Stages is fatigue failure: I. crack initiation at high stress points (stress raisers) II. propagation (incremental in each cycle) III. final failure by fracture Stage I – propagation • slow • along crystallographic planes of high shear stress • flat and featureless fatigue surface Stage II – propagation Crack propagates by repetitive plastic blunting and sharpening of the crack tip.
Fatigue is the catastrophic failure due to dynamic (fluctuating) stresses. It can happen in bridges, airplanes, machine components, etc. The characteristics are: • long period of cyclic strain • the most usual (90%) of metallic failures (happens also in ceramics and polymers) • is brittle-like even in ductile metals, with little plastic deformation • it occurs in stages involving the initiation andpropagation of cracks. Cyclic Stresses These are characterized by maximum, minimum and mean stress, the stress amplitude, and the stress ratio.
Fundamentals of Fracture Fracture is a form of failure where the material separates in pieces due to stress, at temperatures below the melting point. The fracture is termed ductile or brittle depending on whether the elongation is large or small. Steps in fracture (response to stress): • Crack formation • Crack propagation Ductile Fracture Stages of ductile fracture – Initial necking – Small cavity formation (micro voids) – Void growth (ellipsoid) by coalescence into a crack – Fast crack propagation around neck. Shear strain at 45o – Final shear fracture (cup and cone) The interior surface is fibrous, irregular, which signify plastic deformation. Brittle Fracture There is no appreciable deformation, and crack propagation is very fast. In most brittle materials, crack propagation (by bond breaking) is along specific crystallographic planes (cleavage planes). This type of fracture is transgranular (through grains) producing grainy texture (orfaceted texture) when cleavage direction changes from grain to grain. In some materials, fracture is intergranular. Fracture occurs due to stress concentration at flaws, like surface scratches, voids, Impact Fracture: Impact fractures can best be described as a flute or strip of material that was cleanly sheared from a projectile point. The most common type of impact fracture starts at the tip of a point and runs down one blade edge possibly reaching the shoulder of a point. Some points were reworked into a useable point after having been damaged by an impact fracture. Normalized tests, like the Charpoy and Izod tests measure the impact energy required to fracture a notched specimen with a hammer mounted on a pendulum. The energy is measured by the change in potential energy (height) of the pendulum. This energy is called notch toughness.
Nanostructured materials are those materials whose structural elements—clusters, crystallites or molecules have dimensions in the range of 1-100 nm. These small groups of atoms, in general, go by different names such as nanoparticles, nanocrystals, quantum dots and quantum boxes. Substantial work is being carried out in the domain of nanostructured materials and nanotubes during the past decade since they were found to have potential for high technology engineering applications. One finds a remarkable variations in fundamental electrical, optical and magnetic properties that occur as one progresses from an ‘infinitely extended’ solid to a particle of material consisting of a countable number of atoms. The various types of nanostructured materials which has been considered for applications in opto-electronic devices and quantum- optic devices are nano-sized powders of silicon, silicon-nitride (SiN), silicon-carbide (SiC) and their thin films. Some of these are also used as advanced ceramics with controlled micro structures because their strength and toughness increase when the grain size diminishes. Carbon- based nanomaterials and nanostructures including fullerenes and nanotube plays an increasingly pervasive role in nanoscale science and technology. Today, nanotechnology is being heralded as the next enabling technology that will redesign the future of several technologies, products and markets.
Smart materials are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields. Group of new and state-of-the-art materials now being developed, and expected to have significant influence on present-day technologies, especially in the fields of medicine, manufacturing and defense. Smart/Intelligent material system consists some type of sensor (detects an input) and an actuator (performs responsive and adaptive function). Actuators may be called upon to change shape, position, natural frequency, mechanical characteristics in response to changes in temperature, electric/magnetic fields, moisture, pH, etc. Four types of materials used as actuators: Shape memory alloys, Piezo-electric ceramics, Magnetostrictive materials, Electro-/Magneto-rheological fluids. Materials / Devices used as sensors: Optical fibers, Piezo-electric materials, Micro-electro-mechanical systems (MEMS), etc. Typical applications: By incorporating sensors, actuators and chip processors into system, researchers are able to stimulate biological human-like behavior; Fibers for bridges, buildings, and wood utility poles; They also help in fast moving and accurate robot parts, high speed helicopter rotor blades; Actuators that control chatter in precision machine tools; Small microelectronic circuits in machines ranging from computers to photolithography prints; Health monitoring detecting the success or failure of a product.
Particle-reinforced composites These are the cheapest and most widely used. They fall in two categories depending onthe size of the particles: • large-particle composites, which act by restraining the movement of the matrix, if well bonded. • dispersion-strengthened composites, containing 10-100 nm particles, similar to what was discussed under precipitation hardening. The matrix bears the major portion of the applied load and the small particles hinder dislocation motion, limiting plastic deformation. Large-Particle Composites Properties are a combination of those of the components. The rule of mixtures predicts that an upper limit of the elastic modulus of the composite is given in terms of the elastic moduli of the matrix (Em) and the particulate (Ep) phases by: Ec = EmVm + EpVp where Vm and Vp are the volume fraction of the two phases. A lower bound is given by: Ec = EmEp / (EpVm + EmVp) Concrete The most common large-particle composite is concrete, made of a cement matrix that bonds particles of different size (gravel and sand.) Cement was already known to the Egyptians and the Greek. Romans made cement by mixing lime (CaO) with volcanic ice. In its general from, cement is a fine mixture of lime, alumina, silica, and water. Portland cement is a fine powder of chalk, clay and lime-bearing minerals fired to 1500o C (calcinated). It forms a paste when dissolved in water. It sets into a solid in minutes and hardens slowly (takes 4 months for full strength). Properties depend on how well it is mixed, and the amount of water: too little – incomplete bonding, too much – excessive porosity. The advantage of cement is that it can be poured in place, it hardens at room temperature and even under water, and it is very cheap. The disadvantages are that it is weak and brittle, and that water in the pores can produce crack when it freezes in cold weather. Concrete is cement strengthened by adding particulates. The use of different size (stone and sand) allows better packing factor than when using particles of similar size. Concrete is improved by making the pores smaller (using finer powder, adding polymeric lubricants, and applying pressure during hardening. Reinforced concrete is obtained by adding steel rods, wires, mesh. Steel has the advantage of a…
Injection moulding (United States Injection Molding) is a manufacturing technique for making parts from thermoplastic material. Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape. The mold is made by a moldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. It is the most common method of production, with some commonly made items including bottle caps and outdoor furniture. The most commonly used thermoplastic materials are polystyrene (low-cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings), nylon (chemically resistant, heat-resistant, tough and flexible – used for combs), polypropylene (tough and flexible – used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser). Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding. Extrusion moulding is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods, and fibres.The machine used to extrude materials is very similar to an injection moulding machine. A motor turns a screw which feeds granules of plastic through a heater. The granules melt into a liquid which is forced through a die, forming a long ‘tube like’ shape. The shape of the die determines the shape of the tube. The extrusion is then cooled and forms a solid shape. The tube may be printed upon, and cut at equal intervals. The pieces may be rolled for storage or packed together. Shapes that can result from extrusion include T-sections, U-sections, square sections, I-sections, L-sections and circular sections. One of the most famous products of extrusion moulding is the fiber optic. Blow molding or blow moulding is a manufacturing process by which hollow plastic parts are formed. In general, there are three main types of blow molding; Extrusion Blow Molding, Injection Blow Molding, and Stretch Blow Molding. Extrusion blow molding In Extrusion Blow Molding (EBM), plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected. Injection blow molding The process of Injection Blow Molding (IBM) is used for the production of hollow glass and plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve bottles. The process is divided into three steps: injection,…
The tensile modulus decreases with increasing temperature or diminishing strain rate. Obstacles to the steps mentioned in strengthen the polymer. Examples are cross-linking (aligned chains have more van der Waals inter-chain bonds) and a large mass (longer molecules have more inter- chain bonds). Crystallinity increases strength as the secondary bonding is enhanced when the molecular chains are closely packed and parallel. Predeformation by drawing, analogous to strain hardening in metals, increases strength by orienting the molecular chains. For undrawn polymers, heating increases the tensile modulus and yield strength, and reduces the ductility – opposite of what happens in metals. Crystallization, Melting, and Glass Transition Phenomena Crystallization rates are governed by the same type of S-curves we saw in the case of metals Nucleation becomes slower at higher temperatures. The melting behavior of semicrystalline polymers is intermediate between that of crystalline materials (sharp density change at a melting temperature) and that of a pure amorphous material (slight change in slope of density at the glass-transition temperature). The glass transition temperature is between 0.5 and 0.8 of the melting temperature. The melting temperature increases with the rate of heating, thickness of the lamellae, and depends on the temperature at which the polymer was crystallized. Melting involves breaking of the inter-chain bonds, so the glass and melting temperatures depend on: • chain stiffness (e.g., single vs. double bonds) • size, shape of side groups • size of molecule • side branches, defects • cross-linking Rigid chains have higher melting temperatures.
Many semicrystalline polymers have the spherulitic structure and deform in the following steps : • elongation of amorphous tie chains • tilting of lamellar chain folds towards the tensile direction • separation of crystalline block segments • orientation of segments and tie chains in the tensile direction The macroscopic deformation involves an upper and lower yield point and necking. Unlike the case of metals, the neck gets stronger since the deformation aligns the chains so increasing the tensile stress leads to the growth of the neck.
The description of stress-strain behavior is similar to that of metals, but a very important consideration for polymers is that the mechanical properties depend on the strain rate, temperature, and environmental conditions. The stress-strain behavior can be brittle, plastic and highly elastic (elastomeric or rubberlike). Tensile modulus (modulus) and tensile strengths are orders of magnitude smaller than those of metals, but elongation can be up to 1000 % in some cases. The tensile strength is defined at the fracture point and can be lower than the yield strength. Mechanical properties change dramatically with temperature, going from glass-like brittle behavior at low temperatures (like in the liquid-nitrogen demonstration) to a rubber-like behavior at high temperatures .In general, decreasing the strain rate has the same influence on the strain-strength characteristics as increasing the temperature: the material becomes softer and more ductile.