From FEA-based study, it was found that in the case of single-sided repair, the stress intensity factor (SIF) at the unpatched surface tends to be higher than that of the untreated panel SIF. Ea Young's modulus of adhesive kI Stiffness of the adhesive kI´ Stiffness after degradation μ Shear modulus.
Introduction and literature review
Introduction to repair technology
- Bonded repair vs mechanical fasteners
- Composite materials: an overview
- Design considerations in adhesively bonded patch repair
- Experimental techniques for strain measurements
- Introduction to fatigue loading and crack growth study
The presence of the crack results in the redistribution of stresses and strains around the crack tip. To characterize the amplitude of stress values near the crack tip, SIF [9] is presented.
Motivation
- Mechanics based study of composite patch repair
- Optimization study of patch repair
- Fatigue crack growth study
- Estimation of whole field strain distribution in repaired panel using digital image correlation
Dae-Cheol and Jung-Ju [55] have performed both numerical and experimental studies on the fatigue crack growth (FCG) behavior of the thick cracked panel repaired with a single-sided composite FRP patch. They also found that the lap lay-up affects the fatigue life of the panel.
Scope and Objectives
Similarly, the damage is also less with the increased shear strength of the adhesive and the width of the patch. To estimate the adhesive peel and shear stress through the thickness of the adhesive layer using DIC technique.
Thesis Layout
Three-dimensional modeling of FCG behavior in both unrepaired and repaired panels is performed using FEA. Experimental estimation of fatigue life for unrepaired and repaired models is performed and compared with the FEA prediction.
Design and Performance Study of
Repaired and Unrepaired Panel Using FEA
- Introduction
- Geometry and material properties
- Finite element modeling .1 Modeling of cracked panel .1 Modeling of cracked panel
- Mesh convergence study
- Comparison of analytical and numerical SIF of the cracked panel
- Finite element modeling of repaired panel
- Variation of SIF in unrepaired and repaired panel (a) (a)
- Variation of normal stress in unrepaired and on repaired panel
- Effect of patch lay-up configuration on repaired panel
- Effect of patch thickness on repaired panel
- Effect of adhesive thickness on repaired panel
- Effect of tapered patch on peel stress distribution
- Closure
The SIF variation (both KI and KII) due to panel thickness for single-sided and double-sided repaired model with varying number of layers is shown in Figure. Therefore, the variation of both KI and KII is not symmetrical across the thickness of the panel. panel.
Optimum Design of Patch Geometry and Dimension
Introduction
Material properties
Finite element modeling of double sided patched panel with different patch shapes
- Circular patch modeling
- Square patch shape modeling
- Elliptical patch modeling
- Octagonal patch shape modeling
By encircling the circular pattern, another circular area is created, so that it sufficiently encloses the circular area of the patch. Similar to the circular patch model, a circular mesh pattern is created around the crack tip, which is then captured in another circular region.
SIF and reduction parameter (R) variation in double sided repair
- Variation of SIF and reduction parameter in circular patch
- Variation of SIF and reduction parameter in rectangular patch
- Variation of SIF and reduction parameter in square patch
- Variation of SIF and reduction parameter in elliptical patch
- Variation of SIF and reduction parameter in octagonal patch
3.7(a) and (b) it can be observed that as the chip diameter increases, the overlap area increases, so the SIF decreases. 3.8(c) it can be found that R is higher for the rectangular piece of fixed width and increasing length.
Performance of different patch shapes on panel having different crack inclination angles in double sided repairinclination angles in double sided repair
It is found that R is maximum at all slope angles in the case of extended octagonal and rectangular patch shape. 3.12, it is clear that patch shape influences SIF and its impact is different for different crack slopes.
Comparative study of different patch shapes on SIF reduction
Patch area in mm2 Rectangular patch Circular patch Twisted elliptical patch Elongated octagonal patch Square patch. Spot area in mm2 Rectangular patch Circular patch Twisted elliptical patch Elongated Octagonal patch Square patch.
Comparative study of patch shape on SIF reduction in single sided repair
Dimensional optimization of octagonal patch
In the current optimization study, the crack tip mesh was modified to reduce the calculation time. Here, the crack tip mesh has a total of 3456 elements with 16 radial, 36 circumferential and 6 through-thickness elements.
Estimation of optimal patch dimensions from mechanics based approach .1 Influence of patch thickness .1 Influence of patch thickness
- Influence of patch length on J-integral value, peel and shear stresses
Further to see the effect of splice length on shear and peel stresses, the distribution of adhesive shear stress and peel stress is plotted against lap length (see Fig. In general comparison, splice length of 70 mm is chosen taking into account the minimum integral value J , shear and peel stress.
Chip dimensions such as length, width, and thickness are considered design variables. If the solution is optimal and convergence is achieved, the program terminates with optimal design variables.
Optimal solution from GA based approach
A non-inferiority solution is one in which an improvement in one objective requires a degradation of another. The patch dimensions obtained from both the mechanics-based approach and the GA-based approach are in good agreement with the CRMS guidelines as shown in Table 3.5.
SIF estimation using VCCT technique in optimal repaired configuration The single sided and double sided repaired panel is modeled with the arrived optimal patch The single sided and double sided repaired panel is modeled with the arrived optimal patch
- SIF variation through the thickness of panel using VCCT technique
3.22, it is observed that the SIF reduction is highest in the case of double-sided repair and it works very efficiently. Moreover, it is observed that optimal repaired configuration reduces SIF by 35% in double-sided patch-repaired model and by 10% in case of single-sided patch-repaired model compared to the original configuration.
Introduction
An extensive amount of analytical, numerical and experimental research has been conducted to understand the behavior of adhesive joints [69, 70]. Later, an extensive study with DIC is carried out to obtain the surface tension distribution of the repaired panel over the entire field.
Digital image correlation : an overview
DIC works on the comparison between two images of a sample coated with a random speckle pattern in the undeformed and deformed states [21]. The differences in the positions of the center of the reference subset and the center of the deformed subset give the displacement vector in the plane at the point P, as shown in Figure 1.
Material characterization of Al 2014-T6 alloy .1 Specimen Geometry
- Preparation of speckle pattern
- Tensile properties of Al 2014-T6 alloy
The uniaxial tensile load is applied along the longitudinal direction of the test samples using the displacement control mode with a transverse speed of 1 mm/min. Camera heights are adjusted on tripods to provide a full view of the specimen.
Specimen Fabrication .1 Fabrication of cracked panel .1 Fabrication of cracked panel
- Fabrication of repaired panel
From the rectangular patch, the elongated octagonal patch is applied by filling the rounded edges with the help of metal templates and filing tools as shown in Fig. Care is taken while attaching the patch to the cracked panel so that the fibers in the patch are kept parallel to the loading direction.
Whole Field Strain Prediction .1 Strain prediction in cracked panel .1 Strain prediction in cracked panel
- Whole field strain prediction in single sided repaired panel
- Whole field strain prediction in double sided repaired panel
For the full field load analysis of the cracked and repaired panel, approximately 140 dots of size are applied to 1 mm2 of the surface (see Fig. Further, the full field load analysis on the patched area of one-sided repair is studied.
Strain Field in the Adhesive Layer
- Peel and shear strain prediction in adhesive layer of single sided repair
- Peel and shear strain prediction in double sided repair
- Shear strain variation along the interface in double sided repair
It can be observed that the peeling load is maximum at the overlap edge of the patch. From Figure 4.15(e) it can be seen that the peel load is maximum at the overlap edge.
Experimental performance of repaired and unrepaired panel
The failure mechanism observed in cracked as well as single and double sided patch repaired panel is shown in Fig. It is observed that the stress in the double-sided repair is lower than the single-sided repair.
Comparison of strength of repaired and unrepaired panel using MTS
For both repaired single-sided and double-sided panels, partial spalling followed by complete panel failure is observed. It has been experimentally confirmed that the peel stress and strain are maximum at the edge of the chip overlap in the adhesive layer where the chip bond is under increased load.
Introduction
In addition, they also estimated the SIF at the crack tip along with the location of the crack front using displacement contours. The crack growth rate is determined by the Paris law, while the crack growth direction is determined by the maximum energy release rate criterion [97].
Determination of Paris law constants by base line tests
In this work, the material constants C and m are determined by quadratic polynomial fitting of the log-log data of da/dN Vs. ∆K is shown in figure. The average values of material constants are given as input for determining fatigue crack growth. speed and fatigue life of repaired panel using FEA.
Introduction to Zencrack
- Finite element modeling of three-dimensional cracked panel
- Finite element modeling of repaired panel using Zencrack
- Interface Modeling : cohesive elements
- Crack growth criterion
- Estimation of SIF from displacements
- Estimation of crack propagation direction
- Fatigue life prediction
A mesh model of the entire plate geometry, including the crack tip blocks, is shown in Fig. The crack growth process includes an assessment of the magnitude and direction of the energy release rate.
Test Procedure using DIC .1 Experimental Setup .1 Experimental Setup
- Comparison of v-displacement fatigue life of un repaired panel using FEA and Experiment
- Comparison of v-displacement and fatigue life of single sided repaired panel using FEA and Experiment using FEA and Experiment
- Fracture mechanism and fracture surface
The v-displacement contours of the single-sided repaired panel at different times corresponding to and 21,300 fatigue cycles obtained from both DIC and FEA are shown in Figs. It is noted that the fatigue life of the double-sided repair (FEA: 46440 cycles, Expt: 46379 cycles) is almost twice that of the single-sided repair (FEA: 22561 cycles, Expt: 21511 cycles) in both the FEA and the experimental predictions.
Closure
Conclusions and Recommendations
Conclusions
The experimental results show that a double-sided repair dramatically improves fatigue life compared to a single-sided repaired panel. The FEA-based investigation showed that the crack front profile of double-sided and unrepaired panel is uniform and symmetrical.
Recommendations for future work
In Chapter 5 the behavior of bonded, repaired panels under constant amplitude fatigue loading is studied. In this work, only the fatigue crack growth behavior of repaired panels under constant amplitude cyclic loading is considered.
Appendix Appendix A
Analytical SIF expression for inclined center cracked specimen
Appendix B
Estimation of energy release rate (ERR) using VCCT
𝑭𝒛𝑵𝒊(𝒘𝑵𝒍− 𝒘𝑵𝒍∗) + 𝑭𝒛𝑵𝒋(𝒘𝑵𝒎− 𝒘𝑵𝒎∗)] (B.7) where Fx, Fy, Fz represent the nodal forces along x, y axis and z, respectively, u, v and w represent displacements along x, y and z and ∆A is the area of the element along the thickness, given as ∆A = b*∆a, where b is the thickness of the element and ∆a is the length of the element along the plane of the crack. Further, L, M and N represent three planes passing along nodes 1, 2 and 3 of element A as shown in fig.
Appendix C
Hand lay-up process
The tensile specimens are tested which provides an indirect means of evaluating in-plane shear modulus and shear strength parameters. For finding shear modulus, shear stress at each data point is calculated using equation C.2 and shear stress at each data point is calculated using equation C.3.
Adhesive thickness measurement
Appendix D
- History of Crack Blocks
- Estimation of fatigue crack growth direction using Virtual crack extension method extension method
- Estimation of mode I interface fracture toughness
- Estimation of mode II interface fracture toughness
- Estimation of interfacial fracture toughness using VCCT technique
The load versus displacement recorded from the DCB sample is shown in the figure. The load versus displacement recorded from the ENF sample is shown in the figure.
Appendix E
Comments
This degree of plane deformation is 1 when the radial distance from the crack tip is 0.5 times the thickness of the panel at the center line. This counteracts the bending stresses present at the non-lapped surface of the single-sided repaired panel.
Appendix F
Comments
In their study, the Mindlin plate element is used to model the host plate and the patch, viz. the aluminum plate and patch are modeled using a 2D four-node planar element. Furthermore, the presence of transverse shear strain in Mindlin's plate theory provides a bilinear approximation of the displacement through the thickness of the aluminum plate.
Appendix G
Comments
As per the reviewer's suggestion, on page 32, line 15 from the bottom: “steadied” is replaced by “steady” in the revised manuscript. At the reviewer's suggestion, we included Figure 5.4(b), which shows the relationship between traction and separation, in the revised dissertation.
Design, analysis and performance of adhesive bonded composite patch repair on cracked aluminum aircraft panels. Characterization of the effects of debonding on fatigue crack growth behavior in an aluminum plate with a bonded composite patch.
