Fiber Longitudinal Direction Research Articles

Material removal and damage formation mechanisms induced by periodic indentation during ultrasonic vibration-assisted scratching of SiCf/SiC composites

Fiber-reinforced ceramic matrix composites (CMCs) exhibit high hardness, brittleness, heterogeneity, and anisotropy. This study explores the material removal and damage formation mechanisms during ultrasonic vibration-assisted scratching of SiCf/SiC composites through experimental and micro-finite element simulations. The research reveals that high-frequency periodic indentation-separation of abrasive grain influences stress transfer and crack propagation, which are affected by the interfacial layer and fiber orientation. Crack deflection plays a crucial role in material removal and determines the amplitude of normal scratching force. Stress transfer interference primarily governs damage formation, with more severe damage occurring along the longitudinal and transverse fiber directions due to secondary stress concentration and crack deflection. In contrast, scratching along the perpendicular fiber direction requires higher normal forces due to residual adjacent phase materials, but results in more controlled material removal and less damage. The study provides insights into optimizing the ultrasonic vibration-assisted machining of CMCs to minimize damage.

Effect of carbon fiber and thermoplastic resin on laminates under lightning and dielectric strength tests

AbstractThe incorporation of carbon fiber (CF) and thermoplastic resin in wind turbine rotor‐blade represents a challenge for lightning events because CF conducts electricity and the behavior of thermoplastic resins is not well known. A comparative study with laminates with epoxy or thermoplastic resins was performed, using lightning and dielectric strength tests. Lightning tests revealed that resins behaved similarly; the current preferentially flowed in fiber direction and in zones with high contact between fibers (e.g., criss‐crossed fibers at panel edges); the level of through‐thickness damage was inversely related to panel thickness; current flow did not affect the tensile properties in fiber direction, but produced a drop in bending strength; the adequate integration of earth terminals in CF laminates was critical for a proper lightning discharge. In dielectric tests, it was concluded that panels with glass fiber covering CF laminates behaved as through‐thickness insulators; the thermoplastic reduced the laminate's dielectric strength, while the incorporation of CF, or laminate aging in a salt spray chamber showed no difference. Electrical resistance tests showed that the results were not dependent on the type of resin, but on the level of fiber‐to‐fiber and fiber‐to‐terminal contact. The lowest linear resistance was obtained in the longitudinal fiber direction, followed by the transverse and through‐thickness ones.

Experimental characterisation of the local mechanical behaviour of cellulose fibres: an in-situ micro-profilometry approach

The accurate mechanical characterisation of fibres of micrometric length is a challenging task, especially in the case of organically-formed fibres that naturally exhibit considerable irregularities along the longitudinal fibre direction. The present paper proposes a novel experimental methodology for the evaluation of the local mechanical behaviour of organically-formed (aged and unaged) and regenerated cellulose fibres, which is based on in-situ micro-tensile testing combined with optical profilometry. In order to accurately determine the cross-sectional area profile of a cellulose fibre specimen, optical profilometry is performed both at the top and bottom surfaces of the fibre. The evolution of the local stress at specific fibre locations is next determined from the force value recorded during the tensile test and the local cross-sectional area. An accurate measurement of the corresponding local strain is obtained by using Global Digital Height Correlation (GDHC), thus resulting in multiple, local stress–strain curves per fibre, from which local tensile strengths, elastic moduli, and strains at fracture can be deduced. Since the variations in the geometrical and material properties within an individual fibre are comparable to those observed across fibres, the proposed methodology is able to attain statistically representative measurement data from just one, or a small number of fibre samples. This makes the experimental methodology very suitable for the mechanical analysis of fibres taken from valuable and historical objects, for which typically a limited number of samples is available. It is further demonstrated that the accuracy of the measurement data obtained by the present, local measuring technique may be significantly higher than for a common, global measuring technique, since possible errors induced by fibre slip at the grip surfaces are avoided.

Strongly polarized light from highly aligned electrospun luminescent natural rubber fibers

We report the production of luminescent natural rubber micro/nanofibers with high polarized photoemission. The electrospinning technique was used to produce aligned mats from natural rubber. The fibers have diameters that vary between 200 nm and 2.5 μm and with a mean diameter size around 820 nm. The produced fibers were parallel aligned, presenting optical transparency and pronounced birefringence due to polymeric chain alignment. Luminescent elastomeric fibers were produced using a benzothiadiazole-based organic luminescent small molecule dispersed in the natural rubber precursor solution before electrospinning. The photoluminescence spectrum of the fibers indicates that the vibrational resolution is higher than for this luminescent molecule in solution and membrane. This observation was attributed to the molecular packing inside the anisotropic fiber structure, which reduces the degrees of freedom and the molecular interactions of the dye. Therefore, resulting in pronounced linear polarized emission, which could still be improved by stretching along the fiber longitudinal direction, where a dichroic ratio of 13.0 and anisotropy factor of 0.98 were obtained. The experimental observations were supported by theoretical modelling. Such a system offers great potential for technological applications like optical sensors to monitor changes in the emission degree of polarized light when exposed to different ambient conditions.

High-speed inspection of delamination defects in unidirectional CFRP by non-contact eddy current testing

The anisotropic bulk conductivity of carbon fibre reinforced polymers is a challenge for eddy current testing (ECT). The conductivity is markedly higher in longitudinal fibre direction compared to the transverse directions. This means that ECT has mainly been used for detecting imperfections that affect the direction and magnitude of the longitudinal conductivity. That is, fibre misalignment and broken fibres respectively. Damage in the matrix, such as delamination, affects the already low transverse bulk conductivity and is therefore very difficult to detect using ECT.A customized eddy current probe developed for detecting delamination damage in carbon fibre reinforced polymer (CFRP) composite at 4 m/s is presented in this paper. The design of the probe is based on numerical simulation of eddy current flow in the anisotropic CFRP composite. It was found that the optimal coil configuration has a current flow parallel to the conducting carbon fibres, but in two in-line coils with opposite current flow direction operating at 1 MHz frequency. This forces the eddy currents between the probes to flow perpendicular to the fibres, making the probe sensitive to changes in transverse conductivity caused by delamination. The simulated probe configuration was validated experimentally, and the constructed inspection device was able to detect real delamination damage in a CFRP component with demand for high-quality inspection.

Innovative concepts for the usage of veneer-based hybrid materials in vehicle structures

A promising approach for the development of sustainable and resource-saving alternatives to conventional material solutions in vehicle structures is the use of renewable raw materials. One group of materials that has particular potential for this application is wood. The specific material properties of wood in the longitudinal fiber direction are comparable to typical construction materials such as steel or aluminum. Due to its comparatively low density, there is a very high lightweight construction potential especially for bending load cases. Structural components of the vehicle body are exposed to very high mechanical loads in the case of crash impact. Depending on the component under consideration, energy has to be absorbed and the structural integrity of the body has to be ensured in order to protect the occupants. The use of natural materials such as wood poses particular challenges for such applications. The material characteristics of wood are dispersed, and depend on environmental factors such as humidity. The aim of the following considerations was to develop a material system to ensure the functional reliability of the component. The test boundary conditions for validation also play a key role in this context. The potential of wood–steel hybrid design based on laminated veneer lumber and steel was investigated for use in a component subjected to crash loads such as the door impact beam. The chosen solution involves a separation of functions. A laminated veneer lumber-based beam was hybridized with a steel strip on the tension side. The steel strip was designed to compensate the comparatively low elongation at fracture of the wood and to ensure the integrity of the beam. The wooden component was designed for high energy absorption due to delamination and controlled failure during the impact, while maintaining the surface moment of inertia, i.e. the bending stiffness of the entire component. This approach was chosen to ensure the functional safety of the component, avoid sudden component failure and utilize the high potential of both materials. The tests carried out provided initial functional proof of the chosen solution. The hybridization achieved significantly higher deformations without sudden failure of the beam. In addition, bending capabilities were increased significantly compared to a beam without hybridization. In comparison with a state-of-the-art steel beam, the hybrid beam was not able to achieve the maximum deformation and the target weight of the hybrid beam. Further optimization of the hybrid beam is therefore necessary.

Mechanical properties of multiwall carbon nanotubes/unidirectional carbon fiber-reinforced epoxy hybrid nanocomposites in transverse and longitudinal fiber directions

Unidirectional carbon fiber-reinforced epoxy (UCFRE) is suffering from weak transverse mechanical properties and through-thickness properties. The effect of different amount (0.1, 0.3 and 0.5 phr which is proportional to 0.09, 0.27 and 0.46 wt%, respectively) of multiwall carbon nanotube (MWCNT), on transverse tensile properties, flexural strength, fracture toughness in transverse and longitudinal fiber directions, interlaminar shear strength and lap shear strength of UCFRE has been investigated. Dicyandiamide was used as a thermal curing agent of epoxy resin. MWCNT was dispersed in the epoxy resin by ultrasonic instrument and their dispersion state was investigated by scanning electron microscopy (SEM). The curing behavior of epoxy resin and its nanocomposites was assessed by differential scanning calorimetry. Results show that transverse tensile strength, modulus and strain-at-break were increased by 28.5%, 25% and 14%, respectively by adding 0.1 phr of MWCNT. Longitudinal flexural properties of UCFRE was not changed by adding different amount of MWCNT. Although longitudinal flexural strength was increased by 5% by adding 0.1 phr of MWCNT. Fracture toughness in transverse and longitudinal fiber directions was increased by 39% and 9%, respectively at 0.3 phr of MWCNT. Results also show that interlaminar shear strength and lap shear strength were increased at 0.3 phr of MWCNT by 8% and 5%, respectively. These increases in mechanical properties were due to the good adhesion of fibers to the matrix, interlocking and toughening action of MWCNT as revealed by SEM.

Tensile and fatigue properties of fiber-reinforced metal matrix composites Cf/5056Al

Fiber-reinforced metal matrix composites have mechanical properties highly dependent on directions, possessing high strength and fatigue resistance in fiber longitudinal direction achieved by weak interface bonding. However, the disadvantage of weak interface combination is the reduction of transversal performances. In this article, tensile and fatigue properties of carbon fiber-reinforced 5056 aluminum alloy matrix (Cf/5056Al) composite under the condition of medium-strength interface combination are carried out. The fatigue damage mechanisms of Cf/5056Al composite under tension–tension and tension–compression loads are not the same, but the fatigue life curves are close, which may be the result of the medium-strength interface combination.

Mechanical properties of nanoclay-filled kenaf and hybrid glass/kenaf fiber composites

Kenaf is a natural fiber that can be used as an alternative reinforcement material due to its excellent properties such as low density and high strength. In this study, kenaf composites were developed by hybridizing with glass fiber and modified with nanoclay to enhance its mechanical properties. The woven type of glass fiber was placed on top and bottom of the kenaf composite. Three different percentages of nanoclay contents, i.e. 1 wt%, 3 wt%, and 5 wt% were modified with epoxy resin to fabricate nanoclay-filled Kenaf (KFRP) and nanoclay-filled hybrid Glass/Kenaf (GKFRP) composites. The composites were prepared by dispersing nanoclay into epoxy using the three-roll mill machine, and the kenaf fiber was wound using filament winding machine. The flexural test and tensile test in longitudinal (0°) and transverse (90°) fiber direction were performed to study the effect of hybridizing glass fiber and adding nanoclay into KFRP composites. The results show that hybridization of kenaf fiber with glass fiber enhanced flexural and tensile properties in both fiber directions. The addition of 1 wt% and 3 wt% nanoclay also improve the tensile and flexural properties of KFRP composites; however, the addition of 5 wt% nanoclay reduces the mechanical performance of composites.

Glass fiber/polypropylene composites with potential of bone fracture fixation plates: Manufacturing process and mechanical characterization

Mechanical properties and manufacturing processes of Glass Fiber/Polypropylene (GF/PP) composites for application of flexible internal long bone fracture fixation plates have been investigated. PP/Short Chopped Glass Fiber (PPSCGF), PP/Long Glass Fiber (PPLGF) and PP/Long Glass Fiber Yarn (PPLGFY) were used in fabrication of the fixation plates. The PPSCGF and PPLGF plates were made by the heat-compressing process and Three-dimensional (3D) printing method was used to make the PPLGFY ones. The values of Young’s modulus, tensile strength, flexural modulus and strength, and impact strength of the PPSCGF in the fiber longitudinal direction were found to be [Formula: see text]GPa, [Formula: see text]MPa, [Formula: see text]GPa, [Formula: see text]MPa and [Formula: see text]kJ/m2, respectively. Where, these values for the PPLGF were to [Formula: see text]GPa, [Formula: see text]MPa, [Formula: see text]GPa, [Formula: see text]MPa, and [Formula: see text]kJ/m2 and for the PPLGFY were to [Formula: see text]GPa, [Formula: see text]MPa, [Formula: see text]GPa, [Formula: see text]MPa and [Formula: see text]kJ/m2. These have been found to be in close agreement with the human bone properties. Furthermore, the strength and modulus values of the plates were reasonable to be used as a bone implant applicable for bone fracture reconstructions. Hence, the study concluded that the GF/PP composites are useful for load-bearing during daily activities and would be recommended as a choice in orthopedic fixation plate applications. It will help the researchers for development of new fixation designs and the clinicians for better patient’s therapy in future.

Highly strong and flexible composite hydrogel reinforced by aligned wood cellulose skeleton via alkali treatment for muscle-like sensors

Strong and flexible hydrogels have attracted increasing scientific interest due to their “soft-and-wet”properties, which are similar to those of human soft tissues. In this study, a bioinspired, facile method to fabricate a composite hydrogel with a naturally high-strength skeleton structure that shows comparable mechanical performance to those of muscle, tendons and ligaments is reported. The method includes extracting an aligned cellulose skeleton directly from wood by delignification and then compositing with polyacrylamide (PAM) by in-situ chemical polymerization. During these processes, the natural wood skeleton is well preserved and sufficiently high tensile stress is created along the longitudinal direction (L-direction) of wood cellulose fiber, which leads to a highly anisotropicstructure in the prepared PAM/delignified wood (PDW) hydrogels. Although the aligned cellulose skeleton exhibits an efficient strengthening effect, the obtained PDM hydrogels lack the desired flexibility and are easily broken during the harsh deformations. To achieve flexiblility, a simple alkali treatment was applied to the delignified wood which provided the developed PAM/alkali-treated delignified wood (A-PDM) hydrogels with superflexiblility. As a result, the A-PDM hydrogels show excellent tensile properties with Young’s modulus, tensile fracture strength and elongation of 145.52 ± 2.32 MPa, 16.47 ± 1.40 MPa and 15.99 ± 1.81%, respectively. Such favorable mechanical performance is employed to assemble a muscle-like sensor by incorporating the A-PDM hydrogel with ionic Na2SO4 to detect diverse macro-scale human motions. This study provides a facile strategy for designing strong composite hydrogels with superflexibility, opening an effective route for the development of new wood nanotechnology and various functional wood-derived materials.

Temperature dependence of statistical fatigue strengths for unidirectional carbon fiber reinforced plastics under tension loading

The formulation for time- and temperature-dependent statistical static and fatigue strengths for carbon fiber reinforced plastics laminates is newly proposed based on the physically serious role of resin viscoelasticity as the matrix of carbon fiber reinforced plastics. In this study, this formulation is applied to the tensile strength along the longitudinal direction of unidirectional carbon fiber reinforced plastics constituting the most important data for the reliable design of carbon fiber reinforced plastics structures which are exposed to elevated temperatures for a significant period of their operative life. The statistical distribution of the static and fatigue strengths under tension loading along the longitudinal direction of unidirectional carbon fiber reinforced plastics were measured at various temperatures by using resin-impregnated carbon fiber reinforced plastics strands as specimens. The master curves for the fatigue strength as well as the static strength of carbon fiber reinforced plastics strand were constructed based on the time–temperature superposition principle for the matrix resin viscoelasticity. The long-term fatigue strength of carbon fiber reinforced plastics strand can be predicted by using the master curve of fatigue strength.

Visualization of microfibrillar elements in cross-section of polyacrylonitrile fiber along the fiber spinning line.

The microfibrils served as the structural elements in polyacrylonitrile (PAN) fiber, which played an important role in the quality of the PAN precursor fibers. Their morphologies were examined by the scanning electron microscopy (SEM), atomic force microscopy (AFM) and high-resolution transmission electron microscope (HRTEM). The microfibrils existed in all of PAN fibers and arranged evenly in the cross-sections. Furthermore, the pores existed between the microfibrils. The unoriented microfibrillar network was already formed in nascent fiber during coagulated process. Although the microfibrillar network was elongated and the microfibrils oriented along the fiber longitudinal direction during the spinning process, the interconnected microfibrillar network was still existed in the fiber transverse section. Furthermore, the transverse connection of the microfibrils was reinforced and the small microfibrils were tended to aggregate into the large fibrils. For mechanical performance of PAN fibers, their tensile strength increased to 708 MPa and the elongation at break decreased to 15.5%. PAN fibers exhibited ductile rupture during the mechanical test and the microfibrils served as reinforcing elements.

The microstructures and mechanical properties of ultra-high-strength PAN-based carbon fibers during graphitization under a constant stretching

Commercial ultra-high-strength PAN-based carbon fibers (T1000G) were heat-treated at the temperature range of 2300–2600 °C under a constant stretching of 600 cN. After continuous high-temperature graphitization treatment, microstructures, mechanical properties and thermal stability of the carbon fibers were investigated. The results show that the T1000G carbon fibers present the similar round shape with a smooth surface before and after graphitization, indicating the carbon fibers are fabricated by dry–wet spinning. In comparison, the commercial high-strength and high-modulus PAN-based carbon fibers (M40JB and M55JB) present elliptical shapes with ridges and grooves on the surface, indicating the carbon fibers are fabricated by wet spinning. After graphitization treatment from 2300 to 2600 °C under a constant stretching of 600 cN, the Young’s modulus of the T1000G carbon fibers increases from about 436 to 484 GPa, and their tensile strength decreases from about 5.26 to 4.45 GPa. The increase in Young’s modulus of the graphitized T1000G carbon fibers is attributed to the increase in the crystallite sizes and the preferred orientation of graphite crystallites along the fiber longitudinal direction under a constant stretching condition. In comparison with the M40JB and the M55JB carbon fibers, the graphitized T1000G carbon fibers are easier to be oxidized, which can be contributed to the formation of more micropores and defects during the graphitization process, thus leading to the decrease in the tensile strength.

Mesenchymal stem cell interacted with PLCL braided scaffold coated with poly-l-lysine/hyaluronic acid for ligament tissue engineering.

The challenge of finding an adapted scaffold for ligament tissue engineering remains unsolved after years of researches. A technology to fabricate a multilayer braided scaffold with flexible and elastic poly (l-lactide-co-caprolactone) (PLCL 85/15) has been recently pioneered by our team. In this study, polyelectrolyte multilayer films (PEM) with poly-l-lysine (PLL)/ hyaluronic acid (HA) were deposited on this scaffold. After PEM modification, polygonal (PLL) and particle-like (HA) structures were present on the braided scaffold with no significant variation of fibers Young's modulus. Wharton's jelly mesenchymal stem cells (WJ-MSC) and bone marrow mesenchymal stem cells (BM-MSC) showed good metabolic activity on scaffolds. They presented a spindled shape along the fiber longitudinal direction, and crossed the fibers to form cell bridges. Collagen type I, collagen type III, and tenascin-C secreted by MSCs were detected on day 14. Moreover, one-layer modified scaffold presented increased chemotaxis. As a conclusion, our results indicate that this braided PLCL scaffold with one-layer PEM modification shows inspiring potential with satisfying mechanical properties and biocompatibility. It opens new perspectives to incorporate growth factors within PEM-modified braided PLCL scaffold for ligament tissue engineering and to recruit endogenous cells after implantation. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3042-3052, 2018.

Effect of strain rate on mechanical properties of the bamboo material under quasi-static and dynamic loading condition

As a kind of fiber-reinforced bio-composite, bamboo material exhibits superior mechanical performance. In present work, quasi-static and dynamic Split Hopkinson Pressure Bar (SHPB) compression experiments at the nominal strain rate range of 500 s−1–2200 s−1 were carried out to investigate the effect of strain rate on mechanical behaviors of the bamboo material. In consideration of the effect of fiber directions on mechanical behaviors, the specimens were crushed with the loading direction parallel to the fiber longitudinal direction and perpendicular to the fiber direction, respectively. The experiments under both loading directions exhibited remarkable strain rate sensitivity. The yield stress for the specimens crushed along the fiber longitudinal direction increased from 38.59 MPa to 94.25 MPa, as the strain rate varied from 1.85 × 10−3 to 2200 s−1, while it increased from 7.18 MPa to 42.04 MPa for the specimens crushed perpendicularly to the fiber direction. Final deformation photographs and scanning electron micrographs of the bamboo microstructures clearly illustrated their damage morphologies and various failure mechanisms, including fiber buckling, fiber fractures, PCs collapse, delamination and crack propagation at different strain rates and loading directions. These results may provide a novel perspective on the mechanical behaviors of bamboo microstructure and design of new bio-inspired structural materials.

Strength evaluation of unidirectional carbon fiber-reinforced plastic laminates based on tension–compression biaxial stress tests

Biaxial stress tests of carbon fiber-reinforced plastic (CFRP) laminates were performed to investigate failure criteria under biaxial loads. Specimens of unidirectional CFRP laminates were subjected to a tensile load in the longitudinal fiber direction and a compressive load in the transverse fiber direction. An exclusive jig was used to perform biaxial stress tests with a commonly used single-axis testing machine. Measurements were obtained by controlling the displacement ratio between compressive and tensile displacements. The critical tensile and compressive stresses were then calculated using a constitutive equation. The critical longitudinal tensile stress markedly dropped with increasing the compressive load. The failure criteria of the biaxial stress tests were expressed as the ellipse, of which the major and minor axes were the longitudinal tensile/transverse compressive strengths or fracture strains, respectively. Scanning electron microscope observations suggest that fiber/matrix interfacial debonding due to the compressive load could decrease the critical longitudinal tensile stress.

Properties of natural fibre composites for structural engineering applications

Abstract Since the last two decades, many researchers have refocused on using natural fibres, as reinforcements for cementitious and polymer based structural materials. The use of natural fibres for structural applications is not modern. Long before a century ago people in many small towns in China and Korea already mixed straws with mud to build walls in villages. However, at that time there was no systematic way to study the fundamental mechanism and interpret how natural fibres strengthen structures and what processes should be adopted to maximize the performance of natural materials. Currently, nevertheless natural fibre reinforced polymer (NFRP) composites have been widely used in automotive and building industries, it is still a room to promote them to high-level structural applications like primary structural components of aerospace and maritime structures. It is difficult to evaluate the quality of natural fibres, which generally extracted from the nature, and thus it is challenging to develop a generic formula to predict the structural and mechanical properties of NFRP composites. Hydrophilic and hydrophobic properties of natural fibres and polymers, respectively, cause poor bonding interaction at interface. Traditional shear-lag model was popularly used to study the stress transfer mechanism between fibre and matrix of advanced composites. However, such model is not applicable to NFRP composites due to the imperfect shape of nature fibres along their longitudinal direction and irregular shape of fibres' cross section. In this paper, analysis on different aspects in related to the use of natural fibres for real life engineering applications is given. Basic analytical models focusing on the stress transferability in composites are also discussed to provide an insight for researchers and engineers to understand the design and requirements of using natural fibres for structural applications in the future.

Thermo-oxidative stability and remarkable improvement in mechanical performance for styrenic-based elastomer composites contributed from silane-treated pineapple leaf fiber and compatibilizer

Composites of styrenic based thermoplastic elastomer reinforced with pineapple leaf fiber were prepared by melt mixing on a two-roll mill, sheeted out with preferred alignment of the fiber and molded in the form of composite sheet. The effects of silane treatment and compatibilizer on thermo-oxidative stability and mechanical properties were investigated. The silane-treated fiber and compatibilizer were found to enhance the thermo-oxidative stability of polymer. The mechanical properties of elasomer matrix were clearly enhanced with fiber loading. The role of surface treatment and compatibilizer was effectively functioned only along the longitudinal direction of fiber. Young’s and dynamic storage moduli (E′) along the fiber direction for the treated fiber composites with and without compatibilizer were comparable and much higher than those of the untreated fiber system. Tensile stress at 10–350% for the treated fiber composite was much higher than those of the matrix and untreated fiber composite. The tensile stress was further improved by the addition of compatibilizer. The obtained results suggested that both silane-treatment and compatibilizer were essential in prolonging the further improved tensile stress at high strain.

Sensitivity characteristics of broadband fiber-laser-based ultrasound sensors for photoacoustic microscopy.

High-frequency fiber laser sensor is a new acoustic detector for photoacoustic imaging. However, its performance has not been thoroughly studied. Here, we present a comprehensive characterization of a fiber laser sensor for photoacoustic imaging. Ultrasound waves deform the fiber laser cavity and induce frequency changes in the heterodyning output signal. The sensitivity peaks at 22 MHz, which is associated with an azimuthal mode number l = 2 and a radial mode number n = 1. The broadband acoustic sensitivity in terms of frequency shift is 2.25 MHz/kPa and the noise-equivalent pressure reaches 45 Pa with a sampling rate of 100 MHz. The 3-dB bandwidth is 18 MHz for spherical-wave detection. We characterized the spatial distribution of acoustic sensitivity. The sensitivity along the fiber longitudinal direction varies with the laser spatial mode and is determined by the grating and cavity parameters. The sensitivity at the azimuthal direction presents a |cos(2θ)| dependence as a result of fiber core asymmetry. In the radial direction, the sensitivity is inversely proportional to the square root of the distance between the source and the detector. The acoustic sensitivity can be enhanced by reducing the cavity length. We experimentally show that a short sensor can enhance the contrast and penetration depth of PAM than a long one.