Special Topic Review: Full-Field Imaging Techniques for Assessing Mode II Fracture Toughness of Carbon Fiber-Reinforced Polymer Laminates

Abstract

To replicate delamination at the specimen and substructure levels, simulated defects are often introduced into test specimens in research. Therefore, it is crucial to clarify the mechanical behavior of these defects within the laminate. Full-field imaging techniques are applied to investigate the influence mechanism of artificial defects in carbon fiber-reinforced polymer (CFRP) composites.

In this study, center-cracked tension (CCT) specimens were selected to evaluate the Mode II fracture toughness of laminated composites through simple tensile tests. Two batches of specimens, both of the IM7/8552 type, were prepared with 5 μm-thick steel films inserted as artificial defects. In the first batch, the steel film inserts were coated with Frekote mold release agent, while in the second batch, the inserts were embedded directly into the laminate without coating. Additionally, a third batch of specimens with a [0₄, 90]s layup was prepared.

To obtain full-field temperature and displacement data of the test specimens, thermal elastic stress analysis (TSA) and digital image correlation (DIC) were employed simultaneously. The 90° layup enhances the utilization of thermal contrast, as well as anisotropic mechanical and thermal properties. Monotonic loading was first applied to the specimens until failure, during which DIC was used to capture the strain distribution at the onset and failure stages of damage. Meanwhile, acoustic emission (AE) technology was employed to assess the onset of damage, and fracture toughness was indirectly evaluated through load drops.

The research results indicate that full-field imaging techniques can determine how mold release agents and layup structures affect the onset and propagation of damage. Non-adiabatic thermoelastic responses effectively observe subsurface damage in specimens. Finally, the study proposes a new method for assessing fracture toughness based on the temperature rise value during failure events.

1. Introduction

To simulate delamination in composite laminates, artificial defects are typically introduced into test specimens. The reliability of the measured fracture parameters depends on the accuracy with which these defects replicate real failure mechanisms. Therefore, it is essential to present such defects realistically. One widely adopted configuration for studying Mode II fracture behavior is the center-cracked tension (CCT) specimen, first proposed in the early 1990s. The CCT specimen is a unidirectional laminate in which several layers are transversely cut to form an artificial crack. This notch induces local stress concentrations, leading to the initiation of four interlaminar matrix cracks that propagate unstably under tensile loading. The Mode II fracture toughness (GII) is calculated using an energy-based formula that depends on parameters such as specimen width (W), thickness (t), Young's modulus (E), the ratio of cut layers (x), and the critical load (P) triggering unstable crack propagation.

In the early application of this test method, test repeatability was poor due to the asymmetric and irregular nature of crack propagation. This issue primarily stemmed from the use of only transverse layer cutting as the means of damage initiation. Subsequent studies introduced a modified version of the specimen with two artificial delamination structures added at the pre-crack location. The introduction of inserts serves two main functions: first, to promote symmetry at the mid-plane of the laminate (the intermediate plane in the thickness direction), thereby stabilizing the crack propagation path; and second, to minimize mixed-mode effects.

Le Cahain et al. conducted a comparative study on the influence of different insert materials on fracture toughness test results. The findings indicated that when steel inserts were used, the obtained data most closely resembled the behavior of specimens without inserts. Scalici et al. further analyzed the impact of artificial delamination on testing through a combination of experiments and numerical simulations, ultimately determining the minimum crack length value required to eliminate mixed-mode I/II effects.

In addition to characterizing static fracture properties, center-cracked tension (CCT) specimens have also been used in fatigue testing. A clip-on extensometer was employed during testing to monitor the rate of delamination propagation, providing data support for the establishment of semi-empirical crack propagation models. The fatigue loading process allows for the use of thermal elastic stress analysis (TSA), which relies on the cyclic stress state and utilizes the reversible relationship between stress and temperature to characterize the stress field. For orthotropic layups, temperature changes primarily depend on the applied stress amplitudes in the principal material directions (∆σ₁, ∆σ₂), as well as material properties such as the coefficients of thermal expansion (α₁, α₂), density (ρ), and specific heat capacity at constant pressure (Cp).

Few studies have applied TSA in full-field techniques to analyze crack propagation in carbon fiber and glass fiber laminates during CCT testing. This study aims to enhance the understanding of CCT specimen behavior by employing digital image correlation (DIC) and TSA. The main objectives include: 1) Monitoring the damage process without directly observing the crack front using non-adiabatic thermoelastic responses; 2) Integrating full-field stress and strain data to gain a deeper understanding of the influence of artificial delamination in the testing environment; and 3) Assessing fracture toughness using full-field data by directly evaluating the energy released during fracture.

2. Materials and Methods

In this study, three groups of specimens were prepared using IM7/8552 carbon fiber/epoxy resin prepreg, with 20 mm-long and 5 μm-thick steel inserts used to create artificial delamination structures. The primary objectives of the research were to investigate the influence of Frekote® mold release agent (manufactured by Loctite Corporation, Rocky Hill, Connecticut, USA) on the mechanical behavior of artificial delamination and to analyze the impact of layup scheme adjustments on full-field test results.

Three specimen configurations were designed for the experiments (the numbers in square brackets represent the layup directions): (1) [0₄,0]s: Steel inserts coated with Frekote® mold release agent; (2) [0₄,0]s: Steel inserts directly cured (without mold release agent coating); (3) [0₄,90]s: Steel inserts coated with Frekote® mold release agent (note: the underlined numbers represent the layers that have been transversely cut). Test specimen strips measuring 200 mm (length) × 10 mm (width) were cut from the prepared laminates, with geometric parameters consistent with those in literature [3] to facilitate direct comparison with reported fracture toughness data.

To meet the requirements of full-field testing, the specimens underwent the following preprocessing: First, a layer of matte black paint was applied (for infrared thermography), followed by the creation of a fine white speckle pattern through spraying (for digital image correlation, DIC), enabling simultaneous acquisition of the specimen's surface strain and temperature fields. Additionally, two acoustic emission sensors were mounted on each specimen.

The full-field imaging test system consisted of two parts: a stereo digital image correlation (DIC) system comprising two 12-megapixel FLIR Blackfly white-light cameras (equipped with 25 mm lenses), and a Telops Fast M3K infrared camera (equipped with a 50 mm lens) for temperature data acquisition. DIC images were processed using commercial software MatchID, while thermal elastic stress analysis (TSA) data were analyzed using a custom in-house algorithm that performed time filtering at the selected loading frequency to extract thermoelastic signals.

The mechanical testing included two components: (1) Monotonic tensile testing (displacement rate of 0.5 mm/min) until specimen failure, used to assess Mode II fracture toughness (GII); and (2) Cyclic loading testing (load range of 3.5 ± 3 kN) with loading frequencies ranging from 0.5 to 30 Hz to capture the influence of artificial delamination on the specimen's fatigue performance.

3. Results and Discussion

Figure 1 presents the results of monotonic tensile tests on unidirectional specimens. A total of six specimens were tested, with three using steel inserts coated with mold release agent and the other three using uncoated steel inserts.

The load-strain curves obtained through digital image correlation (DIC) all exhibited a distinct "load drop" phenomenon, which directly corresponds to the onset of unstable crack propagation (Figure 1a). This result confirms the effectiveness of DIC technology in identifying the onset and progression of damage in composite materials. Infrared thermography results revealed localized temperature increase regions on the specimen surface (Figure 1b), which originated from the energy release during crack propagation. Acoustic emission test results (Figure 1c) further supported the damage process, showing a significant surge in acoustic emission energy during the peak load and subsequent load drop stages.

Figure 1. (a) Load-strain curve; (b) Thermal energy released during damage propagation; (c) Total acoustic energy throughout the testing process

The Mode II fracture toughness was calculated using Equation (1) and averaged over three specimens of each type. The specimens with Frekote-coated inserts exhibited a fracture toughness of 1271 J/m², while those with uncoated inserts showed 1224 J/m². These results are very close to the 1203 J/m² reported in literature [2]. Although there was a slight increase in the fracture toughness of specimens with coated inserts, no significant difference was observed. In addition to the established load drop method in Equation (1), a new method for assessing the energy release rate was introduced. This method is based on the local temperature rise in the delamination region (Figure 1b). Under the assumptions of symmetric crack propagation along the thickness direction, instantaneous and uniform crack front advancement, and that all fracture energy is dissipated in the form of heat, the Mode II fracture toughness (GII) can be expressed as follows:

The fracture toughness calculated using this method was highly consistent with the results obtained from Equation (1). Specifically, the average fracture toughness for specimens with coated inserts was 1282 J/m², while for those with uncoated inserts, it was 1222 J/m².

Figure 2 presents the full-field ∆T/T₀ (temperature change rate) results obtained through thermal elastic stress analysis (TSA) for unidirectional layup specimens under a 0.5 Hz loading frequency, covering three types of specimens: (a) specimens with Frekote-coated inserts, (b) specimens with uncoated inserts, and (c) specimens with a [0₄,90]s layup and Frekote-coated inserts.

Figure 2. Comparison of full-field ∆T/T₀ distributions obtained through thermal elastic stress analysis (TSA) for specimens with different layups under a 0.5 Hz loading frequency: (a) [0₄,0]s layup (with Frekote mold release agent), (b) [0₄,0]s layup (without Frekote mold release agent), (c) [0₄,90]s layup (with Frekote mold release agent)

Due to the mismatch in the coefficients of thermal expansion (CTE) between the longitudinal and transverse directions of the selected carbon fibers—where the transverse CTE is two orders of magnitude higher than the longitudinal CTE—introducing a 90° layup at the center of the laminate significantly enhances thermal contrast. These 90° layups generate a stronger thermoelastic response, acting as built-in heat sources, thereby further optimizing the detection, tracking, and visualization of damage using thermal elastic stress analysis (TSA) technology.

4. Conclusion

This study utilized center-cracked tension (CCT) specimens to investigate the influence of artificial delamination on the Mode II fracture toughness and full-field response of carbon fiber-reinforced polymer (CFRP) laminates. The results indicate that when using 5 μm-thick steel inserts, the application of Frekote® mold release agent has a minimal impact on the assessed fracture toughness.

The study confirms that digital image correlation (DIC) and infrared thermography techniques are capable of detecting the onset of unstable damage propagation. Additionally, this study proposes a new method for assessing fracture toughness based on the temperature rise value during failure events—a method that shows high agreement with the traditional load drop-based approach.

Furthermore, when performing thermal elastic stress analysis (TSA) on carbon fiber specimens, the introduction of a 90° layup significantly enhances thermal contrast, thereby improving the detection capability for subsurface delamination.