Dear Richard Chukwuma Meribe, from your writings I assume that you are working with an FEM program. However usually you are free to choose the Units as you like, but it is advised to choose SI Units (m kg s ...) othervise conversion can be dificult at times. Some mechanical Engineering FEM programs work with mm length untis. Which program are you working with? Regarding your 2nd Point: it means that the calculation of an iterative step towards a numerical solution didn't converge (usually reducing the error values for your new mesh and the derived quantities like dislacement, strain and Forces). As the values didn't go down the program stopped after a certain pre-set Maximum value of Iteration attemps. You may try a forced remeshing at this state of calulation or choose a different starting mesh right from the beginning. Your third Point is on the notations of stress (s11 ... s33) tensor and Youngs modulus Matrix values. In an 3 by 3 Matrix you have 3 lines and 3 columns with 9 values in total. The diagonal values s11, s22 and s33 (similarly for E11 to E33) are the normal stresses (tensile or compressive) on the faces of a cubic unit volume of material. While s11 to s33 depend on the load conditions, E11 to E33 are the same for the ideal case of an homogenous isotropic material. The off-diagonal stress Tensor values like s12, s23, s13 decribe the shear stresses which depend on the shear modulus Gij. For simple , homogeous materials like ductile metals the shear stress values are symmetric to the diagonal s11 to s33 (i.e. s12=s21; a.s.o.).

The Max and Min In-plane stress are related to the given coordinate system and describe the largest and smallest values for e.g. the s11, which could be the highest tensile (+) or only the lowest compressive stresses (-). For the min stresses it is the other way around. It is the same for the max / min strains. For the Max and Min Principal Stress the stress Tensor will be evaluated regarding the highest normal stresses (e.g. for s11) and the lowest value for s22, respectively s33 (plese refer to the Mohrs stress cycle for 2D and 3D).

Your questions are telling me, that you have not sufficient knowledge about strength of materials or mechanical Engineering Basics. I would advise you to consult a book on the mechanics of materials and Basic mechanical Analysis. 

Good luck and have fun!

hello Dr Ruediger Rentsch,

Thank you very much for your helpful response to my question.Actually I am just a beginner in Abaqus and I am still struggling with understanding some of the signs,language and notations.Though I have come a long way within a short time.And I need some little more understanding with it so I can complete my project.

However,regarding the units of length( mm or m ),If I should choose any unit,how can I confirm the unit I have chosen and working with?for example,I want to work with micrometer and millimeter in one model with different components in (micrometer and millimeter),how can I know the exact unit am working with? Because I always get confused.Please let me also know if the  Global Size of the viewport has influence on the units to chose. 

 Thanks

Dear Richard Chukwuma Meribe,

In opposite to other FEM programs ABAQUS has no built-in system of units. Specify all unit data in consistent units. Some common systems of consistent units:

SI: m, N, kg, s, Pa, J, kg/m3

SI (mm): mm, N, tonne (1000 kg), s, MPa, mJ, tonne/mm3

US Unit (ft): ft, lbf, slug, s, lbf/ft2, ft lbf, slug/ft3

Thank you again Dr Ruediger Rentsch,

Konichi wa, Meribe-san. If the answers to your questions are / were helpful, it would be nice to acknowledge it by giving an positive voting. By the way, do you know Prof. Dr. Sh. Sakamoto at Kumamoto Univ. ?  Best regards, Ruediger 

Hahaahahhah...Konichiwa Rentsch sensei!

Ofcourse I know Sakamoto sensei! but I am new in kumamoto Univ.So I doubt he really know me.Unless he remember the Nigerian guy from kyushu University recommended to him some time ago by Prof Noguchi of Solid Mechanics,Fracture/fatigue Lab.

And please am sorry I didnt vote for the answer earlier as I was little  nostalgic  of applying the answers and advice you gave me,lol.Your answers were great and so helpful.

Again thanks! 

Hello Richard, thanks for your message. Dr. Sakamoto is my PhD mate from Keio University were we spent 4 years together at Prof. Insaki's Lab, while Prof. Inasaki was still at Keio. Regards, Ruediger Rentsch.

Its really feels good to know to someone that was once in my situation as a foreign PhD student here in Japan.Well I can see you picked up Japanese before you left.

For real,Its a pleasure meeting you.

Hello Richard, since you came from  Prof Noguchi's Lab at Kyushu Univ. I guess you must have learnt quite some Japanese as well, didn't you? Actually I made my PhD at Keio in English although I was a Kind of fluent on daily life Topics, chatting and casual talk, but not more than that. I was studying hard and learnt a lot, but I had a excellent time and was often treated very Special. I guess I was the most exciting time, very busy, very intensive regarding work / science and learning (culture). Fortunately Kyushu is not so cold, if not even warm, during this time. Will you stay in Japan over Christmas and New Year? Greetings to you, from cold (5° C and rainy) Germany - there is some snow in southern Germany. Ruediger

Sure! My Japanese have actually improved.Same as you were,I am having also some sort of special treatment.Thanks to sensei and lab mates.

Yea Kyushu is not as cold as Hokaido but it is cold.Coming from a Tropical Nigeria, 5°C of Kumamoto is not funny.It is also raining here.I guess am getting used to it though,because my first winter experience was worst.I am yet to experience snow here in Kyushu.Its usually flurries.

Yea I actually dont have any plan of  traveling so i think my Xmas and New year is gonna be here.

Dear Richard,

In addition, I sent to you the interesting materials that deal with this subject.

I hope this helps.

http://www.amesweb.info/StressStrainTransformations/PlaneStressPrincipalStressCal/PrincipalStressCalculator.aspx

@Rüdiger Rentsch I think you misunderstand his question on the notions. Although he puts E11,E22,E33, E12 in the text, actually he means strains, that is, LE11,LE22,LE33,LE12. It is not modulus. 

Good job

thank you

https://www.researchgate.net/post/Maximum_shear_stress_in_Abaqus

https://books.google.iq/books?id=We6sBAAAQBAJ&pg=PA294&lpg=PA294&dq=determine+which+stress+and+strain+orientation+is+correct+to+assign+to+model+analysis?&source=bl&ots=AW3NQwp3OM&sig=ACfU3U3DJIr4PiepIy3hEDlx4771gcG3ig&hl=ar&sa=X&ved=2ahUKEwiQ36Xj8cnpAhWHmBQKHcsbDgQQ6AEwBHoECAsQAQ

https://books.google.iq/books?id=4natDwAAQBAJ&pg=PA441&lpg=PA441&dq=determine+which+stress+and+strain+orientation+is+correct+to+assign+to+model+analysis?&source=bl&ots=hts9agA3yw&sig=ACfU3U1pthGxrnlmLNopI6qH_RvuqAI0Fw&hl=ar&sa=X&ved=2ahUKEwiQ36Xj8cnpAhWHmBQKHcsbDgQQ6AEwBnoECAUQAQ

Richard Chukwuma Meribe Since Abaqus doesn't have an inherent set of units it's the users responsibility to ensure they are consistent (see https://info.simuleon.com/blog/units-in-abaqus or abaqus help (https://help.3ds.com/2019/english/dssimulia_established/simacaemodrefmap/simamod-c-conventions.htm?contextscope=all). So if you will be using mm in your model then Mass is in tonnes, stress is in MPa, and density is in tonne/mm^3.