Mohamad Ghazi Droubi

Acoustic emission monitoring of abrasive particle impacts on carbon steel

The estimation of energy dissipation during particle impact is a key aspect in evaluating the abrasive potential of impact process. Whereas numerous numerical and analytical approaches exist, aimed at explaining observed wear phenomena, few researchers have attempted to measure the energy dissipation during impact. This article reports the results of systematic acoustic emission (AE) energy measurements aimed at detecting the amount of energy dissipated in a carbon steel target during airborne particle impact. Three experimental arrangements were used to investigate three impact regimes; low velocity–low mass (impact speeds of 1–2.5 m/s and masses of 4.9 × 10−6 to 2.3 × 10−4 g), low velocity–high mass (sphere masses of 0.001–2 g), and high velocity–low mass (impact speeds of 4–16 m/s). Within each of these regimes, both single-particle and multiple-particle impacts were studied in order to investigate the effect of overlapping events. Two parameters, particle diameter and particle impact speed, both of which affect the energy dissipated into the material were investigated and correlated with AE energy. The results show that AE increases with the third power of particle diameter, i.e. the mass, and with the second power of the velocity, as would be expected. The diameter exponent was only valid up to particle sizes of around 1.5 mm, an observation which was attributed to different energy dissipation mechanisms with the higher associated momentum. The velocity exponent, and the general level of the energy were lower for multiple impacts than for single impacts, and this was attributed to particle interactions in the guide tube and/or near the surface leading to an underestimate of the actual impact velocity in magnitude and direction.

Acoustic emission method to study fracture (Mode-I, II) and residual strength characteristics in composite-to-metal and metal-to-metal adhesively bonded joints

ABSTRACT Failure behaviour of two types of adhesively bonded joints (composite-to-metal, metal-to-metal) has been studied under failure modes (Mode I: double cantilever beam (DCB) and Mode II: three-point end notch flexures (3-ENF)) using acoustic emission (AE) technique. The bonded specimens were prepared using two types of adhesive bond materials with three variations of adhesive bond quality. The effect of the presence of interfacial defects along the interface on the residual strength of the joint has also been studied. It was possible using the maximum AE amplitude method to select the AE events of mechanical significance. However, it proved difficult to propose a definitive AE trait for the mechanical phenomena occurring within specific AE event signals, for all adhesive types, bond qualities, and substrate configurations, therefore, all specimen combinations. There was a notable shift in spectral energy proportion as the AE source of mechanical significance varied along the specimen length for specimen combinations. However, it was difficult to confirm this distinctive trait for all specimen combinations due to difficulty in confirming the location and exact mechanical source. The proposed measurement technique can be useful to assess the overall structural health of a bonded system and may allow identification of defects.

Cyclic Nanoindentation and Nano-Impact Fatigue Mechanisms of Functionally Graded TiN/TiNi Film

The mechanisms of nanoscale fatigue of functionally graded TiN/TiNi films have been studied using multiple-loading cycle nanoindentation and nano-impact tests. The functionally graded films were sputter deposited onto silicon substrates, in which the TiNi film provides pseudo-elasticity and shape memory behaviour, while a top TiN surface layer provides tribological and anti-corrosion properties. Nanomechanical tests were performed to investigate the localised film performance and failure modes of the functionally graded film using both Berkovich and conical indenters with loads between 100 μN and 500 mN. The loading history was critical to define film failure modes (i.e. backward depth deviation) and the pseudo-elastic/shape memory effect of the functionally graded layer. The results were sensitive to the applied load, loading mode (e.g. semi-static, dynamic) and probe geometry. Based on indentation force–depth profiles, depth–time data and post-test surface observations of films, it was concluded that the shape of the indenter is critical to induce localised indentation stress and film failure, and generation of pseudo-elasticity at a lower load range. Finite-element simulation of the elastic loading process indicated that the location of subsurface maximum stress near the interface influences the backward depth deviation type of film failure.