Eric Jon Moskala

Deformation rate dependence of the essential and non-essential work of fracture parameters in an amorphous copolyester

Abstract The plane stress fracture toughness of an amorphous copolyester (aCOP) was determined at ambient temperature as a function of the deformation rate (ν = 1, 10 and 100 mm min−1) by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. It was established that both specific essential (we) and non-essential or plastic work of fracture (wp) are composed terms linked to yielding (subscript y) and necking (subscript n), respectively. The essential terms, i.e. we,y and we,n, did not change with increasing ν. This indicates that by increasing ν no alteration in the initial plane stress conditions occurred. The slopes of regression lines fitted for the wy versus ligament (i.e. β′wp,y) and wn versus ligament (β″wp,n) data tended to increase with increasing ν. The overall shape parameter of the plastic zone (β) slightly decreased with the deformation rate. The plastic zone was formed by cold-drawing and not via true plastic deformation as evidenced by annealing-induced (just beyond tg) shape recovery.

Thickness dependence of work of fracture parameters of an amorphous copolyester

The fracture toughness of an amorphous copolyester (COP) of different sheet thickness (0.5, 3 and 6 mm) was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. It was shown that this COP meets the basic requirement of the EWF concept, viz. full ligament yielding (marked by a load drop in the load-displacement curve) prior to the crack growth, in the thickness range studied. Based on the well-resolved yielding both the specific essential (we) and non-essential work of fracture (wp) were split in the contributing terms related to yielding (wI), and necking and fracture (wII). It was found that we is likely to be independent on the thickness range when plane stress conditions prevail, and thus represents a material parameter. This finding is at odds with previous results suggesting that we is thickness dependent. The development and size of the plastic zone were studied by light microscopy and infra-red thermography. The latter technique overestimated the plastic zone in thicker sheets by not differentiating between pure and diffuse yielding.

Molecular dependence of the essential and non-essential work of fracture of amorphous films of poly(ethylene-2,6-naphthalate) (PEN)

Abstract Plane stress fracture toughness of amorphous films of poly(ethylene-2,6-naphthalate) (PEN) with various molecular weights (MW; characterized by the intrinsic viscosity, IV) was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. These PENs met the basic requirement of the EWF concept: full ligament yielding, which was marked by a load drop in the force–displacement curves of the DDEN-T specimens, preceded the crack growth. This “load mark” allowed us to partition between yielding and necking. The yielding-related EWF terms were not affected by the MW of the resins and thus served for the comparison of the results. It is argued that the EWF response is governed by the entanglement network density. The role of entanglements was substantiated by showing that the “plastic” zone developed via cold drawing and not by true plastic deformation. On the other hand, MW influenced the necking related EWF terms. High MW PENs failed by stable crack growth, whereas low MW resins experienced unstable crack growth (more exactly a transition from stable to unstable crack growth) in the necking phase. This was traced to the load distribution capacity of the long entangled chains. Attempts were also made to estimate the essential and non-essential work of fracture parameters and their constituents from uniaxial tensile tests performed on dumbbells.

A fracture mechanics approach to environmental stress cracking in poly(ethyleneterephthalate)

Environmental stress cracking of amorphous poly(ethylene terephthalate) in aqueous sodium hydroxide was studied by using a fracture mechanics approach. Compact tension specimens were machined from injection-molded plaques and used to determine creep crack growth rate as a function of applied stress intensity factor. The effects of caustic concentration and polymer molecular weight on creep crack growth behaviour were determined. Fractographic analysis of the fracture surfaces showed that crack growth proceeded by the formation of novel discontinuous growth bands. The structure and mechanism of formation of these bands are discussed.

Interrelation between energy partitioned work of fracture parameters and the crack tip opening displacement in amorphous polyester films

The essential work of fracture (EWF) method is one of the most promising relatively new techniques for the fracture characterization of ductile polymers, thin films and toughened composite materials [1–7]. According to the EWF theory, the zone in which a crack runs into the material bulk can be divided into two separate regions: a process plane where the actual crack runs and a plastic zone which surrounds the process zone. In the plastic zone various processes take place which are related with energy dissipation mechanisms such as cavitation, crazing, shear yielding, etc. Consequently, the total work required to fracture a pre-cracked specimen can also be divided into parts associated with each of the two zones mentioned above. Therefore we can write

The effect of temperature on the fatigue crack propagation behaviour of cellulose esters

Abstract The fatigue crack propagation ( FCP ) behaviour of a plasticized cellulose acetate-propionate (CAP) was determined as a function of temperature. Changes in FCP behaviour with temperature were analogous to changes produced by varying plasticizer concentration. A temperature-plasticizer concentration equivalence based on their effect on yield strength was proposed to predict the FCP behaviour of CAP over limited plasticizer concentration and temperature ranges.