Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from ?40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.     

Characterization of isothermal and cyclic thermal damage of EB-PVD TBCs with the help of the 3D-DIC technique

The thermal barrier coatings (TBCs) are working under complex elevated temperature loading conditions. In the present work, a damage model for the isothermal and cyclic thermal loads was developed to quantify the failure process of the coatings subjected to isothermal and cyclic thermal exposures. Effects of different damage mechanisms, such as thermal exposure, thermal cycling, aluminum migration, and thermal dwell times, were experimentally and computationally studied. The digital image correlation (DIC) technique was introduced to evaluate degradation of TBCs. The complex damage can be quantified with the help of the DIC strain variations. The introduced model provides a general method to estimate the remaining life.     

Anisotropic oxidation and weight loss in PMR-15 composites

Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. The thermo-oxidative behavior of the composite is significantly different from that of the fiber and matrix constituents as the composite microstructure, including the fiber–matrix interphases/interfaces, introduces anisotropy in the diffusion behavior. In this work, unidirectional G30-500/PMR-15 composite specimens were aged at elevated temperatures in air resulting in oxidation propagation parallel and perpendicular to the fibers. Four different specimen geometries were chosen such that different surface area ratios (i.e., ratios of surface area perpendicular to the fibers to surface area parallel to the fibers) were obtained. Weight loss and volumetric changes were monitored as a function of aging time to study the high-temperature anisotropic oxidation process. Optical micrographs were taken on polished internal sections and viewed in the dark-field mode to measure the degree, depth and distribution of thermal oxidation development from surfaces perpendicular and parallel to the fibers. An empirically based weight loss model is investigated and used to predict weight loss in unidirectional and woven composites.     

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

Increase of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La₂Zr₂O₇) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.     

Effects of vacuum thermal cycling on mechanical and physical properties of high performance carbon/bismaleimide composite

The aim of this article was to investigate the effects of vacuum thermal cycling on mechanical and physical properties of high performance carbon/bismaleimide (BMI) composites used in aerospace. The changes in dynamic mechanical properties and thermal stability were characterized by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA), respectively. The changes in linear coefficient of thermal expansion (CTE) were measured in directions perpendicular and parallel to the fiber direction, respectively. The outgassing behavior of the composites were examined. The evolution of surface morphology and surface roughness were observed by atomic force microscopy (AFM). Changes in mechanical properties including transverse tensile strength, flexural strength and interlaminar shear strength (ILSS) were measured. The results indicated that the vacuum thermal cycling could improve the crosslinking degree and the thermal stability of resin matrix to a certain extent, and induce matrix outgassing and thermal stress, thereby leading to the mass loss and the interfacial debonding of the composite. The degradation in transverse tensile strength was caused by joint effects of the matrix outgassing and the interfacial debonding, while the changes in flexural strength and ILSS were affected by a competing effect between the crosslinking degree of resin matrix and the fiber-matrix debonding.     

Influences of thermal cycling and low-energy impact on the fatigue behavior of carbon/PEEK laminates

Effects of low-energy impact and cyclic thermal loading on fatigue behavior of carbon fiber reinforced polyetheretherketone (carbon/PEEK) laminates have been examined. The fatigue behavior of the virginal composites, low-energy impacted composites, and low-energy-impacted and thermally exposed composites were investigated. Cyclic thermal loading was performed in the temperature range between 60 and −60°C. Residual tensile strength was measured to aid in understanding the influence of low-energy impact on the retention of tensile strength. Fatigue testing involved a stress ratio of 0.1, with a frequency of 3 Hz. The Weibull distribution function was used to evaluate the ultimate tensile strength and fatigue life. S–N curves were plotted and the influence of thermal cycling and the low-energy impact on the fatigue sensitivity of the carbon/PEEK laminates was investigated. Stiffness variation during fatigue testing was monitored and differences in stiffness reduction for three test conditions were compared. C-scan was used to investigate the damage zone under different low-energy impacts and to understand damage propagation during fatigue testing. Moreover, scanning electron microscopy (SEM) was used to examine the fracture morphologies of carbon/PEEK composites in both tensile failure and fatigue failure conditions.     

Effect of vacuum thermal cyclic exposures on unidirectional carbon fiber/epoxy composites for low earth orbit space applications

The effects of the low earth orbit environment on three types of unidirectional high-modulus carbon fiber (M40 J, M55 J and M60 J)-reinforced composites were determined in detail. The synergistic environmental factors were the vacuum environment and thermal cycling. Cyclic thermal loading was performed in the temperature range between 120 °C and ?175 °C for up to 2000 cycles under the high-vacuum state of 1.3^?3 Pa. The material responses were characterized through an assessment of the physical, thermal and mechanical property changes. It follows from the experimental results presented that the synergistic actions of the vacuum and the thermal cycling on the composite property degradation can be attributed to the formation of microvoids and interfacial sliding at the fiber–matrix interface in the early stages of cycling. The implications of these degradation processes based on the dependence of composite properties on vacuum thermal cycling are also discussed.     

Tensile behavior of SiC/C and Rene'41 following isothermal exposure and thermal fatigue

The performance of a coated silicon carbide/carbon composite under isothermal and thermal fatigue conditions was investigated. The material studied is known as Ceracarb which consists of eight-harness satin weave Nicalon® silicon carbide cloth reinforcement, a carbonaceous matrix, and a silicon carbide composite coating. This advanced composite is being considered for replacing the nickel based superalloy Rene'41, as the exhaust nozzle components on military afterburning turbine engines. Thermal fatigue experiments, performed in the laboratory using a thermal cycling test system, were intended to roughly simulate the thermal excursions of an afterburning exhaust nozzle. Several thermal profiles were used to characterize the role of temperature, number of cycles, temperature range, and time at temperature, on the room temperature residual tensile strength of the material. The same thermal profiles were also conducted on test specimens of Rene'41 in order to compare its durability in the laboratory simulation test set-up to the composite. Both materials showed no loss in strength from the as-received condition following thermal testing. However, the Rene'41 showed evidence of microstructural instability at the maximum test temperature of 1093°C (2000°F) which did affect the toughness of the material. While the results from this study showed that both materials retained strength when thermally exposed in the laboratory under no loads, thermal testing under load may provide a more realistic view of how the materials perform in the afterburning exhaust nozzle application.     

碳/酚醛防热复合材料烧蚀行为的数值模拟

碳/酚醛复合材料被广泛地应用于钝头体表面,是飞行器优秀的热防护材料。为了准确地预测其烧蚀性能,本文从复合材料的组成物纤维和基体的角度出发,基于能量、质量守恒和热分解方程,考虑了烧蚀过程中材料热属性的非线性变化和烧蚀面的退缩,分别计算了纤维和基体的烧蚀性能,预测了烧蚀过程中防热复合材料的温度分布、密度变化、质量损失规律及热属性和线烧蚀率等。结果表明：碳/酚醛复合材料的烧蚀是各种因素相互作用、相互影响的高度非线性过程;烧蚀过程中材料结构具有不均匀的温度分布,烧蚀面区域材料密度衰减最大并且材料的质量损失和损失率几乎呈线性增加;纤维和基体的烧蚀行为存在明显差异,分别预测两者的烧蚀性能,可以为热防护材料的设计提供更加准确的参考和依据。     

Thermal fatigue behavior of squeeze cast, discontinuous alumina-silicate fiber-reinforced aluminum alloy (A356) composite

Microstructural damage mechanisms owing to thermal cycling and isothermal exposure at elevated temperature are studied for a short alumina-silicate fiber-reinforced aluminum alloy (A356) composite produced by pressure casting. The tensile strength of the metal matrix composite is found to degrade considerably in each case. An X-ray double-crystal diffraction method is employed to study the mechanisms of recovery in the matrix. The fractal dimension of the X-ray “rocking curves” for individual grains in the composite reflects the substructure formation owing to the rearrangement of dislocations into subdomain walls. Recovery by polygonization is more pronounced in the case of thermal cycling than for equivalent isothermal exposure. The residual stresses in the matrix that provide the fiber clamping force undergo more relaxation in the case of isothermal exposure. The two competing damage mechanisms, thermally activated recovery by polygonization and relaxation of clamping stresses in the matrix, result in identical strength degradation (25%) for both thermal cycling and isothermal exposure.     

Thermal-mechanical fatigue behaviour of alumina fibre/aluminium-lithium composite

Metal matrix composites are candidates for elevated temperature applications. For this reason, it is important to understand their behaviour under thermal-mechanical fatigue conditions. Thermal cycling of a composite material creates thermal stresses in the composite because of thermal expansion mismatch between the fibre and the matrix. This can lead to plastic deformation of the matrix, interface damage, and fibre fracture. Mechanical cyclic loading of the composite during thermal cycling can aggravate the situation even more. A computer-controlled servo-hydraulic thermal-mechanical fatigue test system was used to perform tests on -Al₂O₃(FP)/AI-2%Li metal matrix composite specimens. The volume fraction of unidirectionally aligned fibres was 35%. The tests performed were free-expansion tests, fast and slow thermal fatigue tests, and isothermal fatigue tests. Large reductions in the composite strength were observed under thermal fatigue conditions. This degradation can be attributed to observed microstructural damage of the fibre/matrix interface and fibre fracture.     

Modeling thermal cycling induced micro-damage in aluminum welds: An extension of Gurson void nucleation model

Mechanical properties and damage mechanism of 5A06 aluminum alloy welded joint under thermal cycling condition were investigated. Microstructural and fractographic observation demonstrate that void nucleation around the second phase particles is the dominant factor for performance decrease. A modified Gurson void nucleation model was presented to characterize the effect of thermal stress assisted voiding based on the micromechanical analysis of a cell model. This model was successfully implemented in the finite element code to describe the void evolution under thermal cycling conditions.     

形状记忆合金短纤维增强弹塑性基体复合材料的力学行为

基于Eshelby等效夹杂方法和Mori-Tanaka的平均化理论推导了针对SMA短纤维增强弹塑性基体复合材料的细观力学模型。利用此模型,分析了这种复合材料的力学行为,讨论了材料温度、纤维体积分数和纤维特征形状等参数对复合材料残余应力和残余应变的影响。这对复合材料的分析和设计都有重要的意义。      

Experimental and FEM study of thermal cycling induced microcracking in carbon/epoxy triaxial braided composites

The microcrack distribution and mass change in T700s/PR520 and T700s/3502 carbon/epoxy braided composites exposed to thermal cycling was evaluated experimentally. Acoustic emission was utilized to record the crack initiation and propagation under cyclic thermal loading between −55 °C and 120 °C. Transverse microcrack morphology was investigated using X-ray computed tomography. The differing performance of two kinds of composites was discovered and analyzed. Based on the observations of microcrack formation, a meso-mechanical finite element model was developed to obtain the resultant mechanical properties. The simulation results exhibited a decrease in strength and stiffness with increasing crack density. Strength and stiffness reduction versus crack densities in different orientations were compared. The changes of global mechanical behavior in both axial and transverse loading conditions were studied. By accounting for the obtained reduction of mechanical properties, a macro-mechanical finite element model was utilized to investigate the influence of microcracking on the high-speed impact behavior.     

基于激光扫描法辨识热环境下纤维/树脂基复合材料损耗因子

提出了激光扫描法辨识热环境下纤维/树脂基复合材料损耗因子。首先，以该类型复合材料薄板试件为例，基于复模量法对其在热环境下的振动响应进行了理论求解；然后，建立了复合材料薄板的激光扫描框架模型，并在分别通过激光扫描法和复模量法获得其振动响应的基础上，利用最小二乘法构造响应相对误差函数，进而辨识获得热环境下纤维/树脂基复合材料在纤维各个方向的损耗因子。接着，在明确了热环境下复合材料损耗因子辨识原理的基础上，总结并概括出一套合理、规范的辨识流程。最后，搭建了基于激光扫描热环境下复合材料薄板振动测试系统，并以TC500碳纤维/树脂基薄板为研究对象，在常温到300℃的高温环境下对其前4阶共振响应进行了实际测试，并通过第1阶共振响应数据对损耗因子进行辨识。结果表明，在温度从常温上升到300℃时，纤维/树脂基复合材料的损耗因子呈现不断增大的趋势。另外，还将辨识出的100℃下对应的材料损耗因子代入到理论模型中，计算得到了复合材料薄板试件在该温度下的第2、3、4阶共振响应的理论结果，通过与相同温度下实验测试获得的第2、3、4阶共振响应进行对比可知，两者的偏差在1.4%~13.8%之间，进而验证了所提出的辨识方法的有效性和实用性。     

Determining a critical strain for APS thermal barrier coatings under service relevant loading conditions

Despite the huge progress made in recent years in analysing the degradation behavior and the reliability of thermal barrier coating systems, there is still some deficit in the capability to predict damage evolution in terms of crack initiation and crack growth, which ultimately leads to macroscopic delamination and spallation of the coating system. In order to obtain this prediction capability, a fundamental understanding of the damage evolution processes under isothermal, thermo-cyclic and under thermo-mechanical loading conditions has to be developed.The aim of the presented work is to determine the critical strain, i.e. the strain at which cracking initiates, and to analyse the evolution of a network of cracks for widely used atmospheric plasma sprayed (APS) thermal barrier coating (TBC) systems. The TBC system has been exposed in our study to service relevant loading conditions, namely to thermal gradient mechanical fatigue (TGMF). TGMF tests for in-phase as well as out-of-phase loading cycles were performed on hollow cylindrical specimens made of the single crystal super alloy CMSX-4, loaded mechanically in 〈0 0 1〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8 wt.% Yttria partially stabilized Zirconia (YSZ) TBC. The CoNiCrAlY bond coat was deposited by Low Pressure Plasma Spraying (LPPS), while the ceramic top coat was deposited using the APS process. The loading cycles were chosen to represent an industrial gas turbine engine. Critical strains measured for delamination (within the ceramic coating or at the CoNiCrAlY – TBC interface) and through cracking, i.e. segmentation of the ceramic top coat was determined using a special compression test equipped with in situ acoustic emission technique. The mechanical testing was performed at room temperature after TGMF exposure. In order to study the impact of thermally grown oxide (TGO), specimens have been TGMF tested in the “as received” conditions as well as after isothermal aging (up to 3000 h at 1000 °C). To correlate the signal obtained by acoustic emission (AE) with the evolution of (micro-) cracks, the specimens have been carefully sectioned and investigated by standard metallographic means.The measured critical strains are used as a data basis for a strain-based lifetime model developed for isothermal and cyclic oxidation as well as thermo-mechanical loading. The lifetime model considers two failure modes, namely delamination and (vertical) through cracking.Metallographically obtained crack patterns within the TBC system have been incorporated into finite element models to quantify stress–relaxation as a consequence of damage evolution in the TBC system.The observations show that thermal gradient fatigue loading under in-phase loading leads to a shorter lifetime compared to out-of-phase loading.For the delamination mode, the critical strain values of the model are in good agreement with the experimental data of the TGMF experiments. The modeled critical strain for through cracking, on the other hand, is consistently lower than the experimentally determined failure strains, implying that the model describes the failure situation in a conservative manner.     

热阻塞效应在有机硅树脂-碳纤织物复合材料烧蚀防热中的作用

为评价热阻塞效应对有机硅树脂-碳纤织物复合材料防热的贡献,根据有机硅树脂的烧蚀防热机理建立热响应过程数学模型,预测了有机硅树脂-碳纤织物复合材料的背面温度以及有机硅树脂的热物性参数,重点分析了热阻塞效应对有机硅树脂-碳纤织物复合材料防热性能的影响。结果表明在400 kW/m2的热流烧蚀下,有机硅树脂-碳纤织物复合材料40 s前热阻塞效应大部分来自有机硅树脂分解产生的引射气体,40 s后则完全来自于炭燃烧产生的引射气体;阻塞因子在10 s时达到最小,此刻阻挡了121.6 kW/m2的热流进入有机硅树脂内,在整个烧蚀过程中热阻塞效应减少了4.1%的总热量进入有机硅树脂内;在热物性参数中,热阻塞效应对有机硅树脂密度影响最大,导热系数和比热容次之;与增加逸出气体质量流率相比,延长有机硅树脂逸出气体的时间更能显著提高热阻塞效应,达到更好的防热效果。      

Effect of surface treatment on thermal stability of the hemp-PLA composites: Correlation of activation energy with thermal degradation

The thermal behavior of hemp-poly lactic acid composites with both untreated and chemically surface modified hemp fiber was characterized by means of activation energy of thermal degradation. Three chemical surface modification employed were; alkali, silane and acetic anhydride. Model-free isoconversion Flynn–Wall–Ozawa method was chosen to evaluate the activation energy of composites. The results indicated that composites prepared with acetic anhydride modified hemp had 10–13% higher activation energy compared to other composites. Further, among the three surface modifications, acetic anhydride resulted in higher activation energy (159–163 kJ/mol). Fourier transform infrared spectroscopy supported the findings of thermogravimetric analysis results, wherein surface functionalization changes were observed as a result of surface modification of hemp fiber. It was concluded that, higher bond energy results in higher activation energy, which improves thermal stability. The activation energy data can aid in better understanding of the thermal degradation behavior of composites as a function of composite processing.     

Viscoelastic and thermal behavior of woven hemp fiber reinforced poly(lactic acid) composites

This study examined the physical behavior of hemp/poly(lactic acid) (PLA) composites, particularly the thermal properties and viscoelastic behavior. Twill and plain woven hemp fabrics were used as reinforcements and hemp fabrics-reinforced PLA composites were produced using a film stacking method. The coefficient of thermal expansion of the composites decreased sharply with increasing the volume fraction of fiber. The twill structure was found to be suitable for reinforcing a PLA resin with higher impact strength and better mechanical properties than the plain woven. The viscoelastic properties of the composites including the storage modulus, loss modulus and loss tangent were also examined by dynamic mechanical analysis. In addition, morphological analysis was performed using scanning electron microscopy.     

Effect of thermal cycling on the crack-growth resistance of fiber glasses

We study the influence of thermal cycling on the crack-growth resistance and fracture of fiber glasses based on epoxy binders unidirectionally reinforced by solid or hollow fibers. Specimens were subjected to 15 thermal cycles according to the following procedure: First, they were held for 5 min in liquid nitrogen at 77°K and then, for 60 min, in air at 293°K. Prior to failure, the structure of the specimens in the initial state and after thermal cycling was studied with an optic microscope. The fracture surfaces of destroyed specimens were analyzed with an REM-200 electron microscope. It was discovered that, after thermal cycling, the crack-growth resistance of fiber glasses with solid fibers is lower than the crack-growth resistance of fiber glasses with hollow fibers. In addition, for the same reinforcement, the type of the applied lubricant practically does not affect the degree of decrease in crack-growth resistance. We detected various types of damages to the matrix and fibers caused by thermal cycling and analyzed the morphology of their types. On the basis of the obtained results, we proposed a mechanism of fracture processes in fiber glasses subjected to thermal cycling.Kharkov Engineering-Pedagogical Institute, Kharkov. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 40–46, September – October, 1995.