Hello Arumy's, this is simple. If you know the dimensions of the specimen, simply imagine you place the specimen on a graph paper with the leftmost bottom corner as the origin of the graph I.e. [0,0], then all other numbers are read off from that reference. Note your x coordinates are the horizontal and y coordinates are the vertical axis. You have to determine all of them for yourself. Hope it makes sense.
Hi @YugSth, thanks for your comment. I am not sure I completely understand the question. Usually, elastic properties are extracted before damage occurs, in the linear elastic section of the stress-strain curve. With damage, obviously, we are beyond the elastic limit and there is no need to worry about elasticity. I am not sure if you asking about compression after impact values, which in this case, you determine a new stress-strain curve from a material subject to compression testing after it has first been impacted. If you can explain more, than I will be able to help you.
Hi Dr Okereke, I find something very interesting regarding the dogbone coupon for brittle material: the fracture normally occurs at the point where the grip curve meets the straight gauge length. From FEM, your first simulation also shows that the necking (or high strain and fracture part) occurs exactly at the point where the grip curve meets the straight gauge length. Do you have any idea why the necking/failure does not occur right in the middle of the gauge length? Is it due to the material property or due to the dogbone shape? Really appreciate it if you can provide any comments!
Hello @Ziyuan, thanks for the comment. You are right that the transition from straight to curved is usually a trigger for fracture. This is because that change in cross-section (at the transition) encourages increased stress concentration there. Therefore, with increase stress concentration, that region will reach the fracture stress of the material before the mid-section of the material, earlier, thereby failing earlier than the guage section. The way out is to follow recommended specimen design standard for whatever material type you are testing. What the standards try to do is to recommend a gradual change in cross-section from the straight guage to the curved grip ends. This will mean a large fillet radius. As a result, any stress concentration around that area will be dissipated/reduced by the large fillet angle. I am not sure what your specimen design is but that is a possibility. Secondly, you can also make your gauge section much thinner/smaller than the gripped ends... as a result, you will have that thinner region (smaller area) approach the fracture strain much earlier than any other part in the specimen... thereby encouraging failure in the gauge section. If those do not work, then think about the whole experimental setup such as ensuring loading is truly uniaxial and the specimen is set parallel to the loading direction correctly. Good luck in the work, and those are my simple suggestions to help.
@@MichaelOkereke Hi Dr Okereke! The samples I made based on ASTM D638 are epoxy (thermoset plastic) samples and short carbon fibre reinforced PLA samples. Short carbon fibre reinforced thermoplastic is a trend for modifying 3D printing filament. Both of them shall follow ASTM D638 or equivalent standards. The material I have tested are normally brittle with little plastic behavior. Most of them break at the transition from straight to curved. Two month ago, I tested the 3D printed coupons by using Carbon fibre PLA. 19 out of 20 samples break at the transition point!!! As you know, D638 is really the most common standard. However, within D638 it only provides two standards types of coupon shape... Do you reckon D638 had better to be reviewed? I know we cannot do anything, but it is interesting and critical-thinking about this issue. Furthermore, when tutoring the undergraduates, we do hundreds of aluminum and cold-rolled steel dogbone-shaped coupons for lab demo. To be honest, it is more common that the fracture occurs at the transition point than right in the middle... Is the dogbone coupon really scientific? lol The solution I proposed is to use straight coupons with aluminium end tabs, which can be referred to ASTM D3039/D3039M for carbon fibre reinforced plastics. For straight coupons, at least, the fracture will occur at gauge length with the most severe defect.
Hello, I am just picking up this now. Dogbone specimens are certainly scientific. The idea is to force failure in the gauge section. I am not sure why you are getting failure in the transition. Maybe your loading conditions is not purely uniaxial. I do not think you should mix standards. Also, you can try and model your specific experimental setup and study the stress history/contour plot, to see if there is a place where the stress is concentrating leading to the premature failure in the transition zone.
Hello @Oriomia, thanks for your interest in the channel. I am not sure about your question, as I already have a video about hybrid composite. Maybe, you can go see that here: th-cam.com/video/1rdbKix_biw/w-d-xo.html, unless there is a different question you want me to investigate, then please let me know.
Hello Dr. Michael Thank you for the content and the smooth work. Would it be possible to make a video on tensile and bending of composites specimen and composite beams or plates? Also, is there a chance for composite damage simulations? One more request: is there a possibility to give a good example of calculating the failure criteria such as Hashine damage or any other of your choice. Best Regards
Hello , This is OK. I will like to make such video for other test schemes. I haven't done damage simulations of composites... maybe in future, Yes. That can be done for hashin damage but there are resources on this in YT.
@@MichaelOkereke I have a structure consisting of small bar and cables I want to do the non-linear geometric analysis by Abaqus Can you give me your email
Hello @tarek metrouni, this might be something that we have to discuss in more details. If you want my help, then please book a one-on-one consultation with me via this link: cmvideos.ck.page/products/one-on-one-consultation
I especially like it when you explain everything so nicely. I wish you a lot of success with the channel professor.
Thank you! 😃
Hello Dr. Micheal.
I was wondering how to get the coordinate numbers before designing the specimen?
Hello Arumy's, this is simple. If you know the dimensions of the specimen, simply imagine you place the specimen on a graph paper with the leftmost bottom corner as the origin of the graph I.e. [0,0], then all other numbers are read off from that reference. Note your x coordinates are the horizontal and y coordinates are the vertical axis. You have to determine all of them for yourself. Hope it makes sense.
Thanks for the video, sir. Could you please make a video on how to get elastic properties of unidrectional composites at a given values of damage?
Hi @YugSth, thanks for your comment. I am not sure I completely understand the question. Usually, elastic properties are extracted before damage occurs, in the linear elastic section of the stress-strain curve. With damage, obviously, we are beyond the elastic limit and there is no need to worry about elasticity. I am not sure if you asking about compression after impact values, which in this case, you determine a new stress-strain curve from a material subject to compression testing after it has first been impacted. If you can explain more, than I will be able to help you.
Hi Dr Okereke, I find something very interesting regarding the dogbone coupon for brittle material: the fracture normally occurs at the point where the grip curve meets the straight gauge length. From FEM, your first simulation also shows that the necking (or high strain and fracture part) occurs exactly at the point where the grip curve meets the straight gauge length. Do you have any idea why the necking/failure does not occur right in the middle of the gauge length? Is it due to the material property or due to the dogbone shape?
Really appreciate it if you can provide any comments!
Hello @Ziyuan, thanks for the comment. You are right that the transition from straight to curved is usually a trigger for fracture. This is because that change in cross-section (at the transition) encourages increased stress concentration there. Therefore, with increase stress concentration, that region will reach the fracture stress of the material before the mid-section of the material, earlier, thereby failing earlier than the guage section.
The way out is to follow recommended specimen design standard for whatever material type you are testing. What the standards try to do is to recommend a gradual change in cross-section from the straight guage to the curved grip ends. This will mean a large fillet radius. As a result, any stress concentration around that area will be dissipated/reduced by the large fillet angle. I am not sure what your specimen design is but that is a possibility.
Secondly, you can also make your gauge section much thinner/smaller than the gripped ends... as a result, you will have that thinner region (smaller area) approach the fracture strain much earlier than any other part in the specimen... thereby encouraging failure in the gauge section.
If those do not work, then think about the whole experimental setup such as ensuring loading is truly uniaxial and the specimen is set parallel to the loading direction correctly.
Good luck in the work, and those are my simple suggestions to help.
@@MichaelOkereke Hi Dr Okereke! The samples I made based on ASTM D638 are epoxy (thermoset plastic) samples and short carbon fibre reinforced PLA samples. Short carbon fibre reinforced thermoplastic is a trend for modifying 3D printing filament. Both of them shall follow ASTM D638 or equivalent standards. The material I have tested are normally brittle with little plastic behavior. Most of them break at the transition from straight to curved. Two month ago, I tested the 3D printed coupons by using Carbon fibre PLA. 19 out of 20 samples break at the transition point!!! As you know, D638 is really the most common standard. However, within D638 it only provides two standards types of coupon shape... Do you reckon D638 had better to be reviewed? I know we cannot do anything, but it is interesting and critical-thinking about this issue.
Furthermore, when tutoring the undergraduates, we do hundreds of aluminum and cold-rolled steel dogbone-shaped coupons for lab demo. To be honest, it is more common that the fracture occurs at the transition point than right in the middle... Is the dogbone coupon really scientific? lol
The solution I proposed is to use straight coupons with aluminium end tabs, which can be referred to ASTM D3039/D3039M for carbon fibre reinforced plastics. For straight coupons, at least, the fracture will occur at gauge length with the most severe defect.
Hello, I am just picking up this now.
Dogbone specimens are certainly scientific. The idea is to force failure in the gauge section.
I am not sure why you are getting failure in the transition. Maybe your loading conditions is not purely uniaxial.
I do not think you should mix standards. Also, you can try and model your specific experimental setup and study the stress history/contour plot, to see if there is a place where the stress is concentrating leading to the premature failure in the transition zone.
Thank you Dr. and please if you can do the same job for composite special hybrid composites.
Hello @Oriomia, thanks for your interest in the channel. I am not sure about your question, as I already have a video about hybrid composite. Maybe, you can go see that here: th-cam.com/video/1rdbKix_biw/w-d-xo.html, unless there is a different question you want me to investigate, then please let me know.
Hello Dr. Michael
Thank you for the content and the smooth work.
Would it be possible to make a video on tensile and bending of composites specimen and composite beams or plates? Also, is there a chance for composite damage simulations? One more request: is there a possibility to give a good example of calculating the failure criteria such as Hashine damage or any other of your choice.
Best Regards
Hello ,
This is OK. I will like to make such video for other test schemes. I haven't done damage simulations of composites... maybe in future, Yes. That can be done for hashin damage but there are resources on this in YT.
@@MichaelOkereke Thank you for the response. Keen to watch your videos. Regards
Thank you, Dr. Obi. If you don't mind, I will like to connect with you later. Thanks again
A small error in the dimension [132, 0]
Hello
How are you ?
Can you help me
Hello Tarek, I am good. Thanks. How do you want me to help you?
@@MichaelOkereke I have a structure consisting of small bar and cables I want to do the non-linear geometric analysis by Abaqus
Can you give me your email
Hello @tarek metrouni, this might be something that we have to discuss in more details. If you want my help, then please book a one-on-one consultation with me via this link: cmvideos.ck.page/products/one-on-one-consultation
@@MichaelOkereke Hello Dr. Okereke, how to plot these displacement force curves?