Differential continuum damage mechanics models for creep and fatigue of unidirectional metal matrix composites

Three multiaxial isothermal continuum damage mechanics models for creep, fatigue, and creep/fatigue interaction of a unidirectional metal matrix composite volume element are presented, only one of which will be discussed in depth. Each model is phenomenological and stress based, with varying degrees of complexity to accurately predict the initiation and propagation of intergranular and transgranular defects over a wide range of loading conditions. The development of these models is founded on the definition of an initially transversely isotropic fatigue limit surface, static fracture surface, normalized stress amplitude function and isochronous creep damage failure surface, from which both fatigue and creep damage evolutionary laws can be obtained. The anisotropy of each model is defined through physically meaningful invariants reflecting the local stress and material orientation. All three transversely isotropic models have been shown, when taken to their isotropic limit, to directly simplify to previously developed and validated creep and fatigue continuum damage theories. Results of a nondimensional parametric study illustrate (1) the flexibility of the present formulation when attempting to characterize a large class of composite materials, and (2) its ability to predict anticipated qualitative trends in the fatigue behavior of unidirectional metal matrix composites. Additionally, the potential for the inclusion of various micromechanical effects (e.g., fiber/matrix bond strength, fiber volume fraction, etc.), into the phenomenological anisotropic parameters is noted, as well as a detailed discussion regarding the necessary exploratory and characterization experiments needed to utilize the featured damage theories

A differential CDM model for fatigue of unidirectional metal matrix composites

A multiaxial, isothermal, continuum damage mechanics (CDM) model for fatigue of a unidirectional metal matrix composite volume element is presented. The model is phenomenological, stress based, and assumes a single scalar internal damage variable, the evolution of which is anisotropic. The development of the fatigue damage model, (i.e., evolutionary law) is based on the definition of an initially transversely isotropic fatigue limit surface, a static fracture surface, and a normalized stress amplitude function. The anisotropy of these surfaces and function, and therefore the model, is defined through physically meaningful invariants reflecting the local stress and material orientation. This transversely isotropic model is shown, when taken to it's isotropic limit, to directly simplify to a previously developed and validated isotropic fatigue continuum damage model. Results of a nondimensional parametric study illustrate (1) the flexibility of the present formulation in attempting to characterize a class of composite materials, and (2) the capability of the formulation in predicting anticipated qualitative trends in the fatigue behavior of unidirectional metal matrix composites. Also, specific material parameters representing an initial characterization of the composite system SiC/Ti 15-3 and the matrix material (Ti 15-3) are reported

Link between the microstructure and the durability of polycrystalline materials: a fatigue damage model in an aluminium alloy

In polycrystalline alloys, fatigue damage is strongly influenced by the microstructure. Nowadays crystal plasticity models are used in order to take into account the crystallography and microstructural mechanisms but there is no consensus on crack initiation sites and their most significant mechanisms. The present work combines experimental tests and numerical simulations in order to understand and predict the physical mechanisms that lead to crack formation in high cycle fatigue in high-strength aluminium alloys for aerospace applications. The numerical simulations include a two parameters kinematic hardening. Experiments highlight the importance of two phenomena in fatigue crack initiation in connection with the microstructure. The first aspect is the surface roughness [1]; and our simulations succeed in putting forward the intrusion/extrusion phenomenon. The interest of large deformations in simulations is also discussed because of their effect on grain re-orientation and thus in surface roughness. The second phenomenon is progressive deformations; and the model achieves to account for it through local ratchetting and its effect on the crack initiation. We also intend to model stress relaxation, as its role is yet to be determined. In order to be able to extrapolate the mechanical behaviour over a large number of cycles, it is important to find the stabilized cycle [2]. Parallel simulations allow this to be done for representative crystalline aggregates. Finally, different macroscopic and mostly microscopic fatigue initiation parameters [3] are compared such as the Fatemi-Socie parameter, the stored energy or the commonly used cumulative plastic strain. It leads us to multiple fatigue site initiations, which we can compare with experimental results. The aim is to more accurately predict the site of fatigue crack initiation and the predominant mechanisms in fatigue crack initiation

A calibration procedure for the assessment of work hardening part I: Effects of the microstructure and load type

This paper presents a methodology to define and quantify the level of work hardening locally in a material. The methodology is proposed after a thorough experimental study based on three complementary experimental techniques for microstructural characterizations: microhardness, X-ray diffraction (XRD) and Electron Backscatter Diffraction (EBSD) applied on Inconel 718 samples. In our analysis, several loading histories including single tension, single compression, high strain rates and low cycle fatigue have been investigated. The effects of the microstructure have been further investigated by modifying the size of the grains and the size of the strengthening precipitates. Experimental tests have also been simulated to choose a model variable able to represent work hardening. A reciprocal link between work hardening and experimental characterizations has then been established. Correlation curves have been proposed that enable to quantify the level of work hardening from the knowledge of the experimental data. Accuracy and complementarity of the three experimental approaches are discussed as well as the impact of the microstructure of the material on the measured quantities

A calibration procedure for the assessment of work hardening Part II: Application to shot peened IN718 parts

The objective of this paper is to discuss the application of the calibration methodology exposed in the previous part to shot-peened Inconel 718 specimens. Shot peening is commonly used to increase the fatigue life of critical parts such as Inconel 718 turbine discs. This surface treatment induces residual stresses, work hardening and possibly, gradients of microstructures that, in turn, affect fatigue life. Work hardening is a quantity that represents a set of physical and mechanical phenomena related to the level of disorder reached in the microstructure of the material. Work hardening is seldom taken into account in fatigue life assessment mainly because it is not possible to characterize this quantity directly. We propose to use the calibration methodology (see part I of this paper [1]) on samples shot peened with several conditions. The three complementary experimental techniques (microhardness, XRD and EBSD) are then used to determine through correlation curves the work hardening gradients. The meth-odology for characterizing the work hardening within shot peened specimens is first presented. A dis-cussion of the applicability of the method in this context is then provided. The results obtained for the different characterization methods and microstructural effects are analyzed in two different sections. Finally, the influence of shot peening conditions on residual stresses and on work hardening is dis-cussed, showing the interest of the proposed procedure to obtain a real picture of the mechanical state after shot peening

Computation of coarse grain structures using a homogeneous equivalent medium

This paper deals with the improvement of concentration relations when some basic hypothesis of homogenization techniques are no longer valid inducing important errors in the deduced local fields. In the first part of this work we propose a new formalism of the standard concentration rules developed in the homogenization of periodic media framework. In a second part, we insist on the fact that a homogeneous equivalent medium replacing a coarse grain material is in fact expected to be a generalized continuum. Additional stiffnesses must be attributed to the unit cell and special non-homogeneous boundary conditions and their periodic counterparts are proposed. The resulting HEM turns out to be a Cosserat continuum

Introduction to Life Prediction Methodologies for Materials and Structures

International audienceIn his book "Citadelle" Antoine de Saint Exupéry wrote "Intelligence examines the material while the mind only sees the ship". Such a quote sets the context of this issue of ONERA’s Aerospace Lab Journal entitled "Life prediction Methodologies for Materials and Structures" very well. In order to predict the mechanical integrity and lifetime of complex components and structures, it is essential to understand the physical mechanisms that control local deformation and damage phenomena at the relevant scales in the material. This issue of the Aerospace Lab Journal addresses this problem by providing a forum for some of the leading members of the mechanics of materials community to report on recent theoretical and computational modeling approaches to describe the mechanics of materials at scales ranging from the atomistic, through the microstructure, and up to the continuum. A broad range of materials and physical phenomena are addressed, including metallic and composite materials, dislocation behavior, crystallographic slip, fatigue and damage, and other macroscopic and structural behavior

Onera’s multiaxial and anisothermal lifetime assessment for engine components

International audienceThis paper presents the calculation methods developed at ONERA to assess the lifetime of aeroengine structures subjected to multiaxial thermo-mechanical loadings. The complexity of the steps required in this process has grown over the past years, in order to take advantage of extremely accurate observations in materials and more precise experimental results. The research activities in the field of fatigue are presented within a framework that covers microstructural observations, mechanical testing, the development of constitutive equations and fatigue damage models associated with numerical calculations. All of these steps are today closely interrelated in the complex analysis of lifetime predictions. A fundamental aspect is highlighted: the simultaneous consideration of multiaxial and anisothermal loadings, which requires specific developments. These points are detailed, illustrated by many examples of applications and positioned with respect to research works in the literature