Session: 12-16-02: Drucker Medal Symposium

Paper Number: 99731

99731 - High Throughput Fracture Mechanics of Graphene Sheets via Decoupled Force and Deformation Measurements 

Nanomaterials such as graphene and carbon nanotubes are considered among the strongest engineering materials with intrinsic strength of over 100 GPa. However, their mechanical properties, e.g., strength, modulus and fracture toughness is highly sensitive to flaws, leading to a wide range of measured strength values far below that of the intrinsic strength. The challenge of scattered data, a challenge well known to mechanics community, is often met by collecting a satitistically large number of data sets and proper statistical analysis. On the other hand, experimental challenges of mechanical testing individual nanomaterials remains a formidable challenge in identifying the magnitude of this scatter. The nanomechanical experiments often require laborious sample manipulation, often under high resolution imaging techniques (e.g., SEM) often with high end robotic stages. To address these limitations, we have developed a high-throughput fracture toughness characterization of graphene sheets which relies on applying known displacement fields on multiple graphene samples with precracks each bound to an opening of a TEM greed. In this method, the graphene sheets will be loaded by stretching the TEM grid inside an SEM chamber and monitoring the local displacements in real time. The method was applied to fracture mechanics characterization of bi-layer graphene which was obtained via stacking single layer graphene made via chemical vapor deposition.

Nanomaterials such as graphene and carbon nanotubes are considered among the strongest engineering materials with intrinsic strength of over 100 GPa. However, their mechanical properties, e.g., strength, modulus and fracture toughness is highly sensitive to flaws, leading to a wide range of measured strength values far below that of the intrinsic strength. The challenge of scattered data, a challenge well known to mechanics community, is often met by collecting a satitistically large number of data sets and proper statistical analysis. On the other hand, experimental challenges of mechanical testing individual nanomaterials remains a formidable challenge in identifying the magnitude of this scatter and using statistical approaches to rationalize the measurements (such as Weibull distribution). The nanomechanical experiments often require laborious sample manipulation, often under high resolution imaging techniques (e.g., SEM) with high end robotic stages. To address these limitations, we have developed a high-throughput fracture toughness characterization of graphene sheets which relies on applying known displacement fields on multiple graphene samples with precracks each bound to an opening of a TEM greed. In this method, the graphene sheets will be loaded by stretching the TEM grid inside an SEM chamber and monitoring the local displacements in real time. The mechanical characterization approach decouples force measurements from displacement measurements, and utilizes the elastic modulus in conjunction with the known displacement boundary conditions to estimate the stress intensity factor at the onset of crack growth (i.e., the fracture toughness). In combination with the proposed highthrough experimental loading approach, we will be able to establish statistically sound fracture toughness data base for multilayered graphene sheets with various angles between initial crack and far-field loading directions. By using this approach, as many as 10-15 individual bilayers of graphene can be tested at once, significantly improving the fidelity in the measurements. The method was applied to fracture mechanics characterization of bi-layer graphene which was obtained via stacking single layer graphene made via chemical vapor deposition. Both layers were initially grown on cupper foils, and then transferred on to a polymer film. The graphene-polymer films were stacked together from the graphene side, and placed on a TEM grid. The polymer was then thermally removed, leaving behind the bilayer graphene on TEM grid.

The nanomechanical experiments have revealed that the fracture toughness of bi-layer graphene can in some cases exceed that of single layer graphene, potentially due to compressive stresses that developes in bi-layer graphene when lattice vectors of two layers are not aligned. The measured toughness was then fitted to a revised Weibull distribution which is suited for nanomaterials, and the characteristic size of the defects which leads to such strength distribution was obtained accordingly.

Presenting Author: Mohammad Naraghi Texas A&M University

Presenting Author Biography: Mohammad Naraghi is an associate professor in the department of Aerospace Engineering, Texas A&M University. His research is on the general area of multifunctional nanomaterials.

Authors:

Usama Arshad Texas A&M University
Yanxiao Li Missouri S&T University
Congjie Wei Missouri S&T University
Chenglin Wu Missouri S&T University
Mohammad Naraghi Texas A&M University