Yiu-Wing Mai

Effects of rate, temperature and absorption of organic solvents on the fracture of plain and glass-filled polystyrene

The rate/temperature dependent fracture behaviour of plain and glass-filled polystyrene has been investigated over the crack speed (a) range of 10−6 to 10−2 m sec−1 and in the temperature (T) range of 296 to 363 K. TheKc (a, T) relationships obtained, whereKc is the stress intensity factor at fracture, are shown to follow those given by the Williams/Marshall relaxation crack growth model and the toughness-biased rate theory. Crack propagation in both materials is shown to be controlled by aβ-relaxation molecular process associated with crazing. Crack instabilities observed in plain polystyrene are analysed successfully in terms of isothermal-adiabatic transitions at the crack tip. Fracture initiation experiments are also conducted in which the effects of organic liquids on the fracture resistances of both plain/glass-filled polystyrene have been determined. Good correlations betweenKi2 (Ki being the crack initiation stress intensity factor) and δs, solvent solubility parameter, of various liquid environments have been obtained, which give a minimumKi2 value at δs ≈ δp, where δp is the solubility parameter of the polymer. For a given temperature, liquid environment and crack speed, the glass-filled polystyrene is shown to possess greater resistances to crack propagation than plain polystyrene.

Fracture initiation and crack propagation of acrylonitrile-butadiene-styrene (ABS) in organic solvents

The effects of organic liquid environments on the fracture behaviour of acrylonitrile-butadiene-styrene (ABS) have been investigated. Fracture initiation experiments showed thatKi2, (Ki being the stress intensity factor at crack/craze initiation), could be meaningfully correlated with the solvent solubility parameter (δs) of the different liquid environments and had a minimum value atδs=δp, whereδp was the solubility parameter of ABS. For the range of organic liquids used, hydrogen bonding did not have any significant effects on the correlations. It was demonstrated that theKi2−δs correlations could also be usefully extended to other materials such as plain and glass-filled polystyrenes. At a common crack speed (å), the fracture toughness (R) values in “crazing” liquids (i.e. alcohols) were greater than those in “cracking” solvents (i.e. acetone, benzene, toluene, etc.) which usually caused a “dissolution” effect on the plastic. From crack propagation experiments, and using fracture mechanics analyses, definiteR(å) andKc(å) relationships for ABS immersed in toluene, carbon tetrachloride and methanol were determined. These experimental results showed that crack propagation was relaxation controlled and agreed well with a recent theoretical analysis due to Williams and Marshall for environmental crack and craze growth in polymers. Finally, SEM pictures were presented to show the remarkable differences in the fracture morphologies of ABS in both “crazing” and “cracking” liquid environments.

Scale effects and crack propagation in non-linear elastic structures

Abstract New scaling laws are proposed for crack propagation in geometrically similar non-linear elastic structures. The stresses to cause cracking in large non-linear elastic structures are even smaller than those in larger linear structures.

Thermal-stress resistance and fracture toughness of two tool ceramics

The thermal-stress resistance and fracture strength behaviour of two oxide ceramics (a hot-pressed pure Al2O3 and a composite ceramic NTK-HC2) subjected to severe thermal shocks have been investigated. The damage resistance parameter (KIC/σf)2 for both ceramics is also determined for a wide range of temperatures (25° to 800° C) and cross-head rates (10−2 cm min−1 ∼ 2.5 m sec−1). Fracture strength behaviour of these two oxide ceramics is shown to follow Hasselmans model where the instantaneous strength loss at the critical quenching temperature may be calculated using appropriate (KIC/σf)2 values to give good agreement with experimental results. Repeated shocks show some further degradation in the retained strength for both ceramics so that these materials are susceptible to thermal fatigue. It is found that both materials possess similar resistance to crack initiation (i.e. similar ΔTc and retained strength after shocking through ΔTc) but the pure oxide ceramic has higher resistance to crack propagation.

Fracture toughness and thermal shock of tool and turbine ceramics

The behaviour of some commercial tool carbides and turbine ceramics has been investigated in regard to resistance to crack initiation, crack propagation and retained strength after thermal shock. New data are provided, particularly measurements of the fracture toughness of these materials at actual operating temperatures (up to 1200° C). Many of the materials did not follow the generally accepted Hasselman theory for thermal shock in ceramics, and instead of showing a discontinuity in retained strength at some critical quenching temperature difference, their residual strengths fell gradually at temperatures lower than their supposed critical quenching temperature. This behaviour is explicable when high temperature toughnesses, strengths and moduli are used in the damage resistance parameter (ER/gsf2). It seems that materials not following the Hasselman model suffer cumulative damage with increasing number of shocks. Sub-critical crack growth occurs even if (KIC/gsf)2 values are constant, and such damage, which reduces the room temperature retained strength, is enhanced by (KIC/gsf)2 decreasing at temperatures below ΔTc. In contrast, materials obeying Hasselmans model appear to have a constant (KIC/gsf)2 below ΔTc and for some temperature range above. Only then are “one-shock“ characterizations of materials possible, otherwise, the retained strength depends upon the number of prior shocks. Experiments are also reported which describe the effects of rate of testing on the unshocked and shocked mechanical properties of ceramics. Oxidation is shown to influence the results in a manner not obvious from single shock tests.

Cracking stability in tapered DCB test pieces

With the increasing acceptance of DCB test pieces for fracture toughness determinations, many investigators are now interested in modifying the basic design to suit their own requirements. The arms are no longer parallel but contoured or for simplicity of manufacture, tapered so that a constant crack extension force R or stress intensity factor K is available under constant applied loads for a certain range of crack length. This particular design is very often employed in studies of environmental cracking and of standard fracture toughness testing of materials. However, experiments have shown that if not properly designed, the test piece may fail catastrophically under either monotonic increasing load or displacement. Further, the crack length over which K or R is invariant may be too short for experimental usefulness; and added to this, the crack may veer out of the arms during propagation. Since relatively little work has been done on cracking stability in these tapered DCB specimens, we have, in the present note, attempted to illustrate some of the results we have found for this particular problem.