cambridge 2
DESCRIPTION
dimensional analysisTRANSCRIPT
1
PART 1A EXPERIMENTAL ENGINEERING
DIMENSIONAL ANALYSIS LABORATORY EXPERIMENT No. 1
STRUCTURES
Objectives
The main objective of this laboratory is to explore the power of dimensional analysis in the context of structures and mechanical systems. More specifically, the lab is designed to convey two primary engineering applications for dimensional analysis:
1) Experimental deduction of theoretical results about physical behaviour. 2) Physical modelling of complicated problems where analytical or computational solutions are
not feasible.
Introduction
The laboratory is divided into two main parts. In the first part, a small scale experiments will be conducted which explore the use of dimensional analysis to explain theoretical concepts using simple structures (Application 1). During the second part, we will explore how dimensional analysis can be used to build an experimental model to solve a complicated real‐world problem (Application 2). Dimensional analysis requires a set of basic units to work from. In this lab, we will use the following unit system (SI Units):
Mass (M) = kilograms
Length (L) = metres
Time (T) = seconds
PART I: Simple Models
In medieval times, the Master Builders knew that if a small scale model of their cathedral stood up, then the real thing would stand up also. This is a peculiarity of structures which are essentially daringly‐balanced piles of effectively‐rigid stones. However, things are not so simple for designers of modern structures – and here `structures’ is taken in its widest sense to include not just buildings but also aerospace and automotive structures, micro‐ and nano‐structures and even biological structures. The first experiment considers one important phenomenon that governs the design of many structures (as your IA Structural Design Course will attest) – that of elastic buckling.
2
Experiment 1: The Elastic Buckling of Thin Struts or How Dimensional Analysis allows you to get much more out of a single experiment and Why the analysis of modern structures can no longer be done by geometry alone. __________________________________________________________________________________ When thin struts are loaded in axial compression, they buckle laterally. This can occur in a purely elastic fashion (i.e. when the load is removed the strut returns to its original shape – there is no permanent plastic deformation). Unlike for stone structures, the buckling behaviour of thin members depends on a material property – the Young’s Modulus of Elasticity E (otherwise known as the elastic modulus). This is one reason why geometry alone is not sufficient in the design of modern structures. Given that we are investigating elastic buckling, it should be no surprise that the key material property is the elastic modulus. The elastic modulus is a measure of how stiff the material is. Stiff materials (like steel) have high elastic moduli; more flexible materials (like polycarbonate) have lower elastic moduli. The elastic modulus of a material is typically determined by observing how much a specimen stretches when it is loaded in tension within its elastic range. It is defined as the ratio of stress to strain:
strain
stressE
where stress σ is the force per unit cross‐sectional area = T/bt and strain ε is the fractional extension = extension/(original length) = e/L
The units of stress are N/m2, whereas strain ‐ being a ratio of lengths ‐ is dimensionless. It follows from dimensional consistency that the units of Young’s Modulus are N/m2.
3
Your task: By doing experiments on stainless steel struts of various lengths, predict the loads at which aluminium alloy struts of differing thicknesses will buckle. Youngs Moduli:
Stainless steel: E = 193 x 109 N/m
2
Aluminium alloy: E = 70 x 109 N/m
2
You may use the space below to develop your proposed solution. (Hint ‐ the clue is in the title of this lab exercise. Approach the problem using dimensional analysis.)