Berechnungs- und experimentelle Verfahren zur Charakterisierung thermischer Mikroeigenspannungen in Stückverbunden

Theoretical and Experimental Methods for the Determination of Thermal Residual Microstresses in Particle Reinforced Metal Matrix Composites Particle reinforced metal matrix composites (PMMCs) comprising quasi ceramic hard phases in a metal matrix are used for wear protection in industry. During cooling residual microstresses emerge due to differences in the physical and mechanical properties of the metal matrix and the hard phases. Furtheron, the amount and distribution of the residual microstresses are influenced by the hard phase size, shape and distribution. For characterizing the residual microstress state theoretical methods (analytical and FEM calculations) as well as experimental methods (dilatometry, Micro-Moiré-methods, X-ray diffraction, angle dispersive neutron diffraction, neutron time-of-flight spectroscopy, torsion pendulum tests, differential calorimetry and ultra sonic absorption test) are used and assessed.     

Eigenspannungsmessungen an unterschiedlich abgekohlten,kugelgestrahlten und torsionsschwellbeanspruchten Proben aus Federstahl

Residual Stress Measurements on Differently Decarburized, Shot Peened and Cyclic Torsional Loaded Specimens of a Spring Steel In addition to previous fatigue tests with the spring material 55 Cr 3, X-ray residual stress measurements were carried out on differently decarburized, shot peened and cyclic torsional loaded specimens. It was shown that longer duration of decarburization leads to higher residual tensile stress in the surface zone for not peened specimens. Shot peening, however, effects almost equal residual compressive stresses in the surface zone in spite of different starting conditions, with a slight decrease for specimens with a long duration of decarburization, which corresponds to the results of the fatigue tests. By cyclic torsional loading with mean stress the residual compressive stresses due to shot peening are diminished in the axial and circumferential directions. A main stress system is now to be found with an angle of 45° to axis, whereas the residual stresses in the s?₁-(tensile direction) retain their values, those in the s?₂-(compressive) direction are decreased distinctly.     

Untersuchungen zur Dauerfestigkeit carbonitrierter Proben aus den Einsatzstählen 18CrNiMo7‐6und 20MnCr5

The endurance limit of carbonitrided specimens of 18CrNiMo7‐6 and 20MnCr5 case hardening steels were examined under bending conditions. Four variants were manufactured differing in heat treatment and surface conditions. Besides conventional gas carburised variants, carbonitrided specimens with a low and a raised nitrogen content of 0.2 and 0.45% at the surface as well as a carbonitrided and ground variant were examined. The specimens were tested with stress ratios R = –1 and 0. Depending on the nitrogen content cyclic loading evokes compressive residual stresses at the specimens' surfaces, caused by elastic‐plastic deformation. Specimens of the ground variant show additionally a decrease of the residual austenite content when cycled. The induced compressive residual stresses are substantially responsible for high endurance limits at both stress ratios and are the key for understanding the beneficial effect of carbonitriding in this regard.     

Residual stress effect on impact properties of Gr/Al metal matrix composite

The effects of residual stress on the impact properties of the unidirectionally reinforced P 100 Gr/6061 Al metal matrix composites with different thermal histories have been investigated using an instrumented impact test method and scanning electron microscopy. The cantilever impact generally causes tensile failure at the notch and compressive loading on the opposite side of the specimen. The specimens with yield tensile matrix residual stresses have planar fracture surfaces and low impact energy due to the contribution of tensile residual stress. The specimens with small residual stresses have moderate impact energy because debonding between fibre and matrix or fibre/matrix separation also serves as an additional mechanism for energy absorption. The specimens with higher compressive matrix residual stresses have the largest maximum load of all the specimens with the same matrix treatment. The specimen with matrix compressive yield residual stress has the maximum impact energy owing to a stepwise fracture surface. It can be concluded that good impact properties of composite materials can be obtained by choosing a suitable thermal history to modify the deleterious tensile matrix residual stress.     

FATIGUE CRACK GROWTH AND CLOSURE THROUGH A TENSILE RESIDUAL STRESS FIELD UNDER COMPRESSIVE APPLIED LOADING

Abstract— —Fatigue crack growth and closure through a tensile residual stress field under an applied compressive loading is investigated by carrying out various applied stress ratio tests ranging from R = 0 to R = It is found that even under applied compressive loading, fatigue crack growth rates are well correlated with the effective stress intensity factor range and the behaviour of crack closure through a tensile residual stress field is uniquely controlled by an effective stress ratio which takes account of residual stresses. Consequently, the method of predicting fatigue growth rates, using da/d N vs Δ K data from residual stress-free specimens, can be successfully applied to crack growth through a tensile residual stress field. However, previously used simple assumptions may lead to non-conservative estimates of crack growth rates.     

Effect of pressure rolling on the residual stress state of a particulate-reinforced metal matrix composite

The large difference between the coefficients of thermal expansion of the matrix alloy and the particle in a metal matrix composite gives rise to residual stresses in the material. In the present work, the effect of pressure rolling on the residual stress state of a silicon carbide particle-reinforced 2014 aluminium alloy has been investigated. The three-dimensional stress state measured in both phases-matrix and reinforcement-has been determined using an X-ray diffraction technique. A twofold effect of pressure rolling on the residual stresses was observed. On the one hand, compressive macrostresses as large as - 250 MPa were induced. On the other hand a significant reduction in pseudo-macrostresses was measured where the plastic deformation reaches a maximum. A modified Eshelby model was used to predict quantitatively and qualitatively the residual microstresses after heat treatment and pressure rolling respectively.     

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.     

The effect of residual stresses arising from laser shock peening on fatigue crack growth

Residual stresses have in the past been introduced to manipulate growth rates and shapes of cracks under cyclic loads. Previously, the effectiveness of shot peening in retarding the rate of fatigue crack growth was experimentally studied. It was shown that the compressive residual stresses arising from the shot peening process can affect the rate of crack growth. Laser shock peening can produce a deeper compressive stress field near the surface than shot peening. This advantage makes this technique desirable for the manipulation of crack growth rates. This paper describes an experimental program that was carried out to establish this effect in which steel specimens were partially laser peened and subsequently subjected to cyclic loading to grow fatigue cracks. The residual stress fields generated by the laser shock peening process were measured using the neutron diffraction technique. A state of compressive stress was found near the surface and tensile stresses were measured in the mid-thickness of the specimens. Growth rates of the cracks were observed to be more affected by the tensile core than by the compressive surface stresses.     

Axial fatigue of a gas-nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

Fatigue testing under fully reversed axial loading (R=?1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

Analysis of microstress in neutron irradiated polyester fibre by X-ray diffraction technique

Microstresses developed in the crystallites of polymeric material due to irradiation of high-energy particle causes peak broadening and shifting of X-ray diffraction lines to lower angle. Neutron irradiation significantly changes the material properties by displacement of lattice atoms and the generation of helium and hydrogen by nuclear transmutation. Another important aspect of neutron irradiation is that the fast neutron can produce dense ionization at deep levels in the materials. The polyethylene terephthalate (PET) fibre of raw denier value, 78.2, were irradiated by fast neutron of energy, 4.44 MeV, at different fluences ranging from 1×10⁹ n/cm² to 1 × 10¹² n/cm². In the present work, the radiation heating microstresses developed in PET micro-crystallites was investigated applying X’Pert-MPD Philips Analytical X-ray diffractometer and the effects of microstresses in tensile strength of fibre measured by Instron have also been reported. The shift of 0.45 cm⁻¹ in the Raman peak position of 1614.65 cm{−1} to a higher value confirmed the development of microstresses due to neutron irradiation using micro-Raman technique. The defects due to irradiation were observed by SEM micrographs of single fibre for virgin and all irradiated samples.     

A survey of test methods for multiaxial and out-of-plane strength of composite laminates

This review paper gives an overview of test methods for multiaxial and out-of-plane strength of composite laminates, with special consideration of non-crimp fabrics (NCF) and other textile systems. Tubular and cruciform specimens can provide arbitrary in-plane loading, while off-axis and angle-ply specimens provide specific biaxial loadings. Tensile and compressive out-of-plane strength may be determined by axial loading of specimens with a waisted gauge section, while bending of curved specimens allow determination of the out-of-plane tensile strength. Tests suited for out-of-plane shear strength include the short beam shear test, the inclined double notch test and the inclined waisted specimen. Testing of arbitrary tri-axial stress states using tubular or cruciform specimens with superimposed through-the-thickness loading is highly complex and significant problems have been reported in achieving the intended stress states and failure modes. Specific tri-axial stress states can be obtained by uniaxial loading of specimens with constrained expansion, as in the die channel test.     

Synchrotron diffraction measurement of residual stresses in friction stir welded 5383-H321 aluminium butt joints and their modification by fatigue cycling

This paper presents two‐dimensional information on the residual stresses in 8 mm 5383‐H321 aluminium plates joined by double pass (DP) friction stir welding (FSW). It considers the inherent variability in residual stress magnitudes along 0.5 m lengths of weld pass, and their modification under a sequence of applied fatigue loads. This represents one of a planned series of experiments aimed at illuminating the effects of fatigue cycling on residual stress fields. In this particular case, the magnitudes of the bending fatigue loads (R= 0.1) were chosen to correlate with the measured proof strengths of the weld metal (approximately 160 MPa) and the parent plate (approximately 260–270 MPa). In four‐point bend S–N tests at R= 0.1 on 40 mm wide FS welded specimens of this alloy and plate thickness, these peak stress levels correspond to lives of around 10⁵ cycles and 10⁷ cycles, respectively. Results from the work indicate that significant variability exists among welded plates in peak compressive stress magnitudes (a range of perhaps ?50 MPa to ?140 MPa), although peak tensile stresses were relatively low and more consistent (from around 0 to 30 MPa). Fatigue loading accentuates the peak‐to‐valley stress change and causes an overall translation of the stresses to become more positive. Peak tensile stresses increase several‐fold during fatigue cycling.     

Evaluation of Equibiaxial Compressive Stress Introduced into Austenitic Stainless Steel Using an Eddy Current Method

Equibiaxial compressive residual stress is introduced into steel after peening in order to improve both its resistance to stress corrosion cracking and its fatigue strength. Thus, a nondestructive and relatively quick method to evaluate the equibiaxial compressive residual stress in a surface layer modified by peening is required in order to evaluate the peening intensity needed to enhance the integrity of structural components. The purpose of the work reported here is to establish an eddy current method to evaluate equibiaxial compressive stress which can be applied to the residual stress introduced into various non-ferromagnetic materials after peening. To this end, hydraulic jacks were used to elastically deform specimens of the austenitic stainless steel, Japanese Industrial Standard (JIS) SUS316L, thereby introducing an equibiaxial compressive stress. In the case of SUS316L steel, stress-induced martensitic transformation is rare. The electromagnetic properties of these specimens were then measured. In addition, the eddy current signals from peened specimens were compared with these. The results demonstrate that it is possible to establish a method for evaluating the equibiaxial stress utilizing eddy current signals.     

Fatigue characteristics of SAE52100 steel via ultrasonic nanocrystal surface modification technology

Ultrasonic nanocrystal surface modification (UNSM) technology is a novel surface modification technology that can improve the mechanical and tribological properties of interacting surfaces in relative motion. UNSM treatment was utilized to improve the wear resistance fatigue strength of slim bearing rings made of SAE52100 bearing steel without damaging the raceway surfaces. In this study, wear and fatigue results that were subjected to different impact loads of the UNSM treatment were investigated and compared with those of the untreated specimen. The microhardness of the UNSM-treated specimens increased by about 20%, higher than that of the untreated specimens. The X-ray diffraction analysis showed that a compressive residual stress of more than 1,000 MPa was induced after the UNSM treatment. Also, electron backscatter diffraction analysis was used to study the surface structure and nanograin refinement. The results showed that the rolling contact fatigue life and the rotary bending fatigue strength of the UNSM-treated specimens increased by about 80% and 31%, respectively, compared to those of the untreated specimen. These results might be attributed to the increased microhardness, the induced compressive residual stress, and the nanocrystal structure modification after the UNSM treatment. In addition, the fracture surface analysis showed that the fish eye crack initiation phenomenon was observed after the UNSM treatment.     

基于宏微观分析的碳纤维增强高分子复合材料强度性能表征

微观力学强度理论(MMF)是一种新型的基于物理失效模式的复合材料强度理论。通过对碳纤维/树脂(UTS50/E51)复合材料单向层合板进行纵向、横向静载拉伸、压缩和弯曲试验, 得到层合板的基本力学性能和宏观强度指标。建立了碳纤维增强树脂基复合材料微观力学模型, 获取树脂基体和纤维不同位置的机械载荷应力放大系数和热载荷应力放大系数。结合获取的应力放大系数及试验测得的单向层合板宏观强度, 计算出层合板组分的MMF强度特征值。绘制了基于MMF强度理论的层合板破坏包络线, 并与Tsai-Wu失效准则预测结果进行对比。实现了对UTS50/E51层合板MMF强度特征值的表征。     

Fatigue crack propagation in tensile residual stress fields of welded joints under fully compressive cycling

Fatigue crack propagation rates in centre-crack-typed transverse butt-welded joints were measured at a constant stress intensity factor range obtained by decreasing the applied and mean loads on test specimens. The propagation rate was maintained constant except at extremely compressed stress ratios. Fatigue crack propagation properties under compressive loading were found to be similar to those under tensile loading. Only under highly compressive cycling did crack propagation rates decrease.     

The effect of overloading sequences on landing gear fatigue damage

In service, landing gear can be subject to unexpected hard landing load, which is beyond the design domain. The consequences due to overload can affect the design life of a landing gear to some extent. In this paper, the effect of overload and different loading sequences in random spectra on fatigue damage are investigated, using strain–life based fatigue analysis methods. The discussions are emphasised on the effect of loading sequences on residual stress and mean stress, especially the effect of overload on the fatigue damage of subsequent cycles. In addition, different fatigue analysis techniques in commercial fatigue analysis packages are reviewed and compared. The analysis indicates that the overload effect is stress state dependent and dominated by local residual and mean stress. A ‘Begin’ overload in a load spectrum would cause more damage in the local compressive yield area and an ‘End’ overload within a spectrum will worsen the tensile yielding area. It is suggested that the load sequence effect should be considered in common fatigue analysis if local yielding would exist before or after overloading.     

The effect of residual stress on adhesion-induced instability in micro-electromechanical systems

Adhesion and residual stress play a critical role in the performance and reliability of MEMS (micro-electromechanical systems). The effects of residual stress on the pull-in instability of the micromechanical structure were examined using thin plate theory and energy criterion. It is shown that the critical gap between the film and the substrate is a function of the ratio of residual stress to bending rigidity and the adhesion energy. Such instability is mitigated by the presence of in-plane tensile residual stress in the film. But, it is aggravated by the presence of compressive residual stress. In addition, the effect of compressive residual stress is much more significant than tensile residual stress.     

Residual stress effects and t → m transformation in ion-implanted yttria-stabilized zirconia

Ion-beam treatment affects the near surface region of ceramic materials and is a potential technology in altering the surface microstructure and properties. In this study, a preliminary surface modification in yttria-stabilized zirconia was investigated through the employment of ion implantation. Fracture toughness and hardness were measured and evaluated by the indentation method for specimens implanted by direct As⁺ bombardment. With the aid of residual stress measurement by X-ray diffraction, the properties of the implanted specimens can be related to the residual compressive stress induced by irradiation effects. Thermal stability of zirconia is improved due to the suppression of the tetragonal to monoclinic transformation in the presence of the residual compressive stress.