Bert L. Smith

Impact Damage Resistance and Tolerance of Honeycomb Core Sandwich Panels

The damage resistance and tolerance of flat [(0/45),/core/(45/0), ] sandwich plates with honeycomb core subjected to low-velocity impacts using hemispherical steel impactors has been investigated experimentally. The effects of impactor diameter on the impact behavior, resulting impact damage states, and residual strength under in-plane compressive loading was of particular interest. The impact responses characterized in terms of peak impact force was observed to be dependent on the facesheet type, core thickness, and impactor size, but was found to be independent of the boundary support conditions. The smaller impactor produced damage states characterized by residual dent depths that were comparable to the core thickness, accompanied by visible facesheet fractures. The larger diameter impactor produced damage states with large core damage regions but with dent depths less than the facesheet thickness. Under in-plane compressive loading, depending on the impact damage state, contrasting failure mechanisms involving net-section fracture and buckling failure were observed. A reduction in compressive strength up to 60% of the undamaged strength has been observed.

Strength of 2024-T3 Aluminum Panels with Multiple Site Damage

An aging aircraft accumulates fatigue cracks commonly referred to as multiple site damage (MSD). For ductile materialssuchas2024-T3aluminum,MSDcracksmaylowerthestrengthsignie cantlybelowthatwhichispredicted by conventional fracture mechanics or net section yield failure methods. An analytical model generally referred to as the linkup model (or the plastic-zone-touch model )has previously been used to describe the MSD phenomenon. However, the linkup model is only accurate for limited geometric cone gurations. Two modie cations to the linkup model were developed through regression analysis of test data obtained from the literature and from experimental results conducted in this investigation. The modie ed models show signie cantly improved correlation with the test data over a wide range of cone gurations for e at 2024-T3 aluminum panels with MSD at open holes. Nomenclature a = lead crack half-length an = nominal lead crack half-length c = MSD crack length D = hole diameter L = ligament length ` = half-length for MSD crack and hole, c + D/2 t = panel thickness W = panel width b a = correction to stress intensity of the lead crack, b a/`b W b a/` = correction to stress intensity of the lead crack for the effect of the adjacent MSD crack b b = correction to stress intensity of the adjacent MSD crack for the effect of an open hole b ` = correction to stress intensity of the adjacent MSD crack, b `/ab b I (c/`) b `/a = correction to stress intensity of the adjacent MSD crack for the effect of the lead crack b W = e nite-width correction to the stress intensity of the lead crack, I [sec(p a/w)] r c = critical stress for ligament failure based on

Strength of Stiffened 2024-T3 Aluminum Panels with Multiple Site Damage

Two modie edlinkup modelsweredevelopedfordetermining thecritical stressbased on linkup (ligamentfailure ) of2024-T3aluminumpanelswithmultiplesitedamage.ThesemodelsweredevelopedforusewithstandardMilitary Handbook for Metallic Materials and Elements for Aerospace Vehicle Structures (MIL-HDBK-5G )yield strength values. For this investigation, ligament failure stresses predicted by these models are compared with test stresses determined from a variety of stiffened panels including single-bay panels with the lead crack centered between stiffeners and two-bay panels with the lead crack centered beneath a severed stiffener. The stresses predicted by the modie ed linkup models correlate well with the test data. The results of this investigation should add to the understanding of the extent to which nonlinear behavior can be modeled with simplie ed engineering models. Nomenclature a = lead crack half-length an = nominal lead crack half-length c = multiple site damage (MSD) crack length D = hole diameter Fcol = collapse stress ` = half-length for MSD crack and hole, cC D/2 L = ligament length t = panel thickness tS = stiffener thickness W = panel width WS = stiffener width

Linkup Strength of 2024-T3 Bolted Lap Joint Panels with Multiple Site Damage

A modified linkup model has been developed for determining the residual strength of 2024-T3 aluminum panels with multiple-site damage. This model was developed by semi-empirical analysis of test data from flat unstiffened open-hole panels. The model was later validated with test data from 36 open-hole stiffened panels. During the investigation, 36 bolted lap joint panels with different crack configurations were tested to further validate the previously developed modified linkup model. Stress intensity factors for the modified linkup model for the different crack configurations were determined from finite element analysis with the FRANC2D/L code. The residual strengths predicted by the modified linkup model correlate well with the test data, as was the case with the earlier studies of the stiffened panels. The results for the bolted lap joint panels show a sudden reduction in the residual strength at the early stages of multiple-site damage followed by a gradual linear decrease with increasing multiple-site damage crack size. The results also demonstrate that a stress-intensity-factor-based solution can be formulated with empirical analysis of test data from a simple configuration and then used to analyze more complex configurations.

A Nondestructive Testing Technique for Composite Panels Using Tap Test Acoustic Signals and Artificial Neural Networks

The increased use of composite materials and their relatively high cost and limited availability make it essential to develop low cost, effective nondestructive testing and inspection techniques (NDT/NDI). One of the oldest and widely used NDT/NDI methods is the coin tap test. The objective of this research was to determine if the sound signals generated by tapping a composite sandwich panel could be classified by an artificial neural network (ANN) as originating from damaged or non-damaged areas on the panel and if possible, to make accurate damage level assessments. Tap sound signals were recorded from several test panels using an ordinary condenser microphone and related equipment. Two separate signal-preprocessing techniques were employed, one using Fourier transforms and one using Wavelet transforms. Wavelet transformation of the signals tended to produce the best results. Artificial neural network configurations were developed using the backpropagation-learning algorithm that correctly classified damaged vs. undamaged signals with 100% accuracy. The results further showed the potential of this process for accurately predicting the damage level present to within ±10%. Overall, the results showed the potential for using a combination of signal characteristic analysis with ANNs trained to recognize and classify the characteristics of simple tap test acoustic signals as an effective, low cost NDT/NDI technique.

Link-Up Strength of 2524-T3 and 2024-T3 Aluminum Panels with Multiple Site Damage

The aluminum alloy 2524-T3 is replacing 2024-T3 for many applications because 2524-T3 has a greater fracture toughness while retaining the same strength. Much attention has been given to the effect of multiple site damage on 2024-T3 aluminum; however, very little has been reported about 2524-T3. Twenty-two panels of 2524-T3, each with a different crack configuration, were tested for critical (linkup) strength, and the results were compared with an identical set of previously tested 2024-T3 panels with MSD. The panels were 24 in. wide with a midspan row of 0.25-in.-diam holes at 1-in. pitch. Each panel had a central lead crack with collinear MSD cracks emerging from both sides of the adjacent holes. A comparison of the results showed the 2524-T3 panels to average approximately 27% greater strength than the 2024-T3 panels. The linkup or plastic-zone-touch model used to predict the critical (link-up) strength of the panels was found to be highly conservative. Consequently the test data were used for a semi-empirical analysis to develop a modified link-up model for 2524-T3, similar to the one previously developed for 2024-T3. The average error between the critical strengths from testing and those predicted by the link-up model was approximately 20%, whereas that for the modified link-up model was approximately 3%.

Strength of 7075-T6 and 2024-T3 Aluminum Panels with Multiple-Site Damage

Much attention has been given to the development of technology for the purpose of determining the strength of aluminum panels that have multiple-site damage. The linkup model has been investigated because of its simplicity, and a number of modified linkup models have been presented. However, most of the attention has been given to 2024-T3 aluminum. Little attention has been given to 7075-T6 because it is a more brittle material with lower fracture toughness, making it more suitable to be analyzed by conventional linear elastic (brittle) fracture mechanics technology. The work presented here involves the study of 7075-T6 panels with multiple-site damage. Both the classical linear elastic fracture model and the linkup model are shown to be highly inaccurate. A modified linkup model and a modified brittle fracture model have both been developed by empirical analysis. Both of these models appear to have a high degree of accuracy over a wide range of crack geometry. The modified linkup model for 7075-T6 is then compared with a previously developed modified linkup model for 2024-T3. This comparison shows that 2024-T3 panels with multiple-site damage have greater strength than 7075-T6 panels, especially for panels with small ligament lengths.

Effect of Dents on Crack Growth in Aluminum Alloy Under Constant-Amplitude Loading

a0.5in.spherical hardened-steelindenterhead.Dentdepths rangedfrom0.03to0.0325in.,measuredontheconvex side of the specimen. A starter notch of 0.3 in. was produced at the edge of the specimen with a jeweler’s saw blade. The specimen was fatigue-loaded under constant-amplitude loading to produce an initial crack length of 0.37 in., at whichtimereadingsofcracklengthvscyclesbegan.Thesameconstant-amplitudecyclicloadingusedtoproducethe initialcracklengthwasusedduringthetesting.Thecracklengthsweremeasuredwithanopticalmicroscopeat160 magnification. Nine specimens were tested, including three replications for each of the three conditions. Crack growth data are given in both tabular and graphical forms for all specimens. Crack growth rate data are also presented in graphical form. The overall crack growth in the dented specimens was significantly greater than in the pristine specimens. It was also faster, on average, in the reworked specimens; reworking, in general, did not recapture the life displayed by the pristine specimens.

Residual Strength of Panels with Cracks Based on a Plastic Hinge

The residual strength of a panel with a crack is dependent on the mode of failure. Generally, the panel will exhibit a brittle-fracture mode of failure (based on linear elastic fracture mechanics) or a mode of failure based on yielding. A common practice is to assume an elastic, perfectly plastic, material behavior, along with a net-section-yield criterion for panels that fail by yielding. However, the net-section-yield criterion is not appropriate for configurations such as panels with asymmetric geometry or loading, because equilibrium is violated. For asymmetric cases such as panels with edge cracks or off-center cracks, a plastic moment or hinge must exist, the details of which may be determined by satisfying equilibrium. The purpose of this study is to compare the experimental residual strengths of panels with edge cracks and off-center cracks with the residual strengths determined from the assumption of elastic, perfectly plastic, behavior and a plastic-moment (hinge) criterion. The material chosen for this study is 2024-T3 aluminum alloy. This material is commonly used for aircraft skin and it has a high fracture toughness, and so the mode of failure will less likely be brittle fracture. An elastic, perfectly plastic, material model was assumed, and expressions were developed for a plastic-hinge failure for panels with edge cracks as well as off-center cracks. A large number of panels with either edge cracks or off-center cracks were tested to failure, and the results were compared with the results obtained from the equations developed from the plastic-hinge assumption. The results from the plastic-hinge assumption compared favorably with the experimental results.

The Effect of Dents on the Stable Crack Growth in 2024 -T3 Bare Sheet Aluminum Alloy under Constant Amplitude Loading.

The purpose of this stud y was to determine the effect of dents on the stable crack growth in 0.04 inch thick 2024 -T3 bare aluminum sheet. The test specimens were either pristine dented or reworked. The edge -cracked pin -loaded specimens of 8 inches in width were tested at consta nt amplitude loading with a stress ratio of 0.2 producing stable crack growth of close to 4 inches completely through two dents on the crack line. Dents were produced with a drop tower having a one inch spherical hardened steel indenter head. Dent depths ranged from 0.03 inch to 0.0325 inch measured on the convex side of the specimen. A starter notch of 0.3 inch was produced at the edge of the specimen with a jewelers saw blade. The specimen was fatigue loaded under constant amplitude loading to produce a n initial crack length of 0.37 inch at which time readings of crack length vs. cycles began. The same constant amplitude cyclic loading used to produce the initial crack length was used during the testing. The crack lengths were measured with an optical microscope at 160X magnification. Nine specimens were tested including three replications for each of the three conditions. Crack growth data is given in both tabular and graphical form for all specimens. Crack growth rate data is also presented in grap hical form. The overall crack growth in the dented specimens was significantly greater than in the pristine specimens. It was also, on the average, faster in the reworked specimens; reworking, in general, did not recapture the life displayed by the pristi ne specimens.