Structural approach to enhance the fracture resistance of high-strength metallic materials

Abstract

The paper proposes an approach to form (by special heat treatment) a structure and phase composition of martensitic steel that would enhance its ductility in the high-strength state. A correlation is experimentally established between the stability of fracture resistance and ductility in linear and plane stress states. The behavior of the scatterband of the ultimate strength with increase in the degree of ductility is analyzed. The possibility of predicting the ultimate strength from the lower edge of its scatterband is demonstrated with an example of a circular cylindrical shell Keywords: structural approach, martensitic steel, ductility, scatterband of ultimate strength, linear and plane stress statesIntroduction. Design of low-weight structures used in aircraft and space-and-rocket technologies permit high stresses. Such structures are made of high-strength materials. Because of their low ductility, such materials are highly sensitive to the presence of defects, which results in a significant scatter in strength. The allowable stress might appear to fall within the scatterband of the ultimate strength.The ductility of a material can also be substantially reduced by accumulated microdamage caused by radiation [1, 15], compound stress [4,6,16], and low temperatures [6]. This is manifested by an increased scatter in hardness (as a strength parameter) throughout the service life [7]. Because of the high reliability demands for such structures, it appears important to enhance the stability of fracture resistance by optimizing the structure of high-strength materials during manufacture to make them more ductile.The structural characteristics of high-strength materials responsible for fracture can be specified by means of structural mechanics [17] using the basic relations of physics of strength and fracture [10,13,21]. Among such characteristics are experimentally justified structural dependences of strength, ductility, and ductile-brittle transition temperature and dislocation failure criteria, which generally indicate how the structural parameters of a material should be changed to enhance its fracture resistance [9,18].For example, the Cottrell-Petch criterion defines conditions to optimize the structure of multiphase systems at the meso-, micro-, and nanolevels with regard to the size of matrix grains and subgrains and the volume fraction and morphology of strengthening precipitates, which determine the ability to relax stress concentrators by local yielding rather than by cracking. For some classes and states of structural metals, we experimentally established, using structural approaches [5,18], a correlation between a tensile ductility index (maximum percent elongation as a measure of stress relaxation, the opposite of cracking) and the size of the crack-tip fracture process zone.The approaches of thermal and thermomechanical structural optimization were tested against maraging steels. This had the effect of enhancing stability and fracture resistance with a simultaneous reduction in the weight of welded shel...

Predicting the strain hardening characteristics of ageing alloys under combined loading

“…The results were compared with those obtained in integrated studies of the strength characteristics and structures of metastable and stable states of Kh16N5D3 steel in the cases of like and opposite directions of preloading and reloading. The structural characteristics (intrinsic fringe broadening b and the lattice distance of the matrix a-phase) of maraging Kh16N5D3 steel and the morphology of hardening phase precipitates were analyzed using the X-ray diffraction technique (FeK a -radiation; DRON-2.0 general-purpose X-ray diffractometer) and transmission electron microscopy (microscope UÉMV-100K) according to the procedures from [2,9,12].…”

“…The kinematic nature of strain hardening is mainly observed in the range of small plastic strains that are insufficient for the formation of dislocation forests and their tangling, which hinder reverse slip and its softening effects. As the prestrain increases, which complicates the dislocation structure of the material [10,12], its resistance to reverse loading to the yield point enhances, thus changing strain hardening from mainly kinematic to mixed (isotropic and kinematic). This is reflected on the curves R Z p ( ) e 1 and a Z p ( ) e 1 for 12Kh18N10Ò high-ductility austenitic steel for e Z p 1 > 2% (Fig.…”

Stress-strain state and durability of mechanically inhomogeneous welds under low-cycle loading

“…Note that some issues of fracture resistance of metals are considered in [10], and the fatigue strength of metals and composites under asymetric tension-compression and high-cycle bending is studied in [15,16].…”

Two-parameter failure criterion for elastoplastic bodies with mode I cracks

“…Primarily, it was necessary to correct and estimate the stress state near a crack tip by retaining the subsequent regular terms of the stress field. The results obtained within the framework of the K T I - [6,16], J Q - [18,20,22], and J A -2 [17,21] concepts and other approaches [5,14,15] made it possible to estimate the effect of the regular terms of the stress field on the classical fracture strength characteristics. However, these two-parameter approaches failed to formulate a failure criterion that would relate these parameters.…”