Low-velocity impact damage can drastically reduce the residual strength of a composite structure even when the damage is barely visible. The ability to computationally predict the extent of damage and Compression-After-Impact (CAI) strength of a composite structure can potentially lead to the exploration of a larger design space without incurring significant time and cost penalties. A high-fidelity three-dimensional composite damage model, to predict both low-velocity impact damage and CAI strength of composite laminates, has been developed and implemented as a user material subroutine in the commercial finite element package, ABAQUS/Explicit. The intralaminar damage model component accounts for physicallybased tensile and compressive failure mechanisms, of the fibres and matrix, when subjected to a threedimensional stress state. Cohesive behaviour was employed to model the interlaminar failure between plies with a bi-linear traction-separation law for capturing damage onset and subsequent damage evolution. The virtual tests set up in ABAQUS/Explicit were executed in three steps, one to capture the impact damage, the second to stabilize the specimen by imposing new boundary conditions required for compression testing and the third to predict the CAI strength. The observed intralaminar damage features, delamination damage area as well as residual strength are discussed. It is shown that the predicted results for impact damage and CAI strength correlated well with experimental testing without the need of model calibration which is often required with other damage models.
A woven Five-Harness Satin (5HS) weave with AS4 carbon fibres, and unidirectional high strength IMS60 carbon fibres were used to manufacture hybrid laminates, using resin infusion, to assess their performance in low velocity impact tests. Load/energy-time curves and load-displacement curves were extracted from the experimental data, and non-destructive Cscanning was performed on all pre-and post-impacted specimens to quantify the extent of damage incurred. A finite element-based computational damage model was developed to predict the material response of these hybrid unidirectional/woven laminates. The intralaminar damage model formulation, by necessity, consists of two sub-models, a unidirectional constitutive model and a woven constitutive model. The built-in surface-based cohesive behaviour in Abaqus/Explicit was used to define the interlaminar damage model for capturing delamination. The reliability of this model was validated using in-house experimental data obtained from standard drop-weight impact tests. The simulated reactionforce and absorbed energy showed excellent agreement with experiment results. The post-impact delamination and permanent indentation deformation were also accurately captured. The accuracy of the damage model facilitated a quantitative comparison between the performance of a hybrid unidirectional/woven (U/W) laminates and a pure unidirectional (PU) carbon-fibre reinforced composite laminates of equivalent lay-up. The hybrid laminates were shown to yield better impact resistance.
This study aims to evaluate the applicability of failure criteria and damage evolution methods in the finite element analysis of composite laminates under lowvelocity impact. Implemented by the user-defined VUMAT subroutine in ABAQUS, various progressive damage models are used to predict damage initiation and accumulation in a T700GC/M21 composite laminate. Cohesive elements are inserted between adjacent plies to capture interface delamination. The applicability of damage models is investigated by comparing the global mechanical response and distribution of various damage modes. A new variable, equivalent damage volume, is introduced to quantitatively describe the predicted damage when using different models. The numerical results establish that Hashin and Puck failure criteria generate matrix compression damage in more layers of the composite. Maximum stress and Tsai-Wu criteria are not preferred due to their improper predictions in terms of damage area and permanent deformation of the laminate. As for damage evolution laws, the equivalent strain method provides faster stiffness degradation of the laminate and a smaller area 2 of matrix damage compared with the predictions of the equivalent displacement method.
The development of the latest generation of wide-body passenger aircraft has heralded a new era in the utilisation of carbon-fibre composite materials. One of the primary challenges facing future development programmes is the desire to reduce the extent of physical testing, required as part of the certification process, by adopting a 'certification by simulation' approach. A hierarchical bottom-up multiscale simulation scheme can be an efficient approach that takes advantage of the natural separation of length scales between different entities 2 (fibre/matrix, ply, laminate and component) in composite structures. In this work, composites with various fibre/matrix and interlaminar interfacial properties were fabricated using an autoclave under curing pressures ranging from 0 to 0.8 MPa. The microstructure (mainly void content and spatial distribution) and the mechanical properties of the matrix and fibre/matrix interface were measured, the latter by means of nano-indentation tests in matrix pockets, and fibre push-in tests. In addition, the macroscopic interlaminar shear strength was determined by means of three-points bend tests on short beams. To understand the influence of interfacial properties on the intralaminar failure behaviour, a high-fidelity microscale computational model is presented to predict homogenized ply properties under shear loading. Predicted ply material parameters are then transferred to a mesoscale composite damage model to reveal the interaction between intralaminar and interlaminar damage behaviour of composite laminates.
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