Damage Characteristics in Glass Fiber-Reinforced Epoxy Resin Composites Under Continuous Wave and Pulsed Laser Modes

Abstract

Composite materials have been extensively utilized in unmanned aerial vehicles (UAVs) and other aerospace applications due to their unique advantages, while laser countermeasures have emerged as a critical approach in anti-UAV warfare. Different laser modes produce significantly different effects. In this study, we have mainly considered the ablation and damage characteristics of composite materials affected by continuous wave (CW) and pulsed laser (PL) modes. Through comparative analysis of damage morphologies and elemental variations, the damage characteristics and mechanisms of composite materials have been studied, and thermodynamic models have been established. The results demonstrate that the damage produced using the CW mode is primarily thermal ablation, induced through power intensification, whereas the PL mode predominantly causes thermal stress fractures via energy concentration. This investigation provides fundamental references for optimizing laser-based counter-UAV systems.

1. Introduction

Composite materials have been extensively employed in the aerospace [1,2,3], military equipment [4,5], and automotive industries [6,7,8] due to their superior properties, including lightweight characteristics, high strength-to-weight ratio, corrosion resistance, and fatigue durability. Compared to conventional metallic materials, composites show higher specific strength and stiffness, allowing manufacturers to significantly reduce structural weight while enhancing equipment performance and operational endurance [9,10]. For instance, carbon fiber-reinforced polymers (CFRP) and glass fiber-reinforced polymers (GFRP) are widely adopted in unmanned aerial vehicle (UAV) airframe structures [11,12]. The design flexibility and anisotropic nature of composites further enable performance optimization for specific applications. In laser countermeasure scenarios, however, composites demonstrate distinct ablation behaviors compared to metals, owing to their unique thermophysical properties. High-energy laser weapons can precisely target critical components of UAVs, inducing functional failure or structural collapse through localized ablation, thereby offering an effective countermeasure solution [13,14].

Composite materials typically consist of a reinforcement phase (carbon or glass fibers) and a matrix phase (resin, ceramic, or metal), which show significant disparities in thermophysical properties [15]. Under laser irradiation, heterogeneous thermal responses arise due to differential absorption rates among constituents, leading to localized temperature gradients [16]. Interfacial debonding, or slippage, is frequently induced at phase boundaries, owing to a coefficient of thermal expansion (CTE) mismatch. The ablation process involves coupled thermo-mechanical effects, in that thermal phenomena (expansion, decomposition, and melting) interact synergistically with mechanical responses (stress concentration, crack propagation, and failure), further complicating the ablation mechanisms [17]. Recent studies have advanced the understanding of laser–composite interactions. Liu Haiyan et al. demonstrated the exceptional laser ablation resistance of SiC/PBI/PBO composites under CW irradiation at 30 MW·m⁻² [18]. Yan et al. provided critical insights into ablation characteristics through a comprehensive investigation of interactions between a CW laser and lightweight quartz fiber-reinforced phenolic (LQFRP) composites [19]. Wu Dongjiang et al. pioneered the identification of dual ablation states in ceramic matrix composites (CMC), offering theoretical guidance for material selection in laser countermeasures [20]. Furthermore, Xianshi Jia et al. discovered that under the condition of long focusing, even when the CW power density is reduced to 1%, efficient ablation of alumina ceramics can still be achieved by means of the craters formed by the ablation of carbon particles on the material surface [21]. Zhou Li et al. studied the ablation mechanism of laser-induced filaments in alumina ceramics [22] and the damage performance of femtosecond laser-induced air filaments on alumina ceramics [23].

Nevertheless, there are some challenges that need further research, including the following: (1) The diversity of composite materials leads to inconsistent ablation behaviors, which necessitates systematic studies; (2) structural complexity obscures ablation mechanisms, limiting the applicability of existing theoretical and experimental approaches; and (3) insufficient understanding of thermo-mechanical coupling effects hinders the accurate prediction and control of ablation processes [24]. In this study, we focus on elucidating the laser-induced damage characteristics of glass fiber-reinforced epoxy resin (GF/EP) composites through theoretical analysis and simulation modeling. Our main research objective is to reveal the characteristics and influencing mechanisms of CL and PL parameters on ablation effects.

2. Experimental Setup

Two types of lasers were used in this experiment: a CW laser and a PL. The output power range of the CW laser (8000 W-APP fiber laser, 1064 nm, Sichuan Zhongjiu Daguang Technology Co., Ltd., Chengdu, Sichuan, China) is 50 W to 2000 W. The Gaussian beam profile of the PL (Nd: YAG laser, Ltd., Beijing Leibo Laser Technology Co., Ltd., Beijing, China) has a wavelength of 1064 nm, a pulse width of 12 ns, and a frequency range of 1–5 Hz; its output energy stability is ≤3%. The PL beam shows a Gaussian intensity distribution with a spot diameter of 1.2 mm, while the CW has a spot diameter of 1.0 mm.

In Figure 1, the distance between the laser and the focusing mirror is 60 cm, and the distance from the focusing mirror to the target material is 20 cm. During the experiment, the pulsed laser remains stationary, while the CW moves at a certain speed. The beam splitter (with a transmission to reflection ratio of 8:2) divides the laser pulse into two paths, one for ablation and the other for energy monitoring; the former is focused onto the sample through a lens with a focal length (f) of 20 cm, and the sample is installed on a computer-controlled three-axis translation stage for precise adjustment during laser action.

Regarding the measurement of the ablation morphology, we adopted the VHX-2000 optical microscope produced by KEYENCE Company (Osaka, Japan). Its magnification is between 50 and 5000, and it can integrate and splice the sample patterns with height gradients. It can synthesize the surface morphology of the cleaning pit after paint removal in two and three dimensions. It has a depth measurement function and can measure the depth of the cleaning pit after cleaning. The damage mechanism is determined by observing the surface morphology after cleaning.

The experimental material is composed of glass fiber-reinforced epoxy resin composite material, with a structural configuration of alternating layers of epoxy resin and glass fiber rods (calcium carbonate glass).

3. Results and Discussion

3.1. Damage Characteristics of PL

As shown in Figure 2, the ablation morphology of composite materials using a PL (1 Hz) shows a dark color, and the area increases with increasing pulse energy and number of actions, but the growth rate gradually slows down.

The microstructural characteristics (Figure 3a–e) of the effects of one, three, and five pulses at 85 mJ are as follows: The ablated area had a large number of blocky structures, with obvious fracture marks on the edges and no obvious melting marks. The damage characteristics at 100 mJ (Figure 3f–i) were similar to those at 85 mJ, only with more blocky structures and fracture marks at the edge of the damage pit. Overall, for lower-pulse laser energy, the main damage area was on the outermost epoxy resin layer, rather than the underlying calcium carbonate rod layer. In addition, the fracture marks indicate that the epoxy resin layer was subjected to stress-induced damage. The microstructures of the ablation zone were able to cause an increase in the absorption of incident light, resulting in a gray macroscopic color change.

Higher laser pulse energy was chosen for further analysis of damage morphology. At 175 mJ, the epoxy resin layers were peeled off with the increasing number of pulses (Figure 4a), gradually revealing the tubular structure of the calcium carbonate rod layer that crisscrossed vertically and horizontally (Figure 4b,c). Some tubular structures were broken (Figure 4e,f), and the fish scale-like residual structures remained on its surface (Figure 4d). At 250 mJ, the ablation depth deepened with the number of pulses, but it mainly presented a block structure, whereas tubular depressions were found only after the third pulse, indicating that the tubular structure fell off, and, for the fifth pulse, the epoxy resin part on the other side was further damaged. In summary, the ablation depth increased with the energy and number of laser pulses; in this way, the epoxy resin layer and the calcium carbonate tubular layer were successively damaged by thermal stress, and the main damage characteristics were fracture damage.

3.2. Damage Characteristics of CW

For the CW mode, the ablation experiment was carried out using scanning irradiation with a moving speed of 1 cm/s. The entire ablation area presented a concave shape, and ablation depth gradually increased with laser power (Figure 5). The ablation and carbonization of irradiated surfaces resulted in macroscopic blackening.

Composite materials have multiple structures, and the ablation characteristics of CW action are more complex, with the ablation targets mainly including the epoxy resin on the surface layer and the calcium carbonate tubular structure in the central part. The overall morphology of the ablation presented a carbonized depression in the middle and a clearly ablated layered structure at the edges. The degree of ablation became increasingly severe with increasing laser power and began to exhibit different ablation characteristics. For the lower power of 50 W, the laser action mainly caused melting damage, such as the surface melting of calcium carbonate tubes, and a large amount of block-shaped ablation material was distributed at the edges. For the higher power of 60 W, a high-density particle distribution of ablative material was formed, while larger ablative particles were distributed at the edges, indicating a higher degree of ablation in the middle under laser action. Further increasing the power to 70 W caused the fracture site of the calcium carbonate tube to melt completely, and the edge was mainly eroded by high-density particles. For the highest laser power of 100 W, the calcium carbonate tube was ablated into a spherical shape, with high-density micro- and nanoparticles distributed throughout.

3.3. Elemental Content Analysis

During the laser ablation process, the target element undergoes complex processes, such as high-temperature ablation, ionization, and oxidation, resulting in changes in elemental content; on the contrary, these changes can also be used to invert the mechanism of laser ablation. The elements of the materials were tested in the regions affected by the PL (Figure 4h) and CW (Figure 6A) modes and compared with the original materials to infer the corresponding damage mechanisms (Figure 7).

The overall elements were analyzed (Figure 7a) and found to include Ca, Si, Mg, O, and C. It can be seen that the proportion of elements after PL irradiation is close to that of the original material, while, for CW irradiation, the proportions of Ca, Mg, and O were significantly lower than those in the original material. The carbon-to-oxygen (C/O) ratios of the raw materials, PL ablation, and CW ablation were 1.6, 1.5, and 3.5, respectively (Figure 7b). The above outcomes can be used to invert the ablation mechanism, including thermodynamic ablation, high-temperature ionization, etc. If the element ratio remains basically unchanged, the main damage characteristic is fracture caused by thermal stress; an increase in the C/O ratio indicates that the damage was caused by high-temperature ablation or ionization. At high temperatures, carbon acts as a reducing agent and can reduce MgO or CaO to metallic Mg and Ca, while some of the materials are oxidized into CO/CO₂ gas. Due to its strong adhesion, carbon is more likely to be retained in the ablated area, resulting in a significant increase in the relative C/O ratio. Based on the above analysis, it can be seen that the ablation mechanisms of PL and CW irradiation are thermal stress and thermal ablation (high-temperature ionization), respectively.

4. Theoretical Research

4.1. Thermodynamic Analysis Model

The experimental results indicate that there are significant differences in the damage mechanisms between the PL and CW modes. A high-energy PL can rapidly generate high thermal gradients within the material, inducing thermal stress-induced damage, which can lead to local thermal stress concentration, resulting in interlayer delamination of epoxy resin and calcium carbonate tube fractures. From microscopic observation, there are sharp fracture marks in the laser-treated area, and the elemental composition is similar to that of the original material, indicating that the main damage is physical fracture. For CW lasers, continuous irradiation with the laser causes the material to be in a high-temperature state, leading to thermal decomposition and carbonization of the epoxy resin and calcium carbonate rod layer; thus, thermal ablation dominates. The ablation depth increases with the laser power, and a porous carbide layer is formed on the surface; the thermodynamic models with which to conduct quantitative research were established. For the PL mode, the transient thermal stress coupling model needs to be established, and the thermoelastic wave equation was used to describe the transient temperature gradient and thermal stress distribution; this model integrates mechanical properties such as Young’s modulus and tensile strength to evaluate damage in stress concentration areas. For the CW mode, a steady-state thermal field distribution model was established based on the Fourier heat conduction equation, which combines material thermophysical parameters (such as thermal conductivity and specific heat capacity) and ablation threshold to analyze the temperature field evolution and ablation depth at different powers. COMSOL Multiphysics^® 6.3 Simulation software was selected and utilized to systematically analyze the Multiphysics coupling mechanism of laser action through thermodynamic modeling. The properties of the composite materials are shown in

Table 1. Properties of composite materials.

Absorption Ratio (1064 nm) /(m−1)	Thermal Conductivity /(W·m−1·K−1)	Specific Heat Capacity /(J·kg−1·K−1)	Density /(kg·m−3)	Thermal Expansion Coefficient /(K−1)	Melting Point /(K)
6.35 × 105	0.4	1181	1891	15.6 × 10−6	983

, and the laser parameter settings are presented in

Table 2. Properties of composite materials.

PL Energy Setting /(J)	PL Action Time /(Number of Pulses)	CW Energy Setting /(W)	CW Action Time /(s)	Boundary Condition
25 mJ	Act on a pulse	50 W~100 W (With intervals of every 100,000, a total of five groups)	1 s	Neumann boundary conditions

.

4.2. PL Effects

Under the action of a pulsed laser, composite materials rapidly absorb laser energy and cause a rapid increase in temperature. Compared with the glass fiber layer, the external epoxy resin layer exhibits significantly higher temperatures (Figure 8).

Under severe temperature changes in materials, there is a large temperature gradient on the surface and inside, resulting in uneven expansion and significant thermal stress. The stress variation characteristics of a single calcium carbonate rod under PL irradiation are shown in Figure 9.

The stress experiences a drastic rise and fall, which exceeds the fracture threshold between 0.22 ms and 1.3 ms. In summary, with a PL, rapid deposition of laser energy can cause a temperature surge and induce a sudden increase in thermal stress, which leads to fracture damage in the material.

4.3. CW Effects

The temperature distribution simulation of materials under CW irradiation is illustrated in Figure 10, which shows the heat conduction process and ablation behavior of a CW on composite materials, where the heat gradually diffuses into deeper layers of the material through thermal conduction. The temperature inside the material gradually rises with laser power, leading to ablation, melting, and even carbonization of the material.

The different damage mechanisms of PL and CW irradiation on glass fiber-reinforced epoxy resin composite materials were verified through simulation, as shown in Figure 11. PL irradiation mainly causes physical damage to material surfaces through thermal stress, while CW irradiation induces material ablation and carbonization through thermal conduction, leading to chemical damage. The simulation results provide a theoretical basis for optimizing laser processing technology or improving the laser damage resistance of composite materials.

5. Conclusions

This article outlines our systematic study of the damage characteristics of glass fiber-reinforced epoxy resin composite materials under the actions of PL and CW irradiation, revealing their corresponding damage mechanisms. The laser energy of a PL rapidly deposits on the material, causing a sharp increase in surface temperature and forming a large temperature gradient, resulting in thermoelastic vibration and stress failure; its function is to peel off block shaped objects, with obvious fracture marks on the edges. With a CW laser, on the other hand, spherical protrusions appear on the ablated surface, and, as the laser power increases, the protrusions gradually form a complete spherical shape. The above research results can provide a reference for laser countermeasure technology.