AN3431

Mechanical construction considerations

Doc ID 019017 Rev 1

9/18

Figure 4.

Stress-strain curve

1.

Legend

1: True elastic limit (crystallographic defect movement), which occurs for a very low stress

2: Proportionality limit

4: Offset yield strength, usually defined as

ε = 0.2 %

: Engineering stress

ε: Engineering strain

A: Undeformed cross-sectional area

F: Uniaxial load

L: Underformed length

l: Elongation

E: Young’s modulus

The conductive top cover should be chosen so that the physical properties of the material

(its type and thickness) allow enough elongation when applying a force (stress) but, should

not cause a permanent bend (i.e. the stress should not exceed the elastic limit).

The top cover is usually a metal but, can also be a conductively coated material or plastic.

The most common metal for light touches (80-150 g) is Aluminium. Table 1 shows Young’s

modulus and the yield strength for several common materials.

Table 1.

Young’s modulus and yield strength of common materials

Material

Young’s modulus

-E-

(GPa)

Yield strength

-

(MPa)

Aluminium

69

95

Steel

200

250 to 1650

Copper

100 to 128

70

Polycarbonate

2.6

70

Titanium

105 to 120

730

ε