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  1. #21
    Kung-Fu Joe's Avatar
    Join Date
    Sep 2007
    New Jersey, USA
    Quote Originally Posted by MaverickZ
    So his friend stating that this property of concrete is the same as for wood is incorrect. Wood is anisotropic, concrete is isotropic.
    Again, I'm certainly a novice (at best) on the subject, but here's a quick Abstract that I found on the anisotropic properties of concrete:

    Quote Originally Posted by CNagy
    I used to hear these explanations all the time and, invariably, the people giving the explanation miss the point entirely. Telling me that there is much harder concrete out there to break (with the implication that what I do is thus some sort of sham or parlor trick or is easy) is the same as telling a power lifter that there are heavier weights out there to lift.
    The "parlor trick" aspect is not really my intention, here. Most martial artists know that even with these caveats, power breaking requires a good deal of training and technique. However, a large number of non-martial-artists still look at Breaking as if it's some sort of measure of super-strength.

    The mysticism that has been associated with Eastern martial arts creates a base of misconception in the eyes of a number of people. They know that "wood is really, really hard" and that "karate guys can break wood." They then see that guys like Boxers don't do any board-breaking in their training, and make the false conclusion that a Karate punch is stronger than a Boxing punch.

    Understanding the basic principles behind Breaking does a great deal towards overcoming the misconception that someone can fight well just because they can break a board.


    EDIT: Forgot to attribute the second quote to CNagy, originally.
    Last edited by Kung-Fu Joe; 11/15/2007 9:43am at .

  2. #22

    Join Date
    Oct 2003
    white boy jiujitsu
    Quote Originally Posted by Kung-Fu Joe
    Again, I'm certainly a novice (at best) on the subject, but here's a quick Abstract that I found on the anisotropic properties of concrete:
    Very interesting. I'm no CivE so I claim no expertise in concrete or cement, I was just going on what I picked up in my materials classes. This abstract gives a hint on why concrete may be anisotropic, but I don't know if it implies that this anisotropy happens after being used in construction or during creation.

    Abstract In many existing structures strength variations of about 2030% between upper and lower levels have been registered. These differences are attributed to bleeding and segregation of concrete.
    In the present work standard specimens and structural elements (2.00 m high) were molded with concretes of different bleeding capacity and velocity.
    The influence of bleeding on hardened concrete characteristics was studied: visual appreciation of localized defects, determination of absorption, specific gravity, compressive and tensile strength, and strain measurements.
    With a high bleeding velocity the formation of vertically-oriented channels was observed. With a high bleeding capacity water accumulated under coarse aggregates causing on drying approximately horizontally-oriented fissures.
    These localized defects are responsible for concrete anisotropic behaviour. The presence of defects was not detected in standard specimens owing to their small volume and height.
    According to the measured bleeding parameters a decrease of strength of up to 30% in the upper levels and a difference of about 25% according to the loading direction were registered. From the evaluations realized it comes out that when the bleeding of concrete is high the results of tests made on molded specimens are not representative of the real state of the structure.

  3. #23

    Join Date
    Dec 2003
    North Carolina
    "An isotropic material is one whose properties are independent of orientation or direction. That is, the strengths across the width and thickness are the same as along the length of the part, for example. Most metals and some nonmetals can be considered to be macroscopically isotropic. Other materials are anisotropic, meaning that there is no plane of material property symmetry. Orthotropic materials have three mutually perpendicular planes of property symmetry and can have different material properties along each axis. Wood, plywood, fiberglass, and some cold-rolled sheet metals are orthotropic.

    One large class of materials that is distinctly nonhomogenous (i.e., hetereogeneous) and nonisotropic is that of composites (also see below). Most composites are manmade, but some, such as wood, occur naturally. Wood is a composite of long fibers held together in a resinous matrix of lignin. You know from experience that it is easy to split wood along the grain (fiber) lines and nearly impossible to do so across the grain. Its strength is a function of both orientation and position. The matrix is weaker than the fibers and it always splits between fibers."

    - Pg 71, Norton "Machine Design"

    "Bone material is a composite of collagen and hydroxyapatite. Apatite crystals are very stiff and strong. The Young's modulus of flourapatite along the axis is about 165 GPa. This may be compared with the Young's modulus of steel , 200 GpA, Aluminum, 6061 Alloy, 70 Gpa. Collagen does not obey Hooke's law exactly, but its tangent modulus is about 1.24 GPa. The Young's modulus of bone (18 GPa in tension in human femur) is intermediate between that of apatite and collagen. But as a good composite material, the bone's strength is higher than that of either apatite or collagen, because the softer component prevents the stiff one from brittle cracking, while the stiff component prevents the soft one from yielding.

    The mechanical propreties of a composite material (Young's modulus, shear modulus, viscoelastic properties, and especially the ultimate stress and strain at failure) depend not only on the composition, but also on the structure of bone (geometric shape of the components, bond between fibers and matrix, and bonds at points of contact of the fibres). To explain its mechanical properties, a detailed mathematical model of bone would be very interesting and useful for practical purposes. That such a model cannot be very simple can be seen from the fact that the length of bone does correlate with the mass density of bone; but only loosly. Amtmann (1968, 1971), using Schmitt's (1968) extensive data on the distribution of strength of bone in the human femur, and Amtmann and Schmitt's (1968) data on the distribution of mass density in the same bone (determined by radiography), found that the correlation coefficient of strength and density is only .40-.42. Thus one would have to consider the structural factors to obtain a full understanding of the strength of bone. Incidentally, Amtmann and Schmitt's data show that both density and strength are nonuniformly distributed in the human femur: the average density varies from 2.20 to 2.94 from the lightest to the heaviest spot, while the strength varies over a factor of 1.35 from the weakest place to the strongest place."

    -- Fung Y.C., Biomechanics: Mechanical Proeprties of Living Tissues.

    More information available upon request.

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