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  • 11
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    Effect of adherend thickness and mixed mode loading on debond growth in adhesively bonded composite joints (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Symmetric and unsymmetric double cantilever beam (DCB) specimens were tested and analyzed to assess the effect of: (1) adherend thickness, and (2) a predominantly mode I mixed mode loading on cyclic debond growth and static fracture toughness. The specimens were made of unidirectional composite (T300/5208) adherends bonded together with EC3445 structural adhesive. The thickness was 8, 16, or 24 plies. The experimental results indicated that the static fracture toughness increases and the cyclic debond growth rate decreases with increasing adherend thickness. This behavior was related to the length of the plastic zone ahead of the debond tip. For the symmetric DCB specimens, it was further found that displacement control tests resulted in higher debond growth rates than did load control tests. While the symmetric DCB tests always resulted in cohesive failures in the bondline, the unsymmetric DCB tests resulted in the debond growing into the thinner adherend and the damage progressing as delamination in that adherend. This behavior resulted in much lower fracture toughness and damage growth rates than found in the symmetric DCB tests.
    Keywords: COMPOSITE MATERIALS
    Type: NASA-TM-88992 , NAS 1.15:88992 , AVSCOM-TR-86-B-2
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  • 12
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    Fatigue crack initiation and small crack growth in several airframe alloys (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The growth of naturally-initiated small cracks under a variety of constant amplitude and variable amplitude load sequences is examined for several airframe materials: the conventional aluminum alloys, 2024-T3 and 7075-T6, the aluminum-lithium alloy, 2090-T8E41, and 4340 steel. Loading conditions investigated include constant amplitude loading at R = 0.5, 0, -1 and -2 and the variable amplitude sequences FALSTAFF, Mini-TWIST and FELIX/28. Crack growth was measured at the root of semicircular edge notches using acetate replicas. Crack growth rates are compared on a stress intensity factor basis, to those for large cracks to evaluate the extent of the small crack effect in each alloy. In addition, the various alloys are compared on a crack initiation and crack growth morphology basis.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-102598 , NAS 1.15:102598 , AVSCOM-TM-90-B-001
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  • 13
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    The growth of short cracks in 4340 steel and aluminum-lithium 2090 (1990)
    In:  CASI
    Details
    Publication Date: 2017-10-02
    Description: The objectives were to investigate crack initiation characteristics and short crack growth behavior for Al-Li 2090 and for 4340 steel and to evaluate the ability of a closure-based crack-growth model to predict fatigue crack growth rates and total fatigue lives for the steel. Single-edge-notched tension specimens of each alloy were used to obtain the short crack growth rate information via an acetate replica technique. In addition to constant amplitude loading, tests on the steel were conducted using the Felix/28 variable amplitude spectrum (a shortened form of a standard loading sequence for fixed or semi-rigid helicopter rotors). The short crack growth rates were compared to those for long cracks grown under similar loading conditions. Metallurgical features associated with crack initiation are discussed. For Al-Li 2090 under R = -1 loading, the short cracks grew well below the long crack threshold and grew at acute angles to the loading axis. For 4340 steel under constant amplitude loading at R = 0.5 and 0 and for the Felix/28 spectrum loading, short-crack growth rates agreed well with long-crack growth rates, even near the long-crack threshold. A slight short-crack effect, growth below the long-crack threshold, was observed at R = -1. Fatigue lives were found to depend on the size and type of initiation site, especially for the Felix/28 loading sequence. A semi-empirical crack-growth model incorporating crack-closure effects was used to predict crack growth rates and total fatigue lives of notched 4340 steel specimens. An initial defect size and shape typical of those identified in this steel was assumed for the life predictions. For all loading conditions, reasonable agreement was found between measured and predicted values for both crack growth rates and fatigue lives.
    Keywords: STRUCTURAL MECHANICS
    Type: AGARD, Short-Crack Growth Behaviour in Various Aircraft Materials; 30 p
    Format: text
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  • 14
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    The role of peel stresses in cyclic debonding (1982)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: When an adhesively bonded joint is undergoing cyclic loading, one of the possible damage modes that occurs is called cyclic debonding - progressive separation of the adherends by failure of the adhesive bond under cyclic loading. In most practical structures, both peel and shear stresses exist in the adhesive bonding during cyclic loading. The results of an experimental and analytical study to determine the role of peel stresses on cyclic debonding in a mixed mode specimen are presented. Experimentally, this was done by controlling the forces that create the peel stresses by applying a clamping force to oppose the peel stresses. Cracked lap shear joints were chosen for this study. A finite element analysis was developed to assess the effect of the clamping force on the strain energy release rates due to shear and peel stresses. The results imply that the peel stress is the principal stress causing cyclic debonding.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-84504
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  • 15
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    Cyclic debonding of unidirectional composite bonded to aluminum sheet for constant-amplitude loading (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Cyclic debonding rates were measured during constant-amplitude loading of specimens made of graphite/epoxy bonded to aluminum and S-glass/epoxy bonded to aluminum. Both room-temperature and elevated-temperature curing adhesives were used. Debonding was monitored with a photoelastic coating technique. The debonding rates were compared with three expressions for strain-energy release rate calculated in terms of the maximum stress, stress range, or a combination of the two. The debonding rates were influenced by both adherent thickness and the cyclic stress ratio. For a given value of maximum stress, lower stress ratios and thicker specimens produced faster debonding. Microscopic examination of the debonded surfaces showed different failure mechanisms both for identical adherends bonded with different adhesive and, indeed, even for different adherends bonded with identical adhesives. The expressions for strain-energy release rate correlated the data for different specimen thicknesses and stress ratios quite well for each material system, but the form of the best correlating expression varied among material systems. Empirical correlating expressions applicable to one material system may not be appropriate for another system.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TN-D-8126 , L-10480
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  • 16
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    Fatigue life and crack growth prediction methodology (1993)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The capabilities of a plasticity-induced crack-closure model and life-prediction code to predict fatigue crack growth and fatigue lives of metallic materials are reviewed. Crack-tip constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta(K(sub eff))) under constant-amplitude loading. Some modifications to the delta(K(sub eff))-rate relations were needed in the near threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and in some cases total fatigue lives, for several aluminum and titanium alloys under constant-amplitude, variable-amplitude, and spectrum loading. Fatigue lives were calculated using the crack growth relations and microstructural features like those that initiated cracks. Results from the tests and analyses agreed well.
    Keywords: COMPOSITE MATERIALS
    Type: NASA-TM-109044 , NAS 1.15:109044 , AGARD Structures and Materials Panel Workshop on an Assessment of Fatigue Damage and Crack Growth Prediction Techniques; Sep 29, 1993 - Sep 30, 1993; Bordeaux; France
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  • 17
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    Debond propagation in composite-reinforced metals (1975)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Strain energy release rates were used to correlate cyclic debonding between metal sheets and composite reinforcement. An expression for the strain energy release rate was derived and applied to fatigue test results for three material systems: graphite bonded to aluminum with both a room temperature and an elevated temperature curing adhesive; and, S-glass bonded to aluminum with an elevated temperature curing adhesive. For each material system, specimens of several thicknesses were tested with a range of fatigue loads. Cyclic debonding was monitored using a photoelastic technique. A close correlation was found between the observed debond rates and the calculated strain energy release rates for each material system.
    Keywords: COMPOSITE MATERIALS
    Type: Fatigue of composite materials; Symposium; Dec 03, 1973 - Dec 04, 1973; Bal Harbour, FL
    Format: text
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  • 18
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    Debond propagation in composite reinforced metals (1974)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Strain energy release rates were used to correlate cyclic debonding between metal sheets and composite reinforcement. An expression for the strain energy release rate was derived and applied to fatigue test results for three material systems: graphite bonded to aluminum with both a room temperature and an elevated temperature curing adhesive, and S-glass bonded to aluminum with an elevated temperature curing adhesive. For each material system, several thicknesses were tested with a range of fatigue loads. Cyclic debonding was monitored using a photoelastic technique. A close correlation was found between the observed debond rates and the calculated strain energy release rates for each material system.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-X-71948 , ASTM Symp. on Fatigue of Composite Mater.; Dec 02, 1973 - Dec 07, 1973; Bal Harbour, FL; United States
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  • 19
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    Cyclic debonding of adhesively bonded composites (1982)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The fatigue behavior of a simple composite to composite bonded joint was analyzed. The cracked lap shear specimen subjected to constant amplitude cyclic loading was studied. Two specimen geometries were tested for each bonded system: (1) a strap adherend of 16 plies bonded to a lap adherend of 8 plies; and (2) a strap adherend of 8 plies bonded to a lap adherend of 16 plies. In all specimens the fatigue failure was in the form of cyclic debonding with some 0 deg fiber pull off from the strap adherend. The debond always grew in the region of adhesive that had the highest mode (peel) loading and that region was close to the adhesive strap interface.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-84577 , NAS 1.15:84577 , Intern. Symp. on Adhesive Joints: Their Formation Characteristics, and Testing; Sep 12, 1982 - Sep 17, 1982; Kansas City, MO; United States
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  • 20
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    Geometrically nonlinear analysis of adhesively bonded joints (1982)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A geometrically nonlinear finite element analysis of cohesive failure in typical joints is presented. Cracked-lap-shear joints were chosen for analysis. Results obtained from linear and nonlinear analysis show that nonlinear effects, due to large rotations, significantly affect the calculated mode 1, crack opening, and mode 2, inplane shear, strain-energy-release rates. The ratio of the mode 1 to mode 2 strain-energy-relase rates (G1/G2) was found to be strongly affected by he adhesive modulus and the adherend thickness. The ratios between 0.2 and 0.8 can be obtained by varying adherend thickness and using either a single or double cracked-lap-shear specimen configuration. Debond growth rate data, together with the analysis, indicate that mode 1 strain-energy-release rate governs debond growth. Results from the present analysis agree well with experimentally measured joint opening displacements.
    Keywords: STRUCTURAL MECHANICS
    Type: NASA-TM-84562 , NAS 1.15:84562 , AIAA/ASME/ASCE/AHS Structures, Structural Dyn., and Mater.; May 10, 1982 - May 12, 1982; New Orleans
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