Dip Test Technique Research Articles

Structure, mechanical properties and modelling of polypropylene for different degrees of crystallinity

The objective of this study, dealing with the physico-chemical and mechanical characterisation of isotactic polypropylene, is to develop at room-temperature, a pre-necking micromechanical behaviour model. Spherolites in the injected samples are of 40 μm mean diameter and do not have preferential orientation. This is also true for the monoclinical-type crystal structure which induce an isotropic mechanical behaviour on the macroscopic scale. The WAXD measurements coupled with tensile tests show that the crystallinity does not evolve in the considered strain range. Polypropylene mechanical behaviour is of elastoviscoplastic type with a viscoelastic characteristic even for small strain. The study of several specimens having undergone annealing has shown that polypropylene mechanical behaviour, as well as the appearance of damage, are greatly dependent on the crystallinity and the thickness of the crystalline lamellae. Tensile tests using the dip test technique and loading-unloading were also carried out. They highlighted the effect of internal stress on the viscoplastic behaviour and indicate that the yield stress range decreases with tension. At the end of this study, we propose a pattern to represent the specific morphology of the polypropylene. In distinguishing the behaviour associated to amorphous and crystalline phases in a generalised self-consistent scheme, we simulate the elastic properties of the polypropylene, as functions of its crystallinity.

Role of Eutectic Intermetallic Phase on Creep Strength in a Die-Cast Mg-Al-Ca Alloy: Evaluation by Internal Stress Measurement

The internal stress during high temperature creep was investigated for a die-cast Mg-Al-Ca alloy AX52 (X representing calcium) at 473 K through the strain-transient dip test technique. The microstructure of the alloy is characterized by the eutectic intermetallic phase covering the primary � -Mg grains. The eutectic intermetallic phase plays dual roles in enhancing the creep strength from the viewpoint of the internal stress. First, the eutectic intermetallic phase sustains the stress-independent component of the internal stress with the magnitude of 15 MPa, resulting in the decrease in the effective stress. And second, it lowers the creep rate by two orders of magnitude at a given effective stress by reducing the mobile dislocation density and/or glide velocity of dislocations. [doi:10.2320/matertrans.48.97]

Internal Stress during Creep in a Die-Cast AM50 Magnesium Alloy at 473 K

The internal stress during high temperature creep was investigated for the die-cast AM50 magnesium alloy at 473 K through the strain-transient dip test technique. The microstructure of the alloy is characterized by fine α-Mg grains together with the β-Mg 17 Al 12 particles located on the grain boundaries. The internal stress is proportional to the applied stress, when the applied stress ranges below the yield stress. The ratio of the internal stress to the applied stress is 0.36 for the alloy, which is about half of those for conventional pure metals such as Cu, α-Fe and Ni. The small internal stress is a characteristic feature in the creep for the alloy.

Dip-Test Internal Stress and the Composite Model of Creep Deformation

The composite model of plastic deformation is regarded as a realistic approximation of creep behaviour at elevated temperatures in a well-developed substructure consisting of dislocationdense subgrain boundaries (hard regions) and subgrain interiors (soft regions) with relatively low dislocation density. In the present contribution, the model is applied for an estimation of internal stresses that are experimentally measured by the dip-test technique. Two situations are considered within the model: (i) the density of moving dislocations is the same in both hard and soft regions and (ii) the density of moving dislocations is proportional to the local density in the respective region. The model enables to express the internal stress in terms of microstructural variables found by independent microscopic observations. It is shown that the magnitude of volume fraction of hard and soft region in the composite model has only a small effect on the value of internal stress.

Internal stress and heterogeneous dislocation structure in creep

The present work deals with the problem of microstructural interpretation of internal stress measured in high-temperature creep by the strain-transient dip-test technique. The development of microstructure in the course of creep is considered a transition from uniformly distributed dislocations to a well-developed substructure. The substructure is supposed to have a composite character that consists of cell or subgrain boundaries with few dislocations in cell or subgrain interiors. Model equations for the relation of internal stress to the parameters of the dislocation structure are discussed and examined with reference to experimental data. The evolution of internal stress in creep, evaluated using different formulae, is compared with the evolution of macroscopically measured internal stress. The use of applied-stress dependences of microstructure parameters permits quite realistic estimates of the values of internal stress in steady-state creep.

Constitutive description of creep curves based on internal stress evolution

A constitutive description of creep curves is derived from the time evolution of the internal stress in creep determined by means of strain transient dip test technique and from the kinetic equation between this stress and the instantaneous creep rate. Several constitutive equations have been tested on creep curves of aluminium. It is shown that fitting of the curves and its statistical judgement cannot be used as the only criterion for an investigation of the evolution and role of the internal stress in creep. Such procedure must be completed by another independent procedure. As an appropriate procedure, the measurement of the internal stress in the steady state creep has been successfully applied.

Choice of evolution equation for internal stress in creep

The internal stress values in the course of the primary and the steady-state stages of creep in a Fe3wt.%Si alloy were measured by a technique analogical to the strain transient dip test technique. Two types of evolution equation, i.e. phenomenological and derived from dislocation kinetics, were examined by correlating them with the experimental data. In the application of the second group of equations, characteristics of the real dislocation structure were taken into account. The results of structure investigations are discussed in relation to the concept of internal stress in both homogeneous and heterogeneous dislocation structures, in the latter case using the composite model of the dislocation structure.

An estimate of internal stress evolution in the primary creep based on the composite model of dislocation structure

Abstract The paper gives an estimate of the average internal stress which can be measured by the strain transient dip test technique in the heterogeneous dislocation structure consisting of cells or subgrains bounded by low-angle subboundaries. The structure is described by the two-component model of hard and soft regions and the process of creep is considered an iso-strain rate deformation process. An evolution of the internal stress is derived from the evolution of dislocation structure characteristics in the normal primary creep and is evaluated for alpha iron from experimental data which were published earlier by the present author ( Philos. Mag., 25 (1972) 865; 28 (1973) 891, 1099). As an illustration of local conditions, local stresses and local strain rates are evaluated. The results document a growth of the mean internal stress in creep and changes of local stresses in the hard and the soft region. The internal stress values are relatively low and do not approach the value found experimentally in the steady-state creep which reaches the value of long-range internal stress in the soft region. A suggested interpretation of this discrepancy is based on an assumption that the actual steady-state creep has not yet been reached in the region of constant creep rate.

Constant structure creep in metals after stress reduction in steady state stage

Constant structure creep in several metals and alloys was investigated in structures corresponding to the steady state creep conditions. Results were interpreted by means of the back stress concept. In accord with postulates of the strain transient dip test technique, the residual effective stress is considered the controlling stress for dislocation glide. The suggested interpretation of constant structure creep data allows to obtain some information about parameters of the dislocation kinetics in the steady state creep. From the interpretation, a pronounced applied stress dependence of the mobile dislocation density as well as a specific relation between the activation area and the effective stress in steady state creep under various creep conditions result.

Internal and effective stresses in high-temperature creep evaluated from transient dip tests and dislocation bowing

Because creep of metals and alloys is modelled on the basis of microstructural observations, it has been shown that there is a difference between the mathematical treatment of high-temperature deformation and the real material behaviour. One idea to consider is to split the applied stress into a part depending on the substructure (the internal stress which has to be reached to start dislocation motion) and a part describing the resistance to the glide motion of dislocations (the effective stress). For ferritic chromium steel these quantities have been measured by means of the stress transient dip test technique. This leads to mean values of internal and effective stresses for the whole specimen. Additionally, local stresses acting on individual dislocations are evaluated from dislocation bowing for a wide range of applied stresses. The results show that the ratio of internal to applied stress decreases with increasing applied stresses, which, on the other hand, causes a large increase of effective stresses. Dislocation bowing stresses show a similar dependence. Compared to the results of dip tests, the determination of local stresses leads to less accurate results and to a large deviation of results within small regions of one specimen. Therefore, it is only valuable for comparison purposes.

On the relation between dislocation structure and internal stress measured in pure metals and single phase alloys in high temperature creep

A survey of experimental studies of a relation between dislocation structure and internal stress σ i measured by means of the strain transient dip test technique in high temperature (mostly steady-state) creep of various metals and single phase alloys is given. Application of various approaches, i.e. direct evaluation of σ i from quantitative structure data, correlation of σ i and parameter of dislocation strengthening Gb √ ϱ ( G is the shear modulus, b Burgers vector length and ϱ dislocation density), stochastic model of σ i in a three-dimensional dislocation network, reveals the problem of strengthening by a homogeneously distributed dislocation density from several points of view. Attention is paid to the strengthening effect of polygonized substructure. Discussion of the presented results leads to conclusions about various sources of internal stress in the dislocation structure and substructure formed in creep and about the influence of temperature on dislocation structure in relation to the temperature dependent measured internal stress.

Internal stresses in the model of hard and soft regions

The simple “composite” model of material with inhomogeneous dislocation structure is extended to provide information on strain response to stress changes in steady-state creep. This extension enables us, therefore, to simulate the so-called dip-test technique and to calculate operationally defined internal stress. It is shown that this internal stress corresponds to the back stress acting in soft regions. The model is also convenient for the calculation of strain transients.

The back stress concept in power law creep of metals: A review

The idea that the plastic deformation of crystalline solids takes place under only a part of the applied stress and not under the entire stress (represented by the difference between the applied stress σ and a back stress σ B) is used to discuss the well-known discrepancy between the natural third-power dependence of the steady state creep rate on the applied stress and the experimentally observed significantly stronger dependence in metals and solid solution alloys exhibiting metal class behaviour. The back stress is identified with the internal stress measured by a dip test technique and introduced into the third-power law creep equation. This approach has led to a satisfactory, although essentially phenomenological, interpretation of the third-power law. Some results of complete unloading experiments in steady state creep as well as the results of internal stress measurements during backward straining after the complete unloading are presented and discussed on the basis of the model of soft and hard regions of dislocation structure. It is suggested that the internal stress measured in steady state creep of metals and solid solution alloys exhibiting metal class behaviour represents a level of internal backward stresses acting in the soft regions (cells or subgrains), whereas in the hard regions (cell walls or subboundaries) large, forward internal stresses act. The dislocation processes in the hard regions most probably govern the dependences of the creep rate on both the temperature and the applied stress. However, the forward internal stresses acting in hard regions are related to the backward internal stresses acting in soft regions (and thus to the measured internal stress) through the dislocation substructure. Argon and Takeuchi's theory of internal stress and creep is briefly described and compared with some experimental results including the measured internal stress. It has been found that this theory does not account satisfactorily for the experimental results. Some other attempts to reconcile the natural third-power law with the significantly stronger applied stress dependence of the steady state creep rate observed experimentally are briefly discussed. The need for a sound theory of internal stress and more detailed knowledge of dislocation processes in cell walls and/or subboundaries in creep for greater understanding of creep mechanisms is emphasized.

Transients in the creep of a 16Cr12Ni2.5Mo austenitic steel I: Kinetics

The transient creep of an austenitic 16Cr12Ni2.5Mo stainless steel in the temperature ranges 873–1023 K was investigated by the stress change technique which was applied after a steady state at the stress σ 1 had been reacheed. Two types of experiments were used: type A experiments consisted in an applied stress reduction from ηs 1 to σ 2, the stress σ 2 ranging from σ 1 = 2 MPa (where σ r is the residual stress) to 0.95 σ 1; type B experiments consisted in a stress reduction from σ 1 to σ r = 2 MPa for the hold time t r ranging from 10 to 10 3 s and s subsequent increase in stress from σ r to σ 2 < σ 1. In type B experiments the stress σ 2 ranged from 0.1 σ 1 to 0.5 σ 1. Also, the time dependence of the internal stress σ 1 measured by the strain transient dip test technique after the applied stress reduction from σ 1 to σ r = 2 MPa was determined. Experiments of both type A and type B showed that probably the most important role is played by the disintegration of subgrain boundaries, leading to a temporary increase in the density of mobile dislocations. The rapid decrease in internal stress during the hold time at the reduced stress σ r of 2 MPa after a steady state has been reached at the applied stress σ 1 ⪢ σ r is explained by the fact that the major, if not the dominant, component of the internal stress σ i arises from the bowing-out of the links of the three-dimensional dislocation network.

Internal stress and structure in creep of cold prestrained Fe–21 Cr–32Ni alloy at 975 K

The effect of different levels of cold prestrain on some creep characteristics at 975 K of an Fe–21Cr–32Ni alloy was investigated. The prestrain influences the steady-state creep rate, the structure, and the internal stress measured by the strain-transient dip test technique. This internal stress is identical with that calculated from the dislocation strengthening. The steady-state creep rate of the alloy depends on the effective stress only, without respect to the level of prestrain.

New technique for evaluating long range internal back stresses

A new technique is proposed for determining a measure σ i of the long range internal back stresses in a deformed material. It is based on the inelastic back strain measured as the difference between the maximum observable and the instantaneous back strain accompanying a reduction in stress. Stress reduction tests have been done during steady state creep of Al-11% Zn at 523 K and stress σ to determine the stress dependence of the creep rate at constant structure, the duration of back flow after stress reduction, and σ i. In contrast to the conventional (dip test) technique the new technique yields a constant value of σ i σ ≈ 0.5 for steady state creep. After a large stress reduction σ i falls rapidly below the reduced stress. The stress dependences of the creep rate at constant structure and the duration of back flow can both be described in terms of thermally activated glide with activation areas which depend on σ and are nearly equal to each other at a given σ.

Effective stress in steady-state creep in alpha Cu–Zn solid solutions

Abstract Effective stress in steady-state creep in alpha Cu–10% Zn and Cu–30% Zn alloys in a temperature interval 723 to 973 K was measured using the strain transient dip test technique. The measured effective stress was found to be higher at higher zinc concentration than at any given applied stress. The apparent internal stress [sgrave] = [sgrave] – [sgrave]∗ is related rather to dislocation distribution than to dislocation density as compared in both alloys investigated. The apparent activation energy obtained, where is the steady-state creep rate, represents a purely phenomenological quantity which cannot be compared with the activation energy of dislocation glide. The same applies to the activation area determined on the assumption that the moving dislocation density is proportional to the total dislocation density. It is concluded that high-temperature creep in the alloys investigated is controlled by recovery due to dislocation climb and annihilation, and that the recovery is not driven by the appa...

Effect of Molybdenum on some basic creep characteristics of alpha zirconium in a temperature interval of 350° to 600°C

Creep in three ZrMo alloys (0.1, 0.8 and 2.0 wt.% Mo) was investigated in a temperature interval of 350° to 600°C. The parameter of applied stress sensitivity of steady-state creep rate was found to increase with applied stress, reaching values as high as 50 at 350°C and high applied stresses. Apparent activation energy of creep Q is applied stress and temperature independent for the alloy Zr - 0.1 wt.% Mo only. For the alloys of higher molybdenum concentrations it depends on both applied stress and, especially, temperature. Values of Q as low as about 35 kcal/mol at 350°C and as high as about 130 kcal/mol at 600°C were found for Zr - 2.0 wt.% Mo alloy. Relations between steady-state flow stress and temperature indicate a significant contribution of an athermal deformation mechanism at low temperatures and high steady-state strain rates. The mean effective stress measured by strain transient dip test technique increases with increasing temperature and the effective stress to applied stress ratio increases with applied stress reaching values as high as 0.5. Possible creep rate controlling mechanisms are discussed. It is suggested that the creep rate is controlled by recovery though the current recovery creep theories of dispersion strengthened systems are not in satisfactory agreement with the results obtained. Creep strengthening effect of molybdenum is compared with that of niobium with the conclusion that it is significantly stronger, especially at higher testing temperatures.

Effective stress in steady state creep in an Al-5.5 at.% Mg solid solution

Using the strain transient dip test technique the mean effective stress σ ∗ was measured in steady state creep in an Al-5.5 at.% Mg solid solution. The mean effective stress increases slightly with increasing temperature. The σ ∗ σ ratio (σ is the applied stress) reaches values as low as about 0.4 at both low applied stresses and temperatures and as high as 0.94 at both high temperatures and applied stresses. The parameter m ∗ characterizing effective stress sensitivity of dislocation glide velocity was found to be close to one at lower effective stresses, while at higher effective stresses it increases linearly with σ ∗ . The apparent activation energy of dislocation glide was found to increase slightly with the effective stress; to σ ∗ = 0 a value of 23.7 kcal mol −1 corresponds. Dislocation core drag of solute atoms was suggested as a process controlling dislocation glide velocity in creep of the alloy investigated.

Steady-state creep in alpha iron as described in terms of effective stress and dislocation dynamics

Abstract Using the strain transient dip test technique the mean effective stress [sgrave]* was measured in steady-state creep of alpha iron in a temperature interval 400 to 700°C. At temperatures 400 to 450°C the mean effective stress was found to increase with increasing temperature, while at temperatures 550 to 700°C it is temperature-independent. At intermediate temperatures a more complicated behaviour of mean effective stress was observed. The parameter m* = (∂ ln v g/∂ ln [sgrave]*) describing the effective stress sensitivity of dislocation glide velocity v g was estimated, assuming the moving dislocation density ρ m to be a constant fraction of the density ρ of dislocations unbound in sub-boundaries. The contribution to the effective stress dependence of steady-state creep rate of the effective stress dependenoe of ρm was found to be considerably smaller—at least at high effective stresses—than that of v g. In the temperature interval 550 to 700°C the apparent activation energy of dislocation glide Q g is close to the activation enthalpy of self-diffusion ΔH SD. Non-conservative motion of jogs was, therefore, suggested as the mechansim controlling the dislocation glide. This suggestion is supported by the magnitude of the activation area as well as by its effective stress dependence. In the temperature interval 400 to 450°C the apparent activation energy of dislocation glide is lower than that of recovery, i.e. than ΔH SD. The mechanism controlling the dislocation glide in this temperature interval has not been identified. In the intermediate temperature interval both mechanisms controlling dislocation glide operate; at low effective stresses and higher temperatures that characteristic for the interval 550 to 700°C predominates, while at higher effective stresses and lower temperatures the mechanism controlling glide in the interval 400 to 450°C becomes dominant.