Sheldon Mostovoy

An evaluation of the short rod technique to measure the fracture toughness of polymers

The investigation of the fundamental variables which influence the fracture toughness of structural plastics is greatly hampered by a large amount of scatter and uncertainty associated with the fracture toughness measurement. A major part of the problem is due to a lack of adherence to ASTM Standard E399, mainly with regard to the requirement for a fatigue crack. A razor-blade arrested crack, which is often blunted, is common practice in the plastics field. It is also common to ignore size (plane strain) and precise machining requirements. The short rod (SR) method was evaluated as a potentially more precise and simpler fracture toughness measurement. This toughness measurement is made on a slowly moving and presumably sharp crack, and the geometry of the sample enforces plane strain conditions. Toughness measurements on compact tension (CT) specimens via ASTM E399 were performed on one-inch (25 mm) samples of poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and polysulphone (PSO). Also, a constant compliance method using a contoured double cantilever beam (CDCB) was used to evaluate the toughness of PS, PC, and PSO, but in general we did not achieve stable crack growth. The used samples were then fabricated into SR specimens and their toughness measured. The CT and CDCB methods agreed with each other for PSO and PC, but for PS the CDCB method gave high values. It is argued that the SR method should be compared to the other methods without using a plasticity correction. Then the SR method agrees well with the CT method for PSO and PS and is 15% higher for PC. The PMMA SR results were invalid. Differences between the methods are explained in terms of crack blunting, rate effects, non-homogeneity, residual stresses and the global nature of the crack front. The SR method has promise for polymer evaluation but more experience and evaluation is needed. The method is unique in the ability to study the effects of thermal history and of the environment on fracture toughness.

Crack Closure Effects on Fatigue Crack Growth Thresholds and Remaining Life in an HSLA Steel

The effects of crack closure on the near-threshold corrosion fatigue crack growth behavior of Mil S-24645 HSLA steel and its weld metal have been investigated in air, ASTM seawater at the free corrosion potential, and ASTM seawater at -0.8V and -1.0V (SCE) using frequencies of 10, 2, and 0.2 Hz, and a stress ratio, R = 0.1. Remaining life, in the presence and absence of crack closure, has been estimated as a function of applied stress range for a structure containing a 3-mm-deep surface semi-elliptical flaw.

Investigation of Hydrogen Assisted Cracking in Pressure Vessels

Hydrogen assisted cracking (HAC) has been investigated in high strength 4140 and low strength Z17D pressure vessel steels, charged at −50 mA/cm2 in 1N H2 SO4 + 25 mg/1 As2 O3 and tested under three-point bend decreasing load. The HAC growth rate for Z17D steel (1.4×10−7 cm/s) was found to be approximately two orders of magnitude slower than that of 4140 steel (3.3×10−5 cm/s), while the threshold stress intensity factor for Z17D steel (∼37 MPa√m) was significantly higher than that of 4140 steel (∼7 MPa√m). This research will show that a single analytical model, based on the hypothesis that hydrogen both reduces crack resistance (R) and increases crack driving force (G), can explain HAC in 4140 and Z17D steels. The model predicts the hydrogen concentration required to initiate HAC as a function of the stress intensity factor and yield strength of the steel. Hydrogen-induced reduction of R was found to dominate HAC in 4140 steel, while hydrogen-induced reduction of R was combined with an increase in G for HAC cracking of Z17D steel.Copyright