tag:blogger.com,1999:blog-13887692.post2782073658066186217..comments2024-03-13T02:16:08.135-04:00Comments on Cozy Beehive: Deceleration And Force Of A Helmeted Head ImpactRon Georgehttp://www.blogger.com/profile/18394865788996482667noreply@blogger.comBlogger36125tag:blogger.com,1999:blog-13887692.post-25766345898666191672011-12-20T02:59:32.769-05:002011-12-20T02:59:32.769-05:00n1 conversation.n1 conversation.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-13887692.post-2653995305387679612009-07-10T18:40:57.731-04:002009-07-10T18:40:57.731-04:001) I think its very tough to quantify how much co...1) I think its very tough to quantify how much compression actually occurs - its not a quantity you can ascertain without a look at the microstructure post crash and EPS tends to crack after a point anyway - which is not necessarily a bad thing.<br /><br />2) Yes and no, the brain being a squishy thing behaves strangely when it whacks into a solid - the best analogy i can think of is a block of jello in slow motion. You can observe torsional movement as the thing bounces off a surface.<br /><br />4) Skull stiffness is variable depending on the part of the skull but is several orders of magnitude greater than anything else involved in the system, also the degree of deformation that is needed to appreciably change the contact patch is pretty huge - remember the "pressure" is transmitted in terms of stress waves and thus the skull contact patch geometry has a different influence than say the geometry of the impactor (which is profound in that it generates very different stress concentrations).<br /><br />The case might be different for a child, whose skull is significantly less stiff however.Unknownhttps://www.blogger.com/profile/00403695097675640092noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-85703686345074248652009-07-06T13:04:12.427-04:002009-07-06T13:04:12.427-04:001. My understanding is that even in collisions tha...1. My understanding is that even in collisions that did result in injury to the brain there was little compression of the foam. If that is correct then SURELY the foam is too stiff? I would personally rather have a helmet that "runs out" of cushion (just) before I get brain damage, than one which will still have travel left long after the acceleration on my head becomes critical.<br /><br />2. I understand that angular acceleration is often thought to be far more serious than direct impact, but arent we still looking at (broadly) a situation in which the initial angular acceleration is still the critical factor?<br /><br />4. My point is that the deformation of the skull will effect how the helmet is loaded rather than the brain. In turn, a lower pressure on the helmet would necessitate a less stiff foam.gsport georgehttps://www.blogger.com/profile/10070707288565019602noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-82327116829697879792009-07-06T10:33:55.546-04:002009-07-06T10:33:55.546-04:001) Current foam helmets do not completely compress...1) Current foam helmets do not completely compress because that is an explicit design feature for reasons I have stated above - you do not want your foam approaching dense packing.<br /><br />2) That is patently incorrect. There is significant conflicting theories and research on the precise wounding mechanics. There is some good data showing it is the torsion on the brain, and not the first impact into the skull which is the biggest problem.<br /><br />3) Car crash testing involves measuring accelerations on dummies in various locations (head, chest) that are dynamic analogs to the human body, as well as accelerations in the vehicle.<br /><br />That is quite different from what is going on in helmet testing where the whole head is treated as a unified mass.<br /><br />I said previously acceleration is a good tool because strain gauges and the like are tough to use directly in impact.<br /><br />4) I think you are mistaken if you think elastic deformation of the skull will bring a significant drop in stress in the skull. Also the wounding is caused by different accelerations of the brain WRT to the skull, so you are more interested in the stress states in the brain AND and the stress states/wave dynamics in the skull-helmet system - they are somewhat separate.<br /><br />5) We are, but you are arguing for stuff (like softer foams) which isn't supported by the lit ;)<br /><br />A slightly softer foam to a specific point might be better but you're always going to be fighting that stress riser point with any crushable system.Unknownhttps://www.blogger.com/profile/00403695097675640092noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-34956843477147492272009-07-06T08:30:43.174-04:002009-07-06T08:30:43.174-04:00Jason I think you are making this far more complex...Jason I think you are making this far more complex than is necessary.<br /><br />1. I do not believe that existing helmets ever achieve a state where the foam is "fully" compressed. We arent talking about elastomer foams here but typically expanded polystyrene and the foam often breaks before it compresses even a fraction as much as it could.<br /><br />2. Whilst the brain will undoubtedly rattle about inside the skill, which seems to be your main concern; the initial impact and decceleration will surely be the worst of these wouldnt you agree? Broadly, if the brain is damaged by the first impact then the helmet has failed, and if the brain survives the first impact/deceeleration then it will most likley survive any subsequent waves that are set up.<br /><br />3. An examination of decceleration is quick and easy, and I would argue that it is an established means of gauging "suvivability" and is what is used in the design and testing of car impact protection systems.<br /><br />4. Whilst the human skull will not flex much, it will flex a lot more than a steel headform. When we are dealing with two mating curves, a slight defelction of the skull will correspond to quite a large change in the local radius of curvature which will make quite a difference in the contact area, and therefore pressure and therefore stress in the material.<br /><br />5. Arent we arguing the same point? vis that the existing tests and helmets could be improved?gsport georgehttps://www.blogger.com/profile/10070707288565019602noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-17983308456321727282009-07-05T16:44:05.994-04:002009-07-05T16:44:05.994-04:00Sorry for the late reply.
@ Gsport:
I think you&...Sorry for the late reply.<br /><br />@ Gsport:<br /><br />I think you've got some valid points but you are missing a huge aspect of foam behavior - as foam compacts beyond a certain point it becomes useless.<br /><br />Ron's treatment is entirely based on the concept of trading space for time - which is perfectly valid but it ignores the fact that once the foam reaches dense packing (all the voids are collapsed) its only providing impedance attenuation of the stress waves - of course if you are a driving a shock into the foam all its doing is impedance attenuation, but I think most crashes remain largely acoustic.<br /><br />You're falling into a similar pitfall by assuming that a helmet that pancakes out to a dramatic degree would better protect the head. The reality is that if you look at the dynamic stress strain curves of a foam you'll observe they are largely elastomeric: there is a large range of strains for which the stress remains relatively constant after which it starts to rise precipitously (this corresponds with densification).<br /><br />You want your foam to be operating as much as possible in that plateau region, otherwise you risk the stress being quite high and the foam doing nothing. Going to a softer foam might involve requiring a significantly greater thickness for impact protection.<br /><br />I also think you are heavily over-estimating the compliance of the skull - its fairly rigid in comparison to both the foam and especially the brain.<br /><br />@Ron:<br /><br />I have issues with the anvil testing - for one it ignores the most important dynamics: the brain bouncing and torquing inside the skull, the second is that it ignores the rest of the neck and body attached. I think they could do much better with tissue simulants for a scalp and brain with a rigid headform (metal is probably fine) and an actual neck<br /><br />again, I think its somewhat academic, since as I said before the limitation is comfort and ergonomics not really testing.Unknownhttps://www.blogger.com/profile/00403695097675640092noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-22066081020612014732009-07-01T18:47:36.647-04:002009-07-01T18:47:36.647-04:00Ron, the design brief said the objective was to cr...Ron, the design brief said the objective was to create something for minor bumps and falls. The issue of buckling seems real, so my question is ...after a minor fall, is it elastic enough to come back into its original shape? Who knows. Its still not a bad idea, and it looks like it will really excel in the cooling of the head.Ronhttps://www.blogger.com/profile/16268869622833968439noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-33736222045793814452009-07-01T06:52:55.705-04:002009-07-01T06:52:55.705-04:00The honeycomb/folding design looks really neat, yo...The honeycomb/folding design looks really neat, you can see comfort and ventilation being very good. However I think there is an inherant problem with using a member that will buckle, as I would think these ribs will. The stiffness will start at a maximum, tail off slightly as it begins to buckle, then drop almost to zero once buckling really starts. I suppose with this shape, as the first ribs to hit buckle, others will come into play as they make contact, so that may counteract it somewhat, however you are giving up any protection in the initial zone..<br /><br />Chances of selling a helmet that looks like that and ties on with a ribbon like a bonnet to serious cyclists in any discipline? Pretty much zero I would think...gsport georgehttps://www.blogger.com/profile/10070707288565019602noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-9979110209207630962009-06-30T16:45:24.311-04:002009-06-30T16:45:24.311-04:00Jason, Gsport : Do you have any comment on the use...Jason, Gsport : Do you have any comment on the use of honeycomb structure for helmets? One was shown recently on the Yanko Design blog and the design is being pursued by some French firm named DoYouVelo?<br /><br /><a href="http://www.yankodesign.com/2009/06/29/no-stiff-helmet-for-my-head-please/" rel="nofollow">See here</a> for more details.Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-42740473239310828062009-06-30T06:41:39.362-04:002009-06-30T06:41:39.362-04:00Jason said:-
"Creating a headform of similar...Jason said:-<br /><br />"Creating a headform of similar stiffness and structure is not enough - what is dramatically more important is replicating the hugoniots, impedances.<br /><br />You could very well do all that (at significant cost to the consumer or taxpayer - we're trying to get people to wear the things remember?), however you then have to ask yourself what you are going to gain from it all. The major impediment to helmet performance is not the testing apparatus but what a cyclist is willing to wear due to comfort and aesthetics.<br /><br />Design constraints are the real problem, not the diagnostics.<br /><br />There is a reason they talk in terms of accelerations instead of stresses and strain rates - it is significantly easier to quantify the kinematics rather than the structural and wave dynamics."<br /><br /><br /><br />In my opinion, one of the biggest design constraints right now is that the helmet should cushion an unyielding steel headform, this means that the amount of helmet material involved in the collision is restricted to a very small volume over a pretty small contact patch. With a real skull, the skull would deform slightly (like a bike tyre) to create a larger contact patch with a larger volume of helmet material involved in the decceleration.<br /><br />As a result (in my opinion) a better helmet would have a slightly softer foam that in the worst case scenario impact we would see almost fully flattened over a large area by the riders head. This would result in a more flexible possibly lighter helmet that would likely be more comfortable and easier to "fit" too, all of which is likely to improve adoption.<br /><br />Sure there is a huge amount that is unknown about head injuries, the role of angular acceleration is the obvious one, however what IS known is that:-<br /><br />1. The human head is not a rigid steel headform.<br />2. People dont wear helmets due to issues with fit, feel and comfort (not just how they look).<br />3. Helmet manufacturers design to pass the tests.<br /><br />I would be much happier using a helmet that had proved it could deccelerate a flexible head (with a similar stiffness to a human skull) at 100G from 20mph (where the strain in a fibreglass headform made to be a similar stiffness to a human skull did not exceed 75% of "yield strain") and had "used up" all it's protection than:-<br />Using a helmet designed simply to prevent an unyielding steel headform decellerating at more than 300G from 6.3m/sec (14mph).<br /><br />Using a similar calculation to Ron's initial one, you could pass the Snell test (which most helmets dont pass) with 7mm of cushioning (travel), wheras a 100G from 20mph test would need 40mm of travel, which might be made up of 5 or even 10mm of skull deflection and the compression of an inch and a half of helmet...<br /><br />Wouldnt you?<br /><br />In summary, my point is that we dont have to create a "perfect" test, just a "much better" one, and I honestly dont think that that is THAT hard.gsport georgehttps://www.blogger.com/profile/10070707288565019602noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-58689533015751674552009-06-30T00:03:51.759-04:002009-06-30T00:03:51.759-04:00Interesting discussion. May I also add the mythbus...Interesting discussion. May I also add the mythbuster's use pig cadavers. The ethical implications of using animal body parts are many. I still think if you want to save a human's life, its best to test on a human cadaver.Stan Slevinskinoreply@blogger.comtag:blogger.com,1999:blog-13887692.post-15954291121807990892009-06-29T23:54:43.388-04:002009-06-29T23:54:43.388-04:00Jason. Look into "Stylene". I have read ...Jason. Look into "Stylene". I have read a few research papers that used this material to simulate human scalp. Sort of a rubber. I'm not sure if there's anything out that that can mimic human skull that is made of 22 bones. In bone grafting, they use some clever materials to mimic the porous structure of bone. Might this help?<br /><br />By the way, do you sincerely believe the drop anvil test on the metal head form done by helmet companies today is inadequate? What do you propose they do more?Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-72240949088504453042009-06-29T23:33:15.482-04:002009-06-29T23:33:15.482-04:00Jason said : Mythbusters doesn't do real scien...<b>Jason said : Mythbusters doesn't do real science though, most of the research and R&D in blast loading is conducted with relatively standard crash test dummies with accelerometers and surface mounted pressure transducers - in those cases you are interested more in what the body form sees through protection systems.</b><br /><br />I didn't say they're doing real science. Sometimes, approximations are needed by engineers and common people to test, prove, make a product etc. According to the Mythbuster guys (who themselves have worked in special effects for sometime), ballistics gel closely simulates the density and viscosity of human muscle tissue and is used for testing the performance of firearms ammunition. I was only asking whether something of this nature would be far better than using a rigid, metal headform. Throw some ideas around.Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-35667246580080515182009-06-29T23:12:36.033-04:002009-06-29T23:12:36.033-04:00@ gsport,
It is actually very difficult to get ca...@ gsport,<br /><br />It is actually very difficult to get cadavers for destructive testing in engineering.<br /><br />Creating a headform of similar stiffness and structure is not enough - what is dramatically more important is replicating the hugoniots, impedances.<br /><br />You could very well do all that (at significant cost to the consumer or taxpayer - we're trying to get people to wear the things remember?), however you then have to ask yourself what you are going to gain from it all. The major impediment to helmet performance is not the testing apparatus but what a cyclist is willing to wear due to comfort and aesthetics.<br /><br />Design constraints are the real problem, not the diagnostics.<br /><br />@Ron<br /><br />Mythbusters doesn't do real science though, most of the research and R&D in blast loading is conducted with relatively standard crash test dummies with accelerometers and surface mounted pressure transducers - in those cases you are interested more in what the body form sees through protection systems.Unknownhttps://www.blogger.com/profile/00403695097675640092noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-75565143013187199772009-06-29T19:42:53.061-04:002009-06-29T19:42:53.061-04:00There's a reason those mythbusters use ballist...There's a reason those mythbusters use ballistics gel to simulate human heads when blowing them up.Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-17293163556316594852009-06-29T19:29:40.268-04:002009-06-29T19:29:40.268-04:00Jason,
You are right, EXACTLY modelling a human h...Jason,<br /><br />You are right, EXACTLY modelling a human head is always going to be very difficult. However, to throw up our hands and use a steel headform with a single accelerometer seems like a poor second best.<br /><br />It wouldnt be THAT difficult to create a fibreglass "skull" (of a suitable stiffness) with strain gauges AND accelerometers, covered in a "gel" and do tests to compare it to how a REAL human head (from a donated body) perform under the tests. We would then have valuable data about how the artificial test-skull compares to a real one, and subsequent tests on "test-skulls" whilst not by any means, perfect, would be substantially more accurate than existing tests on steel heads that are virtually unyielding. <br /><br />Helmet design might then advance to try to match the performance of the helmet to the behaviour of the skull and address some of the issues with existing helmets on real heads...gsport georgehttps://www.blogger.com/profile/10070707288565019602noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-14626146391987908492009-06-29T14:52:56.019-04:002009-06-29T14:52:56.019-04:00Gsport said at 6:37pm : The car is actually likely...<b>Gsport said at 6:37pm : The car is actually likely to yield far more that the foam of the helmet. I have never seen the foam of a helmet actually squashed significantly and certainly not flat and I very much doubt that the elastic region of the deformation (which wouldnt be obvious after the event)is significant, wheras I have put my head into the odd car panel and left a very noticable dent that probably accounts for a good couple of inches of travel.</b><br /><br />You're right about the denting. Check out <a href="http://www.liveleak.com/view?i=e0a_1245871431" rel="nofollow">this video </a>I found of a racing cyclist rear ending into a parked car by the side of the road. I did not expect the car to deform as much as the video shows.Ronhttps://www.blogger.com/profile/16268869622833968439noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-6153431881950894502009-06-29T13:20:46.621-04:002009-06-29T13:20:46.621-04:00At Jason : The complexities are immense if you wan...At Jason : The complexities are immense if you want to get really deep into this stuff. I presented a point mass discretized model here, and the level of abstraction is sufficient for any person not interested in too detailed analysis. Hopefully we can get back together again for detailed analyses. I've got to check out some literature before that first.Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-66662648424423973122009-06-29T10:38:52.250-04:002009-06-29T10:38:52.250-04:00Gabbie : Glad you're enjoying it.Gabbie : Glad you're enjoying it.Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-26359611928199338002009-06-28T22:25:34.286-04:002009-06-28T22:25:34.286-04:00I really really like your blog,Ron. Its very diffe...I really really like your blog,Ron. Its very different&interesting.Keep up the good work.<br />GabGabbienoreply@blogger.comtag:blogger.com,1999:blog-13887692.post-75863099956944531142009-06-27T18:45:32.488-04:002009-06-27T18:45:32.488-04:00Vary unique blog.Bicycles math!Vary unique blog.Bicycles math!Major Taylorhttp://marshalltaylor.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-13887692.post-7883881800260819432009-06-27T14:03:01.659-04:002009-06-27T14:03:01.659-04:00@Ron
Most FEA codes that specialize in dynamic mo...@Ron<br /><br />Most FEA codes that specialize in dynamic modelling employ some sort of proprietary "crushable foam model" that is variously based on engineering literature for constitutive relations, EOS, etc.<br /><br />There is a lot of lit out there - you are probably well served using google scholar and searching stuff like<br /><br />"Foam Crush Model"<br /><br />The FEA manuals often source the lit they used as they often work with the researchers in question too.<br /><br />I found this paper especially helpful though:<br /><br />http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V3K-3WRC6B1-5&_user=458507&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=941550657&_rerunOrigin=scholar.google&_acct=C000022002&_version=1&_urlVersion=0&_userid=458507&md5=bc66aba5999f054af6b0ee7b8096ca7b<br /><br />@gsport,<br /><br />the problem with your seemingly easy solution is that it is anything but easy. You need a material that closely matches the dynamic behavior of human tissues - said tissues are highly non-linear, visco-elastic materials.<br /><br />Also you need to capture the detailed geometry and structure of the head<br /><br />In short matching materials and structure to the real thing close enough so that you can measure stress-strain directly is very difficult.<br /><br />Furthermore you've got another problem - injury thresholds and mechanisms are poorly understood, you've got a complex interplay between the medical people and the engineers trying to work out "engineering" values for medical phenomena without being able to do direct analysis on the real, live system.<br /><br />Much of the data about thresholds is forensic and highly empirical based on "feel".<br /><br />There is a reason they talk in terms of accelerations instead of stresses and strain rates - it is significantly easier to quantify the kinematics rather than the structural and wave dynamics.<br /><br />It is more functional to create a headform that mimics the bulk dynamics (ie similar mass, neck compliance, etc for which there is more lit) and speak in terms of kinematics, than it is to go through from a detailed "mesoscale" stress/strain and wavedynamic analysis - the latter is naturally picking up steam especially as bulk concepts like energy conservation don't accurately predict behavior for anything but low speed impacts (ie low strain rates)Unknownhttps://www.blogger.com/profile/00403695097675640092noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-29737246787113527112009-06-27T10:38:45.096-04:002009-06-27T10:38:45.096-04:00The point is that as long as the way helmets are t...The point is that as long as the way helmets are tested is stupid (solid steel headform) and the manufacturers are designing to pass the standards rather than to be good helmets, we will have helmets that are not a good match to our needs. In this day and age it should be a doddle to construct a more realistic head with stress gauges implanted, and make serious measurements of protection levels.gsport georgehttps://www.blogger.com/profile/10070707288565019602noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-51812319291519719462009-06-26T20:42:02.889-04:002009-06-26T20:42:02.889-04:00@ Gsport 6:37pm : The maximum tolerable decelerati...@ Gsport 6:37pm : The maximum tolerable deceleration of the human head is 300g, provided it is applied for just milliseconds on the time scale. This is what I read.Ron Georgehttps://www.blogger.com/profile/18394865788996482667noreply@blogger.comtag:blogger.com,1999:blog-13887692.post-79818105610702152542009-06-26T20:28:03.448-04:002009-06-26T20:28:03.448-04:00Ron, your comment about using ballistics gel to si...Ron, your comment about using ballistics gel to simulate the head may not be practical as high vibrations in it might make the mean accelerations difficult to compute in a test simulation. Thanks.John K.noreply@blogger.com