Forcing the issue: testing gecko-inspired adhesives

Materials are traditionally tested either by imposing controlled displacements and measuring the corresponding forces, or by imposing controlled forces. The first of these approaches is more common because it is straightforward to control the displacements of a stiff apparatus and, if the material suddenly fails, little energy is released. However, when testing gecko-inspired adhesives, an applied force paradigm is closer to how the adhesives are loaded in practice. Moreover, we demonstrate that the controlled displacement paradigm can lead to artefacts in the assumed behaviour unless the imposed loading trajectory precisely matches the deflections that would occur in applications. We present the design of a controlled-force system and protocol for testing directional gecko-inspired adhesives and show that results obtained with it are in some cases substantially different from those with controlled-displacement testing. An advantage of the controlled-force testing approach is that it allows accurate generation of adhesive limit curves without prior knowledge of the expected behaviour of the material or the loading details associated with practical applications.

A New Stretch-Forming Process Based on Loading at Discrete Points and its Numerical Investigation

A new stretch-forming process based on discretely loading for three-dimensional sheet metal part is proposed and numerically investigated. The gripping jaw in traditional stretch-forming process is replaced by the discrete array of loading units, and the stretching load is applied at discrete points on the two ends of sheet metal. By controlling the loading trajectory at the each discrete point, an optimal stretch-forming process can be realized. The numerical results on the new stretch-forming process of a saddle-shaped sheet metal part show that the distribution of the deformation on the formed surface of new process is more uniform than that of traditional stretch-forming, and the forming defects can be avoided and better forming quality will be obtained.

Definition of Strain Amplitude for Cyclic Model to Simulate Actual Damping of Soils under Irregular Loadings

The determination of damping constitutes an essential part of the cyclic characterization of soils. While because of the damping of soils is nonlinear, ie., strain-dependent, the definition of shear strain amplitude under irregular loading process construct the key point of the damping based cyclic models. This paper introduces a new damping-based model (DBM) for nonlinear soil behavior simulation and discusses on the effect of shear strain amplitude definition on model behavior. Both qualitative and quantitative analysis re-sults show that the reversed hysteresis loading curve is significantly influenced by the pre-supposed loading amplitude and generally large pre-proposed shear strain amplitude will lead to low reversed loading trajectory. Analytical comparison among several definitions indicates that defining the maximum reversal point of history as the loading amplitude performs the best.

Design and Optimization of Loading Trajectory for S-Skin Stretch Forming Process by Simulation

Stretch forming is one of the primary methods in skin forming process. Uniform strain distribution and springback are main factors which affect the precision of air skin. In the article the stretch forming process based on S-skin was analyzed. Firstly the parameters ranges of the loading trajectory were designed through the analytic method. Secondly the initial loading trajectory was optimized through finite element numerical simulation. The optimization processes was performed through FET software integrated with the optimization arithmetic. The motion parameters of jaw and machine’s instructions were selected as design variables. Optimization mathematics model was set up which objective is to reduce springback and improve the strain distributes uniform degree. During optimization the maximum main strain and thickness thinning rate of elements were restricted in permissive range. The forming degree of each stage was rational distributed, and the reasonable loading trajectory was founded. The result shows that the reasonable loading trajectory is including pre-stretch, wrap, press and after stretch. After optimization the strain distributes uniformly and the maximum main strain is between 3％～5％. The maximum stretching rate which appears in the shoulders area is less than 6%. In the concave area in which the insufficiency deforming can be occurred easily the strain achieves about 3%, and the deformation is enough. After optimization the unloading springback is decreased distinctly. The average springback of all elements is 0.47mm which reduces 30% compare with before optimization. The result meets the manufacture requirement.