The research team, led by Mohammad Reza Khoshravan Azar with Amir Javadzadeh Khoei and Mehrdad Dadashzadeh, focused on Mode I fracture, the opening mode in which a crack’s faces separate perpendicular to the crack plane. This loading condition is directly relevant to pressurized aircraft fuselages, where hoop stresses drive cracks along skin panels. Aluminum 2024-T3, the workhorse fuselage alloy prized for its damage tolerance, served as the base sheet in every configuration and was riveted to four companion materials: the softer 2024-T0 temper, commercially pure 1100 aluminum, corrosion-resistant 5052-H3, and the high-strength but relatively brittle 7075-T6 aerospace alloy.

Specimen preparation was deliberately rigorous. Every joint was built from two sheets cut to identical dimensions of 100 by 37 millimeters using a CNC-controlled EV30 fiber laser cutting machine, which delivers smooth edges and minimal thermal distortion in thin sheet metal. A two-row riveted configuration was assembled with 2024-T3 rivets 3.17 millimeters in diameter, and the rivet holes were dimpled rather than deeply countersunk so that the rivet heads sat flush with the sheet surface, a practice that mirrors aerospace requirements for aerodynamic smoothness. After riveting, a V-shaped edge pre-crack, 5 millimeters long with a 0.8-millimeter tip height, was laser-machined at mid-width of the bonded region to promote symmetric propagation.

Mechanical testing followed ASTM E399 guidance on a Zwick/Roell universal testing machine with a 100-kilogram-force class capacity, run in displacement control at a crosshead speed of 1 millimeter per second at approximately 27 degrees Celsius. Each configuration was tested three times to verify repeatability, and two 12-megapixel cameras filming at 60 frames per second captured crack initiation and growth from both sides of the specimen. Frame-by-frame analysis converted the footage into precise crack-length histories, which were then correlated with force-displacement curves. From these data the team calculated the stress intensity factor, using the classical single-edge-cracked finite-width plate solution with its polynomial geometric correction factor, and the strain energy release rate under a plane-stress assumption.

The results exposed a striking hierarchy of behaviors. Monolithic reference specimens of 2024-T0 and 2024-T3 established baseline fracture responses: the ductile T0 temper held its 5-millimeter crack stationary for roughly 100 to 110 seconds before gradual stable extension, peaking near 2.4 kilonewtons with a smooth post-peak softening, whereas the stronger T3 sheet reached about 7 kilonewtons but exhibited a shorter stable growth window and a steeper load drop. The strain energy release rate of the T3 specimen climbed to roughly 23 to 24 newtons per millimeter, nearly three times the peak of the T0 material, confirming that higher strength came bundled with a higher crack-driving force.

The dissimilar riveted joints told a more nuanced story. The 2024-T0 plus 2024-T3 combination delayed measurable crack initiation for about 150 seconds and sustained a peak load near 10 kilonewtons, suggesting that pairing a ductile layer with a stronger one redistributed stress and postponed instability. Even more dramatic was the 1100 plus 2024-T3 joint, in which commercially pure aluminum held the crack at its initial 10-millimeter length for approximately 180 seconds and the specimen carried nearly 12 kilonewtons at peak. However, once growth began, the transition to fracture was abrupt, indicating that the soft layer delayed initiation but could not arrest propagation once the crack-driving force matured.

Two configurations bracketed the extremes of joint behavior. The 2024-T3 plus 5052-H3 specimen never developed a meaningful crack history at all: failure initiated in the riveted region itself, driven by local stress concentrations around the fasteners rather than by the engineered pre-crack, and rose smoothly to about 10 kilonewtons without a pronounced post-peak propagation stage. Because no measurable crack extension occurred along the intended path, the researchers could not extract reliable stress intensity or energy release values for this pairing, and they excluded it from the comparative fracture analysis, noting that fastener-dominated failure is a well-documented hazard in mechanically fastened lap joints.

At the opposite end, the 2024-T3 plus 7075-T6 joint delivered the most powerful and the most dangerous performance. It withstood loading for an extraordinary 220 to 230 seconds without any crack extension, then suddenly jumped the crack from 5 to roughly 35 millimeters in a single short interval. Its peak load of about 20 kilonewtons was the highest recorded, roughly double that of several other configurations, and its fracture mechanics parameters dwarfed the field: the stress intensity factor climbed to a maximum of 2123.6 megapascal root millimeters and the strain energy release rate peaked near 62 newtons per millimeter, before collapsing sharply to low levels. This combination of high stiffness, high load capacity, and near-total absence of stable crack growth is characteristic of 7000-series alloys, whose elevated strength is consistently accompanied by reduced damage tolerance under opening-mode loading.

The authors are careful to frame their quantitative results within the limits of classical linear elastic fracture mechanics. Because the specimens contain dissimilar materials, riveted connections, and localized plasticity that violate ideal homogeneous linear-elastic assumptions, the calculated stress intensity factors and energy release rates are presented as comparative engineering parameters rather than exact descriptions of the crack-tip field. A more rigorous treatment, they note, would require elastic-plastic finite element analysis, J-integral evaluation, cohesive zone modeling, or phase-field methods, and future work should also incorporate scanning electron microscopy fractography, fatigue and mixed-mode loading, and additional joint geometries.

Even with those caveats, the experimental dataset offers practical guidance for lightweight structure design. The work demonstrates that ductility in one sheet can buy valuable crack-initiation delay, that alloy pairing can be tuned to trade peak load against fracture warning time, and that certain combinations redirect failure into the riveted zone entirely, shifting the design problem from crack growth to fastener stress concentration. As aerospace and automotive industries increasingly mix alloy tempers and gauges within single assemblies to optimize weight and corrosion performance, this kind of side-by-side fracture data on real riveted joints provides a rare experimental foundation for assessing how the weakest link in a hybrid structure will actually behave when a crack arrives.

The alloy families represented in the study span much of the practical aluminum design space. The 2000-series aluminum-copper alloys, which include both tempers of 2024 used here, are prized for fracture resistance and damage tolerance, which explains their dominance in fuselage skin applications. The 7000-series zinc-based alloys, exemplified by 7075-T6, offer a superior strength-to-weight ratio but carry well-known penalties in brittleness and corrosion resistance, while the 5000-series magnesium-bearing alloys such as 5052 are typically reserved for environments where moisture and corrosive exposure dominate material selection. Commercially pure 1100 aluminum, though structurally weak, serves as a useful ductile benchmark, and its inclusion allowed the researchers to isolate how a very soft companion sheet alters crack-driving forces in the stronger base material.

The choice of riveting rather than welding as the joining method is itself scientifically significant. Fusion welding of aluminum alloys introduces residual stresses, heat-affected zones, and local softening that can confound fracture measurements, whereas mechanical fastening preserves the parent alloy properties in each sheet. This makes riveted dissimilar joints an attractive platform for studying material-mismatch effects in a relatively clean form, since the primary sources of inhomogeneity are the alloy interface and the fastener holes rather than a thermally altered microstructure. The trade-off, as the 5052-H3 configuration demonstrated, is that fastener holes become competing stress concentrators capable of diverting failure away from the engineered crack path entirely.

The experimental methodology also connects to a long lineage of fracture research on thin aluminum sheet. Classical studies dating back to the early 1970s established how specimen width, thickness, and crack-length ratio influence residual strength and stress intensity factors in pre-cracked plates under plane-stress conditions, and later work on 2024-T3 documented the transition between flat and slanted fracture modes in thin sheets. By applying the single-edge-cracked finite-width plate solution with its polynomial geometric correction factor, the present study anchors its comparative parameters in this established analytical framework, even while acknowledging that dissimilar materials and riveted constraint push the problem beyond strictly valid linear-elastic assumptions.

For engineering practice, the findings suggest that fracture assessment of hybrid riveted assemblies cannot rely on data from monolithic sheet tests alone. The same base alloy paired with different companions produced initiation delays ranging from roughly two and a half to nearly four minutes under the test conditions, and post-peak responses ranging from gradual softening to instantaneous crack jumps of tens of millimeters. This spread implies that damage-tolerance calculations for mixed-alloy fuselage panels, automotive body structures, or lightweight transit vehicles should incorporate joint-level fracture data, since the local elastic mismatch across a riveted interface appears capable of either cushioning or amplifying the crack-driving force depending on which side of the joint the crack approaches from.

Subject of Research: Experimental fracture mechanics study of crack growth in riveted dissimilar aluminum alloy joints under Mode I tensile loading

Article Title: Crack growth and fracture characteristics of riveted dissimilar aluminum joints under mode I loading

Article References: Azar, M. R. K., Khoei, A. J., & Dadashzadeh, M. (2026). Crack growth and fracture characteristics of riveted dissimilar aluminum joints under mode I loading. Journal of Materials Science: Metallurgy, 1(1), Article 20. https://doi.org/10.1007/s44492-026-00021-1

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00021-1

Keywords: riveted joints, dissimilar aluminum alloys, Mode I fracture, crack growth, stress intensity factor, strain energy release rate, aluminum 2024-T3, aluminum 7075-T6, fracture mechanics, aerospace structures, linear elastic fracture mechanics, rivet failure