NpjComput.Mater.：探索含能材料中剪下帶的形成機制：分子動力學

海歸學者發起的公益學術平臺

分享資訊，整合資源

交流學術，偶爾風月

剪下帶是由高剪下應力引起的區域性塑性區域。剪下帶的形成能夠適應材料的進一步變形，這可能會導致材料損傷的積累，以及流動應力的潛在減少。剪下帶存在於晶體金屬、陶瓷、金屬玻璃和分子材料等多種材料類別中。總的來說，當材料受到區域性剪下而發生軟化時，通常會出現剪下帶，形成更易受到材料快速流動影響的區域。這將導致區域性的塑性不穩定性，降低材料的偏應力分量，並允許持續變形。分子固體通常被應用於衝擊載荷等超高應變率環境中，由於其複雜的晶體堆積和晶體缺陷形成機制認識的不足，對剪下帶形成的研究提出了巨大的挑戰。分子晶體中初始的剪下帶成核過程尚不明確。

Fig. 1 | Molecular center of mass renderings at 50 ps after maximum compression for all particle velocities. Only molecules with a shear strain above 0.15 are rendered. Molecules are colored by shear strain.

來自美國洛斯阿拉莫斯國家實驗室理論部的Brenden W. Hamilton等人，對含能材料環三亞甲基三硝胺（RDX）中衝擊波誘導的剪下帶形成進行了分子動力學模擬，用於評估剪下帶的成核過程。他們發現，在高壓下，剪下帶的初始形成位點（稱為“胚胎”）在剪下帶形成和生長前會大量形成並迅速降低偏應力，從而抑制了塑性變形。壓縮狀態釋放後，這些胚胎癒合並恢復到材料的結晶相，形成一個可逆的過程。這與低壓衝擊明顯不同：低壓衝擊中，塑性變形導致剪下帶顯著增長，且衝擊釋放後增長仍在繼續。對剪下分子的聚類分析表明，剪下帶是一個大型的、相互連線的網路。高壓系統中會產生數百個小型、獨立的團簇，其大小和剪下幅度並不會隨著時間的推移而顯著增長，但仍然會降低驅動塑性的剪下應力。總的來說，高壓下剪下帶的消失對RDX的機械強度和機械化學動力學至關重要。

Fig. 2 | Correlations between the shear strain at 2.5 ps after a material section is shocked vs. the shear strain of that material at 50 ps after maximum compression. Each point represents a 1 nm3 Lagrangian bin.

這些結果將對高爆炸藥、高超聲速系統等高應變率應用材料的模擬和開發產生廣泛的影響。該文近期發表於npj Computational Materials10: 147 (2024)，英文標題與摘要如下，點選左下角“閱讀原文”可以自由獲取論文PDF。

High pressure suppression of plasticity due to an overabundance of shear embryo formation

Brenden W. Hamilton & Timothy C. Germann

High pressure shear band formation is a critical phenomenon in energetic materials due to its influence on both mechanical strength and mechanochemical activation. While shear banding is known to occur in a variety of these materials, the governing dynamics of the mechanisms are not well defined for molecular crystals. We conduct molecular dynamics simulations of shock wave induced shear band formation in the energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) to assess shear band nucleation processes. We find, that at high pressures, the initial formation sites for shear bands, “embryos”, form in excess and rapidly lower deviatoric stresses prior to shear band formation and growth. This results in the suppression of plastic deformation. A local cluster analysis is used to quantify and contrast this mechanism with a more typical shear banding seen at lower pressures. These results demonstrate a mechanism that is reversible in nature and that supersedes shear band formation at increased pressures. We anticipate that these results will have a broad impact on the modeling and development of high-strain rate application materials such as those for high explosives and hypersonic systems.