Nathalie Godin

Experimental study of crackling noise: conditions on power law scaling correlated with fracture precursors

We discuss the results of a series of experiments performed on heterogeneous materials where we have measured the acoustic emission (AE) signals preceding macroscopic failure that correspond to precursory rupture events. We mainly focus on polyurethane foams whose heterogeneities (pores) constitute arrest points for microcracks. The released AE energy is always power law distributed independently of the material porosity, the loading history (creep and tensile) and the mechanical properties (from brittle to ductile with increasing temperature). This is also the case for time intervals between events except in tensile tests when the material is brittle. We also highlight two key physical behaviors. The first is the occurrence of a unique behavior for a cumulative number of events and the cumulative energy in creep tests. The second is that the statistical properties are more influenced by the mechanical behavior than the microstructure of the material. Finally we discuss the problems which are still open for connecting these results with theoretical modeling of AE.

Monitoring of Martensitic Transformation in Austenitic Stainless Steel 304 L by Eddy Currents

A nondestructive eddy current technique has been applied to cyclic-strain induced martensitic transformation (MT) monitoring in austenitic stainless steel 304 L. During plastic deformation, the austenite transforms locally in martensite. Transformed martensite distribution depends on the geometry of the specimen under test (rectangular section in this work): transformation occurs first in the corners, later close to the flat free surfaces, and finally in the bulk. The obtained value is then representative of a small volume of the sample. Local measurements with an eddy current probe proved to be sensitive to this kind of transformation and even quantitatively after calibration. This article proposes a procedure to calibrate the eddy current response in order to determine the fraction of martensite formed under the probe without the need of X-ray measurements.

Static Fatigue of a 2.5D SiC/[Si-B-C] Composite at Intermediate Temperature under Air

Non-oxide ceramic-matrix composites (CMCs) are subjected to be used in aeronautic applications which require very long duration (up to 100 000h) of materials at high temperatures and under air. Recently a self-healing [Si-B-C] matrix has been developped to enhance strongly the lifetime of CMCs under air. The aim of this work is to study the mechanical behaviour of a SiCf/[Si-B-C] composite with a self-healing matrix under static fatigue, and to determine its lifetime. During the mechanical tests, acoustic emission is detected in order to characterize the damage of the composite in addition to the measurement of the longitudinal deformation of the composite. The analysis of acoustic emission follows a non-supervised procedure of classification. Each event of acoustic emission is described by a set of several parameters, and the total activity can be divided in four classes. The assignment of each class to a damage mechanism is required to follow the spread of damage during fatigue and to determine the mechanisms controlling the failure of the composite.