First load takes ~15 seconds; cached on subsequent loads.
Material Database
Interaction Law
When to use each curve
Linear (n=1): conservative baseline. No test data, or first-pass sizing with low bypass ratios.
Quadratic (n=2): typical for CFRP — supported by most coupon test databases. Best default for fastened CFRP lap-joints with balanced laminates.
Cubic (n=3): less conservative near the knee. Use only with substantiated coupon data for the specific material and lay-up.
Safety Factor
Applied Loads
Bearing Peaking Factor
FEA Forces - Tension
FEA Forces - Compression
Allowables (B-Basis, ksi) - per environment
Show curves:
Results - all environments
How to read the chart.
The plot shows the bearing-bypass interaction envelope for each environment: above each curve is failing, below is passing.
The X-axis is bypass stress with compression on the left and tension on the right; the Y-axis is bearing stress.
Tension allowable is OHT (Open Hole Tension); compression allowable is BIID (Barely Visible Impact Damage compression - the damaged-state allowable).
The black circle marks the applied tension state at (σbp(T), σbr(T)) × FS; the black diamond marks the applied compression state at (-σbp(C), σbr(C)) × FS.
Markers are plotted at the DESIGN stress (limit × FS), so the visual gap from the marker to the envelope is the actual margin.
The two markers can sit at different y values when the worst-bolt bearing load differs between the tension and compression load cases.
Margin lines. Dotted lines drawn from the origin through each applied point out to the envelope of the selected environment. The line length from origin to applied point is the design stress; the segment past the applied point (when MS > 0) shows the available margin. The MS value is annotated at the envelope endpoint. Change the "Margin line on chart" selector in the sidebar to switch which env's envelope is used.
Compression bypass allowable. The BIID (BVID damaged compression) value is the most conservative and is the default. OHC and FHC are also available in the selector. The active selection is highlighted in the allowables table column.
MS ≥ 0 passes; MS < 0 fails (red); 0 ≤ MS < 0.15 is amber (low margin).
Calculation sheetMaterial databaseTheory and limitations
Interaction equation: peak failure of a bolted composite joint under combined bearing and net-section bypass loading is approximated by the power-law envelope
(σbr / Fbr)n + (σbp / Fbp)n = 1
with the exponent n selected per program convention. For each environment the bypass allowable Fbp = OHT in tension and Fbp = BIID in compression (BIID is the damaged-state compressive allowable - much more conservative than UNC or OHC).
Reserve factor: RF = (1/I)1/n where I is the interaction index. Margin of safety MS = RF - 1.
Safety factor (FS): applied stresses are multiplied by FS before the interaction check, i.e. e_br = (σbr × FS) / Fbr and e_bp = (σbp × FS) / Fbp. Equivalently, MS = RFunfactored / FS - 1. Default FS = 1.25 covers a typical ultimate-strength margin requirement against B-Basis allowables; adjust per your program structural criteria.
Bypass area for tubes: Abypass = πDt - the bypass load is shared around the entire annular cross-section.
For flat laminates: Abypass = w·t (net section width times thickness).
Bearing area: Abearing = d·t (single bolt diameter times laminate thickness). A peaking factor Kp accounts for non-uniform contact stress from bolt-hole clearance.
Limitations: the envelope is an empirical curve fit to coupon test data, not a first-principles strength model. For highly off-axis lay-ups or thick laminates, finite-element analysis with progressive damage may be required. BIID values assume standard impact energy per CMH-17; programs with custom impact threats should re-test. Single-shear vs double-shear bearing allowables differ - select the column matching your joint configuration.