Recent advances of interfacial micromechanics in fiber reinforced composites using micro-Raman spectroscopy are given. The faced mechanical problems for interface design in fibrous composites are elaborated from three optimization ways: material, interface, and computation. Some reasons are depicted that the interfacial evaluation methods are difficult to guarantee the integrity, repeatability, and consistency. Micro-Raman study on the fiber interface failure behavior and the main interface mechanical problems in fibrous composites are summarized, including interfacial stress transfer, strength criterion of interface debonding and failure, fiber bridging, frictional slip, slip transition, and friction reloading. The theoretical models of above interface mechanical problems are given.
Polymer-matrix fibrous composites have been widely used in the aerospace and industrial locomotives fields. There exist many interfacial phenomena and all kinds of defects including inclusions, pores, and layer shrink zones in fibrous composites during design and manufacturing processes. Fiber fracture and interface debonding will appear inside the material in service and result in early fatigue, aging, damage, and failure, so it is a hidden danger to risk a major engineering accident. With development of composite material science and aerospace industry applications, the light-weight and high-tough carbon fibers have been widely applied to fibrous composites. Therefore, many researchers coming from physics, chemistry, materials, mechanics, and engineering are attracted by many basic mechanical problems in fibrous composites, such as mechanical properties characterization of high-performance fiber, fiber/matrix interface debonding, fiber bridging, fiber fracture, and matrix cracking [
Besides the effects of fiber surface treatments on fibrous composites have been studied; the basic problems of microscopic interfacial stress transfer and failure were focused in decades [
With unique advantages of nondestructive, noncontact, and high spatial resolution (1
Based on the outline of interface mechanics design, interface evaluation method, and fine characterization techniques of fibrous composites, the research progress on the interface mechanics by MRS is introduced in the paper, including the interfacial stress transfer, interfacial debonding and strength failure criterion, fiber bridging, interface friction, and slip transition. At last, some theoretical models on those interface mechanics problems are summed up.
The mechanical properties of fibrous composites are closely related to the interface control process, material compound, material properties, and interfacial failure modes, so these are very important for the interface mechanics design and optimization of fibrous composites. However, there is no effective criterion yet to optimize the performance of interface mechanics. Based on the existing interface mechanics models and numerical analysis works, it is possible to get the interfacial mechanical parameters and material parameters having no interface failure and to give the reference of the process controls and material options for the optimization of interface mechanical properties [
The design of interface mechanics in fibrous composites should consider the process technology, materials and environment, and other complex factors. It mainly consists of three optimization approaches: material optimization, interface optimization, and computation optimization, as shown in Figure
Interface design routes for fiber composites.
The selection and optimum combination of materials is the most commonly used method for the interface design of fibrous composites. Through choosing fiber and matrix resin having specific properties, the composite laminates are formed by curing according to a certain volume ratio and fiber laying manner. Thus, the loading capacity of fibrous composites can be improved by means of the excellent mechanical properties of fibers. Typically, the macroscopic mechanical tests are used to characterize the interfacial properties of fibrous composites. There are a lot of works to get the interfacial shear strength and other interface parameters, such as the fiber critical length obtained by single fiber fragmentation test and the relationship between fiber pullout force and displacement by single fiber pullout test [
The interface mechanical parameters obtained by the macroscopic mechanical tests are the average results for characterizing the macroscopic performance of interface bonding capability. However, it is difficult to get the fine stress distribution along the fiber/matrix interface and to observe the interfacial debonding and failure processes. Therefore, the development of microscale measurement methods is necessary [
Nowadays, it has been recognized that the way of traditional compound optimization is insufficient to improve the whole mechanical properties of fibrous composites and then the researchers turned to the interface bonding ability to improve the mechanical properties of fibrous composites. Due to different mechanical properties, compound process, and geometric conditions on the fiber/resin interface, there are thermal, mechanical, chemical, and physical coupling effects existing on the interface. Resultantly, different interfacial structures and characteristics appear and affect the fiber/resin interface bonding capacity, and then the fibrous composites exhibit different macrophysicochemical and mechanical properties [
As shown in Figure
Interface optimization ways.
If the interface strength of fibrous composites is too low, the fiber is easy to debond, pullout, break, and fail. On the contrary, if the interface strength is very high, the stress between the fiber and matrix cannot be relaxed and the brittle fracture would occur at the interface. Therefore, the interface design can be optimized by considering the best comprehensive mechanical properties. The interfacial mechanical properties and geometrical parameters are regarded as design variables, and then certain optimization method such as genetic algorithm combines with the finite element analysis to find the best design variables. This is the fast optimization path of interface mechanical performance in fibrous composites.
However, the design variables of composite interface microstructure are not continuous so that the derivative-type optimization method will fail in the case. It is also noted that the uncertainty of initial value limits the capacity of optimization method converging to the global optimum. In addition, the existing mechanical models are imperfect to describe the micromechanical behavior of the composite interface. The mechanical properties of interface layer, residual stress, and stress singularity are the difficulties to constrain the numerical computation [
The macroscopic damage and failure criteria for fibrous composites do not consider the micromechanical properties of interface, such as fiber stress distribution, stress concentration, shear strength, and frictional shear stress on the debonding interface. In addition, there is still lack of a common understanding about the influence of interfacial microstructural parameters and physicochemical properties on the interface micromechanical properties. Currently, the research on microscale experimental mechanics characterization of the interface failure is not only the most difficult and crucial problem but also the important content of interface mechanical evaluation in fibrous composites, as shown in Figure
Evaluation tests and characterization methods for micromechanical properties of interface.
The interfacial shear strength is a commonly used parameter to evaluate interfacial bonding quality, fiber/matrix stress transfer efficiency, and the effect of fiber surface modification. The important parameter can be obtained by single fiber micromechanical testing experiments. One kind of these experiments is realized by applying the external load to single fiber, such as fiber pullout test [
During the implementation and application of these interface evaluation tests for the characterization of micromechanical properties, it is difficult to ensure the integrity, repeatability, and consistency of the interface evaluation. The experimental results of fiber pullout test, fiber fragmentation test, and fiber push-out test vary widely at the same external conditions. Even using the same test method, the experimental results among different laboratories still have differences [
However, the deeper reason is that the differences of many conditions (i.e., interface characteristics) in these interface evaluation tests are neglected, such as interface structure, geometric shape and dimension, and boundary and surface treatment. The testing specimens employed in the different interface evaluation methods have different geometric parameters, such as the droplet contact angle and the embedded fiber length in the microdroplets tension test. Even with the same specimen preparation procedure, it is difficult to ensure that all samples have a uniform geometry and dimension size, which affects the repeatability and consistency of the interface micromechanical parameters characterized by the interface evaluation test. The latest research of microdroplet tension test shows that the microdroplet conformations with different contact angles affect the interfacial shear stress distribution and stress transfer efficiency [
Commonly, the interfacial shear strength obtained by the interface evaluation tests is used as an important characteristic parameter in the interface failure models and is an average value for characterizing the interface bonding properties. It cannot completely describe the details of the interfacial stress transfer and interfacial debonding failure processes. Therefore, more sophisticated real-time experimental data are required to quantitatively and completely characterize the micromechanical behaviors of the fiber/matrix interface [
The testing methods having the ability to carry out the microscale fine characterization, including MRS and digital photoelasticity, digital image correlation, and speckle interferometry. These methods are most likely the first application to completely characterize the micromechanical properties of fiber reinforced composites. MRS measurements have unique advantages at the microscales: nondestructive, noncontact, high spatial resolution (1
When the fiber is under deformation, it causes the movement and deformation of Raman spectrum [
Raman spectra of (a) fiber/epoxy droplet specimen compared with pure epoxy and Kevlar 49 fiber [
The interfacial stress transfer behavior between the fiber and matrix in fibrous composites is a major mechanical problem including several successive stages: the interface intact bonding, interface debonding, interface completely debonding, and fiber pullout. The elastic stress transfer in bonding area and the frictional shear stress transfer in debonded area have been widely recognized. In the process of interfacial debonding and extension, the interface mechanical parameters of bonding shear stress, debonding friction shear stress, and interface debonding length continuously evolve, and the macropulling force or stress is also changed accordingly. At present, the main interface mechanics problems in fibrous composites discussed are as follows: the elastic stress transfer, partial debonding stress transfer, interface failure criterion and fiber bridging, and so on.
One end of single fiber embeds in epoxy matrix, as shown in Figure
Single fiber pullout specimen [
The stress distribution along the embedded fiber cannot be obtained by the above equation, so it cannot be used to study the stress transfer between the fiber and matrix. Cox’s shear-lag model [
Piggott’s model [
Figure
(a) Fiber axial stress and (b) shear stress distributions along fiber of pullout specimen under different strain levels [
The interfacial shear stress (ISS) along the embedded fiber is further given from (
As shown in Figure
In the fiber pullout experiment, the aspect ratio
If the applied strain continues, the fiber fracture failure occurs when the fiber stress
It can be seen that the fiber/matrix interface is more likely to fail if the fiber strength
When the applied load further increased in the fiber pullout test (Figure
(a) Fiber axial stress and (b) shear stress distributions along fiber of pullout specimen under 1.6% strain [
Using the simple Cox’s shear-lag model, the frictional stress transfer in the debonding interface can be easily analyzed. Assuming a linear distribution of the interfacial friction stress, a two-stage model of the interfacial friction shown in Figure
The interface frictional shear stresses on the debonding segments are given by the combination of (
It can be seen that the frictional shear stress plays the role of stress transfer on the debonding interface and can be described as the multistage constant distribution in this study.
If the load continues to be applied, the interface debonding failure propagates forward. According to strength failure conditions (
As shown in Figure
Bridging fiber and interfacial debonding during crack opening [
For the case of unloading after the formation of bridging fiber, a reverse slip will occur on the debonding segment and the fiber retraction results in residual interfacial friction stress, as shown in Figure
Slip transform for bridging fiber after reloading [
The reverse slip happens on the fiber debonding segment before reloading (Figure
Raman measurements along the bridging fiber in Figure
Stress distributions on the bridging fiber under different loads [
During the reloading of bridging fiber, the reverse slip in debonding segment gradually transformed into the forward slip so that the debonding fiber reloaded until the fiber stress eventually reached the maximum in the bridging segment. In fiber bridging segment, the ISS is zero due to the constant fiber stress. Setting a positive constant
Stress transfer model of bridging fiber under reloading.
In the fiber debonding segment, the interfacial frictional shear stress (Figures
At initial reloading stage (Figure
At middle reloading stage, (Figure
At completely reloading stage (Figure
It can be predicted that the debonding interface will continue to extend if the ISS at the bonding/debonding transition point reaches the interfacial shear strength. With the further increase of reloading, the fiber stress in the maximum stress plateau region will reach the fiber tensile strength so that the bridging fiber will fracture. This is the strength criteria for the bridging fiber.
For fibrous composites with stable interface, it can be seen from the above analysis that the physical and chemical nature of the interface determines the interface bonding ability, namely, the interface shear strength. The matrix crack across the fiber will cause interfacial debonding and form the bonding segment, debonding segment, and bridging segment. The fiber stress transfer among these segments has relationship with the interface bonding performance, interface friction, interfacial shear strength, and fiber strength. The balance between them determines whether the bridging fiber is stable or unstable. Once the balance is broken, the bridging fiber cannot exist stably and then transforms into the broken fiber [
The interfacial mechanical design problems faced in fibrous composites elaborated from three ways of the material optimization, interface optimization, and computational optimization. The physical, chemical, geometric, and mechanical properties at microscale have a great impact on the interface behaviors. They are necessary to develop new experimental methods for reasonable evaluation on fiber/matrix interface by fine experimental testing and characterization to improve the interface micromechanical model. Micro-Raman spectroscopy was used to study main mechanical problems in fibrous composites, including the elastic stress transfer and failure criteria of well-bonding fiber, the frictional shear stress transfer behavior of partially debonded fiber, the slip transformation, and stress transfer models of bridging fiber during reloading. These works show that micro-Raman spectroscopy has ability to evaluate the stress transfer behavior of fiber/matrix interface.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by National Basic Research Program of China (no. 2014CB046506), National Natural Science Foundation of China (nos. 11172054, 11272232) and Fundamental Research Funds for the Central Universities (no. DUT14LK11).