The objective of this paper was to investigate the fracture behavior of compact and middle-crack tension specimens made of 6.35 mm thick 2024-T351 aluminum alloy. Tests were conducted on compact tension, C(T), specimens ranging in width from 51 to 152 mm and on middle-crack tension, M(T), specimens ranging in width from 76 to 1016 mm. Most of the M(T) specimens were allowed to buckle during the fracture tests. Three-dimensional, elastic-plastic, finite-element analyses (FEA) using the WARP3D code with straight-front cracks were used to show transferability of the critical crack-tip-opening angle (CTOA) fracture criterion from small laboratory specimens, such as the C(T) specimen to large M(T) specimens. A single value of critical CTOA (Ψc = 6.35°) was determined from the FEA (with a straight-crack front) by matching the average failure load for the 152 mm C(T) specimens. CTOA analyses were then conducted on the various width C(T) and M(T) specimens. The WARP3D analyses with the critical CTOA predicted the experimental failure loads within ±3% for all specimens. The δ5-resistance curves from the analyses of the C(T) and M(T) specimens were nearly identical up to the maximum load. Beyond maximum load, the δ5-resistance curves for the C(T) specimens began to deviate rapidly; whereas, the δ5-resistance curves for the M(T) specimens only deviated slightly.

Processing and properties of composite-to-composite bonding using Scotch-Weld TM AF-555M structural adhesive were investigated. Bonding surfaces of T800H/3900-2 composite were prepared by co-curing the dry and wet peel-plies. Surface topologies of the peel-plies and the co-cured composite surfaces were examined by microscopy, contour mapping using a coordinate measuring machine equipped with a ruby sphere probe, and contact angle goniometry. Curing of the adhesive was conducted in an autoclave or vacuum press at 177°C (350°F) for 2 h under 310 KPa (45 psi). Common bagging practices for composite fabrication in an autoclave were followed. It was found that a prolonged vacuum application (i.e., overnight) prior to the application of temperature and pressure was a critical element to produce porosity-free, high-quality bonds with this adhesive system. Following this procedure, a strong bond line was consistently produced, which routinely provided a single-lap shear strength more than 10% higher than the nominal value of the adhesive (i.e., 35.9 MPa or 5200 psi) when tested at room temperature. An adhesive failure mode at the interface was noted on the fractured surfaces of specimens with strong bonds whereas a premature cohesive failure mode was more evident for the specimens with weaker bonds, probably due to porosities in the bond lines. Photomicrographs showed that the weak single-lap shear strengths occurred on specimens with significant porosity in the bond line, apparently caused by entrapped air from insufficient vacuum application prior to curing. The results of this study are discussed herein.

Fatigue crack growth rate testing is performed by automated data collection systems that assume straight crack growth in the plane of symmetry and use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which for cracks that propagate out-of-plane, under-estimates the crack growth rates and over-estimates the stress-intensity factors. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on twodimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys.Results indicate the correction process works well for high ∆K levels but fails to capture the mixed-mode effects at ∆K levels approaching threshold (da/dN ~ 10 -10 meter/cycle).

Fatigue crack growth rate testing is performed by automated data collection systems that assume straight crack growth in the plane of symmetry and use standard polynomial solutions to compute crack length and stress-intensity factors from compliance or potential drop measurements. Visual measurements used to correct the collected data typically include only the horizontal crack length, which for cracks that propagate out-of-plane, under-estimates the crack growth rates and over-estimates the stress-intensity factors. The authors have devised an approach for correcting both the crack growth rates and stress-intensity factors based on twodimensional mixed mode-I/II finite element analysis (FEA). The approach is used to correct out-of-plane data for 7050-T7451 and 2025-T6 aluminum alloys.Results indicate the correction process works well for high ∆K levels but fails to capture the mixed-mode effects at ∆K levels approaching threshold (da/dN ~ 10 -10 meter/cycle).