Daniel Camas

Numerical and Experimental Analysis of Crack Closure

The fatigue life of metallic materials is strongly influenced by crack closure effects. Finite element (FE) methods allow the study of crack closure with great detail and can provide valuable information about phenomena occurring in the bulk of the material. In this work the distribution of stresses through the thickness of a cracked specimen has been studied using 3D FE simulations. It was found that the transition between the interior of the specimen (plane strain) and the surface (plane stress) differs from that predicted by 2D plane stress models. In addition, an attempt is presented to experimentally validate the results at the surface level. For this purpose full-field image correlation technique was utilized. This allowed direct comparison between the displacement field predicted by the numerical simulations and the experimental results measured by digital image correlation.

Key Aspects in 3D Fatigue Crack Closure Numerical Modelling

Since long time, fatigue crack closure has been studied by means of finite element models. Initially by bi-dimensional models and recently, due to the higher computational capabilities, the use of three-dimensional models has been extended, providing a wider comprehension of the problem. Starting with the methodology used for 2D cases, a specific methodology for 3D models has been developed. Key parameters affecting the model have been analyzed and recommendations have been established. The numerical accuracy is evaluated in terms of crack closure and opening values. They main issues studied are the material behaviour, the loading cycles and crack growth scheme, the contact simulation, the meshing and the element size at the crack tip and along the thickness, the plastic wake computed and the opening and closure definition considered. This paper summarises the main learning and recommendations from the latest numerical modelling experience of the authors.

Numerical Analysis of the Pivot Node in Fracture Problems

Recent studies have allowed us to identify a narrow region of the thickness of the crack front in fracture problems that presents interesting characteristics for the numerical-experimental correlation. Taking the three-dimensional distribution of the stress intensity factor (K) as a reference, we observe how it remains invariant and independent of the main factors influencing this type of analysis. This article presents a summary of how to identify this point through the numerical simulation of the problem and its relationship with parameters such as thickness, load level or angle of curvature. The simulations are carried out with the ANSYS software in an aluminium CT specimen subjected to a fracture loading process in mode I.

Numerical Determination of Fatigue Threshold from CTOD

The experimental procedure to obtain valid and comparable values of ΔKth is laborious and time consuming. The objective of this work is to determine ΔKth using a numerical approach. The CTOD is plotted versus load, and the fatigue threshold corresponds to the onset of plastic deformation. A parametric study was developed in order to understand the effect of material parameters on ΔKth.

Numerical Study of the Influence of the Crack Front Curvature in the Evolution of the Plastic Zone along the CT Specimen Thickness

This paper presents a numerical study of the influence of the load level and the crack front curvature on the plastic zone in the area close to the crack front. The aim of the work is to determine the influence of these parameters on fatigue crack closure. For this, a CT aluminum specimen has been modelled tri-dimensionally and several finite elements calculations have been made considering a large combination of the variables under consideration.