G. Malakondaiah

In-plane anisotropy in the fracture toughness of an Al-Li 8090 alloy plate

Abstract Fracture toughness of an Al-Li 8090 alloy plate was studied in L-T, L + 45° and T-L orientations. A significant in-plane anisotropy exists, with the fracture toughness in the L + 45° orientation being 32% lower and that in the T-L orientation being 45% lower than the fracture toughness in the L-T orientation. The observed behaviour is explained on the basis of Chen and Knotts model that relates the toughness of the alloys to the tensile properties and the characteristics of the void initiating particles. The analysis has revealed that the anisotropy in the in-plane fracture toughness arises because of the variation in yield strength, work hardening exponent and interparticle spacing with notch orientation. The variation in yield strength and work hardening exponent is caused by the crystallographic texture, whereas the variation in interparticle spacing primarily results from the highly unrecrystallized pancake microstructure of the alloy.

Influence of polycrystal grain size on fracture toughness of and fatigue threshold in Armco iron

Influence of polycrystal grain size on ductile fracture toughness of and fatigue threshold stress intensity in Armco iron has been studied over a grain size range 40 to 1050 μm. Both ductile fracture toughness and fatigue threshold stress intensity have been found to decrease with increasing grain size and the variation in either case follows a relationship similar to that proposed by Hall-Petch for strength. The variation of toughness with grain size can be understood in terms of plastic zone size whereas the fatigue threshold behaviour in Armco iron appears to be controlled by the critical value of crack tip opening displacement range.

Influence of microstructural features on the fracture resistance of aluminium-lithium alloy sheets

Abstract The effects of various microstructural features on the plane-stress fracture resistance ( K R ) of ternary (AlLi 2090) and quaternary (AlLi 8090) aluminium-lithium alloys are illustrated, discussed and summarized. These microstructural features include (i) the nature of intragranular precipitates (which control the texture governed planar deformation), (ii) the presence of grain boundary precipitates and (iii) presence of coprecipitates. Suitable modifications in the alloy chemistry, processing and thermo-mechanical treatments, that would enhance the plane-stress fracture resistance of these alloys are suggested based on these observations.

An analysis based on plastic strain energy for bilinearity in Coffin-Manson plots in an AlLi alloy

Fatigue failure of a component occurs through an irrecoverable energy dissipating process. The alloy dissipates most of the plastic strain energy as heat, the other forms being vibration and acoustic emission. Some energy will be absorbed by immobile life defects and by surface damage processes. In order to gain greater understanding of the material response to fatigue damage, it is necessary to propose a physical quantity with energy dissipation as a fatigue damage parameter. Two such widely used fatigue damage parameters are the average plastic strain energy per cycle ([Delta]W[sub p]), the area under the hysteresis loop during low cycle fatigue and the total dissipated energy (W[sub f]), the sum of the areas of all the loops before failure. Each alloy has a certain capacity to dissipate the plastic strain energy. When this limit is attained, the cracks which originated during the earlier cycling will propagate and failure occurs. This paper presents the results of the analysis of the low cycle fatigue data obtained in the case of a quaternary Al-Li-Cu-Mg alloy. The aim of this analysis is to provide further understanding on the bilinear nature of the Coffin-Manson power law relationship, which has already been reported by several researchers.

Effect of crack deflection and branching on the R-curve behaviour of an Al-Li alloy 2090 sheet

The methodology to predict the increase in crack growth resistance due to crack deflection and branching is described. These procedures are discussed with respect to an Al-Li alloy 2090 sheet in T6 condition, the R-curve tests on which revealed that the crack path in L-T orientation was deflected and branched while it remained nominally straight in T-L orientation. The resistance to slow stable crack extension in L-T orientation was found to be significantly higher as compared to that in T-L orientation. Such behaviour is rationalized in terms of crack deflection and branching. The observed variation in the crack path morphology and the resulting R-curves in L-T and T-L orientations can be understood in terms of the microstructural features of the alloy.

On the micromechanisms responsible for bilinearity in fatigue power-law relationships in aluminium-lithium alloys

Aluminium-lithium alloys, like many other aerospace structural alloys, exhibit bilinearity in power-law relationships between high strain, low cycle fatigue life (in terms of number of reversals to failure, 2N{sub f}) and plastic strain amplitude ({Delta}{epsilon}{sub p}/2) or average stress amplitude ({Delta}{sigma}/2) or average plastic strain energy per cycle ({Delta}W{sub p}). In the present paper the micromechanisms leading to bilinear fatigue power-law relationships, especially the Coffin-Manson (C-M) relationship, in aluminium-lithium alloys are compiled and discussed.

Effect of heat treatment on fatigue crack growth in Ti6Al3.5Mo1.9Zr0.23Si alloy

Abstract The effect of heat treatment on fatigue crack growth in Ti6.5Al3.5Mo1.9Zr0.23Si has been studied. Recrystallization annealing (960 °C/2 h) followed by a water quench from within the α + β field (875 °C) leads to a significant improvement in the fatigue crack growth resistance as compared to a conventional heat treatment, owing primarily to a transition which occurs at ΔK =20 MPa √m. The improvement in fatigue crack growth resistance can be attributed to the presence of twinning as the primary mode of deformation in the metastable β-phase.

Effect of laser surface treatment on fatigue crack growth resistance in an FeMnAl austenitic steel

Abstract Laser surface treatment (LST) results in significant improvement in fatigue crack growth resistance of an FeMnAl austenitic steel. The observed benificial effect is attributed to extensive crack closure as a result of compressive residual stresses originating from LST. The structure-sensitive nature of the crack growth operative in the base microstructure at low ΔK levels (up to ∼ 40 MPa m 1 2 with LST.

Low Cycle Fatigue Behaviour of an Al–Li Alloy

ABSTRACT The low cycle fatigue behaviour of a commercial Al-Li alloy 8090 has been studied in the as-received (T851) condition in both longitudinal and long-transverse directions. The material exhibits cyclic stability at lower strain amplitudes and cyclic softening at higher strain amplitudes. The cyclic strain hardening coefficient (n′) is seen to vary with strain amplitude and the alloy exhibits two stage cyclic work hardening behaviour. The fatigue life - plastic strain data display a bilinear nature of Coffin - Manson plots. Fractographic analysis indicates change in fracture mode with applied strain amplitude. These observations provide a possible explanation for the observed transition in terms of change in both deformation mechanism as well as fracture modes.

Secondary Processing Effects and Damage Mechanisms in Continuous-Fiber Ceramic Composites

Continuous-fiber ceramic composites (CFCC) are a relatively new area of composite materials in research. Therefore, the feasibility of using abrasive waterjets (AWJ) as a secondary CFCC process was explored. Continuous fiber-reinforced advanced ceramic composite materials have been machined with the AWJ piercing and cutting process. The topography and morphology of the machined surfaces were evaluated with surface profilometry and scanning electron microscopy (SEM). The surface characteristics, in terms of roughness and the micro-mechanisms of material removal, were evaluated. The AWJ surface characteristics and associated damage to the CFCC were compared with that of a conventional diamond saw cut surface. The AWJ generated surface was found to be significantly different from that of the diamond saw machining surface and the micromechanical behavior of the material removal was strongly dependent on the fiber orientation.