SOLID-SHELL FINITE ELEMENT MODELS FOR EXPLICIT SIMULATIONS OF CRACK PROPAGATION IN THIN STRUCTURES

Crack propagation in thin shell structures due to cutting is conveniently simulated using explicit finite element approaches, in view of the high nonlinearity of the problem. Solidshell elements are usually preferred for the discretization in the presence of complex material behavior and degradation phenomena such as delamination, since they allow for a correct representation of the thickness geometry. However, in solid-shell elements the small thickness leads to a very high maximum eigenfrequency, which imply very small stable time-steps. A new selective mass scaling technique is proposed to increase the time-step size without affecting accuracy. New ”directional” cohesive interface elements are used in conjunction with selective mass scaling to account for the interaction with a sharp blade in cutting processes of thin ductile shells

Static Upper/Lower Thrust and Kinematic Work Balance Stationarity for Least-Thickness Circular Masonry Arch Optimization

This paper re-considers a recent analysis on the so-called Couplet–Heyman problem of least-thickness circular masonry arch structural form optimization and provides complementary and novel information and perspectives, specifically in terms of the optimization problem, and its implications in the general understanding of the Mechanics (statics) of masonry arches. First, typical underlying solutions are independently re-derived, by a static upper/lower horizontal thrust and a kinematic work balance, stationary approaches, based on a complete analytical treatment; then, illustrated and commented. Subsequently, a separate numerical validation treatment is developed, by the deployment of an original recursive solution strategy, the adoption of a discontinuous deformation analysis simulation tool and the operation of a new self-implemented Complementarity Problem/Mathematical Programming formulation, with a full matching of the achieved results, on all the arch characteristics in the critical condition of minimum thickness

Analytical and numerical DDA analysis on the collapse mode of circular masonry arches

The purely-rotational collapse mode of circular masonry arches is investigated, through the guideline of new analytical solutions, by a Discontinuous Deformation Analysis (DDA) numerical tool. The so-called Couplet–Heyman problem, of finding the minimum thickness of a circular masonry arch with general angle of embrace standing under self-weight, is addressed, both analytically and numerically. The main scope of the study is assessing the validity of different analytical solutions that can be derived for the problem. Starting from classical Heyman’s solution, different recently-found solutions based on the true line of thrust (locus of pressure points) are first independently re-derived. Then, multiple experiments on discretised arches are performed, which show that the numerical results are in very good agreement with theory

Static shakedown theorems in piecewise linearized poroplasticity

A fully saturated two-phase solid or structure subjected to variable, in particular cyclic, external actions is described as a nonhardening poroelastoplastic material with piecewise linearized yield loci. With reference to a multifield finite element model, sufficient and necessary conditions for shakedown are established by the static Melan's approach. Shakedown analysis by linear programming is briefly discussed

Non-linear programming numerical formulation to acquire limit self-standing conditions of circular masonry arches accounting for limited friction

The modern rational limit analysis (LA) of masonry arches typically takes off from classical three Heyman hypotheses, a main one of them assuming that sliding failure shall not occur, like linked to unbounded friction. This allows for the computation of the least thickness of circular masonry arches under self-weight (Couplet-Heyman problem) and of the associated purely rotational collapse mode, by different analytical and numerical approaches. The aim of this work is to further investigate the collapse of circular masonry arches in the presence of limited friction. Here, the normality of the flow rule may no longer apply and the whole LA analysis shall be revisited. A new computational methodology based on nonlinear programming is set forward, toward investigating all possible collapse states, by jointly looking at admissible equilibrium configurations and associated kinematic compatibility, in the spirit of the ‘uniqueness’ theorem of LA. Critical values of friction coefficient are highlighted, marking the transitions of the arising collapse modes, possibly involving sliding. Uniqueness of critical arch thickness is still revealed, for symmetric arches of variable opening, at any given supercritical friction coefficient allowing the arch to withstand, despite the visible role of friction in shifting the final appearance of the collapse mode

Assessment of residual stresses and mechanical characterization of materials by "hole drilling" and indentation tests combined and by inverse analysis

Hole Drilling (HD) tests are frequently employed as “quasi-non-destructive” experiments, for assessments of residual stresses in metallic components of power plants and of other industrial structures. With respect to the present broadly standardized HD method, the following methodological developments are proposed and computationally validated in this paper: assessments of elastic and plastic parameters by indentation exploiting the hole generated by HD tests; employment of “Digital Image Correlation” (DIC) for full-field displacement measurements, instead of the strain measurements by gauge “rosettes” usually adopted so far; transitions from experimental data to sought parameters by inverse analyses based on computer simulations of both tests and on minimizations of a “discrepancy function”. Interactions between the two experiments are here investigated, besides the elastic parameters transition from indentation (IND) to HD test interpretation. The main advantage achievable by the procedure proposed herein is reduction of additional “damage” and cost due to usual experimental procedures for diagnosis of structural components (e.g. frequently adopted “small punch” experiments or laboratory tension tests)

Fundamentals of direct methods in poroplasticity

A nonlinear initial-boundary-value coupled problem, central to poroplasticity, is formulated under the hypotheses of small deformations, quasi-static regime, full saturation, linear Darcy diffusion law and piecewise-linearized stable and hardening poroplastic material model. After a preliminary nonconventional multifield (mixed) finite element modelling, shakedown and upper bound theorems are presented and discussed, numerically tested and applied to dam engineering situations using commercial linear and quadratic programming solvers. Limitations of the presented methodology and future prospects are discussed in the conclusions