Unloading Yield Research Articles

Shakedown analysis on the integrity of cement sheath under deep and large-scale multi-section hydraulic fracturing

In the process of deep shale gas exploitation through horizontal drilling and large-scale multi-section hydraulic fracturing, the cement sheath can easily fail due to the high peak pressure and the frequent and violent fluctuations. In this paper, firstly, the deformation characteristics of the cement was studied based on the uniaxial and triaxial compression experiments at different temperature conditions. Then, mechanical model of cement sheath was set up. Based on Melan's shakedown theorem, shakedown load of the cement sheath under the periodic alternating loads is obtained and then converted to the internal pressure of the casing. Results shows that: (1) Compared with that at 25 °C, the compression resistance and elastic modulus of cement reduced significantly under 130 °C. The cement exhibits obvious plastic flow characteristics and can be regarded as a rational elastic-plastic material. (2) During the cyclic loading and unloading process, new plastic strains are continuously generated, the width of hysteresis loops is continuously reduced, and the hysteresis loop becomes denser, which indicates that the accumulation rate of plastic deformation is continuously decreasing. (3) The shakedown load is the smaller of the loading and unloading yield pressures, and reverse yielding tend to occur in the depths. (4) The shakedown load of the cement sheath has a positive linear and non-linear relationship with the cohesive force and the friction angle, respectively. (5) the shakedown load has a positive linear relationship with the external pressure. The external pressure, as the most influence of the calculation result of the shakedown load, accounts for close to 80%. The research could provide valuable theoretical and engineering guidance for the design and construction of shale gas well cement.

Investigation of the unloading yield effect in 7075 Al alloys based on microstructural and digital image correlation analysis

The unloading yield effect (UYE), which indicates that the reloading flow stress overshoots the monotonically loaded flow stress during uniaxial tensile test, was investigated by evaluating three aluminum alloy 7075 sheets with different precipitate states: solution heat-treated (SHT), aged for 20 min (A20, underaged), aged for 80 min (A80, peak aged) and aged for 240 min (A240, overaged). The precipitate states of A20 and A240 specimens were characterized by transmission electron microscope (TEM) and small-angle neutron scattering (SANS) analyses. The UYE was exhibited only in artificially aged specimens, but not in the SHT specimen. It was confirmed that the root cause of UYE is the interaction between the precipitate and dislocation during unloading. In addition, it was observed that strain instability is inevitably generated during stress overshoot owing to temporal geometric softening. The temporal strain instability diffused to the neighboring region in the case of A20 specimen, but that of A240 specimen directly influenced the premature localized necking or failure, resulting in a significant reduction in elongation.

Effect of fluoride on mechanical properties of NiTi and CuNiTi orthodontic archwires: an in vitro study.

ABSTRACTIntroduction: After debonding, white spot may appear on the area below the bracket, which is the early clinical sign of carious lesion. There is increased caries risk underneath and adjacent to orthodontic bands and brackets, which call for maximum use of caries preventive procedures using various fluoride application methods. Objective: The aim of the study was to evaluate alterations in the mechanical properties (modulus of elasticity and yield strength) in loading and unloading phases for different orthodontic archwires (nickel-titanium [NiTi] and copper-nickel-titanium [CuNiTi]) when exposed routinely to fluoride prophylactic agents for a predetermined period of time. Methods: Preformed rectangular NiTi and CuNiTi wires were immersed in fluoride solution and artificial saliva (control) for 90 minutes at 37ºC. After immersion, specimens were tested using a 3-point bend test on a universal testing machine.Results: There is a significant reduction in the unloading yield strength when the NiTi and CuNiTi wires were exposed to APF gel. Conclusion: The result suggests that use of topical fluoride agents affect the mechanical properties of the wires, leading to increase in treatment duration. Fluoride prophylactic agents must be used with caution in patients undergoing orthodontic treatment. Injudicious use of these agents may cause corrosive effects on the orthodontic wire surfaces, with alteration in their mechanical properties.

Investigation of the Unloading Yield Effect in Aluminum and Magnesium Sheet Metal Alloys at Room Temperature

The reduction of CO2-emissions and the lowering of fuel consumption are two main objectives in the automotive industry. To reach these targets, conventional materials like deep drawing steels are substituted by new modern lightweight materials such as aluminum and magnesium alloys. During forming of sheet metal parts, the material experiences a plastic deformation, which can affect the part quality regarding the amount of springback or the occurrence of stretcher strain marks. In this context, a time dependent change of the material behavior can emerge after removing the part from the forming tool. Within this contribution, the influence of pre-strain and the unloading yield effect on the subsequent mechanical behavior of the aluminum alloy AA7020 and the magnesium alloy AZ31B are investigated. Additionally, the time dependency of mechanical properties is analyzed for different aging times from 5 seconds to one week after pre-straining. The results show a significant increase of unloading yield effect with increasing pre-strain during uniaxial tension.

Comparison of the Mechanical Properties of Three Commercial Orthodontic NiTi Round Archwires

To compare mechanical properties of three commercial NiTi orthodontic round wires, three commercial brands of NiTi round wire (Nic-China, Ormco-USA, and Smart-Thailand) with sizes 0.014’’, 0.016’’, and 0.018’’were studied. Five specimens each size of each brand were used to test mechanical properties; unloading force (N), spring back (mm), and yield strength (N/mm) with three-point bend test using an Instron Universal Testing Machine. Kolmogorov-Smirnov test and one-way ANOVA were employed to test the differences among groups with statistical difference at p<0.05.The average unloading force from lowest to highest were Ormco, Smart and Nic with 0.014”, Smart, Ormco and Nic with 0.016”and Smart, Nic and Ormco with 0.018”, respectively. The Nic brand had the highest value of unloading force, spring back, and yield strength in all wire sizes, except unloading force 0.018” Ormco and spring back 0.018” Smart. There were no statistically significant differences in unloading forces among all wire sizes. The three brands of commercial orthodontic NiTi wires presented similar unloading force, spring back, and yield strength properties. These mechanical properties are related to lower rates of deformation and are appropriate to be used in the initial phase of orthodontic treatment.

Strain hardening in Fe–16Mn–10Al–0.86C–5Ni high specific strength steel

We report a detailed study of the strain hardening behavior of a Fe–16Mn–10Al–0.86C–5Ni (weight percent) high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) during uniaxial tensile deformation. The dual-phase (γ-austenite and B2 intermetallic compound) HSSS possesses high yield strength of 1.2–1.4 GPa and uniform elongation of 18–34%. The tensile deformation of the HSSS exhibits an initial yield-peak, followed by a transient characterized by an up-turn of the strain hardening rate. Using synchrotron based high-energy in situ X-ray diffraction, the evolution of lattice strains in both the γ and B2 phases was monitored, which has disclosed an explicit elasto-plastic transition through load transfer and strain partitioning between the two phases followed by co-deformation. The unloading-reloading tests revealed the Bauschinger effect: during unloading yield in γ occurs even when the applied load is still in tension. The extraordinary strain hardening rate can be attributed to the high back stresses that arise from the strain incompatibility caused by the microstructural heterogeneity in the HSSS.

Unloading yield effect in a twin-roll-cast Mg–3Al–1Zn alloy

The yield point produced by unloading and reloading during tension was observed in a twin-roll-cast Mg–3Al–1Zn alloy. The magnitude of the unloading yield drop increased with increasing strain. Repeated stress relaxation tests were performed to quantitatively analyse the evolution of mobile dislocation density at 20 and 150 °C. The exhaustion rate of mobile dislocations at 20 °C is ∼11 times higher than that at 150 °C, and is considered to be responsible for the unloading yield effect at 20 °C.

Plastic Deformation Influence on Material Properties of Autofrettaged Tubes Used in Diesel Engines Injection System

According to the DIN2391 standard, the DIN1.0406 steel is used to manufacture high-pressure injection tubes of diesel engines. The parts are autofrettaged during the manufacturing process to increase operating pressure and fatigue life. The autofrettage process is affected by loading–unloading cycle. In Bauschinger effect (BE) phenomenon, plastic deformation causes a loss in unloading yield strength. The ratio of unloading yield strength to the loading yield strength is called Bauschinger effect factor, BEF. In this paper, plastic deformation influence on the loading and unloading behaviors of DIN1.0406 steel is studied considering the BE. Uniaxial tension–compression experimental data are used to figure out a suitable model to study the BE. To carry out these experiments, a servohydraulic Instron machine is used. The sample tubes having inside diameter of 2.4 mm and outside diameter of 6 mm were made based on the standard ASTM E8M-97a. The tests were carried out using the total strain of 4.31%. Another important purpose of this paper is to investigate the effect of the amount of plastic strain on loading–unloading Young's modulus. Finally, the behavior of DIN1.0406 steel is compared with the steels such as DIN1.6959, HY 180, and PH 13-8Mo used in tubes.

Research of Rock Mass Unloading Constitutive Model and its Engineering Application

Based on the results of triaxial unloading tests，the stress-strain characteristics and failure characteristics of rock mass have been discussed. Considering unloading degree as an important parameter，an elastic-brittle-plastic mechanical model has been established according to the unloading stress condition，which considers that unloading yield function varies linearly between Griffith with Hoek-Brown criterion after rock mass receives plastic deformation caused by unloading. Taking jinshajiang excavation slope of a hydropower station as engineering background, the three-dimensional continua fast Lagrangian analysis software FLAC3D is employed in the slope stability analysis under the excavation, and the distribution characteristics of the slop deformation is studied.

Phenomena in interrupted tensile tests of heat treated aluminium alloy 6061

Interrupted tensile tests were performed in alloy 6061 extrusion after solutionizing and artificial aging treatments. Samples in the peak-aged, under-aged, over-aged and solutionized conditions were studied. Two phenomena were apparent in the stress-strain curves of interrupted tests. They are unload-reload hysteresis loops, and the yield effect on reloading which is referred to as the unloading yield effect. The width of a hysteresis loop increased with prestrain and aging time until the peak-aged strength was attained. Over-aging caused the loop width to decrease. A mechanism based on relaxation of the back stress in a dislocation pile-up is used to explain the hysteresis loop phenomenon and its relationship to prestrain and aging. The unloading yield effect also increases as the prestrain is increased.

The Effect of Surface Layer on the Deformation Process of Copper Single Crystals

The effect of surface layer on the deformation process of copper single crystals with various tensile axes has been investigated. The dislocation density of the surface layer is slightly higher than that of the interior at the yield point and it increases gradually with increasing strain. In the interior, increasing rate of dislocation density is higher and the dislocation tangles are well developed compared with those in the surface layer. The continuation time of slip traces observed by electron microscopy increases with increasing exposure time of specimens in air after electropolishing. Furthermore, the amount of unloading yield drop decreases when the surface layer is removed by electropolishing. It is concluded from the results that the surface layer acts as the barrier against the mobile dislocation before the yielding but it promotes the deformation after the yielding.

Surface and size effects in unloading yield point

Abstract The removal of a surface envelope by chemical polishing of an unloaded crystal, previously deformed plastically in tension, is shown to reduce the unloading yield point. In addition it is shown that the yield point changes continuously with stress and a maximum effect is obtained at a given stress which depends on the crystal size. Furthermore the value of the maximum is a function of crystal size. On the basis of these results it is argued that the unloading yield point can probably be attributed to the non-uniform flow stress distribution which exists near the surface in deformed crystals.

The Temperature Dependence of the Flow Stress of Copper Single Crystals

AbstractIt is shown that the Cottrell‐Stokes ratios of copper single crystals are not constant during tensile deformation in stages II and III at temperatures from 77 to 443°K. However, there appears to be a fixed relationship between the Cottrell‐Stokes ratio and the flow stress (as measured at a standard temperature). This relationship is independent of crystal orientation and the temperature or stage of deformation. The relationship is similar to that previously reported for polycrystalline copper [1], although not quantitatively identical. These relationships have been interpreted as evidence that the obstacles responsible for temperature‐dependent hardening are formed by the action of stresses from arrays of primary dislocations. The nature of the unloading yield point and its relevance to the reversible change in flow stress are also discussed.

Yield phenomena in polycrystalline copper

Abstract Unloading and ageing experiments are reported for polycrystalline copper deformed in tension at 294°K, 196°K and 79°K. The magnitude of the yield effect was investigated as a function of stress, purity, grain size, amount of unloading and the time under reduced load. It was concluded that both the initial unloading yield and the subsequent ageing yield could be attributed to the pinning of dislocations by point defects generated during deformation.

Unloading effects in crystals—I the unloading yield point effect

The orientation dependence of the “unloading yield effect” in copper single crystals was investigated. The maximum relative increase in flow stress due to unloading was observed to be orientation independent. An orientation dependence of the variation of the unloading yield with prestrain was observed. The role of secondary slip systems in establishing the unloading yield effect was investigated utilizing combined tensile-torsion deformation. A mechanism for the establishment of the unloading yield effect based on a forest dislocation interaction is proposed.