Calculation
lü = F / (b · τzul)
This pre-sizing step sets only the nominal mean stress; beyond a certain length, additional overlap adds little real capacity because of the uneven stress distribution.
lü = F/(b·τzul) (Vorbemessung)
Required overlap length for an adhesive joint
This pre-sizing step sets only the nominal mean stress; beyond a certain length, additional overlap adds little real capacity because of the uneven stress distribution.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Required overlap length lü — Approximate minimum overlap length for the chosen allowable stress.
- Transmitted force F — Axial force to be transmitted through the bond line.
- Bond width b — Width of the bond line, transverse to the load direction.
- Allowable nominal shear stress τzul — Bond strength divided by safety factor, τzul = τKB/S; guideline values from the adhesive datasheet.
Example
5,000 N at a 25 mm bond width and an allowable stress of 10 MPa give a required overlap length of lü = 5,000 N / (25 mm · 10 MPa) = 20 mm.
Assumptions and limits
Pure pre-sizing based on nominal mean stress; actual edge stress concentration, peel stresses and adherend stiffness are excluded and require separate verification.
Understand Required overlap length for an adhesive joint
This calculator finds the approximately required overlap length of an adhesive joint from the transmitted force, bond width and an allowable nominal shear stress.
What does this quantity describe?
Rearranging the nominal shear stress of a lap joint, τ = F/(b·lu), gives the overlap length required for an allowable stress τallow: lu = F/(b·τallow). The allowable stress usually comes from the adhesive's bond strength divided by a safety factor.
This is like sizing a beam's cross-section for an allowable bending stress: given the load and the allowable stress, the stress formula is solved for the needed geometric quantity, rather than checking stress for a given geometry.
Formula and variables
lü = F / (b · τzul)
lu = F/(b·τallow)F = lu·b·τallowτallow = F/(lu·b)
| Symbol / input | Meaning |
|---|---|
| Required overlap length lü | Approximate minimum overlap length for the chosen allowable stress. |
| Transmitted force F | Axial force to be transmitted through the bond line. |
| Bond width b | Width of the bond line, transverse to the load direction. |
| Allowable nominal shear stress τzul | Bond strength divided by safety factor, τzul = τKB/S; guideline values from the adhesive datasheet. |
Choose the inputs correctly
Transmitted force F, bond width b and allowable nominal shear stress τallow set the required overlap length. τallow should come from the specific adhesive's bond strength divided by an appropriate safety factor, not a blanket assumption.
How to use the calculator
Enter the force to be transmitted, the available or desired bond width, and the allowable stress to get the required overlap length. Then check the result against the literature rule of thumb lu ≈ (10-20)·t (t = smallest component thickness) as a plausibility check.
Worked example
5,000 N at a 25 mm bond width and an allowable stress of 10 MPa give a required overlap length of lu = 5,000 N / (25 mm · 10 MPa) = 20 mm.
Understand the result and units
A required length of 20 mm at 25 mm width gives a length-to-width ratio of 0.8 — a plausible value for a compact, well-utilized lap joint. Overlaps markedly longer than the lu ≈ (10-20)·t rule of thumb suggests add little further capacity because of the uneven stress distribution.
Force is given in N, width and overlap length in mm, and allowable stress in MPa (N/mm²).
Cross-checking with the thickness rule of thumb
Besides the force-based calculation, the literature cites an independent rule of thumb for good utilization of bond strength in light metals: lu ≈ (10 to 20)·t, where t is the smallest thickness of the overlapped parts. If the force-based length deviates strongly from this range — for example because allowable stress is set very low or force very high — a second look is worthwhile: the chosen geometry may be unfavorable for a plain lap joint, and a different joint type or additional design measures may be preferable.
Typical applications
The calculation is used in pre-design to set an initial overlap length for a known load and available adhesive system, before the final joint is checked in detail.
Assumptions, limits and common mistakes
This is a pure pre-sizing step based on nominal mean stress. It captures neither the real stress concentration at overlap ends nor peel stresses, adherend stiffness, temperature or aging effects. For very large computed overlap lengths, check whether a stiffer joint type or multiple bond points would be more sensible, since real capacity does not grow proportionally with length.
Common mistake: A common mistake is using τallow as the manufacturer's bare bond strength τKB without a safety margin — safety factors of 1.5 to 2.5 against τKB are typical. It is also easy to adopt the computed length uncritically, even though beyond a certain length additional area barely contributes to capacity.
Frequently asked questions
Where do I get the allowable stress τallow?
From the adhesive's bond strength in its datasheet, divided by a safety factor appropriate to the application, typically around 1.5 to 2.5.
What if the computed length becomes very long?
Check whether a wider bond, a stronger adhesive, a stiffer joint type or multiple bond points would be more economical, since a very long overlap does not fully use its area because of the uneven stress distribution.
How does the result compare with the lu ≈ (10-20)·t rule of thumb?
Both approaches should land in the same order of magnitude; a large mismatch signals an unfavorable combination of load, area and part thickness for the chosen joint type.
Can I vary width instead of length?
Yes, since AK = b·lu is symmetric in b and lu, the other dimension can be enlarged to reach the same area when space is limited in one direction.
Is a longer overlap always safer?
Not proportionally: beyond a certain length, additional area in the middle barely contributes to load transfer, so real safety grows more slowly than the nominal calculation suggests.