Science Buddies: "Ask an Expert"

Hello,
I am testing domes and compression. I tried to build domes with plaster of paris but they cracked when the molds were removed. I am now making domes with paper mache. All will have the same thickness, perhaps 5 cm. Do you think this is an approppriate medium. I do not live near anyone with woodworking abilities or tools. HELP ME Please before I waste more time and effort. Thank You Bob

I think making domes of paper mache should work fine. A wall thickness of about 5cm should be good if the diameter of your domes are about 50cm. You probably don't want the thickness of the dome to be more than 10% of the diameter, or they will have the properties more of a solid than of a dome.

One thing you did not indicate in your post is what your hypothesis is and how you are going about testing it. Is it you plan to see how much load they will carry? If you are doing something like this, you should consider variables such as wall thickness relative to diameter, and the geometry of the domes, that is are they full hemispheres or parts of hemispheres? You could also study how their strength might be affected by incorporating some material such as straw in the paper mache mix, or some other material they you think might change the strength of the mixture.

Whatever you decide to do, have fun!

Thanks for the interesting ideas. My hypothesis was "if a smaller diameter dome( a hemisphere) is able to withstand large compression values, than a larger diameter dome will be able to withstand larger compression values". Rather than diameters, maybe I should research adding different buliding materils as you suggested. Have the paper mache as the control and various other added materials as the variables.
I really like that idea, I really want to start my testing now that I have a better direction for my experiment. I now have a different hypothesis forming in my head. Thanks Bob

I'm glad to hear my ideas were helpful, Bob.

Be sure you take into account the thickness of your domes with respect to diameter. If "t" is thickness and "D" is diameter, you should consider evaluating the factor "t/D", using different combinations of thickness and diameter.

It would be great to know how this works out for you.

Good luck.

Hello Again,
I have now have 20 molds all 22.29 cm in diameter, 70 cm in circum, all heights are the same. My concern is wall thickness. Is a 10 % ratio necessary. I have spent 12hrs layering the molds and I have a thickness of about 1cm which is only 5%. Is is ok? I have not found any info which explains dome diameter and wall thickness. I have plain paper mache as controls, and added diffrent materials as variables for testing. My hypothesis is the molds with jypsum added to the paste will be stronger then the molds then the straw, the sand, and the plain paper mache domes. All materials were weighed out exactly. Does this sound like a good experiment to you. I still have time to start over. If the walls are thick enough I will be adding weights for wall compression testing dome strength. Thanks for any imput!

You are making good progress!

There is nothing magic about the dome thickness ratio of 10% that I mentioned. That was to give you a general idea of what might work. I think your 5% thickness will work fine.

Your idea about using the variety of strengthening materials you have selected is very good. This will give you plenty of samples to compare.

So as I understand it, you have chosen 4 materials and have made 20 samples. I presume you have made 5 of each material types. This is an excellent experiment design. It will give you the opportunity to calculate average load to break each material type. Also, keep track of how much variability you observe in the load each material can carry. That is, record all your readings and look at how much spread there is in your results.

One thing to pay special attention to is how you will apply load to your samples and how will you determine how much load it takes to "break" them. Will you be looking for the first crack that starts, or how much load it takes to make them deflect a certain amount, or will you be looking for catastrophic failure? You need to give this some thought. Regarding how you load them, being hemispherical, it will be difficult to apply load in a consistent manner to each sample unless you have some sort of device to help you apply the load in the same location.

Keep up the good work. I wish you success.

Hello, I read your comment and you ask how can I know that I keep the weights in the same place? Well, this is simple, I put an eyewhole for the occulous so that I can put the weights on top of them. I am testing to see when the dome cracks and I hope you have more ideads to make this project better!

P.S. The eyewhole was made from 5/8 inch washers!

Hi,
Domes are ready for testing. I told you that I placed 5/8" steel washers for the oculus and covereed with the papermache, so when I put the weights on top of the dome, it should be evenly spreadout through the dome. Sound good? Thanks so much for all your ideas, questions and time.
Excitedly, Bob

Hi, Bob,

If everything is nice and evenly spaced, you should be good to go. Glad to see you're very enthusiastic about your work! I wish I could also see pictures, since it sounds as if your project is coming along brilliantly!

Hi. thank you for all your help. I did the experiment and it was a success!!! The jypsum proved to be the strongest material in the paper mache domes!!! I will tell you if I win at science fair and thanks again for all your kind help!!!

Hello,
Just want to say thanks again. I received an honorable mention at the SJSF, held at Richard Stockton State College in NJ, this past Saturday. There were more than 800 kids there. I really wanted to place third at least so I could go on to the DelVal competition, but I am very pleased to have won this award. Thanks for all your help. Bobby O

Congratulations, Bobby. Well Done!

We're all glad we could help. It's great to see young people such as yourself so involved with science and engineering.

I hope you enjoyed it enough to go for another project next year.

i am also know starting a science fair project on testing dome strength. My procedure is due very soon, and i have to start my research and get my materials.
My question is how would you make the dome so that you are able to test its strength? Could you also include an image? I also made a new forum post regarding my project. here is link https://www.sciencebuddies.org/science- ... 37&t=12289