An investigation into variations in roughnessinduced crack closure in high strength aircraft alloys under fatigue loading

In recent decades the use of structural adhesive joints in the aerospace industry has increased considerably thanks to their high strength-to-weight ratio, low stress concentration and capacity to join different adherends. There is increasing interest in damage due to low-velocity impacts produced in adhesively bonded components and structures by vibrating loads. This type of loading is known as impact fatigue. The main aim of this paper is to investigate damage evolution in adhesive joints subjected to impact-fatigue and to compare this with damage evolution in standard fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). In this work, adhesively bonded lap joints were subjected to multiple tensile impacts tensile and it was seen that this type of loading was extremely damaging compared to standard fatigue. A number of methods of studying damage evolution in bonded joints subjected to fatigue and impact fatigue loading have been investigated and various parameters have been used to characterise these processes. Two modifications of the accumulated time-stress model [1-4] are proposed and it is shown that both models provide a suitable characterization of impact-fatigue in bonded joints.

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The effect of mean axial and torsional stresses on the fatigue strength of 34CrNiMo6 high strength steel

MATEC Web of Conferences ◽

10.1051/matecconf/201930016004 ◽

2019 ◽

Vol 300 ◽

pp. 16004

Author(s):

Luis Pallarés-Santasmartas ◽

Joseba Albizuri ◽

Nelson Leguinagoicoa ◽

Nicolas Saintier ◽

Jonathan Merzeau

Keyword(s):

High Strength Steel ◽

Fatigue Behavior ◽

High Strength ◽

Fatigue Loading ◽

Experimental Results ◽

Shear Stresses ◽

Mean Stress ◽

Theoretical Predictions ◽

Loading Case ◽

Fracture Appearance

The present study consists of a theoretical, experimental and fractographic investigation of the effect of superimposed static axial and shear stresses on the high cycle fatigue behavior of a 34CrNiMo6 high strength steel in quenched and tempered condition (UTS = 1210 MPa), commonly employed in highly stressed mechanical components. The Haigh diagrams for the axial and torsional cases under different values of mean stress were obtained. In both cases, experimental results showed that increasing the mean stress gradually reduces the stress amplitude that the material can withstand without failure. The results of the present tests are compared with the theoretical predictions from Findley, based on the maximum damage critical plane; and the methods of Marin and Froustey, which are energetic based criterions. Froustey’s method shows the best agreement with experimental results for torsional fatigue with mean shear stresses, showing a non-conservative behaviour for the axial fatigue loading case. Macro-analyses and micro-analyses of specimen fracture appearance were conducted in order to obtain the fracture characteristics for different mean shear stress values under torsion fatigue loading.

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Damage Characterization of Nano-Interleaved CFRP under Static and Fatigue Loading

Fibers ◽

10.3390/fib7020013 ◽

2019 ◽

Vol 7 (2) ◽

pp. 13 ◽

Cited By ~ 4

Author(s):

Mohamad Fotouhi ◽

Cristiano Fragassa ◽

Sakineh Fotouhi ◽

Hamed Saghafi ◽

Giangiacomo Minak

Keyword(s):

Acoustic Emissions ◽

Weight Ratio ◽

Laminated Composite ◽

High Strength ◽

Fatigue Loading ◽

Fatigue Tests ◽

Limiting Factor ◽

Delamination Toughness ◽

Delamination Resistance ◽

Laminated Composite Materials

The use of high strength-to-weight ratio-laminated fiber-reinforced composites is emerging in engineering sectors such as aerospace, marine and automotive to improve productivity. Nevertheless, delamination between the layers is a limiting factor for the wider application of laminated composites, as it reduces the stiffness and strengths of the structure. Previous studies have proven that ply interface nanofibrous fiber reinforcement has an effective influence on delamination resistance of laminated composite materials. This paper aims to investigate the effect of nanofiber ply interface reinforcement on mode I properties and failure responses when being subjected to static and fatigue loadings. For this purpose, virgin and nanomodified woven laminates were subjected to Double Cantilever Beam (DCB) experiments. Static and fatigue tests were performed in accordance with standards and the Acoustic Emissions (AE) were acquired during these tests. The results showed not only a 130% increase of delamination toughness for nanomodified specimens in the case of static loads, but also a relevant crack growth resistance in the case of fatigue loads. In addition, the AE permitted to relate these improvements to the different failure mechanisms occurring.

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