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«A Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Aeronautics and Astronautics University of ...»

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Simulated Hail Ice Mechanical Properties and Failure Mechanism

at Quasi-Static Strain Rates

Jonathan M. Swift

A Thesis

submitted in partial fulfillment of the

requirements for the degree of

Master of Science in Aeronautics and Astronautics

University of Washington



Paolo Feraboli

Robert Breidenthal

Program Authorized to Offer Degree:

Aeronautics and Astronautics

Executive Summary

Hail is a significant threat to aircraft both on the ground and in the air. Aeronautical engineers are interested in better understanding the properties of hail to improve the safety of new aircraft. However, the failure mechanism and mechanical properties of hail, as opposed to clear ice, are not well understood. A literature review identifies basic mechanical properties of ice and a failure mechanism based upon the state of stress within an ice sphere is proposed.

To better understand the properties of Simulated Hail Ice (SHI), several tests were conducted using both clear and cotton fiber reinforced ice. Pictures were taken to show the internal crystal structure of SHI. SHI crush tests were conducted to identify the overall force- displacement trends at various quasi-static strain rates. High speed photography was also used to visually track the failure mechanism of spherical SHI. Compression tests were done to measure the compression strength of SHI and results were compared to literature data. Fracture toughness tests were conducted to identify the crack resistance of SHI. Results from testing clear ice samples were successfully compared to previously published literature data to instill confidence in the testing methods. The methods were subsequently used to test and characterize the cotton fiber reinforced ice.

1 Table of Contents Executive Summary

Table of Contents

List of Figures

List of Tables

I. Introduction

II. Background

A. Mechanical Properties of Ice

B. Stress Within a Sphere Under Compression

III. Specimen Production

A. Simulated Hail Ice Production

B. SHI Disc Production

C. High Speed Camera SHI Production

D. Fracture Toughness and Compression Cube Specimen Production

IV. Experiments and Results

A. SHI Disc Internal Crystal Photographs

B. Spherical Crush Tests

1. Test Description and Procedure

2. Test Results

3. Initial Data Analysis

4. Conclusions

C. High Speed Photography

1. Test Description and Procedure

2. Data Reduction and Analysis

3. Conclusions

D. Fracture Toughness

1. Test Description and Procedure

2. Data Reduction and Analysis

3. Conclusions

E. Compression Strength


1. Test Description and Procedure

2. Data Reduction and Analysis

V. Conclusions



A. Appendix A: Raw 2.5 in (63.5mm) Test Data, “Slow” Strain Rate = 0.01 1/s................. 68 B. Appendix B: Raw 2.5 in (63.5mm) Test Data, “Fast” Strain Rate = 0.4 1/s


3List of Figures

Figure 1: Global Hailstorm Frequency

Figure 2: DC-9 Hail Strike Damage, 7 May 1998

Figure 3: Boeing 737 Hail Strike Damage, 14 Aug 2003

Figure 4: Clear Ice Stress-Strain Behavior

Figure 5: Ice Compressive Strength Compilation

Figure 6: Ice Tensile Strength Data Compilation, 14°F (-10°C)

Figure 7: Types of Crack Propagation

Figure 8: Clear Ice KIC Rate Dependence

Figure 9: Clear Ice KIC Data Compilation

Figure 10: Hail Density Compilation

Figure 11: Radial and Hoop Stresses Along z-axis in Sphere (Half Sphere Shown) under Compression, with Stress Elements

Figure 12: Compression Estimate Parameter Diagram

Figure 13: SHI Mold

Figure 14: Clear and Cotton-SHI Pictures

Figure 15: SHI Disc [0.1 in (3 mm) thick, 2.1 in (55 mm) diameter] Showing Crystal Structure 27 Figure 16: SHI Disc with Cracks Highlighted

Figure 17: Instron 8801 Machine (L) with SHI Test Setup (R)

Figure 18: SHI Thermal Picture, Pre Fracture

Figure 19: Simplified Data Curves, 2.5 in (63.5 mm) SHI, Strain Rate = 0.01 1/s

Figure 20: Simplified Data Curves, 2.5 in (63.5 mm) SHI, Strain Rate = 0.4 1/s

Figure 21: Work to Failure, 2.5 in (63.5 mm) SHI, Strain Rate = 0.01 1/s

Figure 22: Work to Failure, 2.5 in (63.5 mm) SHI, Strain Rate = 0.4 1/s

Figure 23: Estimated Compression Strength, 2.5 in (63.5 mm) SHI, Clear Ice

Figure 24: Estimated Tensile Strength, 2.5 in (63.5 mm) SHI, Clear Ice

Figure 25: Est Tensile Strength, 2.5 in (63.5 mm) SHI, Strain Rate = 0.4 1/s

Figure 26: Est Compression Strength, 2.5 in (63.5 mm) SHI, Strain Rate = 0.4 1/s

Figure 27: 2.5 in (63.5mm), Clear-SHI Quasi-static (Strain Rate = 0.4 1/s) Force-Displacement Curve with Corresponding High Speed Camera Photos

4 Figure 28: 2.5 in (63.5mm), 4g Cotton-SHI Quasi-static (Strain Rate = 0.4 1/s) ForceDisplacement Curve with Corresponding High Speed Camera Photos

Figure 29: Fracture Toughness Beam Specimen and Test Setup

Figure 30: Notched Beam Thermal Picture, Pre-Fracture

Figure 31: Three Point Bend Test Geometry

Figure 32: Clear Ice KIC Rate Dependence, Experimental and Literature Data

Figure 33: KIC Literature Comparison

Figure 34: KIC Rate Dependence

Figure 35: KIC Relationship to Cotton Percentage by Mass, with Trendline

Figure 36: KIC Relationship to Cotton Fiber Volume, with Trendline

Figure 37: KIC Relationship to Cotton Mass for 2.5 in (63.5 mm) Sphere, with Trendline......... 54 Figure 38: Compression Test Setup

Figure 39: Cube Thermal Picture, Pre-Fracture

Figure 40: Clear Ice Compression Strength Literature Comparison

Figure 41: Compression Strength Relationship to Cotton Percentage by Mass, with Trendline. 60 Figure 42: Compression Strength Relationship to Cotton Fiber Volume, with Trendline........... 60 Figure 43: Compression Strength Relationship to Cotton Mass for 2.5 in Sphere, with Trendline

Figure 44: Clear and Cotton Cube Specimens, Post-Fracture

5List of Tables

Table 1: Basic Properties of Isotropic Ice, 3°F (-16°C)

Table 2: SHI Cotton Fill Amounts

Table 3: Quasi-Static SHI Crush Test Matrix

–  –  –

Weather is a significant influence on an aircraft’s operating environment. As aircraft become more common, it becomes increasingly important for engineers to account for severe weather to protect people and goods. One well documented weather related threat to aircraft is hail. Hail is a form of precipitation associated with thunderstorms. Under the correct conditions, the moisture in a thunderstorm can freeze into ice. Hail is a layered accumulation of ice and trapped air greater than 5mm in size [1]. Though the formation process is not completely understood by meteorologists, it is believed that hail forms when updrafts and downdrafts push ice crystals between freezing and nonfreezing layers in a thunderstorm. Additional moisture adheres to the crystal while the surface repeatedly freezes and melts. This changes the relative air content of the ice crystals, creating a layered, onion-like cross section. The ice then falls as hail when the internal winds are no longer strong enough to keep it airborne [2]. Depending upon the conditions, hail can vary widely in size and density, and presumably, in mechanical properties.

Though not every thunderstorm produces hail, they do occur more frequently in certain parts of the world, as can be seen in Figure 1. Engineers need to account for two types of hail strikes: during flight and on the ground.

–  –  –

Though pilots actively avoid the dangerous weather systems responsible for creating hail, hail strikes during flight can and do occur. These strikes tend to impact the leading edges of the aircraft at flight velocities, and may put the crew and passengers at risk by seriously damaging flight systems. Two incidents are sufficient to illustrate the threat. The first occurred on 7 May 1998 near Calhoun, Georgia [4]. The crew of AirTran Airlines Flight 426 attempted to fly through a gap in a line of thunderstorms when the airplane entered an area with severe turbulence and hail. The crew made an emergency landing at the Chattanooga Metropolitan Airport with a passenger and flight attendant seriously injured. The DC-9 aircraft landed with a partially shattered windshield, missing radome, and damage to the pitot system, wings, tail, and engines.

Pictures of the damage are shown in Figure 2.

–  –  –

The second occurred on 14 August 2003 on EasyJet Airlines Flight 903 near Geneva, Switzerland [6]. As reported by the Swiss Aircraft Accident Investigation Bureau (BFU), the Boeing 737 encountered a hailstorm shortly after takeoff, forcing the crew to return to Geneva for an emergency landing. The hail severely damaged the radome, windshield, and leading edges of the horizontal and vertical surfaces, as can be seen in Figure 3.

–  –  –

Aircraft can also be damaged by hail on the ground. In these cases, the hail tends to strike perpendicular to the aircraft surfaces, such as on the top of the wing or fuselage, at much lower velocities than experienced during flight. Hailstorms can shut down airport operations and

–  –  –

International Airport when hail damaged over 100 aircraft [8]. Storms like that disrupt travel and prompt unscheduled inspections and repairs on damaged aircraft, increasing costs for both airlines and passengers.

This hail threat grows due to the increasing use of composite materials in aircraft. The latest passenger airliners from Boeing and Airbus are increasingly made of carbon fiber reinforced plastics (CFRP). For example, both the Boeing 787 [9] and Airbus A350 XWB [10] are approximately 50% composite materials by weight. While using composites in aircraft structures offer numerous production and performance advantages over traditional metals, it must be understood that composites are damaged and fail differently than metals.

Currently, there are few regulations which deal with mitigating the threat specifically due to hail strikes for metallic, let alone composite aircraft. In the United States, the regulations governing aviation are found in the Code of Federal Regulations (CFR), Title 14: Aeronautics and Space, Chapter 1: Federal Aviation Administration (FAA). Parts 23 and 25 specifically deal with the airworthiness of small commuter and large transport class aircrafts, respectively [11].

Part 25 does not directly address the impact of hail strikes on aircraft structures, though section

25.571 states, “An evaluation of the strength, detail design, and fabrication must show that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage, will be avoided throughout the operational life of the airplane” [12]. This includes hail strike damage. Section 25.773 specifically mentions that an aircraft must be able to safely land in the event of hail damage to the windscreen [13]. ASTM Standard F320-10 [14] outlines a test procedure for meeting the requirements outlined in section 25.773 and will be referenced frequently in this paper simply as ASTM F320.

–  –  –

While many basic properties, such as size and density, have been studied, it is difficult to obtain hailstones suitable for testing. Most hail research for aeronautical applications has split along two different approaches. In the first, researchers assume that hail has the same mechanical properties as solid ice. Thus, this research focuses on studying the properties and creating models of ice, as distinct from hail, for experiments under conditions in which aircraft operate.

In the second, researchers use the test method outlined in ASTM F320, which assumes that hail is less dense and tougher than clear ice. Cotton is added to the test ice specimens at 12% concentration by mass to approximate these two properties. Unfortunately, until the mechanical properties of actual hailstones are determined, neither approach can be verified. In this paper spherical test specimens representing hail will be referred to as Simulated Hail Ice (SHI). SHI can consist of either clear ice (clear-SHI) or cotton fiber reinforced ice (cotton-SHI).

This thesis consists of several related parts. The first is to conduct a literature review on the mechanical properties of ice and the failure mechanism of spheres under compression.

Clear-SHI samples were tested for comparison to literature data to establish confidence in the testing methods. The same methods were subsequently used to test cotton-SHI outlined in ASTM F320 which may more accurately model the hail threat to aircraft. It should be noted that there is no previously published data on the mechanical properties of cotton-SHI. Testing focused on the most important properties related to the failure mechanism of spheres under compression.

–  –  –

The study of ice plays a significant role in many diverse engineering disciplines. Ice is a part of professional winter sports, influences the design of Artic oil drilling platforms, and can destroy electrical power lines. The study of ice from a mechanical engineering standpoint effectively began during World War Two when Geoffrey Pyke proposed using icebergs as natural aircraft carriers [15]. While there has been significant research into ice properties, little has focused on hail and even less on fiber reinforced ice.

As water freezes into ice, the molecules arrange into an ordered, crystalline form. The crystalline structure of ice can vary depending upon the freezing conditions. For example, Artic sea ice tends to consist of columnar shaped crystals while the crystals in glacial ice tend to be randomly arranged [16]. Previous research indicates that ice with relatively small, randomly arranged crystals is approximately isotropic. For this kind of ice, the basic mechanical properties are shown in Table 1.

–  –  –

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