The strength of the bolt isn't that important in an impact. It is the ability of the bolt to absorb energy and to stretch that will make the difference. If you look at Hans' seat belts you can see that the straps are deliberately long as that allows for more stretch, which absorbs more energy. The standard inertia seatbelts use the same technique, which is why the strap runs freely through the top mounting and down to the inertia mechanism housing on the floor. Short lengths of seat belt, such as fixed belts attaching to a roll hoop directly behind the drivers seat are not a good idea.
Going back to the bolts, if you look up some mechanical property tables for bolts you will see that as well as Tensile strength (strength at failure) and Yield strength (strength at elastic limit) there are usually figures given for Elongation after failure and impact energy. These are the figures that are of interest when selecting bolts for mounting impact critical components such as seats, seatbelts, roll-hoop, subframes and suspension. As Alan posted, you want bolts that will stretch without breaking. You also want bolts that will absorb energy as the kinetic energy that the seat belts and seat anchor bolts absorb will be energy that would otherwise go into you!
So comparing the 8.8 HTS bolts that I suggested to the much stronger 12.9 bolts:
8.8 bolts: Tensile strength 800MPa, elongation after failure 12%, impact energy 30Joules.
12.9 bolts: Tensile strength 1200MPa, elongation after failure 8%, impact energy 15Joules.
These figures can be found on several websites for example:
http://www.fullermetric.com/technical/i ... rties.aspxIn other words, the 8.8 bolt will absorb twice the energy and stretch by 50% more than a 12.9 bolt of the same size.
The Titanium bolts that Jin mentioned are stronger still. The most common grade of Titanium, i.e. that used for fasteners in the aircraft industry, is Grade 5, aka Ti64. This has a notched tensile strength (across the threaded part) of 1450MPa and an ultimate bearing strength (across the shank) of 1860MPa, the elongation after failure is 14% but the impact energy is just 17Joules (source
http://www.aerospacemetals.com/titanium ... -4911.html ) So they are good from the stretching point of view, but not so good from the energy absorption point of view.
The comparable figures for Stainless Steel fasteners are harder to find. The commonly found A2-70 grade (sold in Screwfix, Toolstation etc) has a Tensile strength of around 700MPa and the less common A4-80 grade has a tensile strength of around 800MPa, but the elongation and impact figures are harder to find. In fact the international standard for stainless fasteners, ISO 3506 doesn’t refer to impact energies and gives elongation not as a percentage, but as a factor of diameter 0.4d for A2-70 and 0.3d for A4-80.
“Stretch like toffee, snap like carrots” was how a senior engineer described stainless fasteners to me once. However, when strength and impact resistance are not issues, but corrosion resistance is then stainless fasteners are the best solution. Use a stainless nyloc nut with a little copper-slip on the thread and you should have no problems unscrewing the nut ten, or twenty, years later.
Alistair