Hubert Zalewski

2025-02

Established hygrothermal knockdown factors for CFRP allowables — temperature and moisture uptake combined effect on in-plane shear strength, transverse tensile strength, and compressive strength from coupon test data.

Challenge: Moisture uptake testing takes 6+ months for fully saturated specimens — test data was only available for intermediate moisture levels. Had to use Fick's second law model to project equilibrium moisture content and apply linear knockdown interpolation.

Learned: Knockdown factor development requires a test plan designed for the full environmental envelope, not just ambient conditions. Retrospective moisture soak corrections from intermediate data introduce significant uncertainty — test to the actual use condition if possible.

2024-10

Thermal residual stress analysis after composite cure — CLT and ANSYS ACP analysis of stresses locked in during cool-down from cure temperature (180°C) to ambient. Assessed impact on first-ply failure margin.

Challenge: Thermal residual stress in CFRP can be 15-20% of the transverse tensile strength — not negligible. Combining them with mechanical loading requires superposition, but failure criterion application must use the combined stress state, not separate margins.

Learned: Thermal residual stresses in composites reduce the mechanical load margin non-uniformly depending on the loading direction. Always include cure temperature thermal analysis before assessing first-ply failure for cured CFRP structures.

2024-05

Open hole tension and compression analysis of quasi-isotropic laminate — stress concentration factor Kt from ANSYS ACP, Whitney-Nuismer point stress criterion for notched strength prediction.

Challenge: Whitney-Nuismer characteristic length (d0) for the laminate was not available in design data — required calibration from open hole tension test data. The d0 value varied significantly with notch diameter, limiting the criterion's applicability.

Learned: The Whitney-Nuismer criterion is useful for design screening but requires notch-size-dependent calibration for accurate life prediction. For certification purposes, test data at the actual hole diameter is required regardless.

2023-12

Interlaminar shear stress analysis for CFRP laminate — comparison between ILSS values from short-beam shear test (SBS) and predicted interlaminar shear stress from ANSYS ACP. Studied effect of stacking sequence on ILSS.

Challenge: ACP shell model cannot capture through-thickness shear stress accurately near ply drop-offs — 3D solid elements required in these regions. Hybrid solid-shell model was necessary for accurate ILSS prediction at ply drop locations.

Learned: Shell elements are insufficient for interlaminar stress assessment, especially near ply drops, free edges, and joints. Transition to 3D solid elements in critical regions is necessary for valid ILSS prediction.

2023-08

Failure criterion comparison study — Tsai-Wu, Tsai-Hill, Puck, and Hashin criteria applied to UD-CFRP laminates under combined biaxial and shear loading. Compared against test data from 45-degree off-axis specimens.

Challenge: Puck criterion requires material parameters (inclination parameters p12+, p22+, p12-, p22-) that are rarely tabulated in material certificates. Had to use values from Puck's original publications for IM7/8552, then verify sensitivity.

Learned: Puck criterion is more physically informative than Tsai-Wu (distinguishes fibre failure from inter-fibre failure) but requires additional material parameters. For design-allowable work, Tsai-Wu with conservative strength values is often preferred for its simplicity and testability.

2023-03

Developed Excel-based CLT implementation for rapid laminate analysis — A, B, D matrix calculation, in-plane and bending stiffness, thermal coefficients of expansion, and first-ply failure using Tsai-Wu criterion.

Challenge: Excel CLT tool validation against ANSYS ACP results revealed a systematic error in my thermal expansion CTE calculation — B-D coupling was incorrectly handled for non-symmetric laminates. Fixed and re-validated against 12 reference laminates.

Learned: CLT implementation for non-symmetric laminates is more complex than symmetric ones. The B-matrix (bending-extension coupling) must be included for accurate thermal response. Many simplified tools ignore this and give wrong results for asymmetric layups.