On Eventual Applicability of Plans in Dynamic Environments with Cyclic Phenomena

Planning and acting in dynamic environments deals with non-deterministic events that might change the state of the environment without consent of the agent. In the worst case, some events might cause the agent to become ``trapped'' in a dead-end state, which in practice might mean damage or destruction of the agent. Presence of non-deterministic events often considerably increases the number of alternatives that might occur in a single step and hence traditional non-deterministic planning techniques might not scale. In this paper, we address a class of problems where non-deterministic events represent ``cyclic phenomena''. If they interfere with the agent, they might be dangerous for it (e.g. ships cruising through the area of AUV operations). We present techniques that initially analyse the problem whether it falls within this class by considering the notion of event reversibility and if so, these techniques generate a plan such that encountered unsafe states, in which the ``cyclic phenomena'' might interfere with the agent, can be eventually crossed without any risk of ``falling'' into a dead-end state. Our approach is evaluated in the AUV and Perestroika domains.

ACL surgical innovation cycles: what goes around, comes around

Over the past 45 years, the ‘gold standards’ for ACL surgery have evolved considerably. Many discoveries in the field of ACL surgery seem to come and go, in cycles. The aim of this paper was to confirm these cyclic phenomena by methodically researching the ACL literature. It proposes a bibliometric research of the literature over the last 45 years to assess temporal changes in publications on 3 topics of high interest in ACL surgery for which we hypothesised a cycle phenomenon: surgery to address anterolateral knee instability, ACL repair and prosthetic ACL reconstruction grafts. We searched PubMed from 1975 to 2019 with different equations and different Boolean operators. Review results are presented as graphs depicting the evolution of the annual publication number over the total number of PubMed results that year. Graph analysis confirmed cycles of interest in these 3 fields. When this analysis was put into perspective with the major historical articles in each field, cycles seem to have 5 typical phases: hope, inflation, disillusion, eclipse and rebirth. The initial hope-inflation and rebirth phase trend peak interval was approximately 35 years. Rebirth trend peak slope trajectories for each topic seemed to display more rapid and stronger rise times than the initial trend peak slope trajectories. These cyclic phenomena that were observed suggest that knee surgeons need to better balance early enthusiasm and hope for innovative surgical practice efficacy with the guidance of both scientific rigour derived evidence and history.

Elastoplastic response of as-built SLM and wrought Ti-6Al-4V under symmetric and asymmetric strain-controlled cyclic loading

Purpose The purpose of this paper is to examine the mechanical behaviour of additively manufactured Ti-6Al-4V under cyclic loading. Using as-built selective laser melting (SLM) Ti-6Al-4V in engineering applications requires a detailed understanding of its elastoplastic behaviour. This preliminary study intends to create a better understanding on the cyclic plasticity phenomena exhibited by this material under symmetric and asymmetric strain-controlled cyclic loading. Design/methodology/approach This paper investigates experimentally the cyclic elastoplastic behaviour of as-built SLM Ti-6Al-4V under symmetric and asymmetric strain-controlled loading histories and compares it to that of wrought Ti-6Al-4V. Moreover, a plasticity model has been customised to simulate effectively the mechanical behaviour of the as-built SLM Ti-6Al-4V. This model is formulated to account for the SLM Ti-6Al-4V-specific characteristics, under the strain-controlled experiments. Findings The elastoplastic behaviour of the as-built SLM Ti-6Al-4V has been compared to that of the wrought material, enabling characterisation of the cyclic transient phenomena under symmetric and asymmetric strain-controlled loadings. The test results have identified a difference in the strain-controlled cyclic phenomena in the as-build SLM Ti-6Al-4V when compared to its wrought counterpart, because of a difference in their microstructure. The plasticity model offers accurate simulation of the observed experimental behaviour in the SLM material. Research limitations/implications Further investigation through a more extensive test campaign involving a wider set of strain-controlled loading cases, including multiaxial (biaxial) histories, is required for a more complete characterisation of the material performance. Originality/value The present investigation offers an advancement in the knowledge of cyclic transient effects exhibited by a typical α’ martensite SLM Ti-6Al-4V under symmetric and asymmetric strain-controlled tests. The research data and findings reported are among the very few reported so far in the literature.

Wrinkling of Tubes by Axial Cycling

Circular tubes compressed into the plastic range first buckle into axisymmetric wrinkling. Initially, the wrinkle amplitude grows with increasing load, but induces a gradual reduction in axial rigidity that eventually leads to a limit load instability and collapse. For lower D/t tubes, the two instabilities can be separated by strain levels of a few percent. Persistent stress-controlled cycling can cause accumulation of deformation by ratcheting. Here, the interaction of ratcheting and wrinkling is investigated. In particular, it is asked if compressive ratcheting can first initiate wrinkling and then grow it to amplitudes associated with collapse. Experiments on SAF2507 super-duplex steel tubes with D/t of 28.5 have shown that a geometrically intact tube cycled under stress control initially deforms uniformly due to material ratcheting. However, in the neighborhood of the critical wrinkling strain under monotonic loading, small amplitude axisymmetric wrinkles develop. This happens despite the fact that the maximum stress of the cycles can be smaller than the critical stress under monotonic loading. In other words, wrinkling appears to be strain rather than stress driven, as is conventionally understood. Once the wrinkles are formed, their amplitude grows with continued cycling, and as a critical value of amplitude is approached, wrinkling localizes, the rate of ratcheting grows exponentially, and the tube collapses. Interestingly, collapse was also found to occur when the accumulated average strain reaches the value at which the tube localizes under monotonic compression. A custom shell model with small initial axisymmetric imperfections, coupled to a cyclic plasticity model, is used to simulate these cyclic phenomena successfully.