We are interested in the effects of dynamic fracture of polymeric specimens. The goal is to develop a deeper understanding and predict dynamic crack initiation and propagation of polymers and different types of composites subjected to highly transient loading under extreme conditions.
Some of the samples are conditioned such that they have a considerable amount of water uptake before the impact experiment. Experimental edge on impacts onto samples are performed using projectiles launched from a gas gun. The dynamic response of the impacted samples are obtained using strain gauges with simultaneous ultra high-speed visualization techniques featuring a newly acquired Shimadzu Hypervision HPV-X2 camera that is capable of frame rates up to 10 million frames per second. We use different types of visualization techniques, such as digital image correlation, caustics, and photoelasticity, to probe the response of the samples.
So far, we have studied samples made of PMMA, neat vinyl ester resin, polycarbonate and carbon fiber/vinyl ester. Of particular interest is to obtain fracture toughness values obtained under mode-I, mode-II or mixed mode loading.
This image shows 9 subsequent high-speed photographs visualizing crack propagation in a PMMA sample conditioned at 11% relative humidity. The location of the crack tip is inside the caustic and the technique used to obtain this image is called caustic imaging.
The research undertaken in this project is aimed at forming an experimental and numerical foundation to investigate shock wave attenuation. We are interested in effects of shock attenuation due to obstacles placed in the path of the shock wave. Particularly, we are interested in how large vs small geometrical features of the obstacles influence the degree of attenuation. Furthermore, we are interested in learning more about shock mitigation using liquids, and in particular, passive techniques where liquids are placed in the path of the shock wave. In particular, we are looking into
Specific geometric shapes of liquid sheets to understand the optimal degree of attenuation that can be obtained.
We ask ourselves what the role of Newtonian and non-Newtonian fluids and their mass, as well as thermal and inertial properties are on shock attenuation.
Direct measures of shock wave amplitude, peak pressure, total impulse in addition to quantitative and qualitative ultra high-speed shock wave schlieren photography is being used throughout this project.
This image shows a series of high-speed schlieren photographs. A planar shock wave, propagating from left to right, impacts onto square obstacles placed along the outline of a logarithmic spiral.
This image shows a series of high-speed photographs taken with a Phantom V711 camera. The incident shock propagates from the upper left corner and impacts onto a water wedge. The incident shock Mach number is 1.52 and the water wedge angle is 47 degrees. A regular reflection occurs.
Over the last 70 years many causes of traumatic brain injury (TBI) due to impact have been proposed. These include acceleration-deceleration of the head, intracranial pressure changes, stress waves, relative motion of the brain with respect to the skull, and cavitation. However, these causes utterly fail to describe quantitatively the main cause of injury, which is excessive tissue deformation. In our opinion, it is apparent that there still exists a debate on the main causes of TBI, and injury thresholds still has to be quantitatively understood.
Our long-term research goal is to establish a correlation between quantifiable mechanical parameters, such as deformation, strain and strain rate response, and underlying biological mechanisms responsible for tissue damage and impairment observed after impact events, and thereby improve prevention, diagnosis and treatment of TBI.
We have developed an experimental setup, HAMr, in which brain cells have been impacted under highly controlled settings. Then, both mechanical and biological response have been recorded.
We have also developed a pendulum impactor setup in which more realistic skull geometries can be studied carefully during and after a dynamic impact.
In this project we also use ultra high-speed imaging techniques to capture the dynamic response of the sample.
This image shows GFAP immunofluorescence of mixed glial cultures. Representative immunostaining for astrocytic GFAP in controls (CTL) and impacted cultures. GFAP in green, DAPI in blue. Bar charts show quantification of relative GFAP intensity by integrated density and area coverage. GFAP area was reduced from 12% in controls to 7% in impacted cultures (*, p less than 0.05). n=3, 3 replicates. Scale bar is 100 micro meter.
