Residual Strength Data Research Articles

Residual strength of GFRP at high-cycle fatigue

Degradation of the residual strength of a glass-fiber-reinforced polyester composite of a lay-up typical of the wind rotor blade material is studied at low-cycle fatigue. A gradual reduction of the residual strength is observed as expected for GRP, accompanied by an increasing scatter of strength. The residual strength model based on the strength-life equal rank assumption yields an accurate approximation of experimental data. The strength reduction at a stress level corresponding to high-cycle fatigue (N>10 6 cycles) appears to correlate well with the test results at higher stress levels, which indicates that the strength degradation at the design stress level can be evaluated using low-cycle tests. Assuming that the parameters of the strength degradation model do not depend on the applied stress level, the residual strength data obtained in low stress level tests of comparatively short duration can be used to estimate the average fatigue life at the same stress thus reducing the total test time.

Strength degradation during fatigue of unidirectional and cross-ply SCS-6/Ti-15-3 composites

The majority of previous fatigue studies on Titanium Matrix Composites (TMCs) have investigated fatigue life where specimens are cycled until failure. Although fatigue life is very important to designers, it is not the only cycle-dependent criterion used to characterize the material. Another behavior required to evaluate the material is the residual strength degradation due to cyclic loading. To date, very few investigations have been completed to determine this behavior in TMCs. Of those investigations, none has explicitly determined the dependence of residual strength reduction on laminate orientation. To study this, the residual strength behaviors of SCS-6/Ti-15-3 with laminate orientations of [0] 8 and [0/90] 2s were investigated after being fatigued at 427°C with different frequencies, R-ratios and control modes. Results show that residual strength degradation is primarily dependent on laminate orientation and percent of cyclic life. Different test parameters mainly affect residual strength degradation through the corresponding changes in cyclic life. Micromechanical analysis reveals that the laminate dependence is due to differences in damage morphologies and there is no single parameter, such as axial fiber stress, that can correlate all the residual strength data from the different fatigue conditions.

Residual Strength Degradation of Cross-Ply Titanium Matrix Composite Due to Elevated Temperature

This study investigates the residual strength degradation of cross-ply, SCS-6/Ti-15-3 titanium matrix composite (TMC) due to elevated temperature fatigue. To accomplish this, several specimens were cycled at 427°C to certain fractions of their expected fatigue lives, then monotonically loaded to failure. Fatigue tests were conducted in both load-control mode with a load ratio (Rσ) of 0.05 and strain-control mode with a strain ratio (Re) of − 1. Maximum stresses in the load-controlled tests were 300 and 450 MPa with frequencies of 5 and 10 Hz, respectively. The strain amplitudes ranged from 0.25 to 0.4% in the strain-controlled tests. Rather than being conducted at a constant frequency, these tests were performed at a strain rate of 0.1% per s. Various mechanical responses during cycling are discussed and compared for the load-controlled and strain-controlled tests. After failure, specimens were sectioned and studied using both optical and scanning electron microscopy. Residual strength data were then correlated to the amount and type of each damage mechanism. It was found that the residual strength degradation in cross-ply laminate, when exposed to different fatigue conditions, correlates together with the fraction of the cyclic life left in the composite.

Progressive Damage and Residual Strength of a Carbon Fibre Reinforced Metal Laminate

An experimental study on the residual strength behaviour of a Carbon Fibre Reinforced Metal Laminate (FRML) was conducted. Mechanical tests were carried out to determine the laminate tensile property profiles and residual strength of notched coupon laminates. The effect of notch geometry was studied for both centre cracks and centre circular holes with notch sizes ranging from 10 to 40 mm. The reduction in residual strength for the saw-cut specimens was greater than that for the circular hole specimens. C-scan and optical microscopy associated with a chemical etching technique were used to investigate progressive damage and failure mechanisms in these laminates. Damage modes such as delamination, crack growth and fibre breakage were observed. Stable crack growth was seen in both the aluminium and composite layers for the saw-cut specimens, but no visible crack growth in the aluminium layers in the circular hole specimens was found although cracking in the composite layer occurred. It is concluded that delamination ahead of the notch tip is attributed to cracking of the composite layer. The average stress criterion developed for fibre reinforced composites was applied to correlate residual strength data and good agreement was obtained.

Thermal Shock Behavior of Silicon Nitride Flexure Beam Specimens with Indentation Cracks

The experimental results of thermal shock testing of silicon nitride flexure beam specimens containing indentation cracks are presented. The thermal stress induced by water quenching is much greater in the transverse direction than in the longitudinal direction, resulting in an insensitivity of residual bend strength to temperature differences up to 580°C. This result indicates that a flexure beam configuration is not an appropriate geometry for thermal shock testing when thermal shock behavior is to be evaluated from residual bend strength data.

Discussion on ‘Large landslides in London Clay at Herne Bay, Kent’ by E.N. Bromhead

It is gratifying to the author that his paper has stimulated such interest, and he wishes to thank the discussers for their contributions. In reply to Professor Hutchinson, it is agreed that the summary plot of residual strength data presented as Fig. 10 of the original paper was not a complete ‘state-of-the-art’ picture, although, in retrospect, the phrasing of the summary would tend to suggest that it should have been. This (apparent) claim is a relic of the first draft of the paper, which as it evolved, had to be made shorter, and material not central to the case shear, is accepted. In the context of the original paper, however , the phrasing is correct, since the ‘blue’ London clay is generally so much more deeply buried than the ‘brown’, and is subject to much higher levels of effective stress. Dunbaven describes an interesting experiment. However, the author would like to point out that the ‘weak layer’ in the Herne Bay case is not the Oldhaven Beds, but a basal layer in the London Clay. Failure was in every case confined to the London Clay. This need not alter the general conclusions of the discusser, of course. At Herne Bay, the underlying Oldhaven Beds are stronger than the London Clay, and the shape of the field failure surfaces appear to be controlled to a large extent by the depth of the Oldhaven Beds/London Clay interface below sea level. It would be extremely interesting to see whether this effect could be

Nondestructive Evaluation of Strength Degradation in Glass-Reinforced Plastics as a Result of Environmental Effects

Abstract Destructive and nondestructive tests were conducted on glass-reinforced plastic specimens after degradation by immersion in hot water for various periods. Ultrasonic attenuation was found to correlate well with normalized residual strength data obtained from destructive tests.

A New Ring Shear Apparatus and Its Application to the Measurement of Residual Strength

Synopsis This Paper describes both the design and principles of operation of a new ring shear apparatus, and its application to the measurement of the residual strength of undisturbed and remoulded samples. As the test results differ significantly from those obtained in multiple reversal direct shear box tests, a critical examination is made of all possible sources of error in both measurement and interpretation. The results of tests on five soils, blue London Clay, brown London Clay, Weald Clay, a Norwegian clay and the Cucaracha Shale from Panama, are presented and discussed in relation to other published residual strength data from tests on these materials. The factors controlling the brittleness of soils tested under drained conditions are examined. Cet article décrit à la fois la conception et les principes d'opération d'un nouvel appareil de mesure du cisaillement circulaire par torsion, et de son application à la mesure des résistances résiduelles d'échantillons non pertubés et remoulés. Comme les résultats des essais diffèrent d'une manière significative de ceux obtenus lors d'essais multiples effectés dans une boîte de cisaillement renversé direct, on donne une analyse critique de toutes les sources d'erreurs possibles à la fois dans les mesures et dans leur interprétation. Les résultats d'essais sur cina sols: arsdle bleu de Londres, argile brun de Londrks, argile de Weald, argile Norvégien et schiste Cucaracha de Panama, sont présentés et discutés par rapport aux autres résultats d'essais de résistance résiduelle publiés sur ces matériaux. On examine les facteurs qui contrôlent la fragilité de ces sols dans des conditions de drainage.