Residual Stresses In Composite Research Articles

Carbon fiber pull-out analysis for composite materials by considering interface properties

ABSTRACT An improved shear-lag model is adopted in this paper to study the fiber pull-out by considering interface properties in a representative volume element (RVE). The surface roughness, represented by grooves distributed around the fiber, is introduced to analyze the influence of roughness on fiber pull-out comparing it with a round fiber. The influence of groove shapes is also discussed. The interfacial stiffness is introduced in the improved shear-lag model by the cohesive zone modeling to simulate a practical interface connection. To represent the residual stress in composites, a pressure load is added on the surface of the RVE to analyze its influence on the fiber pull-out. The results show that the proposed theory can accurately predict the stress distribution in the RVE. Furthermore, the maximum reaction force increases with an improvement in roughness and interfacial stiffness, while the pressure can improve the reaction force in the third stage of fiber pull-out process. The proposed theory can predict the stress field and fiber pull-out process accurately. It can improve the efficiency and effectivity of micro-experiment with the reference of theoretical results. The results obtained using the proposed theory in this research can also be useful for the design and analysis of composite materials.

A Microscale Analysis of Thermal Residual Stresses in Composites with Different Ply Orientations.

Composites, such as fiber-reinforced plastics, are produced using layering prepregs with varying ply orientations to achieve enhanced mechanical properties. However, this results in intricate residual stresses, which are influenced by the forming process and ply orientation. In this study, three representative microscopic models-featuring discrete fiber and resin-represent unidirectional, cross-ply, and angle-ply laminates. These models underwent simulations under three different cooling histories using the finite element method. The findings suggest that ply orientation does not significantly influence temperature distribution. However, it significantly impacts the von Mises stress in the fiber closest to the interface between two stacked laminae. This differs from the inter-laminar stresses determined with the macroscopic lamination model. Apart from the free edge, which exhibits a complex stress distribution, the von Mises stress within a unit cell displays a recurring pattern. The magnitude of the von Mises stress decreases as the ply orientation angle increases and shifts when a temperature gradient is present throughout the composite's thickness. This study provides valuable insights into the mechanics of residual stresses at the microscopic level and highlights potential defect areas influenced by these stresses.

Continuous evolution of processing induced residual stresses in composites: An in-situ approach

Processing induced residual stresses are often responsible for causing warping, undesired distortion, dimensional instability, and delaminations in composite structures. Monitoring the evolution of residual stresses throughout the cure is essential to understand their effect on the mechanical behavior of the composites. In the present work, a novel in-situ experimental approach is used to measure the dimensional changes in the laminates during cure and combined with temperature-dependent moduli obtained from Dynamic Mechanical Analysis (DMA) to calculate the continuous evolution of residual stresses during composite processing. Five symmetric and asymmetric layup configurations are analyzed to investigate the effect of ply orientation on the residual stress evolution. The maximum average residual stress of 110 MPa in the longitudinal direction is observed for symmetric cross-ply [0/90]s configuration and the maximum average warpage of 1.46 mm is observed for balanced un-symmetric [30/−30/60/−60] layup configuration.

Investigation of residual stress in composites produced by explosion welding with ultrasound

The paper presents the results of experimental studies of residual stress in homogeneous steel samples obtained by explosion welding with simultaneous exposure to ultrasound. For experimental determination of residual stress in composites, a method based on sequential removal of thin metal layers from the sample surface and measurement of deformations of the sample from the opposite side using strain gages was chosen. It is shown that the introduction of ultrasound in the process of explosion welding leads to both a redistribution of residual stress and a significant decrease in their maximum values measured in the immediate vicinity of the junction zone of steel samples.

Identification of process-induced residual stresses in 3D woven carbon/epoxy composites by combination of FEA and blind hole drilling

Process-induced residual stresses in 3D woven carbon-epoxy composites are studied by blind hole drilling experiments interpreted with finite element (FE) modeling. It is assumed that residual stresses are primarily caused by the difference in thermal expansion coefficients of the constituents which are modelled as temperature-dependent linear elastic solids. The impact of residual stresses is quantified by drilling blind holes in the composite panels and mapping the resulting in-plane surface displacements by electronic speckle pattern interferometry. Mesoscale finite element models are used to correlate these surface displacements with the volumetric distribution of the residual stresses in the composite. This is done by determining the effective temperature drop ΔTeff that results in the same predictions for the surface displacements as experimentally measured. The effective temperature drop approach allows to use linear elastic models while approximately accounting for various nonlinear effects occurring in the material during processing. The models are also used to establish the sensitivity of the predicted results to the exact location of a hole and its depth.

Prediction Models for Distortions and Residual Stresses in Thermoset Polymer Laminates: An Overview

This paper reviews predictive models developed for the development of residual stresses and shape distortions during the manufacturing of thermoset polymeric/composite materials. Different sources that produce residual stresses and shape changes in the laminated panels are described and reviewed. An overview is presented on the characterisation and predictions of the phenomena resulting in residual stresses. The focus will be on the models accounting for the parameters during the cure cycle of the thermoset composite materials published in the literature from 2005 until 2018. The material types covered here range from thermoset adhesives, full composites, and fibre metal laminates. Furthermore, selected works are reviewed on the reduction of the shape changes and residual stresses of composites and fibre metal laminates consisting of thermoset polymers.

Influence of manufacturing process on the strength of composite materials

Residual stresses in laminated composites induced by non-uniform matrix solidification and shrinkage through manufacturing process may lead to the shape distortion and possible damage of the final product. In this work, the phase transition model for matrix material was numerically implemented into Abaqus software to simulate evolution of stiffness properties and development of residual stresses of composite laminate during manufacturing temperature cycle. This simulation shows the presence of significant residual stresses in composites even before mechanical processing of the material. As an example, the residual stresses were computed for the plate with cross-ply layup. As a last step two specimens with presence of residual stresses and without one were loaded up to failure using progressive damage model. Modelling results show essential influence of residual stresses on distribution and development of matrix cracks, failure modes and strength values.

Effect of Residual Stress in Composites onits Interlayer Adhesive Strength

Effect of Residual Stress in Composites onits Interlayer Adhesive Strength

Residual stress and yield strength evolution with annealing treatments in an age-hardenable aluminum alloy matrix composite

We investigated the possibility of minimizing tensile matrix residual stresses in age hardenable aluminum alloy metal matrix composites without detrimentally affect their mechanical properties (such as yield strength). Specifically, we performed thermal treatments at different temperatures and times in an age-hardenable aluminum matrix composite 2014Al-15vol%Al2O3. Using X-ray synchrotron radiation diffraction and mechanical tests, we show that below a certain treatment temperature (250 °C) it is possible to identify an appropriate thermal treatment capable of relaxing residual stress in this composite while even increasing its yield strength, with respect to the as processed conditions.

Thermal residual stresses in FSS layer-embedded composite plates in co-cure bonding

The residual stresses occur in the frequency selective surface (FSS)-embedded composite structures after co-curing due to mismatch between the coefficient of thermal expansions between the FSS and composite materials. It is well known that residual stress has a great influence on the strength and fatigue life of the FSS-embedded composite structures. Numerous researches have been reported about residual stresses in the composite structures. However, studies on the residual stresses in the FSS-embedded composite structure have not been widely investigated. Therefore, in this study, the thermal residual stresses of FSS-embedded composite structures were analyzed using finite element analysis, with considering the effect of FSS pattern and size. Various FSS patterns, such as square loop, gridded square loop, and double square loop, are considered in the analysis. The effects of dimensional change of design parameters of FSS pattern on the residual stresses in the hybrid composites were also investigated.

Imitational modeling of residual stresses in materials employed for the repair of structural steels

Results of the calculations of residual stresses of composites based on polymer binder and glass fiber, which are used in the repair of metal structures of machines, by imitational modeling have been considered. Imitational modeling has been carried out using a special program that enables one to calculate required parameters with a very high rate after choosing initial data from preformed number sets. It has been determined that the modulus of glass plastics has the most sizeable effect on residual stresses.

Analysis of Deformation and Residual Stresses in Composites Processed on a Carbon Foam Tooling

Carbon foam is a low cost, durable, light-weight material with a low coefficient of thermal expansion (CTE) and it has cure process compatibility with aerospace composites. Therefore, carbon foam is being investigated as an alternative to traditional composite tooling material for aerospace applications. However, the dramatic differences in properties between traditional tooling materials and carbon foam makes it difficult to fully predict the performance of this novel material as a tooling alternative. This article is a comparative analytical study of the effects of processing of composite geometries on a carbon foam tooling vs. conventional tooling materials. Simulation is carried out on graphite-epoxy composite system using a carbon foam tooling and the stresses and the resulting deformations are predicted for three different geometric features of composites. The results show that both the lower CTE and modulus of the carbon foam tooling can contribute to a reduction of residual stresses in the part and the tooling; however, the mechanical strength of the foam needs to be considered in the tooling design.

Effects of the coating system and interfacial region thickness on the thermal residual stresses in SiC f/Ti–6Al–4V composites

A three-dimensional finite element model was developed to study effects of the coating system and interfacial region thickness on the distributions of the thermal residual stresses in continuous SiC fiber reinforced Ti–6Al–4V composite. Three coating systems were applied in this study, that is, C coating, C/TiB 2 coating and without coating. The influence of the interfacial region thickness on the thermal residual stresses in the composite was analyzed with concerning the simple case of without coating. Some proposal was given for the interface structure design of the composites. With regard to the thermal residual stresses, the C coating is an advisable choice for the interface structure design of the continuous SiC fiber reinforced Ti–6Al–4V composites. And for the region of the interfacial region thickness from 1 μm to 3 μm, the thicker interfacial region can reduce most of the thermal residual stresses in composites and improve the axial tensile strength of the composite.

Residual Stress in Composites with the Thin-ring-slitting Approach

During polymerization, dental composites develop residual stresses that may compromise the marginal integrity and properties of the restorative. The objective of this study was to use the thin-walled ring-slitting method to measure and compare residual stresses. The hypotheses to be tested were that composites would generate different levels of residual stress based on their specific formulations and slitting times. Rings made from composites (Z100, Herculite, and Heliomolar) were cut at different times (10 min, 1 and 24 hrs) after being light-cured, and stress was measured. Residual stress was higher at the earlier cutting times, except for Heliomolar (alpha < 0.05). For the 10-minute and one-hour cutting groups, stress followed this order: Z100 > Herculite > Heliomolar. Early slitting was better to capture residual stress, and the thin-walled rings showed higher values than thick-walled rings and were better able to discriminate residual stress in composites.

Transverse and Longitudinal CTE Measurements of Carbon Fibers and their Impact on Interfacial Residual Stresses in Composites

In situ transmission electron microscopy (TEM) is utilized to evaluate the coefficients of thermal expansion (CTE) of two polyacrylonitrile (PAN) based (T1000 and IM7) and one pitch-based (P55) carbon fiber in the temperature range 20–1100 C. The transverse morphology of the fibers is studied using field emission scanning electron microscope and selected area diffraction (SAD) patterns in a TEM and is co-related to the fiber transverse CTE. The PAN-based IM7 and T1000 fibers revealed a transversely isotropic structure, which was consistent with their transverse CTE measurements. Even though the pitch-based P55 fiber revealed cylindrical orthotropy, it did not translate to orthotropy in the transverse CTE. Finite element models of unidirectional IM7/977 composite, utilizing the present IM7 CTE values, are compared to experimental lamina scale CTE results where good agreement was obtained for cool-down regime. The models also indicate that the Tresca stresses in the epoxy matrix at the fiber–matrix interface exceed the allowable strength at cryogenic regimes suggesting possible fiber–matrix debonding and transverse matrix cracking.

Measurement of Fiber-scale Residual Stress Variation in a Metal-matrix Composite

The crack compliance, or slitting, method was used to measure a localized depth profile of residual stresses in a metal-matrix composite. The composite consisted of a matrix of Kanthal, a Fe–Cr–Al refractory alloy, reinforced with continuous uniaxial tungsten fibers. The stress measurements involved successively deepening a narrow slit between the fibers in the matrix, and measuring the resulting deformations with a surface strain gage. The depth profile of the in-plane residual stress components was determined from the measured strains using an eigenstrain-based extension of the residual stress calculation scheme normally used for slitting measurements. To validate some of the eigenstrain assumptions, the measured residual stresses were compared with the predictions from a thermomechanical finite element model. The model used a mesh of the actual fiber arrangement in the composite specimen rather than the commonly used unit-cell model. Compared to other techniques for measuring residual stresses in composites, the slitting measurements provided spatial resolution to a small fraction of the fiber diameter, which is useful when validating a thermomechanical model. Coincidentally, this is the first reported slitting method measurements of shear residual stresses.

The effect of machining on the surface properties of SiC/Al composites

The machined surface quality of composites is one of the most important factors affecting the actual application of the composites. In this paper the hardness and residual stress of composites in the surface layer affected by machining are investigated. The results indicate that the work-hardening and residual stress of composites in the machined surface layer have some peculiarities. The machined surface hardness of composites reinforced by SiC particles may be lower than that of the unaffected interior material, but the subsurface is hardened evidently, and the average hardening is more remarkable for composites reinforced by fine particles. In the surface of composites reinforced by coarse particles the residual stress may be released due to structural defects, but for composites reinforced by fine particles there is a tendency for compressive residual stress in the surface. Cutting with a larger removal rate increases the possibility for tensile residual stress in the machined surface layer of the composites.

Characterization of Fiber/Matrix Interfaces Using X-Ray Microtopography

The structure and strength of fiber/matrix interfaces in fiber-reinforced composites are crucial in determining the overall mechanical behavior of these materials. To investigate the integrity and structure of such an interface on the microstructure scale, a model composite consisting of a single crystal Al 2 O 3 fiber in an Al matrix was investigated with X-ray microdiffraction. The intensity of the (30.0) and (22.0) reflections of Al 2 O 3 was monitored in topography mode using an X-ray beam focused by a tapered capillary to spot sizes ranging from 5 to 25 μm. Significant changes in this intensity revealed buried voids along a nominally intact interface. These changes also provided information about the distribution of residual stresses along the interface. The discussion includes advantages and some complications with this technique for measuring residual stresses in composites.

A finite-element analysis of the inelastic relaxation of thermal residual stress in continuous-fiber-reinforced composites

In an effort to develop a methodology for interpreting in situ high-temperature neutron diffraction measurements of thermal residual stresses in composites, a finite-element model was developed and the implications of various material and experimental parameters on the residual stress evolution were studied. The model composite comprised a continuous Al 2O 3 fiber, unidirectionally aligned in a NiAl matrix. The effects of temperature-dependent elastic, plastic or creep properties, fiber volume fraction and cooling/heating rates were explored on the relaxation mechanisms of the residual stress during an initial cooling and a subsequent heating. Thermal path-dependency in the stress evolution was investigated by comparing the cooling and re-heat cycles. The result shows that the effect of the time-dependent deformation (creep) becomes more significant as the fiber content increases and the cooling/heating rate decreases. Furthermore, the path-dependency in stress evolution becomes stronger (i.e. considering the actual thermal history in the model becomes more important) as the total inelastic relaxation increases during the cooling and/or the subsequent re-heating cycles due to; (i) the presence of creep in addition to plastic deformation, (ii) increased fiber volume fraction, and (iii) slower cooling/heating rates. It was also demonstrated that using an estimated stress-free temperature in elastic–plastic models can be problematic in predicting the high-temperature behavior due to; (i) the simple assumption of reduced Δ T, and (ii) the thermal path-dependency.

Measurement of the hardness distribution in sapphire/NiAl composites using continuous indentation

A variety of fibre coatings have been modelled as a means to reduce the thermal residual stresses in sapphire/NiAl composites, including the use of molybdenum as a compliant layer. However, no experimental studies have been performed to quantify the effect of these coatings on the residual stresses. Measuring the effect of the coatings in real composites is a difficult task given the limitations of the most common residual stress measurement methods, such as X-ray and neutron diffraction. In this study, a continuous indentation system was used to measure a spatial hardness distribution both within the molybdenum fibre coatings and the NiAl matrix. Because it has been shown that residual stresses effect the apparent hardness of a material, the hardness distribution should be indicative of the local residual stress state in the sample. The measured hardness distributions showed distinct trends, including an increasing hardness in the molybdenum as the NiAl interface is approached, a substantial increase in NiAl hardness at the fibre/coating boundary, and a decrease in NiAl hardness that is proportional to the distance from the fibre/coating boundary. The potential utility of continuous indentation systems for mapping the residual stresses in composites is briefly discussed. © 1998 Kluwer Academic Publishers