Stiffness Degradation Of Laminates Research Articles

Prediction of matrix-cracking-induced stiffness degradation of cross-ply laminates based on data-driven method

A constitutive model of composite laminates with matrix cracking is essential for the design of liner-less composite vessels. This paper proposes a data-driven approach to predicting stiffness degradation in cross-ply laminates with matrix cracking. A pixelated expression of the stacking sequence was adopted to deal with the difference in the number of cracked plies, as the features for machine learning (ML) should be uniform. The data set for ML training is established by using finite element analysis (FEA), where the material properties, stacking sequences and crack densities are random. The normalised stiffness of the cracked plies of [0n/90m]s and [0/90m/00.5]s laminates is obtained by using the proposed ML method, and they are then compared with the available experiments, a micro-mechanical analytical model and FEA simulations. The ML method successfully predicted the stiffness degradation of the laminates with different numbers of cracked plies and further explored the application of data-driven methods for mechanics of composite materials.

Crack face sliding displacement (CSD) as an input in exact GLOB-LOC expressions for in-plane elastic constants of symmetric damaged laminates

The crack opening and crack sliding displacements of both faces of an intralaminar crack are the main parameters defining the significance of each crack in laminate stiffness degradation, according to the previously published GLOB-LOC approach for symmetric laminates with an arbitrary number of cracks in all plies. In the exact stiffness expressions of this approach, the crack density is always multiplied by crack opening displacement and crack sliding displacement. The dependence of crack opening displacement on geometrical and elastic parameters of adjacent plies was studied previously and described by simple fitting functions. The crack sliding displacement has been analyzed for low-crack densities only and the proposed finite element method-based fitting expressions are oversimplified not including the out-of-plane ply stiffness effects. Based on finite element method analysis, more accurate expressions for so-called non-interactive cracks are suggested in the presented article. For the first time the shear stress perturbations are analyzed and interaction functions are presented with the feature that they always lead to slightly conservative predictions. The presented simple fitting functions, when used in the GLOB-LOC model, give predictions that are in a good agreement with finite element method results and with experimental data for laminates with damaged off-axis plies in cases when crack face sliding is of importance. The significance of including crack sliding displacement in stiffness predictions is demonstrated.

Assessment of failure toughening mechanisms in continuous glass fiber thermoplastic laminates subjected to cyclic loading

Tensile fatigue behaviour of glass fiber/polyamide composites, including unidirectional ([0]8, [90]8) and cross-ply ([02/902]s, [04/904]s and [904/04]s) laminates, was studied and compared to that of similar glass fiber/epoxy composites. The fatigue resistance of cross-ply glass/polyamide was greater than that of glass/epoxy while also exhibiting lower stiffness reduction. To explain this key observation, residual stiffness and residual strength fatigue tests were performed on cross-ply laminates, while optical microscopy was used to measure ply crack density during the different stages of cycling. Testing of the cross-ply laminates at lower peak stresses of 50% of the ultimate tensile strength (i.e., high cycle fatigue regime) revealed partial cracks that did not propagate completely through the width and thickness of plies due to high matrix toughness and other observed toughening mechanisms such as matrix bridging. A micromechanical finite element model with explicit ply cracks was also used to predict laminate stiffness degradation corresponding to observed ply crack densities, revealing that stiffness degradation was overpredicted when cracks were assumed to span the entire specimen width. Additional finite element simulations with partial cracks showed notably less stiffness reduction. These observations suggest glass/polyamide is inherently more damage tolerant than glass/epoxy and may be a suitable replacement for fatigue critical structures.

Development of a synergistic damage mechanics model to predict evolution of ply cracking and stiffness changes in multidirectional composite laminates under creep

A physics-based multi-scale model that couples viscoelasticity and time-dependent damage evolution for general multidirectional laminates subjected to long-term creep loading is developed. The viscoelastic ply behavior is evaluated using a nonlinear Schapery-type viscoelastic model, while a methodology employed within the framework of classical laminate theory is used to predict the corresponding laminate time-dependent response. The evolution of microscopic ply cracks in multiple plies with different orientations during creep loading is predicted using an energy-based approach, and the corresponding laminate stiffness degradation is evaluated using a synergistic damage mechanics-based model that relies on computational micromechanics in lieu of costly experimental data. The developed model is used to predict the evolution of ply cracks and the viscoelastic stress–strain response of various cross-ply and multidirectional laminates under quasi-static and creep loading. Predicted strains, compliance changes and crack density evolutions show excellent agreement with available experimental creep data for different laminate stacking sequences, providing validation for the model. Predictions are also made for two additional multidirectional laminates in order to investigate the effect of off-axis ply angle on the creep response, which demonstrates the versatility of the model and its usefulness in assessing the long-term durability of general multidirectional composite laminates.

Effective transverse modulus of a damaged layer: Potential for predicting symmetric laminate stiffness degradation

The old concept of the effective stiffness of a 90-layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates. It is shown, focusing on the effective transverse modulus, that the expected dependence of this property on composite elastic properties and laminate lay-up is very weak and follows very simple rules. Calculations show that the effective longitudinal modulus and Poisson’s ratio of the layer are not affected at all by intralaminar cracking. Simple fitting curve for effective transverse modulus change with normalized crack density is obtained from analysis of GF/EP cross-ply laminate. It is shown, comparing with FEM results and experimental data, that this expression can be used as a ‘master curve’ in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different unidirectional composites.

Monitoring fatigue-induced transverse matrix cracks in laminated composites using built-in acousto-ultrasonic techniques

This work investigated the use of ultrasonic wave propagation in a composite laminate to monitor transverse matrix cracking. It was found that the change in the signal’s power spectral density correlates to crack density increase, and its sensitivity was dependent on the actuator to sensor orientation. This orientation dependence was found to be the same as that of the laminate’s stiffness degradation. A crack density damage index model was developed that relates signal changes to laminate stiffness degradation due to transverse matrix cracks. With this damage index model, a matrix cracking monitoring system was proposed, implemented, and validated. The developed system requires a calibration test for the composite layup of interest and a minimum of one actuator and two sensors at different angle orientations. The calibration test and general approach steps are described and validated. The system’s crack density estimates match experimentally measured crack density values within established error bounds.

Critical stiffness damage envelopes for multidirectional laminated structures under multiaxial loading conditions

Load-bearing laminated structures undergo complex crack evolution processes and consequent stiffness degradation, which leads to a decline in their overall performance. Thus, consideration of damage evolution is important for stiffness critical applications. In this study, a multi-scale damage model combining synergistic damage mechanics (SDM) with an energy-based damage evolution model is developed for multidirectional laminated structures. The SDM approach combines computational finite element-based micromechanics with continuum damage mechanics, enabling the evaluation of the laminate stiffness. The damage model predicts evolution of sub-critical matrix cracks in different plies under multiaxial loading. Model predictions, which include ply crack density evolution and laminate stiffness degradation, correlate well with available experimental data, while model results for various CFRP and GFRP multidirectional laminates demonstrate its robustness. Additionally, damage envelopes corresponding to pre-selected critical stiffness degradation levels are developed to serve as an alternative to current approaches of final failure envelopes, and are regarded as a useful design tool for stiffness critical composite structures in practical applications.

Research on the Fiber Breakage Damage in Fiber Reinforced Polymer Laminate

In this paper, the single ply fiber breakage model is established based on the distribution function of fiber static strength, and on this basis, the acting process of fiber breakage in FRP laminate is analyzed in detail. According to the experiment data, a method is devised to identify the parameters of the static strength distribution function. Through the references to the classical laminate theory, a computation model of the laminate stiffness degradation process is developed which takes the fiber breakage damage into consideration. The analysis of the results shows that this model can employ only several parameters to describe very well the laminate stiffness degradation process under different stresses.

Effect of Delaminations on Opening of Transverse Matrix Cracks

The use of advanced composite materials for cryogenic fuel tanks reduces the mass of the structure but the leakage of fuel can become a problem. Since the amount of leakage increases with the crack opening, it is essential that we understand the factors that affect the crack opening. In previous studies by the authors, the opening and the degradation behavior of lamina properties due to transverse matrix cracking had been studied extensively. It has been found that the opening of transverse matrix cracks can be directly related to changes in effective moduli. This paper extends the earlier work to include the additional opening due to the delaminations induced at the tips of the transverse matrix crack. Finite element models have been built for seven delamination configurations. The opening and the stiffness degradation of laminates have been obtained as functions of the crack density and the delamination length. The effect of some factors such as adjacent ply orientation, ply thickness, and temperature on the opening have been studied. It has been found that the crack opening due to transverse crack-induced delaminations is still directly related to reduction in effective moduli. The opening increases almost linearly with the delamination length. A simple formula has been developed to predict the opening. The prediction of the opening obtained using this formula agreed very well with the finite element predictions.