8.9 Mechanical Testing: Universal Testing Machine, Hardness & Impact

The Universal Testing Machine

A universal testing machine applies a controlled axial load and records the resulting deformation. The adjective universal reflects its ability to run tension, compression, bending and shear tests by changing fixtures.

Crosshead movement includes machine compliance and grip slip, so strain must be taken from the extensometer, not from crosshead travel, if the elastic modulus is to be measured correctly. This is a standard examination point and a real source of laboratory error.

Standard tensile specimens have a gauge length related to the cross-section so that results are comparable. For a round bar, the common proportional relation is

$L_0 = 5.65\sqrt{A_0}$

Properties Read from a Tensile Test

Ductile materials such as mild steel show necking and a cup-and-cone fracture; brittle materials such as grey cast iron fracture with little elongation on a plane roughly perpendicular to the load. Percentage elongation and reduction in area are the two standard ductility measures, and a material is conventionally regarded as ductile above about 5% elongation.

Hardness Testing

Hardness is resistance to localised plastic deformation. It is quick, effectively non-destructive, and correlates usefully with tensile strength — for steels, roughly $\text{UTS (MPa)} \approx 3.5 \times \text{BHN}$.

Brinell

A hardened steel or tungsten carbide ball of diameter $D$ is pressed with load $P$ for a set dwell time, and the indentation diameter $d$ is measured optically.

$\text{BHN} = \frac{2P}{\pi D\left(D - \sqrt{D^2 - d^2}\right)}$

The denominator is the curved surface area of the impression, not its projected area. Brinell suits coarse-grained and heterogeneous materials such as castings because the large impression averages over the microstructure. It is unsuitable for very hard materials, where the ball itself deforms, and for thin sections.

Vickers

A square-based diamond pyramid with a face angle of 136 degrees is used, and the mean diagonal $d$ of the impression is measured.

$\text{VHN} = \frac{1.854\,P}{d^2}$

Because the indenter is geometrically similar at all loads, the Vickers number is independent of load and gives one continuous scale from soft metals to hardened tool steel. This makes it the most versatile method, at the cost of requiring a well-polished surface and optical measurement.

Rockwell

Rockwell measures the depth of penetration under a major load beyond that under a preliminary minor load, and displays the result directly on a dial with no optical measurement.

The minor load of 10 kgf seats the indenter and eliminates surface irregularities from the reading. Rockwell is the fastest method and dominates production inspection; its disadvantage is that the scales are not directly comparable with one another.

Comparison

Impact Testing

A tensile test loads slowly; many service failures happen fast. Impact testing measures the energy absorbed in fracturing a notched specimen under a sudden blow, and is the standard route to assessing notch toughness.

Both standard tests use a swinging pendulum. The energy absorbed is the difference between the initial and final potential energy of the hammer:

$U = W\,l\left(\cos\beta - \cos\alpha\right)$

where $\alpha$ is the release angle and $\beta$ the swing-through angle.

Charpy versus Izod

This comparison is examined more often than anything else in the topic.

The memory hook: Charpy is a simply supported beam struck opposite the notch; Izod is a cantilever struck on the notch side.

The ductile-brittle transition

Body-centred cubic metals — notably plain carbon and low-alloy structural steels — show a sharp drop in absorbed energy as temperature falls, through a ductile-brittle transition temperature. Above it, fracture surfaces are fibrous and dull with large energy absorption; below it, they are bright and crystalline with very little.

Face-centred cubic metals such as austenitic stainless steel, aluminium and copper show no such transition and remain tough at low temperature. This is why austenitic stainless steel is specified for cryogenic vessels.

The engineering significance is severe and historical: brittle fracture of ship hulls and pressure vessels in cold service traces directly to operating below the transition temperature. For mining equipment in northern Indian winters, or for imported plant designed for warmer service, the transition temperature is a genuine specification issue rather than an academic one.

Factors that raise the transition temperature, and therefore make brittle failure more likely, are increased carbon content, coarse grain size, higher strain rate, sharper notches and greater section thickness. Grain refinement and nickel additions lower it.

Notch sensitivity

The notch is not incidental. It creates a triaxial stress state that suppresses plastic flow, so the notched specimen may fail brittly under conditions where an unnotched one would not. Impact energy figures are therefore always quoted with the notch geometry, and results from V-notch and keyhole specimens are not interchangeable.

Other Tests in Brief

Together with the tensile, hardness and impact tests above, these form the acceptance-testing framework referred to throughout the machine design and materials chapters of Paper-II.