Browsing by Author "Hancock, P."
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Item Open Access Creep Fatigue Analysis of a Nickel Superalloy over the Range 750-1040°C(Cranfield University, 1979-09) de Abreu e Antunes, V. T.; Hancock, P.After the review of the predictive methods in the high temperature fatigue field the characterization of the low cycle fatigue behaviour of a nickel superalloy, t4ARMOO2, is carried out using one of those predictive methods, the Strainranqe Partitioning method. The basic concepts and procedures of this method are reviewed and applied to NARt"O02 over the range 750° - 1040 C. -These'procedures invol ed the determination of the four basic life relationships at P50 C and 1040°C and their use in conjunction with a damage rule were then employed to assess the degree of insensitivity of the four basic strainrange versus life relationships to test temperature. Three damage rules and the assumptions behind them are discussed within the context of their application with the Stra i nrange Partitioning approach. An alternative analysis based on Ostergren's method is studied in a separate chapter and proved to be an easy and helpful method in predicting high temperature lot., cycle fatigue of several superalloys. The characterization of the creep-fatigue behaviour of 1,4F. RP"002 was completed by separation of initiation and propagation periods within two temperature intervals. A situation of earl formation of cracks at the higher temperature intervaa 950°- l040 C with accelerated growth. at lower tennperatures 750 - 250oC showed that great care is necessary when applying MAPP"002 isothermal results to cases where temperature varies.Item Open Access Fracture toughness testing of a weldable steel(Cranfield University, 1981-08) Al-Hadithy, Nabil Abdul-Jalil; Hancock, P.The mechanical anisotropies of two varieties of BS 4360 50D steel were demonstrated for tensile, Charpy V impact and COD testing. The detrimental effect of sulphide inclusions for through thickness behaviour was related to the total inclusion length per unit area and the interaction of inclusions within the plastic zone. Sulphide inclusions in the through thickness orientation were found to increase crack tip constraints, reduce the plastic zone size and the post-yield load bearing capacity of the specimen ligament. This was achieved by the ease of movement of the growing crack tip to various planes containing suitable clusters of inclusions. COD at maximum load was shown to decrease exponentially with increasing inclusion content. For acceptable through thickness toughness it was shown necessary to limit the inclusion density to 1 to 1.5mm/mm2, according to the constraints in its application. For plate or pipe material made from highly rolled steel, low sulphur steel melts must be used. Even rare-earth-modified steels will show unacceptable inclusion densities after high rolling strains in the mill. The inclusion density would increase both in the ZR and ZT through thickness directions. Notches along the rolling plane gave lower toughness levels due to inclusion interactions out-of-plane facilitated by the added length of the inclusion caused by hot rolling. A computer program was included which uses a polynomial fitting technique relating COD to inclusion density to predict through thickness toughness for any given inclusion content. A method was devised for through thickness testing of thin plate materials. Valid maximum load COD values were obtained by friction welding of extensions to undersized specimens. The effect of free surfaces that interacted with the plastic zone at the crack tip were found to be beneficial to crack tip deformations. For these reasons thin plates were shown to be unsusceptible to through thickness failures, such as, lamellar tearing, unless the effects of the free surface were removed by welding attachments to the surface. At upper shelf temperatures for ductile structural materials, it was shown that the measured toughness was related to the size of the plastic zone and, hence, to specimen geometry. The measured toughness related more to the absolute length of uncracked ligament rather than to crack length or crack length to width ratio. To -achieve geometry independence, the ligament length was required to be greater than the characteristic plastic zone size for the specimen thickness and testing temperature. The extensive plastic zone and the small highly strained zone sizes were shown to be dependent on temperature. At high temperatures the crack tip strains increased which resulted in crack blunting and stress dissipation. The work hardened material ahead of the crack tip showed evidence of small crack fissuring which led to stable crack growth. At low temperatures, where brittle fracture occurred, the highly strained region and extensive plastic zone were much smaller in size. Crack blunting was greatly reduced which allowed brittle fracture stresses to be achieved at the crack tip. At intermediate temperatures the crack tip blunting was extensive and it was shown that the amount of ductile cracking required to increase the constraint necessary to achieve brittle fracture decreased with decreasing temperature. Large specimen sizes resulted in high stored energy which, at transition temperatures, drove the ductile crack to catastrophic failures. This resulted in narrower transition regions which effectively shifted the transition to higher temperatures. The transition from ductile to brittle behaviour was considered, from the experimental evidence supplied, not to represent a shift in the state of stress. The through-thickness strain decrease associated with this transition was a result'of the associated decrease in COD. As the COD was shown to vary with specimen geometry independently of specimen thickness and, hence, independ- ently of the state of stress, then the associated change in through thickness strains were also not indicative of a change in the state of stress. A shift in the state of stress was considered to occur only when the through thickness strains varied with respect to COD. The method of load application was shown to have a significant effect on the measured toughness. Generally yielded specimens were shown to be under greater crack tip constraints when the specimen was loaded in bending rather then in tension.Item Open Access The influence of imposed strain rate on fracture of surface oxides.(Elsevier Science B.V., Amsterdam., 1993-01-01T00:00:00Z) Hancock, P.; Nicholls, John R.; Mahmood, K.The mechanical properties of chromium rich scales formed on 304 stainless steel have been investigated as a function of deformation rate and operating temperature. At 900C at slow strain rates < 10-6 per second no cracking was observed at strains up to 10%. At rapid strain rates in excess of 10-4 per second oxide cracking was found to be independent of strain rate and controlled by the fracture toughness of the oxide. In the intermediate region, with strain rates between 10-4 per second and 10-6 in the temperature range 700 to 950C, the behaviour is determined by the creep deformation and fracture mode of the oxide. The mechanism of surface oxide failure is examined and an equation to predict cracking density over the full range of both monotonic tensile and creep fracture modes is suggItem Open Access Influence of strain rate on oxide fracture(Cranfield University, 1988-10) Mahmood, K.; Hancock, P.The ability of metals and alloys to form and retain protective oxide scales is crucial to their stability at elevated temperatures for extended times. Hence the identification of factors that promote or limit the integrity of oxides on high temperature materials has been the subject of intensive investigations. In the present study the mechanical properties of this chromiwm.-rkh scale on 304 stainless steel foil has been investigated in relation to the deformation rates in the substrate. It was shown that heavy cold working (up to 90%) delays the onset of breakaway oxidation and results in a very adherent scale. The cracking behaviour of the scale was found to be strain rate and temperature dependent under slow strain rate conditions when the substrate deforms by creep. No strain rate dependence was observed over the temperature range 700-900°C when faster strain rates (> 10- S sec -1) were applied. The transition between these two responses was found to vary only slightly with temperature between S.Ox10- S sec- 1 and 7.Sx10- S sec -1 ,increasing as the temperature is raised. A new method has been described for determining the fracture behaviour of oxide scale by estimating the composite defect size. From a knowledge of the onset of scale cracking, determined in Sl(U usi ng (h~ acoustic emission technique, it was possible to correlate the measured intercrack spacing with the fracture toughness from which the tensile properties of the scale can be evaluated.Item Open Access Multilayer TiB2/X hard coatings by sputtering deposition(1998-10) Da Silva, Maria de Fatima Oliveira Vales; Hancock, P.; Nicholls, J. R.Titanium diboride has been investigated as a potential candidate for aerospace structures, cutting tools, surface coatings of first-wall components and diffusion barriers in integrated circuit metallization. Titanium diboride is a very stable hard refractory compound but its brittleness is the main drawback. It was possible to lessen the TiB2 brittleness by producing TiB2/X coating designs by the multi-target RF magnetron sputtering process. X is the metal layer (Al, Ti, NiCr, Mo) in the composite system. The influence of the composition wavelength and volume fraction of ceramic has been studied over a range of sputtering conditions. The most suitable multilayer coating design (TiB2/NiCr) on steel substrate, for maximum hardness (18.81GPa) and elastic modulus (304.6GPa) was found to be with a composition wavelength of 50nm and volume fraction of ceramic of 75%. The greatest improvement of the elastic modulus measured by nanoindentation was found to be for a TiB2/Al two-layer coating design either on steel or on aluminium substrate, giving 36.2% and 40% improvement above the rule of mixtures respectively, when compared with TiB2 coatings deposited under the same sputtering conditions. Several pieces of three-point bent apparatus were designed for measuring the inplane elastic modulus of the coatings. The three-point bent test by nanoindenter shows promise as a method for measuring the in-plane elastic modulus on uncoated beams. A comparison between traditional and non-traditional methods of measuring mechanical properties of the coatings was performed in this study. The nanoindentation technique was found to be an appropriate method to measure the mechanical properties of multilayer coating designs.Item Open Access Static and Dynamic Fracture of Structural Steel.(Cranfield University, 1983-11) Smith, Paul; Hancock, P.; Spurrier, J.The present study is concerned with the assessnent of structural steel fracture toughness, as close to real service loading conditions as practically possible in the laboratory, using small scale specimens. The effects of stored strain energy content is evaluated for slow-static and dynamic COD tests for maximum load and cleavage instability. The literature reviews elastic-plastic fracture mechanics and goes on to study the effect of stored strain energy, the COD technique and dynamic testing procedures presently available. Static and dynamic fracture toughness testing using the COD technique is carried out on BS4360 - 50D structural steel in its normalised state. The testing procedures used closely relate to either the BS5762 COD standard or BS5447 plane strain standard. The specimen size tested is 2B =U= 24 mn, with a fatigue notch size of approximately a/W between 0.48 and 0.57. Photographic-macros and SEM fractography were carried out after the specimens were tested to assess the micromechanism processes operative during a fracture test. It is believed the present work is of special significance to determinate structural design using structural steel, for example with liquefied gas pressure vessels. The resulting test data available from this thesis is envisaged to be the closest approach to real service "true limit severity", and consequently is beneficial to fracture prevention technology.