Effect of transformation volume contraction on the toughness of superelastic shape memory alloys

Shape memory alloys (SMAs) exhibit two very important properties: shape memory phenomenon and superelastic deformation due to intrinsic thermoelastic martensitic transformation. To fully exploit the potential of SMAs in developing functional structures or smart structures in mechanical and biomechanical engineering, it is important to understand and quantify the failure mechanisms of SMAs. This paper presents a theoretical study of the effect of phase-transformation-induced volume contraction on the fracture properties of superelastic SMAs. A simple model is employed to account for the forward and reverse phase transformation with pure volume change, which is then applied to numerically study the transformation field near the tip of a tensile crack. The results reveal that during steady-state crack propagation, the transformation zone extends ahead of the crack tip due to forward transformation while partial reverse transformation occurs in the wake. Furthermore, as a result of the volume contraction associated with the austenite-to-martensite transformation, the induced stress-intensity factor is positive. This is in stark contrast with the negative stress-intensity factor achieved in zirconia ceramics, which undergoes volume expansion during phase transformation. The reverse transformation has been found to have a negligible effect on the induced stress-intensity factor. An important implication of the present results is that the phase transformation with volume contraction in SMAs tends to reduce their fracture resistance and increase the brittleness.

Characterization and analysis of delamination fracture and nanocreep properties in carbon epoxy composites manufactured by different processes

Delamination resistance and nanocreep properties of 2/2 twill weave carbon epoxy composites manufactured by hot press, autoclave, and Quickstep^TM process are characterized and analyzed. Quickstep is a fluid filled, balanced pressure heated floating mold technology, which is recently developed in Perth, Western Australia for the manufacture of advanced composite components. Mode I and Mode II interlaminar fracture toughness tests, and nanoindentation creep tests on matrix materials show that the fast ramp rate of the Quickstep process provides mechanical properties comparable to that of autoclave at a lower cost for composite manufacturing. Low viscosity during ramping process and good fiber wetting are believed to be the reasons that this process produces composites with high delamination and creep-resistant properties. Nanocreep properties are analyzed using a Kelvin–Voigt model.

Manufacturing influence on the delamination fracture behavior of the T800H/3900-2 carbon fiber reinforced polymer composites

'Torayca' T800H/3900-2 is the first material qualified on Boeing Material Specification (BMS 8-276) which utilizes the thermoplastic-particulate interlayer toughening technology. Two manufacturing processes, the autoclave process and the fast heating rated Quickstep™ process, were employed to cure this material. The Quickstep process is a unique composite production technology which utilizes the fast heat transfer rate of fluid to heat and cure polymer composite components. The manufacturing influence on the mode I delamination fracture toughness of laminates was investigated by performing double cantilever beam tests. The composite specimens fabricated by two processes exhibited dissimilar delamination resistance curves (R-curves) under mode I loading. The initial value of fracture toughness GIC-INIT was 564 J/m2 for the autoclave specimens and 527 J/m2 for the Quickstep specimens. However, the average propagation fracture toughness GIC-PROP was 783 J/m2 for the Quickstep specimens, which was 2.6 times of that for the autoclave specimens. The mechanism of fracture occurred during delamination was studied under scanning electron microscope (SEM). Three types of fracture were observed: the interlayer fracture, the interface fracture, and the intralaminar fracture. These three types of fracture played different roles in affecting the delamination resistance curves during the crack growth. More fiber bridging was found in the process of delamination for the Quickstep specimens. Better fiber/matrix adhesion was found in the Quickstep specimens by conducting indentation-debond tests.

Predicting the resistance of fired clay bricks to salt attack

The salt attack of Fired Clay Bricks (FCBs) causes surface damage that is aesthetically displeasing and eventually leads to structural damage. Methods for determining the resistances of FCBs to salt weathering have mainly tried to simulate the process by using accelerating aging tests. Most research in this area has concentrated on the types of salt that can cause damage and the damage that occurs during accelerated aging tests. This approach has lead to the use of accelerated aging tests as standard methods for determining resistance. Recently, it has been acknowledged that are not the most reliable way to determine salt attack resistance for all FCBs in all environments. Few researchers have examined FCBs with the aim of determining which material and mechanical properties make a FCB resistant to salt attack. The aim of this study was to identify the properties that were significant to the resistance of FCBs to salt attack. In doing so, this study aids in the development of a better test method to assess the resistance of FCBs to salt attack. The current Australian Standard accelerated aging test was used to measure the resistance of eight FCBs to salt attack using sodium sulfate and sodium chloride. The results of these tests were compared to the water absorption properties and the total porosity of FCBs. An empirical relationship was developed between the twenty-four-hour water absorption value and the number of cycles to failure from sodium sulfate tests. The volume of sodium chloride solution was found to be proportional to the total porosity of FCBs in this study. A phenomenological discussion of results led to a new mechanism being presented to explain the derivation of stress during salt crystallisation of anhydrous and hydratable salts. The mechanical properties of FCBs were measured using compression tests. FCBs were analysed as cellular materials to find that the elastic modules of FCBs was equivalent for extruded FCBs that had been fired a similar temperatures and time. Two samples were found to have significantly different elastic moduli of the solid microstructure. One of these samples was a pressed brick that was stiffer due to the extra bond that is obtained during sintering a closely packed structure. The other sample was an extruded brick that had more firing temperature and time compared with the other samples in this study. A non-destructive method was used to measure the indentation hardness and indentation stress-strain properties of FCBs. The indentation hardness of FCBs was found to be proportional to the uniaxial compression strength. In addition, the indentation hardness had a better linear correlation to the total porosity of FCBs except for those samples that had different elastic moduli of the solid microstructure. Fractography of exfoliated particles during salt cycle tests and compression tests showed there was a similar pattern of fracture during each failure. The results indicate there were inherent properties of a FCB that determines the size and shape of fractured particles during salt attack. The microstructural variables that determined the fracture properties of FCBs were shown to be important variables to include in future models that attempt to estimate the resistance of FCBs to salt attack.

Deformation and fracture characteristics of ferrite/bainite dual-phase steels

The deformation and fracture characteristics of a low carbon Si–Mn steel with ferrite/bainite dual–phase structure were investigated by thermo–mechanical controlled process (TMCP). The results showed that the curves of the instantaneous work–hardening factor n* value versus true strain ε are made up with three stages during uniform plastic deformation: n* value is relatively higher at stage I, decreases slowly with ε in stage II, and then decreases quickly with ε in stage III. Compared tothe equiaxed ferrite/bainite dual–phase steel, the quasi–polygonal ferrite/bainite dual–phase steel shows higher tensile strength and n*value in the low strain region. The voids or micro–cracks formed not only at ferrite–bainite interfaces but also within ferrite grains in the necked region, which can improve the property of resistance to crack propagation by reducing local stress concentration of the crack tips.

Fall and fracture risk in sarcopenia and dynapenia with and without obesity: the role of lifestyle interventions

Due to their differing etiologies and consequences, it has been proposed that the term "sarcopenia" should revert to its original definition of age-related muscle mass declines, with a separate term, "dynapenia", describing muscle strength and function declines. There is increasing interest in the interactions of sarcopenia and dynapenia with obesity. Despite an apparent protective effect of obesity on fracture, increased adiposity may compromise bone health, and the presence of sarcopenia and/or dynapenia ("sarcopenic obesity" and "dynapenic obesity") may exacerbate the risk of falls and fracture in obese older adults. Weight loss interventions are likely to be beneficial for older adults with sarcopenic and dynapenic obesity but may result in further reductions in muscle and bone health. The addition of exercise including progressive resistance training and nutritional strategies, including protein and vitamin D supplementation, may optimise body composition and muscle function outcomes thereby reducing falls and fracture risk in this population.

Prediction of mode I delamination resistance of z-pinned laminates using the embedded finite element technique

A new finite modelling approach is presented to analyse the mode I delamination fracture toughness of z-pinned laminates using the computationally efficient embedded element technique. In the FE model,each z-pin is represented by a single one-dimensional truss element that is embedded within the laminate. Each truss is given the material, geometric and spatial properties associated with the global crackbridging traction response of a z-pin in the laminate; this simplification provides a computationally efficient and flexible model where pin elements are independent of the underlying structural mesh for thelaminate. The accuracy of the FE modelling approach is assessed using mode I interlaminar fracture toughness data for a carbon-epoxy laminate reinforced with z-pins made of copper, titanium or stainless steel. The model is able to predict with good accuracy the crack growth resistance curves and fracture toughness properties for the different types of z-pinned laminate.