Experimental study and comparative numerical modeling of creep behavior of white oak wood with various distributions of earlywood vessel belt

Many researches have been conducted to investigate creep behavior of wood; however, the effects of structure on wood creep behavior remain unclear. Therefore, the effects of existence and distribution of earlywood vessel belt on creep behavior of white oak (Quercus alba L.) wood were investigated by dynamic thermal mechanical analyzer (DMA) with double cantilever bending in this study. Besides, a comparative numerical modeling simulation on strain curves of white oak specimens was completed using Burger and Five-parameter model. Results revealed that instantaneous strain and 45-min strain of specimens decreased with increase in the distance between earlywood vessel belt and stress acting surface obviously. Additionally, instantaneous strain and 45-min strain of specimens remarkably increased with increase in temperature from 20 to 80 °C. An obvious bending creep behavior was observed with increase in temperature from 20 to 80 °C. Both Burger and Five-parameter model can effectively simulate the creep behavior of white oak specimens with R2 values greater than 0.90. Furthermore, Five-parameter model illustrated a better fitting effect than Burger model in the final creep stage due to the introduction of a non-linear creep strain growth expression. It concluded that creep behavior of white oak wood strongly depends on the existence and distribution of earlywood vessel belt.

Smart Inlays for Simultaneous Crack Sensing and Arrest in Multifunctional Bondlines of Composites

Disbond arrest features combined with a structural health monitoring system for permanent bondline surveillance have the potential to significantly increase the safety of adhesive bonds in composite structures. A core requirement is that the integration of such features is achieved without causing weakening of the bondline. We present the design of a smart inlay equipped with a micro strain sensor-system fabricated on a polyvinyliden fluorid (PVDF) foil material. This material has proven disbond arrest functionality, but has not before been used as a substrate in lithographic micro sensor fabrication. Only with special pretreatment can it meet the requirements of thin film sensor elements regarding surface roughness and adhesion. Moreover, the sensor integration into composite material using a standard manufacturing procedure reveals that the smart inlays endure this process even though subjected to high temperatures, curing reactions and plasma treatment. Most critical is the substrate melting during curing when sensory function is preserved with a covering caul plate that stabilizes the fragile measuring grids. The smart inlays are tested by static mechanical loading, showing that they can be stretched far beyond critical elongations of composites before failure. The health monitoring function is verified by testing the specimens with integrated sensors in a cantilever bending setup. The results prove the feasibility of micro sensors detecting strain gradients on a disbond arresting substrate to form a so-called multifunctional bondline.

Biomechanical comparison of intramedullary screw fixation, dorsal plating and K-wire fixation for stable metacarpal shaft fractures

We compared four methods of metacarpal shaft fixation: 2.2 mm intramedullary headless compression screw; 3.0 mm intramedullary headless compression screw; intramedullary K-wire fixation; and dorsal plate fixation. Transverse mid-diaphyseal fractures were created in 64 metacarpal sawbones and were assigned into four groups. Peak load to failure and stiffness were measured in cantilever bending and torsion. We found that dorsal plating had the highest peak load to failure. However, initial bending stiffness of the 3.0 mm intramedullary headless compression screw was higher than that of the dorsal plates. In torsion testing, dorsal plating had the highest peak torque, but there was no significant difference in torsional stiffness between the plate and intramedullary headless compression screw constructs. We concluded that intramedullary headless compression screw fixation is biomechanically superior to K-wires in cantilever bending and torsion; however, it is less stable than dorsal plating. In our study, the initial stability provided by K-wire fixation was sufficient to cope with expected loads in the early rehabilitation period, whereas dorsal plates and IHCS constructs provided stability far in excess of what is required.

Actuator Behaviour of Tailored Poly(thiourethane) Shape Memory Thermosets

In this work, a new family of poly(thiourethane) shape memory thermosetting actuators was developed and characterized. These materials can be easily prepared from mixtures of two different aliphatic diisocyanates and a trithiol in the presence of a latent catalyst, allowing an easy manipulation of the formulation. Rheological studies of the curing process confirm the latent character of the formulations. The glass transition temperatures and the mechanical properties can be modified by varying the proportion of diisocyanates (hexamethylene diisocyanate, HDI, and isophorone diisocyanate, IPDI) with stoichiometric amounts of trimethylolpropane tris(3-mercaptopropionate). The shape-memory behavior was deeply investigated under three different conditions: unconstrained, partially constrained, and fully constrained. Tests were performed in single cantilever bending mode to simulate conditions closer to real complex mechanics of thermomechanical actuators under flexural performances. The complex recovery process in single cantilever bending mode was compared with that obtained using tensile mode. The results evidenced that the amount of recovery force in fully constrained conditions, or energy released during the recovery process in partially constrained, can be modulated by simply changing the proportion of both diisocyanates. A simple model based on Timoshenko beam theory was used for the prediction of the amount of work performed. The reported results are an important guideline to design shape-memory materials based on poly(thiourethane) networks, establishing criteria for the choice of the material depending on the expected application.

Kirschner Wire Fixation in Dorsally Displaced Distal Radius Fractures: A Biomechanical Evaluation

Background There is currently no consensus for the optimum configuration and number of Kirschner wires (K-wires) to use for the stabilization of dorsally displaced distal radius fractures. In this biomechanical study, we compared the load to failure and stiffness of four common K-wire configurations to identify the strongest construct for use in extra-articular dorsally displaced distal radius fractures. Case Description We created a standard distal radius fracture model in turkey tarsometatarsi which was stabilized using two or three K-wires (1.6 mm) in four different configurations. Following a power calculation, 10 fracture models of each configuration underwent testing in cantilever bending and axial compression. Literature Review Recent randomized trials have shown no evidence that volar locking plates are superior to K-wires for the treatment of dorsally displaced distal radius fractures. This has led to an increase in the popularity of much cheaper K-wires. Several different K-wire techniques have been described but there is no strong evidence to determine which is the optimal configuration and number of wires. Clinical Relevance The three-wire interfragmentary configuration was stiffer than the three-wire Kapandji in axial compression and cantilever bending. There was no difference in load to failure in cantilever bending or axial compression. The three-wire interfragmentary technique is the stiffest configuration of K-wires for dorsally displaced distal radius fractures. The two-wire Kapandji technique was significantly weaker than the other configurations, especially in cantilever bending. Conclusion The authors recommend to always use three wires for percutaneous pinning and never to use two intrafocal wires alone.