Due to the lack of fundamental knowledge of the physics behind delamination growth, certification authorities currently require that composite structures in aircraft are designed such that any delamination will not grow. This usually leads to an overdesign of the structure, hampering weight reductions. In real structures, delaminations tend to grow under a mix of modes I and II. Although some studies have tried to assess mixed-mode fatigue delamination, little progress was made in understanding the physics behind the problem. Therefore, this work scrutinizes mixed-mode fatigue delamination growth and examines experimentally the damage mechanisms that lead to fracture. To this aim, mixed-mode delamination fatigue tests were performed at different mode mixities, displacement ratios and maximum displacements. Selected fracture surfaces were analysed after the tests in a Scanning Electron Microscope to gain insight on the damage mechanisms. The physical Strain Energy Release Rate G∗ was used as the similitude parameter, enabling the characterization of fatigue mixed-mode delamination propagation. The results obtained show no displacement ratio or maximum displacement dependence. Furthermore, the energy dissipated per area of crack created is approximately constant for a given mode mixity. However, the analyses of the fracture surfaces and the correlation of the damage features with energy dissipation indicate that different damage mechanisms that might be activated under different loading parameters cause the resistance to delamination to change under a given loading mode.@en

This paper demonstrates how the critical strain energy density in the delamination tip vicinity may be used to explain the physics of delamination growth under mixed mode I/II. A theory previously proposed to physically relate mode I and mode II delamination growth is further extended towards describing the onset of mixed mode I/II delamination. Subsequently, data from the literature is used to demonstrate that this new concept of the critical strain energy density approach indeed explains, based on the physics of the problem, the strain energy release rate level at which crack onset occurs. This critical strain energy density for the onset of delamination appears to be independent of the opening mode. This means that, in order to characterize the fracture behaviour of a laminate, fracture tests at only one loading mode are necessary. Because the load level at which the physical delamination onset occurs at the microscopic level is much lower than the traditional engineering definition of macroscopic onset, further work must reveal the relationship between the macroscopically visible delamination onset, and the microscopic onset.@en

In an attempt to understand quasi-static delamination growth under mixed mode loading conditions from a physics-based perspective, this work first evaluated cracking in isotropic materials. The critical Strain Energy Density (SED) approach is adopted, because physically the onset of crack growth is expected to occur when the energy available near the crack tip reaches a critical value. The main hypothesis of the present paper is that the critical SED for onset of crack growth is constant for a given material, and independent of the loading mode. The relationship derived from this hypothesis therefore relates the physical onset of crack growth and the angle at which that occurs for any opening mode through the SED. To test this hypothesis, results from literature were taken and shear fracture tests on foam specimens were performed, which both were compared with the derived relationship. The excellent correlation demonstrated the validity of the physics-based relationship, which explains the observed differences between mode I and mode II fracture toughnesses and illustrates why concepts like the Stress Intensity Factor (SIF) alone are insufficient to explain the observations. The developed relationship allows to derive the mode II fracture toughness from mode I fracture toughness tests and the material's mechanical properties.@en

In a context of many studies addressing delamination growth in laminated composites, this thesis provides understanding of the underlying physics of this phenomenon. The models currently used to assess delamination growth are phenomenological in nature and rely almost solely on curve fittings and experimental data. These empirical models are used to predict delamination growth rather than aid in understanding it. This lack of knowledge on the physics of delaminations causes problems for both academia and industry. From the perspective of academia, science needs to be built upon fundamental understanding. However, this is currently not the case for delamination growth. Phenomenological trends, for which the reasons are not yet clear, are assumed as fact, and science tries to advance building on these trends. Meanwhile, from the perspective of industry, engineers compensate for the lack of fundamental understanding with conservativeness, overdesign and a great amount of tests, yielding extra costs. Therefore, the present thesis seeks to understand the fundamentals of delamination growth by physically characterising it. This characterisation is performed relating the strain energy dissipated in delamination growth with the delamination growth rate and the damage mechanisms encountered on the fracture surfaces. To this aim, carbon-epoxy unidirectional laminated specimens were manufactured and tested under mode II and mixed-mode static and fatigue loading. Fracture surfaces were analysed on a Scanning Electron Microscope, and the damage mechanisms were identified and correlated to the strain energy dissipated...@en

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue in-plane shear delaminations. ENF tests were performed and the results showed that damage propagates ahead of the main crack tip in a process zone, which dissipates energy that should be accounted for when characterizing mode II delamination growth. It is not possible to measure appropriately the extent of the process zone in an ENF specimen from observations made from its side, and the definition of a crack tip is not advisable in mode II delamination studies. Instead, an effective crack length, which includes the damaged zone ahead of the main crack tip, should be defined. More studies are necessary in order to understand and define appropriately the extent of the process zone in mode II delaminations.@en

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue in-plane shear delaminations. ENF tests were performed and the results showed that damage propagates ahead of the main crack tip in a process zone, which dissipates energy that should be accounted for when characterizing mode II delamination growth. It is not possible to measure appropriately the extent of the process zone in an ENF specimen from observations made from its side, and the definition of a crack tip is not advisable in mode II delamination studies. Instead, an effective crack length, which includes the damaged zone ahead of the main crack tip, should be defined. More studies are necessary in order to understand and define appropriately the extent of the process zone in mode II delaminations.@en

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue delaminations caused by in-plane shear. ENF quasi-static and fatigue tests were performed and the results show that damage propagates ahead of the crack tip in a process zone. Acoustic Emission was used to verify that the process zone dissipates energy which should be accounted for when characterizing mode II delamination growth. The extent of the process zone in an ENF specimen cannot be measured by the means of visual observations made from the side of the specimen. Therefore, the definition of a crack tip is not recommended in mode II delamination studies. Instead, an effective crack length that includes the damaged zone ahead of the crack tip should be defined. More studies are necessary to understand and quantify fracture in mode II delamination growth before developing methods to assess it using fracture mechanics.@en

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue delaminations caused by in-plane shear. ENF quasi-static and fatigue tests were performed and the results show that damage propagates ahead of the crack tip in a process zone. Acoustic Emission was used to verify that the process zone dissipates energy which should be accounted for when characterizing mode II delamination growth. The extent of the process zone in an ENF specimen cannot be measured by the means of visual observations made from the side of the specimen. Therefore, the definition of a crack tip is not recommended in mode II delamination studies. Instead, an effective crack length that includes the damaged zone ahead of the crack tip should be defined. More studies are necessary to understand and quantify fracture in mode II delamination growth before developing methods to assess it using fracture mechanics.@en

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue delaminations caused by in-plane shear. ENF quasi-static and fatigue tests were performed and the results show that damage propagates ahead of the crack tip in a process zone. Acoustic Emission was used to verify that the process zone dissipates energy which should be accounted for when characterizing mode II delamination growth. The extent of the process zone in an ENF specimen cannot be measured by the means of visual observations made from the side of the specimen. Therefore, the definition of a crack tip is not recommended in mode II delamination studies. Instead, an effective crack length that includes the damaged zone ahead of the crack tip should be defined. More studies are necessary to understand and quantify fracture in mode II delamination growth before developing methods to assess it using fracture mechanics.@en

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue delaminations caused by in-plane shear. ENF quasi-static and fatigue tests were performed and the results show that damage propagates ahead of the crack tip in a process zone. Acoustic Emission was used to verify that the process zone dissipates energy which should be accounted for when characterizing mode II delamination growth. The extent of the process zone in an ENF specimen cannot be measured by the means of visual observations made from the side of the specimen. Therefore, the definition of a crack tip is not recommended in mode II delamination studies. Instead, an effective crack length that includes the damaged zone ahead of the crack tip should be defined. More studies are necessary to understand and quantify fracture in mode II delamination growth before developing methods to assess it using fracture mechanics.@en