鋼の疲労における内部応力および有効応力

The stress applied to a specimen does not fully act to deform material. The effective stress which actually acts on dislocations can be related to the internal stress of material. The origin of internal stress and thus effective stress has been discussed here by means of a dislocation model, concluding that internal stress depends on the dislocation structure. Based on this conclusion, fatigue tests were carried out and the following results were obtained:(1) The behavior of internal stress in the fatigue process showed a clear difference between the over-stressed and the under-stressed conditions.(2) The fatigue failure occurred under many-fold multiple repeated stress in two stress levels below the fatigue limit. Such fracture was attributed to the instability of internal stress.(3) The concept of internal stress and effective stress is useful to the comprehensive understanding of fatigue phenomenon.

The objective of the present study is to investigate the effects of fine precipitates of vanadium nitride (VN) on the creep deformation behavior of 12%Cr-15%Mn austenitic steels through the measurement of internal stress and effective stress. These stresses are obtained by strain dip test. The internal stress of VN free materials was unequivocally determined as a function of applied stress. On the other hand, the change of deformation rate during the strain dip test of the VN containing materials was very small and could not be detected in a range of reduced applied stress. Consequently, the internal stress and effective stress of VN containing material showed a band range of values as a function of applied stress. This range indicates that fine and dense precipitates of VN become strong barriers to the dislocation motion in a wide variation of stresses worked on the dislocations. The temperature dependence of internal and effective stresses of the VN containing material was also clarified. The difference between the stress dependence of minimum creep rate at the lower applied stress and that at the higher one was also discussed to be caused by the behavior of internal and effective stresses

Internal stresses during creep of magnesium alloys at 523 K

In creep design, not only secondary creep but also primary and tertiary creep regimes become important for an accurate creep life prediction and need to be accounted for in more sophisticated models. Mechanism-based theories showed a better potential since they usually leads to more accurate prediction over wider range of stress and temperature. It has long been recognized that the mechanics and kinetics of the deformation and recovery processes occurring at the microscale are determined by, amongst other things, the local effective stress while, at macroscopic level, it may still possible to discern an effective stress, which is different from the applied stress that drives the creep strain accumulation, [1]. Recently, the authors proposed an internal stress based model for primary creep, which is independent on the creep rate formulation at the steady state, ensuring the continuity of the creep curve at the transition between primary and secondary creep stages, [2]. In this work, in order to account for all creep stages, this modeling framework is extended further. In particular, for tertiary creep stage, it is assumed that the occurrence of microstructural modifications (such as fine particle dissolution, secondary phase precipitations, etc.) is the process responsible for the reduction of the internal stress and consequent increase of the creep rate as result of the increment of the effective stress. The possibility to derive the internal stress decay function from tertiary creep data is discussed. The proposed model has been applied to P91 high chromium steel and preliminary results are presented in this work.

Assessment of activation volume in 304L SS at high temperatures

The effect of internal stress on the activation parameters for creep of Fe-Ni alloys

Drill pipes in the oil-gas industry play a major role in the extraction of petroleum products. These pipes are the main link in the drilling rig, these pipes create the necessary rotation and transfer the load to the cutting element (chisels), and they also serve to lift working tools from the bottom of the well to the surface. The main goal of the drill pipe is to supply the drilling fluid, which performs the function of cooling the bit. The destruction of such pipes leads to large economic losses, therefore, these pipes must have high quality primarily in terms of mechanical properties. Based on the foregoing, this work is aimed at studying the causes of accidental destruction of steel drill pipes during their operation in the presence of an increased level of internal stresses in them. As the main method for assessing the level of residual internal stresses, a method is proposed for measuring the groove width during the longitudinal opening of ring samples cut from the body wall of drill pipes. This method is distinguished by its clarity and high speed of obtaining the result. The increased level of internal residual stresses, combined with external working loads during operation, leads to a significant reduction in the service life of drill pipes under the influence of corrosion cracking mechanism.

Generalized type III internal stress from interfaces, triple junctions and other microstructural components in nanocrystalline materials

An experimental method to estimate internal stress level during high temperature creep

A model for the flow stress and strain rate sensitivity of a substitutional alloy—Cu-3.1 at.% Sn

Glass-coated amorphous microwires of two different compositions, and , with square hysteresis loops have been prepared and studied. The effect of applied tensile stress on their magnetic properties is reported for both kinds of microwires with radii of the metallic nucleus in the range from 1.9-m and thicknesses of the coating of 3.8-m. The switching field of the studied microwires depends on the sample's geometry (the radius of the metallic nucleus and the thickness of the glass coating), exhibiting a decrease with decreasing thickness of the coating owing to the induced internal stresses. For Fe-rich compositions an increase of the switching field with the applied tensile stress in the range of 0-750 MPa was observed, such an increase being more significant for a thin coating. The classical square root dependence of the switching field on the applied stress is found for Fe-rich compositions for the range of thicker metallic nuclei. Co-rich microwires exhibit quite a different stress dependence of the switching field, exhibiting first roughly stress-independent behaviour and then an increase with the stress. Removal of the glass coating results in a drastic change of the switching fields for both compositions, indicating the existence of strong internal frozen-in stresses. The observed experimental dependences were explained in terms of a dependence of the nucleation field both on external and on internal stresses and by taking into account the dependence of the magnetostriction constant on the strength of the total stresses.

The influence of internal stress fields on the velocity of dislocations in B.C.C. crystals

The stress-time relation during stress relaxation is interpreted in terms of a power relation between dislocation velocity and the effective stress. Such interpretation gives not only the velocity-stress exponent, but also the internal stress in the work-hardened state. The velocitystress exponents thus obtained agree with those obtained from etch pit dislocation velocity measurements. The variation of internal stress with strain shows that work hardening is largely due to the development of internal stresses. The temperature dependence of flow stress arises from the increase of effective stress with decreasing temperature, consistent with a thermally activated process for dislocation motion.

Stress-strain characteristics of Pb0·95Sn0·05Se single crystals grown from the vapour phase have been investigated in the plastic deformation range. Samples were compressed along a (100) direction, with strain rate varying between 1·85 × 10−5 and 1·85 × 10−3 s−1, in the temperature interval 300–573 K. Plastic deformation was found to include all three stages of deformation hardening and to be performed by slip of dislocations on the {110} (110) slip system. In conditions when deformation took place in a single-slip system, internal and effective stress values were determined in the easy-glide region. The stress relaxation method was used, taking into account the hardening in process of relaxation. The effective stress as a function of strain rate was found to obey a power law, while the temperature dependence had a thermally activated character. Values of activation parameters were found as a function of effective stress by thermal activation analysis. The results obtained are interpreted in the paper by a model of dislocation interaction with systems of intrinsic point defects in the lattice.

Copper-2.5 at%Al alloy single crystals oriented for a single slip system were deformed in tension at 600 and 700°C. In the later stage of deformation, the condition of steady state deformation was satisfied; namely the specimens have been deformed at a constant stress, independent of strain. In this steady state deformation stage, the internal stress and the effective stress were determined by the stress dip test. The dislocation substructure formed in this stage were observed by the etch pit technique. The results obtained are as follows:(1) The tensile axis rotated into the direction predicted by the single glide on the (111)[\bar101] slip system. When the tensile axis approached the [001]-[\bar111] symmetry line, the steady state deformation was observed.(2) The strain rate is proportional to the nth power of the steady state stress and the n value is about 5.(3) The ratio of the internal stress to the applied stress is 0.6∼0.9 and depends on the applied stress. The effective stress increases with the applied stress.(4) The parameter m* describing the effective stress sensitivity of dislocation glide velocity was estimated assuming the mobile dislocation density to be proportional to the square of the applied stress and the m* value was found to be 1.1.(5) The subgrains elongating in the [1\bar21] direction are found on the (111) plane. On the (1\bar11) plane the rows of dislocations are found along the direction roughly parallel to the trace of the primary slip plane, and the high dislocation density regions, approximately perpendicular to the slip trace, are observed.