Crack Tip Opening Displacement Fracture Research Articles

Upgrading the mechanical properties of marine structural steel by enhancement of acicular ferrite formation

AbstractThis work reports the results of an investigation of the effect of magnesium addition on microstructure evolution, acicular ferrite formation together with the change in the mechanical properties of micro‐alloyed steel plates. The investigated steel is subjected to high heat input (4.5 kJ mm−1) simulating the thermal cycle of heat affected zones, conducted by induction heating in a computer controlled weld thermal cycle simulator. Both magnesium and magnesium free steel samples have carbon and aluminum contents of 0.08 wt.% and 0.009 wt.%, respectively, are investigated. The change in microstructure, hardness and crack tip opening displacement fracture toughness properties due to magnesium addition is evaluated, and compared with the other steel having the same chemical compositions but without Magnesium. The fracture toughness tests reveal the increase of magnesium steel‘s crack tip opening displacement values. In contrast, the hardness measuring presents the decrease of its hardness due to the decrease of martensite volume fraction and reduction in ferrite grain size. Optical and transmission electron microscopy observations detect some microstructural morphology that can affect the mechanical properties of such steels. Hence, the improved toughness of magnesium containing steel are attributed to both the generation of acicular ferrite nucleated within the matrix, refined microstructure and the bainite dominated microstructure in steel.

Fracture behaviour of diffusion bonded titanium alloys with strength mismatch

The present study considers the effect of strength mismatch on the fracture behaviour of diffusion bonded joints between commercially pure (CP) Ti and Ti-6Al-4V (Ti64), including dissimilar joints and sandwich structures with strength undermatching and overmatching. The aim of the investigation is to determine the influence of the interlayer thickness (for both higher and lower strength interlayers) and the bond quality on the deformation behaviour and fracture toughness of the joints. The influence of mechanical heterogeneity (strength mismatch) on the fracture behaviour of the interface in dissimilar joints was also investigated. Round bars of CP Ti and Ti64 having a diameter of 40 mm were diffusion bonded as dissimilar butt joints and sandwich structures containing lower strength (undermatching) and higher strength (overmatching) interlayers of different thicknesses. Round transverse tensile specimens and standard four point bend (single edge notch bend) specimens were extracted from the joints via spark erosion cutting. The four point bend specimens were fatigue precracked to introduce a sharp crack after introducing machine notches at the centre of the interlayers in the sandwich structures and at the interface in the dissimilar joints, and tested at room temperature. Some specimens were also prepared with the crack positioned away from the interface to determine the effect of notch position on fracture behaviour. The effect of strength mismatch on the crack tip opening displacement fracture toughness parameter of the joints has been evaluated. Crack initiation, crack growth, and crack deviation processes have been examined and fracture resistance curves (R curves) constructed for the joints. These results were used to explain the influence of mechanical heterogeneity of the joints and interlayer thickness on fracture behaviour.

Brittle fracture: Variation of fracture toughness with constraint and crack curving under mode I conditions

The effect of constraint on brittle fracture of solids under predominantly elastic deformation and mode I loading conditions is studied. Using different cracked specimen geometry, the variation of constraint is achieved in this work. Fracture tests of polymethyl methacrylate were performed using single edge notch, compact tension and double cantilever beam specimens to cover a bread range of constraint. The test data demonstrate that the apparent fracture toughness of the material varies with the specimen geometry or the constraint level. Theory is developed using the critical stress (strain) as the fracture criterion to show that this variation can be interpreted using the critical stress intensity factorKCand a second parameterT orA3,whereT andA3are the amplitudes of the second and the third term in the Williams series solution, respectively. The implication of this constraint effect to the ASTM fracture toughness value, crack tip opening displacement fracture criterion and energy release rateGCis discussed. Using the same critical stress (strain) as the fracture criterion, the theory further predicts crack curving or instability under mode I loading conditions. Experimental data are presented and compared with the theory.

Removal of the influence of stress triaxiality on crack tip opening displacement fracture toughness by continuum damage mechanics approach

Experimental data for AS 1405-180, AS 1204-350, HY 80 and C-Mn steels shows that the crack tip opening displacement (CTOD) elastic-plastic fracture toughness at initiation δ c decreases with increasing crack tip stress trifaxiality. This trend is confirmed by the continuum damage analysis in this paper. The dependence of the CTOD parameter at initiation on the local constraint, i.e. the stress triaxiality, provides the motivation to seek parameters that could rank the toughness of steels. Since the local effective plastic strain can be related to the CTOD, a relationship is described between the initiation CTOD toughness and the crack tip constraint, i.e. the stress triaxiality, on the basis of a new local damage theory for ductile fracture. Furthermore, a new constraint corrected toughness parameter δ dc (and corresponding criterion) for ductile fracture is proposed, in which both crack tip deformation and crack tip constraint intensity are taken into account. Several series of experimental data have shown that the parameter δ dc is nearly a constant or independent of the local constraint. It is found that the toughness variation with constraint changes can effectively be removed by use of the constraint correction procedure proposed in this paper.

Improved crack tip opening displacement fracture toughness parameters δdc and δmc for engineering materials and associated criteria for ductile fracture

Improved crack tip opening displacement fracture toughness parameters δdc and δmc for engineering materials and associated criteria for ductile fracture

HAZ fracture toughness of titanium containing steels

AbstractSteels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm−1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.MST/887