Alan R. Kallmeyer

Evaluation of Multiaxial Fatigue Life Prediction Methodologies for Ti-6Al-4V

Many critical engineering components are routinely subjected to cyclic multiaxial stress states, which may include non-proportional loading and multidimensional mean stresses. Existing multiaxial fatigue models are examined to determine their suitability at estimating fatigue damage in Ti-6Al-4V under complex, multiaxial loading, with an emphasis on long-life conditions. Both proportional and non-proportional strain-controlled tension/ torsion experiments were conducted on solid specimens. Several multiaxial fatigue damage parameters are evaluated based on their ability to correlate the biaxial fatigue data and uniaxial fatigue data with tensile mean stresses (R>-1) to a fully-reversed (R=-1) uniaxial baseline. Both equivalent stress-based models and critical plane approaches are evaluated. Only one equivalent stress model and two critical plane models showed promise for the range of loadings and material considered.

Effect of Temperature on the Low-velocity Impact Behavior of Composite Sandwich Panels

Composite sandwich panels constructed from glass-fiber-reinforced facesheets surrounding both foam-filled and nonfilled honeycomb cores are impacted using a drop-weight impactor at three energy levels and three temperatures. The effects of core material, temperature, and impact velocity on the absorbed energy, peak impact force, and damage mechanisms were studied. The foam-filled samples were subsequently subjected to four-point bend tests to investigate the effect of impact velocity and temperature on the damage tolerance and residual strength of the composites. It was found that the temperature can have a significant effect on the energy absorbed and maximum force encountered during impact, although the effect of the impact temperature on the residual bending stiffness and strength of the composites was mixed. In addition, the nature of the core material greatly influenced the damage mechanisms and impact force transfer in the honeycomb sandwich composites.

A Finite Element Model for Predicting Time-Dependent Deformations and Damage Accumulation in Laminated Composite Bolted Joints

To assess and predict the long-term durability of advanced composite joints, a general, nonlinear finite element program was developed and implemented to model the localized time-dependent deformations and damage accumulation in the vicinity of a fastener hole in a polymer matrix composite (PMC) laminate. This code incorporates an elastic-viscoplastic constitutive model for unidirectional, orthotropic laminae, coupled with classical lamination theory, to determine the time-dependent stresses and deformations in the laminate. The accumulation of damage in the composite, and the subsequent deterioration of mechanical properties, is predicted by a set of ply failure criteria and an associated property degradation model. The results from this analysis were compared to experimental data and were found to provide reasonable correlation with time-dependent bolt-hole elongation measurements obtained from a graphite fiber reinforced PMC.

Constant and variable amplitude fatigue behavior and modeling of an SRIM polymer matrix composite

Strain-controlled fatigue behavior of smooth specimens of an SRIM polymer matrix composite under constant and variable amplitude loading was investigated, including the effects of mean stresses/strains. Significant degradation of the macroscopic stiffness was observed during cyclic loading, and SEM examination of the failed specimens revealed the degradation was due to a variety of damage mechanisms, including matrix cracking, fiber/matrix debonding, fiber fracture, and fiber buckling. Fatigue life predictions made using common strain-based models overestimated the experimental results for both constant amplitude and variable amplitude loading. An improved strain-based model for making life predictions using an effective strain amplitude was proposed which was in much closer agreement with experimental results.

Development of a Multiaxial Fatigue Damage Model for High Strength Alloys Using a Critical Plane Methodology

The prediction of fatigue life for metallic components subjected to complex multiaxial stress states is a challenging aspect in design. Equivalent-stress approaches often work reasonably well for uniaxial and proportional load paths; however, the analysis of nonproportional load paths brings forth complexities, such as the identification of cycles, definition of mean stresses, and phase shifts, that the equivalent-stress approaches have difficulties in modeling. Shear-stress based critical-plane approaches, which consider the orientation of the plane on which the crack is assumed to nucleate, have shown better success in correlating experimental results for a broader variety of load paths than equivalent-stress models. However, while the interpretation of the ancillary stress terms in a critical-plane parameter is generally straightforward within proportional loadings, there is often ambiguity in the definition when the loading is nonproportional. In this study, a thorough examination of the variables responsible for crack nucleation is presented in the context of the critical-plane methodology. Uniaxial and multiaxial fatigue data from Ti–6Al–4V and three other alloys, namely, Rene’104, Rene’88DT, and Direct Age 718, are used as the basis for the evaluation. The experimental fatigue data include axial, torsional, proportional, and a variety of nonproportional tension/torsion load paths. Specific attention is given to the effects of torsional mean stresses, the definition of the critical plane, and the interpretation of normal stress terms on the critical plane within nonproportional load paths. A new modification to a critical-plane parameter is presented, which provides a good correlation of experimental fatigue data.

Prediction of Pressure Cycle Induced Microcrack Damage in Linerless Composite Tanks

Abstract : Linerless composite tanks made from continuous carbon fiber reinforced polymers will enable significant mass and cost savings over lined, composite overwrapped tanks. The key technical challenge in developing these linerless tanks will be to choose and/or design the material to resist microcracks that may lead to leakage. Microcracks are known to form in the matrix of a composite due to mechanical stresses transverse to the reinforcing fiber direction. This paper presents an approach for characterizing the accumulation of microcracks in linerless composite tank materials under cyclic mechanical loading associated with multiple fill-and-drain pressure cycles. The model assumes that the rate of microcrack-damage accumulation is related to the microcracking fracture toughness of the material through a modified Paris-law formulation. A key artifact of this model is that microcrack-damage accumulation under cyclic load can be predicted from only two material constants. This damage accumulation model is validated through a series of coupon tests, and an illustrative example is presented to demonstrate how the model can be used to predict the microcracking performance of a linerless composite tank subjected to fatigue cycles.

Development of a multiaxial fatigue damage parameter and life prediction methodology for non-proportional loading

Most of the prior studies on the prediction of fatigue lives have been limited to uniaxial loading cases, whereas real world loading scenarios are often multiaxial, and the prediction of fatigue life based upon uniaxial fatigue properties may lead to inaccurate results. A detailed exploration of multiaxial fatigue under constant amplitude loading scenarios for a range of metal alloys has been performed in this study, and a new methodology for the accurate prediction of fatigue damage is proposed. A wide variety of uniaxial, torsional, proportional and non-proportional load-paths has been used to simulate complex, real-world loading scenarios. Test data have been analyzed and a critical-plane based fatigue damage parameter has been developed. This fatigue damage parameter contains stress and strain terms, as well as a term consisting of the maximum value of the product of normal and shear stresses on the critical plane. The shear-dominant crack initiation phenomenon and the combined effect of shear and tensile stresses on micro-crack propagation have been modeled in this work. The proposed formulation eliminates many of the shortcomings of the earlier developed critical-plane fatigue damage models. It is mathematically simple with substantially fewer material dependent constants, and provides design engineers with a tool to predict the fatigue life of machine parts with minimal computational effort. This life prediction methodology is intended for a wide variety of LCF and HCF loadings on machine parts made of metals including advanced alloys. KEYWORDS. Multiaxial; Fatigue Damage Parameter; Non-proportional loading.

A Micromechanical Damage Model for Carbon Fiber Composites at Reduced Temperatures

Fiber-reinforced composites are seeing increased use in civil infrastructure applications where they may be exposed to moisture, sub-ambient temperatures (below —40°C in northern regions), and reduced-temperature (freeze— thaw) thermal cycling. These environments may induce internal damage in the composite due to residual stresses caused by mismatches in coefficients of thermal expansion between the constituents. In this study, a micromechanical damage model is presented for the prediction of matrix crack development in a unidirectional carbon/epoxy composite resulting from exposure to sub-ambient temperatures. Two thermal loadings are considered: cool-down from the cure temperature of the composite (121°C) to 25°C (▵T= -96°C) and to -50°C (▵T=-171°C). A finite element model is utilized to determine the internal stresses due to differences in the CTE of the constituents, and a shear-lag model is developed to predict subsequent crack spacing in the matrix. The influence of fiber spacing (or fiber volume fraction) is addressed. Model predictions indicate that, at 25°C, the internal stresses are not large enough to cause matrix cracking in this composite. However, at -50°C, the longitudinal tensile stress in the matrix exceeds the strength of the epoxy at most typical fiber volume fractions, which will result in matrix cracking. The shear-lag model was used to predict the subsequent crack spacing, and demonstrated a strong dependence on service temperature and fiber spacing. The model predictions provide good qualitative agreement with experimental observations, and indicate that the development of damage in a composite should be considered in the design of composite structures for reduced temperature environments.