Measurement methods of residual stresses of high strength structural composites

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Measurement methods of residual stresses of high strength structural composites

 

Author: Jiang, Hao
Title: Measurement methods of residual stresses of high strength structural composites
Degree: Ph.D.
Year: 2013
Subject: Residual stresses -- Measurement.
Composite materials.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xxiii, 257 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2616039
URI: http://theses.lib.polyu.edu.hk/handle/200/7010
Abstract: More attention has been focused on generated residual stress in the materials, because it has been found that the residual stress plays significant character on performance of entire structure. Various methods and technologies have been developed to demonstrate the distribution of induced residual stress during the engineering application, especially on those materials with outstanding performance of high strength laminated structure. However, due to the feature of amorphous structure, requirements of both reliability and accuracy could not be satisfied with these measurement methods as they applied on alloys. Thus, it is necessary to perform a research project to investigate and verify the considered measurement method, which could be confidently applied on nanocrystalline structure materials. On the other hand, with the application of this reliable measurement method, the generation of residual stress induced by mechanical treatment is also studied as one important factor of improved performance of materials. In this thesis, residual stress determination of both carbon fiber reinforced epoxy composites and bulk metallic glass (BMG), as two gradient, amorphous structural materials with impressive strength, are discussed with application of incremental hole-drilling method. The investigation of measurement method is started with theoretical solution of coefficients calculated by numerical simulations. Based on theory of elasticity, a minimum size of finite element model has been discussed to achieve approximate boundary condition as the assumption of hole drilling method. To reduce the heavy manual work of coefficients calculations for multiply drilling process, a program is compiled to connect the finite element software to execute all the necessary simulation jobs. With the released strains from the measurement and related mechanical parameters being input into the program, the distribution of residual stress could be obtained by output data. On the other hand, to ensure the reliability of results, the stability of the entire hole-drilling system is tested by examining quality of holes and performance of motion stages so that a package of optimized configuration could be determined.
The measurement of residual stresses on the composite samples supplied by Airbus Company is performed with both strain gauges and non-contacting optical system. To enhance the sensitivity of optical system, a new setup of equipment is developed with smaller laser wave length and larger illumination angle. Results show that the magnitude of residual stress in composites is very small (maximum stress of ~150MPa). The thermal residual stress vertical to the fiber direction exhibited tensile stress constrained by high strength of fiber, while those along fiber direction show compressive stress. The residual stresses of BMGs induced by both thermal and mechanical treatments are measured with incremental hole-drilling method. A well acknowledged method for mechanical treatment on surface, surface mechanical attribution treatment (SMAT), which has been proved to generate large compressive residual stresses on alloys, is performed on the BMG sheets. Results from application of incremental hole-drilling method show that an evident compressive residual stress existed at near surface (with maximum compressive stress of ~-500MPa), while subsequent tensile stress is balanced at the center region of BMG sheets. The magnitude and affected depth are also studied with relationship of controlled parameters of SMAT process and generation of residual stress with numerical simulation.

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