Failure Modes - Assignment Example

FAILURE MODES Name: Course: Instructor: Institution: City: Date Q1. Briefly discuss the main aspects of behaviour of plastics, steel, concrete and wood under normal conditions and under fire conditions. PLASTICS Plastic is a material comprising of synthetic or semi-synthetic organics that are malleable and can be transformed into solid objects with different shapes (Duron, et al, 2008). According to Duron et al (2008), plastics are organic polymers of high molecular mass, and can also consist of other substances. Examples of plastics includes, nylon, polysulfone, phenolic, polyurethane, Acetal, poly-ether-ether-ketone, poly-vinyl-chloride. They behave differently under different temperatures. Under normal conditions plastics has the following aspects; Plastics have low density They have low thermal conductivity for example, acrylic, Plastics under normal conditions can be easily fabricated Some plastics for example, acrylic, are transparent Plastics for example poly-ether-ether-ketone are excellent temperature resistance, excellent flexural and tensile features. Plastics such as Poly-vinyl-chloride, resists both acidic solutions and gases Although plastics are not subject to corrosion, their exposure to large enough fire will make them burn. WOOD Wood is an organic, porous and fibrous structural material extracted from the stems or roots of trees and other woody plants (Brannigan, 2007). According to Brannigan (2007), it possesses the following properties; Wood can shrink, weak to shear, compression and tension forces Wood also has defects such as splits, knots, and non-straight grain that concentrate stress. Wood, if properly portioned, its structures can be ductile especially when kept short. They are tough, fibrous, fire supporting, light and are cut and graded by human beings If sheathing is nailed properly, the plywood sheathing of wood structures can make them tough and earthquake resistant. Although wood is a poor thermal conductor, when exposed to fire it will burn into ashes. Also their exposure to high ultra violet may cause their deformation. STEEL Steel is an alloy of iron and carbon, mainly used in construction and other various applications since it is very strong (Brannigan, 2007). According to Brannigan (2007), steel exhibits the following characteristics; It is ductile. Steel can be stressed beyond its elastic limit and strongly bent, but still consist of enough strength to resist failure. This therefore, makes steel the best structure material. It is tough, light and can be transformed into any shape, although it needs to be fireproofed. It begins to loose strength at a very high temperature. It is able to resist for a long time, tension, compression and shear forces since it is very strong Steel can be effectively connected by bolting and welding. Steel is tough and strong and can only undergo deformation or transformation under very high temperatures or when exposed to strong enough fire. Concrete Concrete is a composite material consisting of mainly, water, aggregate, and cement. Also reinforcement materials such as rebar are sometimes added into the mixture in order to achieve the required properties of the final product (Brannigan, 2007). According to Brannigan (2007), properties of concrete include; Expansion and shrinkage. Under normal conditions concrete has very low coefficient of expansion. However if provision for expansion is made or very large forces created, cracks can be experienced in some parts of the structure which cannot withstand high heat or temperatures. Concrete is strong in terms of compression forces and very weak if subjected to shear and tension forces or high temperatures. A properly reinforced concrete can provide seismically resistant construction especially when the reinforcement is sufficient to resist the shear forces. Elasticity. The modulus of elasticity of a concrete is relatively constant, but starts reducing at high stress level as matrix cracking develops. Concrete when exposed to fire or high temperatures may crack, sag or break into pieces Q2. Identify and discuss the different types of failure modes that can occur within structures. Types of failure modes Failures in the composite structures can either be stiffness dominated or strength dominated Tensile failures According to Brannigan (2007), in tensile failure, engineered composite materials behavior is characterized of stress-strain curves U V W Stress Y Strain U & Y represents tensile strength and elongation at break V represents tensile strength and elongation at yield W represents tensile stress and elongation at break X represents tensile stress and elongation at yield Explanation of terms used in tensile failures Tensile strength is defined as the maximum tensile strength during the test. Yield point is the point where increased strain occurs without increased stressed Strain refers to the change in length per unit Elastic limit is the maximum stress that a material can withstand with temporary deformation Proportion limit refers to the maximum stress that a material can withstand with linear behavior. Signs of Tensile Failure Cracking Cracks in a structure are caused by stress and the environmental state. Cracking is usually enhanced by sustained elevated temperatures, thermal and chemical environment in the presence of strain and ultraviolet rays (Brannigan, 2007). Crazing According to (Brannigan, 2007), crazes appear as clean fractures extending from the structure surface to the composite. They are a combination of fibrils. Also, they are the initial signs of surface tensile failures mainly in plastic materials and coat finishes. Stress Whitening According to Stroup, & Walton (2004), stress whitening mainly occurs in plastic materials that are stretched near their yield point. Regions with high stress in a structure appear to be whitish on the surface. In tensile failure, the ASCE Structural Plastics Design Manual has provided a method that can be used to approximate large deflections and stresses of isotropic plates when subjected to both membrane stress and lending (Stroup, & Walton 2004). Compressive Failure Compressive failure cannot be predicted easily since failure can occur at a very small scale such as, buckling or compression of individual fibres (Brannigan, 2007). Brannigan (2007) argued that, when compressive failure occurs in sandwich panels, the skin faces may be wrinkled or it may cause unstability of the panel. Classic beam theory is a theory used to explain this behavior and it states that, when the loaded face is in compression, the other face is in tension and as a result the core will experience some shear stress distribution profile. According Duron, (2008), the type of compressive failure for example, that a sandwich laminate will initially exhibit is a function of load span, skin to core thickness ratio, the relationship of core to skin stiffness and skin to core bond strength. Large unsupported panel spans will tend to experience general buckling as the primary failure mode. If the core shear modulus is very low compared to the stiffness of the skin, then crimping may be the first failure mode to be observed. Thin skin and poor skin-to-score bonds can result in skin wrinkling of the structures (Brannigan, 2007). Crimpling of the core takes place when shear modulus is too low to transfer load between the skins (Duron, 2008) The panel lacks the required moment of inertia, when the skins are required to resist the entire compressive load without the help from core and therefore it will fail a long thee core. Skin wrinkling is a type of buckling where the skins separate from core and buckle on their own. Sandwich skins can wrinkle symmetrically, anti-symmetrically and or on one side only. Compressive failure may lead to collapse of the stairs, general structure cracking, walls falling outside, damage to the elevators and poisonous gas or material leaks. First Ply Failure This is a type of failure that occurs when the first ply or group of ply fails in a multidirectional laminate. The design limit load corresponds to first ply failure. The total number of plies, relative stiffness of the plies and load sharing among the piles, determines the relationship between the first ply failure and overall failure of the laminate (Brannigan, 2007). For example, in a gel coated structural laminate, its surface is highly stressed region of laminate when subjected to flexural loading, although the gel will have the lowest elongation within the laminate. Therefore, the gel coated layer will be the first to fail, but the laminate capability or strength of carrying the load will to an extent remain unchanged. According to Brannigan (2007), tensile failure may cause general cracking of the component structure, collapse of the stairs, damage to elevators, raptures to the water tanks and damage at the construction joints and at the partition joints. The critical strain of each ply as provided by ABS Guide for Building and Classing High Speed Craft, is given by; ɛcrit Where; = Strength of ply being considered =Modulus of ply being considered = The distance from the bottom of the panel to the neutral axis = The distance from the bottom of the panel to the ply being considered = Thickness of ply under consideration Q3. Discuss the signs of collapse and collapse hazards of different types of construction. Collapse of structures is mainly caused by structural deterioration, natural disasters such as flooding, earthquakes, collusion impact for example from vehicles, explosions for example of flammable liquids and overloading of structural components (Stroup, & Walton,2004). Before collapsing structural components may show various signs, such as; Sagging of floors and roofs. This may be as a result of excessive loading on floors and roofs and heavy debris. Cracks in walls, columns or foundations which may have been caused by extreme forces or earthquake Bulging and separating walls which may result from aftershocks of earthquakes and extreme forces. Missing or broken structural elements for example doors, windows which may have been caused by high winds, heavy rains or snow Falling or sliding plaster and a falling dust which may have been as a result of heavy winds and heavy rains or snow. Swinging doors and windows Unusual sound, wood crushing, creaking and groaning of the elements of component structures According to Duron (2008), hazards that result from collapse may include; Exposure of human body to hazardous materials such as toxic gases, carbon monoxide, corrosive materials, radioactive materials Damage to buildings. This maybe structural or non-structural in nature and may cause permanent displacement of the building. This as a result may result into the owner incurring losses. Collapse may cause Injuries contamination and/or death to the occupants and the people in the surrounding. References Brannigan, F.L (2007). Building Construction, Third edition, National Building Protection Association, Quincy, Massachusetts, Duron, Z.H., Yoder, N., Kelcher, R., Hutchings, A., Markwardt, S., and Panish, R (2008), Fire Induced Vibration Monitoring for Building Collapse. Final Report, NIST GCR 06-885, National Institute of Standards and Technology, Gaithersburg, MD Duron, Z.H( 2008)., Early Warning Capabilities for Firefighters: Testing of Collapse Prediction Technologies, NIST GCR 03-846, National Institute of Standards and Technology, Gaithersburg, MD Stroup, D.W, & Walton, W.D (2004). Structural Collapse, National Institute of Standards and Technology, Gaithersburg, MD Read More