kf Evaluation in GFRP Composites by Thermography

Since the presence of a notch in a mechanical component causes a reduction in the fatigue strength, it is important to know the kf value for a given notch geometry and material. This parameter is fundamental in the fatigue design of aeronautical components that are mainly made of composites. kf is available in the literature for numerous types of notch but only for traditional materials such as metals. This paper presents a new practice, based on thermographic data, for the determination of the fatigue notch coefficient kf in composite notched specimens. The innovative aspect of this study is therefore to propose the application on composite materials of a new thermographic procedure to determine kf for several notch geometries: circular, U and V soft and severe notches. It was calculated, for each type of notch, as the ratio between the fatigue limits obtained on the cold and hot zone corresponding to the smooth and notched specimen, respectively. Consequently, this research activity provides, for the first time, a little database of kf for two particular typologies of composite materials showing a fast way to collect further values for different laminates and notch geometries.

The Effect of Notches on the Failure of Two-Dimensional Nonwoven Fiber Networks

The tearing response of sheets of nonwoven fiber material is investigated. It addresses the question on how notch length and notch geometry is related to the tearing strength and tearing processes. The system considered consists of elastic-brittle fibers connected by strong interfiber bonds. Fiber fracture is the only failure mechanism. For a random fiber orientation case, deformation of the unnotched specimen occurs by long-range fiber chains connecting the load inducing boundaries, and failure is by tearing the cross section. The strength of the notched random fiber sheets is well described by a net section criterion, independent of the notch geometry. For a fiber orientation with symmetry relative to the loading direction, tensile loading is transferred by formation of the X-shaped fiber chains centered in the specimen. The subsequent failure occurs along the fiber chain by shear. Thus, the tearing strength is independent of the notch depth in double-edge notched and single-edge notched specimens, when the presence of shallow notch does not disrupt the force chains in the model. As the notch disturbs the fiber chains, alternative shear failure path forms near the notch tip, leading to a dependence of failure strength on the notch geometry. Then, the failure strength of notched nonwoven networks is described by a shear strength and a notch geometry term.

The effect of opposing notch geometry on the tensile strength of adhesively bonded single-lap joints

Purpose This study aims to investigate the effect of opposing notches formed on the adherends on the tensile strength of an adhesively bonded single-lap joint. Design/methodology/approach Different notch geometries were constructed on adherends and evaluated by using the Taguchi method to obtain optimum notch geometry. Then finite element analysis was conducted considering optimum notch geometries by using the cohesive zone model. Lastly, finite element analysis results were validated experimentally. Findings Experimental and numerical studies revealed that notches formed on adherends increased the tensile strength of the joint. The failure load of the Type-III joint, where the highest increase was observed, increased by 15 per cent. In addition, it was found that the notch shape, length, depth and distance to the overlap area had significant effects on the failure load of the joint. Originality/value This study shows that higher joint strengths can be accomplished by using the same joint configuration by notching adherends.

Notched tensile response and damage mechanism of the GLARE laminate

Glass laminate aluminum reinforced epoxy, as an attractive material for advanced aerospace applications, is most likely to face the challenge of various blunt notched damages. In this paper, the tensile tests of the glass laminate aluminum reinforced epoxy with circular and square blunt notches are conducted. The effects of notch geometry, such as notch diameter, notch corner radius, and off-axis angle, on the laminated tensile behaviors of glass laminate aluminum reinforced epoxy laminate are explored. A characteristic distance function representing the damage-affected region of notch is empirically constructed to predict the residual strength of the circular notched glass laminate aluminum reinforced epoxy based on a modified point stress criterion. For the square notched laminate, the strength is almost equal to that of the laminate with its circumscribed circle notch. Furthermore, the variation of two fracture patterns observed with the notch geometry is studied, and the evolution of the interlaminar delamination damage is analyzed after the metal chemical removal.

A Probabilistic Approach to Assessing and Predicting the Failure of Notched Components

This work presents a probabilistic model to evaluate the strength results obtained from an experimental characterisation program on notched components. The generalised local method (GLM) is applied to the derivation of the primary failure cumulative distribution function (PFCDF) as a material property (i.e., independent of the test type, load conditions and specimen geometry selected for the experimental campaign), which guarantees transferability in component design. To illustrate the applicability of the GLM methodology, an experimental program is performed using specimens of EPOLAM 2025 epoxy resin. Three different samples, each with a specific notch geometry, are tested. As a first scenario, a single assessment of each sample is obtained and the PFCDFs are used to perform cross predictions of failure. Some discrepancies are noticeable among the experimental results and cross-failure predictions, although they are within the expected margins. A possible reason for the disagreement can be assigned to the inherent statistical variability of the results and the limited number of tests per each sample. As a second scenario, a joint assessment of the three samples is performed, from which a unique PFCDF is provided, according to the GLM. In the latter case, a more reliable assessment of the experimental results from the geometry conditions is achieved, the suitability of the selected driving force is verified, and the transferability of the present material characterisation is confirmed.