatures were compared with the results from the simulation model. The results of this work can be used to support the design of UD-ply laminates to improve their ability to be welded via inductive heating. In addition, the results of this work can be used to assist in pre-determining induction welding equipment settings and heating times.

Size effect in curing optimization for thick composite laminates

Size effect in curing optimization for thick composite laminates

Physics-integrated neural differentiable (PiNDiff) model for composites manufacturing

Various manufacturing technologies are being developed to improve the manufacturing of composites owing to their low weight and high performance. The mechanical properties of the composites depend on various variables and parameters of the manufacturing process, which are challenging, if not impossible, to determine and optimize experimentally. Traditional first-principle modeling approaches are not accessible due to the complex physics involved. A hybrid model that combines incomplete physics knowledge with available measurement data within a differentiable programming framework opens up new avenues to tackle the challenges. In this work, a physics-integrated neural differentiable (PiNDiff) model is developed, where the partially known physics is integrated into the recurrent network architecture to enable effective learning and generalization. The merit and potential of the proposed method have been demonstrated in modeling the curing process of thick thermoset composite laminates, whose governing physics is partially given. The proposed PiNDiff model shows the capability to learn unknown physics from the limited, indirect data and, meanwhile, can be used to infer unobserved variables and parameters. The performance of the PiNDiff model has been compared with two state-of-the-art (SOTA) black-box deep learning models, and its advantages over the purely data-driven models and first-principles physics-based models have been discussed in detail. The demonstrated PiNDiff strategy may provide a general strategy to model phenomena where physics is only partially known and sparse, indirect data are available.

Online optimisation and active control of the cure process of thick composite laminates

This paper addresses the development of an active control tool for the in-situ optimisation of the cure process of thick composite parts. The methodology integrates a cure process model with real time monitoring data exploring potential temperature dwells that accelerate the curing while avoiding undesirable thermal gradients. The cure process simulation is based on a Finite Element (FE) model solution validated in the curing of a thick composite flat panel with an average absolute error of about 1.8 °C. The active control tool interacts PID controller of an oven setting a new cure temperature periodically. The implementation of active control tool during the curing of a thick carbon fibre/epoxy cable has the capability to determine in real time optimal solutions minimising process duration whilst also satisfying constraints in temperature overshoot. A process duration reduction of about 70 % can be achieved compared to a nominal cure cycle with a maximum temperature overshoot lower than 9 °C. The active control tool can replace off-line optimisation tools exploiting in situ with real time monitoring to drive the process and minimise the effects of uncertainty.

Automatic ply detection and finite element model generation for composite laminates

This paper presents a novel methodology to detect and isolate individual ply information contained within a relatively low-resolution cross-section image of thick composite laminate specimens. The proposed method can process laminate sample images and construct detailed geometric models in a fast and automated manner with minimal user interaction. The finite element models can be used directly for structural and strength simulations to analyse the effect of waviness defects. The algorithm processes the greyscale sample image and splits it into multiple slices. The initial starting points for each ply were identified by analysing the pixel brightness of the image. The pixel brightness variation was used to identify the different plies in all image slices and a list of possible ply centreline coordinate is generated. The ply centreline points are grouped and connected by selecting the points with minimal distance to the previous one in the ply. A finite element mesh is created for each ply by creating a boundary at the midpoint between two adjacent plies. The geometric information of the isolated plies is then used to create structured finite element models using an in-house meshing algorithm.

In-Plane and Oblique Edge-on Impact on Thick Glass-Fibre/Epoxy Composite Laminates

Thick composite laminates are often required in marine applications to resist high hydrodynamic forces. In this work, damage in thick glass-fibre/epoxy laminates caused by low velocity impacts was investigated experimentally and numerically. Cubic specimens 50 × 50 × 50 mm were manufactured, and the ply stacking directions were orientated at 0°, 22.5° and 45° to the impact plane. The impact damage was localised in the vicinity of the impactor contact area and included an interplay of fibre crushing, matrix cracks, matrix plasticity, and delaminations. Finite element modelling predicted the impact response and the type of damages. The model also quantified the primary energy absorption mechanisms which were by fibre crushing, matrix plasticity and propagation of delamination cracks. The highest impact damage resistance was obtained with the 0° (in-plane) specimen due to the fibres being aligned to the impact loading direction.

Damage prediction of thick composite laminates subjected to low-velocity impact loads

Experimental and numerical studies have been carried out to predict the behavior of composite laminates under low-velocity impact loads. Numerical models were built based on shell elements in order to simulate it efficiently. A strain-based failure theory was employed to predict the damaged area of laminates using a user-defined subroutine. Drop-weight impact tests were performed for verifying the analysis results of the low-velocity impact. The damaged areas were inspected using ultrasonic C-scans, and three types of thick composite laminates were examined. The impactor velocity was measured at time before and after contact with the target. Finally, the proposed numerical model with the failure theory was verified in that it well predicted the impact damage of the three composite laminates chosen for this study.

Effects of moisture ingression on the impact damage resistance and damage tolerance of alumina based oxide/oxide ceramic matrix composites

In this study the effects of moisture ingression on the impact damage resistance and damage tolerance of NextelTM610/alumina-silicate ceramic matrix composites were experimentally evaluated. Thin and thick composite laminates with balanced and symmetric layups were placed inside an environmental chamber at 80°C and 85% relative humidity till moisture saturation was reached. The laminates were then subjected to low velocity impacts using a hemispherical steel impactor at energy levels of 2.5 J and 10 J for the thin and thick laminates, respectively. This resulted in similar damage depths on the impactor side between the two sets of laminates. The resultant damage was characterized non-destructively with the damage metrics of external dent area, dent depth and planar internal damage area quantitatively determined. Impacted laminates were subject to in-plane compression loading and the residual strengths were evaluated. Planar internal damage area was larger for the moisture exposed laminates in comparison to their pristine counterparts for both the thin and thick laminates. Also, larger internal damage area resulted in lower compression-after-impact strength and thus, less damage tolerant laminates. This was true for the pristine and moisture exposed cases for both the laminate thicknesses.

Thick thermoplastic composite laminate consolidation: Experimental observations and numerical approaches

Since the benefits of composite materials have been demonstrated, particularly those relevant to the fields of transports and energy, increasingly high industrial expectations are emerging. As the control of the processes is complex, the thickness of the composite materials does not usually exceed a few millimetres. The aim of the work presented in this paper is to highlight new phenomena related to the oven/autoclave consolidation of thicker thermoplastic composites. In the case of such thick composites, the influence of certain phenomena that are usually neglected becomes important and a physical interpretation becomes necessary (the thermal contact resistance between mold and part and the influence of mold geometry). Here, an experimental approach is compared with numerical simulations of heat transfer and crystallization in order to validate the proposed thermokinetics model. Finally, some numerical observations are discussed and analyzed.

PREDICTION OF PROCESS-INDUCED DEFORMATION IN THICK COMPOSITE LAMINATES USING A PATH-DEPENDENT CONSTITUTIVE MODEL

PREDICTION OF PROCESS-INDUCED DEFORMATION IN THICK COMPOSITE LAMINATES USING A PATH-DEPENDENT CONSTITUTIVE MODEL

Theoretical and finite element approaches to model the phenomenon of high-velocity impact on GFRP plates

New energy-based theoretical models that predict the ballistic behaviour of thin and thick woven composite laminates are presented. These models are formulated for high-velocity impacts, where the boundary conditions do not play a relevant role. This can be assumed as the local structural behaviour is responsible for the ballistic performance. A non-dimensional formulation is used to analyse the influence of material properties and geometrical parameters in the ballistic response of the laminate. The models are physically-based on the energy contribution of different energy-absorption mechanisms. Moreover, a 3D finite element model is also developed by means of  a continuum damage mechanics model that takes into account the different failure mechanisms described in the theoretical models to simulate the performance of the laminate under high-velocity impacts and to validate the hypotheses of the theoretical models.  A comparison between FE and theoretical models is performed by means of energy absorption mechanisms. The predictive capability of the proposed models is verified against experimental results, which were originally conducted in this work. The results obtained show the dependencies between the ballistic response and the non-dimensional physical parameters of the model. Furthermore, the proposed models can be used to see the relative importance of the different energy-absorption mechanisms involved and the comparison of these mechanisms between the theoretical and the FE models can reflect the different roles played by them, depending on the material properties and geometrical characteristics of the laminate. These results highlight the relevance of the in-plane and out-of-plane energy-absorption mechanisms, which rule the penetration process depending on the plate thickness.

Predicting the upper-bound of interlaminar impact damage in structural composites through a combined nanoindentation and computational mechanics technique

Low-energy/speed impact damage of composite laminates is still challenging to simulate due to difficulties in measuring some key material properties. The present study develops an integrated numerical and experimental method for predicting interlaminar impact damage. A nanoindentation technique for measuring the stiffness properties of composites at a small length scale (nanometers) is leveraged to determine damage to composite laminates due to fast (microsecond) projectile impact. Specifically, nanoindentation is employed to measure the contact stiffness of an indenter and aerospace carbon/epoxy IM7/977-3 laminates with four different stacking sequences. Then, through a technique that combines nanoindentation and computational mechanics, an equivalent impact force approach is proposed to predict the upper-bound of interlaminar impact damage at impact energy levels of 5 and 10 Joules. Drop-weight impact experiments are conducted to validate the prediction results. In practical applications, estimating the upper-bound of damage is important for conservative and efficient damage tolerance designs, especially for thick composite laminates.

Multi-Objective Optimisation of Curing Cycle of Thick Aramid Fibre/Epoxy Composite Laminates.

Aramid fibre-reinforced epoxy composites (AF/EP) are promising materials in the aerospace, transportation, and civil fields owing to their high strength, high modulus, and light weight. Thick composite laminates are gradually being applied to large composite structures such as wind turbine blades. During curing, temperature overheating is a common problem in thick composites, which leads to matrix degradation, thermal residual stresses, and uneven curing. This paper proposes a signal-to-noise ratio (SNR) method to optimise the curing cycle of thick AF/EP laminates and reduce the overheating temperature. During curing, the temperature and strain evolution in a thick AF/EP laminate were monitored using fibre Bragg grating sensors. The effects of the curing factors on the overheating temperature of the thick AF/EP laminate were evaluated using the Taguchi method and predicted via the SNR method and analysis of variance. The results indicate that the dwelling temperature is the main factor affecting the overheating temperature. The optimal curing cycle involves an overheating temperature of 192.72 °C, which constitutes an error of 2.58% compared to the SNR method predictions. Additionally, in comparison to the initial curing cycle, the overshoot temperature in the optimised curing cycle was reduced by 58.48 °C, representing a reduction ratio of 23.28%.