A novel variational framework to model the fatigue behavior of brittle materials based on a phase-field approach to fracture is presented. The standard regularized free energy functional is modified ...introducing a fatigue degradation function that effectively reduces the fracture toughness as a proper history variable accumulates. This macroscopic approach allows to reproduce the main known features of fatigue crack growth in brittle materials. Numerical experiments show that the Wöhler curve, the crack growth rate curve and the Paris law are naturally recovered, while the approximate Palmgren–Miner criterion and the monotonic loading condition are obtained as special cases.
•A phase-field model to study the fatigue cracking in brittle materials is proposed.•It features a degradation of the fracture toughness as a history variable accumulates.•The Wöhler and crack growth curves including nucleation and failure are reproduced.•Palmgren–Miner criterion and monotonic condition are obtained as special cases.•It can easily handle complex geometries, crack patterns, boundary and load conditions.
Data-driven fracture mechanics Carrara, P.; De Lorenzis, L.; Stainier, L. ...
Computer methods in applied mechanics and engineering,
12/2020, Letnik:
372
Journal Article
Recenzirano
Odprti dostop
We present a new data-driven paradigm for variational brittle fracture mechanics. The fracture-related material modeling assumptions are removed and the governing equations stemming from variational ...principles are combined with a set of discrete data points, leading to a model-free data-driven method of solution. The solution at a given load step is identified as the point within the data set that best satisfies either the Kuhn–Tucker conditions stemming from the variational fracture problem or global minimization of a suitable energy functional, leading to data-driven counterparts of both the local and the global minimization approaches of variational fracture mechanics. Both formulations are tested on different test configurations with and without noise and for Griffith and R-curve type fracture behavior.
•A new model-free data-driven paradigm for variational brittle fracture is presented.•The fracture behavior is encoded only into a set of material data points.•Results using noisy or noiseless data imitating Griffith or Rcurve models are studied.•Results from both local and global minimization strategies are correctly reproduced.•The crack jumps are automatically reproduced when the solution remains unique.
The present work proposes a novel thermodynamically consistent model for the behavior of interfaces under shear (i.e. mode-II) cyclic loading conditions. The interface behavior is defined coupling ...damage and plasticity. The admissible states’ domain is formulated restricting the tangential interface stress to non-negative values, which makes the model suitable e.g. for interfaces with thin adherends. Linear softening is assumed so as to reproduce, under monotonic conditions, a bilinear mode-II interface law. Two damage variables govern respectively the loss of strength and of stiffness of the interface. The proposed model needs the evaluation of only four independent parameters, i.e. three defining the monotonic mode-II interface law, and one ruling the fatigue behavior. This limited number of parameters and their clear physical meaning facilitate experimental calibration. Model predictions are compared with experimental results on fiber reinforced polymer sheets externally bonded to concrete involving different load histories, and an excellent agreement is obtained.
•A model to study the fatigue behavior of interfaces under mode-II loads is proposed.•The model is thermodynamically consistent and couples damage and plasticity.•A bilinear interface law is assumed under monotonic conditions.•Four parameters are needed, three for the monotonic and one for the cyclic behavior.•Comparison with experimental tests on FRP bonded joints shows excellent agreement.
Data-driven rate-dependent fracture mechanics Carrara, P.; Ortiz, M.; De Lorenzis, L.
Journal of the mechanics and physics of solids,
October 2021, 2021-10-00, 20211001, Letnik:
155
Journal Article
Recenzirano
Odprti dostop
We extend the model-free data-driven paradigm for rate-independent fracture mechanics proposed in Carrara et al. (2020), to rate-dependent fracture and sub-critical fatigue. The problem is formulated ...by combining the balance governing equations stemming from variational principles with a set of data points that encodes the fracture constitutive behavior of the material. The solution is found as the data point that best satisfies the meta-stability condition as given by the variational procedure and following a distance minimization approach based on closest-point-projection. The approach is tested on different setups adopting different types of rate-dependent fracture and fatigue models affected or not by white noise.
•A new model-free data-driven approach for rate-dependent fracture is presented.•The approach is also extended to fatigue.•Results of rate-independent fracture are encompassed as special case.•Examples using noisy or noiseless data imitating different models are presented.•Reference results adopting classical analytical models are correctly reproduced.
A method to consistently identify the thickness of the interfacial transition zone (ITZ) between aggregates and bulk hardened cement paste (HCP) is proposed. Three different criteria to identify the ...boundaries of the ITZ are established, all based on the analysis of the trend of the effective water-to-cement ratio (w/c) close to an aggregate surface. The method is conceived so as to keep at minimum the operator intervention, thus permitting an objective evaluation of the ITZ thickness. The proposed procedure is applied to a series of ordinary Portland cement samples with different w/c ratios obtained using the code CEMHYD3D. The obtained results and the effects of the presence of the ITZ on the unhydrated and hydrated microstructures are analyzed and discussed.
Obtaining the mesostructure of concrete from X-ray computed tomography (CT) requires segmentation of the data into distinct phases, a process complicated by the limited contrast between aggregates ...and mortar matrix. This paper explores the possibility to add baryte or hematite into the concrete mixture to enhance the contrast between cement paste and aggregates in CT, thus allowing for a semi-automatic segmentation. Raw and segmented CT images of plain and modified concrete mixtures are obtained and compared to assess the validity of the proposed approach. Characterization tests are also performed in order to ensure that the concrete characteristics are not appreciably affected by the presence of the enhancers.
Aim
Routine care of low birthweight (LBW) neonates relies on incubators for stabilisation. An earlier study suggested that skin‐to‐skin contact achieves better physiological stability in the ...transition period when compared to incubator care. The aim of this study was to replicate that study with a larger sample.
Methods
A randomised controlled trial with LBW infants (1500–2500 g) randomised at birth, 50 to routine care and 50 to skin‐to‐skin contact, with stabilisation using the Stability of Cardio‐Respiratory system in Preterms (SCRIP) score measured repeatedly over the first six hours of life as the primary outcome.
Results
Newly born infants in skin‐to‐skin contact showed better transition to extra‐uterine life (p < 0.02), with the SCRIP score at 360 minutes in skin‐to‐skin contact being 5.82 (SD 0.66) and in maternal infant separation 5.24 (SD 0.72), p < 0.0001. In extended skin‐to‐skin contact care, infants had significantly less need for respiratory support, intravenous fluids and antibiotic use during the remainder of the hospital stay.
Conclusion
Skin‐to‐skin contact was likely to be an optimal environment for neonates without life‐threatening conditions who weighed 1500–2500 g at birth. By preventing instability that requires subsequent medical treatment, it may be life‐saving in low‐income countries.
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