יחס חוזק/משקל

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יחס חוזק/משקלאו יחס חוזק למשקל, נקרא גם חוזק סגולי (באנגלית: specific strength, strength-to-weight ratio או strength/weight ratio או אפילו strength-to-mass ratio) הוא היחס בין החוזק של חומר (שמוגדר ככוח ליחידת שטח, או המאמץ ברגע הכשל) ובין הצפיפות שלו. בחומרים רכים כמו טקסטיל מדובר על כשל במשיכה. בחומרים קשים מדובר על כשל במשיכה או בדחיסה.

עוד דרך לתאר את החוזק הסגולי של חומר היא לבחון את האורך לכשל, או את אורך הנשיאה העצמי של החומר. נדמיין עמוד או מוט ארוך של החומר, שתלוי רק מלמעלה. ברור, שבאורך מספיק גדול, המקטע העליון לא יוכל לשאת את המשקל וייקרע. לצורך זה, הכוח שגורם לכשל הוא כוח הכבידה של כדור הארץ, שמחושב על פני כל האורך של עמודת החומר לקבלת המשקל בכשל. לדוגמה, חבל טיפוס ששוקל כ-60גרם למטר, יכול לשאת משקל של 2200קילוגרם, בערך. זה אומר שאורך הקריעה הנשיאה העצמי שלו הוא קצת יותר מ-36.5 ק״מ!

ברור, שחבל שלא יכול לשאת את משקלו העצמי לא יתפקד בכלל כחבל...



The specific strength is a material's strength (force per unit area at failure) divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio or strength-to-mass ratio. In fiber or textile applications, tenacity is the usual measure of specific strength. The SI unit for specific strength is Pa m3/kg, or N·m/kg, which is dimensionally equivalent to m2/s2, though the latter form is rarely used.

Another way to describe specific strength is breaking length, also known as self support length: the maximum length of a vertical column of the material (assuming a fixed cross-section) that could suspend its own weight when supported only at the top. For this measurement, the definition of weight is the force of gravity at the Earth's surface (standard gravity, 9.80665 m/s2) applying to the entire length of the material, not diminishing with height. This usage is more common with certain specialty fiber or textile applications.

The materials with the highest specific strengths are typically fibers such as carbon fiber, glass fiber and various polymers, and these are frequently used to make composite materials (e.g. carbon fiber-epoxy). These materials and others such as titanium, aluminium, magnesium and high strength steel alloys are widely used in aerospace and other applications where weight savings are worth the higher material cost.

Note that strength and stiffness are distinct. Both are important in design of efficient and safe structures.

Examples

Specific tensile strength of various materials
Material Tensile strength
(MPa)
Density
(g/cm³)
Specific strength
(kN·m/kg)
Breaking length
(km)
Source
Concrete 2–5 2.30 5.22 0.44
Rubber 15 0.92 16.3 1.66
Copper 220 8.92 24.7 2.51
Polypropylene / PP 25–40 0.90 28–44 2.8–4.5 <ref>http://www.goodfellow.com/E/Polypropylene.html</ref>
(Poly)acrylonitrile-butadiene-styrene / ABS 41–45 1.05 39–43 <ref>http://www.goodfellow.com/E/Polyacrylonitrile-butadiene-styrene.html</ref>
Polyethylene terephthalate / Polyester / PET 80 1.3–1.4 57–62 <ref>http://www.goodfellow.com/E/Polyethylene-terephthalate.html</ref>
Piano wire / ASTM 228 Steel 1590-3340 7.8 204-428 <ref>http://www.matweb.com/search/datasheet_print.aspx?matguid=4bcaab41d4eb43b3824d9de31c2c6849</ref>
Polylactic acid / Polylactide / PLA 53 1.24 43 <ref>http://www.goodfellow.com/E/Polylactic-acid-Biopolymer.html</ref>
Low Carbon Steel (AISI 1010) 365 7.87 46.4 4.73 <ref>תבנית:Cite web</ref>
Stainless steel (304) 505 8.00 63.1 6.4 <ref>תבנית:Cite web</ref>
Brass 580 8.55 67.8 6.91 <ref name="roymech">תבנית:Cite web</ref>
Nylon 78 1.13 69.0 7.04 <ref>http://www.goodfellow.com/E/Polyamide-Nylon-6.html</ref>
Titanium 344 4.51 76 7.75 <ref>תבנית:Cite web</ref>
CrMo Steel (4130) 560–670 7.85 71–85 7.27–8.70 <ref>תבנית:Cite web</ref><ref>תבנית:Cite web</ref>
Aluminium alloy (6061-T6) 310 2.70 115 11.70 <ref>תבנית:Cite web</ref>
Oak 90 0.78–0.69 115–130 12–13 <ref>תבנית:Cite web</ref>
Inconel (X-750) 1250 8.28 151 15.4 <ref>תבנית:Cite web</ref>
Magnesium alloy 275 1.74 158 16.1 <ref>תבנית:Cite web</ref>
Aluminium alloy (7075-T6) 572 2.81 204 20.8 <ref>תבנית:Cite web</ref>
Titanium alloy (Beta C) 1250 4.81 260 26.5 <ref>תבנית:Cite web</ref>
Bainite 2500 7.87 321 32.4 <ref name="Bhadeshia">

52nd Hatfield Memorial Lecture: "Large Chunks of Very Strong Steel" by H. K. D. H. Bhadeshia 2005. on archive.is </ref>

Balsa 73 0.14 521 53.2 <ref>תבנית:Cite web</ref>
Carbon-epoxy composite 1240 1.58 785 80.0 <ref>

McGRAW-HILL ENCYCLOPEDIA OF Science & Technology, 8th Edition, (c)1997, vol. 1 p 375</ref>

Spider silk 1400 1.31 1069 109
Silicon carbide fiber 3440 3.16 1088 110 <ref>Specialty Materials, Inc SCS Silicon Carbide Fibers</ref>
Glass fiber 3400 2.60 1307 133 <ref name="vectran">תבנית:Cite web</ref>
Basalt fiber 4840 2.70 1790 183 <ref>תבנית:Cite web</ref>
1 μm iron whiskers 14000 7.87 1800 183 <ref name="Bhadeshia" />
Vectran 2900 1.40 2071 211 <ref name="vectran" />
Carbon fiber (AS4) 4300 1.75 2457 250 <ref name="vectran" />
Kevlar 3620 1.44 2514 256 <ref name="ngcc">תבנית:Cite web</ref>
Dyneema (UHMWPE) 3600 0.97 3711 378 <ref>תבנית:Cite web</ref>
Zylon 5800 1.54 3766 384 <ref name="Toyobo Co., Ltd.">תבנית:Cite web</ref>
Carbon fiber (Toray T1100G) 7000 1.79 3911 399 <ref name="Toray Composites Materials America, Co., Ltd.">תבנית:Cite web</ref>
Carbon nanotube (see note below) 62000 0.037–1.34 46268–N/A 4716–N/A <ref name="Strength and Breaking">תבנית:Cite journal</ref><ref name="K.Hata">תבנית:Cite web</ref>
Miralon carbon nanotube yarn C-series 1375 0.7–0.9 1100 112 <ref name="NanoComp">תבנית:Cite web</ref>
Colossal carbon tube 6900 .116 59483 6066 <ref name="CCT">תבנית:Cite journal</ref>
Graphene 130500 2.090 62453 6366 <ref name="nobelprize.org">תבנית:Cite web</ref>
Fundamental limit תבנית:Val תבנית:Val <ref name="Brown 2012">תבנית:Cite journal</ref>

The data of this table is from best cases, and has been established for giving a rough figure.

  • Note: Multiwalled carbon nanotubes have the highest tensile strength of any material yet measured, with labs producing them at a tensile strength of 63 GPa,<ref name="Strength and Breaking"/> still well below their theoretical limit of 300 GPa. The first nanotube ropes (20 mm long) whose tensile strength was published (in 2000) had a strength of 3.6 GPa, still well below their theoretical limit.<ref>

"Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes" by F. Li, H. M. Cheng, S. Bai, G. Su, and M. S. Dresselhaus. תבנית:Doi </ref> The density is different depending on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).<ref name="K.Hata"/>

The 'Yuri' and space tethers

The International Space Elevator Consortium has proposed the "Yuri" as a name for the SI units describing specific strength. Specific strength is of fundamental importance in the description of space elevator cable materials. One Yuri is conceived to be the SI unit for yield stress (or breaking stress) per unit of density of a material under tension. So, the units for one Yuri are Pa m3 / kg. This unit is equivalent to one N m / kg, which is the breaking/yielding force per linear density of the cable under tension.<ref>Strong Tether Challenge 2013</ref><ref>תבנית:Cite web</ref> A functional Earth space elevator would require a tether of 30-80 MegaYuri (corresponding to 3100–8200 km of breaking length).<ref>תבנית:Cite web</ref>

Fundamental limit on specific strength

The null energy condition places a fundamental limit on the specific strength of any material.<ref name="Brown 2012"/> The specific strength is bounded to be no greater than c2 ~ תבנית:ValkN·m/kg, where c is the speed of light. This limit is achieved by electric and magnetic field lines, QCD flux tubes, and the fundamental strings hypothesized by string theory.תבנית:Cn

Tenacity (textile strength)

תבנית:About

Tenacity is the customary measure of strength of a fiber or yarn. It is usually defined as the ultimate (breaking) force of the fiber (in gram-force units) divided by the denier. Because denier is a measure of the linear density, the tenacity works out to be not a measure of force per unit area, but rather a quasi-dimensionless measure analogous to specific strength.<ref>תבנית:Cite book</ref> A tenacity of [math]1[/math] corresponds to:תבנית:Cn [math]\frac{1 {\rm \, g} \cdot 9.80665 {\rm \, m s^{-2}}}{1 {\rm \, g}/9000 {\rm \, m}}=\frac{9.80665 {\rm \, m s^{-2}}}{1/9000 {\rm \, m}}=9.80665 {\rm \, m s^{-2}} \, 9000 {\rm \, m} = 88259.85 {\rm \, m^2 s^{-2}} [/math]

See also

References

תבנית:Reflist

External links