CFRP bolted joint preload loss
Preload loss in composite and CFRP bolted joints
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
Composites break the assumptions metal joints are built on
Every rule of thumb in bolted joint design assumes the clamped member is elastic, isotropic and stable. A carbon fibre laminate is none of those things in the direction that matters.
Through-thickness, a CFRP laminate is essentially a polymer with fibres in it. The fibres run in plane. What carries bolt bearing load through the stack is the epoxy matrix, and epoxy is viscoelastic. It creeps under sustained stress, faster when warm, faster when wet, and it does not stop.
Reference joint used throughout
Every figure in this article refers to the same reference joint, so numbers are comparable across articles and against your own calculations.
| Parameter | Value |
|---|---|
| Bolt | M16 × 2,0, property class 10.9, to ISO 898-1 |
| Assembly preload F_V | 70,0 kN |
| Clamp length | 48 mm, steel on steel |
| Bolt stiffness k_S | 1,04 × 10⁹ N/m |
| Member stiffness k_P | 5,71 × 10⁹ N/m |
| Load factor Φ | 0,154 |
| Transverse test | DIN 25201-4:2010-03 Annex B, 2 000 cycles, ±0,45 mm slip |
Stiffnesses are calculated to VDI 2230 Sheet 1 using the standard cone-of-compression method.
The four losses in a composite stack
| Mechanism | Budget | Time constant |
|---|---|---|
| Through-thickness matrix creep | 25 to 80 µm | Months, continuous |
| Resin-rich surface layer flattening | 8 to 25 µm | Hours to days |
| Moisture absorption and redistribution | 10 to 30 µm | Weeks, reversible in part |
| Differential thermal, CFRP to steel bolt | 15 to 40 µm per 100 K | Per cycle |
The thermal term deserves attention because it surprises people. CFRP in-plane expansion is near zero, 0 to 2 × 10⁻⁶ /K, which is why composites are chosen for dimensional stability. Through-thickness expansion is 25 to 35 × 10⁻⁶ /K, higher than aluminium. A CFRP stack against a steel bolt ratchets harder than an aluminium one does.
What it costs
At 0,884 kN per micrometre for the reference joint stiffness, and taking a mid-range composite budget of 55 micrometres:
| Elapsed | Cumulative loss | Residual clamp force |
|---|---|---|
| Day 1 | 14 µm | 82,3 % |
| Day 30 | 31 µm | 60,8 % |
| Day 90 | 43 µm | 45,7 % |
| Day 180 | 51 µm | 35,6 % |
| Day 365 | 58 µm | 26,7 % |
A composite joint at 26,7 percent of design clamp force is not a joint. It is a pin in a hole with a nut nearby, and the load path has quietly changed from friction to bearing.
That transition is the real hazard. A friction-grip composite joint that becomes a bearing-mode joint concentrates load onto the hole edge, and hole-edge bearing failure in CFRP is sudden, not progressive.
Why the usual composite fixes are incomplete
Metal bushings and sleeves. Load-spreading bushings reduce bearing stress and therefore creep rate, and they protect the hole. Standard, correct practice. They do not stop through-thickness creep of the laminate under the bushing flange.
Large-diameter washers. Same argument. Reducing contact stress moves the matrix down its creep curve, which is a real gain, and creep continues.
Belleville stacks. Widely used in composite joint design and genuinely helpful. The spring extends as the laminate creeps, so the same thickness loss costs less force. Force still falls along the spring curve, and the 0,15 to 0,40 mm of travel is a smaller reserve than a composite budget deserves.
Torque to a lower preload. Reduces contact stress and creep rate, at the cost of the very clamp force the joint needs for friction grip. This is a genuine trade, not a fix.
Retorque after run-in. Effective once. Composite creep is continuous, so the interval never ends.
Holding clamp force in a composite stack
The requirement is a reserve of clamp length large enough for a continuous loss mechanism, and a force delivery that does not fall as the reserve is spent.
| Belleville stack | CFR take-up | |
|---|---|---|
| Travel available | 0,15 to 0,40 mm | 0,50 mm |
| Force at end of travel | Substantially reduced | Unchanged |
| Handles continuous creep | Partially | Yes, until reserve exhausted |
| Reserve readable in service | No | Yes |
| Arrests rotation | No | Yes |
Predicted performance, CFRP stack, 55 µm creep budget, ambient, one year:
| Elapsed | ISOKLAMP CFR | Belleville stack | Plain washer + bushing |
|---|---|---|---|
| Day 1 | 98,6 % | 94,1 % | 82,3 % |
| Day 30 | 98,1 % | 85,2 % | 60,8 % |
| Day 90 | 97,7 % | 78,4 % | 45,7 % |
| Day 180 | 97,4 % | 73,1 % | 35,6 % |
| Day 365 | 97,0 % | 67,9 % | 26,7 % |
| Reserve used | 11,6 % | n/a | n/a |
ISOKLAMP figures are predicted by finite-element analysis of the ISK-16 geometry coupled to VDI 2230 Sheet 1 mechanics with a viscoelastic through-thickness matrix model. Baselines are calibrated against published composite relaxation data, sourced on Full residual clamp force dataset.
At 11,6 percent reserve consumed in the first year, the projected service life against creep alone is well beyond the laminate's own design life, and the remaining reserve is readable without disassembling a joint you would rather not disturb. See Preload reserve indicator.
Surface damage is not optional here
A serrated or wedge-lock washer works by biting into the bearing surface. On a CFRP laminate that means severing surface fibres and initiating delamination at exactly the highest-stress location in the joint. It is not a marginal concern, it is a disqualification.
Any securing device used on composite must bear on a flat, continuous, non-penetrating face, ideally through a metallic load-spreading washer. See No surface damage.
Design sequence for a CFRP joint
- Establish the through-thickness creep budget from the resin system's published compliance data at service temperature and moisture content, not at ambient dry.
- Add the resin-rich layer, moisture and thermal terms.
- Convert to clamp force loss using the joint's load factor, remembering that a composite member has much lower through-thickness stiffness than steel, so Φ is higher and the joint is more sensitive per micrometre than the reference figures suggest.
- Check the residual clamp force against the friction grip the design requires.
- If step 4 fails, the choices are lower creep, more reserve, or accepting bearing-mode load transfer as a deliberate design decision rather than an accident.
More on the underlying mechanism at Non-rotational preload loss, and on method selection at Locking methods compared.
Frequently asked questions
Why do CFRP bolted joints lose preload?
Through-thickness load in a carbon fibre laminate is carried by the polymer matrix rather than the fibres, and epoxy is viscoelastic. It creeps continuously under sustained bolt load, removing 25 to 80 micrometres of clamp length over the first months. Resin-rich surface layers flatten, moisture redistributes, and CFRP through-thickness expansion of 25 to 35 × 10⁻⁶ per kelvin drives thermal ratcheting against a steel bolt.
How fast does a composite joint lose clamp force?
For a mid-range 55 micrometre creep budget, analysis predicts 82,3 percent residual at day one, 60,8 percent at 30 days, 45,7 percent at 90 days and 26,7 percent at one year. The critical point is the transition from friction grip to bearing mode, because hole-edge bearing failure in CFRP is sudden rather than progressive.
Can you use wedge-locking washers on composite joints?
No. Wedge-lock and serrated washers secure the joint by biting into the bearing surface. On a CFRP laminate that severs surface fibres and initiates delamination at the highest-stress location in the joint. Any securing device on composite must bear on a flat, continuous, non-penetrating face, ideally through a metallic load-spreading washer.
Do Belleville washers work in composite joints?
They help and are widely used, but the reserve is small relative to the problem. A disc spring gives 0,15 to 0,40 mm of travel and its force falls along the load-deflection curve as it extends, so clamp force declines continuously against a creep mechanism that also runs continuously. Analysis predicts 67,9 percent residual at one year for a Belleville stack against 97,0 percent for a one-way take-up.
Why is CFRP more thermally sensitive than aluminium in a bolted joint?
CFRP in-plane expansion is near zero at 0 to 2 × 10⁻⁶ per kelvin, which is why composites are chosen for dimensional stability. Through-thickness expansion is 25 to 35 × 10⁻⁶ per kelvin, higher than aluminium at 22 to 24. Because bolt clamp load acts through the thickness, a CFRP stack against a steel bolt ratchets harder than an aluminium one.
Take it further
- The ISOKLAMP technical report covers the full derivation, the geometry, and every dataset behind these figures.
- Full residual clamp force dataset gives the residual clamp force numbers for ten securing methods, with sources.
- Design partner programme is open. Eight slots, two per sector. Bring us a joint that keeps failing and we will run a VDI 2230 Sheet 1 analysis on it.
Engineering questions go to engineering@isoklamp.com. An engineer answers, not a form.