Mechanical Properties

Threaded fasteners are the preferred choice for joining two or more components together as compared to welding or adhesives. ln addition to providing a firm connection, fasteners allow quick and easy dismantling if required, particularly if we need to replace components or carry out maintenance operations.

It is important to select the right fastener material based on the tact that a fastener is a mechanical clamp which utilizes the elastic properties of a metal. Under applied loads, a fastener will stretch within limits and return to its original length when the load is removed, however, if the load keeps on increasing, the fastener material will stretch beyond its elastic limit and get permanently deformed. Applying further load will result in the fastener ultimately fracturing and break off.

 

strength-characteristics_clip_image002

Tensile Strength: Over 95% of fasteners are manufactured from either carbon steel, alloy steel or stainless steel and the mechanical property most associated with fasteners is Tensile Strength. It is an expression of the maximum load (load in tension) that a fastener can support prior to, or coincident with its fracture. Tensile strength in the inch system is expressed in pound force per square inch (lbf/in2 or psi) and in the metric system in Newtons per square millimeter (N/mm2) or Megapascals (MPa). 1 N/mm2 = 1 MPa.

 

ScreenShot001

 

Tensile Load: is the maximum load (load in tension) that can be applied to the particular fastener prior to or coinciding with its fracture. For Inch fasteners tensile load is denoted in pound force (lbf) whilst Metric fasteners, it is expressed in kilonewtons(kN).

 

 

Tensile Load is calculated using the formula:

P = St x As

where  P = tensile load (lbf or kN)

St = tensile strength of the material (psi or MPa)

As = tensile stress area (in2 or mm2)

Page 37 rajouter les dessins


Tensile stress area- As (INCH threads) is computed using the formula:

As= o.7854 (o- o.9~43r

where As = Tensile Stress Area

D = nominal thread diameter

n = threads per inch


Tensile stress area – As (METRIC threads) is computed using the formula:

As= 0 7854(D-0.9382P)2

where As = Tensile Stress Area

D = nominal thread diameter

P = thread pitch


 To calcutate Tensile load:

Example 1 (INCH): To calculate the tensile load of a 1/2 -13 UNC Grade 5 Bolt

(Grade 5 bolts have a tensile strength (St) of 120,000 psi and a tensile stress area(As)of 0.1419in2).

If P = StxAs – therefore P = 120000×0.1419

= 170281bf tensile load.

Example 2 (METRIC): To calculate the tensile load of a M12x 1. 75 Glass 8.8 Bolt

(Class 8.8 bolts have a tensile strength (St) of 600MPa and a tensile stress area (As) of 64.30 mm2).

If P = St x As – therefore P = 800 x 64.30 = 67 440 Newtons

= 67.44 kN tensile load.


Ultimate Tensile Stress: is the theoretical minimum point at which a fastener will fracture.

ScreenShot003

 

Elongation : it is a linear stretching of a fastener caused by a tensile load due either to tightening or to an external load.

 

Proof load: is the load (load intension) that the fastener must support without evidence of permanent deformation. For testing purposes, proof load is an absolute value, and for application design, it is taken as a maximum.

Pre-load: is a measure of the axial load imparted on a fastener. lt’s importance in the fastener industry cannot be underestimated, because almost all critical fastener joints are assigned a pre-load value in order to achieve the correct tension in the fastener. For inch fasteners pre-load is expressed in pounds per square inch (psi) or Newton per square millimeter (N/mm2) in metric terms and is commonly accepted to be 75% of proof load.

ScreenShot002

Shear Load: is the load that the fastener can support prior to fracture, when applied at right angles to the fastener axis. A load occurring in one transverse plane is know as single shear which attempts to cut the fastener in two pieces. Double shear is the load applied in two planes which attempts to cut the fastener into three pieces. Shear load will vary depending upon whether the shear plane is a cross the threaded or unthreaded portion of the fastener.

 

ScreenShot002

Torsional Strength: is the torque a fastener can withstand prior to its failure by being twisted off about its axis. Torsional  strength test are usually carried out on self-tapping screws,socket head screws and other fasteners which are tightened (torqued) by the head.

 

Yield Strength: is the load (load in tension) per unit are a that can be applied to a fastener (specimen) at which point the fastener specimen experiences a specific amount of permanent deformation (0.2%) and moves beyond its elastic limit and into the plastic zone. ln the inch system, yield strength is expressed in lbf/in2 and MPa in the metric system.

Yield Point: is the point at which a fastener in tension moves from the elastic range ta the plastic range.

ScreenShot001

Fatigue Strength: A fastener is subjected to a variety of mechanical and thermal stresses when torqued in a fastened joint. During this process, it will constantly stretch and return to its original shape. These stretch-and-return actions are called cycles and can happen many thousands or even millions of times a day. Depending upon the intensity of the stresses, the fastener will develop cracks at its most vulnerable point (maximum stress concentration point) and it will ultimately fracture from « fastener fatigue ». Fatigue strength is therefore the maximum stress a fastener can withstand for a specific number of cycles, prior to its failure.

ScreenShot001

 

Creep Strength: At elevated temperatures, fasteners, under stress gradually elongate and this stretch increases overtime. Changing to heat resistant materials will often eliminate this problem.

Hardness: is a relative term used to compare the ability of one material to scratch another, without itself being scratched by that material. Hardness tests are used on fasteners because the test is quick, easy and nondestructive and because there is a close corelation between hardness and tensile strength particularly in fasteners manufactured from carbon steels.

 

Most fastener class standards specify a maximum hardness range. Exceeding the maximum specified hardness couId result in brittleness and hence place the fastener at risk of failure.

Vickers Hardness: (HV) is a test performed on a material to ascertain its resistance to penetration by the test body. The advantage of the HV test is that the entire hard ness range encountered in the fastener range is covered by this method.  lt is expressed as HV followed by 3 numbers eg. HV130.

Rockwell Hardness: (HR) testing is somewhat similar to the Vickers Hardness method, in so much as, it too uses the penetration test, however, the results are displayed in scales followed by 2 numbers eg, HRB95, HRC32 etc.

Brinell Hardness: (HB) is expressed by HB followed by 3 numbers eg. HB 124

 

 

Hardness Comparison Table

The hardess Comparasion Table is intented as a guide only. It is applicable to fasteners that are manufatures from Carbon Steel, Cast Steel & Low Alloy Steel.

There may be considerable differences for High Alloy Steel such as A1,A2,A3 & A4 and col formed steels used in grades 4.8 ,5.8 &8.8

The vicker testing machine method is extensively used in fastener because it is applicable over a wide hardness range and is the preferred mehod according to ISO 898-1. Rockwell C is used for hard steels, Rockwell B for soft steel.

*Vicker
 »
HV
Brinell  HB Rockwell Tensile Strenght
HRB HRC HRA Mpa Tonf/in2 Lbf/in2
85 80.7 41 270 19.6 39160
90 85.5 48 285 20.7 41336
95 90.2 52 305 22.1 44236
100 95 56.2 320 23.2 46412
105 99.8 335 24.3 48588
110 105 62.3 350 25.4 500763
115 109 370 26.8 53664
120 114 66.7 385 27.9 55840
125 119 400 29.0 58015
130 124 71.2 415 30.1 60191
135 128 430 31.2 62366
140 133 75 450 32.6 65267
145 138 465 33.7 67443
150 143 78.7 480 34.8 69618
155 147 495 35.9 71794
160 152 81.7 510 37.0 73969
165 156 530 38.4 76870
170 162 85 545 39.5 79046
175 166 560 40.6 81221
180 171 87.1 575 41.7 83397
185 176 595 43.1 86297
190 181 89.5 610 44.2 88473
195 185 625 45.3 90649
200 190 91.5 640 46.4 92824
205 195 92.5 660 47.9 95725
210 199 93.5 675 49.0 97900
215 204 94 690 50.0 100076
220 209 95 705 51.1 102252
225 214 96 720 52.2 104427
230 219 96.7 740 53.7 107328
235 223 755 54.8 109503
240 228 98.1 20.3 607.7 770 55.8 111679
245 233 21.3 61.2 785 56.9 113855
250 238 99.5 22.2 61.6 800 58.0 116030
255 242 23.1 62 820 59.5 118931
260 247 101 24 62.4 835 60.6 121106
265 252 24.8 62.7 850 61.6 123282
270 257 102 25.6 63.1 865 62.7 125458
275 261 26.4 63.5 880 63.8 127633
280 266 104 27.1 63.8 900 65.3 130534
285 271 27.8 64.2 915 66.4 132709
290 276 105 28.5 64.5 930 67.4 134885
295 280 29.2 64.8 950 68.9 137786
300 285 29.8 65.2 965 70.0 139961
310 295 31 65.8 995 72.2 144313
320 304 32.2 66.4 130 74.7 149389
330 314 33.3 67 1060 76.9 153740
340 323 34.3 67.6 1095 79.4 158816
350 333 35.5 68.1 1125 81.6 163167
360 342 36.6 68.7 1155 83.8 167519
370 352 37.7 69.2 1190 86.3 172595
380 361 38.8 69.8 1220 88.5 176946
390 371 39.8 70.3 1255 91.00 182022
400 380 40.8 70.8 1290 93.5 187099
410 390 41.8 71.4 1320 95.7 191450
420 399 42.7 71.8 1350 97.9 195801
430 409 43.6 72.3 1385 100.4 200877
440 418 44.5 72.8 1420 103.0 205954
450 428 45.3 73.3 1455 105.5 211030
460 437 46.1 73.6 1485 107.7 215381
470 447 46.9 74.1 1520 110.2 220457
480 465 47.7 74.5 1555 112.8 225534
490 466 48.4 74.9 1595 115.7 231335
500 475 49.1 75.3 1630 118.2 236411
510 485 49.8 75.7 1665 120.7 241488
520 494 50.5 76.1 1700 123.3 246564
530 504 51.1 76.4 1740 126.2 252366
540 513 51.7 76.7 1775 128.7 257442
550 523 52.3 77 1810 131.3 262518
560 532 53 77.4 1845 133.8 267595
570 542 53.6 77.8 1880 136.3 272671
580 551 54.1 78 1920 139.2 278472
590 561 54.7 78.4 1955 141.8 283549
600 570 55.2 78.6 1995 144.7 289350
610 580 55.7 78.9 2030 147.2 294427
620 589 56.3 79.2 2070 150.1 300228
630 599 56.8 79.5 2105 152.7 305304
640 608 57.3 79.8 2145 155.6 311106
650 618 57.8 80 2180 158.1 316182
660 58.3 80.3
670 58.8 80.6
680 59.2 80.8
690 59.7 81.1
*F≥ 98N
Téléphone: 450-420-1839