Lotfi Toubal

Damage Characterization of Bio and Green Polyethylene–Birch Composites under Creep and Cyclic Testing with Multivariable Acoustic Emissions

Despite the knowledge gained in recent years regarding the use of acoustic emissions (AEs) in ecologically friendly, natural fiber-reinforced composites (including certain composites with bio-sourced matrices), there is still a knowledge gap in the understanding of the difference in damage behavior between green and biocomposites. Thus, this article investigates the behavior of two comparable green and biocomposites with tests that better reflect real-life applications, i.e., load-unloading and creep testing, to determine the evolution of the damage process. Comparing the mechanical results with the AE, it can be concluded that the addition of a coupling agent (CA) markedly reduced the ratio of AE damage to mechanical damage. CA had an extremely beneficial effect on green composites because the Kaiser effect was dominant during cyclic testing. During the creep tests, the use of a CA also avoided the transition to new damaging phases in both composites. The long-term applications of PE green material must be chosen carefully because bio and green composites with similar properties exhibited different damage processes in tests such as cycling and creep that could not be previously understood using only monotonic testing.

Bimodal dwell-fatigue Weibull distribution of forged titanium IMI 834

This study addresses cumulative damage and its evolution during the cold dwell fatigue of a near-α titanium alloy. An experimental study was undertaken to examine the evolution of life, strain, strength and damage of 13 titanium IMI 834 samples cut from a single disk forging. The samples were tested in the same dwell-fatigue loading conditions. In the dwell phase, the load is maintained at 80% of ultimate tensile strength (824 MPa, 90% of yield strength) for 30 s. The secant Young’s modulus and inelastic strain at minimum load were measured in order to document the evolution of the irreversible damage against the number of cycles for all specimens. Experimental observations show significant differences in dwell-fatigue life and damage behavior. This mechanical analysis and an analysis of the cumulative Weibull reliability distribution suggest a bimodal dwell-fatigue failure process. Some features of the mechanical behavior can be used to sort the samples according to each of the two failure modes and improve the reliability of the fatigue test campaign.

Study of the Endurance Limit of AA7075 Aluminum Produced by High-Pressure Vacuum Die Casting Analyzed by Classical Whöler Curve

One of the strongest aluminum alloy is AA7075; it has a yield strength of 350 MPa in the T73 heat-treated state, which is 50% stronger than construction steels. The alloy contains a high mass fraction of alloying elements, but no silicon; therefore it is not easily cast into complex shapes by traditional foundry processes since it is prone to shrinkage, hot tearing and has low fluidity. However, the high pressure vacuum die casting (HPVDC) process uses high metal velocities to fill the mold cavity and applies a pressure during solidification to feed shrinkage, which could facilitate the production defect free castings. Casting AA7075 alloy would be highly beneficial to produce complex shapes and reduce assembly steps. This work will focus on AA7075 production by HPDVC process and the effect of grain refiner on the casting defects and mechanical properties of cast AA7075. It is well known that castings endurance limit is highly affected by defects (porosity, oxides, cold shuts, etc.). In this work, the main objective is to study the impact of the HPVDC process on the endurance limit of AA7075, as well as the effect of grain refiner (TiB2) in reducing the casting defects and enhancing the endurance limit of the cast alloy. The endurance limit will be studied through uniaxial tension-tension fatigue tests on T73 heat-treated AA7075 castings. The microstructures of AA7075 alloy cast with and without TiB2 was observed by optical and electron microscopy. Stress controlled fatigue testing up to 106 cycles will be used to trace the Wholer curves and study the effect of TiB2 on the endurance limit.