Fracture Network Localization Preceding Catastrophic Failure in Triaxial Compression Experiments on Rocks

We quantify the spatial distribution of fracture networks throughout six in situ X-ray tomography triaxial compression experiments on crystalline rocks at confining stresses of 5–35 MPa in order to quantify how fracture development controls the final macroscopic failure of the rock, a process analogous to those that control geohazards such as earthquakes and landslides. Tracking the proportion of the cumulative volume of fractures with volumes >90th percentile to the total fracture volume, ∑v90/vtot indicates that the fracture networks tend to increase in localization toward these largest fractures for up to 80% of the applied differential stress. The evolution of this metric also matches the evolution of the Gini coefficient, which measures the deviation of a population from uniformity. These results are consistent with observations of localizing low magnitude seismicity before large earthquakes in southern California. In both this analysis and the present work, phases of delocalization interrupt the general increase in localization preceding catastrophic failure, indicating that delocalization does not necessarily indicate a reduction of seismic hazard. However, the proportion of the maximum fracture volume to the total fracture volume does not increase monotonically. Experiments with higher confining stress tend to experience greater localization. To further quantify localization, we compare the geometry of the largest fractures, with volumes >90th percentile, to the best fit plane through these fractures immediately preceding failure. The r2 scores and the mean distance of the fractures to the plane indicate greater localization in monzonite than in granite. The smaller mean mineral diameter and lower confining stress in the granite experiments may contribute to this result. Tracking these various metrics of localization reveals a close association between macroscopic yielding and the acceleration of fracture network localization. Near yielding, ∑v90/vtot and the Gini coefficient increase while the mean distance to the final failure plane decreases. Macroscopic yielding thus occurs when the rate of fracture network localization increases.

Correlation between Fractal Dimension and Areal Surface Parameters for Fracture Analysis after Bending-Torsion Fatigue

This paper investigates the fracture surface topography of two steel and aluminum alloys subject to bending-torsion fatigue loadings, as well as their susceptibility to fatigue performance and failure mechanisms. Using fracture surface topography data analysis, elements with different geometries were elaborated. A correlation between the fractal dimension, other selected parameters of surface topography such as areal Sx, and fatigue loading conditions was found. Distinctions in particular regions of cracks were also recognized through proving the correctness and universality of the total fracture surface method. The influence of fatigue loading parameters on the surface topography of fatigue fractures was demonstrated. For the analyzed cases, results show that the fractal dimension and standard surface topography parameters represent a correlation between them and loading conditions. As a single parameter, the appropriate loading ratio cannot be outright calculated with fractal dimension, but can be estimated with some approximation, taking into account additional assumptions.

Correlations between crack initiation and crack propagation lives of notched specimens under constant and variable amplitude loading

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.

Bioinspired cementitious-polymer composite for increased energy absorption

Preliminary results are presented on the energy absorbing characteristics of a cementitious-polymer architecture bioinspired by the organic-inorganic composite structure of nacre. The proposed bioinspired architecture consists of an open cell, platelet-shaped 3D-printed thermoplastic lattice filled with high performance cementitious paste. The hypothesis is that, similar to nacre, the platelet arrangement and differences in mechanical properties of the thermoplastic lattice and cementitious platelets would result in increased energy absorption. Initial laboratory scale investigations were performed using notched beam samples subjected to static three-point bending. Stereo-digital image correlation was used to track global strain displacement field and Hillerborg’s method was used to estimate the total fracture energy. The results indicate that this “brick-and-mortar” hierarchy can increase the energy absorbing capacity of the composite by upwards of 2490% compared with the benchmark cementitious specimen. The load-deformation behaviour and total fracture energy of the bioinspired composite were found to be influenced by the platelet arrangement and size and the lattice thickness.

Association of Alcohol Intake and Fracture Risk in Elderly Varied by Affected Bones: A Nationwide Longitudinal Study

Objective Previous studies investigating association of alcohol intake and fracture risk in elderly yielded conflicting results. We first examined the association between alcohol intake and total fracture risk in elderly subjects and further analyzed whether the association varied by fracture locations.Methods This is a nationwide population-based cohort study which included all people aged 66 (n=1,431,539) receiving the National Screening Program during 2009–2014. Time-to-event were defined as duration from study recruitment, the day they received health screening, to the occurrence of fracture.Results Total fracture was significantly lower in mild drinkers [adjusted hazard ratio (aHR)=0.952; 95% confidence interval (95% CI) =0.931–0.973] and higher in heavy drinkers (aHR=1.246; 95% CI=1.201–1.294) than non-drinkers. Risk pattern of alcohol consumption and fracture differed according to affected bones. Similar J-shaped trends were observed for vertebra fractures, but risk of limb fracture showed a linear relationship with alcohol intake. For hip fracture, risk decrement was more pronounced in mild and moderate drinkers, and significant increment was noted only in very severe drinkers [≥60 g/day; (aHR)=1.446; 1.162–1.801].Conclusion Light to moderate drinking generally lowered risk of fractures, but association between alcohol and fracture risk varied depending on the affected bone lesions.

Computer-Aided Fracture Size Measurement in Orbital Fractures—An Alternative to Manual Evaluation

Objective: We aimed to present a novel semiautomated tool for orbital fracture size measurement and to compare the variability of the proposed method with traditional manual measurements. Methods: Maximal anteroposterior (AP) and mediolateral (ML) dimensions of orbital fractures from computed tomography images were measured for 15 patients with unilateral orbital fractures by 2 surgeons manually and with a semiautomatic software. Variability was assessed with Bland-Altman limits of agreement plots and intra-class correlation coefficients (ICCs). Results: The intra-observer ICCs in manual and automatic measurements were high, >0.9. The inter-observer ICCs in manual measurements were 0.926 (AP) and 0.631 (ML) and in automatic measurements 0.989 (AP) and 0.989 (ML). The ICCs for manual and semiautomated variability were 0.899 (AP) and 0.669 (ML). The differences were thus particularly pronounced in the ML dimensions. In addition, with the semiautomated technique, a total fracture area could be measured and compared with the total area of the bony orbit and a 3-dimensional reformatted image could be generated. Conclusions: Intra- and inter-observer variability proved to be very low for measuring fracture maximal AP length and ML width, making both the manual and the semiautomatic methods feasible clinically. The semiautomatic fracture size analysis allows better observer-independent repeatability for fracture size measurements and provides the possibility for total fracture area measurements at any orbital bony site, even in challenging nonplanar topography.

Fracture Analysis of α-Quartz Crystals Subjected to Shear Stress

This study assesses the correlations between the intensity of stress undergone by crystals and the morphological characteristics of particles and fracturing products. The effects of the fractures on the microstructure of quartz are also studied. Alpha quartz, subjected to shear stress, is quickly crushed according to a fracturing sequence, with a total fracture length that is correlated to the stress rate. The shear stress generates a sequence of macro and microstructural events, in particular localized melting phenomena, never highlighted before on quartz and the formation of different polymorphs, such as cristobalite and tridymite together with amorphous silica.

Profile and Areal Surface Parameters for Fatigue Fracture Characterisation

Post-mortem characterisation is a pivotal tool to trace back to the origin of structural failures in modern engineering analyses. This work compared both the crack propagation and rupture roughness profiles based on areal parameters for total fracture area. Notched and smooth samples made of weather-resistant structural steel (10HNAP), popular S355J2 structural steel and aluminium alloy AW-2017A under bending, torsion and combined bending–torsion were investigated. After the fatigue tests, fatigue fractures were measured with an optical profilometer, and the relevant surface parameters were critically compared. The results showed a great impact of the loading scenario on both the local profiles and total fracture areas. Both approaches (local and total fracture zones) for specimens with different geometries were investigated. For all specimens, measured texture parameters decreased in the following order: total area, rupture area and propagation area.