An experimental study on the stability behaviour of curved CFRP panel under axial compression load

The satisfaction and euphoria of successfully completing a task would be incomplete if I did not mention the people who made it possible and whose constant guidance and encouragement crowned my efforts with success. I would like to thank Mr. Naresh Reddy Kolanu, Research Scientist, Engineering Optics Laboratory, Department of Mechanical and Aerospace Engineering, IIT Hyderabad, for his help in conducting the experiments. In this work, the bending and post-bending axial compression behavior of shallow curved CFRP plates is investigated using digital image correlation (DIC) systems.

A new attempt is made by using multiple DIC systems to capture the shear and strain fields over the flat regions of the curved panel in the pre-buckling, buckling and post-buckling regimes. Special load holders are designed to apply uniform compression load to the curved edges of the panel. A finite element modeling (FEM) of the curved CFRP panel using continuum shell element in ABAQUS is performed to study the buckling and post-buckling behavior.

The actual imperfection of the manufactured curved panel is measured using a coordinate measuring machine (CMM). Furthermore, the measured imperfections are given as input for performing Rik's post-buckling analysis and the results are compared with experimental observation for validation. CFRP Carbon Fiber Reinforced Polymer CMM Coordinate Measuring Machine DIC Digital Image Correlations FEA Finite Element Method L Length of Panel R Radius of Curvature UD Unidirectional.

Introduction

Because it will buckle before the material failure of the structure and stop its functionality. In simple words, buckling is out of plane deformation of the specimen or can be defined as the loss of stability of the object due to their geometrical effects rather than material failure. The elastic limit is a point-up return of the structure to its original position after removal of the applied load.

Any such imperfection would critically affect the panel and could eventually lead to catastrophic failure of the structure. Geometric imperfection refers to the deviation of the mid-plane of the panel from its original shape when subjected to no load, i.e. These imperfections may be created by residual stress when the panel is fabricated or may simply be due to limitations in the accuracy of the manufacturing process.

Method for calculating the imperfection is given by the Thornburgh 2006 [1]. This imperfection has a significant adverse effect on the buckling and post-buckling behavior of the curved composite structures.

Figure 1.2: Example of curved composite structure

Literature review

The author noted that the loss of load-bearing capacity due to post-buckling can reach 50% for the [45/90]2s, which is regretted by a sliding simple support for a softening of the response relationship, as in case of thermally loaded [45] /0/90]s it is laminated with a fixed simple support. If the ratio between the geometric imperfection and the thickness of the panel is 0.5, the buckling load is reduced to 35%. Commonly used techniques are based on the knock-down factor applied to the eigenmode obtained from the native buckling analysis of the perfect structure.

First, the regular mesh consisting of the three-dimensional array (horizontal pixel, vertical pixel and coordinate). Second, a smart mesh where the element was adjusted to the radius of curvature of the panel. In this paper, Brown and Sharpe used the global image CMM used to measure the sample coordinate and an automation software, PC-DMIS version 3.7, used to control the CMM.

MeasPanel software written by NASA to act as a graphical user interface (GUI) and provide an easy method of controlling data input and output from the PC-DMIS software.

Objective

As we know, geometric imperfection is largely responsible for the reduction of buckling load. Arbocz [12] investigated a series of shape and size imperfection data obtained using a probe moving over the surface of the structure to determine the points on the outer surface. A method has been developed for importing the imperfection into commercial finite element software. The aim of this thesis is to experimentally investigate the buckling and post-buckling response of curved CFRP panels under axial compression and validate them with the numerical results derived from FEM-based methods.

To investigate the flexural and post-buckling response of the specimen, a finite element model of curved composite panel and numerical analysis have been carried out with the finite element commercial software Abaqus-CAE 2017, and the experiments were carried out with Digital Image Correlation (DIC) technique. To study the buckling and post-buckling response of curved CFRP panels under axial compression loading. Using coordinate measuring machine (CMM) to measure the initial geometric imperfection of the samples.

Use of 3D DIC technique for capturing the buckling and post-buckling response of the test panels.

Problem definition

Thesis layout

Introduction

Details of test specimen

Fabrication specimen

Materials required for fabrication
Matrix material
Procedure
End block Fabrication

First of all, the 200gsm unidirectional carbon fiber sheet is cut from the carbon fiber bundle. To place the carbon fiber on the acrylic sheet, a rectangular area is marked with the help of a marker. Peel Ply - A peel ply is a porous fabric layer used in laying a composite material that draws out excess resin and prevents the bag material from sticking to the carbon fiber sheets.

Perforated Sheet - Perforated sheet is perforated with a uniform pattern of holes that allows the resin to flow evenly over the fiber sheets. Green Mesh-Helps in uniform distribution of the matrix material over the fiber sheet. Now, the whole assembly is covered using the bag sheet and the sides are tightly packed using adhesive tape to avoid any air leakage during vacuum bagging.

It is transparent in nature and allows easy inspection of the laminate as it cures. Flange consists of an infusion tube connected to the jar, consists of matrix material on one side and receives a jar or vacuum pump on the other side. Here an IV tube is used to allow the excess resin to flow back into the collection jar.

During this time, matrix material from the pot slowly enters the composition and spreads uniformly over the entire length and width of the composition. The size of end watch is 210 mm length, 60 mm width and 35 mm thickness and 10 mm shrinkage allowance provided on each side of the mold. To secure the end block to the panel, two grooves of 774 radii are cut into the rectangular block.

One is 2mm wide across the thickness and another is 6mm with up to 30mm of the block's thickness.

Imperfection measurement using Co-ordinate measuring machine (CMM)machine (CMM)

After calibration, the local coordinate has been generated on the one surface of the panel that is in Cartesian coordinate. To measure radius and thickness, 30 curves are generated on each surface at 10mm spacing along the length and 2mm spacing across the width. After the panel is scanned, the measured data is used to calculate the radius and thickness.

Figure 2.4: (a)Mould setup for end block (1) rectangular mould (2) mixture of resin and hardener (b)End block potting setup (1) holding arms (2) end block (3) curved panel (4) long set square

Closure

Introduction

Curved CFRP panel testing

Experimental procedure and equipments details

The top and bottom edges of the panel are clamped in the fixture, while the straight (longitudinal) edges are supported by a simply supported condition. An enlarged view of the specimen mounted in a simple carrier device is shown in Fig. Before performing the experiments, a dot pattern was prepared on the surface of the curved plate to perform the DIC measurement.

Two sets of LED light source were used to illuminate the panel surface during the test. During the test, a camera takes five pictures per SEC. of the deformed panel and these images are then stored in the PC system. The load and displacement value corresponding to each image is taken by the MTS test system using the National Instruments data acquisition system.

VIC snap was software used to capture the images of the test panel during testing.

Figure 3.1: Experimental setup (1) 100 KN MTS Load frame (2) buckling fixture (3) curved CFRP test panel (4) MTS user interface (5)3D-DIC CCD cameras (6) LED light source (7) Image

Post-processing

Failure in the panels

For the cross-layer panel, it is observed that the crack propagates across the width. It then suddenly failed in the transverse direction slightly above the center plane of the panel along the loading side with a load of 12.21 kN. One of the cracks is located near the curved clamping edge and propagates in a transverse direction.

The other crack formed at a distance of one-fourth of the total length of the panel from the loading point and it propagated in the transverse direction.

Closure

Introduction

Finite element modeling of curved CFRP panel

Numerical analysis without initial imperfection (IMP)

Numerical analysis with initial imperfection (IMP)

CFRP laminate properties

Loading condition

Mesh convergence study

Eigen buckling analysis

Buckling loads

Buckling mode shapes

For the deflection analysis, an external load of 1 KN is applied and a linear deflection analysis is performed. For quasi-isotropic s, the out-of-plane deformation remains near the center of the plate for the second and third mode shapes, but occurs at one-quarter and one-third of the length for the mode shape, as shown in Fig.

Figure 4.3: Unidirectional laminate [0] 8 mode shape obtained from eigen value analysis without initial imperfection.

Post-buckling analysis

Closure

Introduction

Numerical results

Eigen buckling analysis results
Post-buckling results
Out-of-plane deformation results
In-plane deformation results

Out-of-plane deformations obtained from the numerical analysis without initial imperfection are shown in Fig. The out-of-plane deformations obtained from the numerical analysis with initial imperfection are shown in Fig. The maximum out-of-plane deformation observed at the center of each panel due to simply supported boundary conditions.

It is observed that the contours obtained from the numerical results are symmetrical along both axes, i.e. In-plane deformations obtained from the numerical analysis without initial imperfection are shown in Fig.

Experimental results

Out-of-plane deformation result

In-plane deformation results

Load versus end shortening

Numeric Numeric with Initial without Initial with Initial without Initial Imperfection Imperfection Imperfection Imperfection Quasi-isotropic.

Comparison of experimental and numerical results

Closure

Conclusion

Future work

Introduction

Procedure

Geometric imperfection sensitivity of curved panels under combined compression and in-plane bending – A study using Adaptive Meshing and DIC. Modeling the effects of geometric imperfections on the buckling and initial post-buckling behavior of flat plates under compression using measured data. Investigations on imperfection sensitivity and deduction of improved knock-down factors for unstiffened CFRP cylindrical shells.