1424 Mechanical Shock and Modal Test Techniques
Applications The effects of shock are important in many engineering applications ranging from appliances to computers to ships to automobiles, trucks and military vehicles to highperformance aircraft and missiles. Shock is often part of the service and/or transportation environment. Military Standards such as MILSTD810 call for shock testing.
The possible effect of shock must be considered for almost every product that has to be shipped and handled. Care can be taken in a controlled environment but during the transportation phase the product within its package must be designed and tested to withstand the anticipated environment.
For Whom Intended Engineers involved with dynamics and structural test applications.
Most engineers need specialized education in order to properly measure, quantize and analyze this generally unfamiliar environment, and to reproduce it in environmental test laboratories. This course is for packaging designers, test laboratory managers, engineers and aides. It also helps quality and reliability specialists and acquisition personnel in government and military activities, and their contractors.
Instrumentation specialists who will measure transportation, service and laboratory shock need this course. Metrologists learn about shock calibration of accelerometers and systems. Project personnel, structure and packaging engineers learn about developmental shock testing. Product assurance and acquisition specialists learn to evaluate shock test facilities and methods, and to interpret shock test specifications.
This course is designed to serve the varied needs of scientists, engineers, aides and senior technicians. The instructor maintains good balance between practical training and theory.
Brief Course Description The course begins with a review of structural and dynamic theory before examining methods of measuring frequency response from the structure under test. The causes and effects of shock are reviewed in detail, including the different shock pulse shapes.
Experimental modal testing is introduced by a brief discussion of theoretical modal analysis. The single degree of freedom (SDoF) model enables us to understand the fundamental concepts of free and forced vibration, natural frequency, resonance and damping. However in MDoF systems, resonance may occur at a number of different frequencies, each of which corresponds to a different pattern or shape of the system's motion. These are known as the natural or normal modes of vibration or mode shapes. There is a differential equation of motion for each degree of freedom; a set of n simultaneous equations is needed to mathematically describe a MDoF system. These equations are usually solved using matrix algebra.
In the experimental method of Modal Testing, the structure is excited by applying forced vibration and measuring the responses, from which the vibration modes are determined and a structural model developed. This is the reverse process to the theoretical method. Various methods of input excitation are discussed, such as shaker and impact hammer. Structural preparation and suspension methods are also examined.
A review of transducers and signal processing equipment is made before discussing analysis methods, timedomain curve fitting. Modal test philosophy including the sequence of steps and practical considerations in undertaking the test are discussed. The tabulation of results and derivation of mode shapes and construction of spatial models (mass, stiffness and damping) are covered before discussing the application of the modal test results.
The Shock Response Spectrum (SRS) is discussed as it relates to shock measurement and testing. The course then covers shock measurements, also calibration. The relative merits of various types of shakers and shock test machines are briefly considered before covering various shock test methods, including pyrotechnic shock testing. Some typical shock test procedures and specifications are described, both military and commercial.
Diploma Programs This course is required for TTi’s Dynamic Test Specialist (DTS) and Mechanical Design Specialist (MDS) Diploma Programs. It may be used as an optional course for any other TTi Specialist Diploma program.
Related Courses See TTi’s Course 142, Mechanical Shock Techniques, and Course 195, Modal Analysis for Structural Validation, which were combined to create this course.
Prerequisites Prior participation in TTi’s Fundamentals of Vibration would be helpful. Participants will need firstyear college mathematics (or equivalent experience) and some facility with fundamental engineering computations. Some familiarity with electrical and mechanical measurements and vibration will be helpful.
Text Each student will receive 180 days access to the online electronic course workbook. Renewals and printed textbooks are available for an additional fee.
Course Hours, Certificate and CEUs Class hours/days for onsite courses can vary from 1435 hours over 25 days as requested by our clients. Upon successful course completion, each participant receives a certificate of completion and one Continuing Education Unit (CEU) for every ten class hours.
OnDemand OnDemand Internet Complete Course 1424 features over 18 hours of video as well as more indepth reading material. All chapters of course 1424 are also available as OnDemand Internet Short Topics. See the course outline below for details.
Course Outline
Chapter 1  SingleDegreeofFreedom and 2DegreeofFreedom (SDoF and 2DoF) Systems

The Single Degree Of Freedom System

The Spring, k

The Mass, m

The Damper, c

Motion of an SDoF System

The Impulse Response Function, h(t)

The Frequency Response Function (FRF)

Structural Dynamic Relationships

Receptance, Mobility, Accelerance

Two Degrees of Freedom (2DoF)

The 2DoF Frequency Response Function

Observations from the 2DoF FRF
Chapter 2  MultipleDegrees of Freedom (MDoF) Systems

The Multiple Degrees Of Freedom System (MDoF)

Mass Matrix, [M]

Stiffness Matrix, [K]

Flexibility Matrix, [H]

Damping Matrix, [C]

Natural Frequencies and Mode Shapes

Modal and Frequency Matrices

Orthogonality and Normalization

Decoupling the Equations

Single Point Excitation and Response

Observations

Mode Shapes

Mode Shapes for a Cantilever

Mode Shapes for a Plate

Mode Shape Animation
Chapter 3  Some Essentials of Signal Processing

Analog to Digital (AD) Conversion

Aliasing

Fourier Transforms

Fast Fourier Transform

Discrete Fourier Transform, DFT

Windowing

Windowing for Continuous, Random Signals

Windowing for Transient, Impulsive Signals

System Identification Using the FFT

Signal Averaging

Coherence

Coherence—What’s Good and What’s Bad?

Some (Almost) Unbreakable Rules of Signal Processing
Chapter 4  Introduction to Shock

What is Shock?

Causes of Shock

Effects and Remedies of Shock
Chapter 5  A Closer Look at Shock

Terms Used in Mechanical Shock

Input Pulse and Response of a Sprung Mass

Typical Complex Shock Pulses

Shock Pulse Shapes

Shock Pulse Shape Parameters—Haversine Shape

Classical Shock Pulse Shapes

Example of 1000 g 1 ms Shock Pulse—Haversine Shape

Critical Frequency Response

Response to Shock Pulse
Chapter 6  Background and Theory of Modal Testing

What Is Experimental Modal Analysis (EMA)?

Why Experimental Modal Analysis?

Theoretical Modes

Stretched String

Rail Car

Experimental Examples

Ship Hull Section

Bridge Deck

Where Does the Modal Model Fit In to the Scheme of Things?

The Time Domain Structural Response

The Frequency Domain

Experimental Modal Analysis (EMA) Procedure
Chapter 7  Modal Test Planning and Setup

Selecting a Test Procedure

SteadyState

Random

Impact

Burst Random / Chirp

Shaker Testing

Impact Testing

Response Transducers

Strain Gages

Laser

Accelerometers

Strain Gage Accelerometers

Charge Accelerometers

Voltage Accelerometers

Voltage vs. Charge Accelerometers

Mounting Accelerometers

Transducer Selection: General Considerations
Chapter 8  Meshing

Meshing, Defined

Meshing Considerations

The “Pretty Picture” Approach

Finer Or Coarser – What’s the Difference?

An Interpolation Example

Practical Aspects of Marking a Mesh
Chapter 9  Setting up the Modal Test

Support the Structure

Support the Structure—Free Boundary

Setting up the Test—Mount the Transducers

Accelerometer Mounting Considerations

Contact Resonance Considerations

Mounting Methods

Stud

Superglue

Beeswax

Magnet

Mounting Base

DoubleMount

Miscellaneous

Setting up the Test—Suggestions for Making Life Easier

Setting up the Analyzer

Random Excitation

Impact Excitation

Windowing the Response Signals

Data Acquisition

Coherence Examples
Chapter 10  Modal Parameter Extraction

Natural Frequencies, Modal Damping, and Modal Constant

Modal Inferposition

Using Single Mode Methods

“Quadrature” Method

“Circle Fit” Method

Modal Residues

Multiple Mode Methods
Chapter 11  Documenting Modal Test Results

Average Coherence Example

Correct the Viscous Damping Coefficients

Tabulate Results

Presenting Mode Shapes

Deflected Shape

Undeflected and Deflected Shapes

Deflected Extremes

Arrows

Persistence

Color Rendition

Animations

Documentation of Results
Chapter 12  The Shock Response Spectrum

Shock Response Spectrum (SRS)

SRS Mechanical Analog

Accepted Definition of SRS

Terms used in SRS Analysis

Developing SRS

SRS Maximax Values

Maximum Response Spectra for Various Shock Pulse Shapes

Some Properties of the SRS

Velocity Sensitive Region of SRS

Damping and SRS

Maximum Response Spectra for Linear SDoF System

Designing with SRS

Absolute and Relative Deflection SRS

The Use of the SRS in Shock Testing

Required Spectrum and Allowable Tolerances

Shock Specifications

Shock Spectrum Analyzers

Measuring and Analyzing Mechanical Shock

Subroutine for the Calculation of the SRS
Chapter 13  Measurement of Shock

Force Sensors

Load Cell Characteristics

Motion—Displacement Trackers

Characteristics of Motion Trackers

High Speed Photography

ElectroMagnetic Induction

Motion—Velocity Sensors

Motion—Acceleration

Seismic Transducers

Seismic Transducer Characteristics

Pendulum Calibration

Dynamic Calibration of Motion Sensors

Cabling

Accelerator Attachment

Accelerometer QuickCheck Calibration

Accelerometer Loading Effect
Chapter 14  Shock Testing

Types of Mechanical Shock Testing

Shock Pulse — Acceleration, Velocity and Displacement

Example

Drop Test Machines

Navy Impact Machines

The “LightWeight” Shock Tester

HighAmplitude, HighFrequency “Impact” Transient Simulators

Impact Shock Simulators

MIPS Table .. Closer Look

Programmable Systems

ModerateLevel and/or MidFrequency Transients

Electrodynamic Shakers

Electrohydraulic (EH) Shakers

Piezoelectric Shakers

Shaker Technologies—Stroke vs. Frequency Range

Electrodynamic and Electrohydraulic Exciters

Optimized Tailoring

Generation of Oscillatory Transients

Decaying Oscillatory Acceleration

Shaker Optimized Cosine (SHOC) Pulses

Least Favorable Response

Pyrotechnic Shock

Simulating the Damage from a PyroShock “Event”

Rupture Energy Fixture

More Realistic Pyroshock Arrangements

Shock Testing Problem Areas

Data As We See It.

What Our System Has To Handle

Drop Machines

Objectives

Pendulum Type Shock Machine

FreeFall Shock Machine

Drop Testing Machine
Chapter 15A  MILSTD810G, Method 516.6, Shock
Chapter 15B  Undex Underwater Explosions And Surface Testing
Chapter 15C  Typical Free Fall Shock Test Specification

Free Fall Drop Test Methods

Tabulation of Test Data

Test Procedures, under 55 lbs.

Test Procedures, under 220 lbs.

Test Procedures, 220–1,000 lbs

Surface Drop Test

Edge Drop Test

Corner drop tests

Equipment Handling

Test Setup Using a Mechanical Device
Chapter 15D  TableTop Drop Shock Test
Chapter 15E  Typical Drop Shock and Vibration Test Specification for Disk Drive Assemblies

Purpose

Associated Documents

Equipment Required

Software

Drive Configuration

Drop Shock Fixturing

Design Verification Testing

Design Maturity Testing (DMT)

Disk Device Shock/Vibration Specification

PCB Shock/Vibration Specification

Pneumatic Drop Test Shock Machine

Orientation of Axes

Drop Shock Test Fixture

Drop Shock Test Fixture Adapter to Shock Table

Typical Drop Shock Pulse
Appendix A  Glossary of Symbols and Units
Appendix B  A Brief Run through an EMA Computer Session

Introduction

Mesh

FRB Program

Plots

Locate Resonances

Curve Fitting

Damping, Mode Shapes
Appendix C  Finite Element Analysis
Appendix D  Matrix Math Revisited
Summary
Final Review
Award of certificates for successful completion
Click for a printable course outline (pdf).
Revised 6/18/2018