Chapter 3

SHAPES, MATERIALS OF TUBES AND ENERGY ABSORPTION

Thin-walled tubes of different geometry and materials have been prevalently used as

collapsible energy absorbers in various kinds of structural applications. Such devices are

designed to collapse progressively for absorbing impact energy in a controlled manner and

converting kinetic energy in to [plastic strain energy in impact situations. There are various

types of sections of tubes they are listed below,

Circle

Square

Triangular

Ellipse

3.1 Circular Tube:

A circular tube is representing an efficient and light crash absorber under loading

condition. In addition, circular tubes under compression are reported to be the most prevalent

components in energy absorbing systems since they provide reasonably constant operating load.

3.1.1Types of analysis:

A number of analytical models for predicting the mean crushing load of progressive

folding of such tubes have been developed under axial quasi static and impact loading

conditions.

3.1.1a Quasi static and impact loading:

The collapse response and energy absorption capability of circular tubes under quasi

static and impact loading with varying parameters namely wall thickness and tube diameter. It

was found that the energy absorption capacity of tubes in impact tests is higher by about 1.56-

12.3% than quasi static tests, whereas the increase in the initial peak load is between 14.33-

40.25%. Moreover, an important finding is that both crush load and energy absorption capacity

increase with increasing thickness and diameter.

3.1.1b Axial Loading:

Circular tube when subjected to an axial load, a circular tube collapses in an efficient

manner when it displays stable progressive collapse and deforms in the axisymmetric ring, non-

axisymmetric or mixed mode. The collapse mode depends primarily on the ratio of diameter to

thickness (D/t), the ratio of length to diameter (L/D) and the material of tubes. In general, thicker

tubes deform via an axisymmetric mode whereas thinner tubes deforms in an non-symmetrical

mode. Figure 3.1 shows typical collapse mode of circular tube under axial loading.

Fig 3.1 Typical collapse modes of circular tube under axial loading

3.1.2 Advantages:

1. A general observation tells us that circular tube has better energy absorption

performance.

2. It represents an efficient and light crash absorber under axial loading.

3.1.3 Disadvantages:

1. When its length is greater than critical length, it deforms in a global Euler

buckling mode, which is an inefficient mode for energy absorption and thus needs

to be avoided in crashworthiness applications.

2. The crush and energy absorption response of circular tubes are significantly

affected by varying geometrical, material and loading parameters.

3.2 Square Tubes:

The deformation mode of square tubes is very different to circular tubes, though the

general characteristics are similar.