A two parameter driving force for fatigue crack growth analysis

A model for fatigue crack growth (FCG) analysis based on the elastic–plastic crack tip stress–strain history was proposed. The fatigue crack growth was predicted by simulating the stress–strain response in the material volume adjacent to the crack tip and estimating the accumulated fatigue damage. The fatigue crack growth was regarded as a process of successive crack re-initiation in the crack tip region. The model was developed to predict the effect of the mean stress including the influence of the applied compressive stress. A fatigue crack growth expression was derived using both the plane strain and plane stress state assumption. It was found that the FCG was controlled by a two parameter driving force in the form of: . The driving force was derived on the basis of the local stresses and strains at the crack tip using the Smith–Watson–Topper (SWT) fatigue damage parameter: D=σ_maxΔε/2.The effect of the internal (residual) stress induced by the reversed cyclic plasticity was accounted for the subsequent analysis. Experimental fatigue crack growth data sets for two aluminum alloys (7075-T6 and 2024-T351) and one steel alloy (4340) were used for the verification of the model.     

A unified method of design for fatigue crack growth resistance in structural materials

It is well established that there are two fatigue crack tip driving forces – the cyclic, ΔK, and the static, K_max. In this study, the effects of each crack tip driving force on crack growth were evaluated for various structural materials. A unified method of design that allows for predicting the response of long and physically small fatigue cracks at positive stress ratios is introduced. Good agreement between predicted and experimental long and physically small fatigue crack growth data was obtained. The importance of this method in material and component design is discussed as part of a contemporary design philosophy.     

不同应力比下Cu/WCp复合材料疲劳裂纹扩展行为及影响机制分析

通过粉末冶金工艺制备了一种高压电触头用Cu/WCp颗粒增强复合材料。研究了不同应力比下Cu/WCp颗粒增强复合材料的疲劳裂纹扩展行为,并结合裂纹闭合模型和两参数驱动力模型分析了应力比对Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率的影响机制。研究结果表明:随着应力比R的增大裂纹扩展速率增大,尤其在近门槛值附近裂纹扩展速率差别最明显。裂纹闭合模型和两参数驱动力模型均可以较好地将不同应力比R下(da/d N-ΔK)关系曲线关联起来,且两参数驱动力模型的相关性更好。这说明导致不同应力比R下Cu/WCp颗粒增强复合材料疲劳裂纹扩展速率差异的原因主要是Kmax引起裂纹尖端单调损伤,其次是裂纹闭合效应。根据SEM断口分析发现高应力比的断面较低应力比的粗糙,低应力比时断口以基体撕裂为主而高应力比时以颗粒基体脱粘为主。     

Prediction of fatigue crack growth under constant amplitude loading and a single overload based on elasto-plastic crack tip stresses and strains

It is generally accepted that the fatigue crack growth (FCG) depends mainly on the stress intensity factor range (ΔK) and the maximum stress intensity factor (K_max). The two parameters are usually combined into one expression called often as the driving force and many various driving forces have been proposed up to date. The driving force can be successful as long as the stress intensity factors are appropriately correlated with the actual elasto-plastic crack tip stress-strain field. However, the correlation between the stress intensity factors and the crack tip stress-strain field is often influenced by residual stresses induced in due course.A two-parameter (ΔK_tot, K_max,tot) driving force based on the elasto-plastic crack tip stress-strain history has been proposed. The applied stress intensity factors (ΔK_appl, K_max,appl) were modified to the total stress intensity factors (ΔK_tot, K_max,tot) in order to account for the effect of the local crack tip stresses and strains on fatigue crack growth. The FCG was predicted by simulating the stress-strain response in the material volume adjacent to the crack tip and estimating the accumulated fatigue damage. The fatigue crack growth was regarded as a process of successive crack re-initiations in the crack tip region. The model was developed to predict the effect of the mean and residual stresses induced by the cyclic loading. The effect of variable amplitude loadings on FCG can be also quantified on the basis of the proposed model. A two-parameter driving force in the form of: was derived based on the local stresses and strains at the crack tip and the Smith-Watson-Topper (SWT) fatigue damage parameter: D = σ_maxΔε/2. The effect of the internal (residual) stress induced by the reversed cyclic plasticity manifested itself in the change of the resultant (total) stress intensity factors controlling the fatigue crack growth.The model was verified using experimental fatigue crack growth data for aluminum alloy 7075-T6 obtained under constant amplitude loading and a single overload.     

Fiber volume fraction effects on fatigue crack growth in SiC/Ti-15-3 composite

The effects of fiber volume fraction (15, 37, and 41%) on fatigue crack growth in unidirectional SiC/Ti-15-3 composite were investigated at room temperature. The effect of fiber volume fraction on the fiber bridging mechanism was studied to support development of physically-based crack growth models. While each fiber volume fraction exhibits similar decreasing crack growth rates prior to fiber bridging induced crack arrest, post-arrest behavior (observed after incrementally increasing the applied stress level) is quite different. After crack arrest, the 15% (37 and 41%) material exhibited higher (lower) crack growth rates and lower (higher) toughness values than the unreinforced matrix. These different behaviors occur because of differences in the amount of fiber bridging during the post-arrest regime. Metallography of interrupted tests revealed the extent of fiber bridging in the crack wake and matrix plasticity ahead of the crack tip. Models for predicting the effective matrix stress intensities were evaluated and compared to experimental data. A fiber pressure model and finite element studies were used to estimate the condition of the bridged fiber zone and associated fiber stresses. Since the vast majority of useful life for these materials experiences fatigue crack growth, these results assist in discerning an optimum fiber volume fraction for structural applications.     

The influence of modified intermetallics and Si particles on fatigue crack paths in a cast A356 Al alloy

Mechanical fatigue tests were conducted on uniaxial specimens machined from a cast A356-T6 aluminium alloy plate at total strain amplitudes ranging from 0.1 to 0.8% ( R = − 1). The cast alloy contains strontium-modified silicon particles (vol. fract. ~6%) within an Al–Si eutectic, dispersed α intermetallic particles, Al₁₅ (Fe,Mn)₃ Si₂ (vol. fract. ~1%), and an extremely low overall volume fraction of porosity (0.01%). During the initial stages of the fatigue process, we observed that a small semicircular fatigue crack propagated almost exclusively through the Al–1% Si dendrite cells. The small crack avoided the modified silicon particles in the Al–Si eutectic and only propagated along the α intermetallics if they were directly in line with the crack plane. These growth characteristics were observed up to a maximum stress intensity factor of ~ K trmax = 7.0 MPa m^1/2 (maximum plastic zone size of 96 μm). When the fatigue crack propagated with a maximum crack tip driving force above 7.0 MPa m^1/2 the larger fatigue crack tip process zone fractured an increased number of silicon particles and α intermetallics ahead of the crack tip, and the crack subsequently propagated preferentially through the damaged regions. As the crack tip driving force further increased, the area fraction of damaged α intermetallics and silicon particles on the fatigue fracture surfaces also increased. The final stage of failure (fast fracture) was observed to occur almost exclusively through the Al–Si eutectic regions and the α intermetallics.     

Effects of changes in temperature on fatigue crack growth of adhesively bonded Al 2080/SiC/20p-2080 Al laminated composites

The effects of changes in test temperature from 25 to – 125°C on the fatigue crack propagation of 2080/SiC/20p-2080 adhesive bonded laminates (ABL) with volume fraction ratio of 60/40 tested in the crack arrestor orientation were investigated. The fatigue behavior of the laminates was significantly different than that of the individual laminae as well as other types of laminates (i.e., diffusion bonded laminates). The fatigue crack growth behavior of the ABL's was significantly affected by the test temperature, particularly when the fatigue cracks approached and entered the adhesive layers. In-situ monitoring of fatigue crack growth and post mortem analyses were used to determine the likely source(s) of the effects of changes in test temperature and differences between the ABL and its constituents.     

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.     

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Abstract

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.     

Fatigue crack closure: A myth or a misconception?

In this paper, we have extended our previous study on fatigue crack closure to examine the phenomenon of crack opening displacement (COD) and its impact on the crack tip fields in both 2D and 3D specimen geometries using full‐field experimental measurements and integrated finite element modelling. Digital image correlation (DIC) and digital volume correlation (DVC) were used to measure the near‐tip material responses on the surfaces (DIC) and the interior (DVC) of the specimens. Materials with elastic‐plastic and large plastic characteristics were chosen for the study, where plasticity‐induced premature contact between the crack flanks is known to occur. Displacement maps around the cracks were obtained using DIC and DVC at selected load increments and were introduced as boundary conditions into the finite element (FE) models to obtain the “effective” crack driving force in terms of J‐integral, and the results were compared with those “nominal” from the standard FE analysis. Both visual observation and compliance curves were used to determine the “crack opening” levels; whilst the impacts of the crack opening on the crack driving force J and the normal strains ahead of the crack tip were evaluated in 2D and 3D. The results from the study indicate that, crack closure, although clearly identifiable in the compliance curves, does not appear to impact on global crack driving force, such as J‐integral, or strains ahead of the crack tip; hence, it may well be a misconception.     

A unified model of fatigue kinetics based on crack driving force and material resistance

Fatigue in Al-alloys is largely a process of crack growth from pre-existing defects occurring by several different mechanisms, each of which dominates a particular rate-driven segment of fatigue kinetics. These include fatigue void formation through interfacial cracking of secondary particulates, crack extension by brittle micro-fracture (BMF) in near-threshold fatigue, slip driven crack growth in the Paris regime and quasi-static crack extension by the well-known micro-void coalescence (MVC) and the less known fatigue void coalescence (FVC). BMF is mean stress and sequence-sensitive.Mechanism selection for fatigue crack extension in each load cycle occurs on the principle of least resistance to crack driving force represented by ΔK and K_max. Crack extension will switch to a different failure mechanism given reduced resistance to that mechanism by comparison to the current one. Increasing driving force will thus force a switch from BMF to shear and then onto MVC or FVC in that order, over each rising load half-cycle. Higher growth rates will therefore always be associated with a mix of all these mechanisms.     

Plastic mismatch effect on plasticity induced crack closure: Fatigue crack propagation perpendicularly across a plastically mismatched interface

In the present work, comprehensive investigation of both theoretical analysis and numerical simulation was carried out to investigate the plastic mismatch effect on plasticity induced crack closure (PICC) behavior and effective fatigue crack tip driving force. During the process of crack tip approaching interface, crack tip load and crack tip load ratio will change, resulting in the change of PICC degree. When the crack propagates towards higher strength side, K_op/K_max increases; when the crack propagates towards lower strength side, K_op/K_max decreases firstly and then increases. The two mechanisms of “interface plastic mismatch effect on nominal fatigue crack tip driving force” and “interface plastic mismatch effect on PICC degree” were compared. The second mechanism must be considered when building crack tip driving force model for describing fatigue crack crossing plastically mismatched interface, because it is more physically factual and maybe more important than the first mechanism.     

Modeling of crack bridging in a unidirectional metal matrix composite

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches; the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, K _eff, was calculated using both models to correlate the measured crack growth rate in the composite. The calculated K _eff from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.     

AN INVESTIGATION OF FATIGUE CRACK GROWTH IN A DUCTILE MATERIAL AT HIGH GROWTH RATES

Abstract— Investigations into tearing-fatigue have been performed using three point bend specimens made of mild steel. A computer controlled testing machine was used which could maintain constant cyclic displacement ranges, or constant cyclic energy input ranges, and hence provide a range of crack tip driving force conditions. Fatigue crack growth rates were measured and compared with the predictions of a model based on the linear summation and non-interaction of fatigue and ductile tearing growth rates. The effects of fatigue crack growth on monotonic crack growth resistance properties were investigated. It is concluded that there is no significant interaction between these two crack propagation mechanisms in this steel. Crack growth rate equations governing propagation in the tearing-fatigue regime are given.     

Designing the fiber volume ratio in SiC fiber-reinforced SiC ceramic composites under Hertzian stress

Finite element method (FEM) analysis and experimental studies are undertaken on the design of the fiber volume ratio in silicon carbide (SiC) fiber-reinforced SiC composites under indentation contact stresses. Boron nitride (BN)/Pyrocarbon (PyC) are selected as the coating materials for the SiC fiber. Various SiC matrix/coating/fiber/coating/matrix structures are modeled by introducing a woven fiber layer in the SiC matrix. Especially, this study attempts to find the optimum fiber volume ratio in SiC fiber-reinforced SiC ceramics under Hertzian stress. The analysis is performed by changing the fiber type, fiber volume ratio, coating material, number of coating layers, and stacking sequence of the coating layers. The variation in the stress for composites in relation to the fiber volume ratio in the contact axial or radial direction is also analyzed. The same structures are fabricated experimentally by a hot process, and the mechanical behaviors regarding the load–displacement are evaluated using the Hertzian indentation method. Various SiC matrix/coating/fiber/coating/matrix structures are fabricated, and mechanical characterization is performed by changing the coating layer, according to the introduction (or omission) of the coating layer, and the number of woven fiber mats. The results show that the damage mode changes from Hertzian stress to flexural stress as the fiber volume ratio increases in composites because of the decreased matrix volume fraction, which intensifies the radial crack damage. The result significantly indicates that the optimum fiber volume ratio in SiC fiber-reinforced SiC ceramics should be designed for inhibiting the flexural stress.     

SiC/Al层状梯度复合材料的制备与抗毁伤特性

用真空热压法制备了SiC/Al层状梯度复合材料;采用热压扩散工艺,实现了不同SiC含量的复合材料间的有效连接.实验表明SiC/Al层状梯度复合材料制备时,添加纯铝过渡层有助于提高层间结合强度.通过穿甲试验对SiC/Al层状复合材料的抗毁伤性能进行了分析,结果表明所制备的SiC/Al层状复合材料具有比纯铝材料更为良好的抗毁伤性能.     

A comparative study between a small time scale model and the two driving force model for fatigue analysis

A comparative study is performed to demonstrate the difference and similarity between the two driving force approach and a small time scale model under both constant and variable amplitude loading. The small time scale model is different from most existing fatigue analysis methodologies and is based on the instantaneous crack growth kinetics within one cycle. The two driving force approach is cycle-based and uses two driving force parameters to describe crack growth rate per cycle under constant amplitude loadings. A simple modified two driving force approach is proposed based on the concept of forward and reverse plastic zone interaction and is used to calculate the fatigue crack growth under general variable amplitude loadings. Extensive experimental data for various metallic materials are used to validate the two driving force model and the small time scale model.     

The influence of microstructure on the fatigue crack growth rate in marine steels in the Paris Region

This paper presents a study on the effect of microstructure on the fatigue crack growth (FCG) rate in advanced S355 marine steels in the Paris Region of the da/dN versus ΔK log–log plot. The environments of study were air and seawater (SW), under constant amplitude sinewave fatigue loading. Fundamentally, three phenomena (crack tip diversion, crack front bifurcation and metal crumb formation) were observed to influence the rate of FCG. These phenomena appear to be a function of the material microstructure, environment and crack tip loading conditions. The three factors retarded the crack growth by reducing or redistributing the effective driving force at the main active crack tip. A crack path containing extensively the three phenomena was observed to offer strong resistance to FCG. In SW, the degree of the electrochemical dissolution of the microplastic zone appears to be an additional primary factor influencing FCG in the steels.     

ARALL (Aramidfaserverstärkter Aluminiumschicht-Verbundwerkstoff) - Ein neuer Hybrid-Verbundwerkstoff mit besonderen Schwingfestigkeitseigenschaften

ARALL (Aramid Fibre Reinforced Aluminum Laminate)-A New Fatigue Resistant Hybrid Composite High fuel expenses and the tendency to build larger aircrafts are two main factors forcing the aircraft engineers to develop structures which allow for higher design stress levels. Higher design stress levels require an increasing concern with the fatigue behavior of the structure. One way to solve the problem is to develop new high strength fatigue resistant materials. In this paper a material of this type is presented: ARALL, an aramid fibre reinforced aluminum laminate. This hybrid material consists of thin sheets of a high strength aluminum alloy which are bonded together. Into the bond-line thin layers of aramid fibres are embedded. As soon as fatigue cracks are initiated in the metallic part of the hybrid composite material during the fatigue loading, the strong and fatigue insensitive aramid fibres remain unbroken behind the propagating crack. They hinder the crack opening and reduce the stress intensity factor at the crack tip in the metal part of the hybrid composite. This mechanism leads to a significant reduction of the crack growth and can be enhanced by introducing favourable residual stresses into the hyb- rid material. For an optimized ARALL material an aluminum sheet thickness of about 0.5 mm and an aramid layer thickness of about 0.25 mm (with a fibre volume content of about 40–50%) are chosen. ARALL decreases the crack growth rates by orders of magnitude, as compared to monolithic aluminum sheets, and is an extremely damage tolerant material.     

城轨列车制动盘SiC_(p)/A356复合材料热疲劳裂纹扩展机理EI北大核心CSCD

为研究制动盘服役温度载荷及材料微结构对SiC_(p)/A356复合材料热疲劳裂纹扩展行为的影响,明确其热疲劳裂纹扩展微观机理,开展SiC_(p)/A356复合材料热疲劳裂纹扩展实验。结果表明:裂纹扩展过程包括由SiC颗粒偏转作用和二次裂纹释放扩展驱动力导致的缓慢扩展阶段和主裂纹与裂纹扩展前端微损伤连接的快速扩展阶段;加热温度较低时,裂纹扩展的“台阶状”特征明显,整体扩展速率较低,裂纹宽度较小,裂纹扩展方式为颗粒断裂、轻量基体撕裂和沿界面开裂;加热温度较高时,“斜直线跃升”阶段更为明显,裂纹宽度较大且扩展速率较高,裂纹扩展以颗粒脱落以及大幅度基体撕裂为主;主裂纹总是通过选择沿SiC颗粒群或者直接穿过α-Al基体以阻力较小的方式向前扩展,Si相承载时极易发生断裂,成为裂纹扩展源,同时裂纹扩展前端的微损伤对其扩展具有引导作用。