Effect of adhesive thickness and surface roughness on the shear strength of aluminium one-component polyurethane adhesive single-lap joints for automotive applications

Factors That Determine the Adhesive Strength in a Bioinspired Bone Tissue Adhesive

“…Tukey or Games-Howell post-hoc analysis was used when variance was homogeneous, or inhomogeneous (Levene's test), respectively. Single comparisons were made with "group" as the independent variable, and "shear strength" as the dependent continuous variable for the following intergroup comparisons: (groups #1-6), (groups #3, 6-12, 33-34), (groups #3, [13][14][15][16][17][18][19][20][21], and (groups #24-32). A comparison of means (ANOVA) was also made between independent variable "group", and dependent variable "failure mode".…”

“…The failure mode was assessed semi-quantitatively by manual scoring, and qualitatively by scanning electron microscopy (SEM). Failure scoring was based upon the percentage of the cube surface coverage, with the least amount of adhesive remaining attached after testing [19]: 0% surface coverage scored as "1" (adhesive failure-the adhesive layer remained unbroken, attached to only 1 surface); 0% < x < 100% surface coverage (mixed mode failure involving adhesive failure at the adhesive/cube interface (adhesive zone) and cohesive failure within the adhesive layer) scored as "2"; a combination of adhesive and cohesive failure, where these failure types occupied distinct regions, hereafter referred to as "combined mode" failure, scored as "3", or 100% (purely cohesive failure-the adhesive layer failed leaving adhesive on both surfaces, while adhesive failure was not observed) scored as "4". The selected samples from each group were investigated with SEM (Merlin field emission microscope, AB Carl Zeiss, Stockholm, Sweden), with a secondary electron in-lens detector, an acceleration voltage of 3 keV, and a 195-pA current.…”

“…The following covariates (uncontrolled factors) were recorded for each sample: application time, average bond thickness, the thickest bond, or thinnest bond thickness, bond unevenness (difference between the thickest and thinnest bond, normalized to the average bond thickness for the sample), and the failure mode ( Table 1). The correlation strength between each individual factor and the outcome measure (adhesive strength), was explored in scatter plots, with replications of a single formulation of PMC ( Figure 6A-D, n = 242 total, n = 12 or 36 per group, 27% PMC, groups #1-13), or a series of PMC formulations with differing amounts of phosphoserine ( Figure 6E-H, n = 12 or 36 per group, 27-90% PMC, groups #3, [14][15][16][17][18][19][20][21].…”

“…No trend was obvious in the sample mean distribution using categorical scatter plots for: cure type, grip force, grip time, or mixing method (data not shown). A second set of scatter plots (aggregated) evaluated the relationship between different strength PMC formulations ( Figure 6E-H, n = 12 or 36 per group, 27%-90% PMC, groups #3, [13][14][15][16][17][18][19][20][21], and: application time ( Figure 6E), average bond thickness ( Figure 6F), the thinnest (red) and thickest bond ( Figure 6G), and bond unevenness ( Figure 6H). A number of studies have investigated the relationship between bond thickness and adhesive strength, with some reporting similar results (poor correlation) [33,34,40], a positive correlation [23,41], or a negative correlation [21,42].…”

“…Three covariates were excluded from the linear regression model, "application time", "thickest bond thickness" and "thinnest bond thickness", because they were collinear with other factors. The second regression model was built using different formulations (phosphoserine mole %) of PMC (Table 4, n = 142 total, n = 12 or 36 per group, 27%-90% PMC, groups #3, [13][14][15][16][17][18][19][20][21]. A stepwise linear regression model revealed that: (a) the correlation between shear strength and the fixed factors "cure condition", "grip duration", and "formulation" was weak (R 2 = 0.216); (b) inclusion of the following covariates, stepwise in separate "blocks", improved the correlation model: average bond thickness (R 2 = 0.237), unevenness (R 2 = 0.243), and the failure mode (R 2 = 0.377).…”