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FEELINGS AND EMOTIONS(I) Fatigue failures in structural and machine members are produced by progressive fracture leading from points of high stress concentrations. Failure produced by the spreading of small cracks until a complete separation or rupture of a part takes place. Progressive fracture may be initiated at irregularities of outline as, for example, abrupt changes in cross section of a member, internal cracks, or slag inclusions. Fatigue failures resulting from small cracks are a serious source of failure since magnaflux or even X-ray examinations are not always effective in detecting the very small cracks that may later lead to failure. The type of fracture produced in ductile metals under fatigue loading differs greatly from the kind of fracture under static loading. With static loading, plastic flow of the material precedes fracture and the ruptured surface shows a fibrous structure produced by the stretching of the crystals. A fatigue crack, however, looks entirely different. That is, a fatigue crack starts at a local defect and spreads progressively until the stressed section becomes so small that the remaining portion cannot resist the load and a sudden fracture results. In fatigue fractures, two zones of failure can be detected - one produced by the gradual development of the crack and the other by the sudden fracture. The zone produced by sudden fracture resembles the fractured surface of a static tension specimen of a brittle material such as cast iron. For this reason the fractured surface in fatigue is said to be of the brittle type. N 17 NATURE OF FATIGUE FAILURES (II) The mechanism of failure by fatigue is partially known, although a satisfactory theory describing fatigue failure on the submicroscopic basis has not been developed. Microscopic examinations of metal specimens subjected to fatigue stressing have been made, using both the ordinary microscope and, more recently, the electron microscope. These examinations show the presence of many small irregular cracks and points of weakness oriented in all possible directions. This random orientation accounts for the wide variation in fatigue strength obtained using different specimens cut from the same piece of material. The location of the crack propagation source varies. Sometimes a small corrosion pit on the surface of the material may act as the nucleus for many cracks which progress in a transcrystalline manner through the structure. The cracks are irregular and follow various paths around regions of stronger material. These cracks sometimes turn at right angles and apparently follow the weaker shear planes in the crystalline structure. In general, the direction of the cracks appears to follow the directions of the weaker crystallographic planes or the directions of the planes of maximum tensile stress, although the former direction is more common than the latter. In the early stages of stressing under fatigue leading conditions, inelastic action occurs which results in submicroscopic permanent slip lines that require an electron microscope for their direction. As repeated stressing progresses, voids develop within the slip bands that are first submicroscopic and later become microscopic in magnitude. With further stressing, the small voids become larger until they join together to form cracks. It is of importance to note that the foregoing nucleation of email microscopic cracks may be produced in the very early stages of stressing.
' N 18 NATURE OF FATIGUE FAILURE (III) A consequence of the manner In which failure by fatigue takes place is the influence of the volume of the specimen or part on the fatigue strength. In general, the larger the volume, the lower the fatigue strength. This observation follows from the fact that the larger the volume, the larger will be the number of critical points of weakness where cracks may start, and hence, the greater the chance of fatigue failure; this relationship is confirmed by experiment. Another consequence of the manner in which failure by fatigue takes place is the statistical nature of the phenomena. That is, since failure by fatigue is dependent upon local sources of weakness rather than the overall resistance of the material (as represented, for example, by the tensile yield strength of a material), considerable variation in fatigue strength results between specimens cut from the same piece of material. The element of variation in strength suggests that the fatigue strength should be stated in terms of the most probable value. For this reason, statistical procedures are used for the interpretation N 19 NATURE OF FATIGUE FAILURES (IV) The behaviour of materials when subjected to fatigue stresses has been investigated in three general ways, based on the size of the specimen used. First there are the basic types of studies dealing with the smallest size specimen, including study of crystals and smaller unite with the aid of ordinary and electron microscopes. These methods are concerned with the mechanism of failure. Various difficulties have resulted in relatively little being done with this method of approach. However, the development of the electron microscope, with the possibilities of greater magnifications, gives promise of future significant results and a better basic understanding of the fatigue phenomena. The second method of studying fatigue deals with the phenomenological approach in which the stress-strain properties of small laboratory specimens are investigated under various kinds of stress conditions. The latter approach will be the one primarily considered here since solid state mechanics is based on experimental results obtained from small macroscopic specimens. The third type of procedure deals with the fatigue behaviour of full-size machine and structural members or the fatigue behaviour of the complete full-scale structure or machine. N 20 SHOCK AND IMPACT PROPERTIES (I) When forces are applied suddenly for very short periods of time, one effect of such forces is to produce stress waves. Loads that are suddenly applied to structures and machines are called shock or impact loads. She influence of such loads is twofold, since suddenly applied loads modify both the stresses and strains produced and the resisting properties of the material. In this chapter both approximate and exact methods of stress and strain analysis for shock and impact loading are considered. In addition, experimental methods for determining the stress-strain properties of materials under shock and impact loading are discussed and the influences of such loading on the stress-strain properties are outlined. Finally, a brief treatment is given on the design of simple members for shock and impact loading conditions. Shock loading does not always involve an impact or collision of bodies but may be a sudden application of force or motion to a structure. A shock or impact loading is produced if the time of application of the load is of the same order of magnitude, or less than the longest natural period of vibration of the construction considered. Usually the terms sudden impact, or shock loadings are used to designate the same type of loading. Sometimes the terms sudden or impulsive are used where the details of the load-time curve are not significant, whereas the term shook is used when the nature of the shock motion is important. N 21 SHOCK ŔND IMPŔŃT PR0PERTIES (II) Design considerations for shock and impact include the characteristics and types of loadings, the kind of structure considered, the properties of the material, and the experimental and theoretical methods of determining the loads, stresses, and strains produced. Manner of shock load application. Shock or impact loads are applied to structures and machines in various ways. These loads include rapidly moving loads, as produced by a locomotive passing over a bridge; direct impact loads, as produced by a drop of a hammer; sudden application of loads, as occurs during the explosion stroke of a gasoline engine and inertia loads, as introduced by high accelerations and transfers of kinetic energy which accompany mechanical shocks, such as in a flywheel. Shock and impact loads can also be classified in terms of sudden application of force, sudden change in velocity, and irregular shock loadings. Usually shock loading involves relatively few cycles of loading. However, in some cases, such as a railroad coupler, the structure must withstand thousands of shock loadings. This type of' loading is sometimes called impact fatigue loading. N 22 SHOCK AND IMPACT PROPERTIES (III) Characteristics of the structure. The damage produced by shock loading is naturally dependent upon the characteristics of the structure. If the time of application of the load is short compared to the lowest natural frequency of vibration of the structure or machine, an impact load is produced, whereas if the time of load application is long, the load is considered to be static. In most cases, if the time of load application is less than one-half the lowest natural frequency of vibration, the load is definitely considered to be a shock or impact load. If this time interval is greater than three time the natural frequency, the load is generally considered to be a static load. The time of loading referred to above is the time required to increase the load from zero to a maximum value. For dynamic shock loads the shape of the load-time curve, particularly the steepness of its rise or fall is important. The impulse (equal to the area under the load-time curve) is a basic consideration in design. On the other hand, for slow loading, the shape of the load-time curve is unimportant since the maximum load is the major design factor.
FEELINGS AND EMOTIONS
PRE-READING ACTIVITIES
I. Answer the questions. Use the words from the box.
1. How do you feel today? Why?
TEXT
II. Read the text. The Rules of Lasting Happiness
Are you searching for the key to lifelong contentment? Psychologist Oliver James lists the five simple ways to find it. Stop the comparisons It is impossible to expect to be happy all the time, but it is possible to feel comfortable. It’s a fact that depressed people make more comparisons with others and as a result feel miserable. Happy people, on the other hand, rarely compare unless it is to learn something positive from someone they admire. Try to do the same. Don’t be afraid of other people’s choices, looks or lifestyles; instead try to realize that their lives have no impact on your life. Appreciate your positives Many fail to enjoy what they have because they spend their lives looking at what they don’t have. Take a look around you. Be grateful for what’s good in your life every day and it will become a habit. Encourage happiness in others by sharing what you have. Live in the present Our lives are full of worries about the past or the future. Why didn’t I have rich parents? When will I have enough money to buy my dream home? Learn to live each day as it comes. If you are not happy with something, change it. Look at your childhood Maybe your childhood wasn’t very happy, deal with it, then move on. Stop blaming your parents and make the best of what you have, and you will feel happier and more in control. Speak your mind Happy people are very honest. They say what is on their mind without fear. As a result, they are not keeping negative emotions inside. If you want to lie to someone, ask yourself why. You will feel happier if you just tell the truth. In the same way, if you have something positive to say, just say it – if you love someone, tell them as much as you can, and if you appreciate somebody’s act of kindness, say so.
VOCABULARY
III. Read and memorize the definitions.
admire - regard with pleasure, wonder, and approval; have a high opinion of smth/smb (be) afraid (of) = scared = frightened = terrified - feeling fear angry – furious, cross appreciate - used to thank someone in a polite way or to say that you are grateful for something they do bored - uninterested, tired of the world contentment = satisfaction - happiness with one's situation in life depressed - very unhappy disappointed – feeling sad by the failure of an expectation emotion – the inner feeling of a person embarrassed - feeling nervous and uncomfortable and worrying about what people think of you encourage - to give someone the courage or confidence to do something envy (be envious) – the feeling of wanting something that someone else has enjoy – like, get pleasure from having or doing something excited – emotional, emotionally aroused fear – the feeling that you get when you are afraid or worried that something bad is going to hate habit – something that you do regularly or usually, often without thinking about it because you have done it so many times before happy - feeling, showing joy; pleased hate – dislike strongly hatred – an angry feeling of extreme dislike for smth/smb grateful - feeling that you want to thank someone because of something kind that they have done (feel) guilty – a strong feeling of shame and sadness because you know that you have done something wrong impatient - restless or short-tempered jealous - suspicious or fearful of being displaced by a rival joy – great happiness and pleasure lie – give false information with the intention of deceiving miserable - extremely unhappy, for example because you feel lonely, cold, or badly treated mood (in good/bad mood) – the way you feel at a particular time negative – harmful, unpleasant, or not wanted nervous - anxious, showing anxiety pleasure – the feeling of happiness, enjoyment or satisfaction that you get from an experience positive – good or useful proud – pleased about something that you have done or something that you own puzzled = confused - feeling an inability to understand relaxed – feeling comfortable, free from strain or tension sad - feeling sorrow; unhappy shame (be ashamed) – (have) the feeling that you have when you are guilty and embarrassed because you have done something wrong stress – a state of worry resulting from pressure caused by the problems of living, too much work etc. (be) surprised = amazed = astonished – get the feeling caused by something unexpected tell the truth – give true, actual, or factual information upset – feeling unhappy and worried worried - feel uneasy or concerned about something; be troubled
Date: 2015-01-02; view: 1155
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