F.V. Antunes

Effect of Interlayer Delamination on Mechanical Behavior of Carbon/Epoxy Laminates

This article presents the results of a current study on the influence of interlayer delaminations on the static and fatigue behavior of composite laminates. The composite was manufactured by a vacuum molding method using 12 balanced bi-directional carbon fiber layers and epoxy resin. Delaminations with different length were artificially introduced. The specimens with dog bone shape were cut from the original plates having 3 mm thickness and fiber weight fraction of 0.66. Static tests were performed in order to study the influence of delamination size on the laminate stiffness and strength. Complementary finite element analysis was carried out showing the influence of angular misalignments of fiber/matrix delaminations on the laminate stiffness. Fatigue tests were performed in load control for R = 0.05 and R = —1, with a loading frequency of 10 Hz, at room temperature. The artificial interlayer delaminations have a negligible influence on the fatigue strength for tensile cycle loadings, but produce significant decreases in strength for R = —1 fatigue loadings.

High temperature fatigue crack growth in Inconel 718

Abstract The nickel base superalloys are extensively used in high temperature applications, so it is important to know their behaviour under conditions of high-temperature fatigue. This paper studies the influence of ΔK, loading frequency, stress ratio and temperature on the high temperature fatigue crack growth rate of nickel base superalloys. This study is based on fatigue tests carried out in corner crack specimens of Inconel 718 at 600°C and at room temperature. Three stress ratios (R = 0.05, 0.5 and 0.8) and loading frequencies ranging from 0.0017 to 15 Hz were considered in the tests. For frequencies below 0.25 Hz, the load wave shape was trapezoidal with different dwell times at maximum load. At relatively high frequencies the propagation is cycle dependent, while for lower frequencies it is time dependent. At intermediate frequencies a mixed crack growth occurs. The transition frequencies from cycle dependent to mixed regime and from mixed to time dependent regime were obtained for each R. The increase of R increases the transition frequencies, i.e., extends the time dependent crack growth to higher frequencies. The increase of R also produces an increase of cyclic crack growth rate for all regimes of crack growth. In the time dependent regime, a higher variation is observed, that can be explained by an acceleration of oxidation damage promoted by the increase of maximum stress. An approach for modelling the high-temperature fatigue crack growth in nickel base superalloys is presented. A good agreement was observed between time dependent fatigue results and mathematical models based on static load results.

Identification of fatigue crack propagation modes by means of roughness measurements

This paper is a study of the applicability of fracture surface roughness measurements to identify the crack propagation mode in nickel base superalloys (transgranular, intergranular or mixed). The results obtained indicated that this technique can be used as an alternative or as a complement to standard fractography. The best roughness parameters for identifying the crack propagation mode are average roughness, mean roughness depth and mean height profile peak (a, Rz and Rpm). Analysis of roughness spectra in terms of the frequency range showed that the amplitude values of the profiles with wavelengths identical to the grain size are significant when propagation is intergranular. The roughness measurements were used to study the influence of loading frequency and stress state on the fatigue crack propagation mode occurring in Inconel 718 tested at 600°C.

Fatigue life predictions in polymer particle composites

Abstract This paper presents a study on fatigue life predictions in three polymer particle composites with different volume fractions of filler and different particle sizes. Central hole notched specimens were analysed using a fracture mechanics approach. A solution for the stress intensity factor of corner cracks at a hole was obtained using the finite element method and considering quarter-circular and quarter-elliptical cracks of different sizes. The solution was compared with a literature solution and significant differences were found. Fatigue crack propagation tests were performed at room temperature and constant loading amplitude, for stress ratios R=0 and R=−0.75. Finally, fatigue lives, crack shape evolution and final crack length were predicted assuming an initial crack size and considering that the crack maintains a quarter-elliptical shape. The comparison with experimental fatigue lives indicated the presence of initial defects larger than the silica particles; however, these large sizes can be explained by the residual stresses measured near the hole.

Stress intensity factor calculation based on the work of external forces

The external forces method is a numerical method for K calculation based on the finite element method. It uses the work of the external forces W for the calculation of the energy release rate and is particularly advantageous when that forces are applied far from the crack front. The method was applied to a corner crack geometry with the objective of studying its accuracy. Good results were obtained for a wide range of virtual crack displacements (0.03% < Δa/a < 6%) considering 4 values of W along with a polynomial regression of order 3. For that choice of parameters the inaccuracy of K is mainly due to FEM errors. A great sensitivity of K to FEM errors was observed, however accurate values of K were obtained, with errors lower than 2 percent. So, the use of the external forces method for the calculation of K is recommended, considering its simplicity and accuracy.

Determination of Elastic Properties by Resonant Technique: A Sensitivity Analysis

The in-plane elastic properties of materials can be determined using an experimental-numerical procedure based on the resonant frequencies of thin beams and plates. The objective of this paper is to study the accuracy of the material constants obtained with this technique. The procedure is presented and the parameters affecting its accuracy are identified. Specimens of epoxy reinforced with carbon fibers and 6082-T6 aluminum alloy were produced and experimental work was developed to obtain resonant frequencies in free-free boundary conditions. A numerical procedure based on FEM was developed replicating the experimental procedure and was used for a sensitivity analysis on the numerical and physical parameters. A great sensitivity relative to geometry was found, which emphasizes the need for ideally shaped specimens and accurate measurements. The influence of elastic properties on resonant frequencies is comparatively lower and varies quite considerably with geometry. The accuracy of experimental frequencies and specific mass was found to have a great impact on material constants.

Influence of Superposition Length on Transverse Impact Response of Single-Strap Adhesive Joints

Adhesive joints are usually designed to carry in-plane loads, but in many cases they are also prone to transverse loading. On the other hand, the impact response of adhesive joints has received limited attention compared to quasi-static loading. Therefore, the present paper aims to study the influence of superposition length on transverse impact response of single-strap adhesive joints. For this purpose, low-velocity impact tests were performed using a drop weight-testing machine with a hemispherical impactor falling at the center of a bi-clamped specimen. The specimens were manufactured using Docol 1000 high-elastic limit steel, with 1.5 mm of thickness, and an Araldite® 420 A/B adhesive (Huntsman Advanced Materials, Everberg, Belgium). The collapse thresholds obtained were 61.6 J, 75.1 J, and 77.5 J, respectively, for adhesive joints with gap length of ℓo = 0, 10, and 20 mm. An adhesive fracture occurred for the three geometries and the cracks initiated at the corner of the joint where the deflection is higher. Joints with higher ℓo have higher impact energies, despite the lower bonding area, as consequence of the lower local deformation. A numerical study was developed and the zero gap (ℓo = 0) gives maximum peel stress.

Assessment of the fatigue life of aluminium spot-welded and weld-bonded joints

In modern machinery and automobile structures weight reduction and increased durability are the main issues in design. In these applications, lap welded and/or bonded joints are widely used; therefore, tools are needed to accurately predict their fatigue life. This paper is concerned with the fatigue strength of single lap joints formed with thin plates of 6082-T6 aluminium alloy using a high strength two-component epoxy adhesive (Araldite 420 A/B from Hunstman). Experimental S–N curves were obtained for resistance spot-welded and weld-bonded lap joints. The fatigue lives of weld-bonded joints were significantly higher than those of resistance spot-welding joints. In addition, fatigue lives were predicted with Morrows modified Manson–Coffin (M/M–C) and the Smith–Watson–Topper (S–W–T) damage equations. Elastic–plastic numerical models were developed, replicating the experimental work, in order to obtain local stress and strain fields. An acceptable agreement was obtained between the numerical predictions and the experimental results. The M/M–C damage equation diverged from experimental results for relatively long fatigue lives, while the S–W–T equation gave good predictions for all fatigue lives.

Extent of the Surface Region in Notched Middle Cracked Tension Specimens

This article aims at evaluating the extent of the surface region in notched Middle Cracked Tension specimens. Firstly, a fully automatic fatigue crack growth technique is developed to obtain stable crack shapes. After that, the stress triaxiality along the crack front is evaluated for different notch shapes. Then, objective criteria are defined to quantify the extent of the surface region from the stress triaxiality data collected. Next, the extent of the surface region is related to the elastic stress concentration factor of the uncracked geometry by a linear relationship. Finally, empirical two-constant equations able to evaluate the extent of the surface region from the thickness, notch radius, notch depth and elastic stress concentration factor are formulated.

Using a standard specimen for crack propagation under plain strain conditions

Purpose – Stress state has a major influence on different phenomena, namely those involving diffusion and plastic deformation (like crack closure and high‐temperature fatigue crack growth, void formation or ductile fracture). The isolation of plane stress and plane strain states is crucial in fundamental studies of material behavior. The isolation of plane stress state is achieved with thin specimens, whilst the isolation of plane strain state is usually done increasing the thickness or introducing lateral grooves. The purpose of this paper is to propose a specimen geometry able to isolate the plane strain state, based on the standard M(T) geometry.Design/methodology/approach – A numerical study was carried out aiming at obtaining a stress triaxiality parameter, h, as a function of different geometrical features of the specimen, such as the notch radius, notch depth and specimen thickness.Findings – Results show that a pure plane strain state is achievable (i.e. 97 percent of specimen thickness has h>0.97...