W. Steven Johnson

Three-Dimensional Finite Element Analysis of the Cold Expansion of Fastener Holes in Two Aluminum Alloys

A three-dimensional finite element analysis is developed for the cold expansion process in two aluminum alloys, 2024-T351 and 7050-T7451. The entire cold working process including hole expansion, elastic recovery, and finish reaming is simulated. Both isotropic hardening and kinematic hardening models are considered in the numerical calculations. The results suggest that a three-dimensional nature exists in the residual stress fields surrounding the hole. There are significant differences in residual stresses at different sections through the thickness. However, residual stress at the surface is shown to remain the same for the different plastic hardening models after the hole has recovered and finish reaming has been performed. The reaming of the material around the hole has slight effect on the maximum value and distribution of residual stresses. A comparison has been drawn between the FEA of average through thickness strain and a previous experimental investigation of strain that utilized neutron diffraction and modified Sachs boring on a 7050 aluminum specimen containing a cold expanded hole, The different methods show very good agreement in the magnitude of strain as well as the general trend. The conclusions obtained here are beneficial to the understanding of the phenomenon of fatigue crack initiation and growth at the perimeter of cold worked holes.

Temperature effects on fatigue performance of cold expanded holes in 7050-T7451 aluminum alloy

Cold expansion of fastener holes has been successfully used for many years to impart beneficial compressive residual stresses. These residual stresses serve to extend crack initiation lives and to slow down growth of small cracks. The 7050 aluminum alloy is being used in supersonic aircraft structures that are subject to high temperatures. The purpose of this investigation is to identify possible problematic stress relaxation around expanded fastener holes in this material. Three tests have been used to assess the effects of thermal exposure on cold expanded fastener holes in 6.4 mm thick 7050-T7451 aluminum plate. The three tests are an initial evaluation of tensile strength, an open-hole uniaxial fatigue test and a modified stress relaxation test. The tensile testing and fatigue testing were performed at ambient conditions on material in the as-received condition and also in a thermally aged condition. Thermal aging consisted of a 5000 h exposure in a laboratory oven set to 104 °C. Stress relaxation testing was performed at temperatures of 71, 82, 93 and 104 °C on a screw driven test frame fitted with an oven. The results of the tests suggest that stress relaxation is present but is not critical at the temperatures tested. The trend in the stress relaxation testing shows that temperatures slightly above 104 °C may be detrimental to residual stress fields.

High temperature hybrid titanium composite laminates: An early analytical assessment

This paper presents a new type of material system that shows great promise for aerospace applications. The concept consists of thin sheets of titanium bonded together with a polymer composite prepreg consisting of a high temperature resin reinforced with high or intermediate modulus fibers. The potential material performance is outstanding in terms of in-plane fatigue and fracture. As with all composite materials, the exact moduli and strength can be optimized for a given application by varying the constituents, their volume fractions and, in the case of the reinforcing fibers, their orientation. The paper includes analytical studies as well as experimental results.

Fracture and Fatigue Tests and Analysis of Composite Sandwich Structure

A composite sandwich system is investigated in this research. Quasi-static fracture toughness and fatigue crack growth experimental and analytical approaches are the focus. The particular system studied is comprised of a Nomex (aramid fiber) honeycomb core with graphite/epoxy facesheets (skins). A modified version of the double cantilever beam (DCB) specimen geometry is used for experimentation. The critical strain energy release rate, Gc, is used to characterize the fracture toughness of the facesheet-core joint. Fatigue crack growth testing is also performed. Novel analytical and experimental techniques are coupled and utilized to address challenges presented by the material system, especially difficult crack visualization. Crack length and growth can be estimated with an empirical approach, employing a compliance calibration. Experiments can also be simulated once several constants are estimated, aiding design. Many of these techniques can be generalized to other adhesive DCB experimentation. Results show that cold tests result in higher fracture toughnesses and slightly slower fatigue crack growth rates than room temperature tests. The hot temperature has less significant impact. Although only a limited amount of very slow growth data (<10−6 mm/cycle) is measured, the material appears to behave with a fatigue threshold of ≈7% of fracture toughness; and a Paris crack growth model is successfully fit with an exponent of ≈3.2. Results also show the failure of this system is always in the aramid paper core material.

Effect of Temperature on Mode I Interlaminar Fracture of IM7/PETI-5 and IM7/977-2 Laminates

The mode I critical strain energy release rates, GIC, of two polymer matrix composites were experimentally measured at temperatures ranging from -196°C to 160°C. The two composite materials investigated in this study were IM7/PETI-5 and IM7/977-2. Double cantilever beam specimens were manufactured with a °[0°7/ ± 3°/0°7] lay-up. The experimental results showed that GI had a strong dependence on temperature above 25°C and minimal dependence on temperature below 25°C. In addition, it was found that IM7/PETI-5 was significantly tougher than IM7/977-2 at all the temperatures investigated. These materials can be considered suitable, in the future, for manufacturing cryogenic fuel tanks in space applications; therefore, understanding critical composite properties such as the interlaminar toughness at such temperatures is very important.

Determination of the nontraditional lay-up influence and loading configuration on fatigue damage development under bearing/bypass loading conditions using radiography

This article presents the effects of several variables on the damage progression within a mechanically fastened graphite/epoxy composite joint. The variables included the composite lay-up, loading configuration (single shear vs. double shear), R-value, stress level, and damage mechanisms observed in each specimen. In situ X-ray of the individual laminates recorded the extent of damage, mostly longitudinal splitting and bearing delamination, as a function of the cycle count. The following lay-ups were investigated: [04/45/03/90/0]s, [45/90/−45/02/45/02/−45/0]s, [±5/65/(±5)2/−65/±5]s, and [±5/65/(±5)2/−65/5/65]s. All of these lay-ups are considered to be ‘hard’ lay-ups, much stiffer in the 0° direction than in the 90° direction. The stress levels at which detectable damage develops was determined. The researchers chose to apply 50,000 cycles at each stress level. Once damage was detected, the stress level was typically raised to 17.25 MPa (2.5 ksi). Another 50,000 cycles was then applied until the bolt hole diameter elongated by 10% of its original length. The damage length vs. stress level is plotted as a way to compare damage onset stresses and growth as a function of lay-up and stress ratio. The new ‘non-traditional’ lay-ups are shown to offer some unique advantages over traditional lay-ups.

Radiographic Determination of the Lay-up Influence and Loading Configuration on Fatigue Damage Development under Bearing/Bypass Loading Conditions

The goal of this academic project was to study the effects of several variables on the damage progression within a mechanically fastened joint. The variables tracked included the composite layup, loading configuration (single shear vs. double shear), R value, stress level, and damage mechanisms observed in each specimen. In-situ x-ray of the individual laminates recorded the extent of damage, mostly longitudinal splitting and bearing, as a function of the cycle count. The following lay-ups were included: [04/45/03/90/0]s, [45/90/45/02/45/02/-45/0]s, [±5/65/(±5)2/-65/±5]s, and [±5/65/(±5)2/-65/5/65]s. The stress levels at which detectable damage develops was determined. The researchers chose to apply 50,000 cycles at each stress level. Once damage was detected, the stress level was typically raised 2.5 ksi. Another 50,000 cycles was then applied until the bolt hole diameter elongated by 10% of it’s original length. The damage length versus stress level is plotted as a way to compare damage onset stresses and growth as a function of lay-up, stress ratio.

Effect of Cryogenic and Elevated Temperatures on Failure Modes of Several Composites Systems

The mode I critical strain energy release rates, GIC, of three polymer matrix composite systems were experimentally measured at temperatures ranging from -196°C to 177°C. The two composite materials investigated in this study were IM7/PETI-5 and IM7/977-2. Double cantilever beam specimens were manufactured with a [0°7/±3°/0°7] lay-up. The experimental results showed a strong dependency of GI on temperature above 25°C and minimal dependence below 25°C. In addition, IM7/PETI-5 was significantly tougher that IM7/977-2 at all temperatures investigated. The third was a primarily unidirectional laminates of IM7/5250-4 bonded together with AF-191M adhesives. This system showed a marked decrease in toughness at lower temperatures and the failure mode dramatically changed. These materials are considered to be candidates for future cryogenic fuel tanks for space applications, so understanding critical composite properties such as the interlaminar toughness at use temperatures is very important.

Strength of Porcine Carotid Arteries As a Function of Strain Rate and Storage Time

The ultimate strength of collagenous blood vessels is important for clinical problems of trauma and plaque rupture. Trauma related to motor vehicle accidents can create strain rates of 100 mm/s. Cyclic fatigue tests may also require high frequencies that may affect the strength properties of the soft tissue. The yield points and ultimate strengths depend on the unfolding of collagen molecules, collagen crosslinks, and the fiber-matrix bonding this composite structure. The non-linear behavior of animal soft tissues makes the determination of a plastic yield point difficult to distinguish. The lack of sources restricts the amount of primary data regarding ultimate strength, strain, frequency dependence, and harvest time dependence. Nonetheless, the mechanics surrounding plaque cap rupture demand measurements of all these parameters.Copyright