Fabrication and curing conditions effects on the fatigue behavior of a structural adhesive

This paper investigates the effects of degassing, curing and post-curing conditions on the tensile quasi-static and fatigue behavior of a cold-curing structural epoxy adhesive. Quasi-static experiments performed at standardized and high displacement rates similar to those imposed to the material under fatigue loading, in order to derive comparable results from both loadings, and tensile-tensile fatigue tests at a fixed stress ratio, were carried out. Digital image correlation, video extensometer, thermal camera, digital camera and microscope were used to record the evolution of the strain and temperature, and assess the damage evolution during and after loading. The results shown that degassing can improve the material behavior when curing at ambient temperatures, since it assists the removal of several small voids from the bulk adhesive. However, when curing the material at high temperatures, immediately after mixing, the degassing does not offer any advantage, and the material's long-term performance decreases.

Load history effects in fibre-reinforced polymer composite materials

Abdolvahid Movahedirad

Today, it is widely accepted that fatigue is one of the most common failure mechanisms in structural components, and pure static failure is rarely observed. Engineering structures are subjected to complex mechanical fatigue loading histories, which cause damage that eventually leads to functional and structural deterioration. One reason for this complexity is that the cyclic loading is accompanied by time-dependent deformations including creep and recovery. Fiber-reinforced polymer (FRP) composite materials are susceptible to time-dependent deformation, due to the viscoelastic nature of their matrix. Therefore, fatigue-creep-recovery interactions can play a role in the overall deformation behavior of FRP composite materials. The limited experimental investigations of FRP composite materials showed considerable effects of time-dependent phenomena on the fatigue behavior. However, no systematic analysis to clearly determine the effect of creep and recovery on developed damage mechanisms of FRP composites has yet been conducted. The aim of this research is to understand the fatigue behavior of glass fiber-reinforced composite (GFRP) materials and establish a reliable methodology to analytically/numerically model the fatigue behavior of FRP composite materials. The fatigue behavior of (±45)2S angle-ply glass/epoxy laminated composite at different stress levels has been investigated in this thesis. It was observed that cyclic-dependent parameters (cyclic creep, fatigue stiffness, and hysteresis loop area) as well as damage development were a function of applied stress levels. In tension-tension continuous fatigue, by decreasing the stress ratio, the damage became more severe, fatigue stiffness exhibited greater changes, the hysteresis loop area became larger and a higher and more uneven self-generated temperature was recorded. The effect of loading interruption on fatigue behavior was studied by subjecting the specimens to tension-tension constant amplitude fatigue loading interrupted at ¿max by creep intervals. At high stress levels, the fatigue life was improved when hold time was short as a result of the fiber realignment caused by the creep strain, while longer hold time caused considerable creep damage and therefore premature fatigue failure of the specimens. The cyclic loading was also interrupted at zero stress level. The stiffness degradation rates and hysteresis loop area decreased during the cyclic loading phase, while specimen recovery occurred after each stress removal. It was observed that specimens loaded under interrupted fatigue exhibited longer fatigue lives than those continuously loaded at high stress levels as a result of partial fatigue stiffness restoration and crack blunting. It was concluded that fatigue design allowables based on continuous fatigue could be conservative. A model based on the theory of viscoelasticity was developed to simulate specimen recovery in viscoelastic materials and predict their cyclic-dependent mechanical properties. The developed model predicted the cyclic-dependent parameters well - the fatigue stiffness, hysteresis loop area, cyclic creep, storage and loss moduli as well as tan(¿).

EPFL2019

Damage propagation and fracture in highly reinforced particulate metal matrix composites

Aude Hauert

This study contributes to our understanding of the damage evolution and fracture behaviour of two-phase materials that combine brittle particles with a ductile matrix. We focus on model composites roughly half-ceramic/half-metal that are produced in-house by infiltrating ceramic powder beds with liquid pure aluminium or aluminium alloy. Like many other materials, these composites exhibit two modes of tensile failure; they fail either by tensile instability, or alternatively they break prematurely, in a brittle fashion. A micromechanical model is established that links damage build-up and composite fracture. This analytical model predicts the tensile curve and the strength of particulate composites that undergo damage in the form of statistical particle fracture. Similarly to models for continuous-fibre reinforced composites, two extreme modes of load sharing (a fully local and a global load sharing mode) are accounted for, which yield a lower and an upper bound for the failure stress. Under local load sharing, the model predicts either of two different failure modes, i.e. brittle failure or failure by the onset of tensile instability, whereas global load sharing induces the latter failure mode only. We show that this model captures the transition from failure by tensile instability to brittle failure that occurs with an increase of matrix strength or with a decrease of particle strength. The local load redistribution upon particle fracture and avalanche-like growth of damage can thus explain the premature brittle failure that is observed in some composites. The direct assessment of the particle strength properties not being possible, these are "back-calculated" using the aforementioned model and tensile tests (with periodic unload-reload cycles to monitor the evolution of Young's modulus) conducted on composites made with different matrices. Using these particle properties, we show in particular that composite strength predicted under the assumption of local load sharing is in good agreement with experiment, and that fracture of particles embedded in a soft matrix is not only governed by the average particle stress, but also by the composite plastic strain. Moreover, tests of axisymmetric notched specimens indicate that damage possibly also depends on stress triaxiality.

EPFL2009

Maltose as an Ecofriendly Modifier of the Buried Interface for Efficient and Stable Inverted Perovskite Solar Cells

Yi Ding, Hong Zhang

A new concept of modifying the buried interface with ecofriendly material to improve the photovoltaic performance of inverted perovskite solar cells (PSCs) is proposed. A low-cost and ecofriendly maltose with multihydroxyl groups is facilely chosen for the proof of our concept. The maltose modification not only improves the hole mobility of hole transporting layer, but also regulates the formation of a high-quality perovskite film with a low density of gain boundary and trap states. The resulting inverted PSCs show a champion power conversion efficiency of 20.65% with negligible hysteresis, accompanied by enhanced thermal and operational stability. This work shows that buried interface engineering with ecofriendly materials opens a new avenue to further improve the efficiency and stability of sustainable PSCs.