Laser powder bed fusion (L-PBF) is an additive manufacturing (AM) process which offers several advantages over conventional manufacturing techniques, including material savings and design possibilities. For these reasons several industrial sectors such as biomedical, aerospace and automotive, are considering L-PBF for customized parts production. Among the processable material palette, Ti6Al4V gained increasing interest due to the high specific strength, and stiffness, and excellent bio-compatibility, and corrosion resistance. However, to include L-PBF Ti6Al4V parts in load bearing applications it is important to properly understand the mechanical properties of AM’ed materials with a specific focus on fatigue behaviour. In this study the fatigue notch sensitivity of Ti6Al4V L-PBF manufactured specimens is investigated in depth. Samples with different notches were subjected to stress relieving heat treatment to generate a fine α+β microstructure. The results were used to define a relation between the perturbed stress field generated by the geometrical discontinuity, the microstructure size and the notch sensitivity. Moreover, the critical distance has been calculated using the theory of critical distance formulated with the line method (LM). The estimated value were adopted to predict fatigue properties of notched components with different notch radii.

In order to measure the interfacial Strain Energy Release Rate (SERR) of adhesively bonded joints with metallic substrates, an alternative to the Tapered Double Cantilever Beam (TDCB) specimen is used here. In particular, the dimensions of one of the two substrates were reduced to create an Asymmetric Tapered Double Cantilever Beam (ATDCB) specimen. The ATDCB specimen has been introduced in the past by other research teams to study the mode I+II cohesive fracture properties of structural adhesives. It is shown the geometric dissymmetry of the ATDCB specimen affects the crack propagation path towards the interface with the smaller (less stiff) substrate. However, this substrate plasticises during the fracture test; hence, a strategy is also presented to evaluate the amount of potential energy spent for this phenomenon and subtract it from the total potential energy induced in the system during the fracture test. After this operation, the SERR of the adhesive under investigation was calculated by means of the Linear Elastic Fracture Mechanics (LEFM) theory before and after water ageing. The results showed a large decrease of the SERR with the increase of the ageing time.

To study the thermal cracking characteristics and mechanism of sandstone after high-temperature treatment, the pore size distribution and micromorphology of sandstone were observed by nuclear magnetic resonance and scanning electron microscopy. Then, based on the Weibull distribution theory, a thermal elastic mechanical model of random heterogeneous rock was established for the rock unit, the thermal stress distribution characteristics of sandstone were analysed, and the thermal fracture mechanism of rock was discussed. The results show that the porosities of the samples increased with increasing temperature, and the proportion of large pores increased significantly when exceeded 400 °C. Particularly when reached 1000 °C, thermal cracking was distributed in a complex network. Additionally, different rock units are in different thermal stress states, which leads to the regional differences in the distribution of rock thermal fracture. When exceeded 400 °C, there were obvious thermal cracks near the outer edge that weakened the mechanical properties of rock.

Ultra-light Mg-Li alloy is a promising alloy in aerospace since it is known to the lightest structural alloy at present, but its fatigue behaviors remain to be explored. This work focuses on very-high cycle fatigue (VHCF) strength and small crack initiation behaviors of an extruded dual-phase Mg-Li alloy (LZ91). The fatigue strength of the LZ91 alloy at 109 cycles is about 78 MPa, and the fatigue ratio is approximately 0.46. Microstructure characterization demonstrates that fatigue crack tends to initiate from the β-Li phase-enriched region. The α-Mg phase, presenting <10−10 >  fiber texture with the basal plane, deforms hardly along the extrusion direction and acts as an enhanced phase compared with the β-Li phase. The deformation discrepancy localizes cyclic plasticity at the Li phase and finally leads to the fatigue crack initiation.

A bio-ceramic coating was prepared on the surface of ZK60 magnesium alloys by micro-arc oxidation (MAO) method. The substrate (BM) and coated (MAO) specimens were pre-corroded in a simulated body fluid (SBF) for 12 h. Strain-controlled and stress-controlled loading modes were used to conduct fatigue tests for the two specimens, respectively. The cyclic deformation behavior of the two specimens with non-corroded and pre-corroded was studied. The mechanism of cyclic deformation under different loading conditions is related to twinning and slip. At the same test conditions, the fatigue life of the non-corroded BM specimen is higher than that of the non-corroded MAO specimen, while the fatigue life of the pre-corroded MAO specimen is higher than that of the pre-corroded BM specimen. A modified total strain energy model is proposed and the precision of life prediction is higher than that of traditional fatigue model.

In this research, the effects of induced delamination on the variation of the mechanical properties of composite laminates subjected to bending loads are investigated using a micromechanical model. For this purpose, the variation of the mechanical properties of delaminated laminates is determined using stress analysis of damaged ply and classical laminate theory (CLT) relationships. Using the proposed model and CLT, the fracture toughness due to induced delamination formation is presented in cross-ply laminates. Subsequently, the variation of strain energy release rate (SERR) is calculated in terms of crack density using analytical and finite element models to detect dominant failure modes in different crack densities. The results are compared with those of matrix cracking propagation. The results obtained by the proposed analytical model are in good agreement with those obtained by existing numerical and experimental approaches. The proposed model can be utilized to predict induced delamination formation in composite laminates subjected to bending loads.

This paper starts with an overview of the application of the three guidelines (GL) of the German Research Association of Mechanical Engineers (FKM). Each of these provides algorithms for calculating fatigue lives of components under constant or variable amplitude loading, however, with underlying different failure criteria, i.e. technical crack initiation life (GL-nonlinear), fatigue crack growth life (GL-fracture mechanics), and total fracture life (GL-linear). This paper introduces the U-Concept which has been evaluated from a large structural durability database. The U-Concept is a small add-on to the Local Strain Approach (LSA) which is the backbone of the GL-nonlinear. It enables 1) to directly calculate the fatigue life to total fracture based on elastic-plastic material behaviour according to the LSA, or 2) to estimate the remaining fatigue life from crack initiation to fracture without a crack growth simulation.

After 100 years of research, researchers continue to consider how time-dependent environmental effects can be included in existing cycle-dependent fatigue crack growth rate (CGR) models.  Early corrosion fatigue (CF) model assumptions were that cycle-dependent CF and time-dependent stress corrosion cracking (SCC) contributions are separable, operate in parallel, are non-interacting.  However, research has shown that CF and SCC may interact synergistically to produce CGRs greater than that obtained by simple summation.              Proposed here is that CF and SCC mechanisms may interact synergistically as each is dependent on the electrochemically-active surface area (ECSA) at a crack tip.  A phenomenological model is developed for the effects of crack tip strain rate, film-rupture and repassivation rates on ECSA.  Analysis of hold-time CGR data provides a method of separating cyclic- and static-load hold-time effects on ECSA.  Five model parameters, each having physical interpretation, are found to have reasonable magnitudes. 

In order to study the fatigue damage and cycle life of TC4 titanium alloy for aero-engine blades under various load conditions, uniaxial fatigue, multi-stage loading and multiaxial fatigue tests were carried out. The critical plane method combined with von Mises criterion was used to study the fatigue life distribution under various loading paths, and the results were verified by experiments. Based on the nonlinear residual strength model and Poisson stochastic process, the reliability prediction of aero-engine compressor blade under maximum continuous working condition was completed. The results show that the fatigue damage and life distribution of TC4 titanium alloy for aeroengine blade under various conditions can be accurately predicted by the method proposed in this paper and the reliability of the blade can still be maintained above 0.9 after it works for 3000 hours under the maximum continuous working condition.