Most materials expand when heated and contract when cooled. In contrast, negative thermal expansion materials exhibit the opposite behavior. In this paper, we present a metamaterial that expands both when heated and cooled. The unit cell configuration is crafted by embedding two isosceles triangular structures composed of high thermal expansion materials within a re-entrant hexagonal honeycomb framework. Initially, the bases of the two isosceles triangles are in contact. The metamaterial exhibits two deformation states under different external loads. Analytical formulas for the equivalent parameters were derived based on classical beam theory within the regime of small deformation. Finite element simulations were conducted to validate the accuracy of these analytical formulas. The influence of geometric parameters on the metamaterial is discussed in detail. The results demonstrate that the metamaterial can exhibit excellent multi-functional properties within a small range of deformation, including instantaneous sign-variable coefficients of thermal expansion and Poisson's ratios, along with significantly distinct tensile and compressive stiffnesses. This metamaterial consistently exhibits expansive characteristics regardless of variations in ambient temperature. Additionally, the material always expands laterally regardless of the sign of uniaxial loads. Based on these properties, the metamaterial can be applied to fasteners in the aerospace industry that require permanent locking, ensuring stability in various temperature and mechanical load conditions.

中文翻译：

---

一种具有瞬时符号可切换热膨胀系数和泊松比的新型超材料

大多数材料在加热时会膨胀，在冷却时会收缩。相反，负热膨胀材料表现出相反的行为。在本文中，我们提出了一种在加热和冷却时都会膨胀的超材料。晶胞结构是通过将两个由高热膨胀材料组成的等腰三角形结构嵌入凹入六边形蜂窝框架内而制成的。最初，两个等腰三角形的底边接触。超材料在不同的外部载荷下表现出两种变形状态。基于经典梁理论，推导了小变形范围内等效参数的解析公式。进行有限元模拟来验证这些分析公式的准确性。详细讨论了几何参数对超材料的影响。结果表明，超材料可以在小变形范围内表现出优异的多功能特性，包括瞬时符号变量热膨胀系数和泊松比，以及明显不同的拉伸和压缩刚度。无论环境温度如何变化，这种超材料始终表现出膨胀特性。此外，无论单轴载荷的符号如何，材料总是横向膨胀。基于这些特性，超材料可以应用于航空航天工业中需要永久锁定的紧固件，确保在各种温度和机械负载条件下的稳定性。