114 C1

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responsible for the selection, specification and quality control of materials

Material Engineer

FACTORS THAT MATERIALS ENGINEER CONSIDER

• Economic Factors • Mechanical properties • Nonmechanical properties • Production • Aesthetic properties

isn't just affected by the cost of materials.

Economic

The local availability of materials can affect the choice. Using readily available materials can reduce transportation costs and environmental impact. In remote areas, the cost of transporting materials may influence the decision to use locally sourced materials.The local availability of materials can affect the choice. Using readily available materials can reduce transportation costs and environmental impact. In remote areas, the cost of transporting materials may influence the decision to use locally sourced materials.

Availability and cost of the raw materials

Cost considerations are crucial for staying within the budget. It includes not only the material cost but also installation, maintenance, and life cycle costs. Balancing quality with cost efficiency is a key decision factor.

Manufacturing Cost

The transportation of materials to the construction site can impact project costs. Materials that are bulky or heavy may require specialized transport and handling.

Transportation

Different materials have different impacts on construction. Steel-frame buildings are easier and faster to build than reinforced concrete buildings, because steel can be made in a shop and assembled on site. Reinforced concrete buildings require more steps and time on site, such as building forms, placing steel and concrete, and removing forms. Precast concrete units are used to avoid some of these problems, especially for bridges.

Placing

The durability of different materials is not the same. Some materials wear out faster than others. This affects how long the structure can last and how much it costs to maintain it. When choosing a material for a structure, the total cost over its lifetime should be considered, not just the initial cost of building it.

Maintenance

• All materials deform in response to loads however the specific response if a material depends on its properties, the magnitude and type of loads, and the geometry of the material. • Whether material "FAILS" under the load conditions depends on the failure criterion.

Mechanical Properties

loads that don't disturb the structure or material through vibrations or shock. That is why static loads are sustained loading of the structure over a period.loads that don't disturb the structure or material through vibrations or shock. That is why static loads are sustained loading of the structure over a period.

Static Loads

loads that disturb the material or structures through vibrations and shocks. This can be classified as periodic, random or transient.

Dynamic Loads

Harmonic or sinusoidal loads (repeating pattern)

Periodic

never repeating loads such as earthquake loads

Random

impulse load (applied over short period) such as vibrations caused by winds and trucks

Transient

When a material is subjected to a load or force, it changes its shape or size. This change is called deformation. Robert Hooke, a scientist, discovered in 1678 that there is a simple mathematical formula that describes how much a material deforms when a force is applied. The formula says that the deformation depends on the material's properties and its dimensions. The effect of the dimensions can be expressed as a ratio to make the formula more general.

Stress-Strain Relationship

Typical uniaxial stress-strain diagrams for some engineering materials:

glass and chalk, steel, aluminom alloys, concrete, rubber

If a material exhibits true elastic behavior, it must have an instantaneous response (deformation) to load, and the material must return to its original shape when the load is removed. Many materials, including most metals, exhibit elastic behavior, at least at low stress levels. Elastic deformation does not change the arrangement of atoms within the material, but rather it stretches the bonds between atoms. When the load is removed, the atomic bonds return to their original position.If a material exhibits true elastic behavior, it must have an instantaneous response (deformation) to load, and the material must return to its original shape when the load is removed. Many materials, including most metals, exhibit elastic behavior, at least at low stress levels. Elastic deformation does not change the arrangement of atoms within the material, but rather it stretches the bonds between atoms. When the load is removed, the atomic bonds return to their original position.

Elastic Behavior

the property of a material to withstand compression of the elongation with respect to its length. It is equal to the longitudinal stress divided by the strain.

Young's Modulus of Elasticity

the ratio of the transverse contraction strain to longitudinal extension strain in the direction of stretching force.the ratio of the transverse contraction strain to longitudinal extension strain in the direction of stretching force.

Poisson's Ratio

Some materials have a linear relationship between stress and strain until a certain point. Beyond that point, the strain increases rapidly with little change in stress. This means that the material can behave elastically or plastically depending on the stress level. The elastic limit is the stress level where the behavior switches from elastic to plastic. When the stress is removed, the material will return to its original shape partially, but some permanent deformation will remain.

Elastoplastic Behavior

Materials that do not undergo plastic Ductile Break deformation prior to failure, such as concrete, are said to be _______

Brittle

materials that display appreciable plastic deformation, such as mild steel, are_______

Ductile

The previous discussion assumed that the strain was an immediate response to stress. This is an assumption for elastic and elastoplastic materials. However, no material has this property under all conditions. In some cases, materials exhibit both viscous and elastic responses, which are known as viscoelastic. Typical viscoelastic materials used in construction applications are asphalt and plastics.

Viscoelastic Behavior

generally associated with long-term deformations and can occur in metals, ionic and covalent crystals, and amorphous materials.

Creep

is associated only with amorphous materials and can occur under short-term load duration.

Viscous Flow

When a material is tested, the testing machine is actually generating a force in order to move or deform the specimen. Since work is force times distance, the area under a force-displacement curve is the work done on the specimen. When the force is divided by the cross-sectional area of the specimen to compute the stress, and the deformation is divided by the length of the specimen to compute the strain, the force-displacement diagram becomes a stress-strain diagram. By manipulating the units of the stress-strain diagram, we can see that the area under the stress-strain diagram equals the work per unit volume of material required to deform or fracture the material. This is a useful concept, for it tells us the energy that is required to deform or fracture the material. Such information is used for selecting materials to use where energy must be absorbed by the member. The area under the elastic portion of the curve is the modulus of resilience(a). The amount of energy required to fracture a specimen is a measure of the toughness of the material, (b). As shown in (c), a high-strength material is not necessarily a tough material

Work And Energy

occurs when a member or structure ceases to perform the function for which it was designed.occurs when a member or structure ceases to perform the function for which it was designed.

Failure

is a common failure mode. A brittle material typically fractures suddenly when the static stress reaches the strength of the material. On the other hand, a ductile material may fracture due to excessive plastic deformation.

Fracture

a falilure well below the strength of the material that is cause by repeated stresses. The number of applications a material can withstand depends on the stress level relative to the strength of the material. Ferrous metals have an apparent endurance limit, or stress level, below which fatigue does not occur. fallure well below the strength of the material that is cause by repeated stresses. The number of applications a material can withstand depends on the stress level relative to the strength of the material. Ferrous metals have an apparent endurance limit, or stress level, below which fatigue does not occur.

Fatigue

This failure happens in ductile materials, and it spreads throughout the whole structure, which results in a total collapse.

General Yielding

a failure of long and slender members that are subjected to axial compression. Although the member is intended to carry axial compressive loads, a small lateral force might be applied, which causes deflection and eventually might cause failure.

Buckling

could be detined as tailure, depending on the function of the member. For example, excessive deflections of floors make people uncomfortable and, in an extreme case, may render the building unusable even though it is structurally sound.

Excessive Deformation

is defined as the ratio of the stress at failure to the allowable stress for design

Factor Of Safety

refer to characteristics of the material, other than load response, that affect selection, use, and performance. There are several types of properties that are of interest to engineers, but those of the greatest concern to civil engineers are density, thermal properties, and surface characteristics

Nonmechanical Properties

is the mass per unit volume of material.

Density

is the weight per unit volume of material.

Unit Weight

is the ratio of the mass of a substance relative to the mass of an equal volume of water at a specified temperature.

Specific Gravity

Practically all materials expand as temperature increases and contract as temperature falls

Thermal Expansion

The amount of expansion per unit length due to one unit of temperature increase is a material constant and is expressed as the

Coefficient Of Thermal Expansion

The surface properties of materials of interest to civil engineers include corrosion and degradation, the ability of the material to resist abrasion and wear, and surface texture.

Surface Characteristics

All materials wear out over time due to various factors. The causes of material wear depend on the type of material and the conditions it faces. Materials that have a regular structure, such as metals and ceramics, wear out by corrosion, which means they lose material either by dissolving or by forming non-metallic layers or coatings. Materials that are made of long chains of molecules, such as asphalt, wear out by degradation, which means they lose their properties because of solvents and ultraviolet light.

Corrosion And Degradation

Civil engineering structures are usually static, so they do not need to withstand much abrasion or wear. This is different from mechanical engineering, where the wear of parts is a major factor in designing machines. However, some structures, such as pavements, do need to resist wear and abrasion from vehicle tires. This affects the skid resistance and safety of the pavements. Therefore, the aggregates used in pavements should have good abrasion and wear resistance.

Abrasion And Wear Resistance

For instance, aggregate particles with smooth texture are desirable for Portland cement concrete, as they facilitate the mixing and placing processes. On the other hand, aggregate particles with rough texture are suitable for asphalt concrete mixtures, as they help to create a stable pavement layer that can withstand the load effects. Moreover, surface texture also affects the pavement performance, as it determines the friction resistance and the skidding potential of vehicles on wet conditions.

Surface Texture

The choice of a material for a specific project depends not only on its suitability for the purpose, but also on its availability, fabrication and construction. These factors affect the cost, time and quality of the project. Sometimes, different regions have different preferences for a material based on the experience and skills of the engineers and workers. For instance, some cities use high-strength concrete for tall buildings, while others use steel. Both materials can work well for this type of structure, but the regional factors influence the decision.

Production And Construction

The appearance of a material is its aesthetic characteristic. Architects usually decide how a material looks. But civil engineers need to help architects make sure that the material's look matches the structure's needs. Some public projects have a part of their budget for artistic design. Civil engineers and architects should work together to make the project more valuable. Sometimes, politics can be harder than technical issues. Engineers should know that they need to think about more than just technical aspects when they choose materials and design public projects.

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