Shear Strain Hardening Research Articles

Nonlinear shear rheology of single and double dynamics metal-ligand networks

We present a systematic experimental study of the shear rheology of metallosupramolecular assemblies based on entangled telechelic star polymers comprising one (single dynamic network) or two (double dynamics network) types of physical bonds with the aim to unravel the role of concentration and strength of these bonds on the nonlinear response. Model dynamic networks functionalized with terpyridine ligands were formed by adding different metal ions with increasing bonding strength, zinc, copper, and cobalt. The dynamics are driven by entanglement/disentanglement processes and a ligand exchange mechanism. Steady-state viscosities of single and double dynamics networks collapse onto a universal curve over a wide range of Weissenberg numbers based on terminal time (up to about 300 for single and 1000 for double), exhibiting stronger shear thinning (with an exponent of −0.76) compared to entangled neutral star polymers. Double dynamics networks consisting of two different metal ions (with different lifetimes) exhibit stronger mechanical coherence (rate-dependent fractional viscosity overshoot) and accumulate larger strain at steady-state flow compared to single-ion counterparts. The shear stress growth function signals exhibit weak, albeit unambiguous shear strain hardening, which becomes more pronounced for stronger associations. They also exhibit double overshoot, which reflects the interplay of association strength and chain deformation. Increasing the strength of associations leads to the failure of the Cox–Merz rule, which is more severe for single dynamic networks. The markedly different behavior of double dynamics networks is attributed to the fact that at sufficiently high ion content, the weaker bond acts as a sacrificial component, which provides local energy dissipation and enhances the overall deformability. This bears analogies with their linear viscoelastic response, which has revealed that the arm disentanglement (delayed due to the reversible bonds) effectively interpolates between the two single dynamic network components, depending on composition. Our results suggest ways to tailor the mechanical properties of this class of materials by judicious choice of the type and content of the ion.

Effects of entanglement and dispersity on shear strain hardening

Polystyrene solutions are the widely used model system to study polymer dynamics and rheology. In the strong nonlinear region, we shear polystyrene solutions of uniform or bimodal distribution up to unprecedented high rates. The unique shear strain hardening takes place on such solutions with high T g solute, irrespective of dispersity. Non-Gaussian stretching of chains is suggested by full recovery even after a shear strain of 60. Shear strain hardening in uniform solutions happens at a universal Rouse-Weissenberg number, 150, regardless of the number of entanglements. The critical stress between traditional stress overshoot and hardening is around 10 4 Pa, for 10% solutions regardless of dispersity or number of entanglements. This systematic study charts the parameters for shear strain hardening. • Systmatic study of shear strain hardening in solution of uniform and bimodal distributed solutions. • A universal Rouse-Weissenberg number for the onset of hardening is identified and the critical stress is 10000 Pa. • Full recovery after a shear strain of 60 demonstrates its elastic origin: non-Gaussian stretching.

Block shear strength in thin-walled lean duplex stainless steel fillet welded connection with base metal fracture

This paper investigates the accuracy of different design equations given in design specifications and literatures in determining the block shear strength of the lean duplex stainless steel fillet welded connections with base metal fracture. It points out the problem inherent in the block shear provision specified in the North American cold-formed design specification, which does not consider shear strain hardening. The previous test results have found that full shear strain hardening can be assumed for the carbon steel and ferritic stainless steel welded connections, but not applicable to the thin-walled lean duplex stainless steel welded connections with a higher ratio of the tensile strength to yield stress. Based on the results of finite element analysis including ductile damage simulation, this study reveals that the block shear strength of the lean duplex stainless steel specimens can be determined accurately using the mean between the shear yield stress and the shear ultimate stress. This paper also investigates the effect of stress triaxiality incorporated in the block shear equations found in the literature through comparative analysis with the bolted connection. The effect is shown to be negligible for the thin-walled lean duplex stainless steel welded connection, although higher stress triaxiality is developed in welded connection than in bolted connection. This paper recommends a block shear equation assuming the normal tensile stress in the net area in conjunction with partial shear strain hardening. The recommended equation was found to be more accurate than other equations for determining the block shear strength of the thin-walled lean duplex stainless steel welded connections.

Material ductility and temperature effects on block shear capacity of bolted connections

The block shear equation in the current Australasian and amended North American cold-formed steel design specifications is based on shear yielding without strain hardening, while that in the new Aluminum Design Manual assumes full shear strain hardening. This study investigates the range of applicability for both assumptions through experimental tests involving ultra-high strength steel bolted connections at room (ambient) and elevated temperatures, where the levels of material ductility are vastly different from each other. The experimental program has found that the block shear capacities of the ultra-high strength steel bolted connections at room temperature can be determined accurately by neglecting shear strain hardening, but full hardening can be assumed for high strength and mild steel connections. Surprisingly, at temperatures above 300 °C, even the assumption of full shear strain hardening for single-row bolted connections is too conservative, by up to 50%. Finite element analysis is used to show that the catenary effect increases with increasing temperature, as does the extent of shear strain hardening. Another finding is that a conventional block shear failure may occur by simultaneous tensile and shear ruptures, if the material ductility is low enough (rather than high enough as believed in the literature). Contrary to intuition, the block shear capacity of a bolted connection failing by simultaneous tensile and shear ruptures has to be computed using the shear yield stress rather than the shear ultimate stress.

Dendronized Polymers with Ureidopyrimidinone Groups: An Efficient Strategy To Tailor Intermolecular Interactions, Rheology, and Fracture

A library of poly(methyl methacrylate)-based dendronized polymers with generation numbers g = 1–3 was prepared, which contain different degrees of dendritic substitution (0–50%) with strongly hydrogen bonding 2-ureido-4[1H]pyrimidinone (UPy) moieties at their respective g = 1 levels. Our rheological and thermal studies demonstrate that the strong intermolecular UPy interactions are suppressed for g = 2 and essentially eliminated for g = 3. Focusing on samples with short backbone degrees of polymerization (Pn ≈ 40), the linear viscoelastic response alters from liquid-like in the absence of UPy to gel-like with ever increasing moduli as the UPy content increases. Nonlinear rheological measurements indicate a transition from ductile to brittle behavior and, in parallel, a transition from shear strain thinning to shear strain hardening. This unique behavior makes UPy-DPs promising candidates for the design of new functional materials.

Investigation on size effect of specific cutting energy in mechanical micro-cutting

Mechanical micro-cutting is one of advanced processes to manufacture the miniature parts. The uncut chip thickness is one of the two important parameters in mechanical cutting. The uncut chip thickness and the cutting edge radius are at the same length scale, and the cutting tool extrudes and shears the workpiece with the round cutting edge during micro-cutting process. Size effect of specific cutting energy in micro-cutting is significant in comparison with that in macro cutting. A slip-field model is proposed in this paper to analyze the deformation process of workpiece material in micro-cutting. Johnson–Cook constitutive model, which includes shear strain hardening, shear strain rate hardening, shear temperature softening, and plasticity strain gradient (PSG) effects, is applied to the calculation of the shear flow stress at the primary shear zone (PSZ). Predicted cutting force using the shear flow stress is validated by the micro-orthogonal cutting experiments. The specific cutting energy is also calculated. It shows that the size effect on specific cutting energy in micro-cutting is influenced greatly by the shear strain hardening, shear strain rate hardening, shear temperature softening, and relative cutting length due to the ratio of uncut chip thickness to cutting edge radius. The plasticity strain gradient has effect on the specific cutting energy when the uncut chip thickness is smaller than 25 μm for micro-orthogonal cutting AISI 1045 steel.

Experimental investigation on the effect of operating speeds on wear and rolling contact fatigue damage of wheel materials

The objective of this study is to explore the wear and rolling contact fatigue (RCF) characteristics of wheel materials under different operating speeds conditions using a small scale wheel/rail facility. The results indicate that the operating speed plays a vital role in the wear and RCF of wheel materials. As the operating speed increases, both the surface hardness and the hardness of the plastic flow layer on the wheel roller decrease while the worn surfaces become rougher. Shear strain hardening of wheel roller decreases with increasing operating speed. Furthermore, the primary damage mechanism of the wheel roller transforms from spalling and slight surface fatigue to severe fatigue cracks as the operating speed increases. Fatigue cracks initiate from the wear surface and grow along with the soft ferrite lines within the plastically deformation layer. RCF damage become more severe and the wear rate of the wheel roller decreases as operating speed increases. Low wear rate can not adequately remove cracks and leads to a visible increase in the angle of fatigue crack propagation and the crack depth. Furthermore, the wear debris is composed of metallic flakes whose primary compositions are iron, Fe2O3, and Fe3O4 and the size of wear debris decreases, but the thickness increases as operating speed increases.

Block shear failure planes of bolted connections — Direct experimental verifications

This paper presents direct experimental verifications of the active shear planes in bolted connections, previously identified by the first author for determining the block shear capacity. The laboratory test results were obtained by independent researchers for specimens where the applied loads were resisted by the “block” in shear only. The first set consists of five bolted connection specimens in the webs of wide flange sections where the tensile resistance planes had been sawn off. The second set consists of ten bolted connection specimens each in one leg of an angle section that had fractured completely along the net tensile plane through a block shear failure. Comparisons among the gross, net, and active shear planes against the independent laboratory test results showed that the critical shear planes of bolted connections were best represented by the active shear planes rather than either the gross or the net shear planes. It is also pointed out that full or almost full shear strain hardening was generally achieved at the ultimate limit state of block shear failure of bolted connections in hot-rolled steel plates or sections, irrespective of the connection length. Verification against independent laboratory test results of tee sections bolted at the web reinforces this point.

HARDENING INTERACTION IN CYCLIC CREEP OF A TYPE 316 STAINLESS STEEL AT 20°C AND 500°C

Abstract— The development of anisotropic material properties due to torsional cyclic plastic straining of tubes with sustained axial loads was examined for a type 316 stainless steel at room temperature and at 500°C. The effect of the cyclic strains and the cumulative ratchetting strains on the axial tensile properties was determined and the results show a significant increase in the tensile strength at both 20°C and 500°C, with more pronounced hardening at the higher temperature. The cyclic shear stress‐strain response of the material is shown to be extremely temperature dependent and the hardening ratio is much greater at 500°C, which is consistent with the dynamic strain ageing observed previously for this material. The ratchetting strains are controlled by the cyclic shear strain hardening, by the axial hardening resulting from the cyclic shear and the cumulative axial strains, and by the ratio of the secondary shear stress to the primary axial stress.