Latest Research

Below are some of the most recent topics that Dr. Merrett’s lab has been exploring!

“The influence of load history on the stability of polymer composite aircraft structures”

Process for Researching Aeroviscoelastic Aircraft Structures

The process for understanding and improving the use of composite polymer structures in aircraft involves many objectives.

Interconnection of Viscoelasticity, Inertia, Aerodynamics, and Control

Aeroviscoelasticity is the intersection of many different fields in aerospace engineering.

Time to Flutter for 50,000 Flight Cycles/T for a Variety of Flight Speeds and Viscosities

Flutter, or vibration, on an aircraft can quickly become catastrophic. Analytical solutions have been created to understand what properties decrease the time to flutter. The image below is one example of an analytical solution.

“Using case studies and build projects as authentic assessments in cornerstone courses”

Abstract

Final year, undergraduate engineering students should be exposed to authentic practices that resemble industry to prepare the students for the field. Given that engineering involves design and implementation, final courses should expose students to both through authentic assessments. Case studies, design and build projects, and openended final exams were implemented in three, fourth-year cornerstone engineering courses in mechanical, and sustainable and renewable energy engineering. The effects of these changes differed between case study based projects and build projects. Case study based projects resulted in higher project performance and better exam performance than build projects. The improved performance is indicative of better knowledge transfer and a result of increased opportunities for discussion and collaboration between students. These results are consistent with the Ashford Rowe model for authentic assessments; however, students were divided on the benefits of the authentic assessments. Students were more resistant to the build projects, as reported through the teaching evaluations, because of the increased work load and scheduling
required for build projects. This study recommends that cornerstone courses implement case study or paper design projects and open-ended, case study-based final exams to achieve authentic assessments. The use of build design projects for cornerstone courses is not recommended.

Example design and build projects completed by students: Martian chronometers undergoing endurance testing.

“Analytical and Experimental Assessment of the Viscoelastic Time-Temperature Superposition Principle for a Thermo-Viscoelastic Wing Spar”

Thermal Strain vs Position Along the Beam at Different Amperage Configurations

Abstract

The time-temperature superposition principle is used in the study of viscoelastic materials, as the principle allows for tests to be performed in a shorter amount of time. Past studies show that the superposition is primarily done by using a shift factor. An analytical model for a thermo-viscoelastic beam is created using the elastic viscoelastic correspondence principle and Duhamel equation of heat conduction. An experiment using an acrylic cantilever beam with a tip mass is conducted to validate the analytical model. The thermal strain of the heated acrylic beam was shown to have a negligible effect on the total strain; however, the mechanical strain was heavily affected by the temperature. The mathematical model outlined had good agreement with the experimental thermoviscoelastic beam in bending. The time-temperature superposition principle is shown to be reliable when comparing two elevated temperatures.


“The Effect of Skin Tension, Indentation Speed and Tip Contact Area on a Shear Modulus and Viscoelastic Relaxation Periods of Porcine Tissue”

Abstract

Some medical devices require deformation of the surface layer of skin in order to function, any device that requires a penetration of skin will also cause deformation before penetration. Indentation of the skin is an easy and non invasive method to determine the mechanical properties of skin tissue. Skin tissue is commonly assumed to be a viscoelastic material. Experiments were performed to determine the mechanical response to indentation and a viscoelastic model was fit to the data to determine the properties. The applied skin tension, indentation speed and indenter tip profile were varied to determine the relative effect of these three independent variables. It was found that indentation speed and indenter tip profile had large effects on the response properties, while the applied skin tension had a weak effect.

Experimental Setup

“Time to flutter theory for viscoelastic composite aircraft wings”

Time to flutter for flying wing with arbitrary viscoelastic material properties

Abstract

Many general aviation aircraft and commercial aviation aircraft are manufactured from high-polymer composite materials. The increasing use of polymer composite materials adds a time dimension to existing flutter analyses of an aircraft wing. Polymer composites exhibit viscoelastic material behaviors such as energy dissipation and a memory effect that influence the physical response of a wing. The proposed theory, derived from Lyapunov stability principles, predicts the time component of flutter, and provides a set of conditions for viscoelastic structural instabilities in general. The addition of the load history in the stability analysis of a structure is a significant change compared to elastic materials, and no standards exist for assessing the lifetime stability of a viscoelastic structure. Rigorous comparisons to the Goland wing, wings with and without stores, and a flying wing illustrate the practical applications of the theory during aircraft design. Further, the theory predicts the time to flutter of each wing for a range of representative viscoelastic materials and flow conditions. The existence and prediction of the time to flutter is the key, original contribution of the theory because no other method is available to predict the time to flutter.