Fatigue Crack Growth Behaviour and Role of Roughness-Induced Crack Closure in CP Ti: Stress Amplitude Dependence

This paper investigated the fatigue crack propagation mechanism of CP Ti at various stress amplitudes (175, 200, 227 MPa). One single crack at 175 MPa and three main cracks via sub-crack coalescence at 227 MPa were found to be responsible for fatigue failure. Crack deflection and crack branching that cause roughness-induced crack closure (RICC) appeared at all studied stress amplitudes; hence, RICC at various stages of crack propagation (100, 300 and 500 µm) could be quantitatively calculated. Noticeably, a lower RICC at higher stress amplitudes (227 MPa) for fatigue cracks longer than 100 µm was found than for those at 175 MPa. This caused the variation in crack growth rates in the studied conditions.

Compressive Fatigue Behavior of Gum and Filled SBR Vulcanizates

The influence of carbon black on physical mechanical properties, compressive fatigue life, and the temperature changes during compression fatigue process of styrene-butadiene rubber (SBR) vulcanizates were explored. A series of unfilled and filled SBR compounds were prepared, and the compressive fatigue behaviors of the vulcanizates were performed on a mechanical testing and simulation (MTS) machine. The top surfaces of the filled SBR were imaged using scanning electron microscopy (SEM) after 105 cycles of compressive fatigue. The filled SBR shows greater compressive fatigue resistance than the unfilled SBR. The incorporation of carbon black into SBR improves the creep resistance. The best compressive fatigue resistance for the filled SBR was achieved by the addition of 30 phr carbon black. With the increase of carbon black content, the energy dissipation and the heat build-up increase simultaneously. Furthermore, SEM images of the vulcanizates suggest that the crack propagation mechanism of the unfilled and the filled SBR was different. For the unfilled SBR, due to periodical compressive stress, the polymer chains can be destroyed, and the cracks can be easily initiated and propagated, showing serious damage on the top surfaces of the specimen. However, for the filled SBR, the carbon black agglomeration around the cracks can greatly delay the generation of the cracks, decrease the fatigue damage, and ultimately improve the fatigue resistance.

Basal Plane Delamination Energy Measurement in a Ti3SiC2 MAX Phase

The {0001} basal plane delamination dominating the crack-wake bridging in MAX phases at a bulk scale has been investigated by studying the small-scale fracture of a Ti3SiC2. In situ micro-double cantilever beam (DCB) tests in a scanning electron microscope were used to grow a stable crack along the basal plane, measure the fracture energy, and study the crack propagation mechanism at the nanoscale. The results show that the fracture energy (10–50 J/m2) depends on small misorientations angles (e.g., 5°) of the basal plane to the stress field. This induces permanent deformation which can be observed once the DCB has been unloaded. The nanoscale study of the crack shows that the plasticity at the crack tip is small, but a number of pairs of dislocations are forming at each side of the crack. Hence, this study helps to explain the enhanced fracture energy values and possible sources of energy dissipation in basal plane delamination, which is the one of the main toughening mechanisms in the bulk fracture of MAX phases.