Bolt Tightness Turbine

bolt thightness

Hy allI want to ask, how much torque to suitable condition for thigtness bolting for steam turbine cassing?thanks before for yours answer

12 days ago

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DavidUnfollow Follow David

David Christiansen • In general, the turbine OEM will provide the casing torquing requirements.

However, when we needed to fall back to some basis for establishing a bolt torque guideline, we based bolt torque for general maintenance activities on bolt stress induced - min 30 ksi, nominal - 60 ksi and max - 90 ksi. This can be easily determined based on tensile stress area of a bolt.

Also, should note that we standardized all bolting to be B7 material - high strength bolting. Other basic control issues, gaskets were garlock or flexitalic, all flanges were designed in accordance with standard ASME requirements (B16.5). Basically, the issue is effective joint design to achieve a sound, leak-free joint.

If you are using other materials, be sure to account for that in the torque table.

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ErickUnfollow Follow Erick

Erick Morgan • Dimas, send me your email address, and i will send you the bolting torques table (ref. Toshiba)

Good luck



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HARIUnfollow Follow HARI

HARI B. • dear erik, please send torues table on [email protected] also.Thanks in advance



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GordonUnfollow Follow Gordon

Gordon Snieder • This question is not answered by the simple number you are seeking. The science of bolt preloading is far more involved than you and most people imagine, so I suggest you retain a qualified professional engineer for the task.



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Erick Morgan • just sent Hari.



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DimasUnfollow Follow Dimas

Dimas Hamonanangan • Dear EricMy e-mail [email protected] before






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SasantUnfollow Follow Sasant

Sasant N • Dear Eric,Could you please forward the same to [email protected] it in advance.



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Erick Morgan • Dimas and SasantJust sent the file, please check your email.

All, please delete your comment which content your email address from the spam maker.



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BilalUnfollow Follow Bilal

Bilal Karaköse • Eric, please send the torque table to [email protected] also. Thanks



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WilliamUnfollow Follow William

William Stephens • You need to look in your turbine manual there should be a section on bolting. Depending on the size length, material and threads per inch will determine the torque or stretch. We do not know the size or manufacture of your turbine or the pressures in involved.

If the joint is not closed in the proper manor then you will have steam leaks which will cut your steam joint.



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EddyUnfollow Follow Eddy




Eddy Alan • the case bolting on a large steam turbine is measured by bolt stretch. the OEM manuals will give you the stretch required for the bolting, the gap of the case is measured and this is added to the stretch(assuming the gap will close when the bolts are stretched), the nuts are tightened to the case, the length is measured cold, the bolts are then heated and the nuts are tightened the flats required for the stretch needed. when the bolts are cool, the length is measured and the stretch is caculated( original length VS new length). this may be needed to be done multiple times until proper stretch is obtained.



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KenUnfollow Follow Ken

Ken Brown • This it too important to rely on stuff from the Internet. Because of the variations in conditions and materials you should follow the OEM recommendations exactly. Bolt heating or hydraulic stretching might be required or possibly they used ultrasonics to measure bolt length or used the turn of nut methods. Torque is likely the least reliable because of possible variations in anti anti-seize compounds, the condition of the stud threads, the condition of the nut threads and the nut face as well as the condition and lubrication of the washer and/or flange surface. Also be careful of substandard and counterfeit fasteners.



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OctaiUnfollow Follow Octai

Octai Dersamet • Hi Gents,



I concur with Mr. Gordon and Mr. Alan,

The bolt torque tightness is a knowledge of general tools of mechanical engineering.First of all from tools of Mechanics of Solid body we learn that ability of materials to withstand to torsional efforts is twice lower than ability to withstand to axial efforts.Therefore selection of bolt+screw material will be in favor of screws in general, to allow when overloads to brake bolt rather then casing which contains inner thread.The bolts for special purpose are designed by OEM's following lot of requirementsThere are two main issues to take care of:1.To do not allow to bolt (or screw ) to "unscrew" 2.To do not increase axial loads above a certain limit which in case of overloading in cyclic type of loads might generate fatigue breaksTherefore the torque of pre-tensioning will depend on bolt minimal diameter.As Mr. Alan said there are two main technics of pre tensioning of bolts:a)Pre tensioning using dynamometric wrench valid up to certain diameters of bolt B)By a controlled "elongation" of bolts and than screwing the nut to a specific angle market on the equipment pads( this technique is used for large main engines )The "elongation " and as well pre tensioning force is done by a hydraulic device and is controlled by pressure measured by gaugeIn case of lacking pre tensioning loads the force is calculated to generate a spinning force higher frictional force in between two "pads" subject to tightening to do not allow "unscrewing"Having the force you could choose most appropriate pre tensioning techniques.As Mr. Gordon said there are as well many other issues to take into. consideration like thermal elongation for equipment subject to heating.In early stage of my careers as graduate engineer I issued many times both technologies of pre-tensioning, calculation of pre tension forces or check the documentation issued by OEM's



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AvishekUnfollow Follow Avishek

Avishek Dubey • Dear Eric, can I have a copy of the doccument [email protected]


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Tapan KumarUnfollow Follow Tapan Kumar

Tapan Kumar Banerjee • Thanks Devid,I with my 40+ year of experience,totally agree with you.Howevver i offer a proven typical power plant kind of solution.If we are talking about joint plane bolts having central heating hole and doing heat tightening, pl do following.After full heat tightening ie achieving '00' position,measured through the central heating hole,please give an additional twist of about 30 degrees through a template.You are safe and this is the full proof safe methodology



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William Stephens • I work for an OEM manufacture and I either calculate the stretch based on the material of the studs and the active length or go by the drawing that has a tablewith the stretch for that position. Then we can document the stretch on a data sheet and attach to the report. We also use degrees of rotation or number of flats but it depends on the diameter and the material. Again it is documented on a data sheet whatwas done and it can also be measured with an extensimeter to verify that the stretch is in spec.

Bill Stephens



Field Engineer

GSII

Cell 307 231 6796



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William Stephens • Before we had hydraulic wrenches we used bolt heaters, 20# sledges, hammer wrenches, and muscle to stretch turbine studs.

Bill Stephens

Field Engineer

GSII

Cell 307 231 6796


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William Stephens • You are absolutely right

Bill Stephens

Field Engineer

GSII

Cell 307 231 6796



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William Stephens • You sound like you have done this procedure many times and you are exactly right.



Bill Stephens

Field Engineer

GSII

Cell 307 231 6796



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TonnyUnfollow Follow Tonny

Tonny Pulungan • dear Dear Eric, can I have a copy of the torque table, [email protected]



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true 40 51 groupItem?seeMo



Erick Morgan • Agree with all... First step is we should open the manual book for the torque need in turbine cassing bolt. What I share here just for reference, not answer for any problem mention on this topic.To prevent any misunderstanding i will not share anymore the bolt torque table (ref toshiba) regarding you should easy find general torque table in Google.com but not for specific bolt used in turbine.



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RickUnfollow Follow Rick

Rick Campanella • My company, HYTORC provides tools, fasteners, and expertise for exactly these types of bolting operations. If anyone emails me, I'll do my best to help.



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JimUnfollow Follow Jim

Jim Taylor, ASQ-CRE, CPE, CPMM • David, did you take into account the thread friction - i.e. lubricated or not?





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ByronUnfollow Follow Byron

Byron Wooldridge • Eddy Alan gave you the best answer, particularly for high pressure and high temperature applications--you need to know the design bolt stretch. Using torque for everyday, run of the mill bolt tightening is fine but when you need precision, bolt stretch or elongation is the preferred method.



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Octai Dersamet • I do not use repetitive hot stretching.The theory of Solid Body states very clear the limits in between a steel is considered in "elastic " deformation.I would remind you gents that yield tensile strength is as well strength corresponding to 0.2 % elongation.Exceeding this limit which for certain types of steel high alloyed with higher level of Chrome , Nickel or Vanadium could be even less will lead to temporary increase of the tensile but with dramatic effect to elongation.This could lead to brittle fracture with devastating effect of rotating equipment's.As well repetitive heat could generate a thermo mechanical treatment , the increase of strength due to elongation over 0.2%.As well as I said in previous comment could lead to overload on casing inner thread.I would also remind you apart of fatigue stress that I talk about , there are tensions release due to vibrations.If we speak about turbine , things become more complicated, depending on temperatures that turbine have during normal working as well elongations occurs and as well some types of "annealing " might occur and as well thermal elongations.To solve elongation issue(as pre tensioning technology) with heating stretching I personally



found dangerous on long term.We use "cold" elongation using hydraulic oil actuated devices.



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William Stephens • I have worked in Romania.

On large Steam and Nuclear turbines it would take very large hydraulic wrenches to cold stretch the horizontal joint studs on the HP shell. The most efficient way to stretch them is with induction heating. Other wise if you tried to stretch them cold you would push the schedule, break your tooling and wear out the craft.



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Erick Morgan • Dimas. Regarding Rick Campanella comment about Hytorc, it would be good solution to you to open your bolt. This tool provide you the number of torque (availabel for turbine casing) you need to open the bolt and powered by hydraulic unit (torque setted in hydraulic unit), not use your muscle and torque wrench.I ever seen the presentation, the Hytorc company in Indonesia named INDOTORC, and i do interest.Now we prepare to issue the Purchase Request.



PS: If you have much technician or helper who looking for overtime alowance, you should not buy this tool, it save your time so much..... I only have his PIN BBM, 26D8A604



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Octai Dersamet • Dear Mr. Stephens,

I am reluctant regarding those who built the nuclear plant in Cernavoda.Living relatively close that nuclear plant , now based on your statement I am afraid of what might hapen in next years.I have had a direct experience of work ethics culture , management and purchasing management of those who work at that plant.In my first year as graduate in Constanta shipyard as PM in Hull repair division I have been assigned to ensure safely delivery of a shaft for the nuclear plant.They told us that shaft it is very important and consequently (based as well on the normal care which should be take to any shaft) , they have asked us to design ,issue handling procedure during shipment from Canada to Romania.They specially chosen the the American company Prudential which has on board 500 tons barge to freight the shaft in a metallic"box" to protect shaft.I have been assigned to design the transportation of shaft.Being longer than opening of hold in the barge , I neeeded to redesign the hatch coamings., to design and "bed" and inside anchoring systems, procedures of lifting and shaft handing.To be sure that any bot controlled bending will occur I asked tensiles to be measured with tensile stain plates (tensiometers)According to some sources the freight costs about 1 million USD.Having such extraordinary measures taken I tracked the delivery of shaft.The American company shipped succesfully, and we anloaded the shaft in shipyards.The shaft remained outside nearby corner of a building in same position for more than 2 years in external ambiental conditions whiout any protection.Than what do you think about specialism,work ethics and culture of entire chain of management of Canadian company which built the plant?What are the relevance of 10 more fitter's manhours consumed for such delicate mechanical operation to such dangerous type of plant?


Once again I do not recomend to bring bolts into "plastic" domain only to save few man hours.Regarding "streching"(at levels of microns) we in shiyard in general use 1000 bars hydraulics press to enlarge the boss of screw propeller ,ensuring by friction transmission of torques of over 7,000,000 Newtonmeters.With such devices we even could breake by cold streching the bolts not to only induce of a pre tensioning of indicative 10% of force coresponding to 0.2 elongation.



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Byron Wooldridge • The hydraulic stretching technique must be conducted with very precise procedures. One does not simply apply as much pressure as one feels like. Same with induction heating of bolts for "hot stretching"--there is a very specific procedure. Depending upon torquing a large bolt is very iffy depending upon the condition of the threads, the condition of the bearing surface of the nut or bolt head, and depending upon what, if any, lubricant is used. My first employer 42 years ago used bolt heating to tension bolts on the 145MW steam turbines in the facility's power plant. The turbines had been in service 13 years before I started working there, over 8 more years at the time I left without a problem related to case bolting.



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SasantUnfollow Follow Sasant



Sasant N • Agree with you all. once hand tightened the bolts the bolts are elongated and tightened to a certain rotation as per OEM designs for each casing.We do provide Induction Bolt Heating services, the rotation angle differs from OEM to OEM.



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Octai Dersamet • Mr. Sasant & Mr. Wooldridge,

I do not question your experience.I will try once again to sustain my point of view.I don't know your background , and I will apologies if I am saying things known .maybe my English wasn't so good and I couldn't explain very well my point.

Please find below a link related to behavior of steels:

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm

Analyzing the braking diagram of steels ("stress-strain curve" )you will notice that up to0.2 % the behavior of steel is "elastic" meaning that steel object will keep stability of geometrical shape.This for mild steel correspond to 235 N/square mm,Up to this value to 370 N/square mm mild steel will stretch to breaking point at ultimate strength 370 N/square mm.This is zone where item works in "plastic" mode loosing geometrical stability.If you heat the material and stretch it you will experience follows:1.You will bring steel close to austenitic phase from diagram FE-Fe3C and consequentlythe bolt will experience a "forging "treatment but by stretching.2.The uncontrolled cooling of bolt might generate an "hard" area where the strength will increase but potential of brittle rupture increase (very dangerous for bolts subject to cyclic loads like in case of turbines )This will lead as well to dramatic reduction of fatigue -stress ability of "treated" bolt3.The stretch will reduce the section area of bolt4.The scope of work which is "pre tensioning" is very difficult to control.


5.The outcome might be only an "adjustment" of bolt into the "link"

@Mr. Wooldridge,The reasons why your bolts doesn't brake was the fact that we degreed engineers when design we use multiple margin coefficients (for calculations inaccuracies, steel non homogeneity, difficulty to make assessment of dynamic loads, etc).This means a generous structural strength reserve.But application like aviation where weight is a key issue will require better calculation and consequently lower margin coefficients



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Byron Wooldridge • Mr. Dersamet:As I stated, when using heating as a method of bolt tensioning, the process must be closely controlled. Typically, there is a very specific amount of turn applied to the nuts when the bolts are hot. The degree of turn corresponds to a specific elongation of the bolt, which in turn, corresponds to a specific tension in these precision bolts. The other check which can be made is to measure the bolt to determine the amount of elongation due to heating.



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Tapan Kumar Banerjee • Mr Byronyou are right all the way.thanks



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William Stephens • I was in charge of the first non union American crew for the 2010 outage at Cernavoda and we finished 4 days ahead of schedule because of the expertise of the crew and engineering staff.

When I stretch studs on a turbine case I use the Engineering manual, the OEM manual provided to the customer and the drawings. When I do not have that information then I refer to the stud drawing and calculate the active length of the stud and with that and the material of the stud the OEM provides a graph for the amount of stretch which I then provide to the Millwrights that are performing the task. The OEM has a better understanding of the properties of the studs usied for the horizontal joints and writes procedures accordingly. There is a tolerance for the proper amount of stretch and we stay inside of that tolerance.

If we don't heat the studs the risk of galls and destroying the stud or nut is very high. The process is also time sensitive because we have to achieve the stretch before the shell soaks up the heat applied to the stud.

The OEM also recommends that the studs be changed out after a x numbers of maintenance cycles and or ultra sound inspection to look for micro cracks.



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Octai Dersamet • Dear Mr. Stephen,

My comment regarding company which built Cernavoda targeted end of 1984 ,now I don't have updated information.Regarding the replacement of bolts after a number of working hours or number of cycles definitely I agree.



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StefanoUnfollow Follow Stefano

Stefano Dondi • Generally in the maintenance guide of manufacturer , you can find all details about the tightening torques .



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Tapan Kumar Banerjee • I am just not able to undertand,why to unnecessarily give technical/engineering input to a subject,which is well elaborated in OEM manuals..............Thus.....With everybody's consent let the subject rest here................



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Octai Dersamet • Dear Mr. Banerjee,

The reasons of such many technical input are plenty.1.The initiator of this discussion from some reason might have not access to OEM's manual because:-plant is old-the documentation hasn't been properly archived ,lost or destroyed2.The training process in engineering sciences has two main route map:-one is "mimetic" one , when trainee learn by following the actions of "mentor" If mentor has no time to explain or has not enough specialism or even worse has a questionable moral behavior and do not explain details of each of his action.-the "demonstrative" way when training goes from assumptions to conclusions and actions are taken following "necessity" (as mother of innovation).This is the way I prefer because the people who perform a tasks know each aspect of planning of the work , understand limitations and as well the importance of task that he performed. Here very few control it is needed because the individual will choose "knowing" the best way to accomplish the task.3.As you might know OEM's "loose" their interest(otherwise very understandable ) after 5 years from commissioning the items.Therefore the field engineers must do a quality job based on their knowledge4.In same respect sourcing of stud or bolt might encounter problem and require that field engineer to make a new material selection.The previous technology stated by OEM's might not always be suitable5.Some application like Mr. Stephens said (nuclear plants) has an enormous potential of risk, any additional "eye" ,especially from field engineer who perform the task will be helpful in risks mitigation.Definitely there are many other reasons.


Romanian degreed engineers formation and university training followed the philosophy of "demonstration" that I said before.This is only way to do not make mistakes , to remember or to bring back to surface apparently forgotten procedures, to generate new ones adapted to materials , technical and environmental conditions ,etc.Definitely the topic is generous I can come with some more inputs about your own method of tightening if some are interested in.



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SuperUnfollow Follow Super

Super Dave Rettke • As been mentioned above...most manuals will have the necessary information on torquing their fasteners to proper stretch/elongation. If that information has been lost, most OEM's keep records of what the torques and how they are achieved and can provide that information, usually without charge.

If you have a situation in which you cannot get the information at all (OEM out of buisness, won't cooperate etc.) then you have no choice but to discover the material compostion of the bolts, try to find out how many times those particular bolts have been used/torqued and then seek qualified engineering support to determine proper stretch or torque values.

You may have to replace all the bolts and start from scratch anyway if the bolts have been torqued too many times. The risk is not worth the chance of hoping it will be all right.



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DimasUnfollow Follow Dimas

Dimas Hamonanangan • dear allthanks in advance will have input that you convey enlightenment

OEM for mechanical guide we have no idea who holds the leadership in the enterprise interchangeably. so for mechanical archival documentation at all I never find.I have first of all to do this, and I do not have experience of this situation, I do activity based instruction.so I tighten the bolts for steam turbine using a torque wrench made by enerpac series w-4000.it's just that I have not been so sure of what type of firming up the bolt with hexagonal head nuts peacefully in bounds 3714.286 60 ft.lbswhere at the time of release and torque bolts using a heater is needed more than the limit 4000 ft.lbs.

Nb. Mr. Super Dave where I can get the OEM manual from the manufacture Dresser-Rand? whether they are still there when the copy was old old equiptment? because I seek from their web page, I do not find it particularly concerning mechanical and sectional drawing per part.



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SuperUnfollow Follow Super

Super Dave Rettke • Dresser Rand website had this page:

http://dresser-rand.com/literature/steam/2104-restored_class.pdf

Hope that helps. You will have to write/call them to find out as their web site is a little lacking in some respects.



true 51 51 groupItem?seeMo


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alokUnfollow Follow alok

alok bhattacharya • Beside David & Eddy's, I agree most of the views. We follow exactly as per OEM's instruction. However where more bolts are used i.e. turbine or air compressors joints, we have numbered every bolts / studs & keep track it's physical condition. It helps if any replacement is required, spare is available. Engaging same technicians who previously worked on it make job simple.

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GSTR ServicesUnfollow Follow GSTR Services

GSTR Services Enterprise • Hi EricCan you send also the torque table to me [email protected]



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ErallUnfollow Follow Erall




Erall Yuniaz • Dear Eric, can I have a copy of the document [email protected]

thanks..



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BradleyUnfollow Follow Bradley

Bradley Piatt • Joint bolts must be stretched not torqued. The goal is to provide the proper preload on the bolt and that is measured by elongation.

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Erick Morgan • to prevent 3th world war, i will not send anymore of bolt torque table to anyone....



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FrankUnfollow Follow Frank

Frank Wszelaki • As the others have stated bolt tightness of a turbine case depends on the OEM's specifications. Depending on the unit, High Pressure, Intermediate or Low Pressure this can varify. Look to your OEM to provide guidance on the torque requirements. As for actually performing the evolution, many strides have been in the industry and there are professional field crews that can work with the site crews to ensure the proper assembly is made. Much more to this question then meets the eye, as others have stated. I would need more information to help you further.



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SACHINUnfollow Follow SACHIN

SACHIN PYASI • Erick sir

Please please send 1 last copy for me

My id [email protected]



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Erick Morgan • Too many people asking bout the torque table.The surprise is one of them work in a company which specialize in bolt.

To prevent 3th world war, i will not send anymore of bolt torque table to anyone....

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Octai Dersamet • Dear Mr. Eric,

Please note that any comment posted here may or may not be accepted.Each of us based on our own paradigm will accept or not the others input.Therefore ,despite of irony, any inputs will not produce catastrophic effects.On the other hand I wish you to never experience a catastrophic failure in a nuclear plant like we experiencedin 1986 at Cernobil , when an enthusiastic engineer in order to reduce time of commissioning that nuclear plant risked his own life but more important put in jeopardy the heath and life of millions of people who has noting in common with that plant.The unwanted effects we felt at few hundreds of kilometers , not only kilometers like is the distance in between Cernavoda plant and mylace.

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Erick Morgan • Thank you Octai for your advice, please note that we should read the comment from above to the buttom one by one (I think this is the important habit while you read the manual or procedure), I already mention will not share anymore, but still comment asking for that.As I see your comment above, you kindly a good engineer which will read the procedure one by one from above to the bottom.Ah, not all here a great job like you in nuclear power plan (might be), I just work in CFPP with little power load.

Good luck to you Octai, and keep your "nu" from your nuclear...

27 minutes ago


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stretched with a tensile force. This modulus is of interest when it is necessary to compute how much a rod or wire stretches under a tensile load.

There are several different kinds of moduli depending on the way the material is being stretched, bent, or otherwise distorted. When a component is subjected to pure shear, for instance, a cylindrical bar under torsion, the shear modulus describes the linear-elastic stress-strain relationship.

Axial strain is always accompanied by lateral strains of opposite sign in the two directions mutually perpendicular to the axial strain. Strains that result from an increase in length are designated as positive (+) and those that result in a decrease in length are designated as negative (-). Poisson's ratio is defined as the negative of the ratio of the lateral strain to the axial strain for a uniaxial stress state.

Poisson's ratio is sometimes also defined as the ratio of the absolute values of lateral and axial strain. This ratio, like strain, is unitless since both strains are unitless. For stresses within the elastic range, this ratio is approximately constant. For a perfectly isotropic elastic material, Poisson's Ratio is 0.25, but for most materials the value lies in the range of 0.28 to 0.33. Generally for steels, Poisson’s ratio will have a value of approximately 0.3. This means that if there is one inch per inch of deformation in the direction that stress is applied, there will be 0.3 inches per inch of deformation perpendicular to the direction that force is applied.

Only two of the elastic constants are independent so if two constants are known, the third can be calculated using the following formula:

E = 2 (1 + n) G.

Where: E = modulus of elasticity (Young's modulus)

n = Poisson's ratio

G = modulus of rigidity (shear modulus).

A couple of additional elastic constants that may be encountered include the bulk modulus (K), and Lame's constants (m and l). The bulk modulus is used describe the situation where a piece of material is subjected to a pressure increase on all sides. The relationship between the change in pressure and the resulting strain produced is the bulk modulus. Lame's constants are derived from modulus of elasticity and Poisson's ratio.

Yield PointIn ductile materials, at some point, the stress-strain curve deviates from the straight-line relationship and Law no longer applies as the strain increases faster than the stress. From this point on in the tensile test, some permanent deformation occurs in the specimen and the material is said to react plastically to any further increase in load or stress. The material will not return to its original, unstressed condition when the load is removed. In brittle materials, little or no plastic deformation occurs and the material fractures near the end of the linear-elastic portion of the curve.

With most materials there is a gradual transition from elastic to plastic behavior, and the exact point at which plastic deformation begins to occur is hard to determine. Therefore, various criteria for the initiation of yielding are used depending on the sensitivity of the strain measurements and the intended use of the data. (See Table) For most engineering design and specification applications, the yield strength is used. The yield strength is defined as the stress required to produce a small, amount of plastic deformation. The offset yield strength is the stress corresponding to the intersection of the stress-strain curve and a line parallel to the elastic part of the curve offset by a specified strain (in the US the offset is typically 0.2% for metals and 2% for plastics).

To determine the yield strength using this offset, the point is found on the strain axis (x-axis) of 0.002, and then a line parallel to the stress-strain line is drawn. This line will intersect the stress-strain line slightly after it begins to curve, and that intersection is defined as the yield strength with a 0.2% offset. A good way of looking at offset yield strength is that after a specimen has been loaded to its 0.2 percent offset yield strength and then unloaded it will be 0.2 percent longer than before the test. Even though the yield strength is meant to represent the exact point at which the material becomes permanently deformed, 0.2% elongation is considered to be a tolerable amount of sacrifice for the ease it creates in defining the yield strength.

Some materials such as gray cast iron or soft copper exhibit essentially no linear-elastic behavior. For these materials the usual practice is to define the yield strength as the stress required to produce some total amount of strain.

True elastic limit is a very low value and is related to the motion of a few hundred dislocations. Micro strain measurements are required to detect strain on order of 2 x 10 -6 in/in.

Proportional limit is the highest stress at which stress is directly proportional to strain. It is obtained by observing the deviation from the straight-line portion of the stress-strain curve.

Elastic limit is the greatest stress the material can withstand without any measurable permanent strain remaining on the complete release of load. It is determined using a tedious incremental loading-unloading test procedure. With the sensitivity of strain measurements usually employed in engineering studies (10 -4in/in), the elastic limit is

In Great Britain, the yield strength is often referred to as the proof stress. The offset value is either 0.1% or 0.5%

greater than the proportional limit. With increasing sensitivity of strain measurement, the value of the elastic limit decreases until it eventually equals the true elastic limit determined from micro strain measurements.

Yield strength is the stress required to produce a small-specified amount of plastic deformation. The yield strength obtained by an offset method is commonly used for engineering purposes because it avoids the practical difficulties of measuring the elastic limit or proportional limit.

Ultimate Tensile StrengthThe ultimate tensile strength (UTS) or, more simply, the tensile strength, is the maximum engineering stress level reached in a tension test. The strength of a material is its ability to withstand external forces without breaking. In brittle materials, the UTS will at the end of the linear-elastic portion of the stress-strain curve or close to the elastic limit. In ductile materials, the UTS will be well outside of the elastic portion into the plastic portion of the stress-strain curve.

On the stress-strain curve above, the UTS is the highest point where the line is momentarily flat. Since the UTS is based on the engineering stress, it is often not the same as the breaking strength. In ductile materials strain hardening occurs and the stress will continue to increase until fracture occurs, but the engineering stress-strain curve may show a decline in the stress level before fracture occurs. This is the result of engineering stress being based on the original cross-section area and not accounting for the necking that commonly occurs in the test specimen. The UTS may not be completely representative of the highest level of stress that a material can support, but the value is not typically used in the design of components anyway. For ductile metals the current design practice is to use the yield strength for sizing static components. However, since the UTS is easy to determine and quite reproducible, it is useful for the purposes of specifying a material and for quality control purposes. On the other hand, for brittle materials the design of a component may be based on the tensile strength of the material.

Measures of Ductility (Elongation and Reduction of Area)The ductility of a material is a measure of the extent to which a material will deform before fracture. The amount of ductility is an important factor when considering forming operations such as rolling and extrusion. It also provides an indication of how visible overload damage to a component might become before the component fractures. Ductility is also used a quality control measure to assess the level of impurities and proper processing of a material.

The conventional measures of ductility are the engineering strain at fracture (usually called the elongation ) and the reduction of area at fracture. Both of these properties are obtained by fitting

the specimen back together after fracture and measuring the change in length and cross-sectional area. Elongation is the change in axial length divided by the original length of the specimen or portion of the specimen. It is expressed as a percentage. Because an appreciable fraction of the plastic deformation will be concentrated in the necked region of the tensile specimen, the value of elongation will depend on the gage length over which the measurement is taken. The smaller the gage length the greater the large localized strain in the necked region will factor into the calculation. Therefore, when reporting values of elongation , the gage length should be given.

One way to avoid the complication from necking is to base the elongation measurement on the uniform strain out to the point at which necking begins. This works well at times but some engineering stress-strain curve are often quite flat in the vicinity of maximum loading and it is difficult to precisely establish the strain when necking starts to occur.

Reduction of area is the change in cross-sectional area divided by the original cross-sectional area. This change is measured in the necked down region of the specimen. Like elongation, it is usually expressed as a percentage.

As previously discussed, tension is just one of the way that a material can be loaded. Other ways of loading a material include compression, bending, shear and torsion, and there are a number of standard tests that have been established to characterize how a material performs under these other loading conditions. A very cursory introduction to some of these other material properties will be provided on the next page.



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