Flaw Characteristics in Dynamic Fatigue: The Influence of Residual Contact Stresses

The effect of residual contact stresses on the dynamic fatigue response of surfaces containing indentation-induced flaws is studied. Indentation fracture mechanics is used to analyze the growth of well-defined "median/radial" cracks in combined residual (elastic/plastic) contact and applied (uniform) tensile fields, and thence to determine strength characteristics. In this way a general formulation is obtained for the fatigue strength at constant stress rate. Experimental confirmation of the essential predictions of the theory is obtained from strength tests on Vickers-indented soda-lime glass disks in water environment. It is thereby implied that residual stresses can have a significant deleterious influence on the fatigue behavior of any brittle solid whose controlling flaws have a contact history. Such effects need to be considered in the design of structural ceramics, most notably where fracture-mechanics calibrations of crack-velocity parameters are used for lifetime predictions.     

Micromechanics of Flaw Growth in Static Fatigue: Influence of Residual Contact Stresses

Residual contact stresses about indentation flaws are demonstrated to have a strong deleterious effect on specimen lifetime in static fatigue. The underlying basis of conventional fatigue analysis is first examined critically and is argued to be deficient in the way the stress intensity factor for the flaws is related to the characteristic parameters of crack geometry and applied loading. In general, it is necessary to incorporate a residual term into the stress intensity formulation. A modified theory of static fatigue is accordingly developed, in which the residual contact stresses play a far from secondary role in the micromechanics of flaw evolution to failure. Strength tests on Vickers-indented soda-lime glass disks in water environment provide clear experimental confirmation of the major theoretical predictions. Implications of the residual stress effect concerning fracture mechanics predictions of lifetimes for "real" ceramic components under service conditions are discussed.     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

Flow behavior of polyacrylamide solution. III. Mathematical treatment

The flow behavior of polyacrylamide solutions was systematically determined over a wide range of temperatures (20–50°C) and concentrations (20–50 ppm) by using a coaxial cylinder viscometer. The results indicated that the rheological behavior of low-concentration polyacrylamide solution behaves similar to non-Newtonian fluids at all these concentrations. The effect of temperature on the consistency coefficient and flow behavior index of polyacrylamide solution of the different concentrations followed an Arrhenius-type relationship. Moreover, the effect of concentration on consistency coefficient and flow behavior index followed an exponential-law relationship at the temperatures used. The rheological constants for the Arrhenius and exponential-law models were determined. The combined effect of temperature and concentration on the coefficient of dynamic shear stress can be represented by a single equation: shear stress = 2.446 × 10⁻⁷exp(0.0639C + 3613/RT)(shear rate)^{2.337 exp(−0.00707C−245/RT)}. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2784–2789, 2001     

Effects of stress concentration and residual stress on vibration fatigue behavior of socket weld

The vibration fatigue behavior in socket weld of 304L stainless steel was investigated using experimental and numerical. The vibration fatigue test results indicate that original socket weld at a higher stress tend to occur toe failures while for the case of lower stress failures tend to originate at the root. In addition, the socket welded with groove can improve the fatigue property of weld root, but it has a bad influence on the weld toe. From the simulation results, the groove can decrease the stress concentration factor of weld root, and improve the fatigue property of root; whereas the groove can also increase the tensile residual stress of toe, resulting in having a detrimental effect on fatigue property of toe.     

Effect of Contact Residual Stress on Proof Testing

Theoretical predictions are made of strength distributions after proof testing accounting for the possibility of a contact residual stress. Assuming a simple power-law relationship between subcritical crack velocity and the stress intensity factor, the amount of crack growth, hence strength degradation, that occurs during a proof stress cycle is determined. The results clearly show that the contact residual stress can strongly influence the after-proof strength distribution so that after-proof strengths can become weaker than the initial. The results are consistent with experimental observations with soda–lime glass; thus, the importance of accounting for the contact residual stress is emphasized.     

The effects of residual stress on the mechanical properties of glassy polymers

Residual stresses can have either beneficial or detrimental effects on the mechanical properties of plastics. Manufacturing processes frequently impose residual stresses on plastics. In this study, controlled thermal processes were used to impose surface compressive stresses on polycarbonate beam samples (6.4 by 12.5 by 80 mm). Resistance strain gage and photoelastic techniques were developed to measure the magnitude of these stored stresses. The compressive surface stresses were found to be between 14 MPa (2000 psi) and 31 MPa (4500 psi) and to vary with process method and cooling rate. The mean fatigue life (in bending) of the treated beam samples was found to improve by a factor of 10 over that of untreated samples. The increase in the fatigue life of the treated samples appears to be directly related to the magnitude of the surface compressive residual stress in the samples. The imposed residual stress, as determined by photoelastic measurements, has not appreciably relaxed after 1 year of storage at room temperature.     

换热器管子-管板液压胀接的有限元模拟

采用ANSYS/CAE软件 ,对锅炉、换热器管子 管板的液压胀接过程进行了模拟。管子与管板孔之间采用面 面接触元以模拟相互之间的间隙及管子产生塑性变形并贴紧管板孔后对管板的作用。通过本文的模拟分析 ,可获得胀接时接头处的弹 塑性应力状态及卸除胀接压力后管子与管板之间的残余接触压力。值得注意的是此接触压力沿管板厚度方向分布是不均匀的 ;在管孔槽处会出现较高的数值 ;在管板内侧处 ,管子的过渡区会出现较大的残余拉应力     

容器特殊法兰的计算

通过使用CONTACT单元对快开特殊法兰进行分析,从而得到了这种特殊形式法兰的受力特点和力学分布规律,然后对这种特殊法兰进行了应力评定以及疲劳寿命计算。工程表明,此方法设计的特殊法兰满足应力和寿命要求。     

Effect of residual stresses on fatigue crack growth behavior of aluminum substrate repaired with a bonded composite patch

Composite patches bonded to cracked metallic aircraft structures have been shown to be a highly cost-effective method for extending the service life of the structures. The fatigue crack growth behavior of pre-cracked 7075-T6 aluminum substrate with the 12.7-mm V-notch crack repaired with boron/epoxy composite patches was investigated. 1-ply, 2-ply, 3-ply and 4-ply composite patches were studied. The residual stresses due to mismatch of the coefficients of thermal expansion between the aluminum plate and boron/epoxy composite patch were calculated based on the classical equation. The effects of the residual stresses and patch layers on fatigue lifetime, fatigue crack growth rate, and fatigue failure mode of the repaired plates were examined experimentally. A modified analytical model, based on Rose's analytical solution and Paris power law, was developed for this research. This model considered the residual stress effect and successfully predicted the fatigue lifetime of the patched plates. Results showed that the composite patch had two competing impacts on the structure. The composite patch could cause residual tensile stress in the aluminum substrate, which could consequently increase the crack growth rate. Moreover, reinforcement with the composite patch could also retard the crack propagation in the aluminum plate. If a 4-ply composite patch was used, it resulted in high residual stresses and effectively would not extend the fatigue lifetime of cracked aluminum plates.     

Performance Analysis of Pulse Generators on Residual Stress of Machined Silicon Steel Using the EDM Process

The cracks in the workpiece specimens can reduce the fatigue life of any machine components. Since the residual stress has a considerable amount of influence on determining crack formation over the machined surface, it is very essential to analyze the residual stress developed in any machining process. However, it is a very tedious process to compute the residual stress over the machined surface. In the present study, an endeavor has been made to measure and analyze the residual stress of machined silicon steel as a workpiece using the EDM process with different energy distribution. The nano-indentation method was used to compute the residual stress produced over the machined surface. From the experimental results, it was found that the uniform energy distribution has produced higher compressive residual stress owing to the tiny and uniform spark energy distribution. It has also been observed that the tool electrode has a considerable amount of influence on determining development of residual stress in the EDM process.     

Cumulative deformation and fatigue of diamond — new developments

The soft impressor method has been used here to investigate the response of type-Ib high-pressure:high-temperature diamond and CVD diamond-coated:silicon carbide cutting tools to repeated loading cycles, i.e. simulated fatigue conditions, at room temperature. A mean contact pressure of 16.4 GPa was required to initiate cracking in the single crystal, when subjected to a single load cycle, and this was reduced to a fatigue mean pressure of 3.5 GPa after 200k load cycles. Similarly, the coated specimen failed at a fatigue mean pressure of 5.4 GPa after 16k load cycles. In addition, it was shown that the corners of the tool failed at fewer load cycles for a given mean pressure than the centre regions, presumably due to a greater degree of residual stress at those corners. Since moderately hard steels are capable of developing these fatigue mean pressures and conventional mechanisms of fatigue are unlikely under these conditions, e.g. those involving significant dislocation movement and interaction, it is suggested that future research should be directed towards establishing the possible role of stress corrosion and/or hydrogen embrittlement in this process.     

Measurement of Indentation Residual Stresses in Materials Susceptible to Slow Crack Growth

Knowledge of the size and distribution of the indentation residual stress field is important when interpreting slow crack growth data for indented ceramic materials. A technique based on compressively loading indentation cracks has been used to measure the wedging residual stresses at radial indentation cracks. The method also gives information on the fatigue limit and can be applied on any ceramic material susceptible to slow crack growth. Soda–lime glass specimens were indented and the resulting residual stresses, wedging the radial cracks, were measured as a function of indentation load. Calculations of K ₀, the fatigue limit, were made for both virgin indentation cracks and cracks aged until saturation. The magnitude of closing stress needed to prevent slow crack growth was found to depend linearly on the indentation load. For example, for indentation loads of 20 and 60 N, the corresponding closing stresses were 14 and 26 MPa, respectively.     

Determination of Subcritical Crack Growth Parameters by In Situ Observation of Indentation Cracks

A technique to determine stress intensity factor—crack velocity (K— v ) relationships for subcritical crack growth from in situ observation of indentation cracks is described. To minimize the effect of residual contact stresses and lateral crack interaction, measurements were made only on cracks that had undergone significant subcritical crack growth. Crack shapes were determined fractographically from crack-arrest markings, produced by temporary unloading during the crack extension process. The subcritical crack growth parameters obtained by this technique were in excellent agreement with those determined from dynamic fatigue and previous studies.     

Experimental investigation on fatigue property at room temperature of C/SiC composites machined by rotary ultrasonic milling

Rotary ultrasonic milling technology (RUM), as a surface strengthening machining method, was proposed to utilize in processing of C/SiC composites for enhancing anti-fatigue performance innovatively. Static tensile, intermittent fatigue and residual strength test were carried out. Due to constant impingement of high-frequency and low-amplitude vibration, surface residual compressive stress was formed near 2 GPa maximally. Axial thermal residual stress in fiber achieved -662.4 MPa proved by loading-unloading test. The peak value of fatigue damage parameter was reduced significantly. RUM surface restrains most of interface cracks because of residual compressive stress, and hinder the growth of fiber cracks for better machined surface quality. The damage accumulation, the first stress redistribution and fiber reinforcement stage were delayed. Average damage rate was decreased by 80.5 %. Residual tensile strength of RUM C/SiC was improved after fatigue, up to 95.8 % of tensile strength. The strengthening effect from RUM on fatigue property of C/SiC is significant and valuable.     

氮化硅陶瓷滚动体的接触疲劳

本文对氮化硅陶瓷滚动体的接触疲劳机理进行了探讨,结果表明:陶瓷材料的接触疲劳仍是疲劳裂纹的形成与扩展过程,但裂纹形成机理不同于金属材料,其累积损伤的具体形式是局部高应力区内的微裂纹,而不像金属材料那样是通过交替滑移使材料局部弱化产生累积损伤的。降低表面粗糙度、倒角处采取圆滑过渡可以提高滚动体的接触疲劳寿命。     

压力容器快开门法兰的接触分析

通过接触单元来数值模拟压力容器快开门法兰的接触过程,得到了其受力特性和应力分布规律,并对其危险截面进行了强度评定和疲劳分析。工程实践表明,按该方法设计的法兰满足结构强度和寿命要求。     

胀接压力与初始间隙对换热器胀接质量的影响

为研究换热管-管板胀接压力与初始间隙对换热器胀接质量的影响,采用胀接参数化模拟研究方法对液压胀接进行可靠性研究,建立换热管-管板胀接的弹塑性参数化几何模型,对不同的换热管-管板胀接压力和初始间隙进行研究,得到换热管残余等效应力、残余接触应力大小及分布规律。研究表明：胀接压力相同、间隙不同时,卸载后残余等效应力和残余接触应力值差别较小;初始间隙相同,胀接压力不同时,卸载后残余应力和残余接触应力值差别较大;换热管的残余等效应力和残余接触应力随着胀接压力的增大而增大,其变化趋势一致;当材料为Ti31钛合金、胀接压力为183 MPa时换热管与管板的初始间隙消除。研究结果为换热器最佳胀接性能优化及胀接质量的提高提供了依据。     

Strength and Fatigue Properties of Optical Glass Fibers Containing Microindentation Flaws

The inert strength and dynamic fatigue properties of fused-silica optical fibers are studied using subthreshold indentation flaws, i.e., flaws without radial cracks. These subthreshold properties differ from those obtained in comparative tests on silica rods containing postthreshold indentation flaws in three major respects: (1) the inert strengths are significantly higher than predicted by extrapolation of the postthreshold data; (2) the slopes of the dynamic fatigue plots are likewise greater, indicating a greater susceptibility of the subthreshold flaws to chemical kinetic effects; and (3) the scatter in strengths is wider. These trends reflect the change in mechanical response reported for optical fibers with "natural" flaw populations in going from ordinary to ultra-high-strength regions. Direct observations of the indentation sites up to the point of failure indicate that the property differences can be interpreted in terms of a transition from propagation-controlled to initiation-controlled fracture instabilities at reduced contact loads. The subthreshold instability condition is modeled qualitatively as a two-step, deformation-fracture process, with strong emphasis on the importance of residual stress fields in parametric evaluations. The relevance of the results to the practical issue of fiber reliability, most notably in connection with the potential dangers of using macroscopic crack velocity data to predict long-lifetime characteristics, is addressed.     

Fatigue response and constitutive behavior modeling of poly(ethylene terephthalate) unreinforced and nanocomposite fibers using genetic neural networks

The constitutive behavior of poly(ethylene terephthalate) (PET) unreinforced (control) and PET fibers reinforced with 5 wt% vapor‐grown carbon nanofibers (VGCNFs) under uniaxial tension and subsequent to fatigue loading has been evaluated utilizing various analytical models. Two types of fatigue tests were performed: (1) Long cycle fatigue at 50 Hz (glassy fatigue) to evaluate fatigue resistance and (2) fatigue at 5 Hz (rubbery fatigue) to evaluate residual strength performance. The long cycle fatigue results at 50 Hz indicated that the PET‐VGCNF sample exhibited an increased fatigue resistance of almost two orders of magnitude when compared to the PET unreinforced filament. The results of the fatigue tests at 5 Hz indicated that the constitutive response of both the PET control and PET‐VGCNF samples changed subsequent to fatigue loading. The large deformation uniaxial constitutive response of the PET and PET‐VGCNF fibers was modeled utilizing genetic‐algorithm (GA) based training neural networks. The results showed that the large deformation uniaxial tension constitutive behavior of both PET unreinforced and PET‐VGCNF samples with and without prior fatigue can be represented with good accuracy utilizing neural networks trained via genetic‐based backpropagation algorithms, once the appropriate post‐fatigue constitutive behavior is utilized. Experimental data of uniaxial tensile tests and experimental postfatigue constitutive data have been implemented into the networks for adequate training. The fatigue tests were conducted under tension‐tension fatigue conditions with variations in the stress ratio (R), maximum stress (σ_max), number of cycles (N), and the residual creep strain (ε_R). POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers