Isothermal Fatigue Tests Research Articles

Reliability of Sn–Ag–Sb lead-free solder joints

The effect of Sb contents (0–10 wt%) on the reliability of Sn–Ag–Sb solder has been investigated with isothermal low cycle mechanical fatigue tests. The solders tested were Sn–3.5Ag with Sb contents of 1.73, 3.85, 5.12 and 10.05 wt%, respectively, in the form of solder balls. Single-lap specimens were used to simulate real solder joints with the solder balls reflowed between two pure Cu substrates. Experimental results showed that under a fixed controlled displacement of 0.025 mm, fatigue life of the as-soldered joint increased with the amount of Sb additions due to less plastic strain the hardened solders produced. Load drop rate in the final stage increased as Sb addition increased. Fatigue cracks initiated at locations between the neck of the hourglass-shaped solder joint and the intermetallic compound (IMC) layer formed between the solder and copper. With the increase of Sb contents, the fracture mode evolved from within the bulk solder to a mixed fracture mode of solder and IMC layer, and finally, along the IMC layer.

Thermal fatigue in hot‐working tools

Hot‐working, in general, is one of the fundaments of the industrialised society and the hot‐work tool, which gives the part its basic shape, is one of the most essential elements. Most hot‐working tools are subjected to extreme temperature variations, which give rise to stresses and strains and may eventually lead to thermal fatigue.Two hot forming applications have been studied, hot forging and die casting. Tools from these applications were examined to determine damage, material response, thermal fatigue crack initiation and propagation. Surface strain measurements during thermal fatigue testing were performed on the tool material to evaluate the surface mechanical strain levels during typical thermal cycling. Isothermal fatigue testing was also performed to determine the stress–strain response in the temperature interval 200–600°C. It was found that thermal fatigue crack initiation occurs within 1% of the tools life in both applications.

Comparison of isothermal mechanical fatigue properties of lead-free solder joints and bulk solders

Low-cycle isothermal mechanical fatigue testing of bulk solders and solder joints of eutectic Sn–37 wt% Pb, Sn–3.5 wt% Ag and Sn–4.0 wt% Ag–0.5 wt% Cu were carried out at room temperature over a wide range of strains (1–10%). The fatigue results of both bulk and solder joints were compared; the eutectic Sn–37 wt% Pb was used as reference. Concerning bulk solders and solder joints, the two lead-free solders showed better fatigue properties than Sn–37Pb. For all three solder alloys tested, bulk solders showed better fatigue performance than solder joints when subjected to higher strains. The situation was the opposite at lower strains, resulting in bulk solders depicting larger values for the Coffin–Manson fatigue exponent and ductility coefficient compared to solder joints.

Thermomechanical Fatigue Behavior of a Directionally Solidified Ni-Base Superalloy

A continuum crystal plasticity model is used to simulate the material behavior of a directionally solidified Ni-base superalloy, DS GTD-111, in the longitudinal and transverse orientations. Isothermal uniaxial fatigue tests with hold times and creep tests are conducted at temperatures ranging from room temperature (RT) to 1038°C to characterize the deformation response. The constitutive model is implemented as a User MATerial subroutine (UMAT) in ABAQUS (2003, Hibbitt, Karlsson, and Sorensen, Inc., Providence, RI, v6.3) and a parameter estimation scheme is developed to obtain the material constants. Both in-phase and out-of-phase thermo-mechanical fatigue tests are conducted. A physically based model is developed for correlating crack initiation life based on the experimental life data and predictions are made using the crack initiation model.

連続繊維強化ＳＰ７００／ＳＣＳ‐６チタン基複合材料の熱機械的疲労と高温低サイクル疲労の非類似性と類似性

Behavior of thermo-mechanical fatigue (TMF) failure of a new generation titanium alloy matrix composite, SP700/SCS-6, was studied, based on a series of investigations on ; (i) mechanical properties under monotonic loading of the SP-700/SCS-6 composite and the matrix alloy; (ii) fiber push-out tests to represent the fiber/matrix interfacial strength at elevated temperatures ; (iii) isothermal low cycle fatigue (ILCF) tests, (iv) interfacial reaction zone to characterize the microscopic damage evolution under the TMF test and the thermal cycle test without external load. A special focus was paid to understand both the mechanisms of TMF failure, and the dissimilarities and the similarities between the TMF and ILCF failures. From the results of these tests, it was shown that the differences of damage characteristics may be concerned with the degradation of the fiber/matrix interface by the crack propagation and reaction layer due to thermal cycles. These systematic investigations showed that the lives and the failure mechanisms of the TMF should be clearly distinguished from those of the isothermal ILCF, whereas there were a few similarities between them. It was also suggested the difference should be primarily originated from the cracking by the repeat of the thermal cycle and the resultant change in fiber/matrix adhesion strength.

Fatigue Reliability Evaluation for Sn–Zn–Bi and Sn–Zn Lead-Free Solder Joints

The use of Sn–Zn–Bi and Sn–Zn solders is increasing because of their low cost and low melting point. Therefore, it is important to ensure the fatigue strength of Sn–Zn–Bi and Sn–Zn solder joints. In this study, the mechanical fatigue strength of Sn–Zn–Bi and Sn–Zn solder joints was evaluated by using chip scale package (CSP) specimens. The influence of surface treatment on the fatigue strength was investigated by using specimens with Ni/Au or pre-flux on a Cu-pad. These specimens were aged at 85, 125 and 150 � C for 500 and 1000 h to investigate the influence of the appearance of intermetallic compounds at the joint interface. Through a series of isothermal mechanical shear fatigue tests and finite element method (FEM) analyses, it was found that the fatigue lives of Sn–Zn–Bi and Sn–Zn solder joints were greatly affected by the aging temperature. In particular, when Sn–Zn–Bi and Sn–Zn solder joints with Ni/Au plating of the Cu-pad were aged at a temperature above 125 � C, the fatigue strength decreased remarkably in comparison with the specimens without Ni/Au plating.

Isothermal and thermomechanical fatigue behaviour of a high temperature titanium alloy

Isothermal and thermomechanical fatigue (TMF) behaviour (including cyclic stress response and number of cycles to failure) of a Ti – 5.6Al – 4.8Sn – 2.0Zr – 1.0Mo – 0.32Si – 0.8Nd (wt-%) hightemperature titanium alloy was examined. The purpose of the present investigation was to understand the effect of temperature fluctuation on the cyclic behaviour and fatigue life of this alloy and to test the suitability of lifetime prediction based on isothermal laboratory data. The results indicated that both the level of peak stress and fatigue life were decreasing with increasing test temperature from 400°C to 650°C in isothermal fatigue (IF) tests. In TMF tests run between 400°C and 600°C, the peak stresses corresponding to 600°C coincide well with that found in IF tests run at 600°C, while a slight increase in cyclic hardening was found for peak stress corresponding to 400°C compared to that found in a 400°C/IF test. This increase in cyclic hardening became more pronounced when the maximum temperature increased to 650°C. Fatigue life in 'out of phase' (OP) condition was found to be shorter than under an equivalent 'in phase' (IP) condition, and this gap increased with decreasing mechanical strain amplitude. The results indicate that lifetime prediction based on isothermal laboratory data may lead to non-conservative results if thermal fluctuations are present in components made of the present alloy.

Notched specimens thermo-mechanical fatigue of a 1CrMoV turbine steel

Isothermal (N-LCF) and thermo-mechanical fatigue (N-TMF) tests have been performed on circumferentially notched bars made of 1CrMoV. The main purpose of the investigation has been to evaluate the suitability of uniaxial fatigue data for the damage assessment of components with stress raising features. Service-cycle N-TMF tests (with low strain rates and long hold periods at peak temperature) reproduced at the notch root the strain and temperature histories of corresponding uniaxial TMF tests performed in previous studies. It is therefore possible to directly compare the notch root endurances with uniaxial data lines. The experimental results have been complemented by detailed finite element calculations and metallurgical analysis from post-test inspections. Under the assumption of a very short crack initiation length, the endurances of the notched tests can be rationalized from the uniaxial data. Measurements of short crack growth within the notch near field have been analysed using stress-strain loops in the notch near field calculated by finite elements. For the same surface equivalent strain range short crack growth is slower in notched tests than in uniaxial tests and is faster in the present N-TMF tests as compared with the N-LCF tests.

Characterization of a fracture specimen for crack growth in epoxy due to thermal fatigue

Finite element simulations are carried out to characterize a new fracture specimen, consisting of an outer circular epoxy ring bonded to an inner circular invar plate for accelerated thermal fatigue testing. Radial cracks are introduced in the epoxy ring. The growth of these radial cracks is correlated to the applied energy release rate G. We studied the dependence of G on the crack length, the specimen geometry and the elastic modulus. For short cracks, G is obtained in closed form. Analysis is carried out to determine the critical thermal buckling load the specimen can withstand. Experimental results show that the fatigue crack growth rate per thermal cycle d a/d N is given by d a/d N = 0.51(Δ G) 0.38 for cycling between 4 and 100 °C but by d a/d N = 0.25(Δ G) 0.24 for cycling between 20 and 85 °C, where Δ G is the difference of the energy release rate between the highest and lowest temperatures during a thermal cycle. More severe thermal cycles produce considerably larger fatigue crack growth rates than less severe ones at the same Δ G. This result also implies that isothermal fatigue tests will probably be inadequate to predict thermal fatigue crack growth in epoxies.

Thixoextrusion of an A356 alloy: microstructural studies and high temperature fatigue behaviour

Isothermal tensile, isothermal and thermomechanical fatigue tests were conducted on thixoextruded and permanent mould cast A356 T6 aluminium alloy over a broad range of temperatures. The results revealed a significant decrease in strength parameters at 280°C, for both materials. The fatigue lives of thixoextruded material were substantially higher than those of permanent mould cast samples under isothermal and thermomechanical fatigue loading. At corresponding mechanical and inelastic strain ranges, thermomechanical out-of-phase cycling were higher than in-phase cycling for the alloy. Due to data scatter, the out-of-phase fatigue lives of the permanent mould cast samples and of the thixoextruded samples appeared to be very similar.

Characterisation of the creep-fatigue behaviour of a 1CrMoV turbine steel

This paper concerns the creep-fatigue lifetime assessment of a 1CrMoV steel. A procedure based on the calculation of fatigue and creep damage fractions and their summation according to a non-linear interaction rule is applied for the endurance evaluation of isothermal fatigue and service-like thermo-mechanical fatigue tests performed with plain (uniaxial) and notched testpieces. In order to avoid the uncertainties associated with heat-to-heat variability, testpieces were manufactured from a single heat, for which the fatigue and creep deformation/endurance properties were specifically characterised. With the availability of a reliable constitutive model, finite element calculations could be used to determine the local stress-strain response at the position of crack initiation in each test. The predictive capabilities of the proposed approach for lifetime calculation as well as the suitability of uniaxial fatigue and creep data for the damage assessment of notched components are evaluated.

Experimental damage mechanics of microelectronic solder joints under fatigue loading

Fatigue damage is a progressive process of material degradation. The objective of this study is to experimentally qualify the damage mechanism in solder joints in electronic packaging under thermal fatigue loading. Another objective of this paper is to show that damage mechanism under thermal cycling and mechanical cycling are very different. Elastic modulus degradation under thermal cycling, which is considered as a physically detectable quantity of material degradation, was measured by Nano-indenter. It was compared with tendency of inelastic strain accumulation of solder joints in Ball Grid Array package under thermal cycling, which was measured by Moiré interferometry. Fatigue damage evolution in solder joints with traditional load-drop criterion was also investigated by shear strain hysteresis loops from strain-controlled cyclic shear testing of thin layer solder joints. Load-drop behavior was compared with elastic modulus degradation of solder joints under thermal cycling. Following conventional Coffin–Manson approach, S– N curve was obtained from isothermal fatigue testing with load-drop criterion. Coffin–Manson curves obtained from strain-controlled mechanical tests were used to predict fatigue life of solder joints. In this paper it is shown that this approach underestimates the fatigue life by an order of magnitude. Results obtained in this project indicate that thermal fatigue and isothermal mechanical fatigue are completely different damage mechanism for microstructurally evolving materials.

Physical properties and life prediction of TMC at elevated temperatures

Abstract Mechanical performance of titanium matrix composites degrades at elevated temperatures because of the changes in physical properties of the composites (i.e. matrix oxidation, embrittlement and phase transformation). These physical properties of the composites should be taken into account in a life prediction model. The present study was conducted to improve an existing life prediction model (local fiber stress failure model). To achieve this goal, isothermal fatigue tests were performed on [0/±45/90] s SCS-6/Timetal 21S composite at temperatures 400 and 500 °C, and with hold times. The composite lives at 400 and 500 °C increased with increase in hold time. It was observed that the matrix oxidation was accelerated under applied stress amplitude. In addition, the measurement of the surface oxide thickness suggested that the oxidation was much easier and faster at 500 °C than at 400 °C. A modification was made on the life prediction model. Since the oxidation was shown to have a significant effect on the composite life, it was factored into the model. The modified model produced a good correlation between modified single parameter value and the number of cycles to failure.

OS10W0196 Fatigue strength evaluation for Sn-Zn-Bi and Sn-Zn lead-free solder joints

In this paper, the authors have investigated mechanical fatigue strength of lead-free solder joints. The use of Sn-Zn-Bi and Sn-Zn solder is increasing because of the advantage of low cost and low melting point. Therefore, it is important to ensure the fatigue strength of Sn-Zn-Bi and Sn-Zn solder joints. In this study, the mechanical fatigue strength of Sn-Zn-Bi and Sn-Zn solder joints has been studied, by using the CSP(Chip Size Package) specimen. In order to investigate the influence of plating treatment on the fatigue strength of the solder joint, the specimens with Ni/Au or Cu plating treatment on Cu-pad were prepared. And these specimens were aged by holding 200, 500, 1000 hours at 85, 125, 150℃, respectively to investigate the influence of the growth of the intermetallic compounds on the fatigue strength. Through a series of isothermal mechanical shear fatigue tests and FEM(Finite Element Method) analysis, it has been found that if the Sn-Zn-Bi and Sn-Zn solder with Ni/Au plating treatment were aged at the temperature above 125℃, the fatigue strength decreased remarkably in comparison with the specimens with Cu plating. By plating with the Ni/Au on the substrate side, the interface in the substrate side was strengthened. But, the interface in the chip side is weakened. It causes the interface fatigue crack between intermetallic compound layer and solder at the chip side, and the decrease of fatigue strength.

On the low cycle fatigue deformation of K40S cobalt-base superalloy at elevated temperature

Low cycle isothermal mechanical fatigue testing K40S cobalt-base superalloy was carried out at 700 and 900 °C with total strain amplitude from ±0.1 to ±1.0 pct. Correlations between microscopic cyclic deformation and cycle stress response with various microstructural phenomena were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the deformation substructure and carbide precipitation. The results show that K40S superalloy exhibited cyclic stress response of cyclic hardening at 700 °C, and of initial hardening followed by softening at 900 °C. In addition, at both temperatures, cyclic hardening during cycle deformation increased with increasing strain amplitude, while the fatigue lifetime decreased. The cyclic stress response behavior can be rationalized based on the mechanisms associated with dislocation–dislocation interactions, dislocation–precipitate (M 23C 6) interactions and interactions between solute atoms and dislocations.

Creep Analysis and Thermal-Fatigue Life Prediction of the Lead-Free Solder Sealing Ring of a Photonic Switch

Thermal reliability of the solder sealing ring of Agilent Technologies’ bubble-actuated photonic cross-connect switches has been investigated in this paper. Emphasis is placed on the determination of the thermal-fatigue life of the solder sealing ring under shipping/storing/handling conditions. The solder ring is assumed to obey the Garofalo-Arrhenius creep constitutive law. The nonlinear responses such as the deflections, stresses, creep strains, and creep strain energy density of the 3-D photonic package have been determined with a commercial finite element code. In addition, isothermal fatigue tests have been performed to obtain the relationship between the number of cycle-to-failure and the strain energy density. Thus, by combining the finite element results and the isothermal fatigue test results, the average thermal-fatigue life of the solder sealing ring is readily determined and is found to be more than adequate for shipping/storing/handling the photonic switches.

The characteristics of fatigue under isothermal and thermo‐mechanical load in Cr–Ni–Mo cast hot work die steel

ABSTRACT High temperature isothermal fatigue (IF) and in‐phase thermo‐mechanical fatigue (TMF) tests in load control were carried out in cast hot work die steel. At the same load amplitude, the fatigue lives obtained in the in‐phase TMF tests are lower than those obtained in the isothermal tests. Observations of fracture surface and the response of stress–strain reveal that cyclic creep in the tensile direction occurs and the intergranular cracks dominate in TMF tests, whereas cyclic creep in the compressive direction occurs and the path of the crack growth is mainly transgranular in IF tests. A model of life prediction, based on the Chaboche law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method, the lifetime prediction gives results corresponding well to experimental data.

Damage mechanisms and life prediction in high temperature fatigue of a unidirectional SiC–Ti composite

Isothermal fatigue tests are performed in the longitudinal direction at 450°C on a unidirectional SiC/Ti composite. Three major damage mechanisms are identified: the matrix cyclic softening which overloads fibers leading to their progressive rupture; the interfacial degradation of these broken fibers and their oxidation by the environment. Damage kinetics are estimated using microscopic observations and acoustic emission. Finally, a micromechanical model is used in order to understand the respective influence of these damage mechanisms. It is shown that, at a sufficient load level, fatigue fracture of the composite strongly depends on the interfacial degradation kinetics and that fiber oxidation by the environment generates more progressive damage and considerably reduces the composite fatigue life compared to loading under vacuum.

An overview of the damage approach of durability modelling at elevated temperature

Lifetime prediction techniques for components working at elevated temperature are revisited. Two damage approaches in which time effects at high temperature are introduced in different ways are discussed in greater detail. First, a creep–fatigue damage model considers the interaction of the two processes during the whole life before macrocrack initiation; and second, a creep–fatigue–oxidation model separates the fatigue life into two periods: during initiation the environment‐assisted processes interact with fatigue, although bulk creep damage only interacts during the micropropagation period. The second model is illustrated by its application to a coated single‐crystal superalloy used in aerojet turbine blades. Its capabilities are illustrated in a number of isothermal and thermomechanical fatigue tests. Anisotropy effects are also briefly discussed and a special test, introducing cyclic thermal gradients through the wall thickness of a tubular component, demonstrates the predictive capabilities for actual engine conditions.

Thermomechanical and isothermal fatigue behaviour of type 316 stainless steel base metal, weld metal, and joint

In phase thermomechanical fatigue (TMF) in the temperature range 573–973 K (300–700°C) and isothermal fatigue behaviour at 973 K in air were studied for type 316 stainless steel using smooth cylindrical specimens machined from base metal, weld metal, and the weld joint (cross-weld). In all joint specimens, fatigue failure occurred in the weld metal region. The lifetimes of weld metal and joint specimens were almost equal and were always inferior to those of base metal specimens. In the base metal, the effect of strain rate on the isothermal fatigue life was not very significant. Although TMF lifetimes were always a little shorter than the isothermal fatigue lifetimes in base metal, the difference was small for the same mechanical strain range and similar strain rate. This may be because the fracture mode for both types of loading was of a similar mixed type. Conversely, a dramatic reduction in lifetime was observed in weld metal and joint specimens under TMF in comparison with isothermal fatigue. This was attributed to the additional damage caused by many independent subsurface cracks at σ phase boundaries and linkage of these cracks with the surface crack, leading to rapid crack propagation. The δ ferrite in the weld metal completely transformed to σ phase in both the isothermal fatigue and TMF tests.