國立虎尾科技大學 |

Understanding the Strain Localization and Progressive Damage Growth at the Free-Edge of Composite Laminates using Digital Image Correlation.

紀錄類型:	書目-語言資料,印刷品 : Monograph/item
正題名/作者:	Understanding the Strain Localization and Progressive Damage Growth at the Free-Edge of Composite Laminates using Digital Image Correlation./
作者:	Tessema, Addis.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2019,
面頁冊數:	153 p.
附註:	Source: Dissertations Abstracts International, Volume: 81-02, Section: B.
Contained By:	Dissertations Abstracts International81-02B.
標題:	Mechanical engineering. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10980327
ISBN:	9781085584807

Understanding the Strain Localization and Progressive Damage Growth at the Free-Edge of Composite Laminates using Digital Image Correlation.
Tessema, Addis.

Understanding the Strain Localization and Progressive Damage Growth at the Free-Edge of Composite Laminates using Digital Image Correlation. - Ann Arbor : ProQuest Dissertations & Theses, 2019 - 153 p.

Source: Dissertations Abstracts International, Volume: 81-02, Section: B.

Thesis (Ph.D.)--University of South Carolina, 2019.

This item must not be sold to any third party vendors.

Understanding the mechanics of Fiber Reinforced Composites (FRC) that guide the design and optimization of laminate structures, have attracted numerous researchers. For the last five decades, various analytical and numerical models have been developed to understand the damage and failure mechanisms in FRC. For long, Classical laminate plate theory (CLPT) has majorly been used as a theoretical guide in the design of composite structures. The CLPT framework is developed based on the consideration of an infinitesimal wide plate that depicts a planar stress condition. Thus, as it considers only the planar stresses, the out of plane interlaminar stresses are absent from the CLPT formulation. However, for a laminate with finite width or geometrical discontinuity a complex stress condition (which includes interlaminar stresses) is expected on the free surface of the discontinuities. In such, researcher has developed models that considered a three-dimensional stress condition, and this models have revealed the great relevance of understanding the stress condition at the free-edge on the design and failure prediction of composite structures. Nevertheless, there is still a need for further experimental investigations on the free-edge stress/strains localization and their association with the damage initiated at the free-edge.In this study a noble experimental technique is developed and applied to study the strain localization at free-edge of composite laminate under a monotonic tensile load. The technique is used to evaluate the local strain variation across the layers of the laminate free-edge along with the applied uni-axial tensile load. The developed experimental technique has incorporated high magnification optical system for digital image correlation (DIC) in which the full field displacement and strain are measured in-situ at micro-scale. Further, based on the DIC results, the initiation and growth of cracks from the free-edge can be captured, thus, the influence of the strain localization on the formation of damage can be investigated. In overall, this dissertation is grouped in to four categories to address the various issues raised regarding free-edge effect.In the first section, a single a quasi-isotropic (±45/ 90/ 0)s laminate is considered and the strain localization and progressive damage formation at the free-edge of the laminate is investigate. It was found that the failure is initiated as a matrix crack in the most off-axis plies (90o plies), and gradually grown toward the neighboring plies and eventually expands to interlaminar delamination. The perturbation of local strain during the initiation and growth of micro-cracks is evaluated, and the influence of strain redistribution over the surrounding plies is analyzed. Further, it was found that the shear lagging induced between two adjacent plies has a significant effect in directing the matrix crack growth and inducing delamination.In the second part, four different laminates with varying laminas stacking arrangement are fabricated, and the influence of the relative location of laminas on the interlaminar strain localization and damage formation is studied. From this study, it is found that all the strain localizations are spotted in the 90o plies for all group of laminates. Following the emergence of strain localizations, the matrix cracks are observed to initiate at these locations. The rate at which matrix cracks emerge and the stresses at which the primary matrix crack initiated are seen to vary for the different group of laminates. Moreover, by extracting the strain along the vertical line that cross half the thickness of the laminate, the strains variation along the different layers of the laminate are obtained. From the transverse strain (εzz) plot, 90o plies have shown a positive strain that is unexpected and contradict with the conventional presumptions where the transverse strain is negative for the ply under a tensile load. Further, dominant interlaminar shear strain (εxz) localizations are observed at the +45o/-45o plies interface for all groups of laminate, and this agrees with the theoretical formulations. Whereas, another set of shear strain localizations are observed at the locations where the matrix cracks from the 90o plies reached the interfaces to the neighboring plies. Delamination which is induced from the tip of the matrix crack is observed only in (-45/+45/90/0)s laminate.In the third part, by taking one stacking arrangement with different fiber angle of the off-axis plies, the effect of ply’s orientation on the local intra-lamina and interlaminar strain localization and damage formation is studied. Here also, the evolution of the axial strain contour over the thickness of the laminate is obtained for the three laminates ((0/-45/+45/90)s, (0/-30/+30/90)s and (0/-15/+15/90)s) from the micro-DIC. From the experiments, it is found that at the free-edge there is a higher interlaminar transverse strain (εzz) is observed for the 45o and 30o laminates and the lowest is recorded for the 15o laminate. While the highest interlaminar shear strain (εxz) is found in the 15o and 30o laminates, and the lowest is obtained in and 45o laminates. Further, the predominant damage, matrix cracking, is observed in the 90o ply for all groups of laminates. However, interlaminar delamination is noted at the interface of O/90 plies for the laminates with fiber angles of 45o and 30o, no sign of delamination is found for the fiber angle of 15olaminate. It is presumed that the presence of significant transverse interlaminar stress has bold effect on the formation of delamination in addition to the stress intensity effect due to the matrix crack.In the last section, investigation is conducted to understand the explicit correlation between local damage, material residual stiffness degradation and loss in thermal conductivity of carbon fiber composite. The finding from this study is expected to fill the gap in visualizing the actual phenomenon during analytical or FEA modeling. A quasi-isotropic carbon reinforced composites is subjected to tension–tension fatigue loading and an in-situ digital image correlation (DIC) technique at high magnification is used to capture the initiation and propagation of micro cracks at different cycles. The thermal conductivity is measured using a modified transient plane source technique. The gradual growth of local plastic deformation as a function of loading cycle is acquired and different damage modes as a function of loading cycle is explored. Further, the study has elucidated the gradual growth of matrix cracking and the formation of inter-ply delamination. The gradual growth of local cracks and its corresponding influence on the change in thermal conductivity of the laminate at different fatigue cycles are acquired. Thereby, the effect of local damages on the heat flow and thermal conductivity of the laminate is understood. Similarly, the change in modulus of elasticity with the crack growth is elaborated. Finally, an explicit correlation between the degradation of thermal conductivity and modulus of elasticity is described.

ISBN: 9781085584807Subjects--Topical Terms:

557493
Mechanical engineering.
Subjects--Index Terms:

Material residual stiffness

Understanding the Strain Localization and Progressive Damage Growth at the Free-Edge of Composite Laminates using Digital Image Correlation.
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500 $a Source: Dissertations Abstracts International, Volume: 81-02, Section: B.
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502 $a Thesis (Ph.D.)--University of South Carolina, 2019.
506 $a This item must not be sold to any third party vendors.
506 $a This item must not be added to any third party search indexes.
520 $a Understanding the mechanics of Fiber Reinforced Composites (FRC) that guide the design and optimization of laminate structures, have attracted numerous researchers. For the last five decades, various analytical and numerical models have been developed to understand the damage and failure mechanisms in FRC. For long, Classical laminate plate theory (CLPT) has majorly been used as a theoretical guide in the design of composite structures. The CLPT framework is developed based on the consideration of an infinitesimal wide plate that depicts a planar stress condition. Thus, as it considers only the planar stresses, the out of plane interlaminar stresses are absent from the CLPT formulation. However, for a laminate with finite width or geometrical discontinuity a complex stress condition (which includes interlaminar stresses) is expected on the free surface of the discontinuities. In such, researcher has developed models that considered a three-dimensional stress condition, and this models have revealed the great relevance of understanding the stress condition at the free-edge on the design and failure prediction of composite structures. Nevertheless, there is still a need for further experimental investigations on the free-edge stress/strains localization and their association with the damage initiated at the free-edge.In this study a noble experimental technique is developed and applied to study the strain localization at free-edge of composite laminate under a monotonic tensile load. The technique is used to evaluate the local strain variation across the layers of the laminate free-edge along with the applied uni-axial tensile load. The developed experimental technique has incorporated high magnification optical system for digital image correlation (DIC) in which the full field displacement and strain are measured in-situ at micro-scale. Further, based on the DIC results, the initiation and growth of cracks from the free-edge can be captured, thus, the influence of the strain localization on the formation of damage can be investigated. In overall, this dissertation is grouped in to four categories to address the various issues raised regarding free-edge effect.In the first section, a single a quasi-isotropic (±45/ 90/ 0)s laminate is considered and the strain localization and progressive damage formation at the free-edge of the laminate is investigate. It was found that the failure is initiated as a matrix crack in the most off-axis plies (90o plies), and gradually grown toward the neighboring plies and eventually expands to interlaminar delamination. The perturbation of local strain during the initiation and growth of micro-cracks is evaluated, and the influence of strain redistribution over the surrounding plies is analyzed. Further, it was found that the shear lagging induced between two adjacent plies has a significant effect in directing the matrix crack growth and inducing delamination.In the second part, four different laminates with varying laminas stacking arrangement are fabricated, and the influence of the relative location of laminas on the interlaminar strain localization and damage formation is studied. From this study, it is found that all the strain localizations are spotted in the 90o plies for all group of laminates. Following the emergence of strain localizations, the matrix cracks are observed to initiate at these locations. The rate at which matrix cracks emerge and the stresses at which the primary matrix crack initiated are seen to vary for the different group of laminates. Moreover, by extracting the strain along the vertical line that cross half the thickness of the laminate, the strains variation along the different layers of the laminate are obtained. From the transverse strain (εzz) plot, 90o plies have shown a positive strain that is unexpected and contradict with the conventional presumptions where the transverse strain is negative for the ply under a tensile load. Further, dominant interlaminar shear strain (εxz) localizations are observed at the +45o/-45o plies interface for all groups of laminate, and this agrees with the theoretical formulations. Whereas, another set of shear strain localizations are observed at the locations where the matrix cracks from the 90o plies reached the interfaces to the neighboring plies. Delamination which is induced from the tip of the matrix crack is observed only in (-45/+45/90/0)s laminate.In the third part, by taking one stacking arrangement with different fiber angle of the off-axis plies, the effect of ply’s orientation on the local intra-lamina and interlaminar strain localization and damage formation is studied. Here also, the evolution of the axial strain contour over the thickness of the laminate is obtained for the three laminates ((0/-45/+45/90)s, (0/-30/+30/90)s and (0/-15/+15/90)s) from the micro-DIC. From the experiments, it is found that at the free-edge there is a higher interlaminar transverse strain (εzz) is observed for the 45o and 30o laminates and the lowest is recorded for the 15o laminate. While the highest interlaminar shear strain (εxz) is found in the 15o and 30o laminates, and the lowest is obtained in and 45o laminates. Further, the predominant damage, matrix cracking, is observed in the 90o ply for all groups of laminates. However, interlaminar delamination is noted at the interface of O/90 plies for the laminates with fiber angles of 45o and 30o, no sign of delamination is found for the fiber angle of 15olaminate. It is presumed that the presence of significant transverse interlaminar stress has bold effect on the formation of delamination in addition to the stress intensity effect due to the matrix crack.In the last section, investigation is conducted to understand the explicit correlation between local damage, material residual stiffness degradation and loss in thermal conductivity of carbon fiber composite. The finding from this study is expected to fill the gap in visualizing the actual phenomenon during analytical or FEA modeling. A quasi-isotropic carbon reinforced composites is subjected to tension–tension fatigue loading and an in-situ digital image correlation (DIC) technique at high magnification is used to capture the initiation and propagation of micro cracks at different cycles. The thermal conductivity is measured using a modified transient plane source technique. The gradual growth of local plastic deformation as a function of loading cycle is acquired and different damage modes as a function of loading cycle is explored. Further, the study has elucidated the gradual growth of matrix cracking and the formation of inter-ply delamination. The gradual growth of local cracks and its corresponding influence on the change in thermal conductivity of the laminate at different fatigue cycles are acquired. Thereby, the effect of local damages on the heat flow and thermal conductivity of the laminate is understood. Similarly, the change in modulus of elasticity with the crack growth is elaborated. Finally, an explicit correlation between the degradation of thermal conductivity and modulus of elasticity is described.
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