Gérard Degallaix

A tribometer for the study of materials under railway braking conditions

This paper presents a braking tribometer developed to study the behaviour of materials used in railway braking up to high speeds and energies. Its design is based on similitude rules which enable to reproduce at reduced scale, the same working conditions of materials tested at full scale. The validation of the testing machine is presented: it is based on the reproduction at the reduced scale of a performance test programme of the Thalys TGV friction materials, which has been previously performed on a test bench at full scale. A comparison of the obtained results with the ones available at full scale is achieved. A study of the friction physical mechanisms carried out from scanning electron microscopy shows that a stable third body, made of plane sheets, controls an S3M3-type velocity accommodation mechanism. These plane sheets are obtained by agglomeration and packing of particles resulting mainly from the Fe-CuSn sintered matrix of the brake pad. A specific study has enabled the genesis of the third body to be understood.

Interactions between Third-Body Flows and Localisation Phenomena during Railway High-Energy Stop Braking

During railway braking, dissipation of high energy due to friction leads to transient and localised thermal phenomena such as hot bands and hot spots. These localisation phenomena interact with third body flows and friction mechanisms activated in the contact. To study these couplings, an experimental approach has been developed, based on an inertial tribometer able to reproduce high thermo-mechanical brake-disc loadings. This paper focuses on the coupling between flows of third-body and hot-band migration. Transient localised thermal phenomena are described for high-energy stop-braking. The monitoring of the disc track during friction in the visible and infrared wavelengths evidences the interactions between third-body flows and hot-band migration. The consequences relatively to load-bearing areas and third-body compaction in the contact are studied, allowing the proposal of a synthetic diagram of the coupling between friction mechanisms, third body flows, hot-band migration and first-body distortion.

Cyclic shearing behaviour of a unidirectional glass/epoxy composite

Results from an experimental program consisting of monotonic and fatigue shear tests on a unidirectional glass–epoxy composite are presented in this paper. Unnotched and tabbed specimens were subjected to interlaminar and intralaminar shearing modes using an appropriate testing device called “Cube Test”. Shear mechanical characteristics are determined and cyclic resistance is analyzed. Fatigue results are in good agreement with three-point-bending tests with predominant shearing studied elsewhere on the same material. Modified Basquin relationship is established to characterize the shearing fatigue strength. Microstructural observations of the fracture surfaces by scanning electron microscopy show that hackle formation is the predominant damage mechanism.

Failure Mechanisms of TGV Brake Discs

The CRISFIS project (supported by ADEME agency) consists in jointly studying the squealing and cracking aspects of the high power disc brakes for TGV very high speed trains. This paper deals with the progress concerning the cracking part. An experimental and modelling strategy is adopted in order to better understand and predict brake disc cracking. Braking tests conducted on an industrial scale-one test bench are presented. In a first step, the influence of the pad-type on the thermal loading applied to the disc is studied by means of an infrared camera and thermocouples embedded in the pads and in the disc. In a second step, the thermal maps extracted from thermographic monitoring are used as input data for thermal-mechanical calculations. Finally, the results of modelling and tests are compared to the damage observed on the brake discs.

Beneficial Effects Induced by High Temperature Cycling in Aged Duplex Stainless Steel

The cyclic behavior of type DIN 1.4460 duplex stainless steels in as-received and aged conditions was investigated at room temperature and at 500°C. At room temperature, the aged samples showed embrittlement effects such as loss of toughness and reduction of fatigue life. At 500°C, beneficial effects of the synergy between temperature and strain cycling were observed. It is proposed that at high temperature in the ferrite the strain cycling can decompose the chemical composition fluctuations, promoting a demodulation of the spinodal decomposition formed by aging.

Thermal fatigue of a 304 L type steel

Abstract Various components of nuclear reactors are submitted to very sharp thermo-mechanical loadings. Thermal fatigue cracking has been clearly detected in auxiliary loops of the primary cooling circuit of Pressurized Water Reactors (PWRs). The study presented here is focused on the 304 L type stainless steel used in PWRs. The thermal fatigue behaviour of this steel has been investigated using a specific thermal fatigue test equipment called SPLASH. This test equipment allows the reproduction of multiple cracking networks similar to those detected during inspections. The present study deals with two points : i) the experimental determination of crack initiation conditions and the morphological description of the growing crack networks; ii) the multiple crack growth numerical simulation, using a Skelton model, and a generalized Paris law. This modelling, in spite of simplified assumptions, gives predictions in good agreement with observations, as far as the evolution of the mean and deepest cracks during cycling are concerned.

Low Cycle Fatigue of a Duplex Stainless Steel Alloyed with Nitrogen

This paper presents first results on the influence of Nitrogen alloying on the low cycle fatigue behaviour at room temperature of an austenitic-ferritic stainless steel. The observed improvement in fatigue resistance by Nitrogen alloying is attributed to the planar dislocation slip, favoured by Nitrogen in the austenitic phase. The duplex steel exhibits, at all strain levels, a hardening-softening accommodation behaviour, which is characteristic of high Nitrogen austenite, and is controlled by the long range stress evolution.

Advances in hysteresis loop analysis and interpretation by low-cycle fatigue test computerization

Low-cycle fatigue test computerization offers new perspectives in the analysis and interpretation of test results. Test control and data acquisition with a microcomputer allow for increased versatility and accuracy in theevaluation of pertinent characteristic parameters. A specific software package, AICYFAP, has been developed. It provides from the acquired data files 43 parameters on each hysteresis stress-strain loop, including conventional parameters, elasticity and work-hardening parameters, and loop shape parameters. The software package has been used in low-cycle fatigue studies for two kinds of materials: AISI 316L-316LN austenitic stainless steels (energy approach) and a duplex stainless steel (continuous damage mechanics approach). The AICYFAP software package appears to be a complementary tool for a better understanding of low-cycle fatigue behaviors.

Thermal and tribological study of a periodic contact under braking conditions

The present work concerns the tribological and thermal studies of periodic contact obtained during successions of railway stop-brakings. Tests with heat accumulation, performed on a braking tribometer, permit the observation of a modification of the friction coefficient, a wear increase and the presence of a large compacted third body layer on the pad surface. An analytic model of thermal conduction is developed for the determination of the disc surface temperature evolution during the succession of stop-brakings. The results obtained using mass temperature measurements during the succession are compared with those estimated by pyrometry; a good agreement is found.

Biaxial thermomechanical fatigue on a 304L-type austenitic stainless steel*

Abstract Various components of nuclear power plants are submitted to very sharp multiaxial thermomechanical loadings, due for instance to the incomplete mixing of flows at different temperatures. As an example, thermal fatigue damage has been detected in auxiliary loops of the primary cooling circuits of Pressurized Water Reactors. In particular, crack networks were observed in in-service pipes submitted to thermomechanical loading resulting from cyclic temperature gradients across the wall-thickness of components in 304 L type austenitic stainless steel. The thermal fatigue behaviour of AISI 304 L type steel has been studied using a specific thermal fatigue test, called Splash, developed in order to reproduce experimentally such thermomechanical biaxial loading in the thickness of parallelepipedic specimens. All tests have been performed at a maximum temperature of 320°C, but with different minimum temperatures. First, the morphological characteristics of the growing networks were analysed, in surface and in depth. Crack initiation is multiple and occurs on sliding lines or at material defects. Crack network stabilization is observed after 400 000 cycles at a temperature of 150°C. The maximum depth is 2.5 mm. Secondly, the stability of the thermal-fatigue cracknetworks previously obtained was investigated under additional isothermal mechanical loading (four-point bend tests). Selection mechanisms of a dominating crack are observed, showing a great influence of shielding effects, branching and tortuous path. Comparison of the dominating crack behaviour with one having a single crack initiated at a notch tip reveals a significant delay effect.