PSI - Issue 33

Jamaloddin Jamali et al. / Procedia Structural Integrity 33 (2021) 832–842 Author name / StructuralIntegrity Procedia 00 (2019) 000–000

833

2

Nomenclature K I

mode I fracture intensity factor

a

crack length

B

specimen thickness

On the other hand, the first studies on synthetic polymers were conducted by scientists and engineers in the middle of the 19th century. Polystyrene, polyethylene, nylon and silicone are a few of the many examples of synthetic polymers. Polymers are slowly replacing metals because they have weight reductions of up to 80 percent and have high stiffness and easier to manufacture and install compared to metals. Nowadays, there is a rapid growth and development in the polymer industry due to their required properties that are replacing metals in many different industries such as the automotive, aerospace electronics and energy industries to name a few. There are many other reasons why polymers are becoming an optimal choice over other materials which include:  Polymers are considered to be resistant to chemicals  Polymers are good insulators of heat and electricity  Polymers are lightweight with significant degrees of strength when compared to metals  Polymers have high stiffness  Polymers have different ways of processing (extrusion, injection molding, mixed with solvents etc.)  Different polymers have different characteristics for the required usage purpose A disadvantage of using polymers is their low fracture toughness compared to metals. Toughness is the ability of the material to absorb energy and experience plastic deformation without fracturing. As for fracture toughness, it is the critical stress intensity factor KIC of a sharp crack in which the material resists further fracture when this sharp crack exists. Another disadvantage of using polymers is that the study of their mechanical properties after yield point and the prediction of the work of fracture is inadequate in contrast to metals. This is because the study of polymers and their technological advances are considered to be far less developed. It is already known that failure in polymers can take place when the stress intensity is low (and lower than the tensile strength) due to creep rupture, fatigue, defects in the material’s structure and finally, environmental stress cracking agents. While this information is key to understanding how polymers behave during failure, there are still missing data in the literature of materials and engineering to fully comprehend polymers. The following research paper explores the work of fracture of polyester by performing different fracture tests as well as looking for ways to increase the fracture toughness. This is done by modeling, simulating and manufacturing fracture testing fixtures and fabricating polymer specimens to perform fracture testing using the manufactured fixtures in order to study their fracture properties to have a better understanding of PMC’s. 2. Background and formulation There are three modes of fracture: when a tensile stress is subjected normal to the plane of the crack, it results in mode I (opening) fracture. Mode II (sliding) fracture is when a shear stress in subjected parallel to the plane of the crack and normal to front of the crack. Mode III (tearing) fracture occurs when a shear stress is subjected parallel to both the plane and front of the crack (Fig. 1). Mode I, is associated with most of the existing polymers and the damage is quite extensive and is of interest by engineers in structural analysis and failure analysis. Studying mode I is the first step in investigating fracture of polymers and will be completed by studying other modes in the next steps. When performing mode I fracture testing of plastics, the most common tests are the compact tension (CT) and single edge notched bend specimens (SENB) to obtain fracture toughness. The standard ASTM D5045 (2014) features standardized tests for both CT and SENB; it obtains the fracture toughness in relation to critical stress intensity and critical strain energy release rate of plastic materials. The conditions and assumptions that should be followed when conducting these tests are: