Identifier

etd-07062005-070755

Degree

Master of Science (MS)

Department

Renewable Natural Resources

Document Type

Thesis

Abstract

In this study, the effect of notch length on impact strength and fracture toughness was examined to exploit the use of wood plastic composite (WPC) as structural materials. Impact and fracture toughness test methods and estimation procedures were carried out. To evaluate the impact strength of WPC, five different notch sizes with two different fiber orientations on the load head were prepared. In terms of fracture mechanics, notch length was converted to stress concentration factor and the relationship between stress concentration factor and impact strength was determined. Fracture surface of impact specimens was investigated to evaluate the fracture mechanism of WPC by scanning electron microscopy (SEM). For the determination of fracture toughness of WPC, a short bar specimen with a rectangular cross section and a different notch size was used. To obtain the stress intensity factor (KI) for the mode I case, a finite element method (FEM) was carried out. The simulation of the stress intensity factor was performed on four successively refined meshes via quarter point elements around the crack tip. By means of an asymptotic analysis, the verification of the simulation was also presented. The experimental results showed that impact strength of WPC was highly dependant upon the fiber orientation and stress concentration factor. However, fracture toughness was independent of the change of the length of the notch. Fracture toughness of WPC was estimated to be 1.79 MPa√m using a four point bending test. The results of the simulation showed that stress intensity factor of WPC was estimated to be 584.9 (kPa√m). The results of normal stress at the crack tip from a common sequence of four successively refined meshes were diverged while the results of the stress intensity factors converged. The strength of singularity for normal stress (σx) was 4.92 which is quite close to that of true singularity (0.5) for the sharp crack tip when Θ=180°. Crack opening angle (COA) of the four point bending specimen was also estimated to be 50° on the basis of strength of singularity determined from finite element method.

Date

2005

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Qinglin Wu

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