Influence of aging mechanisms on material properties - A428

 

Influence of aging mechanisms on material properties
Foundational knowledge article
Document Type	Article
Document Identifier	428
Tags	Physical aging Chemical degradation Thermal degradation
Prerequisites	Material properties Thermal behaviour

Introduction

The aging process in fiber and polymer materials occurs by different factors, as it is mentioned in the previous article. Therefore, this article enters in the same area as Aging mechanisms in fibers and polymer materials.

Significance

Understanding material aging is crucial for readers because it directly affects the performance, safety, and reliability of products over time, as it was mentioned in Aging mechanisms in fibers and polymer materials. It helps readers make informed decisions about material selection, design, and protective measures to extend service life and prevent premature failures with fundamental knowledge. Therefore, this information may be useful for ensuring cost-effective, durable, and safe designs in future structures or composite materials.

Scope

This article discusses the influence of aging on material properties, such as changes in modulus and strength, increased ductility/brittleness, surface degradation, dimensional instability, reduced moisture resistance, and color changes.

Effect of aging mechanisms of polymers on material properties

Polymers are widely used in engineering applications because of their low density, processability, and adjustable properties. Despite their advantages, polymer performance changes over time as environmental and operational factors induce aging processes that alter their chemical structure, morphology, and overall behaviour. Understanding these influences is essential for predicting service life, ensuring safety, and guiding material selection.

Thermal aging

Exposure to elevated temperatures accelerates chain scission, oxidation, and unintended crosslinking reactions, and as the temperature approaches or exceeds the glass transition temperature (Tg), increased molecular mobility further facilitates degradation. As a result, mechanical properties such as strength and modulus are typically reduced due to chain breakage and loss of molecular weight, while ductility may initially increase because of plasticization associated with chain scission but subsequently decreases at more advanced stages of degradation. In addition, dimensional stability is adversely affected, with polymers exhibiting shrinkage, softening, or permanent deformation when Tg is exceeded.

Hydrothermal and moisture-induced aging

Water molecules diffuse into the polymer matrix, causing swelling and plasticization and, in some cases, inducing hydrolysis of susceptible chemical bonds. This moisture ingress leads to a reduction in mechanical properties, with stiffness and strength decreasing, particularly in hygroscopic polymers such as polyamides. While plasticization effects may be partially or fully reversible upon drying, hydrolysis-driven chain scission results in permanent degradation of the polymer network. In addition, moisture exposure compromises surface integrity, as evidenced by the formation of microcracks, voids, and cavities observed under scanning electron microscopy (SEM), which in turn diminishes fatigue resistance.

Oxidative and UV-induced aging

Exposure to oxygen and ultraviolet (UV) radiation initiates free-radical reactions that cause polymer chain scission and unintended crosslinking, which are photo-oxidation processes particularly critical in outdoor applications. As a result, noticeable changes in color and appearance may occur, including yellowing, chalking, and loss of gloss. Surface properties can be adversely affected, with embrittlement and microcracking developing and promoting crack initiation. These molecular-level degradations progressively reduce mechanical performance, leading to declines in tensile strength and elongation at break due to the reduction in molecular weight.

Chemical and environmental stress cracking

Environmental stress cracking results from the synergistic action of specific chemicals, such as solvents, acids, and bases, combined with sustained mechanical stresses, which together promote crack initiation and subcritical crack growth. This degradation mechanism adversely affects material performance by reducing fracture resistance through the formation of crack initiation sites, diminishing long-term durability under load-bearing conditions, and compromising reliability, as environmental stress cracking often appears as an unexpected failure mode in polymers exposed to aggressive environments.

Mechanical loading and fatigue aging

Repeated cyclic stresses progressively induce creep, fatigue, and microcrack growth, leading to cumulative damage over time. As a result, permanent deformation can occur even under moderate loading due to creep and stress relaxation, while fatigue life is reduced as cracks nucleate and propagate. In addition, degradation of dynamic properties, particularly changes in damping behavior and reductions in modulus, adversely affects the vibration performance of the material.

Physical aging

Below the glass transition temperature (Tg), amorphous polymers undergo a gradual structural relaxation toward thermodynamic equilibrium, a phenomenon commonly referred to as physical aging. This process is accompanied by an increase in stiffness as the polymer structure densifies, a concurrent reduction in ductility that promotes more brittle fracture behavior, and subtle changes in dimensional stability, including slight shrinkage and a reduction in free volume within the material.

Combined effects and engineering implications

Real-world conditions usually involve multiple aging mechanisms simultaneously (e.g., heat + moisture + load). This leads to synergistic degradation effects, complicating lifetime predictions. The influence of aging on polymers is multifaceted, altering mechanical, thermal, and surface properties. Degradation mechanisms such as thermal scission, hydrolysis, oxidation, and physical aging reduce stiffness, toughness, and dimensional stability, while accelerating crack initiation and growth. These property changes have direct implications for the design, durability, and safety of polymer-based structures, highlighting the importance of aging studies in materials engineering.

Effects of aging mechanisms of reinforcement elements (e.g., fibers) on material properties

Fibers (glass, carbon, basalt, etc.) are key load-bearing constituents in advanced materials and composites. Their performance over time is strongly influenced by environmental and mechanical aging mechanisms. These processes gradually alter the microstructure, surface chemistry, and intrinsic strength of fibers, which in turn impact their stiffness, toughness, durability, and overall reliability in engineering applications. The main aging mechanisms affecting fibres are outlined below.

Thermal aging

Prolonged exposure of polymer composites to elevated temperatures induces degradation mechanisms such as molecular chain scission, reductions in polymer crystallinity, and oxidative reactions at fiber surfaces. In glass fibers, thermal exposure can promote relaxation of network bonds, whereas carbon fibers may undergo surface oxidation at high temperatures. These mechanisms result in a reduction in tensile strength and elastic modulus, accompanied by dimensional instability and increased embrittlement. As degradation progresses, the fibers lose their effectiveness in load transfer, leading to an overall weakening of the composite system.

Hydrothermal and moisture-induced aging

Moisture exposure affects reinforcing fibers through several degradation mechanisms, as water molecules penetrate fiber surfaces, dissolve leachable constituents, or disrupt existing chemical bonds. Glass and basalt fibers are particularly susceptible to hydrolysis under such conditions, whereas aramid fibers primarily absorb moisture due to the presence of hydrogen-bonding sites within their molecular structure. These interactions lead to measurable property changes, including reductions in stiffness and strength caused by swelling, material dissolution, or stress corrosion cracking, while a resulting increase in fiber surface roughness promotes microcrack initiation and accelerates damage evolution.

Chemical and environmental aging

Contact with acids, alkalis, solvents, or other corrosive environments alters the surface composition of reinforcing fibers, while exposure to oxidative species and ultraviolet radiation further intensifies degradation mechanisms. These chemically and environmentally driven processes lead to surface cracking and discoloration, promote a reduction in interfacial shear strength, and ultimately increase the susceptibility of the material to mechanical failure under applied stress.

Mechanical fatigue and stress-assisted aging

Cyclic or sustained mechanical loading leads to the development of microcracks, creep deformation, and fatigue-induced defects within the material, while simultaneous chemical exposure under applied stress further accelerates environmental stress cracking. As a consequence of these coupled mechanisms, the material experiences a progressive reduction in tensile strength and fatigue life, accompanied by the accumulation of microstructural damage and a corresponding decrease in reliability during long-term service.

Combined effects and engineering implications

Often, fibers experience multiple degradation modes simultaneously. For instance, moisture uptake accelerates stress corrosion cracking in glass fibers, while elevated temperature enhances oxidation in carbon fibers. Such coupled mechanisms amplify property loss more severely than individual processes. These effects have relevant engineering implications, such as durability concerns as fibre degradation reduces stiffness, strength, and load-bearing capacity of fiber-based systems. In addition, material selection must consider for specific environmental exposures. Protective coatings, sizings, or chemical treatments can mitigate aging effects. Finally, changes in reliability due to aging mechanisms directly affects long-term service life predictions, structural safety, and maintenance costs.

References

In the context of knowledge in practice, knowledge refers to the systematic use of science based knowledge in composites manufacturing practice.

There is a distinction between experience based knowledge and science based knowledge:

• Experience based knowledge (‘know-how’) is an understanding of potential outcomes and their relationships that is founded on pragmatism and experience accumulated over time in individual programs, companies and in the industry more broadly.
• Science based knowledge (‘know-why’) is an understanding of potential outcomes and their relationships, based on the important processing physics, that is mature enough to be codified using the appropriate governing laws and constitutive equations.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Literally "without structure", randomly coiled molecular polymer chains.