Bruno Berthel

Local energy analysis of high-cycle fatigue using digital image correlation and infrared thermography

This paper presents the first results provided by an experimental set-up developed to estimate locally the terms of the energy balance associated with the high-cycle fatigue (HCF) of DP 600 steel. The experimental approach involves two quantitative imaging techniques: digital image correlation and infrared thermography. First, a variational method is used to derive stress fields from the displacement fields. Patterns of deformation energy per cycle can then be determined on the basis of stress and strain data. Second, a local form of the heat equation is used to derive separately the thermoelastic and dissipative sources accompanying HCF. Energy balances show that around 50 per cent of the deformation energy associated with the mechanical hysteresis loop is dissipated while the rest corresponds to stored energy variations.

Local strain and damage measurements on a composite with digital image correlation and acoustic emission

Non-linear stress–strain behaviour under monotonic loading can be caused by continuous damage. In order to understand this non-linearity, simultaneous strain and damage measurements were taken on composite specimens using digital image correlation and acoustic emission as these techniques give valuable local information. The latter is essential in the case of inhomogeneous or anisotropic materials, such as continuous fibre composites. On the one hand, digital image correlation gives access to full field strain and on the other hand, acoustic emission recording can be used for damage monitoring and location if, at least two sensors are placed on the specimen under loading. In this work, these two techniques were combined to correlate strain measurements and damage location on a complex composite material during a monotonic tensile test. The composite is a continuous fibre-reinforced friction material used in car clutches. These measures were used to understand the non-linearity of the stress–strain curve of the as-received material as well as detect volume damage after thermal cycling. In depth study of the strain field and acoustic emission events location revealed a correlation between non-homogeneous damage kinetics throughout the specimen and the evolution of the strain distribution.

Effect of Thermal Cycling on the Mechanical Properties of a Continuous Fibre Composite Used for Car Clutch Facings

In dry clutch systems, the clutch facing is an annular shaped continuous fibre composite with organic matrix (thermo set resins) which transmits the torque from the engine to the wheels. In use it is submitted to thermo-mechanical cycling. Due to the composite fibre organisation, the strain field under thermo-mechanical loading is not homogenous. Full field data is needed to describe the material behaviour. Digital Image Stereo-Correlation (DISC) was used to determine the coefficient of thermal expansion (CTE) of the material. To determine the effect of temperature and cyclic loading on the mechanical properties, the composite was subjected to different thermal cycles. The material properties are modified with increasing temperature and number of cycles. These results were confirmed by dynamic mechanical analysis which showed thermal ageing of the resin. The local information given by the strain fields revealed a non uniform evolution of the material properties with thermal cycling.

Energy balance properties of steels subjected to high cycle fatigue

This paper presents an experimental protocol developed to locally estimate different energy balance terms associated with the high cycle fatigue (HCF) of steels. Deformation and dissipated energy are respectively derived from displacement and temperature fields obtained using digital image correlation (DIC) and quantitative infrared thermography (QIRT) techniques. The combined processing of visible and infrared images reveals the precocious, gradual and heterogeneous development of fatigue localization zones. It also highlights the plastic character of dissipative heat sources (i.e. proportional to the loading frequencies), and the progress of fatigue dissipation, observing the drift of the mean dissipation per cycle for a given loading. The substantial of internal energy variations during HCF loading are finally underlined. The paper ends with a discussion on the consequences of such energy balance properties in terms of HCF modeling.