S.S.R. Koloor

Elastic-damage deformation response of fiber-reinforced polymer composite laminates with lamina interfaces

The single-layer and multi-layer finite element models are developed and examined for adequacy in predicting the elastic-damage response of fiber-reinforced polymer composite laminates. A new experimental-computational approach featuring a two-tier mesh convergence analysis of the finite element models is developed. A 12-ply carbon fiber-reinforced polymer composite laminate beam specimen with anti-symmetric layups is designed and loaded to induce matrix damage under significant deflection without catastrophic fracture. A constitutive model incorporating Hashin’s equations for damage initiation criteria, along with an energy-based damage propagation law is employed in the finite element simulation. The results shows that the multi-layer finite element model predicts well the load–deflection curve of the carbon fiber-reinforced polymer composite laminate, while the single-layer model overestimates the elastic flexural stiffness of the specimen by 47 %. During the flexural deformation, matrix damage initiates in the central and edge regions of the critical laminas under compressive and tensile stresses, respectively. The multi-layer finite element model also predicted the matrix-induced interface delamination along the edges of the critical laminas under tension, as observed experimentally. The model demonstrates the adequacy in representing the role of lamina interface in dictating the elastic-damage response of carbon fiber-reinforced polymer composite laminates manufactured by prepreg layups method.

Effect of Strain-Rate on Flexural Behavior of Composite Sandwich Panel

In the past few decades, Composite Sandwich Panel (CSP) technology significantly influenced the design and manufacturing of high performance structures. Although using CSP increases the reliability of structure, the important concern is to understand the complex deformation and damage evolution process. This study is focused on the mechanical behaviour of CSP under flexural loading condition. A setup of three-point bending test is prepared using three support span of 40, 60 and 80 mm. The loading was controlled by three different displacement rates of 1, 10 and 100 mm per minute to examine the effects of strain-rate on bending behaviour of CSP material. The beam span significantly affects the flexural stiffness of CSP panel. The load-deflection response of the panel shows two different portions, that representing equivalent elastic and plastic regions in both the core and facesheets components of CSP. The non-combustible mineral-filled core appears to be nonlinear in the elastic region, at high loading rate. Consequently the failure occurs as the core/facesheets interface suffers debonding.

Mechanics of Composite Delamination under Flexural Loading

The mechanics of interface delamination in CFRP composite laminates is examined using finite element method. For this purpose a 12-ply CFRP composite, with a total thickness of 2.4 mm and anti-symmetric ply sequence of [45/-45/45/0/-45/0/0/45/0/-45/45/-45] is simulated under three-point bend test setup. Each unidirectional composite lamina is treated as an equivalent elastic and orthotropic panel. Interface behavior is defined using damage, linear elastic constitutive model and employed to describe the initiation and progression of delamination during flexural loading. Complementary three-point bend test on CFRP composite specimen is performed at crosshead speed of 2 mm/min. The measured load-deflection response at mid-span location compares well with predicted values. Interface delamination accounts for up to 46.7 % reduction in flexural stiffness from the undamaged state. Delamination initiated at the center mid-span region for interfaces in the compressive laminates while edge delamination started in interfaces with tensile flexural stress in the laminates. Anti-symmetric distribution of the delaminated region is derived from the corresponding anti-symmetric ply sequence in the CFRP composite. The dissipation energy for edge delamination is greater than that for internal center delamination. In addition, delamination failure process in CFRP composite can be described by an exponential rate of fracture energy dissipation under monotonic three-point bend loading.

Linear Elastic Analysis of Bovine Cortical Bone under Compression Loading

Linear elastic response of the bovine cortical bone has been examined under compression load. Experimental and computational methods were used to observe and predict the response of cortical bone. In computational method, two mechanical behaviors of isotropic and orthotropic were considered to simulate the cortical bone deformation. In experimental process, the specimens were designed to show maximum stiffness and strength by specifying osteon direction along loading axis during tests. The tests were controlled by displacement rate of 0.5 mm/minute and the overall stiffness responses of the structures were recorded to extract mechanical properties and also for validation aims. Finite Element Method (FEM) was used to model the linear response of the structure by using ABAQUS6.9EF. The FE results using orthotropic definition shows a good correlation with experimental data. A discussion was given based on overall stiffness and effective stress variation for both mechanical behaviors. In order to design the optimal implant structure, the presented study was proposed for prediction of bone structure deformation that attached to the orthopedic implants.

Effect of Strain Rate Upsetting Process on Mechanical Behaviour of Epoxy Polymer

The advantages of polymer materials such as high strength and stiffness to weight ratio, corrosion resistance and manufacturing flexibility have increased the industry demands to utilize them in high performance applications. Designing polymer structures depends on a high understanding of their hyper-elastic behaviour, therefore investigating the mechanical behaviour of polymers is necessary. In this paper, the nonlinear behaviour of epoxy polymer is examined under upsetting test. The main aim of the study is to analyse the effect of strain rate on the mechanical behaviour of epoxy polymer. The cylindrical polymer epoxy specimen, 20mm in length and in diameter, was manufactured. The upsetting tests provided quasi-static compressive loads which were adjusted in the loading rates of 0.1, 1, 50, 100, 200 and 500 mm/min. The loadings were continued until complete fracture was observed. Each loading rate was repeated for at least 3 specimens to ensure a reasonably good statistical sampling. The average data of each test is used to produce the load-displacement graphs of the specimens, from which stress-strain curves are extracted to show the behaviour of epoxy polymer. The results show a 37% increase of yield stresses when the loading rate is increased from 0.1 to 500 mm/min and the yield strains increase by 26%. The stress-strain curves are nonlinear where the slope increases when the loading rate is raised from 0.1 to 100 mm/min but then decreases when the rate is further raised from 100 to 500 mm/min. The maximum load that can be sustained is increased with loading rate. This can be due to the microstructure deformation response of epoxy polymer. This polymer is categorised as large-strain material by showing exhibiting large deformations under different rates of compression loading.

Effect of Ply Thickness on Displacements and Stresses in Laminated GFRP Cylinder Subjected to Radial Load

The superior feature of composites such as high stiffness against low density have impelled engineers to use this material in automotive, aerospace and building industries. In the past few decades, composites shell has found applications in storage tanks and transmission pipelines. Designing laminated composite shells is challenging because of the complex mechanical behavior when combining laminate and shell theories. In this paper, the study is focused on the effect of lamina thickness on performance of the GFRP cylinder. For this purpose two 12-ply GFRP cylinders are considered with ply sequences of [0/90/45]s. The lamina thicknesses of the composite shell are assumed to be 0.1, 0.5, 1 and 1.5 mm, to evaluation of the mechanical behaviors of the cylinders and identifying one with the highest strength. The 250 mm diameter cylinders are subjected to a uniform radial patch load. A code is written for the solution based on the shell theory and classical mechanics of laminated composite using MATLAB software. The results are validated by comparing the present results with those found in literature. A good correlation justifies the study being extended to include the study on the effect of ply and shell thickness. The procedure is recommended for design and optimization for strength of various sizes of composite pipes

Mechanical Behavior of GFRP Laminated Composite Pipe Subjected to Uniform Radial Patch Load

Cylindrical vessels are widely used for storage and transportation of fluids. Using composites shells can improve the corrosion resistance of the product and reduce weight therefore investigation of the mechanical behavior is important. For this purpose cylinders with 6, 12 and18-ply of GFRP , with symmetric ply sequence of [90/0/90]s, [90/0/90/0/90/0]s and, [90/0/90/0/90/0/90 /0/90]s with layer thickness 1.3 mm and mean radius 250 mm, are considered under uniform radial patch load. The analysis was based on the shell theory and classical mechanics of laminated composites. A code was written using MATLAB software to compute stress and deflection of the cylinder shell. In numerical simulation, each unidirectional composite ply is treated as an equivalent elastic and orthotropic panel. Analysis is focused on the area of cylinder where the patch load is applied. The results show that the analytical prediction compares well with numerical responses of previous literature. The procedure can be used to predict maximum stress and displacement in a multi-layer shell for various types of similar loading.

Evolution Characteristics of Delamination Damage in CFRP Composites Under Transverse Loading

The initiation and subsequent progression of delamination in CFRP composite laminates is examined using finite element method. A 12-ply CFRP composite, with a total thickness of 2.4 mm and anti-symmetric ply sequence is simulated under three-point bend test setup. Each unidirectional composite lamina is treated as an equivalent elastic and orthotropic panel. Interface behavior is defined using cohesive damage model. Complementary three-point bend test on the specimen is performed at crosshead speed of 2 mm/min. The measured load–deflection response at mid-span location compares well with predicted values. Interface delamination accounts for up to 46.7% reduction in flexural stiffness from the undamaged state. Delamination initiated at the center mid-span region for interfaces in the compressive laminates while edge delamination started in interfaces with tensile flexural stress in the laminates. Anti-symmetric distribution of the delaminated region is derived from the corresponding anti-symmetric ply sequence in the CFRP composite. The dissipation energy for edge delamination is greater than that for internal center delamination.

Assessment of compressive failure process of cortical bone materials using damage-based model

The main failure factors of cortical bone are aging or osteoporosis, accident and high energy trauma or physiological activities. However, the mechanism of damage evolution coupled with yield criterion is considered as one of the unclear subjects in failure analysis of cortical bone materials. Therefore, this study attempts to assess the structural response and progressive failure process of cortical bone using a brittle damaged plasticity model. For this reason, several compressive tests are performed on cortical bone specimens made of bovine femur, in order to obtain the structural response and mechanical properties of the material. Complementary finite element (FE) model of the sample and test is prepared to simulate the elastic-to-damage behavior of the cortical bone using the brittle damaged plasticity model. The FE model is validated in a comparative method using the predicted and measured structural response as load-compressive displacement through simulation and experiment. FE results indicated that the compressive damage initiated and propagated at central region where maximum equivalent plastic strain is computed, which coincided with the degradation of structural compressive stiffness followed by a vast amount of strain energy dissipation. The parameter of compressive damage rate, which is a function dependent on damage parameter and the plastic strain is examined for different rates. Results show that considering a similar rate to the initial slope of the damage parameter in the experiment would give a better sense for prediction of compressive failure.

Hyperelastic Analysis of High Density Polyethylene under Monotonic Compressive Load

In the past few decades, design development of high performance machines and devices encouraged industries to utilize advanced materials such as polymers. Special mechanical features of polymer materials such as high strength to weight ratio and etc have increased scientists demands to investigate the nonlinear behaviour of polymers. One of the challenges in mechanic of polymers is to introducing a model that is competent to predict hyperelastic deformations based on long-strain behaviour of polymers. In this study, a comprehensive research is performed on introduced mechanical models for polymer materials. The major attempt was on introducing an appropriate model among the existing models, capable enough to predict mechanical behaviour of high density polyethylene under monotonic compressive load. The procedures of simulation and experimental tests are performed to examine the load capability of the model for high density polyethylene. Several compression tests are performed on High Density Polyethylene cubic specimens to extract the full stress-strain response of the high density polyethylene. Moreover, strain gauge is used in experimental tensile test to determine Poisson’s ratio. Simulation procedure is performed using ABAQUS 6.9EF for a comprehensive analysis and discussion on hyperelastic deformation. The simulation procedure is confirmed and verified perfectly by experimental data. The comparison between experimental result of compression test under monotonic load and finite element simulation of this test is remarkable to know about behaviour of HDPE to use in other structural and mechanical application.