Transcript LESSON 8
Essential reading: SPINČIĆ, A., An English Textbook For Marine Engineers I., Pomorski fakultet, Rijeka 2008. LUZER, J., SPINČIĆ, A., Gramatička vježbenica engleskog jezika za pomorce, Pomorski fakultet, Rijeka 2003. Material properties Material testing KEY WORDS property (n.) - svojstvo, imovina essential - bitan, osnovan enable (v.) - omogućiti particular application - određena primjena, uporaba appropriate design - prikladna izvedba, izrada, konstrukcija distinguish (n.) - razlikovati (se) i.e. = id est = that is etc. = et cetera = and so on - itd. = i tako dalje c.s.a. = cross section area - površina poprečnog prosjeka behaviour (n.) - ponašanje conditions of loading - uvjeti opterećenja determine (v.) - odrediti quantify (v.) - kvantificirati, odrediti ascertain (v.) - ustanoviti, utvrditi stress (n.) - naprezanje strain (n.) - deformacija distort (v.) - iskriviti, izobličiti Knowledge of the properties of a material is essential to every engineer. This enables suitable choice of material for a particular application, appropriate design of the components and their protection from corrosion or damage Physical characteristics of materials are: colour, specific heat, density, thermal and electrical conductivity, coefficient of expansion, melting point… The properties which describe the behaviour of a metal under various conditions of loading are called its mechanical properties: tensile strength, ductility, brittleness, malleability, elasticity, plasticity, toughness, hardness, machinability… Stress is defined as the internal force which tends to resist deformation when subjected to external forces. stress = applied force c.s.a. of material There are different kinds of stress: - Compressive stress - Bending stress - Shear stress - Tensile stress - Torsion stress Strain is distortion od a material, i.e. its permanent change in size and shape, due to stress. 1. Why should engineers have a good knowledge of material properties? 2. What do physical properties describe? 3. What do mechanical properties show and how are they ascertained? 4. Name some mechanical properties of materials. 5. Does the term ‘load’ mean the same as ’stress’? 6. What types of stress do you know? 7. How is stress measured? What is the stress unit? 8. What is strain? Complete the sentences below choosing among the listed terms: brittle, conductor, elastic, hard, heavy, machinable, malleable, rigid, weldable 1. A light material has a low mass; a material having a high mass is ____________ . 2. A flexible material bends easily without breaking. A material that does not bend easily is ____________ . 3. A tough material absorbs a great deal of energy without fracturing; a material that tends to fracture under impact load is ____________ . 4. A soft material can be indented or scratched easily; a material that resists indentation, abrasion or wear is _____________. 5. A plastic material does not return to its original dimensions after forces producing strain are removed; a material that ragains its original dimensions after forces are removed is ____________ . 6. A ductile material is capable of being drawn into wires without cracking; a material that can be shaped by hammering, pressing and rolling is ____________ . KEY WORDS twist (v.) - sukati, uvijati fixed rule - utvrđeno pravilo criterion (n.) - kriterij (pl. criteria) with reference to - vezano uz, koji se odnosi na specimen (n.) - uzorak, primjerak gripped - ukliješten extend (v.) - produžiti uniform - jednoličan yield point - kritična točka, točka popuštanja, prema tekstu removal (n.) - uklanjanje ultimate tensile stress (UTS) - maksimalno vlačno naprezanje proof stress value - granica elastičnosti intersection (n.) - križanje modulus of elasticity - modul elastičnosti The main criterion with reference to metals is the tensile strenght. A specially shaped specimen of standard size is gripped in the jaws of a testing machine and a load gradually appplied to draw the ends of the specimen apart such a way that is subject to tensile stress Stress vs. Strain curve typical of aluminum 1. Ultimate strength 2. Yield strength 3. Proportional limit stress 4. Fracture 5. Offset strain (typically 0.2%) Additional loading of the specimen will produce a uniform increase of extension until the yield point is reached. Up to the yield point of elastic limit, the removal of the load will result in the specimen returning to its original size. Within the elastic limit, stress is proportional to strain, stress strain This constant is known as the moduls of elasticity (E) of the material. constant = 1. Why are materials subjected to testing? 2. How is the tensile stress test carried out? 3. What measurements are taken by the tensile test? 4. What is the linear strain? 5. What is the ‘elastic limit’ of a material? 6. Explain the expression: “stress is proportional to strain”. 7. When must a ‘proof stress value’ be given? How is it obtained? 8. What is called the ‘factor of safety’? 9. What does the term ‘neck’ mean?