Constant Load Creep Tests Research Articles

Primary Creep in Sn3.8Ag0.7Cu Solder. Part I: Theory, Experiments, and Data Reduction

e < 10 � 4 s � 1 ; and temperatures 25� C, 75� C, and 100� C. Creep and stress relaxation measurements showed that transient creep caused faster strain rates during stress relaxation for a given stress than the corresponding minimum creep rate from constant-load creep tests. The extent of strain hardening during primary creep was a function of temperature and strain rate. Data reduction incorporated a fast Fourier transform method to remove spurious data from stress relaxation corresponding to the period of partial strain relaxation during loading.

The stress relaxation and creep behaviour of a manganese-stabilized austenitic stainless steel

The stress relaxation behaviour of 21–4N, a manganese-stabilized austenitic stainless steel, is investigated in terms of the metallurgical state, the application of multiple strain levels during ‘stepped’ stress relaxation testing at 700 °C, the strain level during isostrain stress relaxation tests at 538 °C and 700 °C, and the correspondence with results from constant-load creep tests. The results indicate that for isostrain stress relaxation tests the stress relaxation rate is similar for strains that span both elastic and plastic strain levels. A transition in the stress relaxation behaviour occurs at a stress level approximately equivalent to the tensile stress–strain proportional limit; below this transition the stress–strain rate relationship, or the time predicted for 1 per cent creep strain, obeys a creep power law type of equation. Stress relaxation testing successfully delineates the difference between the creep resistances of two different metallurgical conditions with similar tensile properties using fewer specimens and requiring less time. The time to 1 per cent creep strain determined from the analysis of stress relaxation results is always less than the actual time to 1 per cent creep strain during constant-load creep tests.

Effect of oxidation on creep data: Part 2 – A procedure for assessing the effect of oxidation on constant-stress or constant-load creep curves using the Theta-projection concept

A procedure based on Theta-methodology coupled with the knowledge of oxidation kinetics is envisaged for quantitatively assessing the effect of oxidation on creep curves. The procedure is based on the generation of a series of real constant-stress creep curves, at different stress and temperature levels, in inert atmosphere, where the effects due to oxidation are kept to a minimum level. Stress enhancement factors due to the effect of area reduction on specimen cross-section with plastic deformation and oxidation are defined for constant-stress or constant-load creep testing. These factors can be used in the integration of the strain-rate equation related to the 4θ - parameter analysis, to derive constant-stress or constant-load curves in air using either the strain hardening or time hardening theories. Although systematic constant-stress creep data in vacuum are not yet available to test the methodology effectively, a preliminary simulation is done to demonstrate how the model works, to check its performance and the possibilities of analysis.

Microstructural Evaluation of the Zirconia Plasma Sprayed Coating on Ti-6Al-4V Alloy after Creep Test

The titanium affinity by oxygen is one of main factors that limit the application of their alloys as structural materials at high temperatures. The objective of this work was evaluating the microstructure of the of the zirconia plasma sprayed coating Ti-6Al-4V alloy in creep test. Yttria (8 wt.%) stabilized zirconia (YSZ) (Metco 204B-NS) with a CoNiCrAlY bond coat (AMDRY 995C) was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco. Constant load creep tests were conducted with Ti-6Al-4V alloy in air and in nitrogen atmospheres in uncoated samples and in air in coated samples at stress level of 56 MPa at 700°C. The microstructural analyze show that the predominant mechanism in the test conditions was characterized by formation and coalescence of micro cavities with shape and size varieties. The cavities are corrugated in discontinue regions, like inclusions and points involving dislocation. The size and shape of dimples are governed by number and distribution of nucleated micro cavities and by stress internal level present in the material.

A Comparison between CoNiCrAlY Bond Coat and Zirconia Plasma Sprayed Coatings on Creep Tests

Thermomechanical and electrical properties of zirconia-based ceramics have led to a wide range of advanced and engineering ceramic applications like solid electrolyte in oxygen sensors, fuel cells and furnace elements and its low thermal conductivity has allowed its use for thermal barrier coatings for aerospace engine components. A comparison between CoNiCrAlY bond coat and zirconia plasma sprayed coatings on creep tests of the Ti-6Al-4V alloy was studied. The material used was commercial Ti-6Al-4V alloy. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti-6Al-4V substrates by Sulzer Metco Type 9 MB. Constant load creep tests were conducted on a standard creep machine in air on coated samples, at stress levels of 520 MPa at 500°C to evaluate the oxidation protection on creep of the Ti-6Al-4V alloy. Results indicate that the creep resistance of the ceramic coating was greater than metallic coating.

Effect of coating on the creep behavior of the Ti–6Al–4V alloy

The influence of the plasma-sprayed coatings and of the atmosphere on creep of the Ti–6Al–4V alloy was investigated. Yttria partially stabilized zirconia (YSZ) with CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on a standard creep machine in air and nitrogen atmospheres on uncoated samples and in air on coated samples, at stress levels of 520 MPa at 500 °C, 319 MPa at 600 °C and 56 MPa at 700 °C. Results indicated that the creep rates in nitrogen and of the coated alloy were lower than those of the uncoated in air.

Stress Relaxation Testing For Analysis of 12Cr1MoVG Steel for Creep Strength Evaluation

The design of pressurized components at elevated temperature is limited by available mechanical properties that often do not span appropriate range of stress, time, strain and temperature. To improve the design process, a new approach based on short-time stress relaxation test (SRT) is introduced in steel tensile creep analysis. The experiments are implemented using SRT and tested for creep properties of 12Cr1MoVG steel at 560 °C. The results of creep curves agree well with the traditional constant load creep test data. The possibility to apply SRT approach is shown promising in the design of the pressurized component at elevated temperature.

Influence of Zirconia Plasma-Sprayed Coating on Creep of the Ti-6Al-4V Alloy

The titanium affinity by oxygen is one of main factors that limit the application of their alloys as structural materials at high temperatures. Notables advances have been obeserved in the development of titanium alloys with the objective of improving the specific high temperature strength and creep-resistance properties. However, the surface oxidation limits the use of these alloys in temperatures up to 600°C. The objective of this work was estimate the influence of the plasma-sprayed coatings for oxidation protection on creep of the Ti-6Al-4V alloy, focusing on the determination of the experimental parameters related to the primary and secondary creep states. Constant load creep tests were conducted with Ti-6Al-4V alloy in air for coated and uncoated samples and in nitrogen atmosphere for uncoated samples at 500°C to evaluate the oxidation protection on creep of the Ti-6Al-4V alloy. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti-6Al-4V specimens. Results indicated the creep resistance of the coated alloy was greater than uncoated in air, but nitrogen atmosphere was more efficient in oxidation protection. Previously reported results about the activation energies and the stress exponents values indicate that the primary and stationary creep, for both test conditions, was probably controlled by dislocation climb. Occurred a decreasing of steady state creep in function of the reduction of oxidation process, showing that Ti-6Al-4V alloy lifetime was strongly affected by the atmosphere due the oxidation suffered by the material.

Mechanical Properties for 95.5Sn&amp;#8211;3.8Ag&amp;#8211;0.7Cu Lead-Free Solder Alloy

Mechanical properties for 95.5Sn–3.8Ag–0.7Cu solder alloy were derived from bulk specimen tensile test and lap shear solder joint tests specimen. The tensile tests were carried out at three temperatures (25 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circhbox C$</tex> , 75 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circhbox C$</tex> , and 125 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> ) and at three different strain rates ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.6times 10^-4 hbox s^-1$</tex> , <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.6times 10^-3$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.6times 10^-2 hbox s^-1$</tex> ). Shear tests were carried out at three temperatures (25 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> , 75 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> , and 125 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> ) and at three different displacement rates (0.5, 0.05, and 0.005 mm/min). The specimen gave the highest strength at the fastest displacement rate and at room temperature of 25 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> (0.61 Tm). The lowest strength occurs when the specimen is subjected to the slowest displacement rate at elevated temperature of 125 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> (0.81 Tm). Constant-load creep tests for bulk specimen were also conducted at three different temperatures (25 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> , 75 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> , and 125 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ ^circhbox C$</tex> ), test results were compared with other reported data.

Analysis of the effect of Si content on the creep response of an Mg–5Al–Mn alloy

The present study aims at investigating the effect of small additions (0.3, 0.5, 0.8 and 1.5%) of Si on the creep response of an AM50 alloy. Die-cast samples not previously heat-treated were subjected to tensile tests at room temperature and to constant load creep tests at 423 and 398 K. The effect of chemical composition was investigated by testing five alloys with rising Mg content, from 0 (basic AM50 alloy) to 1.5%. Irrespective of Mg content, the creep curves exhibited a well-defined primary stage followed by a short secondary region and by a short tertiary stage that led to fracture without extensive necking. Addition of 0.3% Si hardly affected creep strain rate, while greater additions induced a substantial increase in creep strength, an effect that was partially offset by a decrease in strain to fracture.

Effect of environment on the creep behavior of the Ti–6Al–4V alloy

The influence of the atmosphere in tensile creep behavior of Ti–6Al–4V has been investigated in the temperature range from 500 to 700 °C and in the stress range from 14 to 520 MPa. Constant load creep tests were conducted in air and nitrogen atmospheres. Primary and secondary creep is being considered. The results show that both steady-state and primary creep follow power-law creep, with stress exponents in the range from 2.23 to 11.73. Analysis of the primary and secondary creep suggest that the creep mechanisms for both creep stages and atmospherics conditions are related to dislocation climb. The relation between primary creep time and steady-state creep rate, obeyed the equation t p ( ε ˙ s ) N = P for both atmospherics conditions at 500, 600 and 700 °C. The higher creep resistance of Ti–6Al–4V was observed in nitrogen atmosphere.

Plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy

The titanium affinity for oxygen is one of the main factors that limit the application of its alloys as structural materials at high temperatures. The objective of this work was to estimate the influence of the plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy, focusing on the determination of the experimental parameters related to the creep stages. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was air plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on the Ti–6Al–4V alloy in air for coated and uncoated samples and in a nitrogen atmosphere for uncoated samples at 600°C to evaluate the oxidation protection on creep of the Ti–6Al–4V alloy. The steady-state creep rate of the coated alloy is smaller than that of the uncoated alloy in air and nitrogen atmosphere. Results about the activation energies and the stress exponent values indicate that the primary and stationary creep, for all test conditions, was probably controlled by dislocation climb. The plasma-sprayed coatings increased the time to rupture and the strain at rupture is smaller than for uncoated samples tested in air.

Creep Behavior of Al-Si Die-Cast Alloys

Commercial, aluminum die-cast alloys are subject to long-term stresses leading to viscoelastic material responses resulting in inefficient engine operation and failure. Constant load creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al-Si and a 17% Si-Al alloys. Rupture occurred in the primary creep regime, with the eutectic alloy having the longest times to failure. Primary creep was modeled by Jt=A+Btn with A, B, and n dependent on stress. Poor creep performance is linked to the brittle fracture of the primary silicon phase as well as other casting defects.

Microstructure and creep behaviour of eutectic SnAg and SnAgCu solders

The paper presents creep data, that was gained on specimens of different microstructures. The three specimen types have been flip chip solder joints, pin trough hole solder joints and standard bulk solder specimens. The bulk solder specimen was a dog-bone type specimen (diameter=3 mm, length=117 mm). The pin trough hole solder joint consisted on a copper wire that was soldered into a hole of a double sided printed circuit board (thickness 1.5 mm). The flip chip solder joint specimen consisted of two silicon chips (4 mm × 4 mm), which were connected by four flip chip joints (one on each corner). SnAg and SnAgCu flip chip bumps (footprint 200 μm × 200 μm, joint height 165–200 μm, centre diameter 90…130 μm) were created by printing solder paste. Constant–load creep tests were carried out on all three specimen types at temperatures between 5 and 70 °C. Creep data was taken for strain rates between 10 −10 and 10 −3 s −1. The specimens were tested in “as cast” condition and after thermal storage. The microstructural properties of the bulk specimens and real solder joints were examined using metallographic sectioning, optical microscopy techniques, and SEM-microprobe analysis. The results of the microstructural analysis were related to the investigated mechanical properties of the solders. Models of SnAg3.5 and SnAg4Cu0.5, that can be used with the ANSYS FEM software package, will be presented.

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

Abstract The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The α 2 (Ti 3 Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/α 2 interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.

Effect of Sb Addition on the Creep Property of AZ31 Alloy

The effects of antimony addition on the microstructures and creep behavior of AZ31 have been studied. Constant load creep tests were carried out at 200°C and an initial stress of 50MPa for AZ31 alloys containing antimony up to 0.84% by weight. Results show that Small additions of antimony to AZ31 effectively decreased the creep extension and steady state creep rates. The steady state creep rate of AZ31 was reduced 2.5 times by the addition of 0.84% of antimony by weight. The main reason for the higher creep resistance is due to the presence of high volume fraction (~20%) of second phases including Mg3Sb2, which effectively hindered the movement of dislocations during the elevated temperature creep.

Thermal Fatigue Life Simulation for Sn-Ag-Cu Lead-Free Solder Joints

A key issue in using alternative lead-free solder is the evaluation of long-term reliability, such as thermal fatigue. In this paper, an estimation method for the thermal fatigue of Sn-Ag-Cu lead-free solder joints using an inelastic constitutive model was studied. First, the mechanical properties of Sn-Ag-Cu lead-free solders were investigated in order to model the inelastic constitutive equation for finite element analysis. Tensile tests and constant load creep tests were carried out. Next, an inelastic constitutive model was investigated. In the modeling, the inelastic strain rate was decomposed into the steady-state part depending on applied stress and the transient part driven by the difference between applied stress and back stress. Finally, in order to investigate the applicability of the proposed constitutive model for Sn-Ag-Cu lead-free solder, finite element analysis was carried out for the solder joints in a quad flat package. The estimation for the fatigue crack initiation was in good agreement with the actual thermal cycle tests for the quad flat package.

Creep strain and creep-life prediction for alloy 718 using the omega method

The creep behavior of Alloy 718 was investigated in relation to the MPCs omega (Ω) method. To evaluate the creep model and determine material parameters, constant load creep tests were performed at different initial stresses in a temperature range between 550°C and 700°C. The imaginary initial strain rate\(\mathop \varepsilon \limits^. _0 \) and omega (Ω), considered to be important variables in the model, were expressed as a function of initial stress and temperature. For these variables, power-law and hyperbolic sine-law equations were used as constitutive equations for the creep of Alloy 718. To consider the effect of γ″ coarsening leading to a radical drop of tensile strength and creep strength at temperatures above 650°C, different material constants at the temperatures above 650°C were applied. The reliability of the models was investigated in relation to the creep curve and creep life.

The evaluation of high-stress creep ductility for 316 stainless steel at 550 °C by extrapolation of constitutive equations derived for lower stress levels

The CDM-based constitutive equations for the creep of 316 Stainless Steel at 550 °C are reviewed. During creep tests carried out under these conditions, it has been observed that as time elapses inelastic straining takes place due to time independent plasticity and to creep. It has been recognised that at high stress levels the time dependent plastic strain accumulated during constant load creep tests forms a major part of the inelastic strain and dominates over the creep strain. Hence, due to the plastic strain the true stress level is not constant during the test. The time independent plastic strain has been evaluated using a stress–strain curve obtained at a high strain rate, and the creep strains have been evaluated for the relevant stress history by integration of the constitutive equations. Minimum creep rates and lifetimes have been extrapolated from low stresses to higher stresses using linear stress versus logarithmic plots. In this way, the creep strain–time history, the minimum creep rates, lifetimes and ductilities have been evaluated. In the stress range 325–450 MPa a lower shelf ductility of 1.1% has been found. The model is also shown to predict the isochronous rupture locus determined from multi-axial test data obtained from a range of different sources.

A time dependent micro-mechanical fiber composite model for inelastic zone growth in viscoelastic matrices

In this work, a fiber composite model is developed to predict the time dependent stress transfer behavior due to fiber fractures, as driven by the viscoelastic behavior of the polymer matrix, and the initiation and propagation of inelastic zones. We validate this model using in situ, room temperature, micro-Raman spectroscopy fiber strain measurements. Multifiber composites were placed under constant load creep tests and the fiber strains were evaluated with time after one fiber break occurred. These composite specimens ranged in fiber volume fraction and strain level. Comparison between prediction and MRS measurements allows us to characterize key in situ material parameters, the critical matrix shear strain for inelastic zones and interfacial frictional slip shear stress. We find that the inelastic zone is predominately either shear yielding or interfacial slipping, and the type depends on the local fiber spacing.