Magnus Burman

Fatigue of foam core sandwich beams—1: undamaged specimens

The present paper addresses the fatigue characteristics of two cellular foam core materials as used in load carrying sandwich structures. The fatigue loading studied is a constant shear stress which corresponds to the main type of loading that the core in a sandwich structure exhibits. Based on results from testing stress-life curves are presented for a number of stress ratios (R) and are fitted to a simple two-parameter Weibull function. The influence of the R-value is emphasized and standard Haigh diagrams are constructed. It is seen that the fatigue behaviour of the core materials investigated herein well can be described in manners similar to that of classical metal fatigue. A thorough investigation is performed on the behaviour of fatigue crack formation and growth.

Fatigue of foam core sandwich beams—2: effect of initial damage

This paper addresses the influence of sub-surface core damage in sandwich beams subjected to fatigue loading. Using results from static and fatigue tests of undamaged beams a model for the prediction of the fatigue life of damaged beams are proposed. Sandwich beams subjected to transverse loading inducing a shear stress field in the core material with two typical sub-surface damages, a flawed butt-joint and a interface disbond, are investigated. The fatigue life of damaged sandwich beams is investigated through numerous tests which are presented in standard S/N diagrams. Curve fitting to the experimental data is performed using a two parameter Weibull function. In order to capture the effects of crack opening and crack closure loads and their relations two different loading ratios are employed. Also, a modified stress life function is proposed using notch factors to predict the fatigue life of damaged beams. The notch factor is calculated as the ratio of the failure loads of damaged to undamaged beams obtained from static tests. The results indicate that the proposed simple approach can be used to accurately predict the reduction in the fatigue life due to the inflicted damages. Also, an alternative and very general method based on point stress criterion is proposed which seems rather promising. The fracture paths are monitored and the fracture initiation in fatigue is discussed.

Fatigue of closed cell foams

This paper deals with fatigue of closed cell foams. The main idea is to use a few simple tests to predict the tension-tension fatigue properties of foams. The required testing consists of crack propagation rate measurements and one tension-tension fatigue test performed at yield stress for the foam. This data can then be combined to construct a synthetic S-N Curve for the foam. Tests on three densities of Divinycell H-grade foam are performed and the results Support this approach. Some preliminary results from two densities of Rollacell WF-grade are given as well. Static properties of foams scale with relative density and once this scaling can be obtained through various static tests and the same scaling appears to be valid for both crack propagation rates and fatigue properties of foams. The implication of this is that once the fatigue behaviour of one relative density foam is established, one can predict the fatigue behaviour of all other relative density foams within the same class of materials.

FRACTURE INITIATION IN FOAM-CORE SANDWICH STRUCTURES DUE TO SINGULAR STRESSES AT CORNERS OF FLAWED BUTT JOINTS

Defective butt joints in sandwich structures create the presence of 90° corners from which fracture is very likely to be initiated due to the stress intensification caused by the geometric and material discontinuity at the corner. In this article the singular stress field, both the singularity and the stress intensity factors, at the bimaterial comer are computed, and the field amplitudes obtained are then used to predict the onset of damage growth in sandwich beams with bimaterUti corners of different material combinations. A parametric study is used to develop an empirical relation between stress intensity factors and geometric and material parameters. Static strength predictions are compared with tests of sandwich beams with simulated defective butt joints, and the correlation is satisfactory. It is concluded that great care must taken in manufacturing to avoid flawed butt joints, which have a large influence on the load-bearing capacity.

Fatigue of Undamaged and Damaged Honeycomb Sandwich Beams

In this paper, the fatigue characteristics of shear loaded honeycomb sandwich beams are experimentally investigated through constant load amplitude tests. Two honeycomb sandwich configurations are test differentiated only by the orientation of the honeycomb core. The fatigue test results are presented in standard S-N diagrams with a best curve fit to the experimental data. Furthermore, the effect of sub-surface damages, such as flawed butt-joints and interface disbonds is investigated. Two loading ratios are applied for all fatigue test series. On the basis of the difference in these test results, the effect of crack opening and closing is discussed. An empirical prediction model for the fatigue life of damaged beams is proposed based on the results from both the fatigue tests of undamaged beams and the static strength of the damaged specimens. The prediction model is verified against the fatigue test results of the damaged beams. The fatigue fracture initiation and propagation for undamaged and damaged honeycomb core sandwich beams are also discussed.

Characterization of Ductile Core Materials

This article describes an experimental investigation of characterization methods for ductile core materials. Full-field optical strain measurement methods are used to determine the strain distributions in standard testing methods such as block shear and four-point beam testing, particularly for highly ductile cores subjected to large deformations. The results show that the stress and strain fields in both block shear and sandwich beam tests are very different to those assumed by the testing standards. The test methods result in complex post yield states of stress in the core materials, meaning the core shear strength and ultimate shear strain should not be calculated by classical methods in the post yield region.

Fatigue of closed-cell foams in compression:

This article deals with fatigue of closed-cell foams under compression loading. Testing is performed on three densities of Divinycell H-grade and three densities of Rohacell WF-grade foam under cyclic compression loading. The fatigue failure is in all cases described as localized crushing of cell layers. The two main observations from this are that the slopes of the stress—life curves are almost all different, both between the foam types and relative density. Secondly, the stress—life relation slopes are considerably flatter than corresponding data in tension. For one particular foam grade, the stress—life relation is almost flat.

Comparative Life Cycle Assessment of the hull of a high-speed craft

A comparative Life Cycle Assessment is performed for different structural material concepts on a 24-m-long high-speed patrol craft. The study is comparative and determines the differences in and sensitivities to environmental impact, especially in relation to the total impact of fuel burn for the different material concepts. The material concepts are aluminium and various composite combinations consisting of glass fibre and carbon fibre with vinyl ester resin both as single skins and as sandwich with a Divinycell foam core. Commercially available standard Life Cycle Assessment software is used for the Life Cycle Assessment calculations. The study shows that regardless of hull material concept, the environmental impact is dominated by the operational phase due to relatively large fuel consumption. In the operational phase, the lightest carbon-fibre concept is shown to have least environmental impact. Considering the manufacturing phase exclusively for the different hull concepts, it is concluded that the manufacturing of the aluminium hull has a somewhat larger environment impact for the majority of Life Cycle Assessment impact categories in comparison to the different composite hulls. The significant impact on the marine and the fresh water aquatic ecotoxicity originates from the aluminium raw material excavation and manufacturing processes. It is shown that the lightest hull, the carbon-fibre sandwich concept, with a 50% structural weight reduction compared to the aluminium design, can be utilized to reduce the fuel consumption by 20% (775 ton of diesel) over the lifetime with significant impact on the dominating environmental aspects considered herein, abiotic depletion, global warming and acidification.