Low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy

2018

Metals

Background-Creep-fatigue damage is generally identified as the combined effect of fatigue and creep. This behaviour is macroscopically described by crack growth, wherein fatigue and creep follow different principles. Need-Although the literature contains many studies that explore the crack-growth path, there is a lack of clear models to link these disparate findings and to explain the possible mechanisms at a grain-based level for crack growth from crack initiation, through the steady stage (this is particularly challenging), ending in structural failure. Method-Finite element (FE) methods were used to provide a quantitative validation of the grain-size effect and the failure principles for fatigue and creep. Thereafter, a microstructural conceptual framework for the three stages of crack growth was developed by integrating existing crack-growth microstructural observations for fatigue and creep. Specifically, the crack propagation is based on existing mechanisms of plastic blunting and diffusion creep. Results-Fatigue and creep effects are treated separately due to their different damage principles. The possible grain-boundary behaviours, such as the mismatch behaviour at grain boundary due to creep deformation, are included. The framework illustrates the possible situations for crack propagation at a grain-based level, particularly the situation in which the crack encounters the grain boundary. Originality-The framework is consistent with the various creep and fatigue microstructure observations in the literature, but goes further by integrating these together into a logically consistent framework that describes the overall failure process at the microstructural level.

Section: Introductionmentioning

confidence: 99%

Crack Propagation Mechanisms for Creep Fatigue: A Consolidated Explanation of Fundamental Behaviours from Initiation to Failure

2018

Metals

“…Hence, this makes the design process inefficient and expensive because a large number of empirical data are required and must be re-fitted for each condition. To improve this limitation, others have attempted to introduce the variables of temperature and frequency into modified models, resulting in the Coffin-Manson-based creep-fatigue models proposed by Solomon [15], Shi [16], Jing [17] and Wong & Mai [18], and the Basquin-based creep-fatigue models developed by Kohout [19] and Mivehchi [20]. However, these models may only be applied at the situations for which they were derived.…”

Section: The Conventional Empirical Methodsmentioning

confidence: 99%

“…The improvements are that: the structure includes the parameters of typical engineering problems, is easily mathematically solved, may be applied in multiple situations on multiple metallic materials, and covers the full range of conditions from pure fatigue to creep fatigue and then to pure creep. In particular, the model provides a more economic method for fatigue-life prediction since less empirical data are required than other empirical methods such as [15,17]. In addition, the model is applicable for engineering design at the initial stage through combining with finite element analysis (FEA) [28].…”

Section: Extension Of the Empirical Modelsmentioning

confidence: 99%

An Explicit Creep-Fatigue Model for Engineering Design Purposes

2018

Metals

Background: Creep-fatigue phenomena are complex and difficult to model in ways that are useful from an engineering design perspective. Existing empirical-based models can be difficult to apply in practice, have poor accuracy, and lack economy. Need: There is a need to improve on the ability to predict creep-fatigue life, and do so in a way that is applicable to engineering design. Method: The present work modified the unified creep-fatigue model of Liu and Pons by introducing the parameters of temperature and cyclic time into the exponent component. The relationships between them were extracted by investigating creep behavior, and then a reference condition was introduced. Outcomes: The modified formulation was successfully validated on the materials of 63Sn37Pb solder and stainless steel 316. It was also compared against several other models. The results indicate that the explicit model presents better ability to predict fatigue life for both the creep fatigue and pure fatigue situations. Originality: The explicit model has the following beneficial attributes: Integration—it provides one formulation that covers the full range of conditions from pure fatigue, to creep fatigue, then to pure creep; Unified—it accommodates multiple temperatures, multiple cyclic times, and multiple metallic materials; Natural origin—it provides some physical basis for the structure of the formulation, in its consistency with diffusion-creep behavior, the plastic zone around the crack tip, and fatigue capacity; Economy—although two more coefficients were introduced into the explicit model, the economy is not significantly impacted; Applicability—the explicit model is applicable to engineering design for both manual engineering calculations and finite element analysis. The overall contribution is that the explicit model provides improved ability to predict fatigue life for both the creep-fatigue and pure-fatigue conditions for engineering design.

“…Not all strain‐life‐based creep‐fatigue models follow the pattern of frequency‐modified Coffin‐Manson equation. For example, Jing et al proposed a temperate‐modified Coffin‐Manson equation (Equation ), where the coefficient and exponent of the Coffin‐Manson equation are functions of temperature.

\frac{∆ ε_{p}}{2} = C_{3} {(2 N_{f})}^{β}

with

\begin{matrix} left \\ C_{3} = 68.79 - 0.34 T + 250.56 / \sqrt{T} \\ β = 1.29 - 0.0053 T + 2.5 / \sqrt{T} \end{matrix}

…”

Section: A Brief Review Of Existing Creep‐fatigue Modelsmentioning

confidence: 99%

Development of a unified creep‐fatigue equation including heat treatment

Fatigue Fract Eng Mat Struct

2017

Background Creep and fatigue damages in metals are known to interact and then lead to aggregated damage. While models exist for fatigue, creep and creep‐fatigue, no models cover all 3 load regimes. Also, a heat treatment–related parameter is not well included in most creep‐fatigue models. Need There is a need to develop a creep‐fatigue equation, which covers the full loading regime from pure fatigue to pure creep, and creep‐fatigue. Also needed is inclusion of a heat treatment–related parameter. Approach The unified creep‐fatigue equation was started from the Coffin‐Manson equation and integrated with the Manson‐Haferd parameter. This equation was validated on Inconel 718. Outcomes The method of deriving the coefficients and the formula of the creep function are demonstrated, and the resulting equation shows a good ability to describe the grain‐size effect and the fully integrated characteristics. Originality Original contributions of this work are the development of a new formulation to represent creep, fatigue and creep‐fatigue in metals. Also the inclusion of grain size—which is a proxy for heat treatment—in the formulation of this equation and in a proposed modified Manson‐Haferd parameter.