Understanding Material Classification and Properties
Classification of Materials: The most general classification of materials is as follows:
a. Metal: Ferrous, Nonferrous
b. Nonmetallic: Organic, Inorganic
Ferrous Metals: Ferrous metals, as their name suggests, have iron as their main component. Their main features include high tensile strength and hardness. The principal alloys are obtained with tin, silver, platinum, manganese, vanadium, and titanium.
The main products of representatives of metallic materials include gray iron castings, malleable iron, steel, cast iron, and white iron.
Nonferrous Metals: Nonferrous metals usually have lower tensile strength and hardness compared to ferrous metals, but their corrosion resistance is superior. Their cost is higher than ferrous materials, but with increased demand and new extraction and refining techniques, costs have considerably decreased, enhancing their competitiveness in recent years.
The main nonferrous metals used in manufacturing are aluminum, copper, magnesium, nickel, lead, zinc, and titanium.
Nonferrous metals are used in manufacturing as complementary elements to ferrous metals. They are also very useful as pure or alloyed materials, which, due to their physical properties and engineering characteristics, meet specific requirements or working conditions, such as bronze (copper, lead, tin) and brass (copper, zinc). Uses include structures, mechanisms, wires, pipes, etc.
Non-Metallic Materials: Materials of organic origin and inorganic origin.
Organic Materials: These are considered organic when they contain plant or animal cells. These materials are usually dissolved in organic liquids such as alcohol or tetrachlorides, do not dissolve in water, and will not withstand high temperatures. Other features include low electrical and thermal conductivity and good resistance to corrosion. Some representatives of this group are plastics, wood, oil, rubber, and paper. Uses include packaged food and wood adhesives.
Inorganic Materials: All those that are not derived from animal or plant cells or related to coal. They can usually be dissolved in water and generally resist heat better than organic substances. Some commonly used inorganic materials in manufacturing are minerals, cement, ceramics, glass, and graphite (coal).
Materials, whether metallic or nonmetallic, organic or inorganic, are almost never found in the state in which they are used. They usually need to undergo a set of processes to achieve the characteristics required for specific tasks. These processes have required the development of special techniques and structures that have provided the sophistication necessary to meet practical requirements. These processes also considerably increase the cost of materials, which can mean several times the original cost of the material, directly impacting the cost of materials and integrated items. Uses include insulation, corrosion protection, and refractories.
The manufacturing processes involved in converting original materials into useful materials for mankind require special studies to obtain the best application development and cost reduction. In engineering, the transformation of materials and their properties holds a special place, as it often determines the success or failure of material use.
Structural Behavior of Materials: As we all know, construction uses various types of materials, some chosen for design aesthetics and others for structural resistance. The impact of fire on these materials will cause them to behave differently according to their composition. In this course, we will study the behavior of materials such as steel, concrete, and wood, which are common to all existing building systems.
• When materials are in their pure state, meaning they have no protection or covering, they suffer from more extreme fire action. Steel, usually subjected to high temperatures, poses a significant risk; heat spreads quickly through it, and when the material supports loads, it can easily collapse.
• Fire: For each material, we can differentiate: 1. A temperature at which the material is gasified (gasification temperature); 2. A temperature at which the material ignites (ignition temperature).
• The Fire Triangle: Leaving a piece of iron in the open, its color changes, and it loses its original characteristics; it is oxidized. This means that the oxygen in the air combines with the iron to produce iron oxide. A fire is a similar phenomenon: the oxygen in the air combines with combustible materials, but in a violent manner. This rapid oxidation is called combustion.
For a material to enter combustion, certain conditions must be met: 1. There must be enough oxygen, which is usually not a problem since the air around us contains it. 2. A second condition is that there is a combustible material. 3. The third condition is that there is enough heat to start combustion.
• The Triangle and More: Once a fire is made, it can often sustain itself without stopping until only ashes remain. To explain this aspect of fire, current science adds a fourth element to the three we’ve seen: chain reaction. When the fire is intense enough, flames and heat are released, facilitating the blend of oxygen and fuel, which creates new and hotter flames. This chain reaction continues as long as there is oxygen and fuel.
• Heat Transfer: A common origin of fire is a relatively small outbreak, which can be transmitted to other objects and places, leading to significant loss. Therefore, it is important to understand how heat is transferred.
• Conduction: Occurs when an object is in direct contact with another. The heat from the hotter object passes into the cooler one (0 Law of Thermodynamics).
• Radiation: The heat from a flame can be felt some distance from the fire itself, as it is transmitted by invisible heat waves (electromagnetic waves) that travel through air or space (like the sun). Therefore, it is not necessary for an object to touch the fire to burn, as heat can “jump” from one place to another through the air.
• Convection: When heat waves pass through a fluid (for example, air, water, oil, etc.), part of their heat warms the fluid, which then tends to move up or cool down. This means that heat generated at one point spreads to another place. This is called convection transmission. For example, if a fire starts on a ground floor of a multi-storey building, the fire heats the air, which rises to the upper floors, carrying gases, smoke, and fire.
Classification of Fires: In our country, Chilean Standard No. 934, National Institute of Standardization, classifies fires into four classes, each assigned a special symbol. These symbols, along with fire extinguishers, help determine whether the extinguisher is suitable for the type of fire you want to extinguish. These classes are:
Fire Class A: Class A fires occur in ordinary combustible materials such as wood, paper, cardboard, textiles, and plastics. When these materials burn, they leave waste in the form of coals or ashes. The symbol used is the letter A in white on a green background triangle.
Fire Class B: Class B fires occur in liquid flammable fuels such as oil, gasoline, and paints. This group also includes liquefied petroleum gas and some greases used to lubricate machinery. Unlike previous fires, these leave no residue when burned. Its symbol is the letter B in white on a red background square.
Fire Class C: Class C fires are commonly identified as “electrical fires.” More precisely, they occur in “equipment or electrical loads,” meaning they are energized. Its symbol is the letter C in white on a blue background circle. When a class C fire occurs, if power is disconnected, it may become class A, B, or D, depending on the materials involved. However, it is often very difficult to be absolutely certain that the power has been completely cut off. Indeed, even if you disable a general board, the facility may still be powered by another circuit. Therefore, you should treat class C fires as such until you can ensure complete assurance that there is no electricity.
Fire Class D: Class D fires occur in dust or chips of light metal alloys such as aluminum and magnesium. Its symbol is the letter D, white, with a star on a yellow background.
Thermodynamics of Fire: The thermodynamics of each fire has a unique behavior depending on the area in which it develops. However, some common characteristics allow for classification and analysis, which are useful for designers who must not lose sight of the fact that fire can create true high-temperature furnaces that destroy the supportive capacity of the structure.
Three Important Factors for Fire Development: 1. Combustible materials, furniture, coatings, electronic equipment plugged into overloaded electrical systems, or flammable materials stored carelessly. 2. Ventilation: The amount of air available determines the brightness of the fire and the speed of combustion.
The performance of ventilation is crucial for temperature scaling. The amount of air available to a fire is critical for its behavior, but the degree of temperature depends on how quickly heat can dissipate. In other words, slow combustion that fails to dissipate heat can create catastrophic conditions. First, the metal components of the structure will lose their supportive capacity. 3. Heat dissipation: This is most dangerous if heat dissipates quickly without adequate ventilation; the temperature can damage the structure and cause collapses.
Additionally, rapid heat dissipation can create dangerous conditions for firefighting personnel, as the natural state of matter will be broken to ignite, and if suddenly supplied with air, we may have a rapidly developing ignition with possible explosive results.
A less malignant scenario will occur in a live fire where heat dissipates rapidly, and in an open fire, it will end when its fuel supply runs out. There will be a greater likelihood of saving the structure with less damage, and firefighting personnel will face less risk.
Finally, the development of a fire depends on the design of the structure, the degree of ventilation, and thus its ability to dissipate heat, the flammability of the contents, and the construction materials.
Steel: Steel is a good conductor of heat, as it is one of the classic forms of heat transfer through conduction. The iron (the majority element in steel) and metal have free electrons, which can spread heat easily through constructed elements (beams, columns, panels, etc.), causing new outbreaks. When steel melts between 1,300 ºC and 1,400 ºC, it loses half of its strength at around 500 ºC. The heat expands easily, causing a beam of 20 m to reach 21 m at this temperature, resulting in structural steel losing two-thirds of its initial strength in proportion to the increase in load it is subjected to, starting to sag and give way, dragging the rest of the supporting elements of the construction.
In general, all metals under the action of heat face a maximum risk of distortion and collapse.
As part of a structural frame, a steel beam may experience local collapse, emphasizing the importance of providing structural protection appropriate to their nature or operational conditions.
The behavior of steel structures does not presuppose the presence of high or abnormal temperatures; even small to moderate fires can produce material deformation.
Passive Fire Protection: In the development of any construction project involving steel structures, the responsible professional must consider the building’s fate, floor area, number of floors, number of occupants, and the restrictions of massive structural elements used, including the thickness of material for fire protection associated with it, the length of protection required, and the critical temperature of failure due to yielding of unprotected steel (550 ºC). Current legislation in Chile considers only the protection of structures against fires of cellulosic character according to the standard curve UL 263 (International Standard Fire Tests of).
International Legislation: In Europe and the U.S., there is ample information and legislation regarding: – Cellulosic and hydrocarbon fires generated. – Past stamps – Seismic isolators.
Concept of Mass: The relationship between the perimeter exposed to fire and the sectional area of an element: NOTE: The massive lists associated with each type of profile are shown in NCh935-1 Of97. NOTE: The higher the value, the more protection is required.
Concrete: Structural reinforced concrete, prestressed, and post-tensioned concrete usually exhibit good resistance, defined by the time before temperature behavior is observed in the spectrum of a fire. Given its composition, structural concrete generally does not collapse in a fire, although it may experience deviations in both position and soil load. Most structures are usually safe enough to restore their normal functions after suffering from fire. However, traction and bending resistance of concrete are the most affected, while compressive strength experiences a reduction of about 80% at around 800 ºC. Even traditionally considered non-combustible materials (like concrete) are not entirely safe against fire. If we consider that a fire can easily reach 600 ºC within 10 minutes of initiation, and 1,200 ºC at 20 minutes, we understand that even concrete is not absolutely safe.
At 1,000 ºC, gravel and cement disintegrate and dehydrate. If temperatures of 1,000 ºC to 1,200 ºC are maintained for approximately three hours, the effects of fire on concrete are certainly harmful. Concrete elements disintegrate at a rate of about four (4) cm per hour, and at these temperatures, the reinforcement fails to fulfill its function.
Concrete can corrode slowly, leading to total destruction, including its reinforcement. Every porous construction element easily absorbs combustion gases, which are acidic. The chemical reaction neutralizes with calcium compounds contained in the structural concrete, forming calcium chloride, a hygroscopic substance that, when combined with the extinguishing water vapor content in the air confined by the structure, is also absorbed by the concrete in calcium and chloride ions. This concrete corrosion occurs very slowly after the fire, continuing migration or penetration of about 0.25 to 2 cm² per day, if environmental conditions are favorable. In this case, the corrosion of steel is much more significant than that of concrete when conditions are favorable. The percentages of chlorine that could damage the concrete are approximately 0.6% of chloride for normal concrete and approximately 0.01% for prestressed concrete.
What is Prestressed Concrete? Prestressed concrete is concrete that has been introduced with compression reinforcement cables or pre-tensioned steel wires before commissioning. Prestressing is usually induced by strands of high-strength steel, which are tensed and then anchored. The strands should be able to prestress concrete based on their adherence to the concrete, as occurs in prestressed concrete. Intentionally leaving ducts can also create a default profile within the element, allowing steel cables to be moved through them, and then applying the prestressing force using hydraulic jacks. Finally, the strands are anchored at the ends. This procedure is known as post-tensioned concrete. When applying this technique, high-strength concrete and steel are used to withstand the enormous stresses induced.
How Does Concrete Behave Before Fire? • Compressive strength remains almost constant up to the critical temperature. • The elastic modulus decreases. • The density decreases.
Consequences of Fire: • “Spalling” is the loss of concrete surface tension due to mechanical stresses induced by temperature gradients. • “Spalling” occurs only in the presence of strong temperature gradients (during heating or cooling). • “Spalling” results from a large number of simultaneous processes. NFPA 921 provides some likely causes: 1. Moisture in fresh concrete. 2. Differential expansion between concrete and steel reinforcements. 3. Differential expansion between concrete and reinforcement and various aggregates.
Wood: Woody plants are composed of water and two types of substances: cellulose and lignin. The percentage of both compounds ranges around 90%, with the rest being minerals, fats, waxes, etc. If a fire occurs, wood, as a structural element, has the peculiarity of absorbing gases and vapors without apparent damage. However, over time, the wood can release acids it gradually absorbed, such as hydrochloric and hydrocyanic acids. The specific risk of wood is its potential to convey the risk of corrosion to surrounding materials. In fires where PVC is present, this circumstance is exacerbated by exposure to wood vapors. Sometimes, the losses can be quite significant over time, which can often lead to confusion regarding these effects.
The charring depth or growth in the coal seam occurs at a rate of 0.8 mm/min during the first 8 minutes. After this, the carbon layer has an insulating effect, and the rate decreases to 0.6 mm/min. Considering the time for initial ignition, rapid carbonization, and then the delay at a constant rate, the average charring rate is about 0.6 mm/min (or 1.5 m/h).
There are differences between species associated with their density, anatomy, chemistry, and permeability. The moisture content is an important factor affecting the rate of carbonization. Density relates to the mass needed for degradation, and anatomical characteristics influence this. Carbonization in the longitudinal direction is twice that in the transverse direction, and chemicals can affect the relative thickness of the coal seam. Permeability affects the movement of moisture driven through the fibers of wood under the coal seam.
Behavior of Wood in Fire: • Describes the capacity of a material to resist fire within certain temperature limits. The materials used in public buildings, houses, and others should be subjected to laboratory tests to be classified according to two criteria: Fire Feedbacks and Resistance to Fire.
Flashover: Flashover is the transition from a fire’s development phase to the phase of a fully developed fire, in which thermal energy release is at its maximum, depending on the fuel involved.
These criteria form the basis of passive fire protection, which aims to minimize fire risk, prevent or limit the spread of fire to other parts of the building or neighboring properties, facilitate evacuation of people who may be inside, and assist in fire suppression. Therefore, it is necessary to consider the materials used, the provision of fire walls, partitions, fire doors, staircases, means of escape, and generally a criterion of compartmentalization and fire resistance, as well as smoke and hot gases, which are always highly toxic.
Reaction to Fire and Fire Resistance: During a fire, two different states must be considered in the design of buildings regarding materials and structures used. There is an initial fire and then a fully developed fire.
The first term represents the response of materials (content) to an initial fire attack and includes properties such as ignition time, flame propagation, heat release, and smoke. These properties are relevant in the initial development of the fire.
The use of liners or coatings, such as wood fuel in buildings, is restricted to limit the fire growth rate, but their contribution is often overestimated concerning the contents of the building.
However, some limitations are necessary, especially in escape routes.
On the other hand, in a fire or fully developed fire, the action of supporting structures and separators (walls) is essential to limit the fire to the room of origin. This is called the fire resistance of the structure.
Another important aspect considered in structural fire safety is the construction details, such as firewalls, ventilation, and fire separators in attics.
Mechanisms of Protection Against Fire in Wood: The fact that wood is a combustible material does not preclude its use as a stable and secure building material. This can be overcome through various mechanisms that seek to delay ignition, prevent the spread of flames, and maintain structural stability. These include:
• Proper construction and architectural designs. The configuration of various elements of a home should be harmonious in terms of safety and aesthetics.
• Using appropriately sized structures. Certain structural elements must have oversized safety margins depending on the time required for rescue and salvage operations during a fire.
• Fire-retardant treatments or flame retardants can increase the ignition temperature of wood and reduce the production of flames that can spread rapidly to other surfaces or devices nearby.
• Applying the approach of compartmentalization, confining the fire to an area to prevent it from spreading to another room.
• Using firewalls, which, as their name implies, are elements that prevent the passage of air or oxygen in certain pockets of the building, thereby preventing the fire from spreading faster.
Legislation and technical regulations place special emphasis on passive protection against the spread of fire as a preventive action, detailing how the composition of components protects structural materials through appropriate coatings and/or different treatment concepts currently used: planimetric and massive partitioning. The first is directly related to proper design and layout of enclosures, where the location of surfaces (indoor, outdoor, and mediators) can confine the fire in its origin, thereby slowing the spread of fire to other areas of the building or other properties. The second expresses the relationship between the outer surface of the element exposed to fire and the cross-section of the same element, ensuring greater heat resistance and preventing premature collapse, thus requiring a massive peak equal to or less than 390m-1 according to NCh 935/1.Of. 97.
Fire Protection Regulations for Buildings: Firefighting, in both prevention and protection facets (prevention measures are taken to avoid a fire), can be carried out in two ways: Active Protection includes activities involving direct action using facilities and means for protection and firefighting, such as evacuation, the use of fire extinguishers, and fixed systems. Passive Protection or Structural Protection includes methods that owe their effectiveness to being permanently present, but without implying any direct action on the fire. These passive elements do not act directly on the fire, but they can compartmentalize its development (walls), prevent the collapse of the building (coated metal structures), or allow the removal and disposal of heat that would make them impossible to function.
Shock Effects: These occur in parts subject to instantaneous or sudden changes in external loads, which may occur by chance. Their failure generally does not accept plastic or brittle deformation, even in those considered ductile metals. In these cases, it is convenient to analyze the behavior of the material under shock or impact. The static tensile test provides correct values of the ductility of a metal, but it is not necessary to determine the degree of toughness and fragility, which work under variable conditions.
Fatigue: In the study of materials in service, such as machine components or structures, it should be noted that the predominant solicitations are generally not static or quasi-static. Instead, they often involve changes in stress, whether tensile, compression, bending, or torsion, which are systematically repeated. The rupture of the material occurs at values significantly lower than those calculated in static tests. This type of break, which occurs over time, is named fatigue and is commonly identified as broken by repeated stresses. Strains may act individually or in combination.
Classification of Fatigue Tests: Overall fatigue tests are classified by the range of load-time and may present as: – Fatigue tests of constant amplitude. – Amplitude fatigue tests variable.
Amplitude Fatigue Tests Constant: Constant amplitude tests evaluate fatigue behavior with predetermined cycles of loading or deformation, generally sinusoidal or triangular, at constant amplitude and frequency. Extension trials are low and high cycle, estimating the capacity to survive fatigue life by the number of cycles to failure (initiation and propagation of the fault) and resistance to fatigue by the extent of stress for a predetermined number of breaking cycles. It is usually referred to as resistance to fatigue at the maximum stress under which the material does not break or that which corresponds to a preset number of cycles of metals or alloys. In this respect, ASTM E defines fatigue limit as the stress corresponding to a very large number of cycles.
Amplitude Fatigue Test Variable: In fatigue, when the amplitude of the cycle is variable, it evaluates the effect of accumulated damage due to the variation of the stress amplitude over time. These tests involve a high number of cycles with load control, depending on the chosen load spectrum, which will be more or less representative of service conditions.
High Number of Fatigue Cycles: The load spectra-time trials arising from constant amplitude loading cycles resemble simple continuous functions, usually sinusoidal. In general, any applied stress cycle may be regarded as resulting from a constant or static load (σ m) and another variable constant amplitude (σ a) pure sine wave.
Hardness: This method allows us to obtain important mechanical properties quickly and non-destructively, enabling the assessment of ready-made parts. Definition: “The more or less resistance a body opposes to being scratched or penetrated by another” or “the degree of hardness of a body compared to another taken for comparative purposes.”
Hardness Method: Static penetration test, bounce test, scratch test, and abrasion and erosion tests.
Penetration Test: Defines hardness and resistance to penetration or deformation resistance, which opposes a material to being pressed by a given indenter under preset loads.
Rockwell Hardness: This is calculated based on the penetration depth and the total load applied, which is not continuous. There is an initial charge and an additional one (which varies according to test conditions). The value is obtained directly from the dial indicator. Hardness is given by the increase in penetration due to the action of the additional burden once it is removed.
Vickers Hardness: This is similar to Brinell hardness, and its value depends on the applied load and the surface of the stamp or mark. The charges vary from 1 to 120 kgf, and the indenter is a diamond-shaped pyramid.
Tension: A body is subjected to simple tension when uniformly distributed normal loads are applied to its cross-sections, causing elongation. The bollard pull test is the best way to determine the mechanical properties of metals, defining their strength and deformability characteristics. Under a simple state of stress, the yield strength or its practical replacement, the maximum load, and the subsequent static strength are based on whose values the allowable or project (σ adm.) can be set. Using empirical methods, the behavior of materials subjected to other types of solicitations (fatigue, hardness, etc.) can be known.
Material Properties: The properties of metals are the individual characteristics of each, defining their capabilities and future uses.
Physical Properties: The most common physical properties include:
Color, Density: The mass of a body per unit volume.
Specific Gravity: This is directly related to density. A dense material has high specific gravity.
Melting Point: The temperature at which a material becomes liquid under a given pressure.
Boiling Point: The temperature at which the vapor pressure of a liquid equals the atmospheric pressure existing on the liquid. At temperatures below the boiling point (PE), evaporation occurs only on the surface of the liquid. During boiling, steam is formed inside the liquid, which rises to the surface in the form of bubbles, creating a characteristic tumultuous boil.
The boiling point is directly proportional to the pressure; boiling points for various elements and compounds relate to normal atmospheric pressure unless otherwise specified.
Mechanical Properties: The mechanical properties relate to how metals react to forces acting on them. Testing is the best way to determine the mechanical properties of a material. The information obtained after performing the appropriate tests will help us choose the most suitable material for a specific utility. The important mechanical properties include:
Elasticity: The ability of some materials to recover their shape once the force that deformed them is removed.
Plasticity: The ability of a material to retain its new shape once deformed. This is the opposite of elasticity.
Ductility: The ability of a material to stretch into threads (e.g., copper, gold, aluminum, etc.).
Plasticity: The ability of a material to stretch without breaking (e.g., aluminum, gold, etc.).
Hardness: The resistance a body has to being scratched or penetrated by another, or the same as wear resistance.
Resilience: The resistance a body has to sudden shocks or stress.
Fragility: This is the opposite of resilience. A material is considered fragile if it breaks into pieces when a force is applied.
Toughness: The resistance a body has to breaking when subjected to slow strain efforts.
Fatigue: Deformation (which can lead to breakage) of a material subjected to varying loads below the fracture point, when they act over a certain time or number of cycles.
Machinability: The ease with which a body can be cut by chip removal.
Acrimony: Increased hardness, brittleness, and resistance in certain metals as a result of cold deformation.
Castability: The ability of molten material to fill a mold.
Tension: The ability of a material to be elongated. If we stretch a wire of 1 mm² on both sides, there comes a time when it breaks. The force required to break the wire at this section is called tensile strength.
Compression: The ability of a material to be compressed.
Coefficient of Linear Expansion: The ability of materials to expand or contract in a linear percentage according to the material or alloy, depending on the temperature to which it is subjected.
Shear: The condition of a material to be broken.
Torque: The force exerted on a material causing a twist when one end is fixed and the other is free.
Buckling: The effort exerted in a column at its upper part, being fixed at the base, causing a twist and praise by the media.
Resistance: This is the cohesive force of the smallest particles (molecules) against mechanical stress.
Chemical Properties: One of the most important is the relative corrosion and oxidation of materials (especially metals). For instance, steel and its alloys are easily oxidized in contact with moisture, while aluminum creates a protective oxide layer that prevents further oxidation. Therefore, materials are usually painted to prevent oxidation and improve their presentation. The choice of material must be made carefully, depending on the application for which it is intended. For example, a spoon used to remove acid in food must be different from one used for chemical reactions that can cause deterioration.
Electrical Properties: The ease with which some metals conduct electrical current.
Thermal Properties: This property describes how a material reacts to heat. Most metals are good conductors of heat. For example, radiators are made of metals that lead heat. On the other hand, fiberglass or polyurethane is used in construction for thermal insulation of walls and ceilings.
Magnetic Properties: Most ferrous metals (iron and its alloys) are attracted by electromagnetic fields; however, others, such as copper or aluminum, are not. Superconductors (made of special materials and cooled in liquid nitrogen) produce large magnetic fields and are named because they offer resistance to the passage of electrical current.