Theoretical limits in detachment strength for axisymmetric bi-material adhesives

Reversible dry adhesives rely on short-ranged intermolecular bonds, hence requiring a low elastic modulus to conform to the surface roughness of the adhered material. Under external loads, however, soft adhesives accumulate strain energy, which release drives the propagation of interfacial flaws prompting detachment. The tradeoff between the required compliance, for surface conformity, and the desire for a reduced energy release rate, for better strength, can be achieved with a bi-material adhesive having a soft tip and a rigid backing. This design strategy is widely observed in nature across multiple species. However, the detachment mechanisms of these adhesives are not completely understood and quantitative analysis of their adhesive strength is still missing. Based on linear elastic fracture mechanics, we analyze the strength of axisymmetric bi-material adhesives. We observed two main detachment mechanisms, namely (i) center crack propagation and (ii) edge crack propagation. If the soft tip is sufficiently thin, mechanism (i) dominates and provides stable crack propagation, thereby toughening the interface. We ultimately provide the maximum theoretical strength of these adhesives obtaining closed form estimation for an incompressible tip. In some cases, the maximum adhesive strength is independent of the crack size, rendering the interface flaw tolerant. We finally compare our prediction with experiments in the literature and observe good agreement.

Manufacturing large shafts by a novel flexible skew rolling process

When manufacturing large shafts with multi-specification and small-batch production, both the conventional forging and rolling process bring a high tooling cost due to heavy forging press or large-size specialized roller. In this study, a novel flexible skew rolling (FSR) process is proposed by adding degrees of freedom to the rollers as compared to the typical skew rolling process. Since each of the FSR rollers has three degrees of freedom (circle rotating, radial rotating and radial feeding), the FSR process can be divided into four stages: radial rolling, rollers inclining, skew rolling and rollers levelling. Therefore, the FSR process can produce various shafts with same rollers via programming different movements. To verify the feasibility of FSR process, a physical investigation corresponding with a numerical simulation for a single-step shaft is undertaken with a Φ80×390 mm C45 steel billet. According to the results from physical experiments and numerical simulations, the FSR formed shaft has a maximum deviation of 0.99 mm, and its microstructure and properties have been improved obviously. Moreover, although there is a tendency of center crack in FSR products as predicted by numerical results, both the transverse and longitudinal section of the physical shaft are free from central cracking. The major forming defects existed on the rolled shaft are knurled pockmarks, surface threads and side cavity, which are the typical defects of the conventional skew rolling and cross-wedge rolling and can be removed by machining. To the authors’ knowledge, this novel process has a good combination of flexible production and less loading, which will be of great engineering significance to reduce the tooling cost in large shafts manufacturing.

A semi-homogenized extended isogeometric analysis approach for fracture in functionally graded materials containing discontinuities

In this paper, we present a semi-homogenized extended isogeometric analysis approach for assessment of stress intensity factor of center cracked functionally graded domain containing discontinuities such as holes and inclusions. In the semi-homogenized approach, the presence of discontinuities is considered in the 30% region in the vicinity of crack, and the remaining part of the domain is modeled using equivalent homogenous material properties. The Heaviside and singular crack tip enrichment functions are employed to model the center crack, whereas crack topology is tracked using level set functions. The domain form of interaction integral approach is employed to estimate the stress intensity factors at the tips of the center crack. Several problems containing discontinuities in 30% portion of the domain are investigated using the semi-homogenized approach. The efficiency and accuracy of the developed method are examined by comparing the computed results with those attained by modeling discontinuities in the entire domain. The advantages of the semi-homogenized extended isogeometric analysis approach are highlighted in terms of higher computational efficiency.

Stochastic mixed mode stress intensity factor of center cracks FGM plates using XFEM

Extended finite element method (XFEM) and second-order perturbation technique (SOPT) were combinedly utilized using interaction integral (M-integral) through partition of unity method to find out the mean and variance of mixed mode stress intensity factor (MMSIF). Uncertain system parameters are considered in material properties, crack length, crack orientation, gradient coefficients in the present study. MMSIF in a numerical example with center crack is computed to validate the accuracy of the presented model. Finally, typical numerical results are presented to examine the different modulus ratios, crack angle, crack length, position of crack and tensile, shear and combined loadings with uncertain system properties on the MMSIF.

Effect Range of the Material Constraint-II. Interface Crack

The selection of fracture behaviors used in the structure integrity assessment has significant implications on the accuracy of the assessment. The effect range of the material constraint is an important factor which effects the fracture behaviors of structures and exists in the different kinds of welded joints with the center crack. However, for the material constraint induced by an interface crack, which also appears widely in the welded joints, it is not clear whether the effect range exists or not. The further study of the effect range of the material constraint for the welded joints with interface crack is meaningful. Thus, in this study, different basic models with interface crack were designed, the fracture behaviors of these basic models under different material constraints were calculated, and the effect range of the material constraint induced by interface crack were studied. This study about the interface crack and the previous study about the center crack provide an additional basis for an accurate structure integrity assessment.