New aluminum alloy design

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Mechanical & chemical driving force affecting crack nucleation

Load history has been known to affect fracture and stress corrosion behavior. The degree to which it affects depends on the severity of the load history. It is known that shop peening can retard the SCC lives markedly in steels. Similarly, prestraining can reduce the KIscc and plateau velocity in high strength steels. These types of experiments are difficult to quantify their effects on the SCC behavior. One can analyze the prestarining effects in a better way by analyzing the effects of single overloads followed by constant applied load to study the behavior. Such experiments can be done by observing the ‘incubation time’ for a crack to initiate in a fatigue precracked sample, at various constant applied loads in a chemical environment. Such experiments have been conducted on 7075 aluminum alloy and a 4130 steel. It is observed that results are similar in behavior. The data indicates the overall behavior can be analysed by suggesting that the total stress at the crack tip is related to the contributions from chemistry of the environment and an additional factor from “internal stress” Hence we can describe the overall data in terms of: KIscc = Kapplied + Kinternal stress + Kenvironment Such trends in the behavior, has been observed in prestrained steel alloys prior to environmental exposure. The general behavior suggests that the internal stress affects the threshold KIscc more than the plateau velocity. The general SCC behavior is affected by both chemistry and internal stress under external static or cyclic loads

Crack nucleation, growth & arrest under subcritical crack growth

Crack nucleation (and growth) can be characterized under static load (or cyclic loads) in the presence of an environment. Chemically assisted cracking is occurs when stresses to nucleate (or propagate cracks) are much lower fracture of a material in an inert (like in vacuum) environment where a material is free from the damaging chemical environment. Similarly, embrittlement occurs if chemically active elements are dispersed internally as in internal hydrogen, metalloids and other embrittling elements. The materials where such embrittlement phenomena occurs are also divergent: pure metals, alloys, and ceramics or glasses. Since materials are used in applications involve various chemical environments under load, the importance of understanding the role of environment in material performance need not be stressed. In fact, there are many analyses in the past highlighting the divergent behaviors in each of the systems and environments emphasizing the specialties specific to a given material/environment system. In addition, many efforts have been made in the past to arrive at some unifying principles governing the embrittlement phenomena. An inescapable conclusion reached on this topic by many is that the behavior is very “complex”. Hence, recognizing the complexity of material/environment behavior, we focus our attention, mainly on metallic systems, in extracting some similarities to arrive at some generic principles involved. The ultimate goal of this effort is to arrive at some self-consistent scheme for incorporating “chemical effects” into a life prediction model for components in service

Fracture toughnes K1c affecting static threshold K1scc

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Unified approach to crack growth and fracture

Unified approach connects the behavior of a smooth specimen, of a notched specimen and of the fracture-mechanics specimen, under inert and corrosive environments, using the unifying principles and the Modified Kitagawa-Takahashi Diagram. The unifying principles are based on the fact that the behavior of short cracks is not different from that of long cracks, and the same thresholds govern the crack growth. Cracks being high energy defects, local internal stresses are required to initiate and grow the cracks in all cases. The internal stresses can be pre-existing as in the case of long cracks or in situ generated or augmented in the case of smooth and notched specimens. Observed variations in the crack growth rates of short cracks from those of long cracks arise due to variations in the types and degrees of pre-existing internal stresses. In aggressive environments, chemical forces provide additional driving forces over and above the mechanical forces. Chemical forces come from the chemical and/or electro chemical potential gradients which may be difficult to determine as they depend on the nature and the extent of the local chemical reactions in the changing compositional gradients. From practical considerations, we show that they can be quantified using the inert medium as a reference. Cyclic loads provides additional factors since crack tip driving forces come from both monotonic and cyclic loads leading to load-ratio R dependence. We provide here a systematic analysis of these factors using our Unified Approach to help in quantification and codification of the kinetics of the crack growth and fracture

Initiation and growth of corrosion fatigue cracks from corrosion pits using elasto-plastic notch analysis

Corrosion pits are known to act as precursors for fatigue crack initiation under corrosive environment. The transition from pit to crack growth under corrosion fatigue is of considerable interest for many engineering structures. Several predictive methodologies have been developed. As the Pits grow with large aspect ratio, they behave like local stress/strain concentrations accentuating the crack initiation and growth. In this paper, we extend our recent analysis of crack initiation at the elastic-plastic notch tip stress fields* to evaluate its applicability to pit to crack transition. Ref: K. Sadananda, A. Arcari, A.K. Vaudevan, Eng. Frac. Mech., 2017, 176 pp.144-16

Fatigue threshold Kmax,th affected by static threshold K1scc

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Use of an inverse life plot for fatigue endurance/limit estimation

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Unusual Fatigue Crack Growth Behavior of Long Cracks at Low Stress Intensity Factor Ranges

In this article, we characterize and review the unusual lack of threshold in fatigue crack growth (FCG) behavior for some alloys at low values of stress intensity factor ranges ΔK and its implications to damage-tolerant design approaches. This unusual behavior was first observed by Marci in 1996 in IMI 834 alloy. Conventional applications of linear elastic fracture mechanics to FCG analysis at constant R-ratio (or Kmax) assumes that (da/dN) decreases monotonically with decreasing ΔK and approaches the threshold value of ΔKth with (da/dN) ≤ 10−7 mm/cycle for a given R (or Kmax). However, instead of ΔK threshold behavior, some materials exhibit plateau or acceleration in da/dN rate with decreasing ΔK for long cracks tested in both constant R and Kmax conditions. This unusual (da/dN)-ΔK behavior is only observed experimentally but not understood and represents a challenge to scientists and engineers to model the safe fatigue life prediction of structures under low amplitude vibrating loads

Role of crack-tip hydrostatic stress in environmental assisted fatigue cracking

Abstract During service, environmentally assisted cracking of light weight high strength Al-alloys is a common cause of failure. For a given alloy, the important parameters associated with environmental cracking are: the maximum load and load range, the aggressiveness of environment, and the frequency of loading. These parameters relate to damages due to cyclic loading (mechanical fatigue) and time (environment). In general, two causes are commonly attributed to overall damage: (1) hydrogen embrittlement, dominant as the yield strength of the alloy increases and (2) anodic dissolution. The mechanical stress due to cyclic loading can develop a high triaxial stress state in notched and cracked components which can help both the crack initiation and propagation stages of fatigue. Cracks can originate from corrosion pits (as stress concentration sites) produced by local anodic dissolution process. In the case of hydrogen embrittlement process, the triaxial stress state at the notch/crack-tip may add to the increase in local hydrogen concentration. In this paper, we attempt to analyze the effects of the hydrostatic stress state, SH, at the crack-tip on fatigue crack growth in aqueous environment where hydrogen embrittlement process can occur. This is done by means of finite element modelling under plane stress and plane strain conditions for both notched and cracked specimens assuming an elastic perfect-plastic or Ramberg-Osgood material behaviours. In an aqueous environment, a passive film formation at the crack tip may induce additional tensile stresses which could enhance the local damage process. The effect of tensile stresses induced by the passive film is simulated by applying constant nodal forces at the surface nodes in tangential direction to the notch-tip curvature. The present computations explore the hypothesis that a different stiffness in compression should be assigned to the material elements having high hydrostatic stress (SH>0.9SHmax). This is based on the premise that the element with high hydrostatic stress SH would lead to a collection of high hydrogen (H) concentration. In particular, it is assumed that the elements with high H concentration (or high SH) would exhibit restrivtive reversed plasticity (in a limiting case no yielding is assumed in compression). The results indicate that the load displacement curve during unloading is strongly affected by the different stiffness of the elements assigned with high H concentrations. Also significant differences between plane strain and plane stress in terms of hydrostatic stresses are demonstrated. In the case of plane strain the peak value and the gradient of the hydrostatic stress distribution are much higher than in the case of plane stress. Finally, the overall trends in the numerical results are supported by the experimental data taken from the literature on high strength Al-alloys