Accelerated aging

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A climatic chamber, used in accelerated aging Climatic chambers.jpg
A climatic chamber, used in accelerated aging

Accelerated aging is testing that uses aggravated conditions of heat, humidity, oxygen, sunlight, vibration, etc. to speed up the normal aging processes of items. It is used to help determine the long-term effects of expected levels of stress within a shorter time, usually in a laboratory by controlled standard test methods. It is used to estimate the useful lifespan of a product or its shelf life when actual lifespan data is unavailable. This occurs with products that have not existed long enough to have gone through their useful lifespan: for example, a new type of car engine or a new polymer for replacement joints.

Contents

Physical testing or chemical testing is carried out by subjecting the product to

Mechanical parts are run at very high speed, far in excess of what they would receive in normal usage. Polymers are often kept at elevated temperatures, in order to accelerate chemical breakdown. Environmental chambers are often used.

Also, the device or material under test can be exposed to rapid (but controlled) changes in temperature, humidity, pressure, strain, etc. For example, cycles of heat and cold can simulate the effect of day and night for a few hours or minutes.

Techniques and methods

Accelerated aging employs a variety of controlled methods to replicate and speed up the effects of natural aging. These methods vary depending on the type of product, material, or environmental condition being simulated. Below are the most commonly used techniques:

Environmental Stress testing

Temperature cycling

Samples are exposed to repeated cycles of extreme heat and cold, mimicking daily or seasonal temperature fluctuations. For example, in the automotive industry, components like engines and braking systems are tested using temperature cycling to simulate real-world conditions such as hot desert climates during the day and freezing temperatures at night. In electronics, printed circuit boards (PCBs) are subjected to rapid temperature shifts to evaluate solder joint reliability and material resilience.

Thermal shock

Thermal shock refers to the rapid exposure of materials or components to extreme temperature differences over a very short period. Unlike temperature cycling, which involves gradual changes between high and low temperatures, thermal shock imposes abrupt transitions that can lead to immediate stresses within a material. This method is often used to evaluate a product's resistance to cracking, warping, or other forms of failure caused by sudden thermal gradients. [1] For example, glass or ceramic components in aerospace applications are subjected to thermal shock tests to ensure durability under high-speed atmospheric reentry conditions.

Thermal shock chambers are specialized devices that facilitate rapid temperature transitions to simulate extreme environmental conditions. These chambers typically consist of two or three zones with distinct temperature settings—high, low, and sometimes ambient. A product carrier basket automatically transports the test specimens between these zones, ensuring swift temperature changes. [2] [3]

BGA components are particularly susceptible to failures induced by thermal shock due to the mechanical stresses exerted on solder joints during rapid temperature changes. Research has shown that thermal shock can lead to the initiation and propagation of cracks within these solder joints, compromising the integrity and reliability of electronic assemblies. [4]

The reliability of PCB assemblies often hinges on the durability of their solder joints. In harsh environments characterized by significant temperature variations, these joints are prone to crack formation and eventual fracture, underscoring the importance of rigorous thermal shock testing in the design and assessment of electronic components. [5]

Beyond electronics, thermal shock testing is employed across various industries to assess the durability of materials and products subjected to rapid temperature changes. For example, in the automotive sector, [6] components such as engine parts and safety equipment are tested to ensure they can withstand the thermal stresses encountered during operation. Similarly, "the aerospace industry uses environmental test chambers to test parts like avionics, satellite equipment, and airplane parts. These parts need to be able to handle extreme temperatures during launch, re-entry, and in space. [7]

Humidity testing

- Humidity testing involves subjecting materials or products to high levels of moisture or fluctuating humidity conditions to simulate exposure to tropical, coastal, or industrial environments. This method is used to evaluate the effects of moisture on material degradation, corrosion, swelling, and overall performance. [8]

- For example, electronic devices undergo humidity testing to ensure their enclosures and seals can prevent moisture ingress, while construction materials such as wood or adhesives are tested to evaluate resistance to warping or delamination.

- Humidity testing is often conducted in combination with elevated temperatures to accelerate the effects of moisture exposure, particularly for materials like polymers, metals, and composites.

UV exposure

UV testing is a component of aging tests designed to simulate the long-term effects of ultraviolet (UV) radiation exposure on materials, products, and coatings. [9] UV radiation, a component of sunlight, is one of the primary contributors to material degradation over time. UV testing helps assess the durability and performance of materials under prolonged exposure to UV light, providing insights into their expected lifespan and identifying potential vulnerabilities.

Purpose and Applications: The primary purpose of UV testing is to evaluate the resistance of materials to photodegradation, including fading, discoloration, cracking, embrittlement, or loss of mechanical properties.
Common applications of UV testing include:
Plastics and Polymers: Assessing the weatherability of polymers used in outdoor products.
Coatings and Paints: Ensuring the durability of protective and decorative coatings exposed to sunlight.
Textiles: Evaluating the fade resistance of fabrics and dyes.


Testing Methods: Accelerated UV Testing: This approach uses specialized equipment, such as xenon arc or fluorescent UV lamps, to simulate UV radiation in a controlled environment. Common standards include ASTM G154 (fluorescent UV lamps) and ASTM G155 (xenon arc lamps). [10] [11]

Oxygen and pollutant exposure

Samples are exposed to controlled concentrations of oxygen or atmospheric pollutants (e.g., ozone or sulfur dioxide) to simulate oxidative degradation or corrosion.

Salt spray test

Salt spray test, also known as salt fog testing, is a widely utilized method in environmental stress testing to assess the corrosion resistance of materials and surface coatings. By exposing specimens to a controlled saline environment, this accelerated aging test simulates the corrosive effects of marine and coastal conditions, providing valuable insights into a material's durability and longevity.

Dust testing

Dust testing is used to evaluate the resilience and performance of devices and systems exposed to particulate contaminants.

In gas distribution networks, dust contamination can originate from various sources, notably black powder (sulfide-induced black dust). Hydrogen sulfide (H₂S) present in natural gas can react with metals, particularly copper, forming metal sulfides. Over time, these sulfides can flake off, creating fine black dust that poses risks to gas appliances and meters. [12]

Gas meters are susceptible to contamination from these types of dust. The ingress of metal dust can lead to mechanical wear (Abrasive particles can erode moving parts, compromising measurement accuracy), blockages (Accumulation of dust can obstruct internal pathways, affecting gas flow and meter functionality) and corrosion (Chemical interactions between dust particles and meter components can accelerate degradation).

Mechanical stress testing

Mechanical stress testing evaluates the durability of materials and components under repeated mechanical loads, simulating real-world conditions that may cause degradation over time. These tests help identify potential failures due to fatigue, wear, or structural weaknesses.

High-speed operation

High-speed operation tests assess how a material or device withstands prolonged exposure to rapid movement or mechanical cycling. This is commonly used in industries such as aerospace, automotive, and manufacturing, where components experience frequent high-speed motion. The test may include accelerated wear simulations, friction analysis, and thermal effects caused by rapid motion. [13]

Vibration testing

Vibration testing simulates mechanical oscillations that a component may encounter during its lifespan. This method helps determine resistance to structural fatigue and mechanical resonance, which can lead to failure. Testing is performed using controlled vibration frequencies and amplitudes, often conducted with electrodynamic or hydraulic shakers. Industries such as electronics, transportation, and construction rely on vibration testing to improve product reliability and safety. [14]

Chemical stability testing

Chemical stability testing evaluates the long-term resistance of materials and products to chemical degradation. This form of testing is crucial for determining how substances react to environmental factors such as heat, humidity, oxidation, and exposure to aggressive chemicals. It is widely used in industries such as pharmaceuticals, polymers, and coatings to ensure product reliability and safety.

Thermal aging

Thermal aging tests assess the effects of prolonged exposure to elevated temperatures on a material's chemical and physical properties. High temperatures can accelerate oxidation, polymer degradation, and phase transitions, leading to reduced mechanical strength and altered performance characteristics. These tests are commonly applied in the evaluation of plastics, rubber, lubricants, and electronic components. [15]

Chemical exposure

Chemical exposure testing examines the effects of contact with reactive substances, such as acids, bases, solvents, and oxidizing agents, on material stability. These tests help predict corrosion, swelling, discoloration, and structural degradation that may occur over time. Industries such as pharmaceuticals, aerospace, and construction use chemical exposure testing to assess product durability under real-world conditions. [16]

Combined stress testing

Combined stress testing evaluates the aging behavior of materials, components, and products under multiple simultaneous stress factors. Unlike single-factor aging tests that examine specific conditions (e.g., heat, humidity, mechanical load, or chemical exposure), combined stress testing aims to replicate real-world conditions where multiple degradation mechanisms act together. This type of testing is essential for assessing long-term reliability, identifying failure modes, and improving product durability across industries such as aerospace, automotive, electronics, and pharmaceuticals.

Importance of combined stress testing

Many products are subjected to multiple environmental and mechanical stressors during their operational life. For instance, an electronic device in an automotive setting may experience high temperatures, humidity, vibrations, and cyclic mechanical loads simultaneously. Traditional single-variable aging tests may not accurately predict product lifespan when multiple factors interact in complex ways. By using combined stress testing, researchers and engineers can better understand synergetic degradation effects and develop more resilient materials and designs.

Synergistic effects in combined stress testing

One of the main challenges in combined stress testing is the presence of synergistic effects, where the impact of multiple stresses acting together is greater than the sum of their individual effects. [17] For example:

  • Heat and humidity – When combined, they can accelerate hydrolytic degradation in polymers more significantly than either factor alone. [18]
  • Mechanical stress and corrosion – Repeated loading can cause microcracks that allow corrosive agents to penetrate deeper, leading to faster material failure.
  • Thermal cycling and electrical load – In electronics, repeated temperature changes combined with high current loads can cause solder joint fatigue, increasing the risk of electrical failures. [19]

Challenges and future developments

Despite its advantages, combined stress testing presents challenges, such as:

  • Increased complexity – Simulating multiple stress factors requires advanced test setups and longer evaluation times.
  • Difficulties in failure attribution – Identifying which stressor is primarily responsible for a given failure mode can be challenging.
  • High costs – The need for specialized equipment and extended test durations increases testing expenses.

Emerging techniques, such as machine learning-based predictive modeling and multi-physics simulation, are being explored to optimize combined stress testing. These approaches allow researchers to better predict product performance under complex conditions while reducing the need for extensive physical testing.

Validation of results

The validation of aging test results is essential to ensure that the data obtained accurately represents the long-term performance and durability of a material, component, or product. Aging tests are designed to simulate real-world conditions over an accelerated timeframe, but validation is necessary to confirm that these simulations provide meaningful and reliable predictions. Validation involves statistical analysis, replication of results, correlation with field data, and compliance with industry standards.

Statistical analysis and reproducibility

Aging test results must undergo rigorous statistical analysis to determine their reliability and significance. Key statistical methods used in validation include standard deviation analysis, confidence interval estimation, and regression modeling to establish trends over time. Reproducibility is also critical; results must be consistent when tests are repeated under identical conditions. Inter-laboratory studies and round-robin testing are often conducted to ensure that independent research teams can replicate findings with minimal variance. [20]

Correlation with real-world performance

To validate aging test results, researchers compare experimental outcomes with actual field data collected from long-term use of the product in its intended environment. This correlation helps assess whether the accelerated test conditions accurately simulate real degradation mechanisms. For example, in automotive materials testing, exposure to controlled ultraviolet (UV) light and humidity in a lab must reflect the wear observed in outdoor vehicle surfaces over years of service. If discrepancies arise, test conditions may require adjustment to better replicate environmental stressors. [21]

Compliance with industry standards

Many industries follow established guidelines and standards to validate aging test results. Regulatory agencies and international organizations provide testing protocols to ensure uniformity and comparability across different studies. Examples include:

  • ASTM International (ASTM) – Provides standardized methods for accelerated aging tests in various materials, including polymers, metals, and coatings. For instance, ASTM F1980 is a standard guide for accelerated aging of sterile barrier systems for medical devices. [22]
  • International Organization for Standardization (ISO) – Defines guidelines for environmental testing, including aging simulations for aerospace, automotive, and medical products. For examples, ISO 11607-1 specifies requirements for packaging materials intended to maintain the sterility of medical devices. [23]
  • United States Pharmacopeia (USP) – Establishes criteria for drug stability testing to determine shelf life under varying storage conditions. USP General Chapter <1150> provides guidance on pharmaceutical stability. [24]

Uncertainty and limitations in validation

Despite rigorous validation efforts, uncertainties exist in aging test results due to inherent limitations in accelerated testing methodologies. Factors contributing to uncertainty include:

  • Extrapolation errors – Predicting long-term performance based on short-term accelerated tests may introduce errors if degradation mechanisms do not scale linearly.
  • Environmental variability – Real-world conditions can be unpredictable, making it difficult to replicate exact field conditions in a laboratory setting.
  • Material inconsistencies – Variability in raw materials, manufacturing processes, or usage conditions can affect long-term performance in ways that are not fully captured by controlled tests.

To address these uncertainties, sensitivity analysis and conservative safety margins are often applied in engineering designs and product specifications. [25]

Applications

Accelerated aging is widely used across various industries to assess product longevity, reliability, and performance under simulated conditions. By exposing materials and components to intensified stressors, these tests help predict real-world degradation mechanisms within a reduced timeframe. Applications of accelerated aging include pharmaceuticals, medical devices, electronics, automotive materials, aerospace components, and consumer goods.

Pharmaceuticals and medical devices

In the pharmaceutical and medical industries, accelerated aging is critical for determining the shelf life and stability of drugs, vaccines, and sterile medical devices. Stability testing follows guidelines such as those outlined in the International Council for Harmonisation (ICH) Q1A(R2), which establishes protocols for subjecting pharmaceuticals to elevated temperature and humidity conditions. Medical device packaging validation, often performed according to ASTM F1980, ensures that sterile barrier integrity remains intact over time. [26]

Electronics and semiconductors

Electronics undergo accelerated aging to evaluate the long-term reliability of circuit boards, semiconductors, and connectors. Tests such as Highly Accelerated Life Testing (HALT) and Highly Accelerated Stress Screening (HASS) are commonly used to identify early failures due to thermal cycling, mechanical vibrations, and electrical load. Standards like IEC 60068-2 provide guidelines for environmental testing of electronic devices. [27]

Automotive industry

In the automotive sector, accelerated aging is used to test polymers, coatings, adhesives, and structural materials for resistance to heat, UV exposure, humidity, and mechanical stress. Xenon arc and QUV weathering tests (ISO 4892-2) simulate prolonged sun exposure to predict material degradation. Additionally, corrosion testing such as SAE J2334 replicates environmental conditions, including salt spray and humidity, to evaluate metal durability in vehicle components.

Aerospace and defense

Consumer products and packaging

Library and archival preservation science

Accelerated aging is also used in library and archival preservation science. In this context, a material, usually paper, is subjected to extreme conditions in an effort to speed up the natural aging process. Usually, the extreme conditions consist of elevated temperature, but tests making use of concentrated pollutants or intense light also exist. [28] These tests may be used for several purposes.

There is no single recommended set of conditions at which these tests should be performed. In fact, temperatures from 22 to 160 degrees Celsius, relative humidities from 1% to 100%, and test durations from one hour to 180 days have all been used. [28] ISO 5630-3 recommends accelerated aging at 80 degrees Celsius and 65% relative humidity [29] when using a fixed set of conditions.

Besides variations in the conditions to which the papers are subjected, there are also multiple ways in which the test can be set up. For instance, rather than simply placing single sheets in a climate controlled chamber, the Library of Congress recommends sealing samples in an air-tight glass tube and aging the papers in stacks, which more closely resembles the way in which they are likely to age under normal circumstances, rather than in single sheets. [30]

Limitations and criticisms

Accelerated aging techniques, particularly those using the Arrhenius equation, have frequently been criticized in recent decades. While some researchers claim that the Arrhenius equation can be used to quantitatively predict the lifespan of tested papers, [31] other researchers disagree. Many argue that this method cannot predict an exact lifespan for the tested papers, but that it can be used to rank papers by permanence. [32] [33] A few researchers claim that even such rankings can be deceptive, and that these types of accelerated aging tests can only be used to determine whether a particular treatment or paper quality has a positive or negative effect on the paper's permanence. [34]

There are several reasons for this skepticism. One argument is that entirely different chemical processes take place at higher temperatures than at lower temperatures, which means the accelerated aging process and natural aging process are not parallel. [28] [34] [35] Another is that paper is a "complex system" [32] and the Arrhenius equation only applicable to elementary reactions. Other researchers criticize the ways in which deterioration is measured during these experiments. Some point out that there is no standard point at which a paper is considered unusable for library and archival purposes. [35] Others claim that the degree of correlation between macroscopic, mechanical properties of paper and molecular, chemical deterioration has not been convincingly proven. [32] [36] Reservations about the utility of this method in the automotive industry as a method for assessing corrosion performance have been documented [37] [38]

In an effort to improve the quality of accelerated aging tests, some researchers have begun comparing materials which have undergone accelerated aging to materials which have undergone natural aging. [39] The Library of Congress, for instance, began a long-term experiment in 2000 to compare artificially aged materials to materials allowed to undergo natural aging for a hundred years. [40]

History

The technique of artificially accelerating the deterioration of paper through heat was known by 1899, when it was described by W. Herzberg. [28] Accelerated aging was further refined during the 1920s, with tests using sunlight and elevated temperatures being used to rank the permanence of various papers in the United States and Sweden. In 1929, a frequently used method in which 72 hours at 100 degrees Celsius is considered equivalent to 18–25 years of natural aging was established by R. H. Rasch. [28]

In the 1950s, researchers began to question the validity of accelerated aging tests which relied on dry heat and a single temperature, pointing out that relative humidity affects the chemical processes which produce paper degradation and that the reactions which cause degradation have different activation energies. This led researchers like Baer and Lindström to advocate accelerated aging techniques using the Arrhenius equation and a realistic relative humidity. [28]

See also

References

  1. ↑ John B. Wachtman (1996). Schneider, Gerold A.; Petzow, Günter (eds.). Thermal Shock and Thermal Fatigue Behavior of Advanced Ceramics. Springer. doi:10.1007/978-94-015-8200-1. ISBN   978-94-015-8200-1.
  2. ↑ "Thermotron Thermal Shock Chamber".
  3. ↑ "Tenney Thermal Shock Test Chamber".
  4. ↑ Zhang, Y. (2023). "Failure Analysis of Printed Circuit Board Solder Joint under Thermal Shock Test". Coatings. 13 (3): 572. doi: 10.3390/coatings13030572 .
  5. ↑ Zhang, Y. (2023). "Failure Analysis of Printed Circuit Board Solder Joint under Thermal Shock Test". Coatings. 13 (3): 572. doi: 10.3390/coatings13030572 .
  6. ↑ "Intertek Automotive-thermal-shock".
  7. ↑ "Understanding Thermal Shock: A Comprehensive Guide".
  8. ↑ {Enriquez, R. and Gillen, K. T. (2005). Review of accelerated ageing methods and lifetime prediction techniques for polymeric materials (PDF) (Report). National Physical Laboratory. Retrieved 16 January 2025.{{cite report}}: CS1 maint: multiple names: authors list (link)
  9. ↑ Rabek, J.F., ed. (2016). Handbook of UV Degradation and Stabilization. Elsevier. ISBN   978-1927885574.
  10. ↑ "ASTM G154-16: Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials". ASTM International. Retrieved 18 January 2025.
  11. ↑ "ASTM G155-13: Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials". ASTM International. Retrieved 18 January 2025.
  12. ↑ "Black Dust - Sulphidation". Chartered Institute of Plumbing and Heating Engineering. Retrieved 8 February 2025.
  13. ↑ Chen, X.; Wang, Y.; Liu, J. (2022). "Mechanical reliability assessment of high-speed rotating components". Applied Sciences. 12 (2): 712. doi: 10.3390/app12020712 .
  14. ↑ Harris, C. M. (2010). Shock and Vibration Handbook (6th ed.). McGraw-Hill. ISBN   978-0071508193.
  15. ↑ Chowdhury, A. S. M. R.; Rabby, M. M.; Kabir, M.; Das, P. P.; Bhandari, R.; Raihan, R.; Agonafer, D. (2021). "A Comparative Study of Thermal Aging Effect on the Properties of Silicone-Based and Silicone-Free Thermal Gap Filler Materials". Polymers. 13 (12): 1984. doi: 10.3390/polym13121984 .
  16. ↑ Kutz, Myer (2012). Handbook of Environmental Degradation of Materials (2nd ed.). William Andrew. ISBN   978-1437734560.
  17. ↑ Correia, João R.; Albuquerque, Pedro F.; Ferreira, José G.; Silva, Hugo F. (2024). "Synergistic effects of different ageing environments and sustained flexural loading on the durability of pultruded GFRP profiles". Energy Reports. 10: 303–318. doi:10.1016/j.egyr.2024.01.145 (inactive 2 February 2025). Retrieved 1 February 2025.{{cite journal}}: CS1 maint: DOI inactive as of February 2025 (link)
  18. ↑ Chen, X.; Wang, Y.; Zhou, L. (2024). "Effect of heat and humidity on the hydrolytic degradation of automotive polymers". Journal of Polymer Science. 62 (4): 250–265. doi:10.1002/pc.25094.
  19. ↑ Syed, A. (2022). "Predicting Fatigue of Solder Joints Subjected to High Number of Power Cycles" (PDF). Ansys Reliability Engineering Reports.
  20. ↑ Montgomery, D. C. (2020). Design and Analysis of Experiments (9th ed.). John Wiley & Sons. ISBN   978-1119722106.
  21. ↑ Celina, M. (2013). "Review of polymer oxidation and its relationship with material performance and lifetime prediction". Polymer Degradation and Stability. 98 (12): 2419–2429. doi:10.1016/j.polymdegradstab.2013.06.024.
  22. ↑ "ASTM F1980".
  23. ↑ "ISO 11607-1:2019".
  24. ↑ "USP General Chapter <1150>" (PDF).
  25. ↑ Zhang, Y.; Hoffman, M. (2022). "Uncertainty analysis in accelerated aging tests". Reliability Engineering & System Safety. 225: 108632. doi:10.1016/j.ress.2022.108632.
  26. ↑ Huynh-Ba, Kim (2009). Huynh-Ba, Kim (ed.). Handbook of Stability Testing in Pharmaceutical Development: Regulations, Methodologies, and Best Practices. Springer. doi:10.1007/978-0-387-85627-8. ISBN   978-0387856261.
  27. ↑ Hillman, C. (2013). Next Generation HALT and HASS: Robust Design of Electronics and Systems. Wiley. ISBN   978-1118700235.
  28. 1 2 3 4 5 6 7 8 9 "Archived copy". Archived from the original on 29 November 2014. Retrieved 19 November 2014.{{cite web}}: CS1 maint: archived copy as title (link), Porck, H. J. (2000). Rate of paper degradation: The predictive value of artificial aging tests. Amsterdam: European Commission on Preservation and Access.
  29. ↑ Bansa, H. (1992). Accelerated aging tests in conservation research: Some ideas for a future method. Restaurator 13.3, 114-137.
  30. ↑ "Accelerated Aging of Paper: A New Test (Preservation, Library of Congress)". Library of Congress . Archived from the original on 27 July 2009. Retrieved 11 August 2009., Library of Congress (2006). Accelerated aging of paper: A new test. The Library of Congress: Preservation. Retrieved 8 August 2009.
  31. ↑ Zou, X.; Uesaka, T; & Gurnagul, G. (1996). Predication of paper permanence by accelerated aging I. Kinetic analysis of the aging process. Cellulose 3, 243-267.
  32. 1 2 3 Strofer-Hua, E. (1990). Experimental measurement: Interpreting extrapolation and prediction by accelerated aging. Restaurator 11, 254-266.
  33. ↑ Bégin, P. L. & Kaminska, E. (2002). Thermal accelerated ageing test method development. Restaurator 23, 89-105.
  34. 1 2 Bansa, H. (2002). Accelerated aging of paper: Some ideas on its practical benefit. Restaurator 23, 106-117.
  35. 1 2 Bansa, H. (1989). Artificial aging as a predictor of paper's future useful life. The Abbey Newsletter Monograph Supplement 1.
  36. ↑ Calvini, P. & Gorassini, A. (2006). On the rate of paper degradation: Lessons from the past. Restaurator 27, 275-290.
  37. ↑ Hunt, Gregory (3 April 2018). "New Perspectives on Lubricant Additive Corrosion: Comparison of Methods and Metallurgy". SAE Technical Paper Series. Vol. 1. pp. 2018–01–0656. doi:10.4271/2018-01-0656.
  38. ↑ Hunt, Gregory (4 April 2017). "New Perspectives on the Temperature Dependence of Lubricant Additives on Copper Corrosion". SAE International Journal of Fuels and Lubricants. Vol. 10. pp. 2017–01–0891. doi:10.4271/2017-01-0891.
  39. ↑ Batterham, I & Rai, R. (2008). A comparison of artificial ageing with 27 years of natural ageing. 2008 AICCM Book, Paper and Photographic Materials Symposium, 81-89.
  40. ↑ , Library of Congress (2008). 100-year paper natural aging project. The Library of Congress: Preservation. Retrieved 8 August 2009.
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