Fracture of soft cellular solids—Case of non-crosslinked polyethylene foam

2019

Polym J

We study the dependence of the fracture surface energy on the pulling velocity for nano-porous polypropylene (PP) sheets to find two components: the static and dynamic ones. We show that these terms can be interpreted respectively as plastoelastic and viscoelastic components, as has been shown for soft polyethylene (PE) foams in a previous work. Considering significant differences in the pore size, volume fraction and Young's modulus of the present PP and previous PE sheets, the present results suggest a universal physical mechanism for fracture of porous polymer sheets. The simple physical interpretation emerging from the mechanism could be useful for developing tough polymers. Equivalence of Griffith's energy balance in fracture mechanics to a stress criterion is also discussed and demonstrated using the present experimental data.

Section: Introductioncontrasting

confidence: 73%

Visco- and plastoelastic fracture of nanoporous polymer sheets

Tomizawa

2019

Polym J

“…As a matter of fact, we confirmed, together with this independence of on d, eqs. (4), (12), and (13) directly in experimental studies on non-crosslinked polyethylene foam, 9) which will be discussed elsewhere. This is clearly the case of the two-dimensional network model as discussed in the above: we can make the bulk elasticity the same if we use the same number of original springs to make networks of the same size (i.e., if the volume fraction is the same).…”

Section: Discussionmentioning

confidence: 99%

“…6) In the case of cellular solids 7) which include wood, cork, plant parenchyma, stereom of sea cucumber, trabecular bone, carbon and polymeric foams and porous materials, theories based on the unit cell structure have been successful to reveal that fracture mechanical properties can be well understood as a function of the relative density (i.e., volume fraction of solid in foam) in particular for hard cellular solids; 7,8) this is confirmed recently even for very soft foams but with different scaling relations. 9) Further progress has been propelled through more inclusion of detailed structures into theory such as imperfection and randomness, which leads to computational modeling based on finite-element method. 10,11) Similar trend prevails in most of modern theoretical treatments of high performance composites, for example, nacre, 12) bone, 13) and carbon nanotube composite.…”

Section: Introductionmentioning

confidence: 99%

Crack-Tip Stress Concentration and Structure Size in Nonlinear Structured Materials

Aoyanagi

2009

J. Phys. Soc. Jpn.

We revisit the standard elastic-plastic fracture theory developed by Hutchinson, Rice, and Rosengren (HRR) and reproduce, by a simple scaling argument, the stress singularity around the crack tip derived by HRR. From the singular behavior thus reconfirmed, we propose a general scaling relation which guarantees an effect similar to the tip-blunting effect: the maximum stress at the crack tip in a structured material can be reduced by increasing the structure size. This proposed relation is explicitly confirmed by numerical calculations performed for a coarse-grained lattice model, and leads to general scaling relations for fracture surface energy and to a possible reinforcement of cellular solids due to the pores.

“…18,19 Recently, a very soft polyethylene foam with E around 1 MPa has been studied, and scaling laws different from those for hard porous materials were established. 20 Here, we study the fracture energy of similar soft foams with changing fracture rate in a wide range to surprisingly find that linear elastic fracture mechanics 21,22 works well for a fixed velocity in the wide range. The velocity-dependent fracture energy is shown to be composed of a static plastoelastic component and another dynamic viscoelastic component, with the latter scaling linearly with the rate.…”

mentioning

confidence: 94%

Fracture of Soft Foam Solids: Interplay of Visco- and Plasto-elasticity

Kashima

2014

ACS Macro Lett.

Fracture mechanical properties of a very soft solidified foam of polyethylene with Young's modulus about 1 MPa are studied by changing stretching velocity in a wide range, by using sheets of the material in order to suppress finite-size effects. Unexpectedly, we find that the fracture can be described well by linear elastic fracture mechanics for a given fracture rate in the wide range. This allows a direct determination of velocity-dependent fracture energy of the soft foam. As a result, we find that the fracture energy is composed of a static plastoelastic component and another dynamic viscoelastic component, elucidating a simple physical interpretation of each component and giving guiding principles useful for practical applications to reinforce industrial polymer materials. Furthermore, we introduce a finite stress criterion for fracture that is similar in spirit to the cohesive zone model and, using our data, demonstrate that this stress criterion is consistent with the Griffith's energy balance. I n nature and in daily life, there is a variety of cellular structures or foam solids, ranging from familiar ones, such as cork, balsa, bread, coral, and apples, 1 to exotic remarkable ones, such as the stereom of echinoderms, 2 the skeleton of a certain sponge, 3 and the frustules of diatoms. 4 Cellular solids are generally lightweight smart materials like spider webs 5,6 and support our life, for example, daily by their insulating or shock-absorbing features. Active studies on foam solids to date have revealed, e.g., (1) simple fracture mechanical properties that are well-understood as a function of the volume fraction of solid ϕ 7−9 and (2) advantages of the porous structure for reinforcement. 10,11 However, experimental studies are limited to hard cellular solids with Young's modulus E larger than 3000 MPa, such as poly(methyl acrylate) (PMA), rigid polyurethane (PU), and cellular glass. In addition, any velocity dependences of fracture properties of foam solids have never been discussed in a systematic way, while such an issue has received considerable attention for adhesive interface, 12−14 flexible laminates, 15 viscoelastic solids, 16,17 and weakly cross-linked gels. 18,19 Recently, a very soft polyethylene foam with E around 1 MPa has been studied, and scaling laws different from those for hard porous materials were established. 20 Here, we study the fracture energy of similar soft foams with changing fracture rate in a wide range to surprisingly find that linear elastic fracture mechanics 21,22 works well for a fixed velocity in the wide range. The velocity-dependent fracture energy is shown to be composed of a static plastoelastic component and another dynamic viscoelastic component, with the latter scaling linearly with the rate. The velocity dependence originates from that of a yield stress that is introduced through the opening distance at crack tips ( Figure 1). Furthermore, we demonstrate that the Griffith criterion can be regarded as a stress criterion: we find with our data that the Griffith's ene...