Periodic Reporting for period 2 - NanoHighSpeed (High-speed Deformation and Failure of Materials at the Nanometer Scale)
Période du rapport: 2022-05-01 au 2023-10-31
A sustainable economy requires safe and durable products, even when these products are subjected to impact loadings, such as mobile phone displays hitting the floor or aircraft engines experiencing a bird strike. The challenge is to avoid brittle failure. This is currently difficult, since little to no fundamental understanding exists about the deformation mechanisms at high strain rates. The main reason is that the experimental techniques currently used for mechanical characterization at high velocities are limited to large, uniform samples and therefore cannot accommodate complex modern materials.
The ERC NanoHighSpeed project will spearhead the development of experimental techniques for investigating the high-speed deformation behavior at the nanoscale. The new development will be based upon nanoindentation testing, a technique that probes the hardness of a sample with a microscopic diamond needle. Nanoindentation testing already grants researchers access to the mechanical properties of a material on a scale of nanometers and micrometers. However, it is currently limited to low loading rates, which are not representative of impacts or collisions. Using innovative hardware and novel experimental methods, the ERC NanoHighSpeed project will turn nanoindentation testing into a powerful tool for high-speed mechanical characterization. This new nanoscale approach will enable us to gain a fundamental understanding of the failure of materials at high strain rates, down to their smallest constituents. This will help develop new materials that can better withstand loads at high deformation speeds. In the long term, safety, environmental and economic benefits will ensue.
The ERC NanoHighSpeed project will spearhead the development of experimental techniques for investigating the high-speed deformation behavior at the nanoscale. The new development will be based upon nanoindentation testing, a technique that probes the hardness of a sample with a microscopic diamond needle. Nanoindentation testing already grants researchers access to the mechanical properties of a material on a scale of nanometers and micrometers. However, it is currently limited to low loading rates, which are not representative of impacts or collisions. Using innovative hardware and novel experimental methods, the ERC NanoHighSpeed project will turn nanoindentation testing into a powerful tool for high-speed mechanical characterization. This new nanoscale approach will enable us to gain a fundamental understanding of the failure of materials at high strain rates, down to their smallest constituents. This will help develop new materials that can better withstand loads at high deformation speeds. In the long term, safety, environmental and economic benefits will ensue.
During the first scientific period, the main focus of the ERC NanoHighSpeed project was on the development of a high-strain rate nanoindenter prototype. The system was designed around fast piezoelectric transducers. Piezoceramics are materials that instantaneously expand upon applying an electrical voltage. In our setup, they are used as fast actuators to indent the samples with a tiny diamond tip. Conversely, when placed under mechanical compression, piezoceramics instantaneously deliver an electrical voltage. This effect is used in our setup to measure fast force transients. In addition to fast transducers, our prototype relies on an ultrafast electronic controller, which allow measurements at 1 MHz, i.e. every microsecond. After overcoming several technical challenges related to the integration of the different components, we engaged in a careful characterization of the dynamic behavior of the instrument and developed a model to correct for machine artefacts. We also optimized the protocols used for controlling the experiments and ruled out a possible hysteresis effect (unwanted dependence of the hardness measurement on the loading path). While we are still in the process of optimizing our high-speed nanoindenter prototype, we have already reached a critical milestone by producing sustained strain rates of 20 000/s. This is 200 times higher than our proof of concept, and sufficient for investigating the high strain rate deformation resulting from a collision.
In parallel to the hardware developments, we started investigating technologically important materials, such as intermetallic compounds. These materials account for the high temperature strength of the turbine blades used in aircraft engines. The underlying physical mechanism, called “strength anomaly”, has been known since the 1970s. However, it had not yet been experimentally probed whether this positive effect persists under the high-speed deformation conditions of a bird strike, which would be a safety concern. The reason is that intermetallic compounds are tiny components found inside a larger superalloy, which generally cannot be produced in sufficient volume for classical characterization methods. Teaming up with Prof. G.M. Pharr (Texas A&M), we carried out high-speed, high-temperature nanoindentations and reached conclusive evidence ruling out the safety concern.
In parallel to the hardware developments, we started investigating technologically important materials, such as intermetallic compounds. These materials account for the high temperature strength of the turbine blades used in aircraft engines. The underlying physical mechanism, called “strength anomaly”, has been known since the 1970s. However, it had not yet been experimentally probed whether this positive effect persists under the high-speed deformation conditions of a bird strike, which would be a safety concern. The reason is that intermetallic compounds are tiny components found inside a larger superalloy, which generally cannot be produced in sufficient volume for classical characterization methods. Teaming up with Prof. G.M. Pharr (Texas A&M), we carried out high-speed, high-temperature nanoindentations and reached conclusive evidence ruling out the safety concern.
The experimental evidence of the persistence of the strength anomaly in intermetallic compounds at high strain rates is our flagship achievement of the first scientific period. As a matter of fact, it was not only a potentially critical issue for airplane transportation, but conclusive experimental evidence has remained elusive since the 1970s. This is because pure intermetallic compounds are only available in small dimensions, with no suitable characterization method available until now. This highlights the strong potential of the new high-speed nanoscale technique for addressing longstanding scientific questions.
Furthermore, while developing the high-speed nanoindenter prototype, we uncovered critical limitations of hardware components and correction procedures used in nanoindentation testing. Reporting these shortcomings will avoid misleading data from being published and used in practical applications.
Furthermore, while developing the high-speed nanoindenter prototype, we uncovered critical limitations of hardware components and correction procedures used in nanoindentation testing. Reporting these shortcomings will avoid misleading data from being published and used in practical applications.