Isothermal Mechanical Fatigue Research Articles

A study of isothermal mechanical fatigue of welded reactor steel (15H2MFA)

As far as the reactor pressure vessel steel is concerned, the isothermal fatigue study of welded joints is helpful to protect the pressure vessel from blasting and save the life of its operators. Furthermore, the study helps to improve the future design of the reactor components and also enthuse to develop new materials. In material science, the fatigue of metal is quite difficult to deal with, and that of welded joint is an even more complex phenomenon. This complexity arises from different main factors: thermal cycles during welding, which will strongly affect the base material and induce residual stresses and distortion, the fusion process with the filler metal leads to heterogeneity to microstructure which will affect or change the original mechanical properties and chemical composition through out of the welded joint. In this study, isothermal mechanical fatigue of welded reactor steel has been contemplated. The samples of the welded joint used in this study are the ‘15H2MFA steel type of VVER-440’ reactor pressure vessel operating at Paks Nuclear Power Plant, Hungary. The reactor vessel is made of low-alloy heat-resistant steel and austenitic steel with a corrosion-resistant inner (clad) surface.

Microstructure, isothermal and thermomechanical fatigue behaviour of leaded and lead-free solder joints

The reliability of solder joints is a critical issue in the microelectronics industry. The requirement of permanent electrical and thermal connections between solder alloys and the various components of a surface mount device is dependent upon the mechanical integrity of the solder and its interfaces.Accordingly, in this paper, the reliability of lead-free, Sn-3.8Ag-0.7Cu, and leaded, Sn-37Pb, solder alloys was investigated under both thermal-mechanical fatigue (TMF) and isothermal mechanical fatigue (IF) conditions. The investigation included material characterisation and fatigue testing on 4-ball grid array (BGA) specimens. The IF tests were carried out under load control at three different temperatures including Room Temperature (RT), 35 °C and 75 °C. Also, a set of ‘not-in-phase’ (nIP), ‘in-phase’ (IP) and ‘out-of-phase’ (OoP) combined thermal and mechanical cycling tests were carried out to investigate the TMF behaviour of the solders. The stress-life curves for each test condition were generated and then compared taking into account the observations on microstructure.It was found that the IF and TMF performance of Sn-3.8Ag-0.7Cu alloy was better than Sn-37Pb alloy when expressed as stress-life curves. Additionally, the Sn-3.8Ag-0.7Cu was less susceptible to the changes in temperature. This study provides a comprehensive insight into the reliability of solder alloys under a wide range of loading conditions.

List of abstracts

. Results obtained in the analysis of single layer and multilayered carbon stacks produced by PLD at low temperature will be presented. The mechanical properties of carbon single layers deposited at various laser pulse fluences, like hardness and elasticity, were determined by means of nano indentation measurements. It will be shown that a wide variety of mechanical properties can easily be adjusted by adapting the process parameters.

High cycle thermal fatigue of austenitic stainless steel

A new experimental setup called FLASH (THErmal Fatigue by LASer or Helium pulses) has been developed to perform thermal fatigue tests on materials used in structures of Sodium-cooled Fast Reactors (SFRs). A set of thermal loadings ranging from 150°C to 200°C prescribed by a high energy laser has been applied to A316L(N) stainless steel samples. Thermal fatigue tests are performed until a macrocrack is formed. The 3D displacement fields of the surface impacted by the laser beam are measured by a hybrid multiview system. A thermomechanical model is used to compute displacement fields that are compared with correlation measurements. These results are compared in terms of strain levels to the fatigue curve determined from standardized isothermal uniaxial mechanical fatigue tests.

Tribo-thermal fatigue of the steel used for the forging die construction

Frequently the durability of the forging dies is firstly determined by the non-isothermal fatigue wear, which causes the cracks appearance on their internal surfaces, much more before their abrasion wear to reach the limit value. In these conditions it is necessary to design the forging dies firstly by the point of view of the non-isothermal fatigue wear. For a correctly choosing and using of metallic material, it is necessary to determine their intrinsic characteristics regarding its cyclic non-isothermal stresses durability. The experimental determination of these characteristics implies a lot of experiments, which are done in specific conditions, different from those used for isothermal mechanical fatigue durability determination. The paper presents the experimental results concerning intrinsic characteristic determination of the forging dies steel. Based on these results there were determined specific equations which characterize this kind of stresses, and the diagrams that represent their graphic image. These data can be used both in designing and exploitation of the forging dies.

A Modified Closed Form Energy Based Framework for Axial Isothermal-Mechanical Fatigue Life Assessment for Aluminum 6061-T6

In the present investigation, the applicability of a previously developed closed form energy based framework to predict low cycle fatigue (LCF) life of aluminum 6061-T6 was extended from room temperature to elevated temperature. The three different elevated temperatures considered in the present investigation were 75 °C, 100 °C, and 125 °C which were below the creep activation temperature for aluminum 6061-T6. Like the room temperature life assessment framework, the elevated temperature life assessment framework involved computation of the Ramberg–Osgood cyclic parameters from the average plastic strain range and the average plastic energy obtained from an axial isothermal-mechanical fatigue (IMF) test. The temperature dependent cyclic parameters were computed for 25 °C (room temperature), 75 °C, and 100 °C and then extrapolated to 125 °C utilizing functions describing the dependence of the cyclic parameters on temperature. For aluminum 6061-T6, the cyclic parameters were found to decrease with increase of temperature in a quadratic fashion. Furthermore, the present energy based axial IMF framework was found to be able to predict the LCF life of aluminum 6061-T6 at both room and elevated temperatures with excellent accuracy.

Cyclic Deformation Behaviour of Cu Multilayered Films on Si Substrates

In the present study the high cycle fatigue behaviour of multilayered Cu films with different thicknesses and microstructure on silicon substrate was investigated. An ultrasonic resonance fatigue testing systems was used to study the isothermal mechanical fatigue behaviour of the Cu multilayers at room temperature and 150°C. Investigations of fatigue damage on the surface of the samples showed distinct slip band formation on preferentially oriented grains of the Cu metallization. The degree of fatigue damage, which could be related to the thickness, grain size and orientation of the Cu films, was evaluated by determination of the slip band density as a function of loading cycles. It was found that with increasing film thickness and grain size the density of deformed area strongly increases, with the majority of surface deformation occurring in the grains with ⟨111⟩ orientation with respect to the film surface. Furthermore increasing the testing temperature resulted in a significant degradation ofthe multilayered film stack due to a considerable increase in the degree of plastic deformation of theCu metallization layer.

Characterization of Solder Joint Reliability Using Cyclic Mechanical Fatigue Testing

This article summarizes the mechanics of two mechanical fatigue methods, cyclic bending fatigue and shear fatigue, in inducing failure in solder joints in package assemblies, and it presents the characteristics of fatigue failures resulting from these methods using example cases of Sn-Pb eutectic and Sn-rich Pb-free solder alloys. Numerical simulation suggests that both testing configurations induce fatigue failure by the crack-opening mode. In the case of bending fatigue, the strain induced by the bending displacement is found to be sensitive to chip geometry, and it induces fatigue cracks mainly at the solder matrix adjacent to the printed circuit board interface. In case of shear fatigue, the failure location is firmly fixed at the solder neck, created by solder mask, where an abrupt change in the solder geometry occurs. Both methods conclude that the Coffin–Manson model is the most appropriate model for the isothermal mechanical fatigue of solder alloys. An analysis of fatigue characteristics using the frame of the Coffin–Manson model produces several insightful results, such as the reason why Pb-free alloys show higher fatigue resistance than Sn-Pb alloys even if they are generally more brittle. Our analysis suggests that it is related to higher work hardening. All these results indicate that mechanical fatigue can be an extremely useful method for fast screening of defective package structures and also in gaining a better understanding of fatigue failure mechanism and prediction of reliability in solder joints.

An Energy-Based Axial Isothermal-Mechanical Fatigue Lifing Method

An energy-based fatigue lifing method for the determination of the full-life and critical-life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature lifing model, which states: the total strain energy dissipated during both a quasi-static process and a dynamic (fatigue) process is the same material property. The axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy dissipated during both a quasi-static process and a dynamic process at elevated temperatures; and (3) the incorporation of the effect of thermal loading into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed to produce full-life and critical-life predictions as functions of temperature, which were shown to have an excellent correlation with experimental fatigue data. For the highest operating temperature, the axial IMF full-life prediction was compared to lifing predictions made by both the universal slopes and the uniform material law prediction and was found to be more accurate at an elevated temperature.

An Energy-Based Axial Isothermal- Mechanical Fatigue Lifing Procedure

An energy-based fatigue lifing procedure for the determination of full-life and critical-life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce fifteen fracture curves at four operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.

Effect of process-induced voids on isothermal fatigue resistance of CSP lead-free solder joints

With the progressive miniaturization of electronic devices, process-induced voids in lead-free solder joints affect the assessment of thermal fatigue resistance. Voids appear randomly in a solder joint, making quantitative evaluation of fatigue life difficult. This study examined the effect of process-induced voids on the thermal fatigue resistance of CSP solder joints. CSP specimens were subjected to isothermal mechanical fatigue tests; specifically, the accelerated thermal cycle test. When a void is small, it has no apparent effect on fatigue life. However, when voids having diameters of at least 30% of solder diameter are located along the crack propagation route, fatigue life is shortened. FEA and Miner’s law for estimation of fatigue life suggest that voids affect not only the crack initiation but also crack propagation. Estimated numbers of cycles to failure agree quantitatively with the experimental results. The effects of the size, location, and number of voids can be extracted by FEA. As voids along the crack path become larger, fatigue life decreases. Moreover, when two voids are located near the corner of a solder joint on the crack path, a 30% decrease in life appears. This result agrees with experimental results reported in several literatures.

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.

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.

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.

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.

Mechanical Bending Fatigue Reliability and Its Application to Area Array Packaging

ABSTRACTFine pitch area array packages are often assembled to flexible printed circuit boards, which require numerous low-frequency mechanical deflections. The interconnect package reliability emphasis has become focused more on isothermal bending fatigue. The interconnect failures due to bending fatigue were found to be different than those which occur if the packages were subjected to accelerated temperature cycling. A test method has been developed to evaluate isothermal mechanical bending fatigue reliability of area array packaging. Mechanical fatigue results of 1.0, 0.8, and 0.75 mm pitch packaging will be presented, demonstrating the effect of package substrate material properties on fatigue life and fracture morphology. Numerical finite element analysis results will also be presented to support and expand the findings in this study.

Low cycle fatigue analysis of temperature and frequency effects in eutectic solder alloy

Low cycle isothermal mechanical fatigue testing of a eutectic alloy 63Sn/37Pb was carried out in a systematic manner over a wide range of frequencies (10 −4–1 Hz) and temperatures (−40 to 150°C) with the total strain set at different values (1–50%). The low cycle fatigue behavior of the eutectic solder was found to be strongly dependent on test temperature and frequency. If the Coffin–Manson model is used to describe such fatigue behavior, the fatigue exponent m and ductility coefficient C in the model are found to be a function of temperature and frequency rather than numerical constants. The plastic flow law was employed to explain the temperature and frequency dependence. The frequency-modified Coffin–Manson model was tried and found to be able to eliminate the frequency dependence of the numerical “constants” but not the temperature dependence. To have a full description of the temperature- and frequency-dependent fatigue behavior, a set of empirical formulae was derived based on the frequency-modified Coffin–Manson model.

High temperature fatigue behaviour of TZM molybdenum alloy under mechanical and thermomechanical cyclic loads

High temperature isothermal mechanical fatigue and in-phase thermomechanical fatigue (TMF) tests in load control were carried out on a molybdenum-based alloy, one of the best known of the refractory alloys, TZM. The stress–strain response and the cyclic life of the material were measured during the tests. The fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal mechanical tests at the same load amplitude. It appears that an additional damage is produced by the reaction of mechanical stress cycles and temperature cycles in TMF situation. Ratcheting phenomenon occurred during the tests with an increasing creep rate and it was dependent on temperature and load amplitude. A model of lifetime prediction, based on the Woehler–Miner 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.

Analytical and Experimental Hybrid Study on Thermal Fatigue Strength of Electronic Solder Joints. 2nd Report. Evaluation by Isothermal Mechanical Fatigue Tests.

An isothermal mechanical fatigue test method and fatigue testing equipment were developed to investigate the low cycle fatigue strength of Sn-Pb eutectic solder joints. Based upon the results of finite element analysis, it is found that the rate of inelastic strain components (plastic strain and creep strain) in the solder joints can be accurately controlled by the displacement rate of the linearactuator of the test equipment. The isothermal mechanical fatigue tests were carried out by controlling the displacement at a very high displacement rate, and a very low displacement rate, respectively ; only plastic strain was generated in the solder joints at the high displacement rate, and only creep strain at the low rate. It is found that the low cycle fatigue life of Sn-Pb eutectic solder joints is not grealty affected by the ratio of inelastic components. Therefore, it can be said that the low cycle fatigue life can be estimated from the equivalent inelastic strain range, and the relationship of the cycles to failure and the strain range follows Coffin-Manson's law. In comparison of the mechanical fatigue strength results with the results of the cyclic thermal tests, it was found that the mechanical fatigue test method can be used as a good accelerated test method for the thermal fatigue strength of Sn-Pb eutectic solder joints.