Mechanical properties and their tests part 1

Material Testing is necessary to be done in order to determine the suitability of a component in a system. Destructive or non-destructive tests may be carried out for this.

Summary

Material Testing is necessary to be done in order to determine the suitability of a component in a system. Destructive or non-destructive tests may be carried out for this.

Things to Remember

  1. The mechanical properties of metals are strength, elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, fatigue, fracture and creep.
  2. Tensile testing, also known as tension testing, is a fundamental material science test in which a sample to be tested is subjected to axial tension until failure.
  3. Elastic Limit: It is the maximum stress value which the material can withstand without causing permanent deformation which remains even after the removal of stress.
  4. In engineering stress-strain curve, it is seen as stress drops down after necking since the curve is based on the original area. In the true stress-strain curve, the stress actually increases after necking since the cross section area of the specimen decreases rapidly after necking.
  5. Hardness is the property of a material that enables the material to resist any plastic deformation, usually by penetration The term hardness may also the ability of a material to resistance to bending,

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Subjective Questions

Q1:

Write a short note on Clobetasol.


Type: Short Difficulty: Easy

Show/Hide Answer
Answer: <h4>Clobetasol</h4>
<p>Clobetasol is a corticosteroid of the glucocorticoid class used to treat various skin disorders including eczema and psoriasis . It comes in shampoo, mousse, ointment and emollient cream presentations. It has very high potency and typically should not be used with occlusive dressings , or for extended continuous use (beyond two weeks). It is also used to treat several autoimmune diseases including alopecia areata , vitiligo , lichen sclerosis, and lichen planus.</p>
<p>&nbsp;</p>
<p><img src="http://www.goodbyepsoriasis.com/wp-content/uploads/2014/03/clobetasol.jpg" alt="Image result for Clobetasol" /></p>
<p><strong>Mechanism of action</strong></p>
<ul>
<li>It promotes glycogen deposition in liver and inhibits glucose utilization by peripheral tissue. It also promotes glucocorticoids. It increases blood sugar level.</li>
<li>It also causes protein breakdown and-and synthesis by inhibiting amino acid by mobilization from a peripheral tissue.</li>
<li>Redistribution of fat occurs in areas over face, neck, and shoulders.</li>
<li>It inhibits intestinal absorption and enhances renal excretion of calcium.</li>
<li>It promotes water retention.</li>
<li>Mild euphoria occurs in normal doses.</li>
<li>Inflammatory responses are inhibited irrespective to the type injury.</li>
<li>It suppresses all types of immunological hypersensitivity and allergic phenomena.</li>
</ul>
<p><strong>Indications</strong></p>
<ul>
<li>Short-term treatment only for severe resistant skin disorder like recalcitrants, eczema, and psoriasis</li>
</ul>
<p><strong>Dose</strong></p>
<ul>
<li>05% cream apply 1-2 times daily for up to 4 weeks</li>
</ul>
<p><strong>Contraindication</strong></p>
<ul>
<li>Cushing syndrome</li>
<li>Peptic ulcer, diabetes mellitus, osteoporosis, epilepsy</li>
<li>Congestive cardiac failure, hypertension</li>
<li>Viral and fungal infection</li>
<li>Tuberculosis and other infection</li>
<li>Psychosis</li>
<li>Renal failure</li>
</ul>
<p><strong>Adverse reaction</strong></p>
<ul>
<li>Osteoporosis</li>
<li>Acneiform eruptions</li>
<li>Allergic contact dermatitis</li>
<li>Burning sensation</li>
<li>Cracking and fissuring</li>
<li>Cushing's syndrome</li>
<li>Dizziness</li>
<li>Dryness</li>
<li>Erythema</li>
<li>Glaucoma</li>
<li>Itching</li>
<li>Irritation</li>
<li>Numbness of fingers</li>
<li>Perioral dermatitis</li>
<li>Pruritus</li>
<li>Secondary infection</li>
<li>Skin atrophy</li>
</ul>
<p><strong>Nursing considerations</strong></p>
<ul>
<li>It should be used in the lowest dose, for the shortest period, of-of time is possible.</li>
<li>The patient should never change or stop the dosage except as directed.</li>
<li>Monitor input and output chart and vital sign. Notify the doctor if patient&rsquo;s blood pressure increased while taking drugs.</li>
<li>Discuss the potential for menstrual difficulties and amenorrhea related to long-term therapy.</li>
<li>The wound may heal slowly because steroids therapy can cause a delay in development of granulation tissue.</li>
<li>Monitor side effects including patient&rsquo;s mood change, depression and report to a doctor as changes take place.</li>
<li>A diabetic patient may need to change in their diabetes medication while taking corticosteroids.</li>
<li>Daily weight the patient. Rapid weight gain is usually an indication of fluid and sodium retention.</li>
<li>Always give H2 blockers with cortisone.</li>
<li>The topical cream should be applied sparingly and gently rub by using gloves.</li>
</ul>
<p>&nbsp;</p>

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Mechanical properties and their tests part 1

Mechanical properties and their tests part 1

Mechanical properties and their tests part

The mechanical properties of metals are strength, elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, fatigue, fracture and creep.

The materials are tested for one or more of the following purposes

  1. To calculate numerically the fundamental mechanical properties of materials like ductility, malleability, toughness etc.

2.To check chemical composition.

3.To determine the suitability of a material for a particular application.

4 To determine information like force-deformation or stress values to draw sets of specifications upon which the engineer can make up his design.

  1. To determine the surface or surface defects in raw materials or processed parts.

The tests on materials may be classified as

  1. Non-destructive tests: E.g. Radiography, ultrasonic, inspection etc.
  2. Destructive tests: E.g. tensile test, impact test, torsion test etc.

Tensile Test

Tensile testing, also known as tension testing, is a fundamental material science test in which a sample to be tested is subjected to axial tension until failure.

Stress-strain curve

During tensile testing of any material sample under study , the stress-strain curve drawn is actually a graphical representation of the relationship between stress derived from measuring the deformation of the sample. like (elongation, compression or any sort of distortion.)

ðœÅ½: Engineering stress 𝜀: Engineering strain 𝐿: Un-deformed length 𝑙: Elongation 𝐴: Un-deformed cross-sectional area 𝐹: Uni-axial load



Ductility: It measures the amount of deformation that a material can withstand without breaking. There are two ways of expressing ductility

Elastic Limit: It is the maximum stress value which the material can withstand without causing permanent deformation which remains even after the removal of stress.

Toughness: The toughness of a material indicates that material’s ability to absorb large amounts of stresses and deforming plastically before breaking.

True Stress-Strain Diagram

True stress-strain curve gives the actual indication of deformation characteristics. This curve is based on the instantaneous dimension of given specimen. This curve is also called as flow curve. In engineering stress-strain curve, it is seen as stress drops down after necking since the curve is based on the original area. In the true stress-strain curve, the stress actually increases after necking since the cross section area of the specimen decreases rapidly after necking.

The above curve shows the difference between real and engineering stress-strain curve.

The above stress-strain curve shows the behavior difference between ductile and brittle substance.

Ductile materials

In Ductile materials, generally, they exhibit a linear stress–strain relationship up to a well-defined yield point. After the yield point, the curve is seen to decrease slightly or very less. If Further deformation continues, the stress increases on account of strain hardening till it reaches the ultimate strength. Just before this point, the cross-sectional area decreases uniformly. This is because of Poisson contractions. The actual rupture point lies on the same vertical axis as the visual rupture point.

Brittle materials

Brittle materials such as concrete do not have any yield point and do not undergo strain-harden. Therefore, we can see an ultimate strength and breaking strength are the same. This is seen from above stress-strain curve for brittle materials. Brittle materials like glass do not show any plastic deformation but they instead fail while the deformation is elastic. One of the interesting properties of a brittle failure is that the two parts that are broken can be reassembled to form the same shape as the original part since there shall be no neck formation like in the case of ductile materials

Temperature effect on stress-strain curve

Temperature affects the stress-strain curve and the fracture and flows properties. Increase in temperature, strength of material decreases but the ductility increases and vice-versa. Thermally forwarded processes undergo deformation ( called dislocation motion) and reduce strength at elevated temperatures. Structural changes can also occur at certain temperature ranges (high temp/ long term exposure) to alter the general behavior. This depends on material nature and properties.

Notch effects and Brittle Behaviour

Notch is defined as any indentation or cut on an edge or surface. The sensitivity of metals and alloys to notch effects is termed as notch sensitivity. The presence of a notch can abruptly change material behavior. Tough, ductile metals or alloys frequently are notch-strengthened and they relatively have notch sensitivity ratio greater than one.

The behavior of materials can be classified into two categories; brittle and ductile. Glass and cast iron fall in the class of brittle materials. Brittle materials often found to have comparatively large Young's modulus and ultimate stresses in comparison to ductile materials.

Hardness Test

Hardness is the property of a material that enables the material to resist any plastic deformation, usually by penetration The term hardness may also the ability of a material to resistance to bending, scratching, abrasion or cutting. Hardness is the material property that varies from material to material.

Main Hardness Testing Methods

There are three general types of hardness measurements.

  1. Scratch Hardness

scratch hardness or the load measured in grams under which 90° diamond will produce a scratch 0.01 mm wide.

  1. Indentation Hardness:

This type of test is carried out in major engineering metals of interest. This includes Brinell, Rockwell, Vickers, Meyer hardness test.

  1. Rebound or Dynamic Hardness:

The indenter is dropped onto the metal surface and hardness is expressed as the energy of impact. The height of the rebound is used as a measure of the hardness of the surface. The surface to be tested should be smooth, free from oil and tube should be truly vertical.

Brinell Hardness Test

This test is the first standardized indentation hardness test done in around 1900. The Brinell hardness test consists in indenting the surface metal off with a steel ball Dia 10mm at a load of 500-3000 kg, depending on the hardness of particular materials. The applied load is subjected for the standard time of 30 seconds, and the diameter of the indentation is measured taking an average value of two readings for the diameter of the indentation made at right angle. The Brinell hardness number (BHN or HB) is denoted as the load P divided by the surface area of the indentation.

Where, F is applied load(kg),

𝐷is diameter of the ball(mm),

Di² is diameter of indentation(mm)

Advantages and disadvantages

  1. Large indentation averages out local heterogeneities of the microstructure.
  2. Different loads are used to cover a wide range of hardness value of commercial metals.
  3. Brinell hardness test is not much influenced by surface scratches and roughness than other hardness tests.
  4. The test has limitation on only small specimens or in critically stressed components where indentation might be a possible site of failure on structure

Rockwell Hardness Test

Rockwell hardness test is the most widely used hardness test and generally preferred due to speed, its freedom from personal error, ability to determine small hardness difference and small size of indentation. The hardness of any substance is measured as per to the depth of indentation under a constant load.

The instructions for Rockwell hardness test are listed below:

  1. Cleaned and well-seated indenter and anvil.
  2. Surface which is clean and dry, smooth and free from oxide.
  3. Flat surface, which is perpendicular to the indenter.
  4. Cylindrical surface gives low readings, depending on the curvature.

5.Thickness should be 10 times higher than the depth of the indenter.

6.Loading speed should be standardized.

7.The spacing between the indentations should be about 3 or 5 times the Dia of the indentation ball.

References:
1. D. R. Askeland, “The Science and Engineering of Materials”, PWS- Kent Publishing Co., Boston,
2. Westerman Table ( IS Standard)

Lesson

Mechanical Properties and their Tests

Subject

Mechanical Engineering

Grade

Engineering

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