R.C. Alderliesten

Experimental and Numerical Investigation of Metal Type and Thickness Effects on the Impact Resistance of Fiber Metal Laminates

The impact response of fiber metal laminates (FMLs), has been investigated with experiments and numerical simulations, which is reported in this article. Low-velocity impacts were carried out to study the effects of metal type and thickness within FMLs. Glare5-3/2 laminates with two aluminum layer thicknesses and a similar FML containing magnesium sheets were impacted by drop weight tests. Also, a major part of this study was to accomplish a dynamic non-linear transient analysis to study the impact response of FMLs using the commercial finite element (FE) analysis code ABAQUS. By reviewing different approaches of modeling constituents of an FML, it is shown that the appropriate selection of elements has more significant role than failure criterion to predict acceptable results for this type of laminate and loading. The good agreement obtained between experimental and numerical results verifies the possibility of relatively simpler simulation by FE-analysis to predict overall response of FMLs under impact loading.

Yield Strength and Residual Stress Measurements on Friction-Stir-Welded Aluminum Alloys

Friction-stir welding is a relatively new joining technology that has great potential for the aerospace industry. To use friction-stir welding to manufacture an airworthy and damage-tolerant structure, the mechanical behavior must be fully understood. This paper presents the experimental results in which the local yield strength and the residual stress profile for friction-stir-welded AA2024-T3, AA7075-T6, and AA6013-T4 were measured. X-ray diffraction was used to measure the residual stresses. The residual stress profiles in all three alloys exhibited high tensile stresses in the center region of the weld. Around the weld the stress levels were reduced to either zero or low-magnitude compressive stresses. Both AA2024-T3 and AA7075-T6 show comparable residual stress profiles, whereas AA6013-T4 exhibits lower-magnitude residual stresses. Digital imaging correlation was used to measure the local yield strength in all three alloys with and without welds. The yield strength profiles obtained with this new technology matched perfectly with the corresponding hardness profiles of the welds. A relation was found between the yield strength profiles and the residual stress profiles, which resulted in understanding of how the appearance of the residual stress profiles is determined by the yield strength profile.

The effect of impact energy division over repeated low-velocity impact on fiber metal laminates

In this study, a comprehensive comparison was drawn between a single impact with certain amount of energy and two impacts with various amounts of energy, the sum of which is the same as the amount of energy in the single impact. A rational plan for repeated impact tests on fiber metal laminates (FMLs) has been designed so as to assess the effect of impact energy division and its sequence, by varying the energy amount in each of the two impacts, on FMLs under repeated low-velocity impact. The total impact energy is estimated by a quasi-static punch test to use a reasonable amount of energy in the repeated impact tests. The impacts are conducted by a drop hammer machine at the same point. Impact parameters and failure modes for all the repeated impact tests are discussed. The results show that the FML specimen responds stiffer in second impact; to understand the reason of this behavior, impact tests on composite specimens were conducted, and comparison of the results of FML and composite specimens showed that the stiffer behavior is due to the elastic–plastic behavior of aluminum sheets which causes strain hardening and creates a dent. Additionally, the effect of impact energy division and its sequence are so influential that failure modes and impact parameters of each arrangement are considerably different from one another.

Applicability of standard delamination tests (double cantilever beam and end notch flexure) for 5HS fabric-reinforced composites in weft-dominated surface:

Currently, the standard delamination tests established by ASTM (available for mode I and mix mode) are limited to unidirectional composites. Although some researchers have conducted delamination tests in woven composites, their information is still limited. In order to understand the propagation behavior and the value of fracture toughness of woven composites, this article evaluated a 5HS carbon/epoxy composite with a weft-dominated surface. Tests were conducted in double cantilever beam and end notch flexure configurations using an energy-based approach for data reduction in modes I and II, respectively. The results were assessed in terms of delamination resistance curves (R-curves). Both delamination modes showed consistent behaviors for extending the application of the standard procedures, as the energy variation can describe well the crack growth dependence of the irregular surface caused by the crimp, which was more pronounced for mode I.

LDA's Applicability for Predicting Fatigue Crack Growth in FML's under VA Loading

This paper presents the experimental and analytical research on the applicability of the Linear Damage Accumulation Approach for fatigue crack growth in Fiber reinforced Metal Laminates under variable amplitude loading. A recently developed constant amplitude analytical prediction model for Fiber reinforced Metal Laminates has been extended to predict fatigue crack growth under variable amplitude loading using a linear damage accumulation rule. The modied model has been compared with crack growth tests on Fiber reinforced Metal Laminates center-cracked tension specimen. In the end it is discussed to what extent or under which conditions the linear damage accumulation predictions are suciently accurate for Fiber reinforced Metal Laminates structures.

Linear Damage Accumulation for Predicting Fatigue in Fiber Metal Laminates

DOI: 10.2514/1.42486 This paper presents the experimental and analytical research on the applicability of the linear damage accumulation approach for fatigue crack growth in fiber metal laminates under variable amplitude loading. A recently developed constant amplitude analytical prediction model for fiber metal laminates has been extended to predict fatigue crack growth under variable amplitude loading using a linear damage accumulation rule. The modified model has been compared with crack growth tests on fiber metal laminates center-cracked tension specimen. In the end, it is discussed to what extent or under which conditions the linear damage accumulation predictions are sufficiently accurate for fiber metal laminates structures. Nomenclature a = total crack length aD = delay distance ai = current crack growth increment a0 = initial/total crack length of previous loading cycle Ccg, ncg = crack growth relation constants Kbridging = bridging stress intensity factor Kfar-field = far-field stress intensity factor Ktip = crack-tip stress intensity factor N = number of loading cycles ND = number of delay cycles NOL = number of overload cycles Smax = maximum stress magnitude SOL = overload stress magnitude R = stress ratio RKtip = crack-tip stress ratio ROL = overload stress ratio � Keff = effective stress intensity factor range

The generation of deformation damage during fatigue loading in Al-Cu alloy studied by the Doppler Broadening technique

We have investigated the evolution of damage during fatigue loading in Al–Cu 2024-T3 alloys using the positron annihilation Doppler Broadening (DB) technique. This technique enables us to monitor in a non destructive way, at the atomic and vacancy level, the formation of deformation defects and their interaction with solute atoms at selected stages of fatigue testing. The changes in the S and W Doppler Broadening parameters are linked to the changes in fatigue behavior at lower stress levels. The material was tested under constant amplitude fatigue loading at four different stress levels and DB tests were conducted at selected stages of fatigue lifetime. The results are compared to those obtained during static tensile tests.

Fractographic Observations on the Mechanism of Fatigue Crack Growth in Aluminium Alloys

Special load histories are adopted to obtain information about the behavior of the moving crack tip during the increasing and decreasing part of a load cycle. It is associated with the crack opening and closure of the crack tip. Secondly, modern SEM techniques are applied for observations on the morphology of the fractures surfaces of a fatigue crack. Information about the cross section profiles of striations are obtained. Corresponding locations of the upper and the lower fracture surface are also explored in view of the crack extension mechanism. Most experiments are carried out on sheet specimens of aluminum alloys 2024-T3, but 7050-T7451 specimens are also tested in view of a different ductility of the two alloys.

RECENT ADVANCEMENTS IN THIN- WALLED HYBRID STRUCTURAL TECHNOLOGIES FOR DAMAGE TOLERANT AIRCRAFT FUSELAGE APPLICATIONS

This paper presents and discusses some of the recent advancements that have been achieved in the development of the FML concept for thin single-aisle fuselage structures. It is shown that the inherent damage tolerant nature of this concept can be further exploited for these structural applications.