Nuri Ersoy

Measurement of Residual Stresses in Layered Composites by Compliance Method

Macroscopic residual stresses in layered composites were investigated by paying particular attention to implement a reliable experimental technique for the measurement of through-the-thickness residual stresses in layered composite plates. Residual stresses in (010/9010) S APC-2 laminate have been measured by two experimental techniques, i.e., by layer removal method, and by compliance method. The latter has been successfully employed up to date for the measurement of residual stresses in isotropic materials and the authors propose an extension of the methods use to layered orthotropic composites. The data obtained by the layer removal and compliance methods are compared to the results obtained by a numerical model developed by other researchers. There is an excellent agreement between the experimental data obtained by the layer removal and compliance methods and the results predicted by the model. The layer removal and compliance methods are also compared in terms of accuracy and practicality.

A Review on the Mechanical Modeling of Composite Manufacturing Processes

The increased usage of fiber reinforced polymer composites in load bearing applications requires a detailed understanding of the process induced residual stresses and their effect on the shape distortions. This is utmost necessary in order to have more reliable composite manufacturing since the residual stresses alter the internal stress level of the composite part during the service life and the residual shape distortions may lead to not meeting the desired geometrical tolerances. The occurrence of residual stresses during the manufacturing process inherently contains diverse interactions between the involved physical phenomena mainly related to material flow, heat transfer and polymerization or crystallization. Development of numerical process models is required for virtual design and optimization of the composite manufacturing process which avoids the expensive trial-and-error based approaches. The process models as well as applications focusing on the prediction of residual stresses and shape distortions taking place in composite manufacturing are discussed in this study. The applications on both thermoset and thermoplastic based composites are reviewed in detail.

Shear-lag Analysis of the Effect of Thickness on Spring-in of Curved Composites

A new analytical solution is presented for spring-in of curved thermoset matrix composites taking into account the low shear stiffness of the material in the rubbery state before it is fully cured. It is shown that shear-lag through the thickness can have a significant effect in reducing spring-in arising from thermal stresses and chemical shrinkage between gelation and vitrification. Results are presented in a nondimensional form that shows the influences of subtended angle, through-thickness shrinkage, the ratio of curved arc length to part thickness, and the ratio of in-plane modulus to rubbery interlaminar shear modulus, providing greater physical insight into the spring-in mechanism. The analysis predicts a significant effect of thickness on spring-in angle. This is verified by experimental measurements on cross-ply carbon-epoxy C-sections cured on a carbon fiber tool, confirming the validity of the new solution.

Modelling manufacturing deformations in corner sections made of composite materials

A three-step finite element model has been implemented to predict the spring-in of L-shaped parts. The material property development during the cure has been modelled as step changes during transitions between viscous, rubbery and glassy states of the resin. The tool-part interaction is modelled as a sliding interface with a constant sliding shear stress. The effect of various material and geometric variables on the deformation of L-Section parts are investigated by a parameter sensitivity analysis. The spring-in predictions obtained by the finite element method are compared to experimental measurements for unidirectional and cross-ply parts of various thicknesses and radii. Results indicate that although a 2D plane strain model can predict the spring-in measured at the symmetry plane fairly well, it is not sufficient to capture the complex deformation patterns observed.

3D finite element model for predicting manufacturing distortions of composite parts

A 3D finite element model has been developed for predicting manufacturing distortions of fibre-reinforced thermosetting composite parts. The total curing process is divided into three steps that correspond to the states that resin passes through during curing: viscous, rubbery, and glassy. Tool–part interaction properties were calibrated by modelling the distortion of a single ply part. For comparison, composite parts of various geometries (L-section and U-section), stacking sequences, thicknesses, and bagging conditions were manufactured. The full field thickness profile and full field distortion pattern were obtained using a 3D laser scanner, which reveals higher and lower resin bleeding and corner thickening locations. The effect of stacking sequence is also examined with the full field distortion pattern. It was found that the parts manufactured under the bleeding condition give higher spring-in and warpage values. The spring-in predictions were well matched to measurements of the manufactured parts.

Failure behavior of composite laminates under four-point bending:

In this study, failure behavior of fiber-reinforced composites under four-point bending is investigated. First, the tests are modeled analytically using the classical lamination theory (CLT). The maximum allowable moment resultants of [θ12]T off-axis laminate as well as balanced and symmetric angle-ply [θ3/−θ3]s composite laminates as a function of fiber orientation angle, θ, are obtained using Tsai-Wu, maximum stress, maximum strain, Hashin, Tsai-Hill, Hoffman, quadric surfaces, modified quadric surfaces, and Norris failure criteria. Second, the same tests are simulated using the finite element method (FEM). Thermal residual stresses are calculated and accounted for in the failure analysis. An analysis is conducted for optimal positioning of the supports so as to ensure that intralaminar failure modes dominate interlaminar (delamination) failure mode. A test setup is then constructed accordingly and experiments are conducted. The correlation of the predicted failure loads and the experimental results is discussed. The quadric surfaces criterion is found to correlate better with the experimental results among the chosen failure criteria for the selected configurations.

Custom Design Multi-Axial Engine Mount Load-Cell Development for Road Load Identification and Fatigue Life Estimation

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Analysis of residual transverse stresses in a thick UD glass/polyester pultruded profile using hole drilling with strain gage and digital image correlation

Process induced stresses inherently exist in fiber reinforced polymer composites particularly in thick parts due to the presence of non-uniform cure, shrinkage and thermal expansion/contraction during manufacturing. In order to increase the reliability and the performance of the composite materials, process models are developed to predict the residual stress formation. The accuracy of the process models is dependent on the geometrical (micro to macro), material and process parameters as well as the numerical implementation. Therefore, in order to have reliable process modelling framework, there is a need for validation and if necessary calibration of the developed models. This study focuses on measurement of the transverse residual stresses in a relatively thick pultruded profile (20×20 mm) made of glass/polyester. Process-induced residual stresses in the middle of the profile are examined with different techniques which have never been applied for transverse residual stresses in thick unidirectional composites. Hole drilling method with strain gage and digital image correlation are employed. Strain values measured from measurements are used in a finite element model (FEM) to simulate the hole drilling process and predict the residual stress level. The measured released strain is found to be approximately 180 μm/m from the strain gage. The tensile residual stress at the core of the profile is estimated approximately as 7-10 MPa. Proposed methods and measured values in this study will enable validation and calibration of the process models based on the residual stresses.Process induced stresses inherently exist in fiber reinforced polymer composites particularly in thick parts due to the presence of non-uniform cure, shrinkage and thermal expansion/contraction during manufacturing. In order to increase the reliability and the performance of the composite materials, process models are developed to predict the residual stress formation. The accuracy of the process models is dependent on the geometrical (micro to macro), material and process parameters as well as the numerical implementation. Therefore, in order to have reliable process modelling framework, there is a need for validation and if necessary calibration of the developed models. This study focuses on measurement of the transverse residual stresses in a relatively thick pultruded profile (20×20 mm) made of glass/polyester. Process-induced residual stresses in the middle of the profile are examined with different techniques which have never been applied for transverse residual stresses in thick unidirectional comp...

Shape optimization of a sandwich plate with a novel core design

2. TENSION TESTS In the first stage of the study, the mechanical properties of the composite material are determined. A procedure is proposed to determine these properties using tension test and acoustic emission (AE) results for specimens with 0/45/ 45/90 and 0/90 layup sequences and a progressive failure model. Using this procedure, longitudinal and transverse tensile strengths, and , shear strength, , together with the stiffness properties can be obtained. Tension tests are conducted according to ASTM D3039 standard testing procedure. The strain of the specimen is measured by a video extensometer. Fig. 2 shows the peak frequency distribution as well as the energy levels of AE hits for a quasiisotropic specimen, 0/45/ 45/90 , together with the load-strain curve. Ply-failure load levels, which are indicated on the graph, are determined considering the changes in the load-displacement diagram as well as the AE signals. Fig. 3 shows a comparison between the experimental load-strain curves of cross-ply and quasi-isotropic tension test specimens and the curves predicted by the progressive model.