Standards to specify damping and loss factor in joint connections through ultrasonic inspection techniques where temperature and humidity in real world conditions are important

Finite element models are used to predict the vibration characteristics of aircraft and automotive vehicles. A large and complex model can be generated to determine the level of vibration which can be transmitted from one area to another. Manufacturers produce simulations of the structure and compare their results to experimental measurements.

One area where there are a lot of unknowns is in the damping level to put into the simulation (the stiffness is usually easy to estimate). In addition, the simulations usually need to be linear in order to be scaled to large vehicle models (even when using high performance computing clusters), however, many of the damping mechanisms include internal friction which has nonlinear effects.

When comparing simulation with experimental measurements on real components, many differences are apparent. For example, there may be variability among the production components, assembly errors or environmental differences during the testing period (temperature and humidity are important as are loading amplitudes).

In this project, a finite element model of a vehicle component or body will be obtained and fitted with small areas which may be predicted to show variability. One such area might be the representation of joints, in terms of the differences in torque settings for bolts or rivets or other effects from manufacture. We wish in particular to examine the industrial use of self piercing rivets with adhesive joints, common with modern aluminium body structures.

If there are changes from one joint to another, the vibration natural frequencies of the body might also change, due to the change of stiffness of the joint or damping in the joint.

In this project a student will have to research the modelling of joints, in terms of their representation for transmission of vibration at frequencies of interest to designers using finite element simulations. High fidelity models can provide very accurate results if you know how the joint is formed, but how do you reduce the complexity of the model while keeping the important physical parameters.

The student will be expected to create a suite of simulations of bolted, adhesive and spot welded joints in large detail and simulate a vibration experiment, and estimate where the damping is being generated. The student will be expected to use MSc Nastran or MSc Marc to carry out studies on representation of joints in vehicle assemblies. You will learn how to carry out frequency response function predictions using solutions 103, 111 108, and how these are used in industry.

In order to characterise the assemblies in real life, we intend to use ultrasonic reflection and transmission measurements and look at mapping methodologies to scale those to lower frequencies, where flexure and compressive strains are important. Ultrasonic excitation at a range of joint stresses lead to material damping, which can be separated from the friction mechanism in the joint.

The PhD will look at the following areas: i) Flexural motion of a joint, ii) ultrasonic reflection coefficients of a joint, iii) damping due to air pumping / acoustic generation and shear motion. The student will create a number of simulation test specimens to measure the shear response of the joint, including current test methods using hysteresis loops.

Research questions to answer include the fundamental damping mechanics in joints, the different simplified models to represent them, nonlinear vs linear behaviour and how these manifest with large linear FE simulations, what the variability in damping is with temperature and humidity of a specimen. Although research focused, the department has an applied focus with industrial partners. 

Loughborough has access to a full size anechoic chamber for experimental measurements, together with a dynamics laboratory with modal test rigs, joint damping rigs and a full series of data acquisition equipment. It also has a significant capability in measurements with staff from a range of signal processing backgrounds. 

You will be working with Dr Dan O'Boy and Dr Andrew Watson from the Department of Aeronautical and Automotive Engineering who have a proven track record working with industrial manufacturers, with applied industrial outputs and interests in developing simplified numerical models. You will have access to vibration and noise experts, control, experimental and machine learning specialists as needed.