David Taylor, Trinity College Dublin, UK
Crack paths in natural materials
Natural materials often fail by cracking because the fracture toughness values of many natural materials are low, compared to their strengths. Problems arise in determining the origin and direction of propagation of cracks, owing to the complex microstructures involved. Two examples are plant stems and hair, which are made from very different materials but share common features in that they are both highly anisotropic and loaded primarily in bending. Cracks may form either on the surface or internally, driven by different tensile and shear stresses, including stresses arising from transverse plasticity. In propagation, the crack path may be transverse, causing a simple flat fracture, but more often longitudinal splitting occurs, including the classic “greenstick” fracture in which an initially transverse crack turns to become longitudinal. Better understanding of these phenomena allowed us to develop a systematic approach to the analysis of human hair damage and led to the development of a prototype self-healing structure inspired by plant stems.
Donato Firrao, Gruppo Italiano Frattura, Italy
Fatigue fractures do not nucleate always in the same manner
Scientific literature usually adopts a constant approach to fatigue fracture nucleation in metals: i.e. surface protrusions follow a long lasting incubation stage; short cracks oblique to the direction of local normal stresses ensue across a few material grains up to the inception of fatigue cracks that propagate normally to the prevailing alternate load direction.
In real situations of mechanical components cyclic loading, fatigue crack nucleation and evolution do not always follow the above classical serene path, since machines work patterns often depart from design forecasts. Hazardous single or multiple operations in an otherwise continuous alternating load sequence induce in machine components local small surface fractures triggered by impact peak overloads. Thus, classical initial by-the-book steps are not at all present. If these local small fractures, appearing in components subjected to a continuous alternate tensile stress, induce a ∆K value that is larger than the material ∆Kth limit, regular fatigue crack growth ensues, so that fatigue fractures featured by the usual beach marks, interspersed with striations zones, can be seen with apt microscopy tools.
An example of a series of fatigue fractures, without the classical nucleation stage, is presented and illustrated with macro- and micro-fractographic analyses. The evolution and convolution of fatigue fractures incepting from multiple tensile fractures triggered by anomalous overload impacts are also described and rationalized.
Guian Qian, Chinese Academy of Sciences, China
Very-high-cycle fatigue behavior and lifetime prediction of Ti-6Al-4V manufactured by selective laser melting
Guian Qian, Y. Li, D.S. Paolino, A. Tridello, F. Berto, Y. Hong
The effect of building orientation on the very-high-cycle fatigue (VHCF) response of Ti-6Al-4V specimens produced through selective laser melting (SLM) process with three different building orientations (0°, 45° and 90°) has been experimentally assessed. The fatigue performance decreases with different building orientations from 0° to 90°. The fatigue crack origin has been found to be always an internal defect both at high-cycle fatigue and VHCF regime independent of building orientations. Size of defects induced fatigue failures and the stress intensity factor range decrease with the number of cycles to failure. For the first time we observed nanoscale grains in the fatigue crack initiation region, which validates the numerous cyclic pressing (NCP) model for crack initiation and early growth. The effect of defect size, stress amplitude, underlying microstructure and their relationship with the size of FGA is described, which has implication in terms of producing other additively manufactured (AM) alloys also with higher fatigue resistance. By considering the VHCF strength at 109 cycles, the median value decreases from 217 MPa (0°) to 201MPa (45°) and finally to 155 MPa (90°), with a 40% reduction from 0° to 90°. The building orientation significantly influences both the defect size and the resulting VHCF response.
Guozheng Kang, School of Mechanics and Engineering, Southwest Jiaotong University, China
Damage mechanism of an extruded magnesium alloy in uniaxial low-cycle fatigue with ratchetting
Z. Wang, S. Wu, G. Kang
This presentation elucidates the relationship between the primary plastic deformation processes and associated damage mechanisms operating in an extruded AZ31 Mg alloy under uniaxial low-cycle fatigue with an occurrence of ratchetting by addressing the microscopic observations on the microstructural evolution during the cyclic deformation. Different mean stresses and stress amplitudes were prescribed to study the cyclic plastic deformations dominated, respectively, by the twinning/detwinning, twinning/detwinning and dislocation slipping, and dislocation slipping only. In each case, the damage was examined by synchrotron radiation micro-computed tomography and scanning electron microscopy; and corresponding plastic deformation mechanism was characterized by electron backscatter diffraction and transmission electron microscopy. Profuse twins were detected in the twinning/detwinning dominated sample and the twinning/ detwinning and dislocation slipping dominated one, while more complex structures and higher dislocation densities were found in the twinning/detwinning and dislocation slipping dominated sample and the dislocation slipping dominated one where the dislocation slipping was plentiful. The twinning/detwinning and dislocation slipping dominated sample had the most damage initiation sites owing to the transgranular damage caused by twins and the intergranular damage induced by the combined effect of <c+a> dislocations and twins, followed by the twinning/detwinning dominated sample because of the low dislocation density, while the dislocation slipping dominated sample had the fewest due to the lack of twins. Shear linkage between the isolated cracks was observed only in the twinning/detwinning and dislocation slipping dominated sample. This work would provide rich evidences for the damage evolution modelling of extruded Mg alloys.
José Alexander Araújo, University of Brasilia, Brazil
Short crack path under fretting conditions
J.A. Araujo, A.L. Pinto, R.A. Cardoso, R. Talemi
The aim of this work is to estimate short crack path for metals under fretting fatigue conditions. Stress and stress intensity factor (SIF) based criteria are considered in the analysis. The novel critical direction method proposed by Araújo et al. associated with multiaxial critical plane models constitute the stress based approach to estimate the short crack orientation. To evaluate wear effects on the prediction of the early crack orientation finite element (FE) simulations are carried considering the geometry update due to the material removal. Also the SIF is numerically computed and a method is introduced to predict the short crack path under such nonproportional loading conditions and severe stress gradients. Other models available in the literature to estimate crack path are compared with the methods here proposed. To validate the analysis, fretting fatigue data for aeronautical aluminum alloys using cylinder on plane contact geometries are considered.
Leslie Banks-Sills, Tel Aviv University, Israel
Comparison of Fatigue Delamination Propagation in DCB Specimens
Banks-Sills L., Simon I., Chocron T.
Double cantilever beam (DCB) specimens composed of carbon fiber reinforced polymer laminate composites were tested. Two material systems were investigated. In the first, the specimens were fabricated from 15 plies of a plain woven prepreg (G0814/913) arranged in a multi-directional (MD) layup . The plies alternated with yarn in the 0°/90° - directions and +45°/-45° - directions with the delamination between these two ply types. For the second material type, the specimens were fabricated from 19 plies with the delamination between a unidirectional fabric and a woven ply with yarn in the +45°/-45° - directions . The remainder of the plies were woven, with yarn alternating between the 0°/90° and +45°/-45° - directions. This laminate was produced by means of a wet-layup. - continue -
Stavros Kourkoulis, National Technical University of Athens, Greece
The problem of predicting entrance of a mechanical system into its critical state.
Applications in the restoration/conservation of stone monuments
Knowledge of the mechanical response of structures at load levels close to those causing failure, as well as their remaining load carrying capacity, is crucial for structural engineers, independently of whether the structures are intact or damaged. In this direction, intensive technological research is carried out worldwide aiming to the development of sophisticated Structural Health Monitoring (SHM) tools, which can record even the slightest deviations from the expected response of the monitored structure, warning, thus, about upcoming local or global failure. Nowadays, the most widely used SHM tools are based on the recording of either acoustic or electrical signals, emitted while structural members are loaded by mechanical stimuli. The proper interpretation of the data acquired by these sensing techniques, in the direction of revealing hidden pre-failure indicators, is the next challenge of the scientific research. Various approaches have been proposed, as it is, for example, the Ib-value analysis, models based on wavelets and Natural Time, analyses based on Non-Extensive Statistical Physics concepts, various log-periodic power-law models etc. In this study, a comparative consideration of the pre-failure indices provided by such approaches is described, taking advantage of raw data (concerning both acoustic and electric emissions), obtained from either elementary or structural experiments with specimens resembling restored structural members of stone monuments.