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Defect tolerance in soft materials
Full text linkAbstractThe ability of materials to withstand defects like cracks, notches or generic geometric discontinuities, is usually indicated as flaw tolerance, and is a crucial aspect of the safety assessment of structural components. Flaw tolerance in soft materials can be substantially different from that in traditional ones. As a matter of fact, the capacity of highly deformable materials to undergo large deformations with a significant rearrangement of the molecular network at the miscroscale in highly stressed regions can enhance such an ability, leading to an erroneous underestimation of their safety level against defect-driven failure, if traditional methods of analysis are employed. In the present research work, the mechanics of highly deformable notched plates is considered from the fail-safety point-of-view. Experimental, numerical and theoretical remarks are made in order to explain the mechanism of defect resistance in such a class of materials from a physically-based point-of-view
Defect sensitivity to failure of highly deformable polymeric materials
No full textThe capability of materials to bear loads even in presence of defects like cracks, notches or generic geometric
discontinuities is usually indicated as flaw tolerance, and is crucial in modern safety design of
structural components. Such a tolerance capability can be remarkable in highly deformable materials
(also called soft materials), usually much more pronounced than in conventional ones. The ability of
highly deformable materials to undergo very large deformations before failure is mainly due to their
noticeable rearrangement of the molecular network with a significant decrease of the internal entropic
state. Neglecting such an entropic effect can lead to erroneous underestimations of the safety level
against defect-driven failure. In the present research, the mechanics of highly deformable materials is discussed
by examining silicone-based notched and cracked plates. Experimental, numerical and theoretical
aspects of the involved phenomena are analyzed in order to provide an explanation of the mechanism of
defect resistance in such a class of materials from a physics-based point-of-view
A fracture mechanics based model for the analysis of seal effectiveness
No full textThe fluid containment in vessels, pipes, containers, etc. often requires the use of seals in order to assure the absence of leak in junction zones. Sealing mechanism is typically achieved through the use of elastomeric elements that form contact with the surrounding rigid materials the containers are made of. A proper design and safety evaluation of the containment capacity of seals requires the careful evaluation of the contact pressure distribution between the soft (seal) and hard (vessel) elements. In the present paper such a problem is considered and solved through contact stress and strain evaluation based on fracture mechanics; numerical and experimental analyses on elastomeric elements are considered in order to verify the proposed modeling procedure. It is shown that the desired safety level against leakage can be ensured on the basis of the classical fracture mechanics parameters when the seal crack tip exists, or through contact strain assessment when the stress singularity vanishes. Such results can be useful in the design of seal shapes and for estimating the pressure to be applied to the sealed bodies in order to guarantee no leaks. Finally, some final relevant conclusions on the present study on leak containment are drawn
Crack paths in soft thin sheets
Full text linkHighly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature as well as in technology. The understanding of their flaw sensitivity is crucial to ensure a desired safety level. Fracture failure in soft materials usually occurs after the development of an uncommon crack path because of the non-classical near-tip stress field and the viscous effects. In a neo-Hookean material, the true opening stress singularity along the crack path (evaluated normal to the crack line) is of the order , while it is of the order ahead of the crack tip, promoting the appearance of a crack tip splitting leading to a tortuous crack. In the present paper, experimental tests concerning the fracture behavior of highly deformable thin sheets under tension are discussed, and the observed crack paths are interpreted according to the crack tip stress field arising for large deformations. The study reveals that higher strain rates facilitate the development of a simple Mode I crack path, while lower strain rates induce a mixed Mode in the first crack propagation stage, leading to the formation of new crack tips. The above described behavior seems to not be affected by the initial crack size
Fracture toughness of highly deformable polymeric materials
No full textA fundamental requirement for safety design of structural components is flaw tolerance. In this field, the soft materials have a unique ability to bear external loads despite the presence of defects, due to their pronounced deformability. Unlike traditional materials, which have an enthalpic elasticity, the mechanical response of a polymer-based material is governed by the state of internal entropy of a molecular network which has a great ability to rearrange the material structure and shape so to minimize the local detrimental effect of flaws. For a correct estimation of the fracture toughness of these materials, a proper knowledge of this entropic effect is needed. In the present research, the mechanical behaviour up to failure of silicone-based cracked plates is examined by taking into account the time-dependent effects. Experimental and theoretical aspects are discussed in order to understand the defect tolerance of such materials
RATE-DEPENDENT FAILURE MECHANISM OF ELASTOMERS
No full textElastomers display a mechanical behavior that is, both in the elastic as well as at the incipient failure, quite
different from that of traditional materials. Their mechanical characteristics makes them attractive to a myriad
of applications ranging from rubber, optical lenses to tissue engineering scaffolds. Their study is therefore
fundamental for understanding and controlling their mechanical response, especially when it involves large deformation,
viscous effects and damage nucleation around defects. In the present paper we consider the mechanical
response up to final failure, of pre-cracked silicone sheets under different strain rates. A simple statistically-based
theoretical model, combined with a failure criterion, is formulated to describe the observed complex mechanical
response. The model is further implemented in a finite element model to provide comparison of the damage
nucleation predicted by the model and those obtained from experimental tests
Defect tolerance at various strain rates in elastomeric materials: An experimental investigation
No full textElastomeric polymers usually show a very low elastic modulus entailing a soft behaviour
and a very high deformation capability. This macroscopic behaviour is determined by their
microstructure which is characterized by a complex and entangled network of very long
linear chains jointed together in some points called cross-links. Because of such a peculiar
microstructure, the mechanical response (often time-dependent) of elastomeric polymers
to external loads or deformations cannot be described by the classical theories developed
for standard materials – such as the ones whose response depends on their crystalline
structure (e.g. metal, ceramics, etc.) – since such polymers usually neglect entropicrelated
effects that are fundamental in highly deformable amorphous materials. In the present
paper, the response of elastomeric plates containing a central crack under a tensile
strain applied at various rates is experimentally analyzed giving particular emphasis on
the observations of the full-field strain maps determined by means of some digital image
correlation techniques. Some relaxation tests are performed, and the defect sensitivity of
the material is discussed in relation to the applied deformation rate
Notch effect in highly deformable material sheets
No full textDefect tolerance is usually understood as the ability of a material to withstand an external load in the presence of a geometrical flaw. The case of a defect represented by a notch (i.e. a geometric discontinuity with finite curvature radius) can be described by the so-called stress concentration factor at the notch root and by the stress gradient in the vicinity of the notch root. Under static loads and within the elastic regime, notch effect in traditional structural materials is simply governed by the initial notch geometry. On the other hand, in highly deformable materials, such as soft matters (biological tissues, colloids, polymers, gels, foams, etc.), notch effect must be evaluated by considering large strain values arising around the notch, responsible for its blunting. In addition, when notches are contained in non-confined plate-like components, a sort of augmented notch blunting might occur as a consequence of local flexural instability of the material plate in the compressed zones. In the present paper, an experimental and theoretical study is discussed for a silicon sheet with different levels of notch severity
Defect sensitivity of highly deformable polymeric materials with different intrinsic qualities at various strain rates
No full textHighly deformable materials, such as elastomers and gels, can withstand very large deformation without failure, and this response is usually insensitive to the presence of macroscopic defects. These polymer-based materials, different from the traditional ones which are usually characterized by an enthalpic elasticity, show a mechanical response which is governed by the state of internal entropy of their molecular network. If fracture energy is large, the noticeable ability of soft materials to rearrange their network at the microscale, to display large deformation and to dissipate energy thanks to their viscoelasticity, allows the minimization of the local detrimental effect of existing flaws. In the present paper, the mechanical behavior of silicone-based edge cracked plates with different crack sizes and severity of the intrinsic flaws embedded in the material is examined by taking into account the time-dependent effects. Experimental and theoretical aspects are discussed to explain the defect tolerance of such materials. The detrimental effect of intrinsic voids is quantified and the beneficial effect due to strain at low rates is analysed. The critical distance is related to the ultimate stretch value, the quality of the material and the crack size
Strain Field Self-Diagnostic Poly(dimethylsiloxane) Elastomers
Full text linkAdvanced applications, involving high risk mechanical systems, require to verify the in-service deformation level in
order to assess their safety and reliability, providing information for repairing or replacing interventions. In the present work, a selfdiagnostic
PDMS elastomer containing a supramolecular detection probe is proposed, enabling to identify the strain intensity in the
polymeric matrix by fluorescence detection. Turn-on fluorescence represents an efficient, sensitive, simple and real time diagnostic
tool to quantitatively detect high-strain regions for the mechanical monitoring of structural elements. The supramolecular complex
– cross-linking the polymer’s chains – provides fluorescence response induced by strain even if present in a very low amount (10-6
mol kg-1), so completely preserving the mechanical characteristics of the matrix. The developed PDMS material is mechanically
tested and the observed fluorescence field is correlated with that obtained by numerical simulations as well as by contactless measurements
performed via the digital image correlation technique (DIC)
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