Toughness improvement of steel X38CrMoV5-1 via alternative manufacturing process and prevention of catastrophic failure in safety parts

X38CrMoV5-1 (AISI H11-1) steel parts are often manufactured by die-casting followed by quenching and tempering and the microstructure is highly process-sensitive. In order to eliminate the presence of pores in the parts, a different though more expensive manufacturing process has been implemented, involving the hot rolling of a plate and machining of samples in parallel and perpendicular to the rolling direction. Heat treatment of the samples was optimised and the mechanical tensile properties—yield strength, ultimate strength and ductility—and Charpy toughness were determined for ten samples. The results showed that samples machined in the longitudinal direction gave a toughness of about 20 J, twice the toughness of samples machined in the transverse direction (10 J) and 3.5 times the toughness of cast samples (6 J). Ultimate strength and yield strength were 1580 MPa and 1225 MPa in the transverse direction, 1505 MPa and 1275 MPa in the parallel direction, and 1400 MPa and 1260 MPa as cast, respectively. In the latter case, the standard deviation was much higher due to the presence of pores. Using Griffith's theory, the need for pore-free machined parts to prevent catastrophic failure due to cleavage, especially in lift safety systems, is also demonstrated.     

A numerical study of fatigue life in non-Newtonian thermal EHL rolling–sliding contacts with spinning

This paper presents a study on fatigue life in non-Newtonian thermal elastohydrodynamic lubrication (TEHL) point contacts with spinning. A numerical procedure is developed and extended to rolling contact fatigue (RCF) life. The results show that the effect of entraining velocity on the RCF life is closely related to ellipticity. The RCF life first decreases steeply and then gradually with increase in slide–roll ratio. However, the RCF life may increase slightly at a large slide–roll ratio. Spinning is beneficial for reduction of longitudinal friction coefficient; however, even for smooth surface contact, the RCF life can be slightly reduced by spinning.     

Brake torque analysis of fully mechanical parking brake system: Theoretical and experimental approach

The effect of the drum brake temperature reduction on the clamping force of the parking brake system has not been well addressed despite the fact that it may result in vehicle roll away. In view to this, a parking brake model that takes into account the temperature reduction of the drum brake has to be developed and more importantly, it must comply with the applicable standards or regulations such as Federal Motor Vehicle Safety Standard (FMVSS) 135. This paper develops a one dimensional (1D) model of leading-trailing drum-type parking brake model. This brake model is then verified with experiments carried out on a test bench that has been verified with the hand brake system in the vehicle. The results from the experiments show a good correlation with the predicted results from the brake model. It is also found that the existing parking brake design meets the standard requirements. Another finding is that the brake torque slightly increases as the drum temperature increases. With the verified brake model, parametric studies can be carried out as one of the tools during the design process. From the studies, it is found that rollaway will not happen even with the maximum vehicle weight and friction coefficient at drum/lining interface above 0.2.     

HF etching effect on sandblasted soda-lime glass properties

Our objective in this work is to study the HF etching chemical treatment effect on the mechanical and optical properties of soda-lime glass eroded with 200 g fixed sand mass. We followed the evolution of these properties in relation to the chemical attack duration.The results show a clear improvement of the measured properties. The strength of the eroded samples is 44.23 ± 0.91 MPa. It increases up to 57.73 ± 1.76 MPa after 15 min of treatment and reaches 181.43 ± 23.69 MPa after 1 h. This last value is much higher than the as received glass strength (117.5 ± 10.48 MPa). The optical transmission of the eroded samples is about 18.5%. During the first 2 min of the chemical treatment, an important drop of the optical transmission (12%) was observed. However, improvement of the transmission was achieved for longer chemical treatment durations. After 8 h of treatment, the optical transmission increases up to 57%. Microscopic observations show that the HF attack causes the opening and the blunting of the surface cracks. In general, the surface state is improved during the chemical treatment.     

Numerical study of friction-induced vibration and noise on groove-textured surface

A numerical study was performed to predict the squeal instability when a sphere sliding on a groove-textured surface through complex eigenvalue analysis, which was discussed with the experimental results. It is shown that the friction system had a propensity to cause squeal instability. Mode coupling was found when squeal occurred. The calculated dominant frequency of squeal was close to the experimental measured one. The numerical model created can be used to predict the squeal instability of the friction system. Sphere sliding along groove showed potential in reducing squeal instability compared to the case of sphere sliding across groove.     

On the mechanism of squat formation on train rails – Part II: Growth

Longitudinal cross-sectioning of squats reveals characteristic features of internal crack front propagation. Leading crack planes propagate over longer lengths and greater depths as compared to more superficial trailing crack planes. A favourite depth of crack propagation occurs in the subsurface (2–3 mm), is related to the residual longitudinal stress profile, and may lead to an internal crack ‘terrace’. Especially during deeper crack propagation and branching oxidation processes are found to be metallurgical drivers of crack growth. Contact surface modification during squat growth can be distinguished between phases of transient local stress redistribution and of dynamic wheel–rail contact. If the hypothesized shearing wedge in the failure mechanism loses its load bearing capacity, this gives rise to a redistribution of normal stresses within the actual contact ellipse and the formation of a hardness envelope along the crack pattern. This may partially explain why maturing squats show decoloured and hardened surface areas bordering the surface-breaking cracks. A second effect occurs for contact patches not matching the failure ‘envelope’: due to the Poisson effect the surface overlying the crack planes settles slightly, experiences reduced contact, and corrosive products, ‘pumped’ from inside the cracks, may accumulate on the surface (as confirmed by SEM-EDX analysis). During progressive growth of the defect the harder and decoloured envelope as well as the original wedge is pressed into the deeper elastic material, accompanied by a gradual expansion of the contact band and a bilateral bridging of the defect. This may cause high-frequency impact, resulting into progressive internal crack growth affecting the global stress response and rail fracture.     

Effect of electrical current on tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy

A block-on-slip ring-type wear tester was used to investigate the tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy under 2 to 6 N applied load and 0 to 20 A electrical current. The sliding speed was maintained at 25 km/h. The wear loss of copper-impregnated metallized carbon increased with greater electrical current. Under a certain applied load, the wear loss with electrical current was minimized. The tribo-layer had an apparent effect on the friction coefficient. The wear mechanisms were complex, consisting of adhesive wear, abrasive wear and arc erosion.     

Elaboration and mechanical characterization of nanocomposites thin films: Part II. Correlation between structure and mechanical properties of SiO2-PMMA hybrid materials

Mechanical properties of hybrid thin films based on SiO₂-PMMA materials were investigated through nanoindentation tests. We demonstrated in the part I of this paper, that nanoindentation is an appropriate technique to characterize hybrid organic–inorganic thin films. Specific procedures of analysis and the use of appropriate models allows to determine reproductive indentation modulus and hardness of the hybrid layers. The mechanical responses of nanocomposites are not only governed by the composition of the layers but also by the nature and the extent of the hybrid interface. Different layers have been studied, constituted by an inorganic and an organic phase that just be physically mixed or covalently connected. The weak (H bonds) or strong (covalent bonds) interactions generated by the hybrid interface lead to nanocomposites which exhibit different mechanical behaviours. Moreover, comparison between layers obtained by in situ inorganic polymerization in PMMA and layers obtained with preformed silica nano-particles have been also investigated to correlate the morphology of the nanocomposites with the mechanical responses.     

Friction and wear at low temperatures

For investigations of the friction and wear behaviour of materials under cryogenic conditions a special test device (Cryotribometer) has been constructed. The friction tests are performed with samples in a pin-on-disc configuration in a He-gas environment at temperatures between 8 and 77 K. Most of the tests were performed with filled and unfilled polymers like polyterafluorethylene (PTFE), polyamide (PA), polyoxymethylene (POM) and polyimide (PI) against steel. Compared to room temperature most polymers have an improved frictional behaviour in dry sliding at low temperatures, due to their increasing hardness and mechanical strength. Some experiments with hard carbon coatings indicate that the running-in and wear behaviour of these materials is influenced mainly by temperature.