Influence of surface roughness on tribological and mechanical properties of micro-milled and laser ablated poly (methyl methacrylate) PMMA organic glass
Introduction
Poly (methyl methacrylate) (PMMA) being an amorphous structure, exhibits high transparency and are widely used for replacement of glass, particularly in aircraft and aviation applications [1]. In recent days, aircraft windows have emphasized the importance of considering fatigue and fracture behavior. Any structural member may fracture at loads well below the nominal yield strength the materials; if it contains a critical size flaw. These flaws or higher stress concentration are normally built into the structures during the manufacturing, processing or in use [2]. Feng et al., 2017 study reported that surface roughness produced in different length scales may not be avoided during property evaluation. In most of the cases, the flaws or stress concentration tends to grow and becomes critical. This type of growth occurs by fatigue processes during cyclic loading of the structure [3]. The fatigue crack propagation behaviors depends on various parameters such as cyclic stress, temperature, specimen geometry (size, shape and notch configuration), microstructures and the machining process etc [4]. Thus, the high stress concentration in the groove or scallops areas leads to mechanical damage; in addition the inherent stress in the equipment due to their layout and/or supporting arrangement will reduce fracture toughness and resistance towards fatigue crack propagation.
Extensive researches concerning the surface roughness effect on mechanical failure are being carried out by many researchers [[5], [6], [7]]. Gerard and Koss reported that high short crack growth rates have most frequently been attributed to crack closure and microstructural effects [8]. Wu et al., reported that the fine surface morphological structures will exhibit significantly higher strength, strain rate, improved fracture toughness and fatigue endurance limit [9]. In addition the Ritchie et al., study also confirmed that microstructure and load ratio effects the fatigue crack propagation; their report revealed that optimum fatigue crack resistance could be achieved with high toughness materials [10]. Similarly, the study of Brostow et al. have also confirmed that the increase of wear rate as a function of reinforcement due to the influence of surface roughness and brittleness [11].
PMMA sheets are easier to manufacture with respect to conventional glass or silicon [12]. After post processing surface grain size play a major role in deciding the failure characteristics. In order to achieve nano-metric surface finish, micromachining and laser ablation techniques are adapted; as they have the tendency to exhibit low force and temperature.
In the present work and attempt has been made to investigate the stress induced during different grooving and scalloping operation via surface roughness. The effect of surface roughness on mechanical wear, fracture toughness and fatigue properties of differently machined PMMA organic glass were also evaluated. In addition the difference in the surface morphology before and after wear was characterized using optical microscopy (OM) and Surface profilometer, similarly the post failure samples of fracture toughness was accessed using Scanning electron microcopy (SEM).
Section snippets
Materials and methods
Poly (methyl methacrylate) (PMMA) sheets are made of casting process, used in the fitment of aircraft window. 3BRD Air force station, Chandigarh has supplied the sheets which are of 3 mm thickness with a square gauge area of 50 × 50 mm. The samples for testing and characterization were prepared from the supplied sheet. In order to prepare the samples two different machining processes were followed namely micro-milling and laser ablation process. The grooving dimension produced on the PMMA is
Surface roughness
Surface also known as surface profile (Sa) is a measurement of surface finish which is essential for confirming a surface's suitability for its function. To determine the irregularities on the machined surface of PMMA, a contact stylus mode type of a profilometer, Taylor Habson (PGI 1240), UK was used. At least five samples were tested in five different places for average surface roughness (Sa) values and an average values are reported.
Wear analysis
Specific wear rate was measured using pin on disc
Surface roughness
Surface roughness measures irregularities in the surface texture. Fig. 2(a–g) and 2(h-n) represents average roughness (Sa) of PMMA grooved using micro-milling process and laser ablation process respectively. The values obtained are depicted in Table 1. The actual part surface formed by the manufacturing process always contains irregularities in the form of peaks and valleys. The specific surface roughness produced depends on various parameters of machining process such as cutting speed, feed
Conclusion
The study analyzed the surface roughness effect on wear, tensile stress, fracture toughness, and fatigue strength of both MM-PMMA and LA-PMMA samples. The average surface roughness (Sa) value was 5.86 μm and 4.54 μm for micro-milled and laser ablated PMMA respectively. The surface roughness was decreased with the increase of wear time due to the lower roughness of counter face, and after reaching minimum roughness the coefficient of friction has increased due to higher adhesion between polymer
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