I am also indebted to Mr. Matta Seshadri, Researcher in IIT Hyderabad, for his kind help and support. I would also like to thank Research Scientist Mr. Naresh Reddy and project staff Mr. Karthikeyan for their contribution in conducting the experimental tests.
Introduction
Thereby both compressive and tensile stress conditions will be present across the thickness of the test piece. But these damage mechanisms do not reduce the bearing capacity of the composite structure.
Literature Review
Composites without hole
The failure mechanisms of the laminates were also studied by experimentally testing the samples under three-point bending and the results obtained from experiments and the micromechanical model were compared. To compare the accuracy of the failure predictions, a 3-point bend test is performed for both layup sequences.
Composites with single/multiple holes
In 2003, S.Temiz [30] performed the stress analysis of composite laminate and laminate without and with holes using the finite element method by writing a computer program to solve the problem. Investigations were carried out using displacement-based finite element methods and Newton-Raphson method for linearization of system of nonlinear equations.
Motivation
Interlaminar damage mode includes delamination, which is the separation of the adjacent layers in the laminate, the photomicrograph image of which is shown in Figure 1.5 (b). It is important to study these complex damage mechanisms and the overall damage behavior of the composite structure for its effective and sustainable design.
Scope
The cause of delamination is due to the fact that the transverse strengths or in other words the interlaminar force parameters are small compared to other force parameters. As discussed in Section 1.1, cuts or holes are essential and unavoidable in structures due to functional requirements such as wiring, piping, assembly, visual inspection, etc.
Objective
There are three types of delamination: Mode I delamination is known as the opening mode which occurs due to tensile loading, Mode II delamination is known as the shear mode which occurs due to in-plane loading and Mode III delamination is known as the tearing mode which occurs due to out-of-plane shear. The research on damage behavior can also be carried out numerically by conducting progressive damage analysis through a reliable progressive damage model and the results obtained from this model can be compared with the experimental results.
Thesis layout
The two-dimensional (2D) digital image correlation technique was used to capture displacements and in-plane strains across the thickness of the specimen. 2) To perform progressive damage analysis numerically through a three-dimensional progressive damage model implemented in a three-dimensional finite element simulation. In the progressive damage model, the Hashin failure criteria were used for failure initiation prediction, the material property degradation model (MPDM) was used for failure evolution modeling, and the Cohesive Zone Model (CZM) was applied for delamination modeling between the composite laminate layers. . 3) Finally the results obtained from the experiments were compared with those obtained from the numerical model for validation.
Introduction
Specimen Geometry
However, in four-point bending loading, there is no shear force in the part of the specimen between the two support rollers located in between, and the bending moment at this part of the specimen remains constant throughout. For CFRP specimens that have multiple cutouts, the length, thickness, and cutout diameter of specimens are the same as for previous specimens, but the width of specimens is 64 mm instead of 45 mm.
Specimen fabrication
After cutting the laminate into various samples, delaminations would be present on the faces that were cut. To remove these delaminations, end milling of the specimens was performed as shown in the figure. After the above processes were performed on the sample, the surfaces of the sample containing its thickness would be rough.
Experimental Testing Setup
The cameras were properly aligned with the specimen so that it could capture images of the specimen's face across the thickness without any bias. The cameras aperture, which is an opening or hole in the camera through which light passes, was tuned to the correct level, so the brightness of the sample images that were captured was at the optimum level. Two 30 W LED sources (7) were also placed, one near the front camera and the other near the rear camera, in order to ensure adequate illumination. The images captured by these cameras were captured from a computer(6) through Vic-snap 2009 software (from Interrelated Solutions Inc.) which was pre-installed on it.
Digital Image correlation technique and its Principle
The accuracy of correlation depends on how randomly the gray and white pixels are distributed in the image of sample, and this randomness depends on how well the speckle pattern is performed on the face of the sample. If the speckle pattern is performed more accurately, the randomness in the gray and white pixels will be more, therefore the uniqueness of the gray value information of each subset will be more and this leads to a better correlation between subsets. For example, step size of two implies that correlation of subsets will occur for every other subset in the sample image.
Results and discussion
Load-Displacement plots
The same procedure can be used when calculating the displacement for other subsets in the image of a deformed sample, thereby calculating the full displacement variation through the sample region of interest. Once the displacement variation has been calculated, stresses in the region of interest of the sample can be calculated by taking appropriate partial derivatives of the displacement. In unidirectional samples, all fibers are in the longitudinal direction, which is not the case in quasi-isotropic panels. Since the stiffness and strength are highest in the direction of the fibers, unidirectional samples in this case are stronger and stiffer than quasi-isotropic samples. .
Damage behavior
No damage was observed in the bottom layer on the tensile side of the specimen. In the case of specimens with single and multiple cutouts, first layer failure also occurs in the upper lowest layer on pressure side and it occurs in the area near hole as the hole acts as a stress increase and therefore there will be stress concentration in this area . It can also be observed that in samples with multiple holes with 2HT and 2HD configuration, failure also occurs in the region between the holes implying that there is interaction between holes.
Closure
Introduction
Creation of model
There was a circular cutout of 10 mm diameter created central to the model of CFRP specimen with cutout. Dimensions of models of CFRP specimens with multiple cutouts include 250 mm along the length, 64 mm in the width and 3.6 mm along the thickness. Two circular cutouts of diameter 10 mm each and their centers separated by 19 mm were created for each of the configurations of multiple CFRP specimens.
Meshing of model
As shown in Figure 3.1, each of the models is symmetric and one may wonder why only a certain part of the sample can be modeled to save computational time. But the answer is, when the defect starts to appear and spread in the sample, the model loses its symmetry and therefore the whole sample is modeled for each instance.
Boundary conditions on model
Material properties
Progressive Damage Model
- Hashin failure criteria
- Material Property Degradation Model
- Cohesive zone model
- Flow chart for implementing PDM
Cohesive zone model (CZM) is a model in fracture mechanics where fracture propagation is considered as a gradual separation of the surfaces in the area in front of the crack tip. The upper and lower surfaces of the contiguous element are referred to as the contiguous surfaces. The flowchart for implementing the three-dimensional progressive damage model is shown in Figure 3.3.
Results and discussion
Load-Displacement plots
If the solution has converged, then the same procedure for PDM as previously described is applied for the next time step. But if the solution does not converge, then the same procedure is performed once again starting from the calculation of stress and strain until the solution converges. When the time step has reached one, the solution is fully completed and the results required by the user can be displayed.
Strain plots
Progressive damage illustration
An illustration of the damage propagation predicted by PDM for a quasi-isotropic specimen on the top four inserts in compression and on the bottom four inserts in tension is shown in Figures 3.18 to Figures 3.26. In the zero-degree layer, more than one failure mode occurs near the support rolls with higher loading. For CFRP specimens with single and multiple holes, the failure modes in each layer are similar to those of CFRP specimens without holes, except for inserts at -45 and 45 degrees in tension, where matrix failure and more than one failure mode occur with uncertainty, which is the dominant failure mode.
The comparison of the ultimately failed specimen from the experiment and that from the PDM on the compressive and tensile side are shown in Figure 3.27 and Figure 3.28, respectively. This implies that the damage modes occurring on the tensile side in PDM are matrix failure and fiber matrix shear failure.
Closure
Conclusion
In sample without hole, damage initiated at the support rollers propagated around this, while in sample with single and multiple holes, damage initiated near hole and propagated in transverse direction and slightly in longitudinal direction. In quasi-isotropic specimens, damage was initiated in the top layer at 45 degree fiber orientation in the form of fiber matrix shear failure and remained the dominant failure mode in this layer and the -45 degree layer. In specimen without hole, damage initiated at the support rolls propagated through the specimen, whereas in specimen with single and multiple hole, damage initiated near hole and propagated in transverse direction.
Recommendations for future work
Embedded FBGs and 3-D DIC for stress analysis of a structural specimen subjected to bending. Composite structures. An investigation of the intralaminar damage and interlaminar delamination of carbon fiber composite laminates during three-point bending using acoustic emission. Bending analysis of laminated composite plates containing two elliptical holes using higher order shear deformation theory.
