 Welcome to the Ph.D. defense of Sharon Kao-Walter!
On 11th of February , you are welcome to Sharon Kao-Walter´s Ph.D. defense at the School of engineering at Blekinge Institute of Technology. Please email henrik.larsson@bth.se not later than 8th of February if you intend to participate.
Would you like to receive a copy of the Ph.D. thesis? Please contact henrik.larsson@bth.se
Place: Blekinge Institute of Technology, room 2304A, North Wing, at Blekinge Institute of Technology, Campus Gräsvik, Karlskrona
Time: Friday the 11th of February 2005, at 1315.
Title: "On the Fracture of Thin Laminates"
Supervisor: Prof. Per Ståhle
Faculty opponent: Professor Ulf Stigh, University of Skövde, Sweden
Examination committee: Professor Solveig Melin Petersson, Lund Institute of Technology, Sweden, Professor Asher A. Rubinstein, Tulane Unviersity, New Orleans, USA and Docent Liu-Ying Wei, Malmö University, Sweden
Deputy member: Docent Claes Hedberg, Blekinge Institute of Technology, Sweden
ABSTRACT
This thesis concerns mechanical and fracture properties of a thin aluminium foil and polymer laminate that is widely used as packaging material. The possibility of controlling the path of the growing crack propagation by adjustment of the adhesion level and the property of the polymer layer is investigated.
First, the fracture process of the aluminium foil is investigated experimentally. It is found that fracture occurs at a much lower load than what is suggested by standard handbook fracture toughness. Observations in a scanning electron microscope with a tensile stage show that small-scale stable crack growth occurs before the stress intensity factor reaches its maximum. An examination using an optical profilometric method shows almost no plastic deformation except for in a small necking region at the crack tip. However, accurate predictions of the maximum load are obtained using a strip yield model with a geometric correction.
Secondly, the mechanical and fracture properties of the laminate are studied. A theory for the mechanics of the composite material is used to evaluate a series of experiments. Each of the layers forming the laminate is first tested separately. The results are analysed and compared with the test results of the entire laminate with varied adhesion. The results show that tensile strength and strain at peak stress of the laminate, with or without a crack, increase when the adhesion of the adhesive increases. It is also found that a much larger amount of energy is consumed in the laminated material at tension compare with the single layers. Possible explanations for the much higher toughness of the laminate are discussed.
Finally, the behaviour of a crack in one of the layers, perpendicular to the bimaterial interface in a finite solid, is studied by formulating a dislocation superposition method. The stress field is investigated in detail and a so-called T stress effect is considered. Furthermore, the crack tip driving forces are computed numerically. The results show that the analytical methods for an asymptotically small crack extension can also be applied for a fairly large amount of crack growth. By comparing the crack tip driving force of the crack deflected into the interface with that of the crack penetrating into the polymer layer, it is shown how the path of the crack can be controlled by selecting a proper adhesion level of the interface for different material combinations of the laminate. |