Stick-slip Tearing Research Articles

A MICROSCOPY INVESTIGATION OF RUBBER COMPOUND CRACK PRECURSORS AND TENSILE FRACTURE SURFACES

ABSTRACT Tensile stress–strain testing is used to investigate the fracture behavior of carbon black–reinforced styrene–butadiene rubber, using 50 replicate specimens. Four vulcanized rubber compounds are studied: a CB-filled SBR with standard mixing conditions (control), the same formulation with intentional poor mixing of the CB, and materials identical to the control material but formed by adding minor amounts of 0.5-mm-diameter glass microspheres (beads)—serving as large model defects/inclusions—using a two-roll mill at two levels, corresponding to average values of 0.78 and 6.24 beads per gauge section region of the tensile test specimen. Microscopy analysis of the resulting fracture surfaces was conducted to complement our recent publication on Weibull failure statistics for distributions of tensile strength and crack precursor size. All 200 fractured specimens from tensile testing at 23°C were imaged with light microscopy and exhibited fracture surfaces characterized by relatively smooth planes perpendicular to the uniaxial loading direction. Most tensile failures originated from the edges of the dumbbell specimens, in line with expectations from fracture mechanics. Light microscopy revealed concentric fracture ring features of high specular reflectance emanating from crack precursors, which are a universal feature of the failure process for these compounds and independent of precursor type, size, or location. Noncontact interferometric microscopy confirmed that the rings resulted from variations in surface micro-roughness, proceeding outward from the precursor as rough–smooth–rough to the edge of the fracture surface. Fracture rings were also observed for tensile tests performed at 80°C. The variation in surface roughness of the fracture surface has parallels to the stick–slip tearing behavior seen for rubbers torn at medium to high rates. To the best of the authors’ knowledge, this is the first time that such striking features have been reported.

Metal-Coordination Complexes Mediated Physical Hydrogels with High Toughness, Stick–Slip Tearing Behavior, and Good Processability

It is challenging to develop hydrogels with a combination of excellent mechanical properties, versatile functions, and good processability. Here we report a physical hydrogel of poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) cross-linked by carboxyl–Fe3+ coordination complexes that possesses high stiffness and toughness, fatigue resistance, and stimulation-triggered healing along with shape memory and processing abilities. The copolymers have randomly dispersed AAm and AAc repeat units, making the physical cross-links with different strength. The strong coordination bonds and their associations serve as permanent cross-links, imparting the elasticity, whereas the weak ones reversibly rupture and re-form, dissipating the energy. Furthermore, a stick–slip instability is observed during the tearing test, which should be associated with the specific nature of metal-coordination bonds that are strong yet fragile. Because of the dynamic nature of coordination bonds, both tensile and tearing mechanical propert...

Influence of nonuniform network on mechanical properties of nano-silica reinforced silicone rubber

Nonuniform networks were introduced into the nano-silica reinforced silicone rubbers, by co-cross-linking of silicone gum containing 0.05 mol% vinyl ( GL) and gum containing 3 mol% vinyl ( GH), so as to improve the mechanical properties. Their network characteristics were investigated by nuclear magnetic resonance measurement, swelling experiment, dynamic mechanical analysis, and rubber process analysis. Nonuniform networks displayed high tear strengths up to 41 kN m−1 and improved tensile strengths and elongations at break. Nonuniform networks with 7–10 phr of GH had optimal mechanical properties, since densely cross-linked domains scattered in base network and imparted maximum heterogeneity. In this structure, dense domains favored high modulus, while dominated long chains contributed to large extensibility. Hence, such structure could display large elongation before fracture while showing twice modulus upturns and stick-slip tearing characteristics, which were demonstrated by Mooney–Rivlin curves, tearing curves, and scanning electron microscopic images.

Elastic–viscous transition in tear fracture of rubbers

There is a widely reported transition in the relationship between the critical strain energy release rate and a critical crack growth rate during the tearing of rubber materials. This transition is explained here for the first time in terms of an elastic–viscous transition phenomenon. The transition thus characterized depends on the balance of elasticity and viscosity of the material and hence the proximity of the test to the materials' glass transition temperature. Therefore two factors dominate the transition behavior, the cross-link density and the visco-elastic energy dissipation. A new elastic–viscous transition diagram is introduced to elucidate the mechanism of this phenomenon, where the diagram consists of three zones, each with a different fracture mode. These are an elastic–brittle fracture zone, a viscous–ductile fracture zone and an intermediate transition zone between the elastic and viscous zones. The transition zone characterized by stick–slip motion is caused in mechanics as results from unstable fluctuations of crack growth rate due to the energy dissipation near the glass transition temperature. The mechanism for the transition to be generated at the crack front is investigated both theoretically and experimentally with a consideration of surface roughness formation and stick–slip tear motion together with frictional sliding of the rubber at the crack tip.

The effect of temperature on the tearing of rubber

The tearing behaviour of rubber is typically characterised using an energy balance approach where the rubber has a geometrically independent relationship between the crack growth rate and the strain energy release rate. This approach works well during steady tearing, such as that encountered with an unfilled, non-strain crystallising rubber such as styrene butadiene rubber. The rate of tearing is both easy to measure experimentally and to then interpret. However, this approach is made more complicated under conditions where the rubber exhibits unsteady crack growth such as stick slip tearing or knotty tearing, which is often encountered with a strain crystallising rubber such as natural rubber or when reinforcing fillers are used. Under these conditions it is not easy to measure the crack growth rate as the value adopted is usually the average of a very rapid tearing rate and a zero velocity tearing rate. The approach adopted in this study characterises the rate of increase in the strain energy with time, T˙, at the crack tip just before the onset of tearing. The relation between this and the critical strain energy release rate T* for the crack propagation is examined over a wide range of tear rates and temperatures for several different rubber materials, including natural rubber (NR), epoxidized natural rubber with half of the isoprene units being epoxidized (ENR-50) and polybutadiene rubber (BR). It is apparent that the critical strain energy release rate, T* is surprisingly independent of the rate of strain at the crack tip for the majority of materials and over a wide range of temperatures. Only when tearing has become very rapid do additional viscoelastic mechanisms result in significant toughening of the rubbers.

The effect of the rate of strain on tearing in rubber

There have been many previous studies on the tearing of rubber materials. The behaviour is typically characterised using a fracture mechanics approach whereby the rubber has a geometrically independent relationship between crack growth rate during tearing versus strain energy release rate. This approach works well under conditions of steady tearing as the crack growth rate is easy to measure. However, this approach is much harder to interpret under conditions where the rubber exhibits discontinuous crack growth behaviour such as knotty tearing or stick slip tearing. Unfortunately, these are the most common tear conditions observed in practice for filled rubbers as well as for some unfilled rubbers, especially those such as natural rubber that are capable of strain induced crystallisation. Under these conditions it is not clear what the actual crack growth rate is as the value typically given results from the average of a very rapid tear rate and a zero velocity tear rate. The approach developed here for the first time is to characterise the rate of increase in the strain energy at the crack tip just immediately before the onset of tearing in a trouser tear test piece. This is measured directly as the time derivative of the strain energy release rate T • . In this work the relation between this and the critical strain energy release rate T crit to propagate the tear is examined. Having developed a new technique to characterise the tear behaviour, it is then evaluated using a range of different materials, including both natural rubber (NR) and styrene butadiene rubber (SBR). This allows the effect of the strain induced crystallisation observed in NR to be evaluated. The level of cross linking is varied to modify the kinetics of the strain induced crystal formation, and carbon black is incorporated into some of the NR and SBR compounds. The new technique is seen to provide a useful way of examining the different reinforcing mechanisms exhibited by all these different rubber materials.

Tearing energies for in-situ reinforced poly(dimethylsiloxane) networks

Tetraethoxysilane was used to end-link functionally-terminated poly(dimethyl-siloxane) chains, and then to serve as the source of reinforcing silica particles upon its hydrolysis and condensation within the elastomeric networks. These in-situ filled elastomers were characterized using transmission electron microscopy, thermogravimetric analysis, and tearing energy measurements. The electron micrographs showed the presence of well-dispersed, unagglomerated silica particles with average diameters rather narrowly distributed over the range 150–250 Å. These reinforcing particles were found to improve the thermal properties of the networks. Specifically, the larger the amount of filler thus incorporated, the higher the onset degradation temperature, and the larger the weight residue at 800°C. Effects were also found with regard to the tearing measurements, with the filled materials showing unstable tearing (stick-slip tearing), particularly at higher tearing rates. In contrast, the unfilled materials showed steady tearing. The tearing energies of the filled materials were significantly larger than those of the unfilled material. A marked increase in the tear strength occurred upon increasing the silica content up to 10 wt.%, followed by smaller improvements with additional increases in silica content.

Prevention of Stick-slip Tearing of a Duralcan AA6061–matrix Composite During Extrusion

Prevention of Stick-slip Tearing of a Duralcan AA6061–matrix Composite During Extrusion

Fracture energies of styrene-butadiene-styrene block copolymers (I). Effects of rate, temperature, and casting solvent

Specimens of styrene-butadiene-styrene (SBS) block copolymers, Kraton D-1102, were prepared by solution casting using three different solvents: toluene, cyclohexane, and a mixture of tetrahydrofuran and methyl ethyl ketone (THF/MEK). Measurements of fracture energies of SBS specimens were carried out at various temperatures and rates using two methods (i.e., a conventional tear test and a recently developed cutting test). Effect of casting solvent on the fracture energy was investigated as well. It was found that stick-slip tearing dominates at low temperatures (−50 ∼20°C). Tear strength increased with decreasing temperature. Eventually, a value of 180 kJ/m2 was reached at −70°C, close to the glass transition temperature of polybutadiene phase. At temperatures higher than 20°C, however, steady tearing was found and the tear strength gradually decreased with increasing temperature. Tear strength was virtually zero at 100°C, above the glass transition temperature of polystyrene phase. Effect of temperature on tear strength is more pronounced than that of tearing rate. In contrast, the intrinsic strength of SBS block copolymers determined from cutting test remains unchanged, about 570 J/m2, over a wide range of temperatures and rates. Specimens cast from THF/MEK solution exhibit yielding phenomena when stretched. Moreover, they possess a relatively larger tear strength, compared to those cast from either toluene or cyclohexane solution. A more continuous polystyrene phase is expected to develop in THF/MEK as-cast specimens which is believed to account for the large tear strength. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2003–2015, 1997

Observations on the Tearing of Elastomers

Abstract 1. Frequency distributions of stick-slip tear arrest and initiation energies have different standard deviations; initiation values show greater variance. Tear arrest frequency distributions are symmetrical and have been described by a normal distribution. The tear-initiation distribution is skewed and has been modeled using both the extreme value and Weibull distributions. As a result, it is concluded that two separate mechanisms are responsible for the creation of the force minima and maxima observed in stick-slip tearing. Consequently, data reduction methods which average these values to estimate tear energy are inappropriate. 2. The direction of skewness of the tear-initiation energies indicates a distribution of the largest extreme. This has been contrasted with constant-rate tensile-rupture data which are distributed according to the smallest extreme. As a result, it is concluded that the process responsible for tear initiation is not a weakest link mechanism, but, rather, depends on a process which structurally inhibits tear propagation. A mechanism of anisotropic tear-tip reinforcement caused by large regional deformation has been discussed as a possible explanation. 3. Tear energies calculated from tear-initiation forces have been shown to be dependent upon the energy stored in simple tensile extension and the volume of material available to be extended. This type of behavior shows that the tear-initiation energy is perhaps a reasonable indicator of strength but does not meet the criterion for an inherent material property. Tear-arrest values have been shown to be independent of these features and are considered unbiased estimators of inherent material tear energies. 4. It has been shown that for experimental purposes, stick-slip tearing can be eliminated and tear-tip diameter can be held constant using a constrained trouser-tear test. This is in agreement with earlier work by Gent and Henry. It has also been shown that the constrained tear energy thus measured is an inherent feature of stick-slip tearing results, requiring only additional data reduction to extract. Consequently, the constrained tear energy can be estimated from stick-slip tearing, and conversely, the magnitude of stick-slip tearing is predictable from a constrained tear energy and simple tensile extension data.

Scanning electron microscopy studies on tensile, tear and abrasion failure of thermoplastic elastomers

The tensile strength, tear resistance and abrasion resistance of four different types of commercial thermoplastic elastomers have been studied and their fracture surfaces examined by scanning electron microscopy. Thermoplastic polyurethane (TPU) showed elastic deformation under tensile fracture, whereas in 1,2 polybutadiene (1,2 PB) the fracture was initiated by craze formation and propagated by tear failure. Styrene-isoprene-styrene block copolymer (K1107) showed ductile type failure whereas in styrene-butadiene-styrene block copolymer containing a higher proportion of styrene and silica filler (K5152), the fracture occurred by shearing action. The tear failure surfaces of the thermoplastic elastomers showed different fracture patterns which could be correlated with the tear strength of the materials. The tear fracture surface of 1,2 PB showed stick-slip tear lines and that of TPU had a broad tear path with vertical striations. The fracture surfaces of K5152 and K1107 had the characteristics of laminar tearing and uninterrupted continuous tearing processes, respectively. The abrasion resistance of the samples was in the order TPU>1,2 PB>K5152, which was manifested through the type of ridge patterns formed on the abraded surfaces. Abraded surfaces of TPU, 1,2 PB and K5152 showed closely spaced stable ridges, widely spaced ridges bridged by elongated fibrils and highly deformed ridges, respectively.

Scanning electron microscopy studies on tear fracture of natural rubber

Tear fracture surfaces of peroxide- and sulphur-cured natural rubber vulcanizates, both unfilled and filled, have been studied using scanning electron microscope. Peroxide-cured unfilled vulcanizate shows stick-slip tear whereas the HAF black-filled vulcanizate gives rise to a layered fracture surface. Tear branching, observed in the unfilled sulphur-cured vulcanizate changes to a high level of tear deviation resulting in a rough surface, with the addition of HAF black. FT black improves tear resistance only slightly and the fracture surface appears comparatively smooth. The low level of polymer-filler interaction in clay filled vulcanizates causes the filler agglomerates to come out of the matrix during the test thus decreasing tear resistance and giving rise to a pitted surface.