High performance carbon and glass fibers are widely used as reinforcements in
composite material systems for aerospace, automotive, and defense applications.
Modifications to fiber surface treatment (sizing) is one of the ways to improve the
strength of fibers and hence the overall longitudinal tensile strength of the
composite. Single fiber tensile tests at the millimeter scale are typically used to
characterize the effect of sizing on fiber strength. However, the characteristic
length-scale governing the composite failure due to a cluster of fiber breaks is in the
micro-scales. To access such micro-scale gage-lengths, we aim to employ indenters
of varying radii to transversely load fibers and use scanning electron microscope
(SEM) with digital image correlation (DIC) to measure strains at these lengthscales. The use of DIC technique requires creation of a uniform, random, and high
contrast speckle pattern on the fiber surface such as that shown in Figure 1. In this
work, we investigate the formation of sub-microscale speckle pattern on carbon
fiber surface via sputter deposition and pulsed laser deposition techniques (PLD)
using Gold-Palladium (Au-Pd) and Niobium-doped SrTiO3 (Nb:STO) targets
respectively. Different processing conditions are investigated for both sputter
deposition: sputtering current and coating duration, and PLD: number of pulses
respectively to create sub-micron scale patterns viable for micro-DIC on both sized
and unsized carbon fibers. By varying the deposition conditions and SEM-imaging
the deposited patterns on fibers, successful pattern formation at sub-micron scale is
demonstrated for both as-received sized and unsized IM7 carbon fibers of average
diameter 5.2 µm via sputter deposition and PLD respectively.
more »
« less
Sub-microscale speckle pattern creation on single carbon fibers for scanning electron microscope-digital image correlation (SEM-DIC) experiments
High performance carbon fibers are widely used as fiber reinforcements in composite material systems for aerospace, automotive, and defense applications. Longitudinal tensile failure of such composite systems is a result of clustering of single fiber tensile failures occurring at the microscale, on the order of a few microns to a few hundred microns. Since fiber tensile strength at the microscale has a first order effect on composite strength, it is important to characterize the strength of single fibers at microscale gage lengths which is extremely challenging. An experimental technique based on a combination of transverse loading of single fibers under SEM with DIC is a potential approach to access microscale gage lengths. The SEM-DIC technique requires creation of uniform, random, and contrastive sub-microscale speckle pattern on the curved fiber surface for accurate strain measurements. In this paper, we investigate the formation of such sub-microscale speckle patterns on individual sized IM7 carbon fibers of nominal diameter 5.2 µm via sputter coating. Various process conditions such as working pressure, sputtering current, and coating duration are investigated for pattern creation on fiber surface using a gold-palladium (Au-Pd) target. A nanocluster type sub-microscale pattern is obtained on the fiber surface for different coating conditions. Numerical translation experiments are performed using the obtained patterns to study image correlation and identify a suitable pattern for SEM-DIC experiments. The pattern obtained at a working pressure of 120–140 mTorr with 50 mA current for a duration of 10 min is found to have an average speckle size of 53 nm and good contrast for image correlation. Rigid body translation SEM experiments for drift/distortion correction using a sized IM7 carbon fiber coated with the best patterning conditions showed that Stereo-SEM-DIC is needed for accurately characterizing fiber strain fields due to its curved surface. The effect of sputter coating on fiber tensile strength and strain is investigated via single fiber tensile tests. Results showed that there is no significant difference in the mean tensile strength and failure strain between uncoated and coated fibers (average increment in fiber diameter of ∼221 nm due to coating) at 5% significance level. SEM images of failure surfaces for uncoated and coated fibers also confirmed a tensile failure of fibers as observed for polyacrylonitrile PAN-based fibers in literature.
more »
« less
- Award ID(s):
- 1915948
- PAR ID:
- 10383986
- Date Published:
- Journal Name:
- Composites Part A Applied science and manufacturing
- Volume:
- 165
- ISSN:
- 1878-5840
- Page Range / eLocation ID:
- 107331
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Digital image correlation (DIC) is a non-destructive and non-contact optical technique to measure deformation and strain of materials. The method is based on optically tracking the displacements of a speckle pattern created on the material surface. In the case of soft tissues such as mouse aorta, there are several advantages to using DIC since it can provide local, rather than global, deformations and it is suitable for large strain measurements, typical of soft tissues taken to failure [1] [2]. For the optimal use of DIC, several requirements should be met for speckle patterning: 1) randomness, 2) high contrast, 3) appropriate size of speckle in the field of view (3-5 pixels), and 4) firm attachment of speckle to specimen during deformation. In previous DIC studies of soft tissues, the methods employed to create a speckle pattern include the use of an airbrush to spray dye or paint on the specimen, or coating the sample with toner powder. However, biological samples must be partially dehydrated before applying paint which may affect the mechanical properties of the specimen, and toner powder is too hydrophobic to adhere well on specimens when submerged in aqueous solution during mechanical testing. In addition, it is difficult to evenly distribute paint or toner powder on the surface of a hydrated biological specimen [2]. Therefore, a novel method utilizing colloidal gold particles to create a speckle pattern on mouse aorta is proposed in this work.more » « less
-
The transverse strength of fiber-reinforced composites is a matrix-dominated property whose accurate prediction iscrucial to designing and optimizing efficient, lightweight structures. State-of-the-art analytical models for compositestrength predictions do not account for fiber distribution, orientation, and curing-induced residual stress that greatlyinfluence damage initiation and failure propagation at the microscale. This work presents a novel methodology to develop an analytical solution for transverse composite strength based on computational micromechanics that enables the modeling of stress concentration due to representative volume elements (RVE) morphology and residual stress. Finiteelement simulations are used to model statistical samples of composite microstructures, generate stress-strain curves,and correlate statistical descriptors of the microscale to stress concentration factors to predict transverse strength as a function of fiber volume fraction. Tensile tests of thin plies validated this approach for carbon- and glass-reinforced composites showing promise to obtain a generalized analytical model for transverse composite strength prediction.more » « less
-
Polymer matrix composites are popular in the aerospace industry due to their high strength to weight ratio. While they have become popular, understanding and predicting their specific damage evolution mechanisms remains a challenge especially in designing with damage tolerance criteria. One challenge often faced is the presence of surface damage either induced during manufacturing, machining, or service of a composite part. While many studies have investigated how quasi-static, low-velocity, and ballistic impact results in damage in the material, there remains a need to further understand the reduction in performance that results from such surface damage. In this work, micro-indentation was conducted on a unidirectional IM7/8552 laminate composite specimen to induce quasi-static impact damage that results in surface damage. The specimen was then loaded in tension to 33% of its expected failure load and imaged using synchrotron X-ray micro-computed tomography to qualitatively investigate the progression of surface damage into sub-surface damage. This work shows that at 33% of tensile failure load, surface damage propagates into delamination and fiber breakage of plies directly sub-surface. This work sheds light on the progression of surface damage at loads less than 50% of the ultimate strength of a unidirectional laminate composite.more » « less
-
Fiber push-in nanoindentation is conducted on a unidirectional carbon fiber reinforced bismaleimide resin composite (IM7/BMI) after thermal oxidation to determine the interfacial shear strength. A unidirectional IM7/BMI laminated plate is isothermally oxidized under various conditions: in air for 2 months at 195 °C and 245 °C, and immersed in water for 2 years at room temperature to reach a moisturesaturated state. The water-immersed specimens are subsequently placed in a preheated environment at 260 °C to receive sudden heating, or are gradually heated at a rate of approximately 6 °C/min. A flat punch tip of 3 μm in diameter is used to push the fiber into the matrix while the resulting loaddisplacement data is recorded. From the load-displacement data, the interfacial shear strength is determined using a shear-lag model, which is verified by finite element method simulations. It is found that thermal oxidation at 245 °C in air leads to a significant reduction in interfacial shear strength of the IM7/BMI unidirectional composite, while thermal oxidation at 195 °C and moisture concentration have a negligible effect on the interfacial shear strength. For moisture-saturated specimens under a slow heating rate, there is no detectable reduction in the interfacial shear strength. In contrast, the moisture-saturated specimens under sudden heating show a significant reduction in interfacial shear strength. Scanning electron micrographs of IM7/BMI composite reveal that both thermal oxidation at 245 °C in air and sudden heating induced microcracks and debonding along the fiber/matrix interface, thereby weakening the interface, which is the origin of failure mechanism.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.compositesa.2022.107331
