The Effect of Foam Core Density on the Flexural and Axial Behaviour of Sandwich Panels of Varying Slenderness with Natural Flax-FRP and Glass-FRP Composite Skins

This study investigates the effect of foam core density and skin type on the behaviour of sandwich panels as structural beams tested in four-point bending and axially compressed columns of varying slenderness and skin thickness. Bio-composite unidirectional flax fibre-reinforced polymer (FFRP) is compared to conventional glass-FRP (GFRP) as the skin material used in conjunction with three polyisocyanurate (PIR) foam cores with densities of 32, 64 and 96 kg/m3. Eighteen 1000 mm long flexural specimens were fabricated and tested to failure comparing the effects of foam core density between three-layer FFRP skinned and single-layer GFRP skinned panels. A total of 132 columns with slenderness ratios (kLe/r) ranging from 22 to 62 were fabricated with single-layer GFRP skins, and one-, three-, and five-layer FFRP skins for each of the three foam core densities. The columns were tested to failure in concentric axial compression using pinned-end conditions to compare the effects of each material type and panel height. All specimens had a foam core cross-section of 100x50 mm with 100 mm wide skins of equal thickness. In both flexural and axial loading, panels with skins comprised of three FFRP layers showed equivalent strength to those with a single GFRP layer for all slenderness ratios and core densities examined. Doubling the core density from 32 to 64 kg/m3 and tripling the density to 96 kg/m3 led to flexural strength increases of 82 and 213%, respectively. Both FFRP and GFRP columns showed a similar variety of failure modes related to slenderness. Low slenderness of 22-25 failed largely due to localized single skin buckling, while those with high slenderness of 51-61 failed primarily by global buckling followed by secondary skin buckling. Columns with intermediate slenderness experienced both localized and global failure modes. High density foam cores more commonly exhibited core shear failure. Doubling the core density of the columns resulted in peak axial load increases, across all slenderness ratios, of 73, 56, 72 and 71% for skins with one, three and five FFRP layers, and one GFRP layer, respectively. Tripling the core density resulted in respective peak load increases of 116, 130, 176 and 170%.

Thirty minutes of handgrip exercise enhances brachial artery flow-mediated dilation in response to two different shear stress profiles.

The walls of blood vessels are lined with a single-cell layer of endothelial cells. As blood flows through the arteries, a frictional force known as shear stress is sensed by mechanosensitive structures on the endothelium. Short and long term changes in shear stress can have a significant influence on the regulation of endothelial function. Acutely, shear stress triggers a pathway that culminates in the release of vasodilatory molecules from the endothelium and subsequent vasodilation of the artery. This endothelial response is known as flow mediated dilation (FMD). FMD is used as an index of endothelial function and is commonly assessed using reactive hyperemia (RH)-FMD, a method which elicits a large, short lived increase in shear stress following the release of a brief (5 min) forearm occlusion. A recent study found that a short term exposure (30 min) to a sustained elevation in shear stress potentiates subsequent RH-FMD. FMD can also result from a more prolonged, sustained increase in shear stress elicited by handgrip exercise (HGEX-FMD). There is evidence to suggest that interventions and conditions impact FMD resulting from sustained and transient shear stress stimuli differently, indicating that HGEX-FMD and RH-FMD provide different information about endothelial function. It is unknown whether HGEX-FMD is improved by short term exposure to shear stress. Understanding how exercise induced FMD is regulated is important because it contributes to blood flow responses during exercise. The study purpose was therefore to assess the impact of a handgrip exercise (intervention) induced sustained elevation in shear stress on subsequent brachial artery (BA) HGEX-FMD. Twenty healthy male participants (22±3yrs) preformed a 30-minute HGEX intervention on two experimental days. BA-FMD was assessed using either an RH or HGEX shear stress stimulus at 3 time points: pre-intervention, 10 min post and 60 min post. FMD and shear stress magnitude were determined via ultrasound. Both HGEX and RH-FMD increased significantly from pre-intervention to 10 min-post (p<0.01). These findings indicate that FMD stimulated by exercise induced increases in shear stress is potentiated by short term shear stress exposure. These findings advance our understanding regarding the regulation of endothelial function by shear stress.