Experimental Mechanics

Description:

Using modern testing equipment, the behavior of several materials including polymers, ceramics, metals, composites and alloys is characterized experimentally across time and length scales. Of particular interest to TAMG is to:

• Couple microstructural changes to bulk mechanical behavior. The characterization and quantification of microstructures combined with targeted experimental testing allows correlations that can be used to explain fundamental material behavior (e.g. small-scale plasticity) and to form realistic lifetime predictions.
• Feed computational modeling. Targeted mechanical testing with parametrically varied extrinsic (geometry and loading rates) and intrinsic (e.g. material chemistry and microstructure) factors is used to create datasets appropriate for creating/calibrating computational models.

Figure: (a) An example of Mode I loading of a compact tension specimen of an aluminum alloy (left) and full field strain profiles around the crack tip obtained in real time. (b) Examples of stress-strain curves obtained from testing Mg alloy specimens cut in two different directions and overlaid by Acoustic Emission activity results (left); corresponding full field surface strain profile revealing strain localization zones near yielding (right).

• Identify damage precursors. Damage is a multiscale process requiring a detailed understanding of the current material state at a number of length scales. Acoustic Emission (AE) has the potential to bridge different scales. To this aim, in-situ Scanning Electron Microscope (SEM) monitoring has been combined with AE monitoring to obtain:
  (i) the material state at the microscale to identify damage precursors for prediction of remaining useful life,
  (ii) a detailed understanding of damage formation. Microscale damage is examined and studied from nucleation to final failure to understand the effects of microstructure on fatigue life for remaining life predictions.

Figure: Microscale in situ SEM investigations are compared with mesoscale experiments demonstrating the multiscale nature of damage. Additionally, AE signals are then compared to SEM images obtained both in real time and post mortem to identify possible damage sources. Damage is seen to form real time and additional damage is revealed on the fracture surface through the volume of the material. SEM is able to identify surface damage for association with AE signals. Further, AE monitoring allows for the identification of damage in the volume of the material as well.

Figure: (top) Crack initiation do to void coalescence and growth to a finite size at failure. (bottom) Crack initiation and growth to catastrophic failure.

Figure: (top) Crack initiation invisible to the SEM during loading is identified post mortem upon examination of the fracture surface. Both precipitate fracture and matrix fracture is observed.

Related Publications: