mechanical properties and their tests part 2

Vickers Hardness Test

Vickers hardness test contains a square-based diamond pyramid as the indenter with the angle included between opposite part or faces of the pyramid of 136°(degree). The Vickers hardness number (VHN) may be defined as the load value divided by the surface area of the indentation.

VHN = 2p sin (𝜃/2)/𝐿^2 = 1.854𝑃/𝐿^2
Where, 𝑃is applied load, kg

𝐿is the average length of diagonals, mm

𝜃is the angle between the opposite faces of diamond = 136°

Knoop Test

The Knoop indenter (diamond-shape) is used for measuring hardness value in a small portion of an area, such as at the cross section of the heat-treated metal surface. The Knoop Hardness Number (KHN) is the applied load divided by the uncovered projected area of the indentation.𝐊𝐇𝐍 =P/𝐴𝑃
𝑃/ 𝐿^2 * 𝐶
Where 𝑃 = applied load, kg

𝐴𝑃 = uncovered projected area of the indentation, mm^2

𝐿 = length of long diagonal,

mm 𝐶 = a constant for each indenter supplied by manufacturer

Micro Hardness Test

Micro Hardness Testing of metals, composites and ceramics and other materials are employed where a 'macro' hardness test is not suitable or usable. This tests can be used to provide required data or information while measuring individual microstructures within a large matrix, or testing very thin foil structure res or while determining the hardness gradient(value of hardness along the length) of any specimen along a cross section.

Impact Test

The impact test is a method for understanding the behavior of the material when subjected to shock loading in bending tension, or torsion. An impact test signifies the toughness value of material which is the ability of a material, to absorb energy during plastic deformation.

The pendulum type impact testing machine is widely used for notched bar impact tests. The following type of impact tests is conducted on this machine.

1. Izod Test

This test uses a cantilever test piece of dimension 10mm x 10mm section specimen having standard 45°(degree) notch 2 mm deep. This specimen is now broken by means of a swinging pendulum which is allowed to fall from a predefined height to cause an impact load on the specimen. This angle inclination of the pendulum after hitting of the specimen is the measure of the impact test.charpy and Izod test

1. 2. Charpy Test

This test is more common than Izod test and it consists simply supported test piece of dimension 10mm x 10mm section. The specimen is loaded on supports or anvil so that the blow of striker is opposite to the notch part.

Initial energy =𝑊ℎ = 𝑊(𝑅 – 𝑅𝑐𝑜𝑠𝛼) = 𝑊𝑅 (1 – 𝑐𝑜𝑠𝛼)

Energy after rupture = 𝑊ℎ’ = 𝑊 (𝑅 – 𝑅𝑐𝑜𝑠𝛽) = 𝑊𝑅 (1 – 𝑐𝑜𝑠𝛽)

Energy used to rupture specimen = 𝑊ℎ – 𝑊ℎ’

= 𝑊𝑅 [(1 − 𝑐𝑜𝑠𝛼) – (1 – 𝑐𝑜𝑠𝛽)] = 𝑊𝑅 (𝑐𝑜𝑠𝛽 – 𝑐𝑜𝑠𝛼)

Where, â„Ž = Height of fall of center of gravity of pendulum/striker 𝛼 = Angle of fall â„Ž’ = Height of rise of center of gravity of pendulum/striker 𝛽 = Angle of rise 𝑅 = Distance from C.G. of pendulum/striker to axis of rotation O. 𝑊 = Weight of pendulum/striker.

This equation is suitable for Charpy as well as Izod test.

Significance of Transition-Temperature Curve

There exist a transition from notch brittle behavior to notch tough behavior with the increase in temperature. In metals, such transition occurs at 0.1 to 0.2 times the absolute melting temperature Tm, while in the case of ceramics materials the transition occurs at about 0.5 to 0.7 Tm. A required criteria is to base the transition temperature on the temperature at which the fracture becomes 100 % cleavage. This temperature is also called as nil ductility temperature. The NDT is the particular temperature at which fracture initiates with no prior plastic deformation. Below the NDT, the probability of ductile fracture is very negligible. At low temperature, the fracture of materials in machine occurs by cleavage but at higher temperature the fracture in materials occurs by ductile rupture.

Fatigue Test

Fatigue failure

Fatigue is defined as the progressive and localized structural damage which occurs while a material is subjected to cyclic loading. .This occurs when a material is subjected to continuous or discontinuous loading and unloading.

If the loads are above a certain threshold value ,tiny microscopic cracks will begin to form at the stress concentrators. At a certain stage this, a crack will reach to its critical size, finally, the structure will suddenly fracture.

Types of Fatigue Loading

Alternating /reversing stress

In this test, the test specimens are cycled to alternative/reversed stress. The cyclic stress varies from certain tensile to same opposite compressive. The mean stress to the specimen under complete cycle is equal to zero.

Repeated Stress

In the repeated stress loading condition the stress to a specimen varies between zero and a maximum tension or compression stress in a cyclic manner.

Combined Steady and Cyclic Stress

A stress condition normally experienced in practice is a cyclic stress imposed on a steady load stress. the specimen is never at zero tension or compression.

S-N Curve

A common and widely used method to measure the resistance to fatigue is the rotating cantilever beam test. One of the ends of a machined cylindrical specimen is clamped in a motor-driven chuck. A weight of predefined mass is suspended from the other end.

The specimen initially has tension force acting on the top surface, while the bottom surface of the specimen is compressed. After the specimen turns 180º (degree), the locations which were originally in tension are now in compression. Thus, the stress at any one point goes through a complete cycle from zero stress to maximum tension stress to zero stress to maximum compression stress. Generally, a series of specimens are tested at different applied stresses to obtain the accurate result.

Preventions

The most effective method of improving fatigue performance is improvements in design:

1. Eliminate or reduce stress raisers by streamlining the part.
2. Avoiding sharp surface tears resulting from punching, stamping, shearing, or other processes.

3.Preventing the development of surface discontinuities during processing.

1. Reduce or eliminate tension residual stresses caused by manufacturing.
2. Improve the details of fabrication and fastening procedures.

Creep Test

Creep is defined as the slow plastic deformation of metals under constant stresses (the applied stress is less than the yield strength at that temperature) or under prolonged stress usually at high temperature.

Example. Moderate creep occurring in concrete is sometimes welcomed because it releases tensile stresses that might otherwise cause cracking. The temperature range where creep deformation may occur differently in various materials. Example, tungsten material requires a temperature in a couple of thousands of degrees before creep deformation can occur but ice will creep out near 0 °C (32 °F). The rate of creep deformation is a function of the material properties, exposure temperature, exposed time and the applied structural load.

The specimen that is to be tested is placed in the electric furnace where it is then heated to a given temperature and is constantly subjected to a loading . The strain variations in the specimen are measured with strain gauges.

Creep and Stress Rupture Curve

As soon as stress is applied, the specimen elastically elongates a small amount Eo depending on the applied stress and the modulus of elasticity of a material at the high temperature

The creep curve can be studied in the following three stages:

(i) Primary Creep: This is the initial part of the curve. In this part, the curve shows a rapid extension of the specimen.

(ii) Secondary Creep: This state of the curve denotes the period of extension at which the creep occurs at a less or more constant rate having its minimum creep rate.

(iii) Tertiary Creep: In tertiary stage, the rate of strain rate or extension accelerates out rapidly leading to rupture finally. In this stage, the uses of alloys should be avoided. Tertiary creep is assumed to be due to necking.

Fracture

A fracture is defined as the separation of an object or material into two or more than two pieces under the action of stress. In crystalline materials, individual fractures or breaks without the body separating into two or more pieces. Depending on the nature of substance which is the fracture, a fracture in the component reduces strength( in most substances) or inhibits propagation of light(optical crystals). Fracture strength which is also called as breaking strength is the value of stress at which a specimen fails due to fracture.

Types of fracture are given below

Brittle fracture: In such fracture, no apparent plastic deformation takes place before fracture of the component. Necking does not occur.

Ductile fracture: In such fracture, extensive or high plastic deformation( also necking) takes place before fracture.

(Sunil Risal, 2014 April; R.K.Rajput, 2009)