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The micromechanisms by which ductile fracture extended from a pre-existing crack was experimentally observed for two classes of forged SA 508 pressure vessel steel. The micromechanisms were related to the measured values of fracture toughness characterised by the resistance to crack initiation and growth. This comparison was only possible with the aid of an accurate fracture resistance test technique which could determine the crack growth toughness from a single specimen. An unloading compliance test system was developed and was used for the construction of crack growth resistance curves. Microstructural parameters determined from a specimen were related to the toughness measured on that specimen and this proved invaluable in isolating the controlling parameters. The effect of orientation and location on the toughness of the materials was assessed. The crack growth resistance was sensitive to the orientation of the crack with respect to the maximum hot working direction and the bands of segregation associated with elongated manganese sulphide inclusions. The toughness was high when the crack plane was perpendicular to the segregation bands and low when the crack plane was parallel with the bands. The location of the crack-tip through the thickness of the forging had a minor effect on the crack growth resistance. A limited study of test temperature, strength level and isothermal ageing was undertaken. Testing within the dynamic strain ageing regime of temperature had a marked effect and reduced the crack growth resistance to below the value at room temperature. Increasing the strength level of one steel by re-heat treating had no effect on the crack growth resistance. Subsequent isothermal ageing treatments also had no effect on the resistance curves. The magnitude and extent of void formation around growing cracks was studied and related to the applied loading. The size, shape and distribution of inclusions was characterised for the materials and orientations used in the fracture tests. Correlations between inclusion parameters and toughness revealed the important microstructural parameters controlling initiation and crack growth. Simple models for initiation and crack growth resistance were developed which take the controlling parameters into account. These models are shown to agree reasonably well with some experimental data.

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The development of precise data for the effect of chemistry and processing variables on the fracture toughness of low to medium carbon cast steels, is paramount to an increased understanding of the fracture behaviour of these important engineering materials. This knowledge would, in itself, lead to improvements in alloy development/selection rationale and increased cast steel usage, particularly in critical areas. / Toward this end, an investigation into the influence of Cr, Mn and Ni additions on the room temperature fracture toughness and flow properties of 0.30, 0.40 and 0.55 wt% C cast alloys was conducted. In order to avoid the chemical and processing complexity normally associated with commercial castings, the study was undertaken on heavy section castings produced in the laboratory from melts composed of commercial purity ingredients. Measurements were conducted on three-point bend test pieces machined from heat-treated castings. The fracture toughness techniques used included LEFM (K(,IC)), J integral and COD ((delta)(,c)). A potential drop method was employed to record crack initiation. / Very high room temperature fracture toughnesses were achieved in the majority of the alloys examined, with only three alloys having K(,IC) values of less than 150 MN.m('-3/2) or COD, (delta)(,c), values of less than 0.1 mm. Further, yield stress ((sigma)(,ys)) values were found to lie within 80-105% of those recorded for commercial castings of comparative composition. / At the 0.30 and 0.40 wt% C levels it was found that the room temperature fracture toughness of the alloys was relatively independent of the microstructure as well as the amount and particular addition element employed, although variations in the tensile properties were noted. / At the 0.55 wt% C level significant differences in the fracture behaviour were caused by the individual element additions. Manganese was found to improve the toughness of the high carbon alloys, while nickel produced little change and chromium had a negative effect on toughness. In each case the influence of these elements was manifested in microstructural differences. / The results of this study are reviewed in the light of the limited data available from commercial cast steels and the need for improved toughness performance without undue loss of yield strength and other properties. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI

In nuclear environments, irradiation hardening and damage have a detrimental effect on materials performance. Among others, fracture toughness of austenitic stainless steels decreases under neutron irradiation. Helium arising from transmutation reactions is one source of embrittlement leading to that decrement and it is here assumed as a case study, austenitic steel 316L being the material under investigation. The experimental reproduction of irradiation hardening effect on yield stress is attempted here by pre-strain under tensile loading at room temperature. The experimental production of porosity is attempted by inducing ductile damage, creep damage or a combination of them. Damage at the microstructural level is analyzed by metallography, fractography, X-ray tomography and quantified by image processing.After calibrating the elastic, the plastic and the porous plastic constitutive equations by the means of tensile tests on smooth and notched specimens, results from damaging experiments are validated by finite element analysis using the Gurson-Tvergaard-Needleman model. The numerical models obtained represent different levels of damage into the material, as induced by the experiments.Material presenting different levels of damage is then machined for fracture toughness evaluation in the shape of sharp-notched round bars. Fracture toughness initiation is inferred from the load vs. displacement plots applying an opportune fracture criterion. In order to test the suitability of the Gurson-Tvergaard-Needleman model, the load vs. displacement results are validated by retrofitting opportune constitutive laws for each “damaged” state. Retrofitting is discussed in relation to the type of damage produced.Results show that the reproduction of the macroscopic effect of irradiation hardening on yield stress may be attempted for 316L by a pre-strain tensile loading at room temperature for levels up to 1.5 dpa or slightly more. These interrupted tensile tests did not give evidence of void volume fraction production. Creep tests at 650 °C showed sensitization at the grain boundaries but not porosity into the matrix. Creep tests at 1000 °C created 1.2% to 1.8% void volume fraction from grain boundary sliding. Finally, one 7% pre-strained specimen was subjected to creep test at 900 °C and stopped at 5% creep strain, without evidence of porosity into the matrix.Fracture toughness tests on the “damaged” states obtained before showed a decrement of fracture toughness initiation when compared with “undamaged” 316L. Specimens with 30% and 40% eng. strain presented a sensible decrement and exhibited a brittle-like behaviour. The differences in porosity size and physical processes involved suggest not stating that a correlation exists with the helium embrittlement effect on the same property. The Gurson-Tvergaard-Needleman model worked for the “undamaged” material. It proved to be not suited for the brittle-like 30% and 40% eng. strain “damaged” materials because it did not capture the experimental progression of damage.In the end, fracture toughness numerical predictions were made using different values of initial void volume fraction. It was argued that, starting from a threshold value, the brittle-like 30% and 40% eng. strain “damaged” materials revert to a ductile behaviour.

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Defect assessment against fracture initiated failure is carried out using fracture characterising parameters determined under quasi-static rates of loading. In practice, however, there are many instances where much higher loading rates prevail such as collision, blast and earthquake damage; and in transport. For these situations the rate sensitivity of the material to fracture should be considered. Fracture toughness tests (COD) have been conducted on C-Mn steel weld metal over a range of temperature and loading rates. The effect of increased loading rates is to reduce the crack-opening displacement whilst changing the fracture behaviour, such change being accompanied by an increase in the ductile-brittle transition temperature. Thermally activated flow is found to be the predominant mechanism governing plastic flow at intermediate strain-rates. It is then possible to evaluate the effect of strain-rate and temperature on the yield strength as a function of rate parameter kT 1n (A/ε), based on the Arrhenius equation. An attempt is presented to model the influence of temperature and loading rate on the fracture initiation toughness, COD. It may be fairly said that numerical results quite well describe the behaviour of the experimentally determined variation of COD with temperature and loading rate over the range 0.1 < K < 10⁶MPam^{1/2 s}^-1 and O < T < 500^oK. Thus, it has been demonstrated that the constitutive surface δI (T, K_I) can be produced numerically using the constitutive relation σy (ε,ε, T). Consideration of the defect sizes for engineering critical assessment showed that a significant reduction in maximum allowable defect size, bar a_max can result as a consequence of increased loading rate.

Thesis (Ph.D.)--Aberdeen University, 1989. / Title from web page (viewed on Mar. 4, 2010). Includes bibliographical references.

The spray cooling system of an operating billet caster has been redesigned with the aim of reducing the formation of mid-way cracks. These cracks are caused by tensile strain which is generated at the solidification front when the surface temperature of the strand rebounds owing to a sharp reduction in surface heat extraction. The objective of the design, therefore, was to achieve a cooling system that would minimize surface temperature rebound of the strand as it passes from one cooling zone to the next. A computer program based on the explicit finite difference method has been used for the design work. The spray design was implemented on one strand of an operating continuous casting machine which produced 10.8 cm square billets. Transverse sections were cut from the test strand and sulfur printed, then compared to sulfur prints of sections taken from an adjacent strand of the same heat but with unmodified sprays. It was shown that with empirical adjustment, the redesigned spray system reduced the severity of mid-way cracks in over 80% of the heats. It was also found that the carbon content and cast structure have a profound effect on the cracking tendency, whereas, the Mn/S ratio (up to 30%) is less effective. Finally, a new design method for sprays has been proposed which may result in a better temperature distribution and may be easier to adjust to suit specific operating conditions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate