Gerald Kress

Fatigue Response of Notched Graphite/Epoxy Laminates

Tests were performed to determine the damage states in quasi-isotropic graphite/epoxy laminates with center holes caused by cyclic tensile loading. The influence of the stacking sequence on the initiation and interaction of damage modes and the relationship between damage, strength, stiffness, and life of the laminates were also studied. X-ray radiography, moire interferometry, and stiffness change were used to monitor damage. Fatigue damage in both laminates began with matrix cracks around the holes leading to delaminations. In laminates cycled at the same percent of notched tensile strength the stacking sequence influenced the density of the matrix cracks and the modes and distribution of the damage. Ply cracking was also caused by the stacking sequence. The damage states in the two laminates produced stiffness changes of 15 to 20 percent, different rates of change in residual strength, and a factor of two to four difference in fatigue life. It was determined that continued cyclic loading produced matrix cracks which led to fatigue of the laminates.

Non-linear constitutive model for plain-weave composites

A non-linear constitutive model for plain-weave composites is developed which is based on the micromechanical behavior of a representative unit cell. The global constitutive relationships are consistently derived from the total strain energy of the system. Compatibility of the constituents geometric non-linearities is fully taken into account where the matrix plays the role of an elastic foundation and is thus modeled as a continuum.

Stiffness reduction in laminate coupons due to the free-edge effect

Finite-width laminate coupons with a multidirectional [±θ]s layup are considered in regard to their effective axial stiffness properties. Owing to the existing free-edge effect the effective coupon stiffness is clearly reduced in comparison with the predictions of classical laminate theory. For the description of this effect a closed-form analysis is presented. An essential characteristic of the analysis is the introduction of appropriate additional deformation modes from which the respective stiffness reduction is derived in a consistent way without involving too much computational effort. For comparison and validation purposes the influence of the free-edge effect on effective coupon stiffness is also investigated by detailed finite element analyses. Good agreement between closed-form analysis and numerical analysis is obtained.

Examination of Hashin's failure criteria for the second world-wide failure exercise

The second world-wide failure exercise is an international activity, organised by Kaddour and Hinton, aiming at a better understanding of the limitations of existing failure criteria to predict failure under triaxial stressing. The present work is a response to an invitation to take part and employ Hashins failure criteria to the 12 test cases defined by the organizers of the second world-wide failure exercise. Methods, based on nonlinear analysis, used to perform the required simulations are explained and the predictions are presented. Observed problems are discussed.

Minimized computational effort for the thick-walled composite tube problem

Exact solutions exist for the problem of thick-walled composite tubes if uniformity of geometric and material properties in the axial and circumferential directions is assumed. If solved directly, the resulting system of equations for the boundary and continuity conditions quickly increases in size with the number of layers. This paper shows how the number of unknowns and the matrix size are significantly reduced if a finite element technique retaining the exactness of the solution is employed. It is also demonstrated in some detail how various loading conditions such as non-uniform temperature distributions due to heat flux across the wall, shrink fits, axial force, etc., can be accounted for by finite element formulation.

Free-Edge Influence on CFRP-Laminate Strength

Stress perturbations at free edges of CFRP-laminates may induce initial failure events at low load levels and accelerated damage growth, eventually reducing ulti mate strength in tensile-test situations. Lower values of measured strength must be ex pected from specimens with free edges than without.

Width influence on stiffness measurements of multidirectional CFRP laminates under uniaxial load

Abstract Stiffness analyses based on classical lamination theory (CLT) are valid only in the absence of free-edge effects. The range of applicability of CLT is therefore limited to cylindrical structures such as payload carriers of launching rockets or satellite telescope tubes and similar constructions. The experimental verification of such predictions is often troublesome if finite-width-test coupons are used: free-edge effects then may cause considerable or even severe reductions in stiffness as predicted by CLT. In practice, confusion is created by not knowing how to discern between the influence of the free edge and other possible causes such as errors in laminate construction, material choice or the fabrication process. The investigation of the influence of free-edge effects on stiffness measurements comprises a finite-element model which is based on a so-called quasi-three-dimensional finite element, a simple theory that follows from the boundary-layer nature of the free-edge problem and predicts the width influence on measured stiffness values, and the comparison with experimental data.

Examination of Hashin’s failure criteria for Part B of the second world-wide failure exercise: Comparison with test data

The Second World-Wide Failure Exercise is an international activity, organized by Hinton and Kaddour [The background to the Second World-Wide Failure Exercise (WWFE-II). 2012.], aiming at a better understanding of the limitations of existing failure criteria to predict failure under triaxial stressing. Part A of the exercise was to apply the failure criteria to the 12 test cases defined by the organizers of second world-wide failure exercise to predict strength. The presently reported Part B is to compare the predictions with the before unknown strength measurements, also provided by the organizers of second world-wide failure exercise. Hashin’s [Failure criteria for unidirectional fiber composites. J Appl Mech 1980; 47(6): 329–334.] model is used in its original form with one exception which is explained in the article. Problems with inhomogeneous stress distributions, arising in certain samples, are addressed. The deviations between predictions and measurements are quantified and explanations for those are given for the individual load cases.

A new aligned short-carbon-fiber-reinforced thermoplastic prepreg

Short-fiber composites offer a significant advantage over long-fiber composites in manufacturing complex structural parts because of their much higher strain capacity in all directions. On the other hand, the drawback of using randomly oriented short-fiber composites is a significant loss in specific mechanical properties. Therefore, to minimize this drawback, it is absolutely necessary that all fibers be aligned. This paper will introduce a new aligned short-carbon-fiber-reinforced thermoplastic prepreg (HTA7/PEI), the manufacturing technique, and the processing parameters for composite plates, as well as the first unidirectional mechanical properties in comparison to a similar long-fiber composite.

Speckle interferometry as a tool for verifying finite element method (FEM) damage models

As a consequence of the recent qualitative improvement of the phase fringe patterns by Thierry Floureux (1993) using electronic speckle pattern interferometry, the ability to precisely measure local displacement fields exists. With this new ability speckle interferometry becomes a tool for many applications where local effects are of great theoretical concern. The technique can be used to test and, if necessary, correct assumptions or simplifications used in theoretical models. This paper shows new applications of the improved speckle interferometric method, one being the detection of the various crack forms such as splits and delaminations (NDT) occurring in the above-mentioned fatigue-loaded specimens and their affect on the displacement field at the surface. The other one is using speckle interferometry as a tool for verifying or, respectively, updating FEM-damage models of fatigued specimens.