Röntgenografische Eigenspannungsmessungen an plasmagespritzten Wärmedämmschichten aus Zirkonoxid vor und nach thermischer und mechanischer Beanspruchung

X-Ray Stress Measurements on Plasma Sprayed Zirkonia Thermal Barrier Coatings Turbine blades for aero engines with plasma sprayed zirconia thermal barrier coatings were subjected to typical component loads as thermal fatigue and creep. Before and after testing residual surface stresses in the coatings were determined by X-ray stress measurements. Different substrate temperatures during the plasma spraying process caused tensile or compressive stresses on the surfaces of the thermal barrier coatings. Coatings with compressive stresses failed after only a few thermal cycles whereas those with tensile stresses sustained 700 to 1500 cycles without visible damage. Thermal fatigue and creep tests lead to stress changes reducing tensile stresses and increasing compressive stresses.     

Die thermische Stabilität von Eigenspannungen in kugelgestrahlten Oberflächen

Thermal Stability of Residual Stresses in Shot Peened Surfaces . In industry shop peening represents a well-known process to improve the fatigue properties of metallic components. However, the beneficial effect of the induced compressive residual stresses can be reduced if a component is subjected to higher temperatures during service. In this work the relief of the residual stresses as a function of time and temperature is calculated for a 12 %-chromium steel. Specifically, the results can be used for the prediction of the stress relief behaviour of gas turbine compressor blades. This model can also be applied to the determination of the mechanisms which are responsible for the residual stress relief. At temperatures above approximately 300 °C in this material the dominating mechanism is diffusion controlled creep whereas at lower temperatures the stress relief is caused by logarithmic creep.     

热循环作用下圆筒基体热障涂层的失效过程分析

用等离子喷涂的方法在高温合金圆筒上制备热障涂层,通过红外线辐照圆筒外部对试件进行加热以及内部通冷却空气强制冷却的方法对热障涂层系统进行了热循环失效试验。利用有限元工具ABAQUS对热障涂层系统中的瞬时温度场与应力场进行了计算以分析热障涂层的失效原因。计算结果表明,试样处于稳态最高温度时以及降温过程开始的很短时间内,陶瓷层中出现较大的周向拉应力,该应力将导致热障涂层出现表面垂直裂纹;陶瓷层与粘接层界面的径向应力不足以引起界面的开裂,界面的起裂来源于垂直裂纹出现后所带来的边缘效应。     

Eigenspannungen in der Oberfläche gas- und badnitrierter Stahlproben

Residual stresses in the surface of gas and bath nitrided steel specimens . Residual stresses in the steels Ck 45, 34 Cr 4, 34 CrAl 6, and Ck 15 were measured by X-ray diffraction. Cylindrical specimens of the heat-treated steels were nitrided in ammonia-gas and in a Tenifer®-salt- bath. The course of residual stresses of Ith order was determined by removing thin layers. With the carbon steel Ck 15 ist shown the effect of tempering on residual stresses in bath-nitrided and quenched specimens.     

Einfluß der Wärmeermüdung im Tieftemperaturbereich (77 K ⇔︁ 373 K) auf mechanische Eigenschaften der (α + β)-Ti-Legierung

Effect of the low-temperature thermal cycling (77 K ⇔︁ 373 K) on certain mechanical properties of a two-phase (α + β)-titanium alloy The paper reports an investigation on the effect of low-temperature thermal cycling (LTTC) within (77 K ⇔︁ 373 K) range on select mechanical properties and crystal lattice parameters of the constituent phase, α and β, of a double-phase titanium alloy as determined at temperatures of 295 K and 77 K. Fatigue tests were conducted under plane bending conditions at stresses exceeding the material endurance limit. It was found that the LTTC effects were closely related to the number of thermal cycles applied: after 100 cycles the fatigue strength at the two temperatures rose by 30% as compared with the no-LTTC condition: after 500 LTTC cycles a pronounced drop in the fatigue strength was observed ′ 87% at temperature of 295 K and 30% at 77 K. Microscopy and X-Ray studies showed that twinning had a significant effect on both the LTTC and mechanical fatigue processes. Residual stresses arising during the LTTC treatment were found to be a stimulating factor in the development of deformational processes occurring in the loaded material.     

Study on the damping behavior of Al/SiC<Subscript>p</Subscript> composite in thermal cycling

A SiC particulate reinforced 1040 commercially pure aluminum was thermally cycled in air between 20 and 300 °C up to 500 cycles. And the damping capacities of the specimens after 50 and 500 cycles were measured against temperature and strain amplitude. Thermal cycling causes the increase in damping, and dislocation damping is the main mechanism. A damping peak was observed in the range of 150–200 °C, which is related to dislocation motion. Thermal cycling leads to the increase in the peak temperature. The activation energy of the internal friction peak was calculated by Arrhenius equation, yielding 1.02 and 1.09 eV for 50 and 500 cycles, respectively. Increase in dislocation during thermal cycling is responsible for the increase in peak temperature and activation energy.     

Effects of thermal cycling on thermal expansionand mechanical properties of SiC fibre-reinforced reaction-bonded Si3N4 composites

Thermal expansion curves for SiC fibre-reinforced reaction-bonded Si3N4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured from 25 to 1400 °C in nitrogen and in oxygen. The effects of fibre/matrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fibre direction were between the measured curves for the strongly- and weakly-bonded composites, but those normal to the fibre direction for both bonding cases were similar to that of the unreinforced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly-bonded SiC/RBSN composites the most, resulting in loss of strain capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fibre and RBSN matrix due to oxidation of the carbon-rich fibre surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the main reasons for the poor mechanical performance of these composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Residual stresses in fibre- and whisker-reinforced aluminium alloys during thermal cycling measured by X-ray diffraction

Residual stresses have been determined using X-ray diffraction in two different metal matrix composites, viz. a squeeze-cast Al-2%Mg matrix with 10, 20 or 26 vol.% Al₂O₃ fibres and an extruded AA 6061 alloy with 25 vol.% SiC whiskers. The composites have been studied after thermal cycling between 240 or 250 °C and room temperature succeeded in some cases by quenching to liquid nitrogen temperature. On the squeeze-cast composite, stresses were measured at room temperature and in situ at 240 °C. X-ray stress determinations were compared with the stress values calculated by a modified Eshelby model for equivalent inclusions. By the model, the stresses can be accurately predicted for both composite systems. Thermally induced plastic relaxation reduces the residual stresses. The degree of reduction depends on the reinforcement volume fraction, the difference in coefficient of thermal expansion between the phases and the magnitude of the temperature drop. At elevated temperature the stresses are less responsive to reiterated quenching and heating.     

Zur Ermittlung verformungsbedingter Makro- und Mikroeigenspannungen durch mechanische und röntgenographische Eigenspannungsanalysen

Analysis of deformation induced residual macro-and microstresses by mechanical and X-ray methods Residual stress distributions in plastically deformed tensile and bending specimens of perlitic steel were analysed using X-ray diffraction technique and incremental holedrilling method. After tensile loading compressive residual stresses are measured by X-ray analysis in the ferrite phase. Consequently X-ray analysis detects compressive microstresses. In the case of bending specimens residual macrostresses are superposed with residual microstresses after unloading. In no case identical residual stress values were measured by X-ray and hole drilling methods. Microstresses can be separated combining both measurement methods. Microstresses after tensile loading were found to be greater than in surface layers of respective bending samples subjected to the same amount of plastic strain.     

THERMAL FATIGUE UNDER MULTIAXIAL STRESSES

Thermal fatigue under multiaxial stresses has been investigated. Circular cylindrical specimens were tested under thermal fatigue which was in-phase with axial mechanical fatigue loading. The axial forces on the specimens were varied throughout the test programme, but all the temperature cycles were identical, so various biaxial stress and strain ratios were obtained. Straight thermal fatigue cracks occurred in different directions and also net-like crazes in various patterns were observed on the surfaces of the specimens. The transient temperature, stress and strain fields have been calculated with a thermal elasto-plastic Finite Element Method. Comparing test results with calculations, it appears that the patterns of thermal fatigue cracks are dependent on the stress state and the plastic strain state, not on the strain state. The direction of cracks is perpendicular to the maximum principal stress and the maximum plastic strain. Net-like thermal fatigue crazes will occur when one principal stress is about the same as the other one and one plastic strain component is approximately equal to another.     

Generation of residual stress and plastic strain in a fracture mechanics specimen to study the formation of creep damage in type 316 stainless steel

Residual stresses are created in type 316H stainless steel fracture mechanics specimens using the process of local out‐of‐plane compression (LOPC). Three sets of LOPC tools are used to create different distributions of residual stress near to the crack tip. Also the tools create different levels of prior plastic strain. Residual stresses are measured using the neutron diffraction method and compared with the stress predictions obtained from finite element (FE) simulations of LOPC. The specimens are then subjected to thermal exposure at 550 °C for several thousand hours. A creep deformation and damage model is introduced into the FE analysis to predict the relaxation of stresses and creation of damage in the specimens. Neutron diffraction experiments are undertaken to measure the relaxed residual stresses and fractographic analysis of thermally exposed samples measured the extent of creep damage. A comparison between measured and simulated results demonstrates that the prior plastic strain has a significant effect on damage accumulation but this is not accounted for in the current creep damage models.     

Messung des Eigenspannungsabbaus durch Analyse des Barkhausen-Rauschens

Measurement of Residual Stress Reduction by Analysis of the Barkhausen-Effect Residual stresses and their variations under fatigue conditions have been measured by analysis of the Barkhausen-Effect. The stresses are proportional to the magnetic parameter M in a definite area. The investigations have been made on a high-strength fine-grained steel. under fatigue condition the relief of residual stresses is finished after a few stress cycles. By superposition of residual and load stresses the longitudinal stresses accept in consequence of the lateral contraction, whereas the transverse stresses are growing up in reason of their orientation to the direction of load stress. A change of the residual stress distribution could not be established.     

Thermal Residual Stresses in W Fibers/Zr-based Metallic Glass Composites by High-energy Synchrotron X-ray Diffraction

Thermal residual stresses in W fibers/Zr-based metallic glass composites were measured by in situ high energy synchrotron X-ray diffraction(HEXRD). The W fibers for the composites were 300,500,and 700 m m in diameter,respectively. Coaxial cylinder model(CCM) and finite element model(FEM) were employed to simulate the distribution of thermal residual stress,respectively. HEXRD results showed that the selected diameters of W fiber had little influence on the value of thermal residual stresses in the present composites. Thermal residual stresses simulated by CCM and FEM were in good agreement with HEXRD measured results. In addition,FEM results exhibited that thermal residual stress concentrated on interface between the two phases and area where the two W fibers were the closest ones to each other.     

Spannungsintensitätsfaktoren für lange axiale Oberflächenrisse in Rohren bei Thermoschockbelastung

Stress Intensity Factors for Long Axial Surface Cracks in Pipes Under Thermal Loading The stresses occurring in the pipes of the primary loop in the case of an emergency cooling may be described conservatively by a thermal shock problem. The influence of these thermal stresses on a long axially orientated surface crack is studied in this paper. By means of the weight function method stress intensity factors are calculated for this loading case.     

Durch veränderliche Betriebslasten in Kerben erzeugte Eigenspannungen und ihre Bedeutung für die Anwendbarkeit der linearen Schadensakkumulations-Hypothese

Residual Stresses Due to Variable Service Loads and Their Importance Regarding the Applicability of the Linear Damage Accumulation Hypothesis . The linear damage accumulation hypothesis often provides unreliable life estimations. To improve its application, residual stresses at the notch root of open hole specimens (K_t = 2,5) of the aluminium alloys AlCuMg 2 (US equivalent 2024 T 3) and AlZnMgCuAg (US equivalent 7075 T 6) were continuously measured during the macro-crack-free life range. Based on the residual stress history a corrected hypothesis was applied.     

Die Prüfung der Temperaturwechselbeständigkeit hitzebeständiger Stähle und Legierungen

Testing the Resistance to Thermal Shocks of Heat-Resisting Steels and Alloys Scaling resistance of heat-resisting stells and alloys is impaired to a large extent by a reiterating change in temperature and is, in particular, a function of the temperature interval and the rate at which the change in temperature occurs. A test stand for thermal cycling is described which permits quenching upto 80 specimens at a time from temperatures as high as 1100 °C in air, oil or water. As many as 30 temperature cycles are possible in one day. Repeated quenching in water causes considerable changes in specimen shape: length fo austenitic specimens of size 10 mm diam. × 90 mm length increased by about 30%, with cracks forming before specimens broke. Ferritic steel specimens are shortened, in some cases to half their initial length. In steels with a mixed austenitic and ferritic structure length increase is particularly marked. Change in length s a function of the rate of change in temperature, as can be seen by comparing the modes of quenching in water, oil and air.     

An inverse method for evaluating weld residual stresses via fatigue crack growth test data

This paper presents an inverse method for calculating the thermal residual stresses in welded specimens via measured fatigue crack growth rates. Firstly, fracture-mechanics superposition law has been used to extract the stress intensity factor due to residual stress contribution from measured crack growth rate. Secondly, a so-called B matrix has been established by performing finite element analysis. Residual stress distribution is then determined by solving linear algebraic equations relating the B matrix and residual stress intensity factors obtained from crack growth test data. The inverse method has been validated by a well-established residual stress distribution and corresponding stress intensity factor, and then applied to an M(T) sample in 2024-T3 alloy with a longitudinal weld. Agreement with the measured residual stresses is reasonably good and reasons for certain differences between the calculated and measured are discussed.     

Über den Einfluß von Eigenspannungen,Nahtgeometrie und mehrachsigen Spannungszuständen auf die Betriebsfestigkeit geschweißter Konstruktionen aus Baustählen

About the Influence of Residual Stresses, Weld Geometry and Multiaxial Stress States on the Oprational Fatigue Strength of Welded Constructions from Structural Steels For a fatigue design of welded structures among other influences also the influence of residual stresses, weld geometries and multiaxial stresses must be taken into account. Knowledge about the influence of residual stresses in the high-cycle fatigue region cannot be transformed unconditionally to the behaviour in the finite-fatigue life region or to variable amplitude loading with exceedances of the endurance limit, because the fatigue behaviour depends also on the stress concentration in the weld toe and the related stress relief. Principally, the fatigue strength is improved by a better weld geometry, e.g. by TIG-dressing, by introduction of radii which are big enough. In order to transform data obtained on specimens to components of bigger size criteria like structural or local equivalent stress, first technically detectable crack and size effects must be considered. For a fatigue life calculation for structures under variable amplitude loading a damage sum of D = 0.5 is recommended. While for the evaluation of multiaxial stress states with constant principle stress directions the von Mises criterion can be applied satisfactorily using structural or local stresses in the weld toe. However, conventional hypotheses fail by an overestimation of fatigue life, when the principle stress directions change, e.g. due to a phase difference between normal and shear stresses. Presently, in such cases only an experimental proof of the fatigue behaviour can be performed.