Himayat Ullah

Experimental and Numerical Analysis of Damage in Woven GFRP Composites Under Large-deflection Bending

Textile-reinforced composites such as glass fibre-reinforced polymer (GFRP) used in sports products can be exposed to different in-service conditions such as large bending deformation and multiple impacts. Such loading conditions cause high local stresses and strains, which result in multiple modes of damage and fracture in composite laminates due to their inherent heterogeneity and non-trivial microstructure. In this paper, various damage modes in GFRP laminates are studied using experimental material characterisation, non-destructive micro-structural damage evaluation and numerical simulations. Experimental tests are carried out to characterise the behaviour of these materials under large-deflection bending. To obtain in-plane shear properties of laminates, tensile tests are performed using a full-field strain-measurement digital image correlation technique. X-ray micro computed tomography (Micro CT) is used to investigate internal material damage modes – delamination and cracking. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus to study the deformation behaviour and damage in GFRP. In these models, multiple layers of bilinear cohesive-zone elements are employed to study the onset and progression of inter-ply delamination and intra-ply fabric fracture of composite laminate, based on the X-ray Micro CT study. The developed numerical models are capable to simulate these features with their mechanisms as well as subsequent mode coupling observed in tests and Micro CT scanning. The obtained results of simulations are in agreement with experimental data.

Analysis of Nonlinear Shear Deformations in CFRP and GFRP Textile Laminates

Carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) woven composites are widely used in aerospace, automotive and construction components and structures thanks to their lower production costs, higher delamination and impact strengths. They can also be used in various products in sports industry. These products are exposed to different in-service conditions such as large tensile and bending deformations. Composite materials, especially ±45° symmetric laminates subjected to tensile loads, exhibit significant material as well as geometric non-linearity before damage initiation, particularly with respect to shear deformations. Such a nonlinear response needs adequate means of analysis and investigation, the major tools being experimental tests and numerical simulations. This research deals with modelling the nonlinear deformation behaviour of CFRP and GFRP woven laminates subjected to in-plane tensile loads. The mechanical behaviour of woven laminates is modelled using nonlinear elasto-plastic as well as material models for fabrics in commercial finite-element code Abaqus. A series of tensile tests is carried out to obtain an in-plane full-field strain response of [+45/-45]2s CFRP and GFRP laminates using digital image correlation technique according to ASTM D3518/D3518M-94. The obtained results of simulations are in good agreement with experimental data.

Analysis of nonlinear deformations and damage in CFRP textile laminates

Carbon fibre-reinforced polymer (CFRP) textile composites are widely used in aerospace, automotive and construction components and structures thanks to their relatively low production costs, higher delamination and impact strength. They can also be used in various products in sports industry. These products are usually exposed to different in-service conditions such as large bending deformation and multiple impacts. Composite materials usually demonstrate multiple modes of damage and fracture due to their heterogeneity and microstructure, in contrast to more traditional homogeneous structural materials like metals and alloys. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These damage modes need adequate means of analysis and investigation, the major approaches being experimental characterisation, numerical simulations and microtomography analysis. This research deals with a deformation behaviour and damage in composite laminates linked to their quasi-static bending. Experimental tests are carried out to characterise the behaviour of woven CFRP material under large-deflection bending. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behaviour and damage in woven CFRP laminates. Multiple layers of bilinear cohesive-zone elements are employed to model the onset and progression of inter-ply delamination process. X-ray Micro-Computed Tomography (MicroCT) analysis is carried out to investigate internal damage mechanisms such as cracking and delaminations. The obtained results of simulations are in agreement with experimental data and MicroCT scans.

Damage in woven CFRP laminates subjected to low velocity impacts

Carbon fabric-reinforced polymer (CFRP) composites used in sports products can be exposed to different in-service conditions such as large dynamic bending deformations caused by impact loading. Composite materials subjected to such loads demonstrate various damage modes such as matrix cracking, delamination and, ultimately, fabric fracture. Damage evolution in these materials affects both their in-service properties and performance that can deteriorate with time. These processes need adequate means of analysis and investigation, the major approaches being experimental characterisation and non-destructive examination of internal damage in composite laminates. This research deals with a deformation behaviour and damage in woven composite laminates due to low-velocity dynamic out-of-plane bending. Experimental tests are carried out to characterise the behaviour of such laminates under large- deflection dynamic bending in un-notched specimens in Izod tests using a Resil Impactor. A series of low-velocity impact tests is carried out at various levels of impact energy to assess the energy absorbed and force-time response of CFRP laminates. X-ray micro computed tomography (micro-CT) is used to investigate material damage modes in the impacted specimens. X-ray tomographs revealed that through-thickness matrix cracking, inter-ply delamination and intra-ply delamination, such as tow debonding and fabric fracture, were the prominent damage modes.

Numerical analysis of the interactive damage mechanisms in two-dimensional carbon fabric-reinforced thermoplastic composites under low velocity impacts

Damage and failure in carbon fabric-reinforced polymer (CFRP) composites under low velocity impacts is investigated using explicit finite element analyses. Three-dimensional finite element models are developed to simulate the deformation behaviour of, and damage evolution in CFRP laminates under such loading conditions. In these simulations, the onset and growth of inter-ply delamination is captured by bilinear cohesive zone elements inserted between plies of the composite. Intra-ply fabric fracture is simulated by defining a transverse layer of cohesive elements at the specimen fracture location. The energies associated with dynamic propagation of these damage mechanisms are also captured by the numerical simulations, demonstrating their potential to model the damage modes as well as their interaction. In this study, a novel damage modelling technique based on the cohesive zone method is proposed for interaction of various damage modes, which is more efficient for coupling between failure modes than a continuum damage mechanics approach. For the first time, it was observed that the pattern of interlaminar damage formation was from the front interface towards the back of the specimen, unlike the traditional back to front one in drop-weight tests. Results of experimental tests performed on a woven CFRP material under low velocity impact in Izod-type impactor at various energy levels are used for comparison with simulations. A satisfactory agreement was found between experiments and simulations confirming the validity of the numerical analyses.

Fracture mechanics based fatigue life estimation of axial compressor blade

Axial flow compressors are used in various power plants employing gas turbine engines. The engine components usually experience fatigue damage due to many cycles of stress reversals as well as defects. It is necessary to predict the life of such components subjected to dynamic loadings and also to investigate allowable crack size present in the components to avoid sudden failure of system. To assess the fatigue crack propagation quantitatively, basic fatigue life estimation is carried out theoretically for failure in both high and low stress regions in axial compressor rotor. Under different damage spectra, the engine components have different fatigue life depending upon the size, location and stress levels at flaw location. As first iteration, fatigue life of axial compressor rotor of AA 2618 material, through crack propagation phenomenon is studied analytically employing finite element analysis of component without flaw. Various theories regarding fatigue life are used to estimate crack growth in components. Paris law used in this study predicts the life of component by integrating initial and final flaw size, calculating rate of crack growth as a function of cycles. Rate of crack growth as a function of cycles is dependent to the stress intensity factor range for a particular material. In second iteration, crack propagation and fatigue life estimation is carried out through finite element analysis by introducing crack in finite element model at middle of the blade height. Study resulted in estimation of critical crack length and life of component in the presence of crack. Also abrupt rise in stress levels due to various sizes of geometric discontinuities (crack) are recorded.

Analysis of impact induced damage in composites for wind turbine blades

Glass fabric-reinforced polymer (GFRP) composites used in wind turbine blades are usually exposed to large-deflection bending impacts caused by wind storms, heavy rainfall, water splashes and hailstones in the offshore; and sand and dust impingement in the desert environments. Such loadings can cause deterioration of structural integrity and load-bearing capacity of the blade structure due to induced damage in the form of matrix cracking, delamination and fibre fracture. These types of damage mechanisms become more detrimental and pose a threat to the fatigue life of the turbine blades. In this work, first the load-bearing and energy absorbing capability of woven GFRP laminates is investigated under impact loading. Experimental tests are conducted to characterise the behaviour of GFRP composites under large-deflection dynamic bending in Izod type impact tests using Resil impactor. Impact tests are performed at various energy levels to determine the ultimate fracture toughness of the laminates. In these tests, the material demonstrated interply delamination damage due to weaker matrix at low energy levels. At higher impact energies, apart from delamination, the material also exhibited permanent deflection instead of catastrophic fabric fracture. The latter was due to the visco-elasto-plastic nature of the glass fibres apart from the thermoplastic matrix. The deformation behaviour and delamination damage ensued by dynamic loading is also studied by developing three-dimensional finite element (FE) model in Abaqus/Explicit commercial package. In FE model, multiple layers of bilinear cohesive-zone elements are defined at the damage locations. Stress-based criteria and fracture-mechanics techniques are used to assess damage initiation and its progression, respectively. Numerical results gave good correlation when compared to the dynamic response observed in experiments. The methodology developed here can be employed in damage tolerant design of wind turbine composite blades subjected to similar impact loading conditions.

Damage and fracture in fabric-reinforced composites under quasi-static and dynamic bending

Fabric-reinforced polymer composites used in sports products can be exposed to different in-service conditions such as large deformations caused by quasi-static and dynamic loading. Composite materials subjected to such bending loads can demonstrate various damage modes – matrix cracking, delamination and, ultimately, fabric fracture. Damage evolution in composites affects both their in-service properties and performance that can deteriorate with time. Such behaviour needs adequate means of analysis and investigation, the main approaches being experimental characterisation and non-destructive examination of internal damage in composite laminates. This research deals with a deformation behaviour and damage in carbon fabric-reinforced polymer (CFRP) laminates caused by quasi-static and dynamic bending. Experimental tests were carried out to characterise the behaviour of a CFRP material under large-deflection bending, first in quasi-static and then in dynamic conditions. Izod-type impact bending tests were performed on un-notched specimens of CFRP using a Resil impactor to assess the transient response and energy absorbing capability of the material. X-ray micro computed tomography (micro-CT) was used to analyse various damage modes in the tested specimens. X-ray tomographs revealed that through-thickness matrix cracking, inter-ply and intra-ply delamination such as tow debonding, and fabric fracture were the prominent damage modes both in quasi-static and dynamic test specimens. However, the inter-ply damage was localised at impact location in dynamically tested specimens, whereas in the quasi-static specimens, it spread almost over the entire interface.

Dynamic large-deflection bending of laminates

Abstract Composite laminates employed in various sports products are usually subjected to large-deflection dynamic bending during their service. The chapter first describes the experimental characterisation of composites as well as investigation of various damage modes ensued under these loading conditions replicated by means of Izod-type dynamic tests. It then describes the development of three-dimensional finite-element models employing a cohesive-zone method to study the onset, progression and interaction of some damage modes observed experimentally. The developed numerical models are capable of simulating damage mechanisms in laminates and their interaction observed in the tests.

Damage analysis of carbon fabric-reinforced composites under dynamic bending

Fabric-reinforced polymer composites used in various applications can be subjected to dynamic loading such as impacts causing bending deformations. Under such loading scenarios, composite structures demonstrate multiple modes of damage and fracture if compared with more traditional, macroscopically homogeneous, structural materials such as metals and alloys. Among damage and fracture modes are fibre breaking, transverse matrix cracking, debonding between fibres and matrix and delamination. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These failure modes need adequate means of analysis and investigation, the major approaches being experimental characterization and numerical simulations. This study deals with analysis of damage in carbon fabric-reinforced polymers (CFRP) under dynamic bending. The properties of, and damage evolution in, the composite laminates were analysed using a combination of mechanical testing and microstructural damage analysis using optical microscopy. Experimental tests are carried out to characterize the behavior of CFRP composites under large-deflection dynamic bending in Izod type impact tests using Resil Impactor. A series of impact tests is carried out at various energy levels to obtain the force-time diagrams and absorbed energy profiles for laminates. Three-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behavior and damage in composites for cases of dynamic bending. In these models, multiple layers of bilinear cohesive-zone elements are placed at the damage locations identified in microscopic study. Initiation and progression of inter-ply delamination at the impact and bending locations is studied numerically by employing cohesive-zone elements between each ply of the composite. Stress-based criteria are used for damage initiation, and fracture-mechanics techniques to capture its progression in composite laminates. The developed numerical models are capable to simulate these damage mechanisms as well as their subsequent interaction observed in tests and microscopy. Simulations results showed a good agreement when compared to experimentally obtained transient response of the woven laminates.