PMMA(poly methyl metha crylate)

POLYMETHYL METHACRYLATE  (PMMA)

INTRODUCTION

Polymethyl methacrylate (PMMA) has many excellent performances such as lightweight, high temperature resistance, high light transmittance, and superior mechanical properties. Thus, the structures made of PMMA are widely used in aviation field.

The mechanical properties of PMMA have attracted considerable attentions since several planes crashed were caused by the cracks in the hatch. The researches in the fatigue and fracture properties of polymer materials started from the 1960s for the strict requirements of strength and reliability. Berry (1961) confirmed that Griffith strength theory could be used to analyze the brittle fracture of PMMA. Recently, this theory and experiments have become the important principles to analyze the brittle fracture of polymer material. Mukherjee and Burns (1971) proposed that the fatigue behaviors of PMMA were determined by three parameters: stress intensity factor amplitude, average stress intensity factor and frequency. Woo and Chow (1984) unified fatigue crack propagation formula of metal aluminum and nonmetal PMMA. They proposed that strain energy release rate amplitude should be used to analyze the crack propagation but not stress intensity factor amplitude. Cheng (1990a, 1990b) studied the influence of temperature and loading rate to tensile strength and fracture toughness of PMMA. Kim (1993, 1994) proposed that fatigue crack growth rate of most polymers increased with the temperature increasing and decreased with the loading frequency increasing. Ramsteiner and Armbrust (2001) solved some fatigue experimental questions of polymers such as the measurement of crack propagation, the influence of the specimen shape and the applied frequency, the measurement with constant or increasing stress intensity amplitude, and the propagating crack as a signal for transitions in internal deformation mechanisms. Yao (2002, 2003, 2004) investigated dynamic fracture behavior of thin PMMA plates with three- and four-parallel edge cracks by means of the method of caustics in combination with a high-speed Schardin camera; dynamic fracture behavior of the thin PMMA sheet with two overlapping offset-parallel cracks under tensile loading using the optical method of caustics in combination with a Cranz-Schardin high-speed camera; and the fracture characterization of a V-notch tip in PMMA material by means of an optical caustics method, respectively. Sahraoui and El Mahi (2009) measured the dynamic fracture toughness of notched PMMA at high impact velocities, where the classical method was limited by the inertial effects. The direct measurements of the specimen deflection were successfully used for the toughness evaluation.Sauer and Hsiao have started to investigate the craze phenomenon of polymer early in 1949. Simultaneously, Kies and his co-workers got some inspiration for the top of PMMA hatch have the better craze resistance, and researched biaxial and multiaxial tension directional PMMA. Directional PMMA is manufactured in the following way: PMMA plate is pulled under directional stresses according to a pre-selected temperature curve including the heating, keeping and cooling. Compared with normal PMMA, directional PMMA has more excellent mechanical properties such as higher pull strength and elasticity module. Some important elements of airplanes are made of directional PMMA plates such as hatches.

The previous studies have made a tremendous contribution to the failure research of PMMA. However, most of the previous researches are based on the isotropic mechanical model. On account of the special processing of directional PMMA, it is difficult to reflect the mechanical properties of directional PMMA with the isotropic mechanical model. The purpose of this paper is to establish an anisotropic mechanical model by isodyne method and measure all the mechanical parameters by the digital image correlation method for the aeronautical directional PMMA. Furthermore, we also utilized the FEM numerical simulation and experimental methods to study the fracture mechanics properties of directional PMMA in two different directions: One is along the direction of directional tension and the other is along the vertical direction of directional tension.

FRACTURE MECHANICAL PROPERTIES

Because of the anisotropic mechanical behaviors in directional PMMA, the fracture mechanical properties of crack propagation along different directions are different. According to the anisotropic mechanical model and mechanical parameters, finite element method (Chan et al., 1970) is used to calculate stress field and strain field around the tip of crack along different directions. One is along the direction of directional tension; the other is along the vertical direction of directional tension. Three point bending specimen is modeled and the size and load of specimen1 and specimen2 The length of the model is 40cm, the width is 10cm, and the length of crack is 5 cm. The area of the crack tip is singularity, so around this area singularity element which is a kind of triangular element of six nodes is used.Directional PMMA is a kind of polymer material. It is composed of molecular chains and brittle material, so the damage of directional PMMA is relative with the deformation. Asthe analysis to the strain around the tip of crack shows that the values of shear stress are small compared with normal stress, but the values of shear strain and normal strain are in the same level, so shear stress could not be neglected. In the same stress state, the strain values around the tip of the specimen2 are larger than that of the specimen1, so the crack along the vertical direction of directional tension extends more easily

Fracture of Polymers

• Fracture strengths of polymers are low compared tometals and ceramics.
• Brittle fracture occurs in thermosetting polymers.Fracture is initiated at stress concentrators (scratches notches, etc). Covalent bonds are severed during fracture
• In thermoplastic polymers, both ductile and brittle fracture are possible Brittle fracture is favored atlower temperatures, higher strain rates, and at stressconcentrators
• Brittle to ductile transition often occurs with increasing temperature

Microstructure Of  PMMA

APPLICATIONS OF PMMA:

 

CONCLUSIONS:

As the anisotropic mechanical properties in the directional PMMA, the fracture mechanical properties of crack propagation along different directions are different which are analyzed by experimental methods. The stain values of the crack tip along the different directions of crack propagation  which indicates that the crack along the vertical direction of directional tension extends more easily than that along the direction of directional tension. Fracture toughness value measured by experiment along the direction of directional tension is larger than that along the vertical direction of directional tension. So the crack along the vertical direction of directional tension extends more easily.