Mechanical properties of Building Materials

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This ppt explains in brief all the mechanical properties of the building materials. Useful for B. Tech Civil Engg. IInd year students.

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BUILDING MATERIALS & CONSTRUCTION Unit -1: Mechanical Properties of Building Materials:

BUILDING MATERIALS & CONSTRUCTION Unit -1: Mechanical Properties of Building Materials By Nauras Saiyed Asst. Professor Department of Civil Engineering

Mechanical Properties :

Mechanical Properties Abrasion Strength Creep Fatigue Hardness Elasticity & Plasticity Ductility & brittleness Toughness Impact Strength Wear

Mechanical properties of a few building materials:

Mechanical properties of a few building materials Hardness of diamond Ductility of copper Elastic behaviour of rubber Cement and bricks – strong in compression Wood and steel – strong in tension

Abrasion:

Abrasion Abrasion is the process of scuffing, scratching, wearing down, marring, or rubbing away. It can be intentionally imposed in a controlled process using an abrasive. Abrasion can be an undesirable effect of exposure to normal use or exposure to the elements. The resistance of a material to the abrasion is found out by dividing the difference in weights of specimens prior to and after abrasion with the area of abrasion Weight loss is a good indication of the abrasion resistance of a product.

Abrasion - Examples:

5 Abrasion - Examples Abrasion causes rocks to become smaller and more rounded.

Abrasion – Examples –Water Spray Test:

Abrasion – Examples –Water Spray Test The above fig. shows relative abrasion qualities of different samples stabilised soil products have good abrasion characteristics. Effect of a spray test on a soil block compacted at 2MN/m 2 Effect of a spray test on a soil block compacted at 8MN/m 2 http://collections.infocollections.org/ukedu/uk/d/Jh2380e/7.4.3.html

Stress:

Stress This is a measure of the internal resistance in a material to an externally applied load. For direct compressive or tensile loading the stress is designated  and is defined as:

Types of stress:

Types of stress Compressive stress Compressive load Tensile load Compressive load Tensile load Tensile Stress

Strain:

Strain We must also define strain . In engineering this is not a measure of force but is a measure of the deformation produced by the influence of stress. For tensile and compressive loads: Strain is dimensionless, i.e. it is not measured in metres, killogrammes etc.

Ultimate Strength:

Ultimate Strength The strength of a material is a measure of the stress that it can take when in use. The ultimate strength is the measured stress at failure but this is not normally used for design because safety factors are required. The normal way to define a safety factor is :

Creep:

11 Creep In many applications, the building materials are required to sustain steady loads for long periods. Under such condition, the material may continue to deform until its usefulness is seriously reduced. Such time-dependent deformation of a structure can grow large and may even result in final fracture without any increase in load

Creep:

Creep Time dependant permanent deformation At constant stress, strain continues to increase

Fatigue:

Fatigue When materials are subjected to a repetitive or fluctuating stress, they will fail at a stress much lower than that required to cause fracture under steady loads. This behaviour is called fatigue.

Fatigue:

Fatigue Fatigue failure is characterized by three stages Crack initiation Crack Propagation Final Fracture

PowerPoint Presentation:

Perhaps the most likely form of cracking to be encountered by engineers is that from fatigue. For a crack to progress there has to be an initiation site and it can be assumed one will always exist, certainly where there has been welding or thermal cutting. There also has to be an alternating tensile stress, preferably with some kind of stress concentration, to provide the frequent input of energy required for crack Progression. Example Checking for fatigue is an obvious condition on structures carrying traffic. This is very pronounced in tracks carrying roller coasters where down forces can be linked to accelerations up to 4 g and where there are multiple cycles of load applied through the many train wheels and with trains passing every few minutes all day long. The condition is so burdensome that track design is dominated by the need to consider fatigue loading and such tracks have a defined life as a consequence

Fatigue failure in a roller coaster:

Fatigue failure in a roller coaster

Difference between Creep & Fatigue:

Difference between Creep & Fatigue Fatigue happens under a repeated (cyclical) load, eg . Repeatedly pressing and releasing a spring. Creep occurs under constant load, eg . A turbine blade at a power plant that sees very long run cycles of months at a time. Also note that fatigue occurs due to fluctuating or reversing loads at a stress level well below the yield point (usually below 60% of yield especially for high strength steels), while creep occurs at a steady and very high load near the yield point (above 80 % of yield).

Hardness:

18 Hardness Hardness is a measure of a material’s resistance to localized plastic deformation (a small dent or scratch). Quantitative hardness techniques have been developed where a small indenter is forced into the surface of a material. The depth or size of the indentation is measured, and corresponds to a hardness number. The softer the material, the larger and deeper the indentation (and lower hardness number).

Hardness:

19 • Resistance to permanently indenting the surface. • Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties. Adapted from Fig. 6.18, Callister 6e. (Fig. 6.18 is adapted from G.F. Kinney, Engineering Properties and Applications of Plastics, p. 202, John Wiley and Sons, 1957.) Hardness

PowerPoint Presentation:

Mohs ’ Scale of Hardness The hardness of stone materials can be determined with the help of this scale.

Elasticity:

Elasticity When a load is applied to a material, there is change in its shape and dimension. The term elasticity is used to indicate the ability of material to restore its initial form and dimensions after the load is removed.

Elastic Deformation:

22 1. Initial 2. Small load 3. Unload Elastic means reversible. Elastic Deformation

Plastic Deformation (Metals):

23 1. Initial 2. Small load 3. Unload Plastic means permanent. Plastic Deformation (Metals)

PowerPoint Presentation:

24 Typical stress-strain behavior for a metal showing elastic and plastic deformations, the proportional limit P and the yield strength σ y , as determined using the 0.002 strain offset method (where there is noticeable plastic deformation). P is the gradual elastic to plastic transition.

Plastic Deformation (permanent):

Plastic Deformation (permanent) From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors. After removal of the stress, the large number of atoms that have relocated, do not return to original position. Yield strength is a measure of resistance to plastic deformation .

Ductility, %EL:

Ductility, %EL Ductility is a measure of the plastic deformation that has been sustained at fracture: A material that suffers very little plastic deformation is brittle . tensile strain, e tensile stress, s smaller %EL (brittle if %EL<5%) larger %EL (ductile if %EL>5%)

PowerPoint Presentation:

• Another ductility measure: • Ductility may be expressed as either percent elongation (% plastic strain at fracture) or percent reduction in area . %AR > %EL is possible if internal voids form in neck.

PowerPoint Presentation:

Lower toughness: ceramics Higher toughness: metals Toughness is the ability to absorb energy up to fracture (energy per unit volume of material). Energy to break a unit volume of material A “ tough ” material has strength and ductility . Approximated by the area under the stress-strain curve . Toughness

Impact Strength:

Impact Strength The impact strength of a material is the quantity of work required to cause its failure per its unit volume. The impact strength is a complex characteristic which takes into account both toughness an strength of a material

Wear:

Wear The failure of a material under combined action of abrasion and impact is known as its wear. The wear resistance is usually expressed as a percentage of loss in weight It is of great importance in deciding the suitability of a material for use in road surfaces, railway ballast, etc.

THANKS:

THANKS

PowerPoint Presentation:

Stress-Strain Diagram Strain ( ) ( D L/Lo) 4 1 2 3 5 Stress (F/A) Elastic Region Plastic Region Strain Hardening Fracture ultimate tensile strength Slope= E Elastic region slope =Young’s (elastic) modulus yield strength Plastic region ultimate tensile strength strain hardening fracture necking yield strength

Important Mechanical Properties from a Tensile Test :

33 Important Mechanical Properties from a Tensile Test Young's Modulus : This is the slope of the linear portion of the stress-strain curve, it is usually specific to each material; a constant, known value. Yield Strength : This is the value of stress at the yield point, calculated by plotting young's modulus at a specified percent of offset (usually offset = 0.2%). Ultimate Tensile Strength : This is the highest value of stress on the stress-strain curve. Percent Elongation : This is the change in gauge length divided by the original gauge length.

Modulus of Elasticity:

Modulus of Elasticity If the strain is "elastic" Hooke's law may be used to define Young's modulus is also called the modulus of elasticity or stiffness and is a measure of how much strain occurs due to a given stress. Because strain is dimensionless Young's modulus has the units of stress or pressure

Fatigue:

Fatigue Stress Strain Failure

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